Geology of the

Aspen 

JL 

15-minute Ouadrangle VSi^r \«/

0EOLOOICAJ. SURVEY PROFESSIONAL P^PER
Geology of the

Aspen 15-minute Quadrangle,

Pitkin and Gunnison Counties,

Colorado

By BRUCE BRYANT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1073

A study of an area at the intersection of the 

Colorado mineral belt with the west flank of the 

Sawatch uplift

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Bryant, Bruce Hazelton, 1930-

Geology of the Aspen 15-Minute quadrangle, Pitkin and Gunnison Counties, Colorado

(Geological Survey Professional Paper 1073)

Bibliography: p. 123

1. Geology Colorado Aspen region. 2. Mines and mineral resources Colorado Aspen region. I. Title.

II. Series: United States Geological Survey Professional Paper 1073 

QE92.A86B79 557.88*41 78-606144

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-03254-6
CONTENTS

Abstract..............................

Introduction ..........................

Location and physical geography 

Previous geologic investigations 

Present investigations .............

Fieldwork and acknowledgments

General geology .......................

Precambrian rocks.....................

Mica schist and gneiss .............

Page

............. 1

............. 2

............. 2

............. 4

............. 4

............. 4

............. 5

............. 7

............. 7

Mixed quartz diorite, pegmatite, quartz monzonite, and

mica schist and gneiss ........................... 7

Biotite quartz diorite ................................. 8

Porphyritic quartz monzonite ......................... 8

Muscovite-biotite granite............................. 9

Quartz monzonite.................................... 9

Age ................................................ 11

Cambrian through Mississippian shelf sequence ............ 12

Introduction ........................................ 12

Sawatch Quartzite ................................... 12

Peerless Formation .................................. 14

Manitou Dolomite ................................... 15

Stratigraphic relations and correlations of Cambrian and

Ordovician rocks................................ 16

Chaffee Group....................................... 17

Parting Formation ............................... 18

Dyer Dolomite................................... 19

Oilman Sandstone ............................... 20

Leadville Limestone ................................. 21

Pennsylvanian through Triassic sequence of the Eagle Basin . 22

Belden Formation ................................... 23

Gothic Formation of Langenheim (1952)............... 25

Eagle Valley Formation .............................. 28

Maroon Formation................................... 29

Environments of deposition of Gothic and Maroon Formations ........................................ 40

State Bridge Formation .............................. 40

Chinle Formation .................................... 44

The nonmarine and marine rocks of the Jurassic and Cretaceous .......................................... 46

Entrada Sandstone .................................. 46

Morrison Formation ................................. 47

Burro Canyon Formation and Dakota Sandstone ....... 49

Mancos Shale ....................................... 51

Laramide igneous rocks .................................. 54

Hornblende quartz diorite ............................ 54

Quartz porphyry..................................... 55

Aplite .............................................. 56

Aplite porphyry ..................................... 56

Composition ........................................ 56

Middle Tertiary igneous rocks ............................ 58

Granodiorite ........................................ 61

Alteration ....................................... 63

	Page

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...... 79

...... 80

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...... 80

...... 80

...... 83

...... 84

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...... 87

...... 88

...... 88

...... 88

...... 90

...... 93

...... 97

Rock glaciers ........................................ 101

Age of the rock glaciers ........................... 101

Landslide deposits ................................... 103

Solifluction deposits ................................. 103

Colluvium........................................... 104

Talus ............................................... 104

Alluvial and debris fan deposits ....................... 105

Alluvium............................................ 105

Economic geology ....................................... 106

Deposits of Laramide age ............................. 106

The Highland tunnel............................. 110

Lenado.......................................... Ill

Ore deposits of middle Tertiary age .................... 113

Iron deposit at head of Cooper Creek............... 113

Disseminated sulfide deposits ..................... 114

Fissure veins .................................... 114

Copper-silver deposits in Dakota Sandstone ........ 115

Silver content of copper sulfides ............... 119

Conclusions ................................. 119

Ore deposits of unknown age.......................... 120

Gold Hill........................................ 120

References cited......................................... 121

Index................................................... 141

Middle Tertiary igneous rocks Continued

Porphyritic granodiorite ..................

Hornblende porphyry .....................

Alaskite .................................

Composition .............................

Tertiary and Quaternary diabase dikes .........

Metamorphism ...............................

Chemistry of metamorphism...............

Conditions of metamorphism ..............

Structure ....................................

Precambrian structures ....................

Phanerozoic structure.....................

Pre-Laramide structural events ........

Late Cretaceous and Paleocene(?) events 

Laramide structures ..................

Faults............................

The Elk Range thrust zone .........

The Castle Creek fault zone ........

Other faults ......................

Folds.............................

Oligocene intrusion and deformation 

Late Tertiary movements..............

Joints, shears, faults, and veins ............

Quaternary surficial deposits..................

Glacial geology ...........................

Moraine deposits .....................

Outwash terrace deposits...............

Age and correlation of glacial deposits ..

m
IV CONTENTS

STRATIGRAPHIC SECTIONS

Page

SECTION 1. Manitou Dolomite, Peerless Formation, and Sawatch Quartzite .................................................... 130

2. Morrison Formation (part), Entrada Sandstone, Chinle Formation, Maroon Formation, and Gothic Formation (part) of

Langenheim (1952) ......................................................................................... 131

3. Dakota Sandstone (part), Burro Canyon Formation, Morrison Formation, Entrada Sandstone, Chinle Formation, and

State Bridge Formation ..................................................................................... 137

4. Dakota Sandstone and Burro Canyon Formation ................................................................. 139

ILLUSTRATIONS

FIGURE 1. Map showing location of the Aspen area.......................................................................... 3

2. Diagram of classification of sedimentary rocks ................................................................... 5

3. Map showing regional geologic setting of the Aspen area .......................................................... 6

4. Photographs of outcrops of mica schist and gneiss ................................................................ 8

5. Diagram of proportions of quartz, plagioclase, and potassic feldspar in rocks of the quartz monzonite unit............. 11

6. Photograph of lower and middle Paleozoic rocks .................................................................. 13

7. Diagram of proportions of quartz, feldspar, and mica and clay in Sawatch Quartzite ................................. 14

8. Photograph of typical thin and irregular bedding in Peerless Formation............................................. 15

9. Diagram of proportions of quartz, carbonate, and mica and feldspar in the Peerless Formation........................ 15

10. Photograph of upper part of Manitou Dolomite showing lenses and stringers of white chert and wavy irregular bedding

planes..................................................................................................... 16

11. Sketch of possible stratigraphic relations of Cambrian rocks in west-central Colorado in a northwest-southwest section

through Aspen............................................................................................. 18

12. Sketch of possible stratigraphic relations of Cambrian rocks in west-central Colorado in a north-south section through

Aspen..................................................................................................... 18

13. Photograph showing contact of Parting Formation of the Chaffee Group with the Manitou Dolomite .................. 19

14. Diagram of proportions of quartz, carbonate, and mica and feldspar in the Parting Formation of the Chaffee Group..... 20

15. Photograph of upper part of the Leadville Limestone .............................................................. 21

16. Photographs showing features of the Belden Formation ........................................................... 24

17. Photographs of breccia in Gothic Formation of Langenheim (1952)................................................. 26

18. Diagram of proportions of quartz, carbonate, and mica and feldspar in the Gothic Formation of Langenheim (1952)..... 27 

19-22. Photographs:

19. Panorama of the Elk Mountains west and southwest from ridge between East Maroon and Conundrum Creeks .. 30

20. Maroon Formation on northeast ridge of North Maroon Peak............................................... 31

21. Features of the Maroon Formation ....................................................................... 32

22. Evaporite in the lower part of the Maroon Formation near Hayden Peak ..................................... 34

23. Diagrams of mineral composition of the Maroon Formation ........................................................ 36

24. Chemical diagrams of analyzed rocks from the Maroon Formation.................................................. 37

25. Summary diagram of 41 azimuths of crossbedding in the Maroon Formation ........................................ 40

26. Photographs showing features of the State Bridge Formation...................................................... 42

27. Photographs comparing typical textures of sandstone in the Maroon Formation with sandstone in the State Bridge

Formation................................................................................................. 43

28. Diagram of proportions of quartz, carbonate, and mica and feldspar in the State Bridge Formation.................... 43

29. Photograph of Gartra Member of the Chinle Formation............................................................ 44

30. Diagram of proportions of quartz, carbonate, and mica and feldspar in the Chinle Formation.......................... 45

31. Photographs of Entrada Sandstone and Morrison Formation ...................................................... 47

32. Photographs of Morrison Formation ............................................................................. 48

33. Photographs showing features of the Mancos Shale ............................................................... 51

34. Photomicrographs of Laramide igneous rocks .................................................................... 57

35. Chemical diagrams for analyzed Laramide igneous rocks .......................................................... 58
CONTENTS V

	Page

FIGURE 36. Panorama of the east end of the Elk Mountains ................................................................... 60

37. Photograph of crosscutting contact of part of the White Rock pluton ............................................... 61

38. Photographs showing features at or near contacts of the White Rock pluton ......................................... 62

39. Photomicrographs of granodiorite of the White Rock pluton ....................................................... 64

40. Photographs of porphyritic granodiorite intrusives ............................................................... 66

41. Photomicrograph of porphyritic grandiorite ...................................................................... 67

42. Photomicrograph of hornblende porphyry ........................................................................ 67

43. Diagram of proportions of quartz, plagioclase, and potassic feldspar................................................ 68

44. Chemical and normative diagrams of analyzed middle Tertiary igneous rocks ........................................ 69

45. Photographs showing areas of thermal metamorphism ............................................................ 73

46. Diagram of chemical changes in red beds of the Maroon Formation during contact metamorphism .................... 75

47. Diagrams of structures in Precambrian mica schist and gneiss in the Hayden Peak quadrangle ....................... 77

48. Map showing some structural features in the Aspen region ........................................................ 78

49. Photograph of west side of Conundrum Creek window ............................................................. 81

50. Photographs showing features of the Castle Creek fault zone ....................................................... 83

51. Photographs showing middle Tertiary faults ..................................................................... 86

52. Diagrams showing orientation of faults, shears, joints, and veins ................................................... 89

53. Map of upper Tertiary and Quaternary rocks of the Aspen area .................................................... 92

54. Map showing glaciers in the Aspen area during the maximum extent of Bull Lake(?) Glaciation ....................... 94

55-58. Photographs:

55. Moraine deposit C and associated features ................................................................ 95

56. Moraine deposit B ...................................................................................... 96

57. Cirque moraines ........................................................................................ 97

58. Outwash terraces in the Roaring Fork River Valley ........................................................ 97

59. Profile sketch along the Roaring Fork River valley showing present stream grade, terrace levels, moraines, and older

gravels .................................................................................................... 98

60. Photographs of neoglacial moraines in the Hayden Peak quadrangle................................................ 100

61. Photographs of rock glaciers .................................................................................... 102

62. Photographs of landslides ...................................................................................... 104

63. Map showing areas of production, prospects, disseminated sulfides, and Oligocene and Laramide igneous rocks ........ 107

64. Cross section along the Highland tunnel......................................................................... 110

65. Geologic map of the Lenado area ................................................................................ 112

66. Photograph showing setting of the Pitkin Iron Corp. mine ......................................................... 113

67. Geologic map of the Eagle Mountain area showing location of prospects along the Snowmass Creek fault zone ......... 117

68. Map showing locations of prospects and localities of analyzed samples, Gold Hill area ................................ 122

TABLES

Page

TABLE 1. Chemical analyses, modes, and norms of Precambrian rocks ....................................................... 10

2. History of stratigraphic nomenclature of Cambrian through Mississippian quartzite-carbonate shelf sequence in Aspen

area ....................................................................................................... 14

3. Chemical analyses and modes of Maroon Formation and hornfels ................................................... 38

4. Fossils in the Mancos Shale ..................................................................................... 53

5. Chemical analyses, modes, and norms of Laramide igneous rocks ................................................... 55

6. Chemical analyses, modes, and norms of middle Tertiary igneous rocks ............................................. 70

7. Relation between mapped structural elements and measured joints, shears, and veins ................................ 90

8. Correlation of Pleistocene and Holocene events, Roaring Fork and Fryingpan River drainages ........................ 99

9. Analyses of some ore samples, Aspen, Richmond Hill, and Lenado districts ......................................... 109

10. Analyses of veins associated with the White Rock pluton .......................................................... 114

11. Estimated modes of mineralized Dakota Sandstone from near the Snowmass Creek fault zone ........................ 116

12. Analyses of mineralized Dakota Sandstone from near the Snowmass Creek fault zone ................................ 118

13. Microprobe analyses of silver-bearing copper sulfides from the Snowmass Creek mine................................ 119

14. Analyses of rock samples from the Gold Hill area ................................................................. 120
VI CONTENTS

METRIC-ENGLISH EQUIVALENTS

[SI, International System of Units, a modernized metric system of measurement]

SI unit U.S. customary equivalent

Length

millimeter (mm) 

meter (m)

kilometer (km)

= 0.03937 inch (in) 

= 3.281 feet (ft) 

= 1.094 yards (yd) 

= 0.621 4 mile (mi) 

= 0.540 0 mile, nautical (nmi)

Area

centimeter2 (cm2 ) 

meter2 (m2 )

hectometer2 (hm2 ) 

kilometer2 (km2 )

- 0.1550 inch2 (in2 ) 

= 10.76 feet2 (ft2 ) 

 1.196 yards2 (yd2 )

= 0.000 247 1 acre 

= 2.471 acres 

= 0.003 861 section (640 acres or 

1 mi2 ) 

= 0.386 1 mile2 (mi2 )

Volume

centimeter3 (cm3 ) 

decimeter3 (dm3 )

meter3 (m3 )

hectometer3 (hm3 ) 

kilometer3 (km3 )

= 0.061 02 inch3 (in3 )

 61.02 inches3 (in3 ) 

= 2.113 pints (pt) 

= 1.057 quarts (qt) 

- 0.264 2 gallon (gal)

= 0.035 31 foot3 (ft3 ) 

= 35.31 feet3 (ft3 )

 1.308 yards3 (yd3 ) 

= 264.2 gallons (gal) 

= 6.290 barrels (bhl) (petroleum, 1 bbl = 42 gal) 

= 0.0008107 acre-foot (acre-ft) 

= 810.7 acre-feet (acre-ft) 

= 0.239 9 mile3 (mi3 )

Volume per unit time (includes flow)

decimeter3 per second 

(dmVs) = 0.035 31 foot3 per second (ft3 /s)

= 2.119 feet3 per minute (ft3 / 

min)

SI unit

Volume per

decimeter3 per second 

(dm3/s)

meter3 per second (m3 /

U.S. customary equivalent

unit time (includes flow) Continued

= 15.85 gallons per minute 

(gal/min) 

= 543.4 barrels per day 

(bbl/d) (petroleum, 

1 bbl = 42 gal)

« \ OK 01 foots rtpr cAprtfiri fff3/o^

= 15 850 gallons per minute 

(gal/min)

Mass

gram (g) 

kilogram (kg) 

megagram (Mg)

= 0.035 27 ounce avoirdupois (oz 

avdp) 

= 2.205 pounds avoirdupois (Ib 

avdp) 

 1.102 tons, short (2000 Ib)

= 0.984 2 ton, long (2 240 Ib)

Mass per unit volume (includes density)

kilogram per meter3 

(kg/m3 )

 0.062 43 pound per foot3 (lb/ft3 )

Pressure

kilopascal (kPa) =r 0.145 0 pound-force per inch2 

(lbf/in2 ) 

i= 0.009 869 atmosphere, standard 

(atm) 

= 0.01 bar 

= 0.296 1 inch of mercury at 

60°F (in Hg)

Temperature

temp kelvin (K) 

temp deg Celsius (°C)

= [temp deg Fahrenheit (°F) +459.671/1.8 

= [temp deg Fahrenheit (°F) 32] /1.8
GEOLOGY OF THE ASPEN 15-MINUTE QUADRANGLE, 

PITKIN AND GUNNISON COUNTIES, COLORADO

By BRUCE BRYANT

ABSTRACT

The Aspen area, located 170 km southwest of Denver, Colo., lies 

at the intersection of the northeast-trending Colorado mineral belt 

and the west margin of the north-trending Sawatch uplift of 

Laramide age; it is within the southwest part of the northwesttrending late Paleozoic Eagle basin.

Precambrian shales and graywackes, perhaps as old as 2 billion 

years (b.y.), were converted to sillimanite-bearing gneiss and 

muscovite-biotite schist 1.65-1.70 b.y. ago. They were deformed into northeast-plunging folds and were migmatized, and they were intruded by quartz diorite, porphyritic quartz monzonite, and granite. 

Muscovite-biotite quartz monzonite intruded this older Precambrian terrane about 1.45 b.y. ago and is the predominant Precambrian rock near Aspen. Uplift, some faulting, and much erosion occurred during the 900-million year (m.y.) interval between emplacement of the plutonic rocks and deposition of Upper Cambrian 

sediments.

From Late Cambrian through Mississippian the region was part 

of a broad area alternately covered by shallow seas or occupied by 

low-lying land. Quartzite, dolomite, and limestone 200-320 m thick, 

comprising the Sawatch Quartzite and Peerless Formation (Cambrian), Manitou Dolomite (Ordovician), Chaffee Group (Mississippian(?) and Devonian), and Leadville Limestone (Mississippian) 

were deposited during this interval.

After an hiatus during which soil formation and solution of the 

Leadville Limestone took place in the Late Mississippian, a thick sequence of marine and nonmarine clastic rocks was deposited in the 

newly developing Eagle basin during the late Paleozoic and early 

Mesozoic. Deposition of about 300 m of carbonaceous shale, 

limestone, dolomite, and minor siltstone and evaporite of the Belden 

Formation began in a shallow sea in Early and Middle Pennsylvanian time. Facies relations indicate that the northwest-trending Uncompahgre uplift southwest of Aspen, if present at that time, had 

very low relief. The overlying Middle Pennsylvanian Gothic Formation of Langenheim (1952) contains calcareous sandstone, siltstone, 

shale, limestone, and evaporite. Its clastic debris, significantly 

coarser than that in the Belden, signals the initial rise of the Uncompahgre uplift bordering the Eagle basin on the southwest; the 

Gothic here lacks the conglomerates and fossiliferous marine 

limestones found closer to the uplift. Red terrigenous clastic rocks 

and minor limestone and evaporite of the Maroon Formation as 

much as 3,200 m thick, deposited mainly hi a fluvial flood-plain environment during the rest of the Pennsylvanian and the Early Permian, indicate withdrawal of the sea caused by further uplift of the 

Uncompahgre highland.

Following an hiatus accompanied by local folding, the red conglomerate, sandstone, and siltstone of the State Bridge Formation 

(Late Permian and Early Triassic) was deposited in a fluviallacustrine environment adjacent to a much-expanded Uncompahgre 

uplift; a significant part of the State Bridge is material recycled 

from the Maroon Formation exposed to erosion on the flank of the

uplift. The State Bridge, absent towards the south, becomes thicker 

and finer grained towards the north.

The Chinle Formation (Late Triassic) rests with angular unconformity on the State Bridge Formation. The Chinle contains a basal 

discontinuous quartz-pebble conglomerate (Gartra Member) and is 

chiefly calcareous siltstone and limestone, with some beds of sandstone and conglomerate composed of fragments derived from the 

limestone beds. The Chinle was deposited on flood plains and in 

lakes by streams. Storms may have disrupted the sediments in the 

lakes producing the limestone pebble conglomerates. The lack of 

feldspar in the Chinle indicates that the nearby part of the Uncompahgre uplift was not a sediment source, or was covered by a deeply 

weathered feldspar-free mantle. The formation, absent towards the 

south, thickens toward the north.

Thicknesses of the Maroon, State Bridge, and Chinle change 

across faults in the Highland Peak quadrangle, showing that the 

faults were active during or after deposition of these units. Uplift 

and erosion of the Uncompahgre highland during the Early Jurassic 

accounts for the zero isopach of the Chinle. This tectonic episode 

clearly had ended prior to deposition of the Morrison Formation 

(Upper Jurassic). The faults were reactivated much later during the 

middle Tertiary and, perhaps, during the Laramide.

The Entrada Sandstone (Upper Jurassic), 30 m or less thick, was 

deposited discontinuously throughout the Aspen area. The Morrison Formation (Upper Jurassic), 122-162 m thick, was deposited 

in swamps on flood plains and in lakes. Sandstone and siltstone of 

the Burro Canyon Formation (Lower Cretaceous), 18-24 m thick, 

then was deposited in a continental environment by streams flowing from the west. This was succeeded by the Dakota Sandstone 

(Lower Cretaceous), 67-73 m thick, which was deposited in a 

shoreline and marginal marine environment. About 1,600 m of 

marine shale, sandstone, and limestone of the Mancos Shale were 

deposited in the Aspen area in the Late Cretaceous; deposition probably continued until the beginning of Laramide tectonism.

Laramide tectonism and intrusive activity began about 72 m.y. 

ago. Three major structural elements produced during this event 

are the Sawatch uplift, the Elk Range thrust, and the Castle Creek 

fault zone. Early in the rise of the Sawatch uplift a thick section of 

sedimentary rock overlying the Belden Formation slid off the uplift 

in a west to southwest direction along the Elk Range thrust. Further uplift caused faulting along the uplift margin and formation of 

the Castle Creek fault zone, which forms part of the uplift margin. 

At Aspen the Castle Creek fault zone has a stratigraphic displacement of 4.2 km. The zone is the principal structure bounding the 

Sawatch uplift south of Aspen, but towards the north it diverges 

from the northeasterly trending margin of the uplift, and instead 

trends northwest to follow a late Paleozoic zone of weakness. A 

fourth major structure, the Roaring Fork syncline, preserves rocks 

as young as Late Cretaceous and is itself cut by the Castle Creek 

fault zone.

Emplacement of the Laramide porphyritic intrusions in the

1
ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Aspen mining district preceded and accompanied faulting. These 

porphyries constitute a bimodal suite: hornblende quartz diorite 

(54-57 percent SiO2 ) and more felsic rocks (70-75 percent SiO2 ). The 

felsic porphyries have low contents of MgO, total Fe, and K2O 

relative to their SiO2 contents. The silver-lead-zinc deposits of the 

Aspen, Richmond Hill, and Lenado districts are spatially related to 

the Laramide porphyries and faults.

About 35 m.y. ago, in Oligocene time, sills, dikes, laccoliths, and 

plutons of granodiorite invaded the area south and southwest of 

Aspen. The largest body, the White Rock pluton, was emplaced 

along the Elk Range thrust and caused uplift and high-angle 

faulting. The Oligocene igneous rocks are a typical calc-alkaline 

suite of the continental interior derived from the lower crust or upper mantle. Their SiO2 content ranges from 55 to 70 percent and the 

rock composition, from diorite to quartz monzonite. A few veinlets 

of aplite cut the main pluton. The more mafic parts of the suite have 

relatively high K2O contents.

The White Rock pluton caused extensive hornblende hornfels 

facies contact metamorphism of the Belden, Gothic, and Maroon 

Formations. Pyroxene, amphibole, garnet, and vesuvianite are 

characteristic minerals of the hornfels. Sodic scapolite commonly 

forms late porphyroblasts. Zeolites, such as chabazite, thomsonite, 

stilbite, and natrolite, coat late fractures and vugs in the hornfels. 

Red beds of the Maroon Formation were converted into green and 

gray calc-silicate hornfels and quartzite. Loss of the volatiles CO2 , 

H2O, and O2 occurred during the metamorphism.

After the main pluton cooled enough to allow fractures to form, 

dikes of porphyritic granodiorite were intruded. After intrusion of 

the dikes, extensive pyritization and locally intensive albitization, 

silicification, and sericitization occurred at two principal centers 

and were accompanied by introduction of Cu and Mo. Fluid inclusion studies suggest that the alteration took place at temperatures 

of 275°-350°C and depths of about 1,000 m.

A contact metasomatic magnetite deposit occurs in marble of the 

Belden Formation just outside the White Rock pluton. Veins 

associated with the middle Tertiary igneous episode are thin and 

discontinuous and contain silver, lead, zinc, and, locally, gold. Carbonaceous beds in Dakota Sandstone contain disseminated bornite 

and lesser amounts of chalcocite adjacent to a fault of this age. The 

bornite contains as much as 1.9 weight percent Ag and the 

chalcocite as much as 2.7 weight percent Ag.

A few diabase dikes that cut the area are probably related to an 

episode of late Tertiary to Holocene volcanism in the region and 

perhaps correlate with a series of basalt flows deposited 1.5 m.y. 

ago in a valley about 370 m above the present valley of the Roaring 

Fork River.

A sequence of glacial moraine and outwash deposits fills the lower 

parts of the main valleys. Deposits on top of Red Mountain, just 

north of Aspen, arid on an interstream divide southwest of 

Snowmass 450 m above the modern Roaring Fork River valley may 

date from the early Pleistocene before much of the valley cutting in 

the upper part of the Roaring Fork drainage. The oldest deposits inside the present valley of the Roaring Fork are now 200 m above the 

present stream and are probably middle Pleistocene in age. The 

next youngest are 100 m above the modern river bed. Moraines and 

terraces dating from the latest full glacial episode are breached only 

by narrow gorges, which are 50 m deep downstream from Aspen. 

Other end moraines are found upvalley from Aspen, and several 

moraines of latest Pinedale age occur in the cirque floors. Rock 

glaciers, some of which may have formed before the Altithermal and 

some of which may have formed after that event, are abundant, and 

a few are moving at rates of tens of centimeters a year. A few 

moraines date from neoglacial times.

INTRODUCTION 

LOCATION AND PHYSICAL GEOGRAPHY

The Aspen, Colo., area is located in the drainage of 

the Roaring Fork of the Colorado River in the southern 

Rocky Mountains about 170 km southwest of Denver, 

Colo., and 20-30 km west of the Continental Divide 

where it follows the crest of the Sawatch Range (fig. 1). 

The southwestern half of the area comprises the 

eastern part of the Elk Mountains, a northwesttrending range of high peaks extending from the head 

of the Taylor River to the Crystal River.

Altitude ranges from 2,256 m on the Roaring Fork 

River to 4,348 m at the summit of Castle Peak in the 

southwest corner of Hayden Peak quadrangle. The 

northeastern half of the area is densely timbered (fig. 

62A) except in the sage-oak brush zones below the 

timber (fig. 33.B). The southwestern half of the area is 

largely above timberline except for the bottoms and 

sides of the major valleys (fig. 19).

Pleistocene glaciation has contributed much to the 

shape of the landscape. The major mountain valleys 

are glacial troughs modified by postglacial alluvial 

fans, talus cover, and landslides. Tributary streams 

occupy hanging valleys above the main troughs. The 

mountains themselves are shaped by numerous 

cirques, some of which contain active rock glaciers, 

and the interstream ridges consist of narrow aretes 

along some stretches. Morainal and outwash landforms dominate the major stream valleys of the Roaring Fork and Snowmass Creek west of Aspen. The 

northernmost part of the area was not glaciated, and 

its landforms are the result of the processes of normal 

stream erosion, landsliding, and downslope movement 

of soil and colluvium.

The climate and, consequently, the vegetation vary 

greatly in short distances. The crest of the Elk Mountains receives more moisture than the ranges farther 

east in the Colorado Rockies. About 130 cm of 

precipitation falls on the crest of the Elk Mountains, 

much of it in the form of snow. The amount of 

precipitation decreases considerably northeast from 

the crest of the Elk Mountains so that the city of 

Aspen has only about 50 cm (Berry, 1959). Just a few 

miles down the Roaring Fork River from Aspen 

precipitation is even less so that the climate there is 

truly semiarid.

Temperatures in the Roaring Fork valley are mild 

except for nights on the valley floor in midwinter, 

which may register 18 °C or lower. The sun warms 

the air into the 6° to +3°C range on most winter 

days. At Aspen 32°C is about the hottest temperature 

recorded; most summer highs are around 28 °C.
107°

INTRODUCTION

106°

50 100 KILOMETERS

FIGURE 1. Location of the Aspen area, southern Rocky Mountains, Colo. Heavy solid outline, Aspen 15-minute topographic quadrangle. Numbers are key to 7.5-minute quadrangles in and adjacent to the Aspen area: 1, Maroon Bells (1960); 2, Hayden Peak 

(1960); 3, Aspen (1960); 4, Highland Peak (1960); 5, Ruedi (1961); 6, Woody Creek (1961); 7, Capitol Peak (1960); 8, Snowmass Mountain (1960); 9, Gothic (1961); 10, Pearl Pass (1961); 11, New York Peak (1960); 12, Thimble Rock (1960).

The summers in the high mountains are short, but 

sustain a luxuriant growth; wildflowers are abundant, 

especially so in the meadows in the high basins where 

the snow melts late. Summer rains in the form of local 

convectional showers or from an occasional frontal 

system are quite variable in intensity and duration, 

and while working there I found conditions to range 

from a daily rain of 1-3 hours in length to just a threat 

of rain almost daily.

Aspen is reached by traveling 64 km southeast on 

Colorado Highway 82 from Glen wood Springs on the 

Colorado River or by traveling 64 km on the same

highway west over Independence Pass (fig. 1) from 

Granite on the Arkansas River. Independence Pass is 

closed during the season of heavy snows from October 

to late May because of its high altitude and the 

numerous avalanche tracks along the route.

From the main road along the Roaring Fork valley 

between Aspen and Snowmass, Colo., paved roads extend up Maroon Creek, Castle Creek, and Brush Creek, 

whereas Owl Creek, Snowmass Creek, and Woody 

Creek have parallel dirt roads. Other roads extend to 

Warren Lake on Smuggler Mountain, 6 km east of 

Aspen, and to the top of Aspen Mountain at the ski-lift
ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

complex 4 km south of Aspen. Jeep roads continue up 

other valleys, such as Express Creek and Castle Creek, 

along the crest of Richmond Hill (all within 18 km to 

the south of Aspen), and up the other developed ski 

areas (not open for public driving). Trails continue up 

the major valleys into the heart of the high country 

that comprises the Maroon Bells-Snowmass 

Wilderness, a part of our Wilderness preservation 

system.

Aspen started as a silver mining town in the 1880's. 

It was briefly the metropolis of western Colorado in 

the early 1890's. The years following the demonetization of silver in 1893 saw a gradual decline in mining 

through World War I. During the following 25 years 

Aspen continued to exist as the county seat of Pitkin 

County, a modest supply center for local ranchers, and 

the site of small mining operations. After World War 

II, the first ski lifts were built, and a new era started 

for the city and the region. A number of cultural attractions, the best known of which is the music school, 

help attract summer visitors. The 1960's saw a 

dramatic increase in the pace of building of ski 

facilities, lodges, condominium apartments, and 

houses, including the development of a whole new 

village, West Village of Snowmass-at-Aspen, on a site 

about 14 km west of Aspen that consisted of only 

meadows and aspen groves in 1964.

The area covered in this report is that of the Aspen, 

Hayden Peak, Highland Peak, and Maroon Bells 

7.5-minute quadrangles.

PREVIOUS GEOLOGIC INVESTIGATIONS

The Hayden Survey produced a topographic and 

geologic map of Colorado in the early 1870's. Reports 

of their work in the Aspen area are given in the Annual 

Report of 1874 (Hayden, 1874; Peale, 1874, Hayden, 

1876; Holmes, 1876). Holmes' report on the geology of 

the northwestern part of the Elk Range is accompanied by a l:125,000-scale geologic map on a 

topographic base and is worthwhile reading, for he 

outlines his reconnaissance impressions of some of the 

major structures of the region. Since the publication of 

that map no more detailed geologic maps of the whole 

region were published until the maps on which this 

report is based.

After the discovery of rich silver ore at Aspen in 

1879 and the development of the mines there, most of 

the geologic work in the Aspen area was concentrated 

in the mining district. Early descriptions of the 

geology in the mines there are by Emmons (1886), 

Brunton (1888), Sivyer (1888), Henrich (1889), and 

Newberry (1890). The classic study of the Aspen mining district was made by J. E. Spurr in 1895 not long 

after the peak in mining activity. Published in 1898 it

remains a valuable source of information about details 

of the geology in many individual mines. Spurr mapped the area of the Aspen 7.5-minute quadrangle in 

reconnaissance and mapped the mining areas at a large 

scale. He also discussed the processes of alteration of 

the carbonate rocks and ore deposition. In a subsequent paper (1909) he developed his ideas on ore 

deposition more fully.

Knopf (1926) extended Spurr's mapping to include 

Richmond Hill and described more recent mine workings and the igneous rocks in that area.

After Spurr's time, no major work was published on 

the Aspen mining district until that of Vanderwilt 

(1935b). Drawing on his work in the Snowmass Mountain area (Vanderwilt, 1937) adjacent to the southwest 

corner of the Aspen region and on examination of some 

features of the Aspen area with D. F. Rohlfing, a mining engineer who lived and worked in Aspen from the 

1880's intermittently through the 1930's, Vanderwilt 

(1935b) revised and updated some of Spurr's concepts 

concerning the structure and ore occurrences in the 

mining district.

The Aspen area has been the subject of various 

specialized commodity reports (Harder, 1909, iron; 

Heyl, 1964, zinc; and Boyd and Bromley, 1953, 

uranium). Volin and Hild (1950) gave the results of a 

U.S. Bureau of Mines drilling program in the Smuggler 

Mine at Aspen.

Unpublished theses by Prather (1964), Waterman 

(1955), Iradji (1955), and Gaskill (1956) include parts of 

the Aspen area. Graton (1958) described the 

petrography of the igneous rocks in Gaskill's thesis 

area, which overlaps the southwest corner of the 

Aspen area.

Stratigraphic studies especially pertinent to the 

Aspen area have been published by Johnson (1944), 

Brill (1944, 1952), Langenheim (1952, 1954), and Poole 

and Stewart (1964).

A brief description of the glacial geology of the 

Aspen area appeared in Richmond, Malde, and Tweto 

(1965).

PRESENT INVESTIGATIONS 

FIELDWORK AND ACKNOWLEDGMENTS

The present study was begun in July 1963. Three to 

four months each summer were spent on fieldwork, 

which was completed in September 1968. The four 

7.5-minute quadrangles comprising the Aspen 

15-minute quadrangle were mapped and the maps were 

subsequently published (Bryant, 1969, 1970, 1971a, 

1972a); these geologic maps should be consulted while 

reading this report. About 19 months were spent in the 

field, 10 months of which I had the help of field
GENERAL GEOLOGY

assistants, Kirby W. Bay (1964), Edward S. Grew 

(1965), Charles E. Beverly (1966), David G. Morse 

(1967), and Stephen G. Pollack (1968). I thank these 

men for their industrious efforts.

I thank Ogden Tweto who introduced me to Colorado geology, F. G. Poole and G. N. Pipiringos for 

consultation on Triassic and Jurassic stratigraphy, B. 

L. Bartleson for consultation on Pennsylvanian 

stratigraphy, K. L. Pierce for advice on Pleistocene 

and Holocene geology, W. A. Cobban and J. E. Gill for 

advice on Cretaceous stratigraphy, and F. E. 

Mutschler, V. L. Freeman, and D. L. Gaskill, who mapped adjacent areas, for cooperation and discussion on 

many aspects of the work. I thank James Babcock, 

Frank Willoughby, and John Heron for discussing 

various aspects of Aspen geology with me. John P. 

Kellogg, then of Richmond Hill Exploration Co., made 

available the results of their exploration work in the 

Highland Tunnel and at Lenado. Pitkin Iron Corp. 

allowed me to visit their mine and use their mine road 

as access in mapping. I thank the Aspen Ski Corp. and 

Highlands Ski Corp. for permission to use the roads 

serving their ski-lift facilities.

The purpose of this report is not to give a comprehensive account of all aspects of the geology of the 

Aspen area but rather to present new data obtained in 

this study and discuss its bearing on former ideas and 

conclusions. Consequently, the stratigraphy and 

petrology of the lower Paleozoic rocks are only very 

briefly treated. More emphasis is given to the upper 

Paleozoic and Mesozoic rocks, the Tertiary rocks, the 

structures of the area, and the Quaternary history. No 

attempt is made to treat the economic geology in a 

comprehensive manner but some new data are 

presented on this subject.

The present study adjoins modern detailed geologic 

study of adjacent areas to the west and southwest 

(Gaskill and Godwin, 1966a, b; Gaskill and others, 

1967; Godwin, 1968; Mutschler, 1970) and to the north 

(Freeman, 1972a, b).

As the study progressed, short topical reports have 

appeared, many of which other workers in the region 

and I prepared in conjunction; and the quadrangle 

maps have also appeared (Bryant, 1966; Bartleson and 

others, 1968; Obradovich and others, 1969; Lipman 

and others, 1969; Bryant, 1969, 1970, 1971a, b, c, 

1972a). The Aspen 7.5-minute quadrangle was the 

basis for a folio of environmental maps (Bryant, 

1972b-i).

Many of the data on composition of the sedimentary 

rocks are presented on a triangular carbonate-quartzmica and feldspar diagram. Figure 2 gives a classification of rocks in terms of that diagram.

Quartz

Quartz 

sandstone 

.10

20

Dolomitic or calcareous

arkose or calcareous

micaceous sandstone

or siltstone

Carbonate Mica and 

feldspar

FIGURE 2. Classification of sedimentary rocks in terms of proportions of quartz, carbonate, and mica and feldspar.

GENERAL GEOLOGY

The southern Rocky Mountains consist of a number 

of Laramide uplifts modified by Tertiary plutonism 

and volcanism and by late Tertiary block faulting. One 

of these uplifts, the Sawatch Range of central Colorado, apparently was originally asymmetric with a 

steeper and more structurally complex west side and a 

relatively gentle and simple east side. In late Tertiary 

time the east side was sundered by a fault system 

along the trend of the Rio Grande trough forming the 

upper Arkansas River valley. Aspen lies where the 

west margin of the Sawatch Range makes a sharp 

change in trend from N. 10° W. south of Aspen to N. 

30° E. to the north (fig. 3).

The Sawatch Range exposes a variety of Precambrian granitic and metamorphic rocks, which occupy 

the eastern quarter of the Aspen area. Overlying them 

is a thin sequence of shelf quartzite and carbonate formations representing several cycles of deposition and 

erosion from Cambrian through Mississippian time.

In late Paleozoic and early Mesozoic time Aspen was 

in the southwest part of a northwest-trending 

basin the central Colorado trough or Eagle basin. 

Detritus from the Uncompahgre uplift to the southwest filled the trough to a depth of 3-5 km in Pennsylvanian and Permian time. The Uncompahgre uplift 

was an at least intermittently positive area until Lake 

Jurassic time when the whole region was covered first 

by thin terrestrial deposits and then by marine deposits 

about 1.6 km thick.
ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

109° 108° 107° 106° 105°

38°

100 KILOMETERS

EXPLANATION 

Upper Tertiary volcanic rocks 

Middle Tertiary volcanic rocks 

Tertiary intrusive rocks 

Tertiary through Paleozoic sedimentary rocks

Precambrian metamorphic,plutonic, and 

sedimentary rocks

EAGLE BASIN

—————— BOUNDARY OF UPPER LATE PALEOZOIC 

TECTONIC ELEMENTS—Name in 

capital letters

——— — BOUNDARY OF COLORADO MINERAL BELT

FIGURE 3. Regional geologic setting of the Aspen area. Rectangular outline, Aspen 15-minute quadrangle. Modified 

from U.S. Geological Survey and American Association of Petroleum Geologists (1961), Tweto and Sims (1963), 

and Mallory( 1972).
PRECAMBRIAN ROCKS

In Late Cretaceous and Paleocene(?) time intrusion 

of porphyries, uplift, and faulting commenced. The 

4-6-km-thick upper Paleozoic-Mesozoic section probably slid southwestward off the Sawatch Range uplift 

about this time. Intrusion of porphyries and emplacement of associated rich Ag, Pb, and Zn mineralization 

were concentrated at a node of most complex faulting 

at the sharp bend in the trend of the Sawatch Range 

uplift. This bend in the margin of the uplift intersects a 

belt of Precambrian shear zones and Tertiary intrusion 

and mineralization extending northeast to Boulder, 

Colo., and is known as the Colorado mineral belt (fig. 

3).

During the Oligocene small to fairly large concordant to discordant bodies of granodiorite invaded the 

upper Paleozoic and Mesozoic rocks. Iron and 

disseminated sulfide deposits and small silver-, lead-, 

and zinc-bearing veins formed at that time.

The region was uplifted and dissected in late Tertiary and Quaternary time and was subject to a 

number of episodes of glacial erosion in Quaternary 

time.

PRECAMBRIAN ROCKS

Precambrian rocks of the western part of the 

Sawatch Range uplift occupy a belt 2-5 km wide along 

the eastern edge? of the Hayden Peak and Aspen 

quadrangles. A reconnaissance map of the Sawatch 

Range by Stark and Barnes (1935) classified the rocks 

in the Aspen area as "Silver Plume granite, Sawatch 

schist and migmatite, and Pikes Peak granite." I have 

mapped these rocks in six units in the Aspen area and 

hesitate to make any firm correlations either with 

units in the Front Range or with units nearby in the 

Sawatch Range, where detailed studies have been 

made in the Mount Harvard quadrangle (Barker and 

Brock, 1965; Brock and Barker, 1972) and the Holy 

Cross quadrangle (Tweto and Pearson, 1964; Tweto, 

1974). Recent reconnaissance mapping and isotopic 

dating, however, indicate that most of the granitic 

rocks of the Aspen area are correlative with the 

Precambrian Y St. Kevin Granite (Tweto and Pearson, 

1964) of the northern Sawatch Range (Tweto and 

others, 1978), but those exposed in the southeastern 

part of the Hayden Peak quadrangle may be older (R. 

H. Moench, oral commun., 1975).

MICA SCHIST AND GNEISS

Mica schist and gneiss occupy an east-westtrending belt less than 1.5 km wide north of Express 

Creek in the Hayden Peak quadrangle. They are intruded by granite on the south and grade into mixed 

quartz diorite, pegmatite, schist, and gneiss on the

north. The rocks are best exposed on the north side of 

the Express Creek valley.

In the schist and gneiss unit pegmatite lenses, sills, 

veinlets, and dikes are widespread and are a fraction of 

a centimeter to 3 m thick (fig. 4). Some contain quartz 

and feldspar crystals as much as 15 cm in diameter and 

muscovite as much as 20 cm in diameter. Sills of 

coarse-grained granite are abundant in the schist and 

gneiss near the contact of the granite. Some bodies of 

biotite-muscovite granite and quartz monzonite occur 

in the schist and gneiss away from the contact.

Layers of the schist and gneiss range in thickness 

from 1 to 10 cm (fig. 4). The schist and gneiss consist of 

muscovite, biotite, quartz, and feldspar. The micas are 

alined and form plates as much as 5 mm long. Alined 

needles of sillimanite or sillimanite altered to 

muscovite are visible in hand specimens. The gneiss is 

generally finer grained and less micaceous than the 

schist.

The mica schist and gneiss have a mica content ranging from 15 to 80 percent. Muscovite is relatively more 

abundant than biotite in most mica-rich rocks. Grain 

size of muscovite is about twice that of the biotite. The 

micas are both synkinematic and postkinematic. In 

some specimens growth of muscovite appears to have 

lasted longer than that of biotite, for a larger proportion of muscovite is less well alined on the foliation 

planes than biotite. Quartz and plagioclase, in grains 

as much as 2 mm in diameter, are the other major constituents. Plagioclase content ranges from none to as 

much as quartz. Its composition is An16_22.

Sillimanite in the form of fibrolite occurs in both 

muscovite and biotite. In one rock examined, it forms 

lenticular aggregates that contain little or no mica.

Secondary minerals are chlorite after biotite and 

sericite after plagioclase. Accessory minerals are zircon, apatite, sphene, opaque minerals, and tourmaline.

Abundance of muscovite, virtual absence of potassic 

feldspar, and presence of sillimanite in the samples examined indicate that these rocks were metamorphosed 

in the sillimanite-muscovite zone of the amphibolite 

facies. Few data are published concerning mineral 

assemblage and metamorphic grade in the Precambrian of the Sawatch Range with which data from 

these rocks might be compared.

MIXED QUARTZ DIORITE, PEGMATITE,

QUARTZ MONZONITE, AND MICA SCHIST

AND GNEISS

A heterogeneous unit of quartz diorite, pegmatite, 

quartz monzonite, and mica schist and gneiss occupies 

the McArthur Mountain area in the east-central part of 

the Hayden Peak quadrangle. Much of the unit is con-
ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

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cealed by a thin colluvial cover composed of conspicuous fragments of pegmatite and the granitic 

rocks and less conspicuous fragments of mica schist 

and gneiss. Pegmatites, especially, form prominent 

outcrops protruding from the colluvium.

Intrusive relations between the rock types in this 

unit indicate that in general the sequence of the rock 

types from oldest to youngest is (1) mica schist and 

gneiss, (2) quartz diorite, (3) quartz monzonite, and (4) 

pegmatite. The various rock types are so intermingled 

and exposures are so poor that they are lumped as one 

unit. In places the mica schist and gneiss component 

has layers of granitic and pegmatitic material and is a 

migmatite. Contacts of the unit with adjacent ones are 

not sharply defined as the adjacent rock types make up 

one of the elements of this unit.

BIOTITE QUARTZ DIORITE

The north side of Ashcroft Mountain, in the eastern 

part of Hayden Peak quadrangle, is composed 

predominantly of medium- to coarse-grained biotite 

quartz diorite cut by dikes of muscovite-biotite quartz 

monzonite and aplite and by dikes of quartz-potassic 

feldspar-muscovite pegmatite as much as 10 m thick. 

The quartz diorite contains some inclusions of mica 

gneiss. The contacts of the quartz diorite with adjacent rock units are not well exposed.

In thin section the biotite quartz diorite is seen to 

consist of anhedral to subhedral plagioclase with a 

grain size as large as 4 mm. The plagioclase has a composition of about An40 but is considerably saussuritized. Quartz occurs in anhedral grains as much as 2 mm 

in diameter interstitial to the plagioclase. Some quartz 

grains have a graphic intergrowth of plagioclase in optical continuity with adjacent larger subhedral 

plagioclase grains. Brown biotite is as much as 6 mm 

long and locally is chloritized. Epidote and muscovite 

are other secondary minerals. Accessories are allanite, 

zircon, apatite, sphene, and opaque minerals.

PORPHYRITIC QUARTZ MONZONITE

Porphyritic biotite quartz monzonite occupies about 

2.5 km2 in the southeast corner of the Hayden Peak 

quadrangle. Reconnaissance mapping shows that the 

pluton extends over 40 km southeastward across

FIGURE 4. Outcrops of mica schist and gneiss, Hayden Peak quadrangle. Rocks of A and B are highly contorted. Folds have northeast-trending axes parallel to mineral lineation in the rocks. B, 

numerous pegmatite lenses are folded along with the layers of 

schist and gneiss. C, Schist filled with lenses and pods of 

pegmatite.
PRECAMBRIAN ROCKS

Taylor Park, and foliation is well developed in most of 

the pluton (R. H. Moench, oral commun., 1975).

The characteristic feature of the porphyritic. biotite 

quartz monzonite in the Aspen area is the presence of 

potassic feldspar grains as long as 3 cm in a matrix of 

plagioclase, quartz, potassic feldspar, and biotite with 

grain sizes of 1-5 mm. The porphyritic quartz monzonite is cut by dikes of pegmatite and equigranular 

fine- to medium-grained granitic rock. In places the 

porphyritic quartz monzonite contains inclusions of a 

fine-grained biotite-plagioclase rock as long as 25 cm, 

which locally make up about 1 percent of the rock.

Under the microscope the potassic feldspar is seen to 

be microcline and includes some string perthite. The 

microcline contains smaller grains of plagioclase, 

quartz, and biotite. In part it appears to replace 

plagioclase, and some myrmekite is present in the adjacent plagioclase. Plagioclase is anhedral to 

subhedral, has a composition of An20-3o, and displays 

normal zoning. Quartz occurs in aggregates of 

anhedral grains that are interstitial to plagioclase in 

part. Anhedral biotite forms aggregates; some grains 

are partly altered to chlorite. Accessories are apatite, 

zircon, sphene, and opaque minerals.

One inclusion examined is composed of 

postkinematic biotite and calcic oligoclase. The 

plagioclase is zoned with one oscillation formed by a 

more calcic intermediate zone. Muscovite, quartz, 

microcline, and opaque minerals are minor constituents of the inclusion.

Chemical analysis of a typical sample of porphyritic 

biotite quartz monzonite (sample no. 1, table 1) confirms that compared to the adjacent granite (sample 

no. 2, table 1) described next, the quartz monzonite 

contains less SiO2 and K2O, and more CaO, FeO, Fe2O3 

and MgO.

MUSCOVITE-BIOTITE GRANITE

A distinctive, homogeneous coarse-grained 

muscovite-biotite granite crops out at the head of Express Creek and near Taylor Lake and Taylor Pass in 

the southeastern part of the Hayden Peak quadrangle. 

It is best exposed in cliffs west of Taylor Pass and on 

glaciated knobs south of Taylor Lake. Where 

weathered it is a light grayish pink because of its content of potassium feldspar, and on erosion surfaces, 

such as near Taylor Pass, it has disintegrated to a 

coarse-grained sand.

The granite contains Carlsbad twinned microcline 

crystals commonly 1 cm long and as much as 3 cm 

long. Quartz is 0.5-1 cm in diameter. Micas are as 

much as 0.5 cm in diameter but in many places occur 

as clots of smaller grains. The granite is cut by

pegmatites and contains lenses and veins of coarsegrained quartz.

Contacts of the granite generally are sharp. 

However, at the contact to the north numerous sills of 

granite and pegmatite penetrate mica schist and 

gneiss along foliation planes. At its contact with porphyritic biotite quartz monzonite south of Taylor 

Lake, the granite grades to porphyritic quartz monzonite, so that no age relation between the two rock 

types could be established. Both rocks are cut by 

pegmatites and medium-grained granitic rock near 

that contact.

In thin section the granite is composed chiefly of 

microcline, quartz, and plagioclase. Microcline occurs 

as anhedral grains, includes grains of quartz and 

plagioclase, and is perthite in some samples. Quartz 

forms anhedral grains, which are weakly strained. 

Plagioclase with composition in the range An20-25 occurs in anhedral to subhedral grains and is sericitized 

or saussuritized in some samples. Biotite generally 

makes up less than 2 percent of the granite and is partly chloritized in some samples. Muscovite also makes 

up only a percent or two of the rock. The proportion of 

the two micas ranges widely. Some rocks contain only 

muscovite and in some biotite predominates. Accessory minerals are zircon, apatite, and opaques. 

Rock from an isolated exposure near the head of 

Cooper Fork (southeast part of Hayden Peak 

quadrangle), mapped as part of the granite unit, contains small garnet grains included in the quartz and 

plagioclase. Mineral composition based on mineral content of the few samples examined from this rock unit is 

quite variable, yet the impression gained in the field is 

that the unit is quite homogeneous. Probably, the unit 

is fairly homogeneous overall, but heterogeneous at 

the scale of a thin section because of the coarse grainsize.

Chemical analysis (sample no. 2, table 1) of a typical 

sample of the unit is very similar to the average 

muscovite-biotite granite cited by Nockolds (1954). 

However, both the mode and the norm would place the 

sample in the quartz monzonite mineral-composition 

field.

QUARTZ MONZONITE

Most of the Precambrian rocks of the Aspen area are 

plutonic rocks of relatively uniform aspect and are 

mapped as muscovite-biotite quartz monzonite. Rocks 

of the unit are locally well exposed on the sides of the 

major valleys, such as the Roaring Fork, Hunter 

Creek, and Difficult Creek, and on the west side of 

Aspen Mountain. However, substantial areas 

underlain by quartz monzonite are covered by moraine 

or colluvium, and the rock is weathered to a grus on the
10 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

TABLE 1.— Chemical analysis, modes, and norms of Precambrian 

rocks

[Major oxides determined by rapid methods by P. L. D, Elmore, 

G. W. Chloe, James Kelsey, J. L, Glenn, Hezekiah Smith, and Lowell 

Artis, U.S. Geological Survey, 1969. Minor elements determined by 

semiquantitative spectrographic methods by Joseph L. Harris, U.S. 

Geological Survey, 1969. Mode by point count; P, present but not 

intersected in counting. Major oxides and CIPW norms in weight percent; mode in volume percent; CIPW norms calculated on the basis of 

analyses recalculated Lo 100 percent after deduction of volatiles. 

Results for minor elements are reported in ppm (parts per million) to 

the nearest number in the series 100, 70, 50, 30, 20, 15, 10, ..., 

which represent approximate midpoints of interval data on a geometric 

scale. The assigned interval for semiquantitative results will include the quantitative values about 30 percent of the time. 

L, detected, but below limit of determination; N, not detected; 

leaders indicate value not determined. Looked for but not found: 

Ag, As, Au, B, Bi, Cd, Mo, Pd, Pt, Sb, Te, U, W, Zn, Ge, Hf , In, Li, 

Re, Ta, Th, Tl, Pr, Sm, Eu]

__.. Granite Quartz monzonite qudrcz

monzonite

Sample No. —————— 1 234

Lab. No. ——— ———— W172424 W172425 W172426 W172427

Major oxides

Powder density ——— 2.68 2.64 2.76 2.76

Minor elements

Be ———————————— 5332

Yb ———————————— 5235

Modes

Potassic feldspar- 6.7 39.8 14.9 14,3

Opaque mineral ——— 1.5 .2 .4 1.4

anite F

TABLE I.— Chemical analysis, modes, and norms of Precambrian 

rocks— Continued

. Granite Quartz monzonite quartz

monzonite

Field No. ——————— 2048 2050 2077 2078

CIPW Norms

Fs ———————————— 2.22 ,26 .87 1.74

SAMPLE DESCRIPTIONS

1. Porphyritic muscovite-biotite quartz monzonite. Subhedral normally 

zoned plagioclase An20-30 as long as 3 mm, anhedral quartz as much 

as 3 mm in diameter in aggregates interstitial to plagioclase in 

part, somewhat perthitic microcline as much as 2 cm in diameter 

containing inclusions of the other minerals, and anhedral biotite in 

aggregates as much as 2 mm in diameter. Absence of large microcline 

from thin section accounts for discrepancy between mode and chemical 

analysis. Sample from cliff, 960 m S. 54° W. of southeast corner of 

Hayden Peak quadrangle.

2. Coarse-grained muscovite-biotite granite. Subhedral plagioclase 

An 25 somewhat sericitized as long as 5 mm, anhedral quartz as much 

as 5 mm in diameter and microcline microperthite as much as 1 cm in 

diameter, containing inclusions of quartz and plagioclase, and 

anhedral biotite as much as 1 mm in diameter in aggregates as much 

as 5 mm in diameter. From blasted talus block at 3,523 m altitude 

on road 610 m north of Taylor Pass, Hayden Peak quadrangle.

3- Biotite granodiorite. Anhedral to subhedral normally zoned 

plagioclase (An 32-20), partly altered, as much as 3 mm in diameter.

Anhedral quartz as much as 3 mm in diameter partly interstitial to 

and partly replacing plagioclase and quite strongly strained. 

Anhedral, partly bent biotite as much as 2 mm in diameter and 

anhedral perthitic microcline as much as 3 mm in diameter. Roadcut 

on Colorado Highway 82; 300 m west along road from east margin of 

Aspen quadrangle.

4- Biotite granodiorite. Anhedral to subhedral plagioclase An25 as 

much as 4 mm in diameter containing patches of microcline, anhedral 

strongly strained quartz as much as 3 mm in diameter, subhedral to 

anhedral microcline as much as 3 mm in diameter, and anhedral 

biotite as much as 1 mm in diameter in aggregates. Roadcut on 

Colorado Highway 82 365 m N. 70° E. from symbol for Difficult 

Campground on Aspen quadrangle.

uplands of Smuggler Mountain 5 km east of Aspen and 

on the south end of Richmond Hill 11 km south of 

Aspen. 

The quartz monzonite weathers orange pink and is 

light gray where fresh. Although the rocks mapped as 

quartz monzonite are fairly uniform in general aspect, 

they display some differences in detail. The proportion 

of potassic feldspar and mafic minerals differs in 

places so that the rock locally has granite or 

granodiorite composition (fig. 5). The usual texture is 

eauieranular and directionless with a grain size of 1-5
PRECAMBRIAN ROCKS 11

Quartz

Plagioclase 90 Potassic 

feldspar

FIGURE 5.—Proportions of quartz, plagioclase, and potassic 

feldspar in rocks of the quartz monzonite unit. Circled points are 

analyzed samples and numbers from table 1; x, dike cutting quartz 

monzonite.

mm. Locally, the rock is porphyritic and has grains of 

potassium feldspar as much as 2 cm long. In a few 

places it has a poorly developed gneissic structure. In 

one area on the north side of Hunter Creek cataclastic 

textures are well developed; the rock ranges from 

blastomylonitic gneiss to phyllonite. Near faults the 

quartz monzonite is altered and brecciated.

Small pegmatite veinlets, pods and stringers, commonly with indistinct margins, occur in scattered outcrops. Larger pegmatites with perthite and muscovite 

as much as 10 cm in diameter occur rarely. Some areas 

of the quartz monzonite, such as north of Warren Lake, 

lack pegmatites. On Smuggler Mountain some locally 

vuggy quartz veins containing hematite occur in the 

granitic rock.

Inclusions of mica schist and gneiss are numerous in 

the southeastern corner of the Aspen quadrangle and 

the northwestern corner of the Hayden Peak 

quadrangle.

Chemical analyses (sample nos. 3, 4, table 1) are of 

samples of the granodioritic side of the range in composition of rocks in this unit (fig. 5).

AGE

A number of recent studies have developed a 

chronology for the Precambrian rocks of Colorado that 

is quite consistent in broad outlines for the area from

the San Juan Mountains to the northern Front Range 

(Bickford and others, 1969; Hansen and Peterman, 

1968; Hedge and others, 1968; Mose and Bickford, 

1969; Peterman and Hedge, 1968). These studies have 

revealed that sedimentary and volcanic rocks, probably deposited 1,750-2,000 m.y. (million years) ago, 

were deformed, metamorphosed, and intruded 

1,650-1,750 m.y. ago. No older rocks have yet been 

found in the Precambrian of the Colorado Rockies. 

Many of the intrusions are foliated, but some are not; 

younger plutonic rocks that were emplaced 

1,400-1,500 m.y. ago are generally unfoliated. The 

region was uplifted and micas became closed systems 

in the interval 1,100-1,400 m.y. ago, except where affected by Late Cretaceous and Tertiary plutonic activity. A major younger Precambrian pluton, known to exist only in the Front Range, is the Pikes Peak Granite, 

which is 1,030 m.y. old.

Rubidium-strontium determinations by Wetherill 

and Bickford (1965) on granitic rocks from the 

Sawatch Range gave a whole-rock isochron at 

1,640±35 m.y. Rubidium-strontium determinations on 

minerals indicated that at about 1,350 m.y. ago extensive redistribution and, in some cases, complete reequilibration of strontium between the mineral phases 

took place. Some results were interpreted to indicate 

intrusion of pegmatites at about 1,450 m.y. ago. 

Rubidium-strontium data from the Kroenke Granodiorite in the central Sawatch Range fit a 1,700-m.y. 

isochron (Barker and others, 1974) and from the Cross 

Creek Granite in the northern Sawatch a 1,710-m.y. 

isochron (Tweto and Lovering, 1977). In the northern 

Sawatch Range, Pearson, Hedge, Thomas, and Stern 

(1966) obtained an Rb-Sr isochron for the St. Kevin 

Granite at 1,470±65 m.y., and Doe and Pearson 

(1969), an age of 1,420±40 m.y. based on isotopic 

dating of zircon. Mica ages range from 1,175 to 1,435 

m.y. and may represent imperfect reequilibration by 

heating associated with emplacement of the St. Kevin 

and cooling by uplift and modification by Laramide 

heating (Pearson and others, 1966).

Currently, the most reasonable interpretation of the 

mica schist and gneiss in the Aspen area is that they 

were laid down as shale and graywacke 1,700-2,000 

m.y. ago and were subjected to deformation and 

sillimanite-grade metamorphism about 1,700 m.y. ago.

The porphyritic quartz monzonite near Taylor 

Lake resembles the Hell Gate Porphyry of Stark and 

Barnes (1935) (R. H. Moench, oral commun., 1974).
12 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Tweto and Pearson (1964) considered the Hell Gate 

Porphyry a textural facies of the St. Kevin Granite. 

However, recent reconnaissance mapping shows that 

the porphyritic quartz monzonite near Taylor Lake is 

contiguous with quartz monzonite with well-developed 

gneissic structure in Taylor Park (R. H. Moench, oral 

commun., 1975). Consequently, that rock and the closely associated granite are considered to be older or 

emplaced during a late stage of the main metamorphism and deformation 1,700-1,650 m.y. ago.

The biotite quartz diorite intruded the mica schist 

and gneiss, perhaps during a late stage or after the 

metamorphism. No absolute age data are available for 

that rock type; it is younger than the mica schist and 

gneiss and older than the quartz monzonite. Intrusive 

rocks of that composition are most commonly 

associated with the episode of intrusive activity 

associated with the metamorphism 1,700-1,650 m.y. 

ago.

Quartz monzonite contiguous with that forming 

most of the granitic rock in the Aspen area fits the 

whole-rock isochron for the St. Kevin Granite (J. C. 

Reed, Jr., oral commun., 1973). In Lincoln Gulch 

(18 km southeast of the Aspen area) quartz monzonite 

clearly intrudes migmatite. That migmatite probably 

formed 1,650 or 1,700 m.y. ago is indicated by the work 

of Wetherill and Bickford (1965).

CAMBRIAN THROUGH MISSISSIPPIAN 

SHELF SEQUENCE

INTRODUCTION

A distinctive sequence of thin quartzite and carbonate formations deposited in a shelf environment 

during Cambrian to Mississippian time forms a belt 

0.4-3.2 km wide between the Precambrian of the 

Sawatch Range and younger sedimentary rocks in the 

Aspen and Hayden Peak quadrangles (fig. 6). The 

wider parts of the belt occur where numerous faults 

cause repetition of outcrops of these formations. Rocks 

of the sequence form East Aspen and West Aspen 

Mountains, features named on the mining district 

topographic base maps (Spurr, 1898), which are prominent peaks when viewed from the town of Aspen, 

although they are only the ends of sharp ridges when 

viewed from above. These rocks form Castle Butte, 

which is the 3,347-m knob west of Tourtelotte Park, 

they form the east valley side of Castle Creek between 

Express Creek and Fall Creek, and they also form the 

walls of the cirque at the head of the south fork of Express Creek about 1.6 km west of Taylor Lake. The 

total thickness of the sequence ranges from about 200 

to 300 m and is 245-275 m on the average.

Table 2 shows the history of the stratigraphic

nomenclature of the Cambrian through Mississippian 

rocks at Aspen so that the reader can relate the work of 

Spurr (1898) and Johnson (1944) to this paper.

SAWATCH QUARTZITE

The Sawatch Quartzite of Late Cambrian age 

(Eldridge, 1894) forms the basal Paleozoic formation in 

the Aspen area. The Sawatch is resistant to weathering and erosion and forms cliffs in many places. It is 

composed predominantly of tan-weathering white 

quartzite with interspersed vuggy brown-weathering 

beds cemented by dolomite or calcite. Its base is 

marked in many places by 0.3-1.8 m of locally crossbedded quartz pebble conglomerate containing pebbles 

as much as 2.5 cm in diameter and some grayish-pink 

microcline grains as much as 1 cm in diameter. A few 

beds of pebbles and granules of quartz occur above the 

base in the lower part of the formation, but in most 

places the grain size is <0.5 mm. Beds of carbonatecemented sandstone in fine-grained vitreous quartzite 

are common in the upper part of the formation.

The upper contact of the Sawatch is drawn below the 

point where yellowish-brown-weathering sandstones, 

sandy limestones, and interbedded shales begin to 

dominate the rock type. This change is generally 

abrupt, but the rocks appear conformable. (See 

stratigraphic section 1, p. 130.)

The thickness of the Sawatch in the Aspen area 

ranges from about 45 to 75 m. Variations in thickness 

may occur through short distances; interpretation of 

Johnson's (1944) Taylor Pass section indicates about 

30 m of Sawatch very close to where I measured 43 m. 

Bush and Bush (1974) showed 33 m of Sawatch near 

Taylor Pass. A thickness of 72 m measured on the 

cliffs on the north side of Woody Creek just east of 

Aspen quadrangle is an unusually great thickness of 

Sawatch for the area.

Just south of the Aspen region Johnson (1944) divided the Sawatch into a lower white quartzite member, 

glauconitic sandstone member, upper white quartzite 

member, and Peerless Shale Member. He believed that 

the glauconitic sandstone member graded into the upper quartzite member northward. Near Taylor Pass 

(p. 312) he found his Peerless Shale Member of the 

Sawatch channeling the top of the glauconitic sandstone member. I did not observe this channeling in the 

section measured for this study, about 1.6 km west of 

Taylor Pass, nor did Johnson report it in his measured 

section near Taylor Pass (1944, p. 358-9). Johnson's 

published sections do not seem to support his statement about the glauconitic sandstone member grading 

into the upper quartzite member in the Aspen area, but 

rather they suggest to me that his upper quartzite 

member grades northward into rocks that I mapped as
CAMBRIAN THROUGH MISSISSIPPIAN SHELF SEQUENCE 13

FIGURE 6.—Lower and middle Paleozoic rocks, 1 km south of Aspen. 

View west across Spar Gulch from 2,822 m altitude on East 

Aspen Mountain. Lower left cliff is Manitou Dolomite (detailed 

view, fig. 10), and upper cliff is upper part of the Leadville 

Limestone (detailed view, fig. 15) and is about 25 m high. These 

rocks are on the east limb of the Aspen Mountain syncline. Sill of 

aplite porphyry crops out on long, sloping ridge beyond Leadville 

cliff and is in the Belden Formation. Wooded ridge on right 

skyline is faulted west limb of the syncline. Mine openings at the

base of the Leadville cliff are at the contact between the limestone 

in the upper part and dolomite in the lower part of that formation—the contact fault of Spurr (1898). The first discovery of ore 

in the Aspen district was along this horizon just outside the view 

to the right. Extensive dumps to the left are from the Visino and 

Bonnybel mines, which are in a complex of faults cutting the 

limestone and dolomite. The lower part of the Leadville and the 

Chaffee Group are poorly exposed on the slope between the two 

cliff-forming horizons.
14 ASPEN 15-MINUTE QUADRANGLE. PITKIN AND GUNNISON COUNTIES, COLORADO

TABLE 2.—History of stratigraphic nomenclature of Cambrian through Mississippian quartzite-carbonate shelf

sequence in the Aspen area

Spurr (1898)

Leadville Limestone

Limestone 

and 

dolomite

Parting quartzite series

Silurian beds

Cambrian sediments

Johnson (1944)

Leadville Limestone

Chaffee Formation Dyer Dolomite Member

Parting Member

Manitou Dolomite

Sawatch Quartzite

This report

Leadville Limestone

Chaffee 

Group

G i Iman Sandstone

Dyer Dolomite

Parting Formation

Manitou Dolomite

Peerless Formation

Sawatch Quartzite

Peerless Formation (Singewald, 1947). Bush and Bush 

(1974) correlated much of the glauconitic sandstone interval in the Sawatch south of the Aspen area with the 

Peerless of the Aspen area. They believed that the 

quartzites above the glauconitic sandstone interfinger 

with Peerless near Taylor Pass and to the north.

Fossils have not been found in the Sawatch Quartzite in the Aspen area.

Thin sections of Sawatch Quartzite show that it is 

composed predominantly of quartz (fig. 7). The quartz 

occurs in single-crystal clastic grains and multigrain 

fragments perhaps derived from vein quartz or quartzite. In some rocks the quartz grains occur interlocking 

as a mosaic; in others they are set in a matrix of finegrained quartz and sericite. Microcline and muscovite 

occur as clastic grains in the matrix. Carbonate grains 

occur between the quartz grains in some rocks. Most of 

them appear to have recrystallized there, but a few 

have rounded outlines suggesting that they are 

detrital. Clastic zircon and opaque mineral(s) are accessory. Tourmaline was found in one section.

PEERLESS FORMATION

The Peerless Formation (Behre, 1932; Singewald, 

1947) overlies the Sawatch Quartzite at the fairly 

abrupt change from predominantly white quartzite 

containing a few carbonate-cemented sandstone beds 

below to predominantly grayish-orange and moderateorange-brown-weathering dolomitic sandstone and 

dolomite—and durky-red to yellowish and lightgreenish-gray shale above. Spurr (1898, p. 5-7) described the beds we now map in the Peerless at Aspen 

but included them in his Cambrian sediments, which

were shown in the atlas that accompanied the 

monograph as Sawatch Formation.

The Peerless contains a few beds of white quartzite 

near the base. Many of the shale beds contain abundant grains of clastic mica as much as 0.5 mm in 

diameter. The Peerless contains some beds of flat pebble conglomerate consisting of fragments of dusky-red 

shaly dolomite in a matrix of sandy dolomite. Beds 

generally range from a fraction of a centimeter to 30 

cm in thickness (fig. 8). The Peerless crops out less 

than the underlying Sawatch Quartzite and overlying

Quartz

Feldspar Mica and clay

FIGURE 7.—Proportions of quartz, feldspar, and mica and clay in 

samples from the Sawatch Quartzite of the Aspen area. Circled 

points represent samples from the base of the formation.
CAMBRIAN THROUGH MISSISSIPPIAN SHELF SEQUENCE 15

FIGURE 8.—Typical thin and irregular bedding in Peerless Formation. 

Darker colored beds are dolomitic shale and shaly dolomite; lightcolored beds are dolomitic sandstone. Scale is 18 cm long.

Manitou Dolomite, but it forms distinct brownweathering rough-textured float derived from beds of 

sandy dolomite.

Much of the Peerless contains a larger proportion of 

carbonate than is apparent in hand specimens, and 

among the sandstones more of the rock is sandy 

dolomite than dolomitic sandstone (fig. 9). In most 

samples clastic grains of quartz and very minor 

amounts of muscovite and microcline, which are as 

much as 3 mm in diameter but generally 0.5-1 mm, are 

set in a matrix of mosaic-textured carbonate. Some 

rocks contain rounded grains of glauconite in places 

altered to hematite and sericite.

Zircon, opaque minerals, and tourmaline are accessory minerals. Between the larger grains is some 

matrix composed of a clay mineral or a very fine grained mica.

The upper contact of the Peerless is transitional into 

the Manitou Dolomite and is difficult to place exactly. 

(See stratigraphic section 1, p. 130.) It is drawn where 

gray dolomite becomes the predominant rock type.

The Peerless as mapped by me is about 30-45 m 

thick. My judgment of its thickness from the sections 

described by Johnson (1944) agrees well with the two 

thicknesses I measured. Johnson (1944) recognized the 

Peerless Shale Member at the top of the Sawatch as 

transitional between the Sawatch and the overlying 

Manitou. However, he did not identify it in his

measured sections except at Ashcroft (1944, p. 357), 

where he assigned some of the beds I mapped as 

Peerless to the Manitou Dolomite. The Peerless as I 

mapped it in the Aspen area consists of the upper part 

of the Sawatch and the lower part of the Manitou as 

they were identified by Johnson in many sections. 

Bush and Bush (1974) extended a unit of dolomitic 

sandstones from Taylor Park into the southern part of 

the Aspen area, and they named that unit the Deadmans Gulch Formation. These rocks were mapped in 

the Peerless Formation by me.

A few graptolites were found in the Peerless Formation by Johnson in his Ashcroft and Taylor Pass sections; and the ones at Taylor Pass were of 

Trempealeauan (Late Cambrian) age.

MANITOU DOLOMITE

The Manitou Dolomite (fig. 6; Cross, 1894) forms the 

prominent flatiron on the end of West Aspen Mountain 

and cliffs in many other places, such as the lower part 

of Castle Butte, and dip slopes along Castle Creek from 

Ashcroft Mountain north to Fall Creek. The Manitou 

consists of gray, medium-gray- to light-gray- to 

yellowish-gray-weathering medium-bedded dolomite, 

with irregular bedding planes. The lower part contains 

a few beds of yellowish-brown dolomite sandstone and 

dolomite and has some shale partings. White chert 

stringers and nodules occur in the upper part of the formation (fig. 10). It also contains some medium-darkgray chert lenses and stringers.

Most of the Manitou contains a small percentage of

Quartz

Carbonate Mica and 

feldspar

FIGURE 9.—Proportions of quartz, carbonate, and mica and feldspar 

in the Peerless Formation in the Aspen area.
16 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Gerhard (1972) analyzed the environment of deposition of the Manitou in central Colorado; he found it to 

range from subtidal to supratidal but to be dominantly 

intertidal, in a section studied near Taylor Pass 

(Gerhard, 1972, p. 21, 23).

STRATIGRAPHIC RELATIONS AND

CORRELATIONS OF CAMBRIAN AND

ORDOVICIAN ROCKS

The stratigraphic relations of the Cambrian and Ordovician formations pose some problems. According to 

Johnson (1944), in the Taylor Park region southeast of 

the Aspen area, the Sawatch consists of four members: 

a lower quartzite member, a middle glauconitic sandstone member, an upper quartzite member, and a 

Peerless Shale Member. In Taylor Park the total 

thickness of the three lower members is 76-90 m, but 

to the east and south they thin to as little as 34 m at 

the west margin of the Garfield 15-minute quadrangle 

about 60 km south-southeast of Aspen. All three lower 

members are identifiable in that section, but the 

Peerless Shale Member is missing. Farther east the 

Sawatch is cut out by an unconformity below the 

Manitou (Dings and Robinson, 1957).

Just south of the Aspen area the thickness of the 

Sawatch Quartzite undergoes an abrupt change, as 

described by Johnson (1944); and the lower part loses 

its distinct tripartite character. In the southern part of 

the Aspen area the Sawatch of this report is about 

30-45 m thick and is overlain by about 30 m of 

Peerless. This relation of relatively uniform Sawatch 

overlain by Peerless continues to the north edge of the 

Aspen area, but thicknesses increase so that at Woody 

Creek 72 m of Sawatch is overlain by 49 m of Peerless.

Johnson (1944, p. 312) stated that the glauconitic 

sandstone member of the Sawatch grades to white 

quartzite so that the upper and lower members lose 

their identity to the northwest and that the Peerless 

Member rests unconformably on the glauconitic sandstone member near Taylor Pass, not far from the section I measured. In that section no stratigraphic break 

was detected either at the base of, within, or at the top 

of the Peerless, although detailed stratigraphic work 

nearby might reveal this unconformity. If the relations 

Johnson described are correct, it is possible that an unconformity may be present in rocks mapped as 

Peerless Formation by me in the Aspen area, as shown 

by Johnson (1944, pi. 1).

Bush and Bush (1974) correlated part of the 

glauconitic sandstone member of Johnson (1944) with 

the Peerless; they named the overlying part of the 

glauconitic sandstone, the upper white quartzite, and 

the Peerless Member the Deadmans Gulch Formation. 

They correlated beds at the top of the Peerless FormaFIGURE 10.—Upper part of the Manitou Dolomite showing lenses 

and stringers of white chert and wavy irregular bedding planes (in 

upper part of photo). Outcrop in Spar Gulch, Aspen quadrangle. 

Compare with figure 6.

silt-sized clastic quartz grains, and some beds contain 

traces of clastic muscovite. The sandier beds near the 

base have a few percent of clastic microcline in addition to much quartz and traces of muscovite. Zircon 

and tourmaline occur in these sandy beds.

The basal contact of the Manitou is difficult to locate 

precisely for as much as 10 m of the well-exposed section of Manitou measured contains beds of dolomitic 

sandstone and shale partings. (See stratigraphic section 1.) The base of the Manitou is drawn where lightgray- to yellowish-gray-weathering dolomite dominates over grayish-orange dolomite, sandy dolomite, 

and dolomitic sandstone. Gerhard (1972, p. 9) studied a 

nearby section and arrived at similar conclusions concerning the base of the Manitou, and Spurr (1898, p. 7) 

found a similar gradation farther north near Aspen.

No fossils were found in the Aspen area in the 

Manitou by Johnson (1944) or me, but data from the 

White River uplift to the northwest show that the 

Manitou is of Early Ordovician age (Bass and 

Northrop, 1963, p. J15).

Thin-section samples of the Manitou are composed 

of carbonate grains that range from 0.01 to 0.3 mm and 

average 0.1 mm in diameter; they contain scattered 

quartz grains less than 0.1 mm in diameter. Muscovite, 

opaque minerals, and zircon occur in trace amounts.
CAMBRIAN THROUGH MISSISSIPPIAN SHELF SEQUENCE 17

tion in the Mosquito Range east of Leadville, Colo., 

and in the southern and eastern parts of the Sawatch 

Range with the Deadmans Gulch Formation. They 

believed that the Deadmans Gulch Formation of the 

Taylor Park region was deposited at the same time as 

the upper part of the Peerless Formation of the Aspen 

and White River uplift areas. In the latter area the Upper Cambrian Dotsero Formation was correlated by 

Bush and Bush (1974) with the Peerless. They found no 

fossils that allow firm time correlations between these 

units in the region.

Northeast of the Aspen area, in the Pando, Colo., 

area, Tweto (1949) described a dolomitic sandstone 

about 3 m thick containing shaly dolomitic sandstone 

beds about 15m above the base of the Sawatch. There, 

the Sawatch is about 56 m thick and is conformably 

overlain by about 30 m of Peerless.

On the White River Plateau near Glenwood Springs, 

Bass and Northrop (1963) measured 157.6 m of 

Sawatch, which contains a 21-m dolomite bed 90.5 m 

above the base. The Sawatch they mapped is overlain 

by the Dotsero Formation, 29 m of dolomite and 

limestone with minor amounts of shale, of Late Cambrian (Trempealeauan) age.

Berg (1960) and Bush and Bush (1974) interpreted 

the Dotsero to be stratigraphically equivalent to the 

Peerless. Berg believed that the Peerless is older to the 

east; paleontologic control for this interpretation was 

indicated (Berg, 1960, fig. 1) but not documented. He 

did state that in Manitou Park on the west side of the 

Rampart Range about 170 km east of Aspen he obtained fossils of late Franconian age (Late Cambrian) 

from the Peerless. Johnson (1944, p. 313-314) stated 

that he collected Trempealeauan fossils from the Peerless in the Aspen area, which would tend to confirm 

Berg's interpretation. Gerhard (1972) agreed with Berg.

In the Garfield quadrangle the Manitou Dolomite of 

Early Ordovician age oversteps the Sawatch and lies 

on Precambrian rocks, owing to pre-Manitou erosion of 

the Sawatch. On the White River Plateau the Clinetop 

Member of the Dotsero Formation immediately 

underlies the Manitou Dolomite and was deposited in 

the intertidal and supratidal zones in a sea regressing 

to the southwest (Campbell, 1976). Thus, the regional 

stratigraphic relations suggest that the conformity 

and gradation between Peerless and Manitou in the 

Aspen area may be more apparent than real, and 

perhaps the channeling reported by Johnson (1944) 

was at the base of the Manitou rather than the 

Peerless.

On the other hand, Tweto and Lovering (1977) 

reported that fossils from the Peerless in the Minturn 

and Holy Cross quadrangles are of Franconian age. 

They correlated the dolomite unit in the Glenwood 

Canyon area (Bass and Northrop, 1963) with the

Peerless and believed that the quartzite beds above 

that unit and the equivalent of the Dotsero Formation 

have been cut out by pre-Manitou erosion in the area of 

the Sawatch Range.

A slightly different interpretation has been given by 

Lochman-Balk (1972, p. 73). She designated the 

Peerless as partly Franconian and partly 

Trempealeauan and believed that the portion now 

preserved is partly older than the Dotsero. According 

to Lochman-Balk, the dolomite in the Sawatch mapped 

in the Glenwood Springs area is older than the Peerless 

and is a tongue from a thick dolomite section in Utah. 

This interpretation is supported in a recent study by 

Campbell, Bickford, and Janssen (1976), which showed 

two dolomite intervals in the Sawatch of the White 

River Plateau to thicken to the southwest1 and to 

pinch out to the north and east.

Possible relations of the Peerless of the Aspen area 

with units of other areas are

1. The Peerless near Aspen is the equivalent of the Peerless of the Manitou Park and Gilman areas and to 

the dolomite and the overlying quartzite in the 

Sawatch and part of the Dotsero of the Glenwood 

Springs area (figs. 1LA, 12). That means that the upper part of the Sawatch of Glenwood Springs intertongues with Peerless between there and Aspen just 

as it apparently does between Taylor Park and the 

Aspen area. At Aspen the Peerless is thicker than at 

Gilman, and less has been removed by preOrdovician or Ordovician erosion. The 

Trempealeauan fossils reported by Johnson (1944) 

from the south edge of the Aspen area are from the 

very top of the Peerless. It is possible that the bottom could be Franconian.

2. The Peerless of the Aspen area is correlative only 

with the Dotsero of Glenwood Springs, and the 

regressive sequence represented by the Peerless and 

the Dotsero is younger in the White River Plateau 

area than at Gilman (fig. IIB). Near Aspen those 

rocks are of an age intermediate between those at 

Gilman and Glenwood Springs. The lack of 

widespread paleontologic control and closely spaced 

stratigraphic sections between the Sawatch Range 

and the White River Plateau precludes a definite 

solution to the problem at this time.

CHAFFEE GROUP

The Chaffee Group (Campbell, 1970; Tweto and 

Lovering, 1977) consists of the Parting Formation, 

which was mapped by Spurr (1898) in the Aspen area,

'Misprint in their abstract stated southeast.
18 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

and the Dyer Dolomite and the Oilman Sandstone, 

both of which he mapped with the Leadville Limestone 

(table 2). At the time of my work the Chaffee was considered a formation and the Parting and Dyer were

NW

Glenwood 

Springs

SE

Aspen

Taylor 

Park

Garfield 

quadrangle

€d

Om

p€

p€

B

50 KILOMETERS

FIGURE 11.—Possible stratigraphic relations of Cambrian rocks in 

west-central Colorado. A, B, Alternative sections on a northwestsoutheast line through Aspen. Diagrammatic sketches, not to 

scale. Cd, Dotsero Formation,Cs, Sawatch Quartzite.Cp, Peerless 

Formation, all of Cambrian age; Om, Ordovician Manitou 

Dolomite; pC, Precambrian rocks. Dashed lines approximate 

paleontologically defined time lines; fr, Franconian Stage; tr, 

Trempealeauan Stage. Wavy line, unconformity. C, Location of 

sections in figures 11 and 12.

members (Kirk, 1931). The Oilman Sandstone was 

mapped as a member of the Leadville Limestone.

A number of fossil collections from the region were 

summarized by Johnson (1944, p. 328-330) and indicate a Late Devonian age for the Chaffee Group. The 

only collection reported in the Aspen area is from East 

Aspen Mountain and consists of fragmentary bones 

and plates of fish (Spurr, 1898, p. 21). We found similar 

material in the Parting on Richmond Hill near the head 

of Queens Gulch. The Late Devonian age of the Chaffee 

indicates a substantial unconformity between it and 

the underlying Manitou of Early Ordovician age. Just 

to the south of Aspen the Harding Quartzite of Middle 

Ordovician age and the Fremont Limestone of Late Ordovician age are found between the Manitou and Chaffee (Johnson, 1944).

Campbell (1970) subdivided the Parting and Dyer of 

the Chaffee into members and was the first to suggest 

that the Chaffee be elevated to group status, the Parting and Dyer to formations. Tweto and Levering 

(1977) transferred the Oilman Sandstone from the 

Leadville Limestone to the Chaffee and designated the 

Oilman, Parting, and Dyer as formations in the Upper 

Devonian and Lower Mississippian(?) Chaffee Group.

PARTING FORMATION

Emmons (1886) gave the name "Parting quartzite 

series" to less resistant beds separating more resistant 

carbonate formations near Leadville. Spurr (1898) extended the name to the Aspen area.

Generally, the Parting Formation does not crop out 

well, although quartzite beds form distinctive float. 

However, good stratigraphic sections of Parting may 

be seen about 1.3 km N. 20° E. of Ashcroft; in various

Taylor 

Park

Garfield 

quadrangle

FIGURE 12.—Possible stratigraphic relations of Cambrian rocks in 

west-central Colorado. Section on a north-south line through 

Aspen. Diagrammatic sketch, not to scale. Cd, Dotsero Formation; Cs, Sawatch Quartzite; Cp, Peerless Formation, all of Cambrian age; Om, Ordovician Manitou Dolomite; pC, Precambrian 

rocks. Dashed lines approximate paleontologically defined time 

lines; fr, Franconian Stage; tr, Trempealeauan Stage. Wavy line, 

unconformity.
CAMBRIAN THROUGH MISSISSIPPIAN SHELF SEQUENCE 19

places on the ridge between the two principal south 

forks of Express Creek (fig. 13); on the southwest side 

of the 3,347-m knob (Castle Butte) west of Tourtelotte 

Park; and on the ridge east of lower Spar Gulch (East 

Aspen Mountain).

The Parting Formation consists of (in approximately 

ascending order although rock types are interbedded in 

many parts of the formation): white to tan poorly 

sorted crossbedded quartzite; gray, greenish-gray, and 

dusky-red shale and dolomitic shale; gray to tan 

siltstone; and grayish-yellow dolomite. Its thickness 

ranges from 15 to 30 m, judging by map relations. According to Johnson (1944), the Parting of the Aspen 

district is transitional between the more quartzitic sections of the northern Sawatch Range, which resemble 

the rocks of the type locality in the Leadville district, 

and more shale- and carbonate-rich sections to the 

south, which resemble the Upper Devonian Elbert Formation of the San Juan region. The Parting is of Late 

Devonian age.

Campbell (1970) divided the Parting into three 

members: A, lower member of medium- to coarse-

>.*,-&%&>• v.,.. ^^3pr ^^^

FIGURE 13.—Ridge between two south tributaries of Express Creek, 

Hayden Peak quadrangle, showing contact of Parting Formation 

of the Chaffee Group with the Manitou Dolomite. Parting consists of easily erodible shales interlayered with resistant sandstone and dolomite, and the formation as a unit tends to form a 

bench between more resistant dolomite above and below.

grained quartzose sandstone with some sandy shale; 

B, a middle member of basal gray shale, succeeded in 

sequence by thin dark-gray irregularly bedded 

dolomite, gray shale, and a thin sandy dolomite and 

dolomitic sandstone; and C, an upper member of shale, 

dolomite, and sandstone. Interpolation of Campbell's 

isopach maps (1970, figs. 5-7, p. 317) indicates that he 

measured 2.4 m of member A, 4.6 m of member B, and 

5.8 m of member C near Ashcroft, probably at or very 

close to section 9 of Johnson (1944).

Thickness variations of the Parting in the Aspen 

area and to the south (Johnson, 1944; Prather, 1964; 

Waterman, 1955; Slebir, 1957), to the southeast (Dings 

and Robinson, 1957), and to the southwest (Vanderwilt, 1937) indicate that trends of the isopachs shown 

by Campbell, at least in Pitkin County, are not significant. The variations in thickness in the region are local 

and numerous, and they appear not to show any 

trends. Within a larger area the more generalized 

trends shown by Baars (1972) are realistic. These 

trends indicate that the source area of the Parting was 

to the north and east as envisioned by Campbell (1970) 

and that the Uncompahgre did not exist as a large 

positive area during the Devonian as postulated by 

Pampe (1970).

In thin section the quartzites of the Parting are composed of rounded clastic grains of quartz as much as

2 mm in diameter in a matrix of finer quartz and carbonate which appears to have recrystallized to various 

degrees. Clastic grains of microcline compose a few 

percent of the quartzites. Muscovite, zircon, tourmaline, and opaque minerals are minor components.

The dolomites generally have a grain size of 

0.02-0.04 mm and contain a few grains of quartz of 

similar size. Muscovite, microcline, zircon, and opaque 

minerals are less common than quartz as accessory 

minerals. Gradational between the dolomite and quartzite are beds of sandy dolomite and dolomitic or calcareous sandstone (fig. 14).

DYER DOLOMITE

The Dyer Dolomite (Behre, 1932) of the Chaffee 

Group was mapped with the Leadville Limestone by 

Spurr (1898; table 2, this report) as an unnamed lower 

part.

The Dyer forms small cliffs and is well exposed on 

the east side of Castle Creek, at Fall Creek, at about 1.3 

km northeast of Ashcroft, and on the cliff west of 

Tourtelotte Park.

In the Aspen area the Dyer consists of thinly 

laminated light-gray and darker gray dolomite and 

contains some beds of dark-gray limestone as much as

3 m thick. It contains some red shale partings and
20 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Quartz

Carbonate Feldspar 

and mica

FIGURE 14.—Proportions of quartz, carbonate, and mica and 

feldspar in the Parting Formation of the Chaffee Group. Dot, 

sandstone; triangle, limestone and dolomite.

layers of flat pebble dolomite conglomerate. The 

dolomite is locally sandy. It ranges in thickness from 

15 to 30 m.

Campbell (1970) divided the Dyer into a lower 

fossiliferous dolomitic limestone and dolomite called 

the Broken Rib Member and an upper thin-bedded finegrained light-gray to dark-gray dolomite called the 

Coffee Pot Member. Interpolation of his isopach maps 

indicates that he measured about 5.5 m of the Broken 

Rib Member and 29.3 m of the Coffee Pot Member near 

Ashcroft. Although the Broken Rib Member of Campbell is not present north of Aspen along the margin of 

the Sawatch Range, it is well developed in the White 

River Plateau, and Johnson's sections suggest that it 

may be recognizable south of the Aspen area.

The isopachs shown in Pitkin County by Campbell 

(1970) for the Dyer are of uncertain validity, as those 

for the Parting. Changes in thickness along northsouth trends are similar to those along east-west 

trends. Isopachs shown by Bloom (1961) are based on 

many fewer sections in the White River Plateau but 

many more along the Sawatch Range. Nevertheless, in 

the Aspen area, they appear to be erroneous because he 

used a thickness of 76 m for a section at Aspen. Even 

the figures of 52 and 69 m reported by Johnson and 

described in his measured sections are, I think, too 

great.

In thin section the dolomites of the Dyer are fine 

grained, ranging generally from 0.01 to 0.05 mm but 

locally as coarse as 0.1 mm. Most sections have a few 

grains of quartz of similar size and few opaque

minerals. In one sample of thinly laminated lightand dark-gray dolomite, the darker beds are coarser 

grained with a grain size of about 0.08 mm as compared 

to 0.01 mm for the lighter gray beds. Quartz occurs in 

the coarser grained beds.

In the Aspen area no fossils were seen in the Dyer. 

Elsewhere, especially on the White River Plateau, the 

lower part (Broken Rib Member of Campbell) is quite 

fossiliferous and has been assigned a Late Devonian 

age, although evidence against a Mississippian age is 

not definite (Bass and Northrop, 1963, p. J25-J26). 

Lack of fossils in the Dyer Dolomite and presence of 

sparse Early Mississippian fossils in rocks in western 

Colorado considered to be equivalent to the Dyer have 

led Tweto and Levering (1977) to designate its age as 

Late Devonian and Early Mississippian(?).

OILMAN SANDSTONE

The horizon of dolomitic sandstone and sedimentary 

breccia within the dolomite section of the Leadville 

Limestone as defined by Eldridge (1894) allowed rocks 

believed to be predominantly Devonian to be separated 

from those of Mississippian age (Behre, 1932). This 

horizon has been mapped with the Leadville Limestone 

in the Aspen area by me and by other workers in the 

Sawatch Range (such as Tweto, 1974). Tweto and 

Levering (1977) now place the Oilman Sandstone as a 

formation in the Chaffee Group because of an unconformity recognized at the top of the Oilman and 

because of the change from primary dolomites in and 

below the Oilman to normal marine limestone above 

the Oilman. I did not recognize the unconformity at 

the top of the Oilman in the Aspen area, and the 

dolomites in the lower part of the Leadville greatly 

resemble the dolomites of the Dyer near Aspen.

In the Aspen area the Oilman Sandstone consists of 

several beds of dolomitic sandstone, sandy dolomite, 

and sedimentary breccia. It ranges from 1.2 to 4.6 m in 

thickness and furnishes dolomitic sandstone float that 

is easily distinguished from the carbonate beds above 

and below. The breccia beds consist of fragments of 

dolomite and, locally, of dark-gray chert as much as a 

few centimeters long in a matrix of sandy dolomite or 

dolomitic sandstone. In thin section the Oilman is seen 

to consist of rounded quartz grains and a few carbonate grains as much as 1 mm in diameter in a very 

fine grained carbonate matrix.

The Parting represents a transgressive sequence 

with several regressive episodes. Detrital sediments 

were derived from a highland northeast of the Sawatch 

Range. The deposition of the lower Dyer in a sublittoral environment marked the maximum advance of 

the Devonian sea. Much of the Dyer above the lower 

part in the Aspen area represents a regressive car-
CAMBRIAN THROUGH MISSISSIPPI AN SHELF SEQUENCE 21

bonate sequence deposited in a shallow sublittoral environment (Campbell, 1970).

After deposition of the Dyer, the region was elevated 

above sea level for a geologically brief time. The lower 

Dyer is latest Devonian and the lower Leadville is 

probably Kinderhookian in age. The Oilman Sandstone 

was deposited by a transgressive sea in Late Devonian 

or Early Mississippian time. Source for the sand grains 

in the Oilman may have been Devonian rocks eroded 

from an uplift of small relief at the present site of the 

Sawatch Range (Nadeau, 1972, p. 99).

According to Banks (1970), the sand grains were 

deposited by winds and were the primary source of 

silica of the chert that formed penecontemporaneously 

with deposition of the dolomite matrix. These beds 

were reworked by wave action to produce the sedimentary breccias that are such a distinctive feature of the 

Oilman.

LEADVILLE LIMESTONE

The Leadville Limestone as mapped by Spurr (1898) 

in the Aspen area included the Dyer Dolomite, the 

Oilman Sandstone, and the Leadville Limestone of this 

report. I included the Oilman Sandstone with the 

Leadville Limestone on my maps (1970; 197 la). Two 

units equivalent to the lower and upper parts of this 

report have been named the Castle Butte and Redcliff 

Members by Nadeau (1972), who also included a 

Oilman Member at the base of his Leadville.

The Leadville forms cliffs and ledges in many places 

in the Aspen area. It consists of gray to dark-gray 

thick- to thin-bedded dolomite and a few beds of 

limestone in the lower part, and blue-gray thick-bedded 

to massive limestone in the upper part.

The lower part of the Leadville contains thinly 

laminated dolomite resembling some beds in the Dyer 

Dolomite and some thicker beds of dolomite. It contains dark-gray chert lenses and stringers. The contact 

between the dolomite and overlying limestone is not 

uniform. In some places, such as the hill west of 

Tourtelotte Park, it is quite abrupt; in other places, 

such as the 3,190-m knob in the Express Creek valley, 

gray limestone and gray to light-gray thinly laminated 

dolomite are interbedded.

The upper massive limestone makes the most prominent outcrops and has a characteristic blue-gray color. 

Some excellent exposures occur on Express Creek and 

again to the south near the southwest branch of Express Creek. The ridge between Spar and Vallejo 

Gulches (Aspen quadrangle) has a number of cliffs of 

Leadville along it (fig. 15). The hill west of Tourtelotte 

Park (Castle Butte) is capped by Leadville and is the 

type locality of Nadeau's Castle Butte Member.

*jtS* *

,N{1ips^ *

^XA\*IX^ «|ffc<fcV^tf*'

:jWT^tt: ri "

X^i^l

-* w*u

FIGURE 15.—Upper part of the Leadville Limestone. "Blue limestone" of Spurr (1898) and Castle Butte Member of Nadeau 

(1972). Thick-bedded coarse-grained biosparite. Cliff about 25 m 

high. (Closeup of cliff in the middle of fig. 6.)

In the Spar Gulch area, the contact between 

dolomite and limestone is quite abrupt. Old mine workings along that contact, which Spurr (1898) interpreted 

to be a bedding-plane fault, suggest that the ore may 

have occurred in cave deposits formed by postLeadville-pre-Belden solution along the contact. An 

exposure where the road into Spar Gulch crosses the 

ridge of Leadville shows an irregular breccia at the 

base of the limestone. In the breccia are large 

fragments of Leadville in a matrix of dark-gray shale 

resembling the overlying Belden Formation. In places 

bedded shale surrounds the fragments, and in other 

places the shale itself is fragmented. These relations 

may be interpreted to show that cavities along the contact below the limestone unit existed at the time of 

deposition of the basal part of the Belden Formation, 

that the cavities were filled or partly filled during 

deposition of the Belden, and that some further solution occurred after deposition of the Belden.

The blue-gray limestone part of the Leadville is ab-
22 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

sent north of the Roaring Fork valley in the Aspen 

quadrangle but is present on Larkspur Mountain 2.5 

km north of the Aspen quadrangle. The absence of the 

limestone member along the 10.4 km distance was interpreted by Spurr (1898) to be due to faulting along a 

bedding-plane fault he called the Silver fault and by 

Vanderwilt (1935b) as an unconformity. Vanderwilt 

(p. 229) stated that the upper contact of the Leadville 

is marked by a conglomerate of chert fragments and 

fine-grained quartzite. Surface exposures of this contact are very rare. I have seen a similar conglomerate 

above the Leadville south of Express Creek, but 

nowhere else. Most prospects and mines near the contact north of the Roaring Fork have dolomite breccia 

and shale in their dumps.

Especially critical are the map relations just north of 

Hunter Creek where Belden is in contact with Sawatch 

and Manitou. These relations appear to be caused by 

faults since there are abrupt changes in what formations are adjacent to the Belden, and they support 

Spurr's fault interpretation of the contact in this area. 

A recent study by Maslyn (1976) interprets this 

feature as having formed by pre-Belden faulting and 

sinkhole formation.

Vanderwilt (1935b) and Johnson (1944) described 

erosional features on the top of the Leadville where exposed in mines near Aspen. Johnson (p. 337) described 

a conglomerate containing Precambrian boulders supposed to be at the base of the Belden. To me such a conglomerate seems anomalous because scattered pebbly 

sandstones I have seen at the base of the Belden are 

composed of mature sediments rich in quartz and chert 

and very poor in feldspar. The character of the Belden, 

described below, and the preservation of Leadville 

around the Sawatch Range suggests to me that preBelden erosion did not cut down to Precambrian rocks 

and that the material described by Johnson, if the 

description is accurate, may have another origin.

Spurr (1898, p. 132) observed in mine workings a 

truncation of the formation by faulting at depth. Spurr 

interpreted the fault, which he called the Silver fault, 

to be a near-bedding-plane fault occurring throughout 

the Aspen quadrangle. I interpret the lack of the blue 

limestone of the Leadville between the Roaring Fork 

and Larkspar Mountain to be due to more local 

faulting along the margin of the Sawatch which is 

moderate to high angle and subparallel to the bedding. 

The possibility of a regional bedding-plane fault will be 

discussed later.

The lower part of the Leadville has few fossils, but 

the blue limestone in places has a large component of 

fossil fragments. The most obvious fossils are corals 

and crinoid stem fragments. Foraminifera are 

widespread. On Richmond Hill at one locality 

brachiopods and bryozoa were seen. Girty (1903) and

Johnson (1944) list fossils found in the Aspen area and 

in the Crested Butte 15-minute quadrangle to the 

south.

The following Foraminifera and algae were identified 

by B. A. Skipp (written commun., 1973) in thin sections of the blue limestone from the Aspen area. One 

section contained Calcisphaera laevis Williamson, 

Calcisphaera pachysphaerica Pronina, Spinoendothyra 

spinosa Chernysheva, Earlandia, Latiendothyra sp., 

Septaglomospiranella sp., Septaglomospiranella 

dainae Lipina, Chernyshinella, Eotubertina, and 

Paracaligella, an assemblage characteristic of 

foraminiferal zone 9 or late Osagean (Mamet and 

Skipp, 1970). Species found in other sections are, in addition to some of those listed, Tuberendothyra, 

Radiosphaera, Parathurammina, Septabrunsiina sp., 

and Palaeospiroplectammina. These forms, according 

to Skipp, are indicative of zones 8 or 9, middle to late 

Osagean. The Leadville is considered to be 

predominantly of Osagean or late Early Mississippian 

age in west-central Colorado, although some basal 

beds may be of Kinderhookian age (early Early 

Mississippian) (Conley, 1972; Craig, 1972).

Some beds of the dolomite in the lower part of the 

Leadville are coarser grained than the thinly laminated 

beds and have a grain size of 0.2 mm. Near ore bodies 

the dolomite is more coarsely recrystallized and has a 

grain size as coarse as 1 mm. The blue limestone consists of shell fragments and Foraminifera as much as 

2 mm in diameter in a matrix of recrystallized calcite. 

Quartz and opaque minerals are the principal accessory minerals in carbonate rocks of the Leadville. 

The petrology of the Leadville has been well described 

by Nadeau (1972) and Conley (1972). Most of the 

features they described can be found in the Leadville in 

the Aspen area. They interpreted the lower dolomitic 

part of the Leadville as having been deposited in a 

relatively sheltered environment in the upper intertidal zone; whereas the blue limestone of the upper part 

formed in a shallow marine high-energy environment.

PENNSYLVANIAN THROUGH

TRIASSIC SEQUENCE OF THE

EAGLE BASIN

In Pennsylvanian time the sedimentation pattern in 

the Aspen region changed markedly from that of the 

preceding part of the Paleozoic. Instead of widespread 

thin relatively uniform quartzite-carbonate formations, thicker, more variable, and geochemically less 

mature sediments were deposited. Aspen lay on the 

southwest side of a northwest-trending depositional 

basin, called the Eagle basin or Central Colorado 

trough (fig. 3). Detritus from bordering uplifts was
PENNSYLVANIAN THROUGH TRI ASSIC SEQUENCE OF THE EAGLE BASIN 23

deposited in thicknesses of as much as several 

kilometers in this trough. In the Aspen region the 

detritus came from the uplift to the southwest, which 

is called the Uncompahgre highland. Deposition began 

in the Early Pennsylvanian in a shallow marine and 

nearshore environment, and the basin margin lacked 

much structural relief. During this time the Belden 

Formation, consisting primarily of shale and carbonate, was deposited. Increasing relief of the Uncompahgre highland produced coarser clastic sediments 

during deposition of the Gothic Formation of 

Langenheim (1952), which includes arkoses, siltstones, 

marine limestone, and shale. Further uplift of the Uncompahgre and filling of the basin led to deposition of 

coarse conglomerates and sandstone of the Maroon 

Formation in a terrestrial environment. Siltstones 

locally dominate the upper part of the Maroon. Some 

deformation and erosion of the rocks in the basin ensued, and further uplift of the Uncompahgre in Permian and Trias sic time led to deposition of the State 

Bridge Formation, which in the Aspen area contains 

conglomerate, sandstone, and siltstone. A period of 

uplift and erosion followed, and then the Chinle Formation of Late Trias sic age was deposited. The uplift 

southwest of Aspen was a positive area either during, 

or more likely after, deposition of the Chinle, for that 

formation is absent on the Uncompahgre highland. Upper Jurassic rocks are the oldest ones which do not 

reflect the influence of Uncompahgre uplift.

BELDEN FORMATION

The Belden Formation (Brill, 1942, 1944) crops out 

in the Aspen area in a north-south belt along the 

margin of the Sawatch Range parallel to and west of 

the older Paleozoic formations and in an isolated area 

on Conundrum Creek in the Hayden Peak and Maroon 

Bells quadrangles. The Belden Formation is generally 

poorly exposed except where contact metamorphism 

has converted it to hornfels and marble. It underlies 

valleys which contain surficial deposits, such as Castle 

Creek from Ashcroft to north of Sawyer Creek; and 

grassy areas, such as Van Horn Park and Annie Basin 

(Aspen quadrangle). The best exposures are in mine 

tunnels, and these usually reveal structural complications not perceived when only surface observations are 

made.

The Belden was mapped as the Weber Formation by 

Spurr (1898) and as Weber(?) Formation by Knopf 

(1926). Brill (1944) recognized that the "Weber Shale" 

of the Aspen area was correlative with Belden Shale of 

the type locality on the northeast side of Eagle basin.

The Belden is composed of relatively thin bedded 

carbonaceous limestone, dolomite, and shale with a few 

beds of sandy shale and sandstone (figs. IQA, B). The

sand content increases in the upper part, and the upper 

contact is transitional with the more sandstone-rich 

Gothic Formation of Langenheim (1952). In nearby 

areas a massive sandstone is used to mark the bottom 

of the Gothic (Langenheim, 1952; Mutschler, 1970; 

Gaskill and Godwin, 1966a). However, in the Aspen 

area such a distinctive sandstone is missing, and the 

contact is gradational. Dark-gray shale is interbedded 

with greenish-gray and tan calcareous siltstone and 

sandstone in the interval of gradation, and the contact 

is placed where the light-colored siltstone, sandstone, 

and limestone become dominant.

The base of the Belden lies on an erosional surface on 

the Leadville Limestone, except between the Roaring 

Fork River and Larkspur Mountain in the Ruedi 

quadrangle, where the contact may be a fault. 

Southwest and west of Aspen residuum on this surface 

is thick enough to form a mappable rock unit called the 

Molas Formation, but in the Aspen area rocks that 

might be assigned to the Molas are difficult to find in 

surface exposures. In the area of the San Juan Mountains, Merrill and Winar (1958) described the Molas as 

consisting of three units: a lower unit of unstratified 

siltstone and mudstone with layers of limestone and 

chert boulders; a middle unit of poorly stratified 

siltstone, mudstone, sandstone, and conglomerate sequence; and an upper unit of well-stratified calcareous 

shale, siltstone, and sandstone. If any Molas occurs in 

the Aspen area, it is the controversial breccia at the 

Leadville-Belden contact, which might represent the 

lower unit. Around the Treasure Mountain dome 

southwest of the Aspen area, as much as 15 m of Molas 

was mapped by Mutschler (1970), Gaskill and Godwin 

(1966a), and Gaskill, Godwin, and Mutschler (1967). 

There, the fragments in the residual breccia consist of 

chert and quartzite.

In the Aspen area the stratigraphic base of the 

Belden unmodified by faulting is exposed in places 

south of Express Creek. One place is on the east side of 

the shoulder on hill 11490 (Hayden Peak quadrangle). 

There, the base of the Belden consists of about a meter 

of medium-bedded dark- to medium-gray dolomite, a 

10-cm bed of sandy dolomite, another meter of lightgray-weathering dolomite, and a lens of sandstone. 

Elsewhere, lenses of coarse-grained sandstone and 

chert-pebble conglomerate, perhaps 0.5 m thick, occur 

at or near the base of the Belden.

At the Taylor Pass road where it crosses Waterfall 

Gulch, siltstone, sandstone, and shale at the BeldenLeadville contact somewhat resemble the Molas Formation.

As Spurr pointed out (1898, p. 30), the lower part of 

the Belden is richer in carbonate (mostly limestone but 

some dolomite) than the upper part. The proportion of 

carbonate to shale varies from place to place, and con-
24 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

tact metamorphism emphasizes the contrast between 

the carbonate-rich parts and the shale-rich parts. On 

Conundrum Creek and at the head of Castle Creek, 

separate units of marble and hornfels were mapped in 

the Belden. On Taylor Peak just south of the Hay den 

Peak quadrangle, large outcrops are divisible into

FIGURE 16.—Features of the Belden Formation. All these exposures 

are in marble near the White Rock pluton (fig. 48). The Belden is 

poorly exposed where it is unmetamorphosed. Darker colored 

beds are finer grained and contain more carbonaceous material 

and silt than the lighter colored beds. A, Thin-bedded marble at 

altitude of 3,392 m east of Cooper Creek, Hayden Peak 

quadrangle. B, Somewhat thicker bedded marble on 

3,900-3,960-m nose on the east side of the ridge north of Taylor 

Peak, Pearl Pass quadrangle. C, Breccia zone at same locality as 

B. Zone apparently parallel to bedding formed before contact 

metamorphism.

three units from bottom to top: marble with a few 

hornfels beds; hornfels, quartzite, and a few marble 

beds; and carbonaceous hornfels. Although neither the 

base nor the top of the formation is exposed there, 

those outcrops probably represent most of the total 

thickness of the Belden.

Thicknesses of Belden Formation range from 122 to 

580 m, probably owing to the effects of faulting in that 

formation. Where faults are not shown, such as in sections B-B' and C-C" of the Hayden Peak quadrangle 

(Bryant, 1970), the formation is almost completely concealed beneath the surficial deposits in Castle Creek 

valley, and it may be highly faulted, judging from exposures in tunnels along the belt. An approximate 

thickness of the Belden is 300 ±150 m. No regional 

isopach maps should be drawn based on sections of the 

Belden in the Aspen area.

The large number of carbonate beds in the lower 

Belden makes it locally difficult to differentiate Belden 

from Leadville. Generally, Belden limestone is darker 

gray, finer grained, and more thinly bedded than the 

Leadville (figs. 15, 16); it may have partings of carbonaceous shale or shaly limestone. Probably, the
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 25

most difficult place to make the distinction is east of 

Van Horn Park where the distinctive blue limestone of 

the Leadville is missing, and the Belden has many beds 

of dolomite and limestone as much as 6 m thick. 

Another problem area of numerous thick carbonate 

beds in the Belden occurs on the ridge at the head of 

the southwest fork of Express Creek.

Breccias are common in the Belden where exposures 

are good, such as in areas of contact metamorphism 

(fig. 16C) and in tunnels. Some of the best natural exposures are of marble on Taylor Peak just south of the 

Hayden Peak quadrangle, where the breccias, in the 

lower part of the exposure, are composed of fragments 

of the adjoining rock in a calcareous matrix now converted to marble.

Anhydrite and gypsum occur in the Belden, probably in the lower part. Knopf (1962) reported 30.5 m 

of gypsum in the Hope tunnel on Castle Creek in the 

northern edge of the Hayden Peak quadrangle. Knopf 

(1926, fig. 1) put the gypsum near the top of the 

Belden, but his section (1926, fig. 5) shows that only 

60-90 m of shale separate the gypsum from the Leadville. His map (Knopf, 1926, fig. 4) showed a welldeveloped fault gouge between what he called the 

Weber(?) and Maroon. Near the south edge of the 

Aspen 7.5-minute quadrangle, the Highland tunnel 

(discussed in a later section) revealed gypsum and 

anhydrite in a breccia about 1.5 km in from the portal, 

and a drift driven on a curving course south of the tunnel intersected about 30 m of anhydrite and gypsum interbedded with limestone and shale. On the Fryingpan 

River, about 13 km north of the Aspen quadrangle, 

Freeman (1972b) mapped in the Belden a unit about 

24 m thick containing evaporite beds.

Extensive fossil collections in the Glenwood Springs 

area indicate that the Belden is Morrowan and Atokan 

in age (Bass and Northrop, 1963, p. J36-J41). Other 

lists of fossils from the Belden are in Girty (1903) 

(called Weber Formation by him), Langenheim (1952), 

and Brill (1944). In the Aspen area some brachiopods 

were found in limestone of the Belden at one locality 

south of the Woody Creek-Hunter Creek divide in the 

Aspen quadrangle.

Isopach maps of the Belden on the southwest side of 

the Eagle basin are poorly controlled (Brill, 1958, 

p. 104). As pointed out previously, the possibility of 

great thinning or thickening, owing to structural effects in the Aspen area, does not permit any firm 

thickness figure to be stated for the area, but the 

Belden is probably between 150 and 450 m thick. In 

the Crested Butte 15-minute quadrangle Langenheim 

(1952) found a thickness of 169 m about 18 km to the 

south of the Aspen area; Mutschler (1970) recorded 260 

m in the Treasure Mountain dome 13 km to the 

southwest of the Maroon Bells quadrangle; and 

Freeman (1972b) measured 183-213 m just to the

north of the Aspen quadrangle. Clearly, the pattern of 

isopachs is not as simple as indicated by Brill (1944, 

fig. 2; 1958). Facies relations in the Belden (Mallory, 

1972) and local data from south of the Aspen area 

(Langenheim, 1952, p. 572) indicate that the Uncompahgre uplift was not a strong positive element in Early Pennsylvanian time, although the Front Range, 

Apishapa, and Pedernal uplifts to the east and south of 

the Uncompahgre uplift apparently were. The zero 

isopach of the Belden southwest of Aspen is due to 

post-Belden erosion. Maps of the Treasure Mountain 

dome (Mutschler, 1970; Gaskill and Godwin, 1966a) 

show that at least some and perhaps all of that erosion 

is post-Maroon and pre-Entrada in age.

Petrographic and X-ray examination of rocks of the 

Belden Formation show that the shales are composed 

of clastic grains of quartz and muscovite in a very fine 

grained matrix of quartz, dolomite or calcite, sericite, 

chlorite, pyrite, and carbonaceous material. Neither 

feldspar nor clay minerals constitute a significant proportion of the specimens examined. Since these rocks 

were buried to a depth of about 6 km before uplift, any 

preexisting clay minerals were probably unstable and 

were converted to mica. The sandstone lenses at the 

base of the formation are composed of quartz and 

chert. Tourmaline and zircon are accessory minerals in 

the sandstones.

GOTHIC FORMATION 

OF LANGENHEIM (1952)

The Gothic Formation of Langenheim (1952) occupies a belt of 0.8-1.6 km wide along the margin of 

the Sawatch Range in the Aspen and Hayden Peak 

quadrangles and crops out in the Conundrum Creek 

area northwest of Hayden Peak and in the southern 

part of the Maroon Bells quadrangle. It is well exposed 

in the Woody Creek valley southwest of Lenado and in 

Lenado Gulch in the Aspen quadrangle, and on the 

cliffs on the west side of Castle Creek in the Hayden 

Peak quadrangle. Southeast of the Hope mine on the 

Castle Creek road is a good roadside exposure.

The Gothic Formation was named by Langenheim 

(1952), and the type locality is on the ridge west of Copper Creek about a mile south of the Maroon Bells 

quadrangle. Langenheim (1952, p. 551) and Bartleson, 

Bryant, and Mutschler (1968) have given reasons for 

using the term Gothic rather than Minturn Formation, 

which is well established in the Vail region on the 

northeast side of the Eagle basin.

The Gothic Formation has transitional contacts with 

the underlying Belden Formation and the overlying 

Maroon Formation. At its base the Gothic Formation 

consists of green, gray, and very light grayish red 

calcareous shale, shaly limestone, sandy limestone, 

and calcareous siltstone interbedded with dark-gray
26 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

carbonaceous shales and siltstones resembling the 

Belden. No well-defined sandstone bed marks the base 

of the Gothic in the Aspen area as it does in the 

Crested Butte region (Langenheim, 1952) and to the 

southwest on the Treasure Mountain dome (Mutschler, 

1970). In the Aspen area the lower contact is drawn 

where light-colored sandstone, siltstone, and silty or 

sandy limestone predominate above the dark-gray 

calcareous and shaly limestones of the Belden. The 

Gothic contains megascopically obvious clastic mica 

grains, which are lacking in the Belden.

The upper contact is marked by a relatively abrupt 

transition in color from dominantly gray, green, and 

brown beds to dominantly reddish gray, grayish-red, 

and pale-red colors in the Maroon Formation. The 

Maroon Formation is more poorly sorted, coarser 

grained, and has fewer limestones than the Gothic.

The Gothic Formation in the Aspen region consists 

of gray, tan, and brown calcareous sandstone and 

siltstone, silty and sandy limestone, and limestone. It 

contains a few thin greenish-gray and grayish-red 

beds, especially near its upper contact. Thin beds of 

dark-gray limestone and silty limestone are scattered 

throughout the formation. Many of the rocks weather 

grayish orange to moderate orangeish brown. Except 

in the southwesternmost exposures conglomerate beds 

are absent; a few pebbles were seen in the inclusion of 

Gothic in the White Rock pluton in the southern part 

of the Maroon Bells quadrangle, and pebbly arkose occurs in the Gothic in the southwestern corner of the 

quadrangle. Ripple marks and mudcracks are found 

locally in the Gothic.

Gypsum and anhydrite occur in lenses at 10 m 

thick in Monument Gulch in the Hayden Peak 

quadrangle. Associated with the evaporites are layers 

of breccia that also are at least 10m thick. These breccias weather to towers that give the gulch its name 

(fig. 17.4). They resemble the stacks described by 

Mallory (1971, fig. 6); their origin is uncertain. Mallory 

suggested that such stacks could be produced by brecciation associated with flowage, accompanying the 

change from anhydrite to gypsum that occurs in the 

evaporite as it is subjected to near-surface environment, or that they may be the remains of filled, recentFIGURE 17 (right).—Breccia in Gothic Formaton of Langenheim 

(1952), Hayden Peak quadrangle. A, View down Monument Gulch 

from altitude 3,377 m at its head, showing the "monuments" composed of brecciated and unbrecciated Gothic Formation. Left 

slope of gulch is a dip slope, and the breccia is at one general 

stratigraphic horizon. A few small exposures indicate that the 

breccia is underlain by 10 m or so of evaporite. B, Closeup of breccia in one of the "monuments"; fragments are calcareous 

siltstone, silty limestone, and limy sandstone in a calcareous 

matrix.
PENNSYLVANI AN THROUGH TRI ASSIC SEQUENCE OF THE EAGLE BASIN 27

ly formed, solution sinks. I have no evidence for their 

origin except that in Monument Gulch they occur in a 

stratigraphic horizon immediately overlying the evaporite, which seems to favor the first hypothesis.

In Monument Gulch the evaporites contain some 

beds of carbonaceous limestone resembling beds in the 

Belden. The breccias in places contain fragments of 

that rock type as well as fragments of sandy or silty 

limestone typical of the Gothic in the Aspen area. The 

breccias contain fragments of silty limestone and limy 

sandstone 5-20 cm in diameter (fig. 17B) but locally as 

much as 60 cm in diameter in a calcareous matrix. 

Some local zones of brecciation lack much calcareous 

cement, some of the rock in the towers is just fractured 

rather than brecciated; in some towers the lower parts 

are more thoroughly brecciated than the upper parts.

In other places in the outcrop area of the Gothic Formation, outcrops of breccia are found without any 

associated evaporites. Perhaps evaporites are present 

but not exposed, or perhaps they have been removed 

by solution. Alternatively, those breccias may have 

been formed by fault movements or by igneous intrusion.

In thin section, rocks of the Gothic Formation are 

seen to consist of various proportions of clastic grains 

of quartz, microcline, plagioclase, carbonate, muscovite, and less abundant biotite and chlorite, and accessory zircon, tourmaline, and opaque minerals, in a 

matrix of very fine grained carbonate, which may contain a small amount of clay mineral. The clastic grains 

are generally <0.5 mm in diameter, and in many of the 

limestones they are 0.05-0.1 mm in diameter. The clastic grains of quartz, especially, are quite angular. Mica 

grains tend to be alined parallel with the bedding.

Composition of rocks of the Gothic Formation ranges 

from limestone, or, rarely, dolomite, to calcareous sandstone and siltstone (fig. 18); but almost no rocks in 

the formation are limestones lacking detrital silicate 

minerals or sandstones lacking carbonate cement. 

Many rocks called sandstone in the field have carbonate contents greater than 50 percent and consequently are sandy limestones.

The farthest southwest exposures of the Gothic Formation in the Aspen area are on a hill south of the head 

of Rustler Gulch in the Maroon Bells quadrangle. Approximately 60-90 m of Gothic are exposed between 

the Elk Range thrust and the overlying Maroon Formation; these exposures consist of light- to dark-gray 

arkose, pebbly arkose, dark-gray silty shale, gray to 

dark-gray limestone, and silty limestone. A local bed of 

dark-gray sandy oolitic limestone near the top of the 

Gothic contains a number of marine fossils and 

overlies a siltstone containing a few plant fragments.

Quartz

Carbonate Mica and 

feldspar

FIGURE 18.—Proportions of quartz, carbonate, and mica and feldspar in the Gothic Formation of Langenheim (1952)) in the Aspen 

15-minute quadrangle. Dot, sandstone; cross, siltstone; triangle, 

limestone or dolomite.

The following fossils from the limestone were identified by Ellis L. Yochelson (gastropods and 

scaphopods) and Mackenzie Gordon, Jr. (rest of fauna; 

pelecypods in consultation with John Pojeta). (USGS 

colln. no. 22251-PC; field sta. 151):

Brachiopods:

Linoproductus cf. L. prattenianus (Norwood and Pratten)

Anthracospirifer rockymontai us (Marcou)

Composita ovata Mather 

Pelecypods:

Palaeoneilo sp.

Myalina sp.

Aviculopecten sp.

Pectenoid pelecypod indet.

Schizodusl sp. 

Scaphopods:

Plagioglypta sp. indet. 

Gastropods:

Euphemites sp. indet.

Bellerophon (Pharkidonotus) sp. indet.

Knightites (Retispira) cf. K. (R.) nodocostatus (Gurley)

Pleurotomacean aff. Hypselentoma sp. indet.

Shansiella carbonaria (Norwood and Pratten)

Colpites cf. C. kansasensis (Sayre)

Naticopsis (Naticopsis) sp. indet.

Naticopsid aff. Trachydomia sp. indet.

Stegocoelia sp. indet.

Murchisonid gastropod indet.

Pseudozygopleurid gastropod indet.

lanthinopsis (Soleniscus) sp. indet.

At least four other indeterminate genera. 

Cephalopods:

Mooreoceras cf. M. normale Miller, Dunbar, and Condra

Domatoceras sp.

Liroceras sp. indet.
28 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Concerning this collection Yochelson (written commun., 1966) stated:

Because so many specimens are exfoliated it is difficult to get a 

precise age. In general terms this is probably Middle or Late Pennsylvanian rather than either Early Pennsylvanian or Early Permian

* * * This collection shows more variety than the gastropods 

previously available in the Leadville region.

According to Gordon (written commun., 1966):

* * * The collection rather closely resembles one that Gordon made 

(from the White Quail Limestone Member of the Minturn Formation) on Radio Ridge, one mile southwest of Ptarmigan Peak in the 

Gore Range * * * A nautiloid Domatoceras n. sp. is a rather spectacular member of the White Quail fauna and the one in collection 

22251-PC appear to be the same species * * * The age of the White 

Quail Member in the Minturn quadrangle is Middle Pennsylvanian 

(Des Moinesian).

A small collection of fossils from a bed of sandy 

limestone in the inclusion mapped in the White Rock 

pluton at the head of East Maroon Creek contained 

bellerophontid gastropod indet., Trepospira sp., and 

other indeterminate gastropods. The material did not 

permit an age determination. Plant fragments were 

also present at the locality.

No other fossils were found in the Gothic Formation 

of the Aspen area except some plant fragments in the 

lower part west of the west fork of the Express Creek.

Elsewhere in the region, Girty (1903) listed a number 

of fossils collected from the lower Maroon (Gothic of 

this report) of the Crested Butte area, and Langenheim 

(1952) found a Des Moinesian fauna in the Gothic Formation. Bartleson (1972) also in the Crested Butte area 

found a fusulinid sequence of Des Moinesian age in 

several limestones that have some continuity along 

strike of the Eagle basin. At the southwest margin of 

the Elk Range thrust sheet, Mutschler (Bartleson and 

others, 1968) found Atokan fusulinids in a limestone in 

an outcrop of Gothic separated by the thrust and the 

Snowmass pluton from the stratigraphic boundaries of 

that formation. This is the only locality as yet in the 

Gothic to produce Atokan fossils.

Neither Bartleson (1968) nor I found any fossils in 

the more basinward sections of the Gothic north of the 

crest of the Elk Mountains.

The apparent thickness of the Gothic Formation in 

the Aspen area ranges from 140 m on the Express 

Creek-Castle Creek ridge to about 900 m near Sawyer 

Creek. Much of the thickness variation may be due to 

faulting: either bedding-plane faults associated with 

the Elk Range thrust, unmapped high-angle faults 

along the margin of the Sawatch Range, or both. Very 

few complete and well exposed sections of the Gothic 

are available for study in the region; so little can be 

said about thickness variations. The Gothic is apparently thinner to the north in the Fryingpan 

drainage (Freeman, 1972b). Yet in the Eagle area nearly equivalent evaporitic and clastic rocks are believed 

to be 2,740 m thick (Mallory, 1971). There, the 

stratigraphic interval represented by that thickness 

depends on interpretation of an horizon continuous 

with the Jacque Mountain Limestone Member which 

marks the top of the Minturn Formation and is probably somewhat higher stratigraphically than the top 

of the Gothic on the south side of the Elk Mountains. 

North of the Elk Mountains mapping suggests that 

the top of the Gothic rises stratigraphically in the 

Hayden Peak quadrangle and descends at the north 

margin of the Aspen quadrangle. The thickest wellexposed section probably lacking internal faulting is at 

Haymaker Gulch near the Taylor River 20 km south of 

the Hayden Peak quadrangle about at the 

margin of the Eagle basin during deposition of the 

(Bartleson, 1968). Even that section is incomplete. 

Consequently, the precise location of the southwest 

margin of the Eagle basin during depositon of the 

Gothic Formation is not known, owing to post-Gothic 

erosion. The greater grain size and conglomerate content of the Gothic in the southernmost exposures indicate that the margin of the basin was in that direction and perhaps not many kilometers away.

All the Gothic of the Aspen area except that south of 

Rustler Gulch is in the upper plate of the Elk Range 

thrust and is a more basinward facies than that of the 

type locality. It has a finer grain size and fewer beds of 

pure limestone than the Gothic to the southwest. Except for a few pebbly beds at the head of East Maroon 

Creek, conglomerates are absent; whereas they are 

common in the outcrops to the west and south. These 

facies relations show that the effects of the ancestral 

Sawatch uplift proposed by De Voto (1972) are minor 

in the Aspen area compared to those associated with 

the Uncompahgre uplift. However, the lack of Gothic 

exposures northwest of the Aspen area precludes a 

thorough three-dimensional facies analysis.

EAGLE VALLEY FORMATION

The Eagle Valley Evaporite was defined by Lovering 

and Mallory (1962) on the northeast side of the Eagle 

basin as the basin center evaporite sequence that intertongues with the Minturn and Maroon Formations. It 

also intertongues with the Gothic Formation on the 

southwest side of the basin (Bartleson and others, 

1968). Bartleson, Bryant, and Mutschler (1968) used 

the term Eagle Valley Formation for rocks that make 

up a transition zone of indistinct color change between 

Gothic and Maroon Formations, whether or not 

evaporites are interbedded.

Rocks at the north edge of the Aspen quadrangle lying between the Maroon and Gothic Formations are 

assigned to the Eagle Valley Formation. They are com-
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 29

posed of interbedded gray, greenish-gray, grayish-red, 

and reddish-brown calcareous siltstone and sandstone, 

and gray silty limestone and limestone. They are 

stratigraphically equivalent to the lower part of the 

Maroon Formation immediately to the south. Along 

strike to the northwest of Larkspur Mountain in the 

Ruedi quadrangle, the rocks mapped as Eagle Valley 

in Woody Creek interfinger with the Maroon Formation, and the bottom of the Maroon Formation is about 

460 m lower straigraphically than south of Woody 

Creek. The Eagle Valley here is probably of Middle 

Pennsylvanian age because it is stratigraphically 

equivalent to the lower part of the Maroon Formation.

MAROON FORMATION

The Maroon Formation is the most widespread rock 

unit in the Aspen region, and it makes up four of the 

five 4,270-m (14,000-ft) peaks in the area (fig. 19). Parts 

of the formation are well exposed in many places on the 

high mountains and in the deep valleys, where they 

produce the spectacular character of the scenery of 

much of the region.

The Maroon Conglomerate was named by Eldridge 

(in Emmons and others, 1894, p. 6), and it included all 

the beds above the Belden (called Weber Limestone by 

him) and below the Morrison Formation in the Crested 

Butte quadrangle. However, the type locality was 

given as Maroon Creek, although no type section was 

published. Spurr (1898) used the term Maroon Formation in the Aspen 7.5-minute quadrangle because conglomerates are rare there. He included the rocks now 

mapped as Gothic Formation by me, and he excluded 

some moderate-red rocks in the upper part. He called 

those upper rocks Triassic but pointed out that the 

contact is gradational and difficult to map. Included in 

the Triassic sandstones of Spurr are the upper part of 

the Maroon, the State Bridge, and the Chinle Formations of the present report. West of Castle Creek in the 

Aspen quadrangle the upper contact of the Maroon 

mapped by Spurr almost coincides with that mapped 

here, but on Red Mountain and along Woody Creek 

hundreds of meters of Maroon Formation were included with the Triassic sandstones.

The Maroon Formation consists of sandstone, 

siltstone, conglomerate, mudstone, and limestone interbedded on scales of 30 cm to 10 m. an incomplete 

section of the Maroon Formation in "Stratigraphic sections" illustrates many of the variations in rock type 

and color occurring in the Maroon. (See stratigraphic 

section 2, p. 131.) Colors in the lower part tend to be 

grayish red and pale red, whereas in the upper part 

reddish-brown and moderate-red colors dominate. This 

color change is probably the one noted by Spurr and

served as a basis for his separation of Maroon Formation from Triassic sandstones. The change in color is 

too gradational and variable along strike to be worth 

mapping, although locally, such as along Woody 

Creek, the change is relatively sharp across strike. An 

additional complicating factor is that a slight amount 

of contact metamorphism darkens the brighter reds to 

grayish-red hues.

No reliable stratigraphic markers were found in the 

Maroon Formation. Where exposures are excellent on 

the high peaks of the Elk Mountains a few sandstone, 

conglomerate, or limestone beds can be traced as much 

as 1.6 km (fig. 20). Sequences of beds may make a 

distinctive outcrop pattern that allows faults and 

relative movement on them to be deciphered on slopes 

where the rocks are well exposed. However, many conglomerate and limestone beds lens out in just a meter 

or a few meters. The whole aspect of the stratigraphic 

section may change in a short distance across strike. 

For instance, at the head of West Maroon Creek the 

ridge on the east side of the valley contains less 

limestone and more sandstone than does the west-side 

ridge. Large areas in the eastern part of the Maroon 

Bells quadrangle and in the adjoining Hayden Peak 

quadrangle lack the numerous conglomerates of the 

measured section in the western part of the Maroon 

Bells quadrangle. In the lower part of the Maroon 

where limestones are relatively numerous, the lack of 

these conglomerates makes determination of the 

Maroon-Gothic contact very uncertain in areas where 

the distinctive color difference between the two formations is destroyed by contact metamorphism.

Bedding-plane features are numerous, especially on 

thin muddy or silty beds (fig. 21E). They include desiccation cracks, current-ripple marks, worm tubes, raindrop impressions, and impressions of plant fragments.

Crossbedding is well developed in the sandstones 

and conglomerates (fig. 21C). Some torrential 

crossbedding occurs. Some of the more massive sandstone units have fine internal laminae (fig. 2L4).

Conglomerate lenses are common in many parts of 

the Maroon Formation. In the lower part of the formation they contain limestone, quartzite, quartz, 

dolomite, and Precambrian basement pebbles indicating that Paleozoic sedimentary rocks contributed 

to the streams which deposited the conglomerate (fig. 

215). (See stratigraphic section 2, p. 131.) White 

limestone pebbles are characteristic of the conglomerates in the lower part of the Maroon and are 

unlike anything in the Cambrian-throughMississippian stratigraphic section. A few limestone 

beds in the lower part of the Maroon resemble these 

pebbles in color and texture. These pebbles must have 

come from some Pennsylvanian sedimentary rocks
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FIGURE 19.—Panorama of the Elk Mountains west and southwest from ridge between East Maroon and Conundrum Creeks, Maroon Bells quadrangle. Highest 

close peak is Pyramid Peak (P); Maroon (M) and North Maroon (NM) Peaks to 

right of Pyramid Peak. This view encompasses part of the type locality of the Maroon Formation. Light-colored rock on the end of ridge in right foreground 

is hornfels; probably granodiorite is subjacent. Two faults cut the same ridge 

just right of the middle of the view. Distant peaks to right are Snowmass

Mountain (S) and Capitol Peak (CP), composed of Oligocene granodiorite of the 

Snowmass pluton. Dips in the Maroon Formation are north away from the 

White Rock pluton, which lies at the left margin of the photograph. A broad gentle fold underlies Pyramid Peak. Sill of porphyritic granodiorite (G) about 

60 m thick on right; laccolith of same forms peak on right skyline; one thin dike visible on Pyramid Peak.

§
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 31

FIGURE 20.—Maroon Formation on the northeast ridge of North 

Maroon Peak and the ridge north of the peak. Looking west from 

3,780 m altitude on the west side of the north ridge of Pyramid 

Peak, Maroon Bells quadrangle. Light-colored beds are coarsegrained sandstones and conglomerates lacking fine-grained red 

matrix. Rocks in the lower part of the valley, especially to the 

right, are metamorphosed adjacent to a small intrusive in Minnehaha Gulch, which lies off this photo to the right. Thin dike of

deposited southwest of the present exposures of the 

conglomerate. Other limestones, which are gray, may 

also, at least in part, have been derived from Pennsylvanian rocks. Langenheim (1952) reported Des 

Moinesian fossils from limestone pebbles in the 

Maroon south of the Aspen area. According to 

Langenheim (1952) and Bartleson (1972), limestone 

pebbles are also numerous and widespread in conglomerates of the Gothic Formation south of the 

Aspen area.

The change from conglomerates containing a 

substantial proportion of limestone pebbles to those 

containing much fewer or no limestone pebbles occurs 

in the middle of the formation in the calculated interval of stratigraphic section 2. However, on the west

hornblende porphyry cuts ridge at left. Thicker dikes of porphyritic granodiorite cut main ridge at notch on left and 

numerous places on ridge crest at far right. They are part of a 

swarm extending to the Snowmass pluton (Mutschler, 1970). 

Small laccolith on ridge crest at far right. Rock glacier in left 

center was one on which movement rate was measured (Bryant, 

1971b). About 4 km of ridge is in view.

end of Red Mountain and along Woody Creek in the 

Ruedi gradrangle, some pebble beds in the upper part 

of the Maroon contain limestone and dolomite pebbles. 

This suggests that locally during the deposition of the 

Maroon the Uncompahgre highland changed shape so 

that at least Pennsylvanian and perhaps lower 

Paleozoic sediments were eroded from a newly uplifted 

part of the highland rather late during deposition of 

the Maroon Formation.

Basement rock types in the Maroon conglomerates 

are mainly quartz-feldspar gneiss, granitic rocks, and 

fine-grained metamorphic rocks. Micaceous schist and 

gneiss and amphibolite are rare. The fine-grained 

metamorphic rocks are in part felsic volcanics with 

well-preserved porphyritic textures resembling those
32 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

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PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 33

studied by J. C. Olson south of the Gunnison River 

(U.S. Geological Survey, 1971, p. A41). These rocks 

contain euhedral phenocrysts of albite and, locally, 

quartz in a very fine grained groundmass of quartz, 

feldspar, and, locally, biotite. Other similar-appearing 

rocks are fine-grained quartz-feldspar-biotite rocks 

that could be blastomylonite, metamorphosed felsic 

volcanics, or siltstones. Some of the coarser grained 

gneisses, which have well-developed granoblastic textures, compositionally resemble the felsic volcanic rock 

and may be from some as yet unstudied area where 

those volcanics are more highly metamorphosed. Some 

of the pebbles might come from the quartzitic and 

granitic units mapped in the Black Canyon of the Gunnison area by Hansen (1971), units that trend 

northeast toward the Aspen area.

The conglomerates typically occur in channels with 

lenticular cross sections, have sharp basal contacts, 

and grade upward into sandstone, siltstone, and, locally, limestone (figs. 21B, C, D). The lenses are several 

centimeters to a meter or two thick; most are 30-60 cm 

thick. Some conglomerates form beds as much as 4.6 m 

thick and are traceable as much as 1.6 km in areas of 

good exposure. The conglomerates usually contain 

pebbles 2-5 cm long, but some contain cobbles as 

much as 25 cm long. The maximum number and 

thickness of conglomerate beds in the Aspen area is 

about in the line of the measured section. (See 

stratigraphic section 2, p. 131). Conglomerate beds are 

relatively rare in the more easterly outcrop areas of the 

Maroon Formation in the Hayden Peak and Aspen 

quadrangles.

Limestone beds are common in the lower part of the 

Maroon Formation, although they make up only a 

small proportion of the formation. (See stratigraphic 

section 2.) They are generally several centimeters to a

FIGURE 21 (facing page).—Features of the Maroon Formation. A, 

Small-scale crossbedding in thinly laminated fine-grained sandstone near base of Maroon at mouth of Hunter Creek valley, 

Aspen quadrangle. B, Conglomerate in lower part of Maroon 

south of the Elk Range thrust. Pebbles of white limestone, gray 

dolomite, and some Precambrian plutonic rock, in cirque south of 

Rustler Gulch, Maroon Bells quadrangle. C, Crossbedded sandstone and conglomerate in the upper part of the Maroon on the 

east side of upper East Snowmass Creek at 3,719 m altitude, 

Highland Peak quadrangle. D, Conglomerate rich in pebbles of 

Precambrian metamorphic and plutonic rock in channel cutting 

thinly laminated pebbly sandstone, upper part of the Maroon, 

southwest corner of the Highland Peak quadrangle. E, Casts of 

worm burrows(?) in thinly laminated micaceous sandstone with 

muddy partings. Upper part of Maroon, north ridge of Highland 

Peak, southwest corner of Aspen quadrangle.

meter thick and medium light gray to medium dark 

gray. Three meters is about the maximum thickness. 

Many of the limestones are silty or muddy and grade 

downward into siltstone, sandstone, and a basal conglomerate and have sharp contact with overlying sandstone or conglomerate. The limestone beds are commonly lenticular. A few thin beds of white limestone 

occur in the lower part of the Maroon in the southern 

part of the area. They are especially noticeable on the 

southwest side of the head of Conundrum Creek. 

Several beds of stromatolitic limestone several centimeters to 0.3 m thick occur in the lower Maroon a 

hundred meters or so stratigraphically above the concordant contact of the White Rock pluton on the ridge 

east of Electric Pass (Hayden Peak quadrangle).

Dolomite is rare in the Maroon, but on the East 

Maroon-Conundrum ridge near the south margin of 

the Maroon Bells quadrangle, a few beds of light-tanweathering dolomite are traceable for about 1.6 km.

In thin section the limestones are micrites. Some 

have patches of sparry calcite, some of which have a 

shape suggesting that they could be repaced fossil 

fragments. Another variety consists of micrite intraclasts cemented by sparry calcite. Terrigenous constituents are almost always present, and all gradations 

to siltstone occur.

Beds of anhydrite and gypsum occur in the lower 

part of the Maroon Formation in the Hayden Peak 

quadrangle. One bed is just a few hundred meters 

above the Gothic Formation on the two shoulders just 

below 3,352 m altitude north of Sawyer Creek. The bed 

is about 6 m thick and can be traced about 450 m along 

strike to the south, where surficial deposits cover it. 

On the second shoulder south of Sawyer Creek spectacular solution cavities occur in that bed. The 

anyhydrite and gypsum are associated with thinbedded silty and micaceous gray limestone.

Near Hayden Peak as many as three beds of evaporite 

crop out, if one assumes no significant faulting. If the 

three occurrences represent the same stratigraphic 

horizon, two faults not shown on the geologic 

quadrangle map are necessary. The abundance of 

talus, the fractured character of the rock, and the 

presence of intrusive material make it difficult to be 

sure whether or not faults are present. Evaporites 

north and west of Hayden Peak form a bed 7-9 m thick 

adjacent to the White Rock pluton (fig. 22A). The bed 

has an irregular contact with underlying hornfels (fig. 

22B). No effects of contact metamorphism are visible 

in the anhydrite except recrystallization and coarsening of grain size compared with similar rock in areas 

lacking metamorphism. This bed continues for at least 

900 m south of where it crosses the ridge crest. A
34 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

FIGURE 22.—Evaporite in the lower part of the Maroon Formation 

near Hayden Peak. A, Outcrop of at least 12 m of gypsum and 

anhydrite at about 3,600 m altitude northwest of hill 13232 on 

ridge north of Hayden Peak. Granodiorite of the White Rock 

pluton forms slope in background. B, Closeup of contact between 

evaporite (white) and hornfels (dark gray) at 4,020 m altitude 

northwest of Hayden Peak. Thin-section study shows that the 

hornfels has the usual composition, and no evidence exists for any 

exchange of material between siltstone and adjacent evaporite 

during thermal metamorphism.

60-cm bed at about 3,900 m altitude on the south side 

of the ridge east of Hayden Peak may be correlative.

At the lower contact of the White Rock pluton west 

of hill 13316 on the ridge north of Hayden Peak is a 

12-m-thick bed of evaporite. This bed seems to be on 

strike with an outcrop of breccia at the contact about 

610 m to the south.

The third evaporite occurrence in the hornfels near 

Hayden Peak is on the northwest side of the 4,156-m 

peak north of Electric Pass.

On the west side of Conundrum Creek valley north of 

Hunter Peak is a bed of breccia in metamorphosed 

Maroon Formation that might represent an evaporitic 

horizon. The bed extends about 2.4 km along the valley 

side; it is less resistant than the surrounding rocks and 

forms shoulders or saddles where it crosses ridges between cirques. The incompetent horizon consists of 

thin-bedded calcareous siltstone, shale, limestone, and 

calcareous sandstone. The breccia is composed of 

fragments of those rocks in a calcareous matrix. In the 

southern exposures of this bed some calc-silicate 

minerals occur in the matrix, which suggests that brecciation took place before intrusion of the White Rock 

pluton. The whole soft zone is as much as 46 m thick, 

but the breccia is only 15 m thick at the most. If this 

zone is equivalent to one of the lower evaporite 

horizons across the valley to the east, some Gothic 

Formation not shown on the geologic quadrangle map 

may exist between it and the thrust fault near the bottom of the valley.

At the top of the Maroon Formation in the Aspen 

region is a unit of siltstone that was not mapped. This 

unit is well developed in the Woody Creek quadrangle 

(Freeman, 197la) to the north, but to the south in the 

southern part of the Highland Peak quadrangle, it interfingers with sandstone. In the Highland Peak 

quadrangle and the western part of the Aspen 

quadrangle the upper 300-360 m of the Maroon Formation is composed of siltstone and fine-grained sandstone. On the ridge east of East Snowmass Creek at 

the farthest south outcrop of Morrison Formation, 

abou 60 m of siltstone are at the top of the Maroon. 

West of fhe Snowmass Creek fault zone in the 

Highland ^ 3ak quadrangle the siltstone is missing, 

owing to pre-Morrison faulting and erosion of the upthrown block.

The siltstone is typically thick bedded to massive, 

blocky weathering, and moderate red to grayish red. 

Locally, it is thin bedded and chippy weathering. Interbeds of fine-grained sandstone are widespread. 

Casts strongly suggesting worm burrows are a typical 

feature of the siltstone (fig. 2 IE").

Thin sections of typical siltstone show that it has
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 35

angular to subangular quartz, plagioclase, and 

microcline grains averaging <0.05-0.1 mm in 

diameter. Other minerals forming clastic grains are 

muscovite, carbonate, tourmaline, zircon, sphene, and 

opaque minerals. Carbonate cements the grains.

On Eagle Mountain in the western part of the 

Highland Peak quadrangle is a distinctive unit of lightgray, tan-weathering sandstone at least 200 m thick in 

fault contact with the uppermost part of the Maroon 

Formation. It resembles no other formation, so it has 

been assigned to the Maroon; and it probably is not far 

out of place stratigraphically, It contains rare interbeds of fine-grained grayish-red sandstone, and the 

nearby Maroon Formation contains a 4-8-m-thick bed 

of well-sorted fine-grained light-gray sandstone like 

that mapped. The sandstone unit is somewhat 

calcareous and micaceous in places. It is thick to thin 

bedded and has well-developed crossbedding. Some of 

it has thin internal laminae within the thicker 

crossbedded units.

The sandstone is composed almost entirely of 

subangular to subrounded quartz grains 0.1-0.3 mm in 

diameter (fig. 23). Other clastic grains include altered 

feldspar, chert, tourmaline, zircon, and opaque 

minerals. A small amount of carbonate cement and 

sericite or clay occurs between the quartz grains.

This unit resembles the sandstone of the Fryingpan 

River (Freeman, 1971a), which lies above the Maroon 

Formation and is believed to have been deposited in an 

eolian environment. This unit within the upper Maroon 

may represent deposition in a similar environment at a 

somewhat different time than the sandstone of the 

Fryingpan River.

In thin section the sandstones of the main part of the 

Maroon Formation range from well sorted to poorly 

sorted. The principal clastic constituents are angular 

to subangular grains of quartz, plagioclase, microcline, 

and in many places carbonate, and flakes of muscovite 

and biotite (fig. 27A). Grain size ranges from <0.05 mm 

to several millimeters. Fragments of chert, siltstone, 

and limestone occur. Coarser grained poorly sorted 

sandstones grade to conglomerates containing a large 

proportion of rock fragments. Common accessory 

minerals occurring as clastic grains are zircon, tourmaline, apatite, and opaque minerals. Less common 

ones are chlorite, sphene, rutile, and amphibole.

The plagioclase grains are altered to varying 

degrees, and most are now albite. Many of the alteration products appear to be clay minerals, but some are 

sericite.

The mica flakes are generally alined parallel with 

bedding. In many thin sections the biotite is partly to 

wholly replaced by opaque mineral, which is mostly

hematite. This alteration is a common feature of these 

red beds (Walker, 1967; Bartleson, 1972) and is responsible for their color contrasts with the Gothic Formation (Bartleson, 1972).

Matrix is composed of carbonate, which is calcite in 

most of the rocks. It may contain some clay minerals. 

The proportion of matrix ranges from almost none in 

some sandstones to more than 50 percent. Siltstones 

have a large proportion of matrix, and through an increase in matrix they grade into micritic limestones.

Clastic mica makes up only a few percent of the 

rocks, but it is conspicuous in most samples. 

Muscovite is dominant, probably because much of the 

biotite is pseudomorphosed by an opaque mineral that 

appears to be hematite.

In polished sections of the analyzed samples of 

unmetamorphosed rocks of the Maroon Formation, 

hematite was the dominant opaque mineral. One section contained a small amount of magnetite, and one 

magnetite grain pseudomorphosed by hematite. A 

very small amount of magnetite or maghemite was 

detected in a few of the analyzed samples by passing a 

hand magnet over powdered samples of the rock. 

Leucoxene-coated opaque grains suggest that some 

titanium-rich opaque minerals may be present.

The rocks of the Maroon Formation range in composition from limestone to arkose, and all gradations 

between these rock types occur (fig. 23A). In general 

the siltstones have less feldspar and more carbonate 

than the sandstones. Field observations and study of 

representative samples in thin section show that the 

Maroon contains more arkose and fewer silty or sandy 

limestones than the Gothic Formation (figs. 18, 23A). 

Sandstones and siltstones of the Maroon are richer in 

plagioclase than potassic feldspar (fig. 235). When 

compared with other sandstones, those of the Maroon 

Formation are much richer in carbonate. Huckenholz 

(1963) compiled the mineral composition of 295 sandstones and found that 95 percent contain less than 10 

percent carbonate.

These compositional features are largely related to 

the source of the detritus composing the Maroon Formation. The abundance of limestone pebbles in the conglomerates of the lower Maroon indicates that both 

clastic limestone particles and CaCO3 in solution would 

be readily available for deposition in the Maroon rocks. 

The small number of basement-rock pebbles examined 

in thin section contained far more plagioclase than 

potassic feldspar. Thus, more plagioclase than potassic 

feldspar was available in the Precambrian terrane exposed to erosion in the Uncompahgre uplift.

Chemical analyses of the sandstones and siltstones 

of the Maroon Formation reflect the mineralogy and
36 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Quartz Quartz

1 /w vvv\/\/v\/\/v

Carbonate Mica and

A feldspar

Plagioclase

B

Potassic 

feldspar

EXPLANATION

• Sandstone

* Limestone

+ Siltstone

A Siltstone unit at top

X Uniform sandstone, Eagle Mountain

FIGURE 23.—Mineral composition of the Maroon Formation. Circled and numbered points are analyzed samples in table 2. A, Proportions of 

quartz, carbonate, mica, and feldspar in rocks of Maroon Formation. B, Proportions of quartz, plagioclase, and potassic feldspar in 

rocks of the Maroon Formation.

show that the composition of the sandstones differs 

significantly from averages of arkoses (tables 3, fig. 

24). K2O/Na2O ratios are generally less than those of 

average arkoses (fig. 24,4). This is a reflection of the 

relatively high plagioclase contents, and it is probably 

related to the plagioclase-rich composition and 

relatively little chemical weathering of rocks in the 

source area. The Al2O3/Na2O+K2O+CaO ratios are 

generally much lower than most sandstones (fig. 24B). 

This is because of the high calcite contents, for 

Al2O3/Na2O+K2O ratios are in the range of most sandstones (fig. 24Q. The closeness of the analyses to the 

ideal muscovite line suggests that the rocks may contain some undetected clay minerals, because modal 

analyses suggest that feldspars dominate over mica. 

Raup (1966) found a variety of clay minerals in similar 

rocks derived from a different source area on the 

northeast side of the Eagle basin. A plot of CO2/CaO 

shows that most of the rocks fall very close to a line for 

ideal calcite (fig. 24D). This shows that most of the 

CaO is combined with CO2 in calcite rather than being 

derived from plagioclase or some other Ca-silicate 

mineral. The high Fe2O3/FeO ratios (table 3) are due to 

the predominance of hematite over other iron-bearing 

minerals in these rocks.

The age of the Maroon Formation is considered to be 

Middle and Late Pennsylvanian and Permian. South of 

the Aspen area the Maroon Formation conformably 

overlies the Gothic Formation, which has Des Moinesian fossils (Langenheim, 1952,1954; Bartleson, 1972). 

No fossils with well-defined time connotations have 

been found in the Maroon Formation in the Aspen area 

or elsewhere on the southwest side of the Eagle basin. 

The top of the Maroon Formation at Glenwood 

Springs is defined as the base of the tongue of Weber 

Sandstone (Brill, 1952). The age of the Weber in Colorado is unknown, but Pennsylvanian and Permian 

fossils have been found in it in northeast Utah east of 

the Yellowstone River (Hallgarth, 1967, p. 183). Since 

the tongue of the Weber Sandstone at Glenwood 

Springs is only 30 m below the South Canyon Creek 

Member of the State Bridge Formation, which has a 

fauna considered to be of Guadalupian age (Bass and 

Northrop, 1950), the Weber tongue is probably Permian, and thus some undetermined thickness of the 

Maroon is Permian. Freeman (197 la) extended these 

stratigraphic relations to Woody Creek just north of 

the Aspen area, where a limestone correlated with the 

South Canyon Creek Member is 18-60 m above the 

base of the State Bridge. This limestone pinches out 16
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN

______I________

37

20

1 2 3 

Na 2 0 IN WEIGHT PERCENT

15

o 

ffilO

fe 

o

9. .7 

• 3

0 5 10 15 20 

CaO+Na 2 0+K2 0 IN WEIGHT PERCENT

B

FIGURE 24.—Chemical diagrams of analyzed rocks from the Maroon 

Formation. Numbers of points refer to numbers of analyzed 

samples, table 3. Averages of T, taphrogeosynclinal, X, exogeosynclinal, and E, eugeosynclinal sandstones (Middleton, 

1960). Square, mean composition of arkose (Pettijohn, 1963);

km northwest of the Aspen area. Locally, in the Woody 

Creek quadrangle an angular unconformity separates 

the State Bridge Formation from the Maroon Formation (Freeman, 1971a, 1972a); in those localities the top 

of the Maroon may be older than elsewhere and may be 

only of Pennsylvanian age. In the Aspen area the unconformity between the State Bridge and the Maroon 

is not obvious; there, the upper part of the Maroon Formation may be of Permian age. However, Hallgarth 

(1967, p. 179) assumed that the Maroon was Pennsylvanian age. Consequently, thickness compilations 

(McKee and others, 1967) show very little Permian 

rock in the Aspen area. 

Plant fragments are fairly widespread in the Maroon

15

I 10 h

O_r\i

< 5

ldea' 

muscovite

«Albite

Microcline*

5 10 15 

Na 2 0+K2 0 IN WEIGHT PERCENT

c

20

25 0 5 10 15 

CaO IN WEIGHT PERCENT 

D

circle, average of five typical arkoses (Pettijohn, 1957). A, 

K2 O/Na 2 O diagram. Line represents ratio of 1.0. B, 

Al 2 O3 /CaO + Na2 O + K2 O diagram; C, Al2 O3 /Na2 O + K2O 

diagram; D, CO2 /CaO diagram. Line represents proportions of 

CaO and CO2 in ideal calcite.

Formation in the Aspen area, but fossils well enough 

preserved for generic identification were not found.

West and north of the Aspen area a few of the 

limestone beds in the lower part of the Maroon contain 

fossils indicating a marine origin (Langenheim, 1954; 

Mutschler, 1970; Freeman, 1971a).

The Maroon is so thick and dips so gently in many 

areas that it is difficult to obtain accurate information 

on thicknesses. In the Aspen area the measured and 

calculated section is 3.2 km thick. Initial dip of rocks of 

the Maroon Formation is not an important factor contributing to errors in thickness calculation here. 

Siltstones have dips similar to coarse sandstones and 

conglomerates (but not to crossbeds), and the dip of
38 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO 

TABLE 3.—Chemical analysis and modes of the Maroon Formation and hornfels

[Major oxides determined by rapid methods by P. L. D. Elmore, G. W, Chloe, Hezekiah Smith, James Kelsey, J. L. Glenn, Samuel Botts, and 

Lowell Artis, U.S. Geological Survey, 1967. Minor elements determined by semiquantitative spectrographic methods by Joseph L. Harris, 

U.S. Geological Survey, 1967, and J. C. Hamilton, U.S. Geological Survey, 1971, Mode by point count: P, present but not intersected 

in counting. Major oxides and CIPW norms in weight percent; mode in volume percent. Minor elements are reported in parts per million 

to the nearest number in the series 100, 70, 50, 30, 20, 15, and 10, ..., which represent approximate midpoints of interval data on a 

geometric scale. The assigned interval for semiquantitative results will, include the quantitative value about 30 percent of the time. 

N, not detected; leaders (——) indicate not determined. In modes, leaders (——) indicate absent. Looked for but not found: Ag, As, 

Au, Cd, Ge, Hf, Hg, In, Li, Pd, Pt, Re, Sb, Sn, Ta, Te, 11, U, W, Zn]

Stratigraphic section 2

Sample No. — — —— — —— 1 

W168739 

ME642

2 

W168736 

ME1978

3 

WL68737 

ME1198

4 

W168740 

MB1042

5 

W168739 

MA440

6 

W169329 

1704b

Traverse from red beds to hornfels,

7 

W169328 

1704a

8 

W169331 

1705b

9 

W169330 

1705a

10 

W169333 

1706b

, Conundrum Basin

11 

W169332 

1706a

12 13 

W169335 W169334 

1707b 1707a

Major oxides

Al2 o 3 ————————

Fe2 0 3 ———————————

MgO —————————————

H2 o7 ———————————— 

Hr\

P 2 Q~ ——————

2.6 

.51 

.10 

.08

.75 

52.0 

.00 

.32

.14 

.25 

.04 

.00

.10 

43.1 

2.3 

2.70

54.7 

6.7 

2,2 

.40

2,5 

15.8 

1.2

1.5

.29 

,71 

.39 

.16

.12 

13.4 

2.66 

2.70

58.9 

7.2 

1.0 

1,0

2,1 

12.6 

.56 

2.0

.40 

1.5 

.29 

.00

.02 

12.2 

2.66 

2.70

70.1 

12.2 

3.9 

.16

1.5

2.7 

2.4 

2.5

.54 

1.3 

.60 

.00

.02 

1.6 

2.54 

2.70

72.7 

6,7 

1.6 

.18

1,0 

7.1 

1.2 

1.5

.48 

1,3 

.54 

.00

.03 

5,0 

2.65 

2.68

53.7 

13,2 

5.5 

1.2

5.6 

7.2 

2.3 

3.0

.21 

2.0 

.81 

.21

,08 

5.0 

2.63 

2.78

56.0 

10.2 

2.8 

.64

2.4 

11.3 

2.9 

2.2

.07 

1.1 

.53 

.15

.08 

9.2 

2.60 

2.75

46.0 

10.2 

3.9 

1.0

4.2 

16.6 

1.6

2.4

.02 

1.0 

.62 

,17

.12 

12.0

2.75

59.1 

10.0 

3.4 

.56

2.1 

10.2 

2.8 

2.1

.26 

1.1 

.60 

.11

.08 

7.5 

2.67 

2.70

61,7 

12.3 

2.9 

2.6

3,6 

6.2 

1.7 

5.6

.18 

.92 

.71 

.12

,09 

1.0 

2.72 

2.75

66.4 

8.7 

1.9 

.96

1.6 

9.4 

2.3 

2.6

.05 

.80 

.44 

.15

.08 

4.6 

2.68 

2.75

57.7 64,1 

9,9 6.7 

.59 2.3 

3.1 1,6

7.2 1.2 

13.2 13.3 

3.2 2.9 

2.9 1.2

.16 .02 

.39 .43 

.95 .12 

.37 .04

.09 .07 

.19 6.0 

2.88 2.69 

2,98 2.75

Minor elements

Nb ——————————

Ni ——————————————

N 

70 

N 

N

N 

70 

N 

N

N 

N 

N 

N

1,500 

20 

N 

N

N 

N 

N

N 

200 

N 

N

20 

5 

N 

3

N 

<30 

N 

7

100 

30 

30 

300

N 

5 

2

N 

200 

N 

N

15 

1.5

N 

N

N 

N 

N 

5

70 

20 

20 

50

N

7 

1

N 

300

N

7

50 

7

N 

N

10 

30 

5 

10

70 

50 

30 

500

N 

10 

3

30 

100 

<1

N

3 

15

N 

N

10 

<30 

3 

5

70 

30 

30 

500

N 

5 

3

N 

500 

1.5 

20

70 

3 

150 

5

N 

50 

N 

15

150 

70 

50 

500

500 

15 

5

N

N 

500

1

7

100 

3

N 

N

N 

<30 

N 

10

150 

70 

30 

300

100 

10 

3

N

N 

300 

1 

10

50 

3 

70

N

N 

<30 

N 

10

200 

50 

50 

200

100 

10 

5

N

N 

70 

1 

7

50 

3

N 

N

N 

<30 

N 

15

150 

70 

50 

500

N 

10 

5

N 

1,500 

1,5 

1.5

70 

50 

50

N

L 

50 

10 

20

300 

100 

30 

150

N 

30 

3 

70

N 

500 

1 

7

50 

3

N 

N

N 

<30 

N 

10

15 

70 

30 

700

N 

10 

3

N N 

500 150 

1 N 

15 N

100 15 

3 7 

N 70 

5 N

N N 

50 <30 

N N 

10 5

150 70 

70 20 

70 20 

1,000 50

100 N 

15 10 

7 2

N N

Modes

Potassic feldspar —

Points counted — ——

0.1

99.9 

P

P

P

773

45,0 

8.6 

6.4 

34.6

1.7

3.7

P 

P 

P

705

43.3 

4.7 

4.2 

43.0

3.6

1.2

P 

P

.1

641

49.7 

21.7 

8.3 

5.5

1.7 

1.6

11.4

P 

P 

.1

691

58.2 

7.6 

7.6 

16.7

1.7

2.4

4.9

.3 

P 

,6

P

——

636

14.5 

11.1 

1.0 

2.4

1.0 

70.0

——

——

525

35.2 

28.3 

16.0 

15.9

.7

3.8 

.1

P

——

738

15.3 

4.4 

1.1 

16.7

2.4 

.3 

24.9 

34.9

——

615

34.2 

31.2 

15.7 

15.9

.1 

P

2.9

P

P

744

24.3 

4,9 

2.6 

5.9

5.5

P 

1.5

P 

55.3

547

31.6 

2.0 

9.2 

23,5

3.0

.3

3.7

.3 

4.8

21.6 

P

709

1.3 41.2 

16.6 13.0 

—— 18.0 

.5 16.0

.1 .1

—— ,1 

P P

80.0 6.3

1.5 

5.3 

617 725
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 39

SAMPLE DESCRIPTIONS

1. Medium-light-gray limestone. Carbonate with a grain size of 0-002 mm except a few lenses with 0.3-mm grain size. A few scattered 

clastic grains of quartz 0.1 mm in diameter, and a few of muscovite. From 30-cm bed 123 m above the base of the Maroon in measured 

section (interval 38, Stratigraphic Section 2). About 3,536 m altitude in gully north of Rustler Gulch in Snowmass Mountain 

quadrangle, about 90 m west of Maroon Bells quadrangle boundary.

2. Grayish-red sandstone. Clastic grains of quartz, plagioclase, microcline, carbonate, and muscovite averaging 0.2-0.3 mm, and as large 

as 0-5 mm, cemented by calcite. About 518 m above base of Maroon Formation (interval 121, Stratigraphic Section 2). From ridge 

between Rustler Gulch and east fork of Rock Creek very close to quandrangle boundary, southwest corner of Maroon Bells quadrangle.

3. Pale-red sandstone. Clastic grains of quartz, plagioclase, and muscovite with a grain size of 0-5-0-2 mm cemented by calcite. About 

274 m above base of Maroon Formation (interval 75, Stratigraphic Section 2). From about 3,688 m altitude in gully north of Rustler 

Gulch about 100 m west of Maroon Bells quadrangle boundary in the Snowmass Mountain quadrangle.

4- Moderate-red arkose. Clastic grains of quartz, plagioclase, microcline, muscovite, and biotite average 0.1-0-4 mm in diameter but 

reach 0.6 mm, occurring 784 m below the top of the Maroon Formation (interval 257, Stratigraphic Section 2). From the boundary 

between the Maroon Bells and Highland Peak quadrangles about 900 m from their west corners.

Angular clastic grains of quartz, plagioclase, microcline, muscovite, and biotite, 0.5-0.1 mm in 

54 m below the top of the Maroon Formation (interval 286, Stratigraphic Section 2). About 3,737 m

5. Moderate-red sandy siltstone.

diameter cemented by calcite, 254 m below the top UL me natuun rurniaLj.un vim-erven ^ou, oLLaLj-gLapn-n^ oe 

altitude in gully west of 4,037—m peak on East Snowmass Creek-Willow Creek divide. Highland Peak quadrangle.

6. Grayish-red (5 R 4/2) siltstone. Quartz as much as 0.4 mm in diameter, muscovite as much as 0.1 mm long, plagioclase, and microcline 

in a matrix of carbonate, clay(?), hematite, and altered biotite. From 3,743 m altitude on east side of Conundrum Basin in Gothic 

quadrangle, 120 m south of the Maroon Bells quadrangle boundary.

7. Grayish-red sandstone. Clastic grains of quartz, microcline, altered plagioclase, muscovite, and carbonate with a grain size of 

0.3 mm cemented by calcite. Same locality as no. 6.

8. Grayish-red siltstone. Clastic grains of quartz as much as 0.08 mm in diameter, muscovite as much as 0.25 mm long, plagioclase, 

potassic feldspar, biotite, and carbonate in a matrix of carbonate, sericite(?), and opaque mineral. From altitude 3,755 m on east 

side of the head of Conundrum Creek 90 m north of south margin of Maroon Bells quadrangle.

9. Pale-red (5R 6/2) arkose. Angular to subrounded clastic grains of quartz, partly altered plagioclase, muscovite, microcline, 

carbonate, and biotite as much as 1 mm in diameter but averaging 0-1-0-2 mm. Calcite cement. Same locality as no. 8.

10. Medium-dark-gray hornblende hornfels. Green hornblende with a grain size of 0.02-0.04 mm. Clastic grains of quartz, microcline, and 

plagioclase averaging 0.04-0-08 mm in diameter but as much as 0.15 mm in diameter. From 3,767 m altitude on the east side of the head 

of Conundrum Creek 335 m north of the south boundary of the Maroon Bells quadrangle.

11. Medium-light-gray metamorphosed sandstone. Clastic grains of quartz, plagioclase, and microcline averaging 0.08-0.25 mm in diameter 

but reaching 1.5 mm in diameter. Sodic scapolite forms sieve-textured porphyroblasts as much as 1.5 mm in diameter. Hornblende has a 

grain size of 0.08-0.1 mm. Same locality as no. 10.

12. Light-greenish-gray (5G 8/1) pyroxene hornfels. Pyroxene with an average grain size of 0-04 mm. From 3,780 m altitude on the east 

side of the head of Conundrum Creek 780 m north of the south boundary of the Maroon Bells quadrangle.

13. Light-bluish-gray (5B 7/1) fine-grained metamorphosed sandstone. Somewhat recrystallized clastic grains of quartz, plagioclase, and 

potassic feldspar averaging 0.3 mm in diameter; containing interstitial garnet, pyroxene, and calcite. Same locality as no. 12.

the Maroon Formation is not detectably different from 

that of the overlying Entrada Sandstone or Morrison 

Formation. Some faults of small displacement cross 

the section measured. However, I estimate that the 

section is accurate to within 150 m stratigraphically. 

This thickness contrasts with the 362 m measured by 

Langenheim (1952) at Copper Creek just south of the 

Maroon Bells quadrangle in the lower plate of the Elk 

Range thrust just 3.2 km south of the section 

measured in the Maroon Bells and Highland Peak 

quadrangles. An intermediate thrust sheet south of 

the White Rock pluton in the Pearl Pass quadrangle 

has Maroon Formation of an intermediate thickness 

reported to be 1,066 m by G. H. Eldridge (in Emmons 

and others, 1894). Other complete sections along strike 

to the northwest are thinner than the one measured in 

the Aspen area (Langenheim, 1954). Since we lack

Stratigraphic markers in the Maroon Formation, we do 

not know how much of the thickness variations are due 

to pre-Entrada structural movements and how much 

are due to variation in thickness of deposition. Reconnaissance mapping by me and V. L. Freeman indicates 

that west of the north-trending fault at the west edge 

of the Highland Peak quadrangle the Maroon is much 

thinner. This thinness is apparently due to pre-Late 

Jurassic erosion of the Maroon in the uplifted block on 

the west side of the fault.

To the north a complete section of the Maroon in the 

southern part of the Ruedi quadrangle is apparently 

4.5 km thick (Freeman, 1972b). Yet five miles farther 

north the Maroon is about 1.52 km thick, and just 

south of the Fryingpan River it is only 0.91 km thick. 

The relative effects of pre-State Bridge structural 

movements, nondeposition, or unmapped Laramide
40 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

structures on this drastic thinning are unknown 

because of lack of stratigraphic markers traceable 

from the margin into the interior of the Eagle basin.

ENVIRONMENTS OF DEPOSITION OF 

GOTHIC AND MAROON FORMATIONS

During deposition of the Gothic Formation shallowwater normal-marine conditions occurred in a belt extending from Taylor Park to the Crystal River valley. 

Sandstone, shale, and fossiliferous limestone were 

deposited in this belt. Northeast of this belt the clastic 

rocks are finer grained, and the limestones lack fossils. 

Bartleson (1972) postulated thinning of the Gothic 

Formation to the northeast, but none is apparent until 

the latitude of the Fryingpan River. Fluvial and transitional marine rocks were deposited southwest of the 

normal-marine rocks and were probably eroded at 

times to contribute material to the Gothic. Facies relations in the Gothic Formation (Bartleson, 1968) are 

quite similar to those of the Minturn Formation 

(Boggs, 1966), but because of lack of stratigraphic 

markers and deposits preserved at the basin margin on 

the southwest side, the story is not as complete as on 

the northeast.

In later Des Moinesian time the Uncompahgre uplift 

expanded its boundaries, and the Gothic deposited adjacent to the highland was eroded to furnish a substantial contribution to the Maroon Formation, which in 

the Aspen region was deposited in a fluvial to transitional marine environment at first and then later probably entirely in fluvial and other continental environments. Maximum depositional thicknesses of the 

red beds of the Maroon Formation are preserved in the 

Aspen region; whereas maximum thicknesses for older 

rocks, time equivalents of the Gothic and the lowest 

part of the Maroon are in the evaporite-bearing basins 

near Eagle and Cattle Creek (southeast of Glenwood 

Springs (Mallory, 1971).

Mineralogical and petrographic studies by Walker 

(1967a, b) and Raup (1966) and the occurrence of 

evaporite deposits in the rocks of the Eagle basin show 

that the red beds were deposited in an arid to semiarid 

environment. Hematite, causing the red color, formed 

by in situ alteration of iron-bearing detrital minerals.

Neither facies relations nor current directions obtained from crossbeds (fig. 25) show any effects of the 

ancestral Sawatch uplift proposed by De Voto (1972). 

Sedimentation in the Aspen region appears to have 

been controlled by the history of the Uncompahgre 

uplift.

FIGURE 25.—Summary of 41 azimuths of crossbedding in the Maroon 

Formation in the Aspen 15-minute quadrangle.

STATE BRIDGE FORMATION

The State Bridge Formation (Donner, 1936, 1949; 

Brill, 1942) has been extended to the Aspen region by 

Freeman (197la). In the Woody Creek quadrangle he 

discovered an angular unconformity separating State 

Bridge from Maroon strata, and he subdivided the 

State Bridge into several units.

In the Aspen 7.5-minute quadrangle the State 

Bridge Formation forms the eastern part of Red Butte 

and a dip slope on the upper part of the Highlands Ski 

area on the ridge between Castle and Maroon Creeks; it 

forms parts of the ridge between Maroon and Willow 

Creeks in the Highland Peak quadrangle. Especially 

well exposed sections are found on the north side of the 

lower Willow Creek valley and the east side of East 

Snowmass Creek. (See stratigraphic section 3, p. 138.)

In the Aspen area no angular discordance between 

the State Bridge and Maroon Formations can be 

demonstrated. Local evidence for disconformity exists. 

In one place, blocks of siltstone from the Maroon Formation as much as 1.5 m in diameter are incorporated 

in the base of the State Bridge Formation. In a number 

of places along the north side of Willow Creek, conglomerate of the State Bridge Formation is in sharp 

contact with siltstone of the Maroon Formation. In
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 41

other places a physical break between the formations 

is not detectable, and siltstone apparently grades upward into sandstone and conglomerate.

The upper contact of the State Bridge Formation is 

an angular unconformity beneath the Chinle Formation. This is shown by the truncation of an unnamed 

sandstone member of the State Bridge in the Aspen 

quadrangle along the west side of the Maroon Creek 

valley. Along the East Snowmass Creek valley the 

Chinle lies on siltstone of the State Bridge to the north 

and cuts down into an underlying conglomerate, and 

finally to the south truncates the entire formation.

The State Bridge Formation consists of grayish-red 

and reddish-brown to orange sandstone and siltstone 

and polymictic conglomerates. A few beds of pinkishgray to white quartz sandstone occur locally. The 

siltstone and sandstone alternate in beds several centimeters thick that have considerable lateral continuity (figs. 26.B, Q. Locally, siltstone intervals are as 

much as 10 m thick. Sandstone beds range from well 

sorted to poorly sorted. The poorly sorted ones have a 

bimodal grain-size distribution. The larger grains are 

of easily visible rounded quartz 1-2 mm in diameter. 

Many of the poorly sorted sandstones have chips of 

siltstone. Conglomerate beds are as much as 9 m thick 

and are composed of fragments as much as 20 cm long 

of granitic rock and gneiss, fine-grained metamorphic 

rock, quartzite, gray limestone, and chert (fig. 26^4). 

Some local conglomerates overlying siltstone beds are 

composed of angular fragments of siltstone in a sandstone matrix. Some conglomerate beds have a lateral 

continuity over a number of kilometers. In the saddle 

south of hill 9970 west of Castle Creek, a few thin 

limestone beds were found.

Parallel oscillation ripple marks are a characteristic 

bedding feature of the State Bridge Formation (fig. 

26.D). Current ripple marks also occur, but the parallel 

ripple marks are absent from the underlying red beds. 

Crossbedding is well developed in some of the conglomerates and sandstones.

At the top of the State Bridge Formation is a finegrained uniform crossbedded moderate-orangeish-pink 

to pale-reddish-brown sandstone that is probably the 

Lower Triassic Sloane Peak Member (Freeman, 1971a). 

This sandstone is well exposed on places on the lower 

part of the Highlands Ski area and on Maroon Creek 

just to the northwest; it is about 18 m thick. It is cut 

out beneath the pre-Chinle unconformity at the west 

margin of the Aspen quadrangle. On Red Butte a 

similar sandstone is yellowish gray and about 30 m 

thick, and about 30 m of State Bridge sandstone,

siltstone, and conglomerate lie between it and the 

Chinle.

The rocks of the State Bridge range from poorly 

sorted to well sorted. Larger clastic grains of quartz 

and feldspar and smaller rock fragments 1-2 mm in 

diameter are predominantly rounded to subrounded; 

whereas the smaller grains are subangular (fig. 27f?). A 

few quartz grains exhibit thin overgrowths. Most of 

the plagioclase grains are highly altered, either due to 

weathering before deposition or syngenetic or 

diagenetic alteration of the grains in sandstone beds of 

the Maroon Formation, which were probably sources 

of much of the material in the State Bridge Formation. 

Some grains of carbonate appear to be clastic. 

Muscovite, biotite, zircon, tourmaline, and opaque 

minerals occur as clastic grains. The rocks contain 

various amounts of cement and matrix. Carbonate 

forms cement, and some of the matrix appears to be 

rich in clay and hematite.

Rock fragments seen in thin section include plentiful 

chert, fine-grained quartz-feldspar metamorphic rock, 

granitic rock, and carbonate rock.

The sandstone correlative with the Sloane Peak 

Member is a very fine grained well-sorted sandstone to 

siltstone composed of subangular quartz 0.05-0.1 mm 

in diameter. A minor amount of microcline and altered 

plagioclase form clastic grains. Opaque minerals, tourmaline, zircon, and, locally, rutile are distributed 

throughout the rock and concentrated in heavy 

mineral seams in some places. The matrix, which is 

sparse, is composed of calcite and clay minerals.

Rocks of the State Bridge Formation are less 

feldspathic and have less carbonate than those of the 

Maroon Formation. (Compare figs. 28 and 23A) The 

Sloane Peak Member is a quartz sandstone, and in that 

respect it is similar to the sandstone in the Maroon 

Formation on Eagle Mountain (Highland Peak 

quadrangle).

No fossils were found in the State Bridge Formation 

in the Aspen region. Near Glenwood Springs a thin 

limestone bed in the State Bridge Formation called the 

South Canyon Creek Member contains Permian fossils 

(Bass and Northrop, 1950). A discontinuous limestone 

bed in the northern part of the Woody Creek 

quadrangle has been correlated with that member 

(Freeman, 1971a). The rocks an arbitrary 30 m above 

the South Canyon Creek Member have been assigned 

an Early Triassic age (M. E. MacLachlan, in McKee 

and others, 1959), since the rocks above that member 

are continuous with the Moenkopi Formation of the 

eastern Uinta Mountains (Stewart and others, 1972a). 

Rocks beneath that member are Permian. Where the
42 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

I 12U v

Lfi

FIGURE 26.—Features of the State Bridge Formation. A, Conglomerate composed mostly of pebbles and cobbles of Precambrian rock. At about 3,350 m altitude on ridge north of Highland 

Peak, Aspen quadrangle. B, Thin-bedded sandstone and siltstone 

south of hill 9970 on ridge west of Castle Creek, Aspen

member is absent it is more undertain how much of the 

State Bridge Formation is Permian and how much is 

Triassic.

The zero isopach of the State Bridge passes through 

the Aspen region on the projection of the zero isopach 

shown by Stewart, Poole, and Wilson (1972a, pi. 4). 

However, in detail, the isopach is highly irregular

quadrangle. C, Bed of coarse-grained crossbedded sandstone in 

thin-bedded sandstone and siltstone near B. D, Parallel ripple 

marks on sandstone bed near base of State Bridge southeast of 

hill 11675 on ridge west of Castle Creek, Aspen quadrangle.

because of movements along north-trending faults in 

the Highland Peak quadrangle during or after State 

Bridge deposition. A good example is the fault that 

crosses the ridge east of Burnt Mountain at 3,210 m 

altitude. East of the fault is about 215 m of State 

Bridge and west of it only about 75 m. That fault 

moved before the deposition of the Chinle Formation in
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN

Quartz

43

FIGURE 27.—Comparison of typical textures of sandstone in the 

Maroon Formation with sandstone in the State Bridge Formation. A, Poorly sorted arkose from about the middle of the 

Maroon Formation, from 3,853 m altitude on the east side of 

Sievers Mountain north of Maroon Lake. Angular to subangular 

clastic grains of quartz, microcline, and plagioclase with scattered 

interstitial calcite. B, Poorly sorted bimodal arkosic sandstone of 

the State Bridge Formation from near the base on the east side of 

East Snowmass Creek (stratigraphic section 3, bed 2; p. 139). 

Rounded to subrounded clastic grains of quartz, microcline, and 

plagioclase with interstitial calcite. Both A and B viewed through 

crossed nicols; both are at same scale.

Late Triassic time because thicknesses of Chinle are 

similar on both sides of it. Other faults affecting present thicknesses of the State Bridge moved before the

Carbonate Mica and 

feldspar

FIGURE 28.—Proportions of quartz, carbonate, and mica and 

feldspar in the State Bridge Formation. Dot, sandstone; circled dot, siltstone; triangle, sandstone correlative with Sloane 

Peak Member.

deposition of the Morrison Formation in Late Jurassic 

time.

The greater abundance and thickness of conglomerates in the Aspen area compared to the Woody 

Creek and Ruedi quadrangles to the north indicates 

that the original margin of the sedimentary basin was 

not far to the southwest and that this margin was controlled by the Uncompahgre uplift. (See stratigraphic 

section 3.) A few crossbed azimuths give northerly 

readings. Much greater thicknesses of generally finer 

grained rocks 29 km north of Aspen on Hardscrabble 

Mountain (Freeman, 1971a) indicate the presence of a 

State Bridge depocenter not previously detected in 

regional stratigraphic studies (Stewart and others, 

1972a, pi. 4). The extent of the depocenter cannot be 

determined because only older rocks are preserved 

along strike in the Sawatch Range and the White River 

Plateau.

Much of the State Bridge Formation consists of 

detritus reworked from the underlying Maroon Formation. This would account for the sphericity of the larger 

quartz and feldspar grains. The pebbles and cobbles in 

the conglomerates could have been reworked from 

those in the Maroon conglomerates, but they may also 

have been derived from the Uncompahgre uplift and 

adjacent Pennsylvanian sedimentary rocks. The facies 

relations suggest that during State Bridge deposition 

the uplift expanded somewhat to the northeast over its 

extent during deposition of the Maroon Formation. An 

angular unconformity along the Castle Creek fault
44 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

zone between State Bridge and Maroon rocks indicates 

that the Maroon Formation in that area should have 

furnished some detritus. The contrast in thickness of 

the Maroon Formation on the two sides of the Castle 

Creek fault zone in the Woody Creek quadrangle 

(Freeman, 1972a) suggests that some of the lower 

State Bridge north of the zone might have been 

deposited during erosion of the Maroon in and south of 

the zone.

Stewart, Poole, and Wilson (1972a) and Cadigan 

(1971) interpreted the environment of deposition of the 

State Bridge as dominantly fluvial or deltaic in an arid 

environment. The South Canyon Creek Member, 

however, represents a period of marine transgression 

into the region late in the Permian. Freeman (oral commun., 1968; 197la) believed that oscillation ripple 

marks and the presence of well-sorted sandstone beds 

indicate deposition in widespread shallow bodies of 

water in a marginal marine environment.

CHINLE FORMATION

The Chinle Formation (Gregory, 1915) unconformably overlies the State Bridge and Maroon Formations 

in the Aspen region. The rocks of that formation were 

mapped by Spurr (1898) as red Triassic sandstones. 

The Chinle is well exposed in gullies on the east side of 

East Snowmass Creek, on the north side of lower 

Willow and Maroon Creeks, and on Red Butte.

The Chinle Formation has been subdivided into three 

members in the Aspen region (Poole and Stewart, 

1964). (For stratigraphic sections see sections 2 and 3 

(this report) and section C-17 (Stewart and others, 

1972b).) At the base are discontinuous beds or lenses of 

quartz sandstone and conglomerate comprising the 

Gartra Member. The Gartra Member is 0-4.6 m thick 

and consists of white to light-gray, and, less commonly, various shades of red quartz sandstone and conglomerate. It is locally mottled very dusky red purple. 

It is mostly a coarse-grained sandstone with scattered 

granules, but in places it contains pebbles and cobbles 

of quartz, quartzite, and chert as much as 15 cm long 

(fig. 29). Less common pebbles are composed of 

granitic rock or siltstone. Crossbedding is well 

developed in the Gartra. Many of the sandstone lenses 

are channels that cut down into weathered State 

Bridge or Maroon rocks.

The mottled member (Poole and Stewart, 1964) 

overlies the Gartra, or where the Gartra is absent, the 

State Bridge or Maroon Formations. It erodes easily 

and forms a bench or is covered by debris from 

upslope. In one place rocks of this member are interbedded in the Gartra Member. The mottled member 

consists of very dark red to dusky-red and grayish-red 

siltstone and claystone and has very pale orange to

FIGURE 29.—Gartra Member of the Chinle Formation. Coarsegrained sandstone and quartz-pebble conglomerate, east side of 

East Snowmass Creek valley, Highland Peak quadrangle.

light-greenish-gray mottling in stringers and lenses. 

Locally, the mottling extends down into the Maroon or 

State Bridge rocks for a few feet. Some orange-red 

chert occurs in the mottled member. The member is 

0-10 m thick.

The red siltstone member (Poole and Stewart, 1964) 

comprises the bulk of the Chinle Formation in the 

Aspen area. It consists of moderate-red and reddishbrown, dark-reddish-brown, and grayish-red siltstone 

and calcareous siltstone. Much of the siltstone is not 

well bedded and has a hackly fracture, in contrast to 

the blocky weathering in the siltstone at the top of the 

Maroon Formation. The siltstone contains beds of 

mottled moderate-red to pinkish-gray limestone and 

pinkish-gray to light-gray limestone and silty 

limestone. Beds of sandstone and pebble conglomerate 

composed of fragments of similar limestone are common. Some are mottled pale red to grayish red and
PENNSYLVANIAN THROUGH TRIASSIC SEQUENCE OF THE EAGLE BASIN 45

light greenish gray. A few beds of moderate-reddishbrown calcareous sandstone occur in this unit.

The sandstones of the Gartra Member are poorly to 

moderately well sorted. The larger grains (1-3 mm in 

diameter) are rounded. They are composed of quartz 

and, less commonly, quartzite and chert. Feldspar 

grains are less abundant, but locally they comprise a 

few percent of the rock. Accessory minerals include 

clastic muscovite, zircon, tourmaline, and opaque 

minerals. Between the clastic grains are clay minerals, 

calcite, or fine-grained silica. Voids occur between 

grains in some rocks.

The limestones and silty limestones are micrite. 

Coarse-grained limestones are composed of clastic 

fragments of micritic limestone and a small proportion 

of rounded quartz grains. Silt-sized quartz grains and, 

rarely, muscovite occur. In some of the siltier rocks 

clastic zircon, tourmaline, apatite, sphene, and opaque 

minerals occur, and locally they form heavy mineral 

partings. Calcareous sandstones have calcite both as 

clastic grains and cement.

Composition of specimens of the Chinle contrasts 

with that of older red beds (fig. 30). Siltstones are not 

well represented in the diagram, but overall the Chinle 

has more calcite and less feldspar that the State 

Bridge and Maroon Formations.

No fossils were found in the Chinle Formation in the 

Aspen area. Stewart, Poole, and Wilson (1972b, p. 

77-8fr) summarized the paleontologic evidence for the 

Late Triassic age assigned to the Chinle.

In the Aspen region the zero isopach of the Chinle 

must pass through the Maroon Bells and Hayden Peak 

quadrangles. No Chinle is recognized on the south side 

of the Elk Mountains in the Maroon Bells quadrangle 

or the Snowmass Mountain quadrangle (Mutschler, 

1970). This isopach is offset by a north-trending fault 

just west of Snowmass Creek. West of that fault the 

Chinle is absent, owing to pre-Morrison erosion. East 

of the fault the Chinle is about 122 m thick. It thins to 

the south to 24 m on the 4,037-m peak on the East 

Snowmass Creek-Willow Creek divide near the south 

margin of the Highland Peak quadrangle. On Maroon 

Creek and the Highlands Ski area, it is also about 

122 m thick. Thus, the 122-m isopach trends about N. 

70° W. to the fault near Snowmass Creek. At that fault 

it is offset to the north. In the Woody Creek 

quadrangle the 122-m isopach passes between 

Snowmass Creek and the west edge of the quadrangle. 

No fault is detected, but thinning to the west is rapid 

(Freeman, 1972a). The Chinle thickens to 274 m in the 

east-central part of the Woody Creek quadrangle. 

These thickness changes help define the southwest 

side of the Chinle depocenter shown by Stewart, Poole, 

and Wilson (1972b, pi. 3). That depocenter is relatively

Quartz

Carbonate Mica and 

feldspar

FIGURE 30.—Proportions of quartz, carbonate, and mica and 

feldspar in the Chinle Formation. Dot, Gartra Member; x, other 

rocks.

well defined on the northeast and southwest but poorly 

defined on the northwest and southeast because the 

Chinle has been removed by erosion in those areas.

Whether the Chinle north of the Aspen area is thick 

because of preservation by post-Chinle and preEntrada deformation or because that area was a structural depression during Chinle deposition is difficult to 

evaluate. The Aspen area exhibits no obvious increase 

in the quartz and feldspar content of the Chinle to the 

south, as one might expect if the Uncompahgre uplift 

were a source area and if the stratigraphic thinning in 

that direction were due to proximity to the margin of 

the basin. The Chinle is a mature sediment from a 

mineralogical point of view and gives no definite 

evidence of derivation from Precambrian rocks of the 

Uncompahgre highland. Most of the material was 

derived by thorough weathering and reworking of the 

underlying Maroon and State Bridge rocks. Contributions from Precambrian basement rocks, if any, were 

thoroughly broken down so that feldspars were converted to clay. The silt and clay were deposited by 

streams of low gradient, and in lakes. Limestone was 

deposited in the lakes, and disruption by storms of 

those limestone beds shortly after deposition may account for the calcarenite and limestone pebble beds. 

These features suggest that if the Uncompahgre uplift 

area was a positive element during Chinle deposition, 

its margin was some distance to the southwest, and it 

had low relief. The isopleth map of the Gartra (Thordarson and others, 1972) and the direction of dip of 

crossbedding in the Gartra (Stewart and others, 1972b, 

pi. 4) suggest that the main directions of transport
46 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

were northwest away from a source lying to the 

southeast rather than one lying to the southwest in the 

direction of the Uncompahgre uplift. However, in 

southeastern Utah and southwestern Colorado isopleths in the Shinarump Member (Thordarson and 

others, 1972) and a much higher feldspar content in 

rocks equivalent to the Chinle (Cadigan, 1957) than in 

the Aspen region indicate that the southwestern part 

of the Uncompahgre uplift was a source area for the 

Chinle in that region.

Fossil content and sedimentary features of the 

Chinle indicate deposition in a terrestrial environment 

by streams and in lakes (Stewart and others, 1972b). 

The Gartra Member is a typical stream deposit, laid 

down on a soil zone on the State Bridge and Maroon 

Formations. The stratigraphic relations (Stewart and 

others, 1972b) and the clay mineralogy (Schultz, 1963) 

of the mottled member indicate that it is an old soil 

zone, in part reworked. Much of the overlying siltstone 

and limestone was probably deposited in lakes and 

flood plains.

THE NONMARINE AND MARINE ROCKS 

OF THE JURASSIC AND CRETACEOUS

ENTRADA SANDSTONE

The Entrada Sandstone (Gilluly and Reeside, 1928) 

forms a thin discontinuous horizon above the red beds 

in the Aspen area. Spurr (1898) included it with the 

overlying Morrison Formation, which he called the 

Gunnison Formation—a name that has since been 

abandoned. He did note that the Gunnison Formation 

contained a basal gray or yellow sandstone.

In the Aspen area the Entrada is well exposed on 

Red Butte and in numerous places along the north 

valley side of lower Willow Creek and on the east side 

of the East Snowmass Creek valley. It crops out as 

small ledges between the smooth slopes formed by the 

Chinle and Morrison Formations and as small cliffs in 

gullies where those more easily weathered formations 

are well exposed (fig. 31A). (See stratigraphic sections 

2 and 3 (this report) and section C-17 (Stewart and 

others, 1972b).)

The Entrada consists of yellowish to pinkish-gray 

crossbedded medium- to coarse-grained sandstone containing scattered 1-2-mm spherical quartz grains and 

sparser grains of pink, red, and dark-gray chert in a 

finer grained matrix. Near its base it locally contains 

quartz and chert pebbles as much as 5 cm in diameter. 

The underlying Chinle is bleached in places to a depth 

of as much as 0.3 m below the unconformity at the base 

of the Entrada (fig. 3LB).

The Entrada unconformably overlies the Chinle Formation and the Maroon Formation. Where mapped in 

the Aspen area it overlies the Chinle; but some thin unmapped lenses of Entrada lie in the southwestern part 

of the Maroon Bells quadrangle, and other unmapped 

lenses may exist at the west edge of the Highland Peak 

quadrangle where the Morrison is shown in contact 

with the Maroon Formation. On Capitol Creek, to the 

west of Snowmass Creek, Entrada rests on Maroon. 

The upper contact of the Entrada is not everywhere 

well defined. Sandstones of similar aspect occur in the 

lower part of the Morrison Formation. I drew the upper contact at the lowest interbedded shale or 

limestone (fig. 3L4). (See stratigraphic sections 2 and 

3, this report.)

Thin-section examination shows the grain size 

distribution to be bimodal even in the finer grained 

sandstones, although that feature is more obvious in 

the field in the coarser grained sandstones. Some rocks 

have spherical grains 0.6 mm in diameter in a groundmass containing grains 0.1-0.2 mm in diameter. 

Clastic grains of microcline and chert show similar size 

relations. Accessory clastic grains are zircon, tourmaline, opaque minerals, and apatite. Small amounts 

of clay minerals occur as matrix, and calcite cement is 

widespread.

The Entrada lacks fossils, but its relations with 

paleontologically dated rocks show that it is Late 

Jurassic in age.

In the Aspen area the thickness of the Entrada 

ranges from 0 to 15 m, except at Red Butte where it is 

mapped as being thicker than 30 m in places. In the 

southwest corner of the Maroon Bells quadrangle the 

Entrada is not everywhere present (fig. 32A), and it is 

less than 3 m thick where present. This contrasts with 

the thin but continuous Entrada around the Treasure 

Mountain dome and to the east (Gaskill and Godwin, 

1966a; Mutschler, 1970). The Entrada is absent in the 

Gunnison region and the eastern part of the Black 

Canyon (Baker and others, 1936; Hansen, 1971). The 

area where it is missing outlines a restricted Uncompahgre uplift (McKee and others, 1956, pi. 5); however, 

it is difficult to say that it is missing either because of 

nondeposition or because of post-Entrada-preMorrison erosion. The Entrada might have been 

deposited only in low areas on the Upper Jurassic surface, or it might have been deposited as a thin blanket 

and only locally removed by the streams that subsequently deposited the Morrison Formation. The 

absence of Entrada on the fault block west of 

Snowmass Creek poses the same question. Farther 

west at Capitol Creek the Entrada is only 4 m thick, 

and State Bridge and Chinle beds are absent.
THE NONMARINE AND MARINE ROCKS OF THE JURASSIC AND CRETACEOUS 47

F'4^^ .^p*^" t# : "rJ?r rfrii"^*^

'&$$&* >Et^iA^ "M^^fc

%^: ^^¥'r:^^-^-

^^^/v'^W^^'il'ife V v*Mpj£&#\' •

--••''•^.yiJ^^-'*-^

MORRISON FORMATION

The Morrison Formation (Cross, 1894) crops out in 

the Aspen area on Red Butte, along the valley side of 

lower Maroon and Willow Creeks, and on the east side 

of East Snowmass Creek. It forms dip slopes on the 

east part of Burnt Mountain and west of East 

Snowmass Creek. Except where cut by gullies and 

slide scars (fig. 325), it is usually poorly exposed and 

forms with the Chinle a soil-covered, grassy slope between the better outcrops of State Bridge or Maroon 

below and Dakota Sandstone and Burro Canyon Formation above. Spurr (1898) included the Morrison in 

the Gunnison Formation, the name applied to rocks 

between the Maroon and Dakota in the Crested Butte 

area by G. H. Eldridge (in Emmons and others, 

1894)—but this name has been abandoned.

In the Aspen area the Morrison conformably overlies 

the Entrada Sandstone, but where the Entrada is absent, it disconformably overlies the Chinle Formation.

FIGURE 31 (left and above).—Entrada Sandstone and Morrison Formation. A, Entrada Sandstone overlies easily erodible siltstone of 

the Chinle Formation. Base of Morrison is mapped at base of 

siltstone beds in middle of photograph. Sandstone above is mapped in the Morrison Formation. Entrada is about 5 m thick. North 

side of Willow Creek valley, Highland Peak quadrangle. B, Contact between Entrada Sandstone and underlying Chinle Formation on west side of gap at the head of Brush Creek, Highland 

Peak quadrangle. Chinle has been bleached for about 15 cm below 

unconformity, and the Chinle is locally squeezed into fractures in 

the Entrada.

At the west margin of the Highland Peak quadrangle 

it lies on the Maroon Formation. On the south side of 

the Elk Mountains in the southwestern part of the 

Maroon Bells quadrangle, it lies on an unconformity 

cutting the Maroon Formation (fig. 32^4).

The Morrison Formation consists of calcareous 

siltstone and sandstone and yellowish-grayweathering limestone and silty limestone grading upward into grayish-red and greenish-gray siltstone and 

claystone. Near the base of the formation is yellowishgray calcareous sandstone containing beds of gray 

shaly limestone. (See stratigraphic sections 2 and 3.)

Some of the basal sandstone has scattered spherical 

quartz and chert grains and resembles the Entrada. 

This resemblance has caused some confusion in location of the base of the Morrison (Baker and others, 

1936; Langenheim, 1957). In places this sandstone is 

crossbedded. If the sandstones contained beds of shaly
48 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

tf*5',, ^V-^v,.

--•.•-">..;•. ^l^-j'.

FIGURE 32.—Morrison Formation. A, Unconformity between Morrison and Maroon Formations in area of contact metamorphism 

at head of Rustler Gulch, Maroon Bells quadrangle. B, Typical exposure of Morrison claystone, siltstone, and sandstone overlain 

by more massive, cliff-forming sandstone of the Burro Canyon 

Formation and Dakota Sandstone near top of outcrop. North side 

of Willow Creek valley, Highland Peak quadrangle. About 140 m 

of strata shown here.

limestone or shale, I included them in the Morrison 

(fig. 3L4). Lenses of gray or mottled red, white, and 

gray chert occur locally in the sandstone.

The limestones range from thin laminations to beds 

as much as a meter thick. They are medium gray on a 

fresh surface and locally contain stringers and lenses 

of medium-gray to medium-bluish-gray chert. They 

grade to siltstone and sandstone beds.

The siltstone and claystone in the upper part of the 

Morrison are locally mottled greenish gray and grayish 

red. In the southwestern part of the Maroon Bells 

quadrangle a chert pebble conglomerate bed about a 

meter thick occurs in the upper part of the Morrison.

The basal sandstones of the Morrison range from 

poorly sorted to well sorted. The poorly sorted ones are 

similar to the Entrada Sandstone and contain larger 

rounded grains of quartz, chert, and microcline 0.5-2 

mm in diameter in a matrix of quartz, microcline, and 

chert grains, 0.1-0.3 mm in diameter. Most of the 

sandstones, however, are better sorted than those of 

the Entrada. The poorly sorted ones are concentrated 

at the very base of the Morrison. At the west margin of 

the Highland Peak quadrangle where the Morrison 

rests unconformably on the Maroon Formation, the 

basal Morrison contains some quartz pebbles, and 

some of the larger clastic grains are angular. Clastic 

muscovite and biotite are also present. The sandstones 

have various amounts of calcite cement. Accessory 

clastic grains of an opaque mineral, zircon, sphene, and 

tourmaline occur. The stratigraphically higher sandstones are fine grained and contain rounded to 

subangular quartz 0.1-0.3 mm in diameter cemented 

by calcite. A few calcite carbonate grains appear to be 

clastic. Microcline and rarely plagioclase also form 

clastic grains.

The limestones are locally biomicrites rich in 

charophyte or ostracod remains. Most of them contain 

various amounts of silt-size quartz grains.

The claystones contain scattered angular clasts of 

quartz 0.02-0.13 mm in diameter and fragments of 

untwinned potassic feldspar and plagioclase. 

Muscovite, apatite, tourmaline, zircon, and opaque 

minerals also form clastic grains. These minerals are 

enclosed in a matrix of clay and minor carbonate. 

Studies in the Colorado Plateau region to the west indicate that most of the clay in the upper part of the 

Morrison is montmorillonite derived from alteration of 

volcanic ash. The siltstones and claystones in the lower 

part of the Morrison generally contain illite, derived by 

erosion of sedimentary source terrane (Keller, 1962).

In the canyon of Snowmass Creek in the Woody 

Creek quadrangle, Baker, Dane, and Reeside (1936) 

found a 3.4-m-thick sequence of sandstone, oolitic
THE NONMARINE AND MARINE ROCKS OF THE JURASSIC AND CRETACEOUS 49

limestone, and shale containing marine fossils, which 

they assigned to the Curtis Formation. This sequence 

was not found in the Highland Peak quadrangle 11 km 

to the south.

In the Aspen area the Morrison is 122-161 m thick, 

and it can be divided into two units: a lower unit of 

sandstone and limestone 36-41 m thick and an upper 

unit of claystone and siltstone and a few sandstone and 

limestone beds. These two units might be correlated 

with the Salt Wash and Brushy Basin Members, 

respectively, but regional studies (Craig and others, 

1955) do not recognize these members as far east as the 

Aspen area. However, these studies (Craig and others, 

1955, p. 137) do correlate a claystone and limestone 

facies east of the limit of the Salt Wash Member with 

that member. The rocks in the Aspen region apparently are gradational between the claystone and lenticular 

sandstone facies of the Salt Wash Member mapped to 

the west (Craig and others, 1955, p. 138) and the 

claystone and limestone facies. The mapped area of the 

Brushy Basin Member is terminated where the Salt 

Wash Member loses its identity (Craig and others, 

1955, p. 156). However, the rock types in the upper 

part of the Morrison at Aspen are identical to those in 

the Brushy Basin Member.

Thickness variations in the Aspen region throw 

some doubt on the configuration of the poorly controlled area of thin Morrison shown parallel to the Uncompahgre highland in regional isopach maps (Craig and 

others, 1955, p. 159). In the Woody Creek quadrangle 

the thickness of the Morrison has been measured as 

162 m at Snowmass Creek (Craig, 1959) but is mapped 

as 90 m in the quadrangle as a whole (Freeman, 1972a). 

If this discrepancy represents the range in thickness of 

the Morrison in that quadrangle, that range is greater 

than in the Aspen and Highland Peak quadrangles. A 

series of sections along a northwest-southeast line on 

the south side of the Elk Mountains show a range of 

91-140 m in the thickness of the Morrison 

(Langenheim, 1957; Vanderwilt, 1935a; D. L. Gaskill, 

written commun., 1968; F. E. Mutschler, written commun., 1970). The evidence is insufficient to define a 

thinning in a southwesterly direction.

Facies and mineralogical studies (Craig and others, 

1955; Keller, 1962; Cadigan, 1967) indicate that the 

Morrison was derived from the west and southwest 

and was deposited by streams in a continental environment, in stream beds, on flood plains, and in lakes. 

Volcanic ash from air falls, in part redistributed by 

sedimentary processes, contributed considerable 

material to the claystones.

No diagnostic fossils were found in the Morrison of

the Aspen region, but the Morrison is famous for its 

well-preserved dinosaurs, which show that it is of 

latest Jurassic age.

BURRO CANYON FORMATION AND 

DAKOTA SANDSTONE

The Burro Canyon Formation (Stokes and Phoenix, 

1948) and the Dakota Sandstone (Meek and Hayden, 

1862) were mapped as one unit in the Aspen area 

because in most of the outcrop belt formed by these 

formations float from the Dakota Sandstone upslope 

from the Burro Canyon Formation covers the siltstone 

and claystone beds of the Burro Canyon. Those beds 

are diagnostic for the distinction between the two formations. The sandstones of the Burro Canyon and 

Dakota form cliffs and ledges along the top of the 

north side of the Willow Creek valley (fig. 325), on 

lower Maroon Creek, and on Eagle Mountain; and they 

form extensive dip slopes throughout the central part 

of the Highland Peak quadrangle.

Rocks of stratigraphic position similar to those of 

the Burro Canyon Formation west of the Colorado 

River are called Cedar Mountain Formation (Stokes, 

1952). Young (1960) extended the term Cedar Mountain to include stratigraphically equivalent rocks to 

the east margin of the Colorado Plateau about 80 km 

west of Aspen. What is here called Dakota Sandstone 

he named the Naturita Formation, and he referred to 

the Cedar Mountain and the Naturita as the Dakota 

Group. The Cedar Mountain at its type locality is 

predominantly shale and mudstone overlying a basal 

conglomerate (Stokes, 1952). The Burro Canyon at its 

type locality contains conglomerate, sandstone, shale, 

limestone, and chert (Stokes and Phoenix, 1948; 

Stokes, 1952) and is more nearly like the predominantly sandstone unit in the Aspen area, which was mapped as Burro Canyon by me. This practice has been 

followed by recent workers elsewhere in the region 

(Freeman, 1972a; Hansen, 1971).

The Burro Canyon Formation consists of light-gray 

to white quartz sandstone with chert- and quartzpebble conglomerate and interbeds of light-bluish-gray 

claystone, silty shale, and sandy siltstone.

The sandstones are generally fairly fine grained but 

contain granules of quartz, chert, and feldspar. The 

sandstones weather grayish orange. The claystones 

and siltstones occur in intervals of comparable 

thickness to the sandstone beds (about 3 m) to thin interbeds in the sandstones. The proportion of sandstone 

varies considerably from place to place. The contact 

between the Morrison and Burro Canyon is sharp and 

is drawn at the base of the lowest sandstone or conglomerate. The conglomerates contain pebbles of
50 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

chert, quartz, and quartzite as much as 2 cm in 

diameter, and they occur at the base of the formation 

in some places and above the base in other places. (See 

stratigraphic sections 2 and 3.)

In the southwestern part of the Maroon Bells quadrangle, contact metamorphism of the Dakota Sandstone has obscured the distinctive colors of the Burro 

Canyon claystone and siltstone interbeds, if any are 

present. In that area a quartz and chert pebble conglomerate commonly occurs at the base of the Dakota, 

and the pebbles are as much as 4 cm in diameter. Locally, conglomeratic beds are found as much as 5 m 

above the base of the Dakota.

The lower part of the Dakota Sandstone consists of 

white, yellowish-gray, light-brown, and gray crossbedded sandstone with a few lenses of quartz- and chertpebble conglomerate. The lower sandstones weather 

grayish orange, and the sandstone at the base forms 

cliffs. A few thin beds of medium-gray to dark-gray 

shale or claystone occur in this interval. The upper 

part consists of dark-gray, in part carbonaceous, sandstone and shale, locally containing plant fragments.

The upper contact of the Dakota is gradational with 

the overlying Mancos Shale. (See stratigraphic section 

4, p. 139.) The contact was drawn above the highest 

thick sandstone bed, although significant intervals of 

dark-gray siltstone and shale occur below that sandstone.

The sandstones of the Burro Canyon and Dakota 

range from poorly sorted to well sorted, and those of 

the Dakota are on the average somewhat better sorted 

than those of the Burro Canyon. Quartz grains range 

from subangular to rounded and form the main part of 

the sandstone. In some of the finer grained sandstones 

(<0.5 mm grain size), the quartz grains are partly intergrown to form an incipient mosaic texture. 

Overgrowths of quartz on the clastic grains are common. Some sandstones have a fine-grained silica cement. Quartz also occurs in granules and small pebbles 

of quartzite and vein quartz, some of which were 

cataclastically deformed.

Chert makes up from a trace to 40 percent of the 

sandstones. It ranges from dark gray to white and tan. 

Red chert occurs in a few places. The chert ranges in 

texture from cryptocrystalline to silt-sized grains in a 

cryptocrystalline matrix. Some appears to be silicified 

siltstone or volcanic rock. Other fragments may be 

silicified limestone.

Potassic feldspar is locally present in trace amounts. 

A light-olive-gray sandstone (stratigraphic section 4, 

bed 26; p. 139) from near the top of the Dakota contains several percent of plagioclase and clastic grains 

of muscovite and biotite.

Tourmaline, zircon, and opaque minerals are common accessory minerals. Some samples of the Dakota

contains carbonaceous material either as discrete 

fragments or as fine-grained material interstitial to the 

clastic grains of quartz. Analyses of several carbonaceous sandstones gave an organic carbon content 

ranging from 0.1 to 0.4 percent.

In the Aspen area the contact between the Burro 

Canyon and the Dakota is at the base of a ledgeforming sandstone, and it is poorly exposed. At the 

Colorado-Utah border, Carter (1957) found a welldeveloped disconformity at the base of the Dakota.

No diagnostic fossils were found in the Burro 

Canyon Formation or Dakota Sandstone in the Aspen 

area. The Burro Canyon Formation is considered to be 

of Early Cretaceous age on the Colorado Plateau to the 

west. The Dakota Sandstone is also of Early 

Cretaceous age both to the east (Waage, 1955) and to 

the west (Young, 1960), and consequently it is 

reasonable to infer an Early Cretaceous age for the 

Dakota in the Aspen region.

In the Aspen area the combined thickness of the 

Burro Canyon and the Dakota is 67-73 m, of which the 

Burro Canyon makes up 18-24 m. Most regional 

studies (Young, 1960; McGookey and others, 1972) 

show an area including the Aspen region where Burro 

Canyon or Cedar Mountain rocks are missing. On the 

south side of the Elk Mountains measured sections of 

the Dakota Sandstone range from 49 to 69 m (Vanderwilt, 1937; F. E. Mutschler, written commun., 1970; D. 

L. Gaskill, written commun., 1968), but no Burro Canyon is definitely recognized. In the southwestern part 

of the Maroon Bells quadrangle, the Dakota is about 

44 m thick, and no Burro Canyon was recognized. 

Whether the Burro Canyon is absent in those areas 

because of nondeposition or because of subsequent erosion is unknown. Farther south along the Gunnison 

River near the Black Canyon, Hansen (1971) mapped 

about 60 m of combined Burro Canyon and Dakota. To 

the west on the White River near Glenwood Springs, 

Bass and Northrop (1963) mapped 45 m of undifferentiated Dakota. However, just 8 km north of the 

Highland Peak quadrangle, Freeman (1972a) mapped 

69 m of Burro Canyon and 60 m of Dakota Sandstone 

for a total thickness of 129 m, almost double that of 

the Aspen area. Perhaps this unusual thickness for 

this region represents the filling of a former valley. 

This hypothesis is supported by a somewhat thinner 

Morrison section there than several kilometers to the 

south.

The Burro Canyon Formation was deposited in a 

continental environment by streams flowing from the 

west. The predominance of sandstones compared to 

the underlying Morrison Formation suggests some 

uplift to the west. However, the source area consisted 

of sedimentary rock for the most part. The debris was 

transported a considerable distance so that only resist-
THE NONMARINE AND MARINE ROCKS OF THE JURASSIC AND CRETACEOUS 51

ant materials survived. The Dakota Sandstone was 

deposited in a marginal marine environment as offshore bars, beaches, and lagoonal and swamp deposits, 

as the sea encroached from the north.

MANGOS SHALE

The Mancos Shale (Cross, 1899) underlies the 

sagebrush and scrub-oak-covered hills in the northern 

part of the Highland Peak quadrangle (fig. 33B) and 

the valley of the Roaring Fork west of Red Butte. It 

also occupies the lower slopes in the upper part of the 

Owl Creek and Brush Creek drainages to the south 

where it is covered by surficial deposits in many areas. 

Spurr (1898) mapped these rocks as the Colorado and 

Montana Formations. In the present study the Mancos Shale was mapped as three units: a thin lower shale 

member, the Fort Hays Limestone Member, and a 

thick upper shale member.

The lower shale member of the Mancos consists of 

dark-gray silty shale and thin beds of gray siltstone 

and fine-grained sandstone. The siltstone and finegrained sandstone beds are numerous in the Mancos 

where it grades into the underlying Dakota Sandstone. 

Near the top are several beds of fossiliferous 

calcarenite that has a fetid odor when broken. This 

horizon is partly equivalent to the Juana Lopez 

Member of the Mancos in the San Juan Basin (Rankin, 

1944; Dane and others, 1966). This fossiliferous zone 

has been recognized southwest of the Aspen area by D. 

L. Gaskill and F. E. Mutschler (written commun., 

1968; 1970), where it contains fossils typical of the 

type locality and the upper part of the reference section of the Juana Lopez described by Dane and others 

(1966). A few 1-4-cm bentonite beds are exposed in the 

lower member of the Mancos. However, the exposures 

of this stratigraphic interval are rather poor because so 

much of it forms a north-facing heavily vegetated dip 

slope partly covered with surficial deposits. One of the 

better places to see part of the interval is near the 

2,769-m shoulder at the head of Sterner Gulch in the 

Aspen quadrangle. The lower member of the Mancos is 

about 122 m thick.

The Fort Hays Limestone Member of the Mancos 

forms the most distinctive lithologic marker in the formation. In many places it forms small ledges or 

cuestas that protrude through surficial deposits and 

above the less resistant shale of the Mancos. It is well 

exposed on lower Maroon Creek, on the west ridge of 

Red Butte, and on Owl Creek. The Fort Hays consists 

of light-gray-weathering dark-gray limestone in beds 

0.3-0.6 m thick. It contains interbeds of shaly 

limestone. Fossils are locally abundant. The most spectacular fossil is the pelecypod Inoceramus deformis,

MB?"

FIGURE 33.—Features of the Mancos Shale. A, Silty shale and shaly 

siltstone containing a few thin beds of fine-grained sandstone in 

the middle of the Mancos (Baculites perplexus zone). Cut along 

former Denver and Rio Grande Western Railroad, Aspen 

quadrangle. B, View north across Brush Creek valley from Cozy 

Point showing typical terrane of Mancos Shale on a south-facing 

slope at 2,440 m altitude and lower and the persistent sandstone 

bed about 12 m thick in the upper part of the Mancos, Highland 

Peak quadrangle.

measuring as much as 10 cm across. The Fort Hays as 

mapped is about 12 m thick. Both the overlying and 

underlying beds contain thinner limy beds; so the contacts are somewhat gradational, although the 

stratigraphic interval containing the thicker limestone 

beds is relatively well defined. 

The upper shale member of the Mancos, about 1,450
52 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

m thick, including the uppermost part in the Woody 

Creek quadrangle (Freeman, 1972a), comprises most of 

the outcrop area of that formation; it is locally well exposed on south-facing slopes and gullies on the 

relatively low hills in the northern part of the Highland 

Peak quadrangle (fig. 33B). It is also locally well exposed along the Roaring Fork River and Colorado 

Highway 82. The upper member of the Mancos consists of dark-gray shale and silty shale, some shaly 

siltstone, and a few lenticular beds of fine- to mediumgrained olive-gray sandstone (fig. 33A). Some thinbedded shaly limestone occurs in the lower part, and a 

few beds of silty limestone and calcareous shale occur 

elsewhere in the upper member. Limestone concretions 

are abundant in places. Sandstone beds may be traced 

as much as 300 m along strike, and they reach a maximum thickness of 7.6 m. They are thick to thin bedded, silty, and carbonaceous, and they contain plant 

fragments. In some places gradations to silty shale are 

exposed along their strike. About 1.2 km of the upper 

member is exposed in the Highland Peak quadrangle.

One sandstone bed in the upper part of the upper 

member of the Mancos was mapped separately. It 

crops out continuously for 1.6 km at the north edge of 

the Highland Peak quadrangle (fig. 33B), and it was 

identified in fault blocks on either side of that outcrop. 

It forms ledges and small cliffs and is about 12m thick 

in its eastern exposures and 6 m thick in the most 

western exposures in the quadrangle. The sandstone is 

olive gray and contains phosphatic nodules 1-3 cm in 

diameter at its top.

The sandstones of the Mancos are generally fine 

grained, silty, and calcareous, and they grade into 

siltstone and shale. They typically contain angular to 

subangular quartz grains 0.05-0.4 mm in diameter. 

Feldspar, predominantly plagioclase, constitutes 

about 10 percent of the sandstone. Clastic grains of 

biotite and muscovite comprise as much as a few percent of the rock. Chert grains of the same size as the 

quartz and feldspar grains make up 5-15 percent of the 

rock. Calcite as clastic grains and as interstitial 

material comprises a substantial proportion of some of 

the sandstones. Some also have clay in the matrix. The 

one sample studied from the mapped sandstone was 

relatively well sorted and lacked matrix and calcite. 

Opaque minerals, zircon, tourmaline, and chlorite are 

accessory minerals.

The marine Cretaceous rocks of the western interior 

of the United States have abundant fossils and 

characteristically contain distinctive, geologically 

short lived cephalopod faunas that are excellent time 

markers. Numerous fossil collections were made in the

Highland Peak quadrangle (table 4). These combined 

with more than 100 collections from the Woody Creek 

quadrangle (Freeman, 1972a) allowed us to correlate 

the mapped sandstone in the upper member of the 

Mancos in the various fault blocks and to determine 

the structure in the Mancos Shale. The stratigraphic 

section shown by Freeman (1972a) applies also to the 

Highland Peak quadrangle. The top of the section as 

exposed in the Highland Peak quadrangle occurs about 

at the top of the Exiteloceras jenneyi zone. None of the 

fossil collections in the Highland Peak quadrangle 

were from the Didymoceras nebrascense zone. 

However, the one Didymoceras stevensoni came from 

about 30 m below the mapped sandstone bed, suggesting the possibility of some overlap between the 

two zones.

In the Woody Creek quadrangle (Freeman, 1972a), a 

mappable sandstone occurs in the Baculites perplexus 

zone. At Cozy Point and along the ridge to the west in 

the Highland Peak quadrangle are one or more sandstones at about the same stratigraphic level. Faults, 

some of which may be unmapped, and stratigraphic 

pinchouts make it uncertain whether the sandstones 

exposed are all the same, although they probably 

represent the sandy zone shown on Freeman's section. 

Other discontinuous sandstones occur in or near the 

Baculites perplexus zone on the hills near the head of 

Wildcat Creek.

The mapped sandstone that lies within the 

Didymoceras stevensoni zone is at the same stratigraphic horizon as a thin sandstone at Thompson 

Creek 32 km west of the Aspen area. That horizon has 

been correlated with the Cozzette Member of the 

Nelsen facies of the Price River Formation (Young, 

1955; Zapp and Cobban, 1960; Warner, 1964). A sandstone at a similar stratigraphic position occurs on the 

south side of the Elk Mountains (D. L. Gaskill, written 

commun., 1968). This sandstone was probably 

deposited in an offshore marine environment during 

regression of the Cretaceous sea. According to Warner 

(1964), the seaward limit of nonmarine deposits of the 

Cozzette Member was about 56 km northwest of the 

Aspen region. This sandstone probably represents an 

initial stage of the late Campanian regression (Haun 

and Weimer, 1960) of the sea designated R3 by Weimer 

(1960). The sandstone in the Aspen area appears to 

pinch out to the northwest (Freeman, 1972a); so it may 

be an offshore bar separate from but the same age as 

the Cozzette.

The sandstone mapped as Mesaverde(?) Formation 

by Freeman (1972a) in the Woody Creek quadrangle 

lies at least one and perhaps as much as three zones
THE NONMARINE AND MARINE ROCKS OF THE JURASSIC AND CRETACEOUS 53

TABLE 4.— Fossils in the Mancos Shale

[Numbers are U.S. Geological Survey Mesozoic locality numbers, shown in Bryant (1971a, 1972a); all identifications by W. A. Cobban. Leaders (—) indicate not present]

Ammonite zones or stratigraphlc horizon

Baculites perplexus

Baculites 

gregoryensi

Didymoceras Exiteloceras 

stevensoni jenneyi

Molluscoidea: 

Bryozoa:

Membraniporoid form— 

Pyriporoid form————

Mollusca:

Cephalopoda:

gregoryensis Cobbanperplexus Cobbanes cf. B. perplexus Cobbanscotti Cobba

sp. B. gilbert! Cobban—————— 

Didymoceras nebrascence (Meek and Hayden)- 

Didymoceras stevensoni (Whitfield)—————

Didymoceras n. sp.————————————————

Holoscaphites gilli Cobban and Jeletzky— 

Holoscaphi tes sp. ———————————————

Holoscaphites n. sp.——————————————

Anapachydiscus complexus (Hall and Meek)- 

Anapachydiscus sp.———————————————

Exiteloceras jenneyi (Whitfield)—————— 

Exitelo sp.

Oxybeloceras sp.- 

Placenticeras sp. 

Menuites sp.———-

(Meek and Hayden)-

Pelecypoda:

noceramus perplexus Whitfield————————- 

noceramus deformis Meek———————————— 

noceramus subcompressus (Meek and Hayden)- 

eramus barbabini Morton (of Meek)————

eramus convexus Hall and Meek—————— 

noceramus pertenuis Meek and Hayden———— 

noceramus aff. I. proximus Tourney———— 

noceramus aff. I. vanuxemi Meek and Hayde

Jnoceramus sp.——-———————— 

Ostrea inornate Meek and Hayde 

Ostrea sp.—————————————

Syncyc2onema hallii (Oabb)———

Ngmphalucina identalis (Morton)-

Lucina sp.————————————————————— 

Phelopteria sp.—————————————————— 

Thetis! circularis (Meek and Hayden)- 

Cymbophora sp. ————————————————

Gastropoda:

Aporrhai s sp.——————————————————

above Exiteloceras jenneyi (W. A. Cobban, written 

commun., 1968). It is probably about the same age as 

the Rollins Sandstone Member, the basal member of 

the Mount Garfield Formation of the Mesaverde 

Group 32 km west of Aspen, but whether that sandstone is marine or nonmarine in the Woody Creek 

quadrangle is uncertain because of limited exposures. 

K-Ar dating of tuffaceous beds in the marine Upper 

Cretaceous rocks of the Western Interior has shown 

that each of the cephalopod zones lasted 0.5-1 m.y. 

(Gill and Cobban, 1966). The Baculites perplexus zone 

occupies about 550 m stratigraphically in the Aspen 

area; whereas the Baculites scotti zone occupies only 

about 122 m. This suggests that much more rapid 

deposition took place during the life range of Baculites

perplexus 76.5-77.5 m.y. ago than at other times during the Upper Cretaceous in this area. The estimated 

age of the base of the Fort Hayes Member is 86 m.y. 

ago (Gill and Cobban, 1966). Only about 300 m of 

sediments were deposited during the 9.5-m.y. time interval from then to the beginning of the Baculites 

perplexus zone.

Exiteloceras jenneyi is estimated to have lived 72 

m.y. ago (Obradovich and Cobban, 1975; Dickinson 

and others, 1968). The lack of diagnostic fossil collections above this zone hinders a definite statement concerning the time of deposition of the youngest marine 

rocks preserved in the Aspen region, but a million or 

two million years later than the lifetime of Exiteloceras jenneyi seems reasonable for the age of the
54 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

youngest rocks preserved. However, marine sedimentation may have reoccurred after that time, for two 

tongues of Mancos marine shale occur in the 

Mesaverde section on North Thompson Creek about 60 

and 180 m above the base of the lowest persistent 

sandstone (Donnell, 1959). That sandstone probably 

correlates with the stratigraphically highest sandstone 

preserved in the Aspen region.

LARAMIDE IGNEOUS ROCKS

In the Late Cretaceous, or possibly early Paleocene, 

sills and dikes of hornblende quartz diorite, quartz porphyry, aplite, and aplite porphyry intruded near the 

margin of the Sawatch Range in the Aspen and 

Hayden Peak quadrangles. K-Ar ages of micas from 

two of these rocks gave ages ranging from 67 to 72±2 

m.y. (Obradovich and others, 1969). Spurr (1898) mapped only two varieties of igneous rocks in the Aspen 

quadrangle: the mafic type he called diorite porphyry, 

and the felsic type he called quartz porphyry. Knopf 

(1926) subdivided these rocks into the following units 

(equivalent units of this report in parentheses): diorite 

porphyry (quartz diorite); albite alaskite porphyry, 

containing phenocrysts of quartz and muscovite 

(quartz porphyry); albite aplite porphyry (aplite porphyry); and aplite (aplite).

Age relations between these rocks types are not well 

established. Knopf (1926) assumed that the quartz 

diorite was the oldest followed by the quartz porphyry, 

the aplite porphyry, and the aplite in that order. The 

only field relation between the rock types observed by 

Knopf was that the aplite cut the quartz porphyry in 

the Hope mine. I saw no well-defined age relations in 

the field, but K-Ar ages of 72±2 m.y. of the quartz porphyry and of 70.0±2.3 m.y. and 67.4±2.2 m.y. of 

aplite agree with the field relations in the Hope mine.

The dominant mode of occurrence of these igneous 

rocks is as sills, which are cut by faults. However, the 

aplite and quartz porphyry body on the east side of 

Richmond Hill in the Hayden Peak quadrangle appears to have been emplaced along a fault. The aplite 

body along Castle Creek in the Hayden Peak 

quadrangle is shown in cross section A-A of Bryant 

(1970) as occupying a fault because field evidence for 

faulting on its east contact was not found. Since that 

contact is not well exposed this judgment was based 

on float, and is thus subject to question: this aplite 

body could be a steeply dipping sill in the Belden Formation bounded by a fault on the east, like the sills 

along strike in the Aspen quadrangle to the north. |

HORNBLENDE QUARTZ DIORITE

Most of the mapped hornblende quartz diorite occurs 

as sills. One thin sill on the north end of Aspen Mountain is near the base of the Sawatch Quartzite. A sill as 

much as 45 m thick on the top and west side of Richmond Hill is in the Manitou Dolomite and the Chaffee 

Group. The sill at the north edge of the Hayden Peak 

quadrangle is at the top of the Leadville Limestone, 

and it was encountered at that horizon in the Hope 

mine about 600 m below its outcrop, where it is about 

45 m thick (Knopf, 1926). Some irregularities in the 

outcrop pattern of the hornblende quartz diorite east 

of the Midnight shaft suggest that some faults may 

have existed before its emplacement and exerted some 

influence on its shape. Sparse float suggests that thin 

discontinuous sills occur at intermediate stratigraphic 

positions between the ones on the north end of Aspen 

Mountain and Richmond Hill. These were shown as a 

thin continuous sill on Spurr's (1898) detailed maps 

and sections. A few small bodies of quartz diorite lie 

along faults; some of them are interpreted to be fault 

slices. Dikes of hornblende quartz diorite occur in the 

Precambrian rocks on Hunter Creek and Smuggler 

Mountain.

The hornblende quartz diorite forms a fairly thick 

soil and does not crop out well. One convenient place to 

see exposures of the rock is on the sharp corner of the 

Queens Gulch road at about 3,048 m altitude southwest of the Midnight tunnel.

The hornblende quartz diorite is a greenish-gray to 

bluish-gray porphyritic rock containing phenocrysts of 

chloritized hornblende as much as 5 mm long and of 

saussuritized plagioclase as much as 1 mm long in a 

fine-grained groundmass of quartz, albite, calcite, 

epidote, sericite, magnetite, and ilmenite. Apatite is a 

common accessory mineral. Sphene and allanite are 

less abundant. Some rocks of the unit have a substantial epidote content in place of calcite, and in some 

rocks epidote or aggregates of epidote form 

pseudomorphs after plagioclase phenocrysts. The 

shape of some of the smaller chlorite grains suggests 

that they may be after biotite. No outcrops of 

unaltered rock were seen.

Chemical analyses (table 5, col. 2) show that these 

rocks are fairly mafic quartz diorites. Where the K2O 

(which amounts to 2 weight percent) is located is uncertain; Spurr (1898) thought that some of the altered 

feldspar was orthoclase, but I could not identify any 

for certain. The K2O may be in fine-grained sericite 

or clay alteration products. Alteration might have 

changed the K2O/Na2O ratios, but the analyses of the
LARAMIDE IGNEOUS ROCKS 55

TABLE 5.— Chemical analysis, modes, and norms of Laramide igneous rocks

[Major oxides determined by rapid methods by P. L. D. Elmore, G. W. Chloe, Hezekiah

Survey, 1967 and 1969. Minor elements determined by semiquantitative spectrographic methods by Joseph L. Harris, U.S. Geological Survey, 1967, and William B. 

Crandell, U.S. Geological Survey, 1968. Mode by point count; P, present but not

volume percent.

number in the s

(1), rock too fine grained to count. CIPW norms calculated on

r elements are reported in parts per million to the nearest 

:ries 100, 70, 50, 30, 20, 15, and 10, ..., which represent

..quantitative results will include the quantitative value about

not found: Ag, As, Au, Bi , Cd , Ge, Hf , Hg , In, Li, Pd, Pt, Re, Sb, Sn, Ta, Ti, 

U, W, Znl

Sample No. —— —— ——

Field No. —— —— ———

S i02 —————————

Fe:,0 3 —— —— ——— ———

CaO ——— ——— ——— ——

Na2 0 ———————— —

T 10 2 —— ——— ——— —

Powder density —— —

1234567 

W171459 W171457 W171456 W171455 W168735 W171458 W168734 

2060 1184 1560 HNMP 1175 2059 1045

51.9 54.1 67.7 69.2 72.4 72.4 73.4 

16.4 15.6 15.9 16.2 14.8 15.4 14.7 

5.2 3.1 2.3 1.3 .55 .67 .87 

5.7 5.0 .40 .20 .78 .48 .48

4.1 2.8 .35 .25 .23 .14 .15 

5.1 5.9 2.4 2.5 2.1 1.3 1.9 

3.0 2.9 4.1 3.4 3.2 3.4 2.9 

2.0 1.9 2.5 2.7 3.7 4.2 3.4

.20 .19 .15 .43 .33 .11 .44 

3.3 2.7 1.1 1.3 .65 1.0 .56 

1.5 1.0 .15 .03 .12 .10 ,12 

.58 .56 .07 .04 .18 .03 .18

.21 .17 .09 .03 .03 .14 .12 

.09 3.1 2.0 1.6 .05 .50 <.05 

2.76 2.77 2.68 2.62 2.62 2.58 2.61 

2.84 2.84 2.72 2.70 2.68 2.70 2.65

Minor elements

B —— ————— —————— —

Co _

Cr——— ——— -——— -

La ———————————

r

N N N 50 N N N 

1,000 1,000 500 70 1,000 3,000 1,000 

N N 1 1.5 1 2 1 

30 15 N N N N N

N 50 N 30 N 7 N 

20 3 10 70 2 1.5 1 

70 70 50 N N N N 

5 5 N N N N 3

7 7 10 15 10 10 10 

5 20 N 20 N N N 

70 5 7 20 10 30 7 

20 20 N N N N 3

1,500 1,000 500 300 300 1,500 300 

200 150 7 N 7 N 7 

30 30 15 15 20 20 20 

150 200 150 50 50 70 50

100 100 N N N N N 

20 20 20 30 10 20 10 

3 3 1.5 1.5 1.5 2 2

Modes

Quartz — ———— —— —— 

Potassic feldspar —

1.1 15.8 34.5 (1) 37.4 (1) 38.0 

46.5 37.7 18.3 —— 29.6 —— 30.3 

17.5 —— 27.5 —— 25.5

25.5 3.2 —— —— .5

Norm

Ap ————— —————— ——

9.96 11.05 30.03 34.92 35.59 34.04 40.37 

1.48 —— 2.35 3.38 2.19 3.02 3.28 

12.35 12.43 15.40 16.65 22.29 25.26 20.46 

26.52 27.17 36.15 30.02 27.61 29.28 24.98

22.48 26.50 11.93 12.67 9.42 6.37 8.40

10.67 7.72 .91 .65 .58 .36 .38

7.88 4.98 1.20 .68 .81 .99 1.28

3.98 2.10 .30 .06 .23 .19 .23 

1.44 1.47 .17 .10 .44 .07 .43

14.94 12.83 .91 .65 1.44 .79 .57 

10.67 7.30 .91 .65 .58 .36 .38

TABLE 5.— Chemical analysis, modes, and norms of Laramide igneous rocks— Continued

Sample 

Lab. No.— 

No. ——————— 

—— —— W171459 

1234567

W171457 W171456 W171455 W168735 W171458 W168734 

Field No. ——————— 2060 1184 1560 HNMP 1175 2059 1045

Recalculated to 100 percent after deduction of H 20 and CO,

SiO. ——————————— 54.24 56.90 70.55 72.20 73.81 73.68 74.73 

A1 2 0 3 —————————— 17.14 17.27 16.57 16.90 15.09 15.67 14.97

Mg0 —— —— — 4.28 3.10 .36 .26 .23 .14 .15

Ca0 ———————————— 5.33 6.53 2.50 2.61 2.14 1.32 1.93

K>0 ———————————— 2.09 2.10 2.61 2.82 3.77 4.27 3.46

Mn0 ———————————— .22 .19 .09 .03 .03 .14 .12

SAMPLE DESCRIPTIONS

1. Altered andesite. Plagioclase laths as much as 0.6 mm long in a matrix of 

chlorite, epidote, sphene, and opaque mineral. From dike. Correlated with

hill 10977 northeast of Warren Lake. Aspen quadrangle.

5 mm long pseudomorphosed by chlorite. Saussuritized phenocrysts of plagioclase

altitude 550 m N. 35° E. of the Midnight shaft, Aspen quadrangle.

3. Gray fairly fine grained rock containing greenish feldspar phenocrysts as much 

as 5 mm long and some pyrite. Sericitized subhedral plagioclase phenocrysts, 

quartz in groundmass as much as 0.3 nun in diameter, euhedral to subhedral pheno— 

crysts of muscovite and carbonate. Aplite porphyry sill on Aspen Mountain.

and Vallejo Gulches. Aspen quadrangle.

hedral to euhedral phenocrysts of plagioclase as much as 1.5 mm long altered to 

albite; euhedral phenocrysts of quartz as much as 1.5 mm in diameter showing

Highland tunnel, North Midnight drift at about 2,652 m altitude, Aspen

phenocrysts as much as 0.6 mm long (composition An S8 to Ani+s) and as laths of 

albite iff the groundmass; quartz 0.1-0.3 mm in diameter with blocky outlines; 

interstitial potassic feldspar 0.1-0.3 mm. Dike of aplite, east side of Castle 

Creek. From 2,926 m altitude 1,160 m N. 20° W. from junction of Sawyer Creek 

and Castle Creek, Hayden Peak quadrangle.

6. Fine-grained light-gray rock with a few scattered phenocrysts of feldspar and 

quartz. Euhedral phenocrysts of plagioclase as much as 1.5 mm long (Anito), 

partly altered; few phenocrysts of quartz with a round outline, chlorite after

Dike correlated with aplite porphyry of the Aspen mining district. From crest

quadrangle.

7. Gray fine-grained rock containing phenocrysts of biotite. Euhedral to subhedral 

phenocrysts and glomerophenocrysts of plagioclase as much as 1 mm in diameter 

(Ani*5-52) and lath-shaped groundmass grains of albite; euhedral phenocrysts of 

biotite as much as 1 mm in diameter; groundmass of plagioclase, potassic 

feldspar, quartz, and muscovite with a grain size of 0.04-0.2 mm. Dike of 

aplite. From east side of top of 3,418-m shoulder on west side of the head of

gave ages of 70.0+2.3 m.y. and 67.4+2.2 m.y. (Obradovich and others, 1969).

more silicic altered rocks do not suggest that. The hornblende quartz diorite of the Aspen area does not conveniently fit the usual classifications because of the 

high K2O content in relation to low SiO2 and high Fe 

content.

QUARTZ PORPHYRY

Quartz porphyry was mapped along the Castle Creek 

fault zone in the southern part of the Aspen quadrangle and in the northern part of the Hayden Peak 

quadrangle. It is part of an irregular sill in the lower 

part of the Belden Formation. These relations are
56 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

(Knopf, 1926), and in the Highland tunnel. In the 

Highland tunnel and on the surface to the south, that 

sill splits into two or more sills. The quartz porphyry 

on Richmond Hill in the Hayden Peak quadrangle appears to occupy a fault, since neither side is concordant 

with the sedimentary rocks, which are displaced across 

the intrusive. Small cross-cutting bodies were mapped 

outside the main porphyry belt about 1.5 km south of 

the portal of the Hope tunnel, a concordant body in the 

fault block south of Fall Creek, and a dike southeast of 

Ashcroft Mountain. Quartz porphyry crops out on the 

road southeast of the Midnight tunnel at 3,280 m 

altitude and on the road northwest of the Hope mine at 

2,990 m.

The quartz porphyry is white to light gray and contains conspicuous euhedral phenocrysts of quartz, and 

locally, muscovite 2.5 mm in diameter and inconspicuous phenocrysts of plagioclase 0.5-2 mm in 

diameter in a groundmass of quartz, feldspar, and, in 

places, sericite 0.02-0.2 mm in diameter (fig. 34A). 

Some of the quartz phenocrysts are embayed, suggesting partial resorption by the magma after their 

formation. The plagioclase phenocrysts are altered to 

albite. Calcite occurs in veinlets and in the groundmass. Pyrite is disseminated in much of the altered 

rock. The dikes and small intrusives are less altered 

than the larger bodies. In addition to quartz 

phenocrysts, they contain very sparse biotite 

phenocrysts in a very fine grained groundmass. The 

dike southeast of Ashcroft Mountain has unaltered 

phenocrysts of plagioclase with An28 composition. The 

small intrusives mapped south of the Hope mine contain a few phenocrysts of potassic feldspar.

APLITE

Aplite was mapped principally in the northern part 

of the Hayden Peak quadrangle where it occurs along a 

fault on the east side of Richmond Hill and as sills in 

the lower part of the Belden Formation. It also occurs 

as dikes on Smuggler Mountain. Aplite forms ledges 

on the lower east side of Castle Creek north of Fall 

Creek in the Hayden Peak quadrangle.

The aplite is a white to gray fine-grained rock having 

a grain size of 0.1-0.3 mm. It consists of somewhat 

blocky quartz and laths of plagioclase with interstitial 

potassic feldspar. The plagioclase is albite. A few 

phenocrysts of unaltered plagioclase with a composition of An43-58 are as much as 1 mm long (fig. 34B). 

Euhedral phenocrysts of biotite as much as 1 mm long 

occur sparsely in places. Muscovite, opaque minerals,

zircon, allanite epidote, chlorite, and calcite are accessory minerals.

APLITE PORPHYRY

The aplite porphyry occurs as a sill in the lower part 

of the Belden Formation in the syncline south of the 

city of Aspen. A fault slice probably derived from the 

same sill occurs on the ridge north of Queen's Gulch. 

This sill is at least a hundred meters thick on the lower 

part of Aspen Mountain but thins to the north. It extends beneath the valley of the Roaring Fork River and 

is cut off by a fault in the mine workings under Smuggler Mountain (Spurr, 1898). Possible feeder dikes for 

this sill are found on the surface and in mine workings 

(Spurr, 1898) at about 2,850 m altitude on the ridge 

west of Spar Gulch and on Smuggler Mountain. Dikes 

mapped in the Precambrian rocks on Smuggler Mountain were also included in this unit.

The aplite porphyry is a very light gray to lightgreenish-gray rock containing phenocrysts of sericitized plagioclase as much as 3 mm in diameter. The only 

unaltered phenocrysts found were from the dikes on 

Smuggler Mountain, and they have a composition 

An30-40. Quartz usually occurs as grains 0.1-0.3 mm in 

diameter in the groundmass, but a few phenocrysts 

1-2 mm in diameter occur locally. The quartz tends to 

be in blocky subhedral grains (fig. 34C). Plagioclase 

laths and interstitial potassic feldspar complete the 

matrix. Sericite and calcite are common alteration products. Pyrite and zircon are accessory minerals.

COMPOSITION

Since the textural varieties of the felsic porphyries 

overlap in chemical and mineralogic composition, they 

will be discussed together. Among the analyzed 

samples, the degree of alteration is quite varied, as indicated by the varied contents of CO2 and H2O+. The 

degree of alteration does not appear to have drastically 

affected the contents of CaO, Na2O, K2O, MgO, and 

total Fe in these rocks. (Compare samples 3 and 4 with 

5, table 5.)

These rocks all have a low content of MgO and total 

Fe. They are light colored because of the scarcity of 

mafic minerals in them. Their total alkali content and 

the ratio between Na2O and K2O are similar to those of 

granodiorite, but their CaO content is lower than most 

granodiorites. Normative and modal quartz are high 

for rocks of similar modal plagioclase and normative 

An content.

Modally, these rocks are leucocratic quartz mon-
LARAMIDE IGNEOUS ROCKS 57

zonites. Chemically, they are granodiorite and quartz 

monzonite judging by proportions of CaO, Na2O, and 

K2O compared with Nockolds' (1954) averages. 

However, MgO and total Fe are lower than for such 

rocks. In the AlkFM diagram (Alk, K2O+Na2O; F, 

FeO+2Fe2O3+MnO; M, MgO) (fig. 35A), these rocks 

are lie near the Alk corner; in the ACF diagram (A, excess A12O3 over that used for feldspar plus ferric iron; 

C, CaO; F, ferromagnesian oxides) (fig. 35B), they lie 

near the AC line; and in the AFK diagram (A, 

Al2O3+Fe 2O3 -(CaO + Na 2O + K2O); K, K2O; F, 

MgO+FeO+MnO) (fig. 35Q they lie near the AK line.

Few quantitative data are available from dated 

Laramide intrusives in adjacent areas. In the Leadville 

district 48 km to the east, all the porphyries are felsic, 

and a number of varieties have been recognized (Emmons, 1886; Emmons and others, 1927; Tweto, 1960). 

At Leadville as well as at Aspen most of the porphyry 

is altered so it is difficult to obtain comparable 

analyses. The light-colored porphyries of the Aspen 

district are most similar chemically and petrographically to the Pando Prophyry of the Leadville 

district, formerly White and Mount Zion Porphyries, 

both terms now abandoned in preference to the name 

Pando, which has been dated as 70 m.y. old (Emmons, 

1886; Pearson and others, 1962).

About 60 km southeast of Aspen in the Garfield 

quadrangle are a number of intrusive rocks. The bulk 

of them comprise an igneous complex, and Mount 

Princeton batholith, which cuts the faults at the 

margin of the Sawatch Range (Dings and Robinson, 

1957) and is probably mid-Tertiary, late Eocene and 

early Oligocene in age (Steven, 1975). The oldest rock

FIGURE 34 (left).—Photomicrographs of Laramide igneous rocks; all 

viewed through crossed nicols; all at same scale. A, Quartz porphyry from 1,681 m inside portal of Highland tunnel, Aspen 

quadrangle. Part of irregular sill in Richmond Hill mining district 

emplaced earlier than much of or all of the faulting along the Castle Creek fault zone. Euhedral phenocryst of quartz with 

somewhat rounded edges in a matrix of albite, quartz, and 

sericite. Also contains phenocrysts of plagioclase not shown. B, 

Aplite from hill 11212 on east side of Richmond Hill, Hay den 

Peak quadrangle. Intrusive apparently emplaced along a fault 

related to the Castle Creek fault zone. Inconspicuous phenocrysts 

of plagioclase and biotite in a groundmass of quartz, albite, 

potassic feldspar, and minor sericite. Laramide igneous rocks are 

seldom this unaltered in the Aspen area. Analyzed sample 7, table 

5. C, Aplite porphyry from about 3,080 m altitude on Aspen 

Mountain. Sill near base of the Belden Formation in the Aspen 

mining district. Euhedral to subhedral phenocrysts of plagioclase 

now altered to calcite and sericite in a matrix of subhedral to 

anhedral quartz and altered plagioclase and potassic feldspar. 

Block-shaped quartz in upper center is a distinctive feature of the 

microtexture of many samples of Laramide porphyry in the 

Aspen area.
58 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

FeO+2Fe2O3+MnO

MgO+FeO+MnO

MgO+FeO+MnO 

(CaO+Na2O+K20)

FIGURE 35.—Chemical diagrams for analyzed Laramide igneous 

rocks (table 5). A, AlkFM diagram; B, ACF diagram; C, AKF 

diagram.

in the area is an undated quartz diorite porphyry older 

than at least some of the movement on the range-front 

faults. Judging by the petrographic description of 

Dings and Robinson (1957), this rock resembles the 

quartz diorite of the Aspen area, if the latter could be 

found unaltered. A highly altered rhyolite is cut by a 

fault and the Mount Princeton Quartz Monzonite 

(Dings and Robinson, 1957), and thus may be as old as 

the Laramide rocks at Aspen. The Tertiary Tincup 

Quartz Monzonite Porphyry believed by Dings and 

Robinson (1957) to be older than the Mount Princeton 

batholith has recently been dated as early Oligocene 

(Cunningham and others, 1977).

The Laramide igneous rocks of the Aspen area constitute a bimodal suite. The quartz diorite contains a 

54-57 percent SiO2 after deduction of volatiles, and the 

felsic porphyries have 70-75 percent SiO2 on the same 

basis (table 5).

MIDDLE TERTIARY IGNEOUS ROCKS

Middle Tertiary igneous rocks of calc-alkaline affinity and predominantly granodioritic composition make 

up a number of plutons, sills, dikes, and laccoliths in 

the Elk Mountains south and southwest of Aspen. 

These rocks are of the same general age and chemistry 

as widespread extrusive and intrusive igneous rocks 

elsewhere in the region (Steven and Epis, 1968; Lipman and others, 1969).

These intrusive rocks were first described by the 

Hayden Survey (Hayden, 1874; Holmes, 1876). Cross 

(1894) called them diorites before the term granodiorite 

was in general use, and his description of the rocks is 

quite accurate. The name diorite caused Spurr (1898) 

to miscorrelate the hornblende quartz diorite of 

Laramide age of the mining district with the rocks of 

the White Rock pluton. Knopf (1926) recognized that 

the intrusives of the Elk Mountains were indeed 

granodiorite and that they differed from the Laramide 

hornblende quartz diorite of the Aspen area. Vanderwilt (1937) discussed the Snowmass pluton west of the 

Aspen area and thought that the three main plutons of 

the Elk Mountains were derived from a single source. 

This idea is supported by the chemical and age data 

now available (Lipman and others, 1969; Obradovich 

and others, 1969).

Graton (1958) described the petrography of a part of 

the White Rock pluton near White Rock Mountain in 

the Gothic quadrangle in an area mapped by Gaskill 

(1956). Cunningham (1976) studied a related intrusive, 

the Oligocene Italian Mountain Intrusive Complex, 

just southeast of the White Rock pluton and 7 km 

south of the Hayden Peak quadrangle, and deciphered
MIDDLE TERTIARY IGNEOUS ROCKS 59

a sequence of intrusions of predominantly 

granodioritic and quartz monzonitic composition.

Since the work of Emmons, Cross, and Eldridge 

(1894), it has been known that the intrusive rocks cut 

the Ruby beds in the northeastern part of the West Elk 

Mountains. Emmons, Cross, and Eldridge considered 

those rocks to be Cretaceous in age, but they are now 

mapped as the lower part of the Wasatch Formation, 

and they are probably of Paleocene age. These relations only place an older limit on the age of the intrusive rocks as post-Paleocene (Godwin and Gaskill, 

1964). K-Ar ages of micas from several of the intrusives in the Elk Mountains fall in the range of 29 to 

34 m.y., or Oligocene (Obradovich and others, 1969).

The Aspen area includes much of one of the major 

plutons of the Elk Mountains—the White Rock pluton 

(fig. 36) dated as 34 m.y. old by Obradovich and others 

(1969). That pluton has an outcrop pattern forming a 

shape resembling a doughnut (fig. 48), although in 

detail the borders on the pluton are highly irregular in 

many places. Rocks of the White Rock pluton form the 

jagged peaks at the head of Castle Creek just south of 

the Hayden Peak quadrangle, part of Conundrum 

Peak, the ridge north of Hayden Peak, Keefe Peak, and 

the crest of the Elk Mountains at the head of East 

Maroon Creek. This pluton is concordant with the Elk 

Range thrust and with the bedding in the Maroon Formation in gross aspect, but its upper contact cuts 

through a thousand meters of the Maroon Formation 

in places (fig. 37). A large number of dikes and sills 

emanate from the pluton, especially near Castle Peak 

and west of Copper Lake.

Detailed mapping available of the southern part of 

the Snowmass pluton (fig. 48; Gaskill and Godwin, 

1966a; Mutschler, 1970) and reconnaissance of its 

other sides indicate that it occupies a position along 

the Elk Range thrust similar to that of the White Rock 

pluton except that it is much more concordant with the 

bedding of the rocks in the upper plate of the thrust. 

Only 4.8 km separate the Snowmass pluton from a 

small granodioritic stock in the West Maroon Creek 

valley, which is probably a cupola of the White Rock 

pluton. It is likely that the Snowmass and White Rock 

plutons are connected at depth along the Elk Range 

thrust.

Extensive areas of contact metamorphism are 

associated with parts of the White Rock pluton. The 

widest areas of metamorphism are adjacent to discordant contacts. On the other hand, the contact metamorphic aureole of the Snowmass pluton is narrow, and the 

contacts generally concordant.

Associated with the White Rock pluton are 

numerous mineralized fracture zones, most of which 

contain quartz, calcite, and pyrite; a few fracture zones

contain galena, sphalerite, and chalcopyrite and have 

silver or gold values. Several areas of the pluton contain much disseminated pyrite and lesser amounts of 

chalcopyrite and molybdenite. The latter mineral is 

concentrated in restricted areas where the granodiorite 

is highly altered (Bryant, 1971c). One contact 

metasomatic iron deposit in marble of the Belden Formation is associated with the White Rock pluton at the 

head of Cooper Creek at the south edge of the Hayden 

Peak quadrangle.

The White Rock pluton generally has sharp concordant to discordant contacts lacking chilled margins (fig. 

38Q with country rock, which usually is not deformed 

adjacent to the intrusion. However, locally, especially 

where the contact is with the Belden Formation, the 

adjacent rock is highly deformed apparently at least in 

part due to force of intrusion (fig. 38A). In places on 

the east side of the White Rock pluton, hornfels adjacent to the pluton is brecciated (fig. 38B), Most of 

these breccias have a matrix of calcareous material 

rather than of granodiorite and resemble in many 

respects the breccias of Monument Gulch (fig. 17). 

However, they are adjacent to the pluton contact 

rather than occurring along a stratigraphic horizon 

above evaporite as in Monument Gulch.

In the western part of the Maroon Bells quadrangle 

three varieties of middle Tertiary igneous rocks were 

mapped: (1) hornblende porphyry occurring as dikes 

and sills, (2) granodiorite in stocks, and (3) porphyritic 

granodiorite occurring as sills, dikes, and laccoliths. At 

least some of the hornblende porphyry is offset by 

faults that probably formed as the granodiorite stocks 

were emplaced. Other bodies of hornblende porphyry 

occupy the same fracture system as the dikes of 

granodiorite porphyry and thus are probably younger 

than the sills that are cut by the faults. Both the hornblende porphyry and granodiorite and many of the 

faults are cut by dikes of porphyritic granodiorite. 

However, some faulting did occur after the emplacement of the sill of porphyritic granodiorite north of 

Maroon Lake. Thus, the relations between faulting and 

intrusions show some sequential development, but in 

detail those time relations are complex.

In the rest of the Maroon Bells quadrangle and in the 

Hayden Peak quadrangle, hornblende porphyry was 

not a distinctly mappable rock type. Some of the thinner dikes and sills next to the contact of the White 

Rock pluton resemble some of the hornblende porphyry, but they grade texturalty into fine-grained and 

medium-grained granodiorite and can be traced into 

the pluton.

The porphyritic granodiorite cuts granodiorite in 

most places as well-defined dikes, some of which have 

chilled margins, showing that the granodiorite was
Ca Co

SB

FIGURE 36.—Panorama of the east end of the Elk Mountains from a high surface on 

Precambrian rocks east of Ashcroft Mountain, Hayden Peak quadrangle. High 

peaks left to right are Castle Peak (Ca), Conundrum Peak (Co), Cathedral Peak (Ct), 

and Hayden Peak (H). Granodiorite of the White Rock pluton (W) forms lower slopes below Castle Peak at left and crest of ridge to the right of Hayden Peak and is roofed by hornfels and red beds of the Maroon Formation. A few small faults are visible 

near the ridge crest left of Cathedral Peak. The white outcrop (B) below the skyline right of center is Belden marble in a fault block adjacent to the pluton shown in

figure 38A. To the right of the pluton the Maroon dips north towards the Roaring Fork syncline, and the little knob (SB) on the far right skyline marks the base of the 

State Bridge Formation. Break in slope between rocky spurs and wooded ridges in 

center and to right mark faultline scarp adjacent to White Rock pluton. Ridge in 

right foreground leads to Richmond Hill and Aspen and represents part of an erosion surface of early(?) Tertiary age. The Elk Mountains were lifted above this surface by intrusion of igneous rocks in the Oligocene. View encompasses about 16 km 

on skyline.
MIDDLE TERTIARY IGNEOUS ROCKS 61

FIGURE 37.—Crosscutting contact of part of the White Rock pluton on west side of Conundrum Peak. Bedded rock is hornfels of the Maroon 

Formation. Sharp peak right of center is Castle Peak and rounded peak on center skyline is Conundrum Peak. Granodiorite contact is 

on left side of Conundrum Peak. Irregular body of granodiorite caps ridge in foreground. Maron Bells and Hayden Peak quadrangles. 

View east across Conundrum Creek valley. Skyline about 2 km long.

cool enough to fracture and to quench the porphyritic 

granodiorite at the time of its emplacement. However, 

one small stock of granodiorite north of Crater Lake in 

the Maroon Bells quadrangle grades to, rather than is 

cut by, the dikes of porphyritic granodiorite mapped to 

the west of it. Also, in a few places in the White Rock 

pluton, ill-defined zones of porphyritic granodiorite 

grade into normal granodiorite. On the southwest side 

of the ridge southeast of Copper Lake near the 

southern margin of the Maroon Bells quadrangle a 

5-7-cm thick dike of very fine grained rock containing 

phenocrysts of hornblende and plagioclase cuts a dike 

emanating from the White Rock pluton. Apparently, 

that rock intruded after the apophysis of the pluton 

had cooled. Consequently, although a general sequence 

of intrusion of somewhat different rock types can be 

demonstrated locally, it is not well defined everywhere.

GRANODIORITE

In the White Rock pluton no well-defined internal 

contacts were mapped despite variations in texture 

and mineral composition in the granodiorite. Locally, 

small areas of more mafic rock (CI 15-20) were noted 

near the margins of the pluton. These did not seem to 

have sharp contacts with normal granodiorite (CI 

5-15). Perhaps more detailed study would allow mapping of variants of the granodiorite in the pluton.

Textures of the granodiorite range from 

hypidiomorphic-granular (fig. 39.E7) to microporphyritic (fig. 39D). The grain size ranges from fairly 

coarse to fine. All gradations between the different textures occur, although the end members are markedly 

different. The finer grained dikes and apophyses adjacent to the White Rock pluton resemble the dikes
62 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

; yf/ 1 '^^ J'"'" 

-A'Vr^Ti^ ^

FIGURE 38.—Features at or near contact of the White Rock pluton, 

Hayden Peak quadrangle. A, Contorted Belden marbles on 

3,751-m knob on Castle Creek-Conundrum Creek ridge. Contact 

of pluton is at change of color on even-crested part of ridge. 

Marbles are in upfaulted block adjacent to the pluton. Dark-gray 

beds are 1-2 m thick. B, Breccia just above the pluton north of

mapped as porphyritic hornblende diorite in the 

western part of the Maroon Bells quadrangle. The 

former grade into coarser grained varieties of 

granodiorite.

In places inclusions are numerous in the White Rock 

pluton. They consist of hornfels derived from the

3,665-m knob on Cooper Creek-Castle Creek ridge. Breccia (right) 

cuts undisturbed hornfels (left). Tree in center about 20 m tall. C, 

Contact of pluton in cirque southwest of Cathedral Lake. Note 

lack of chilled margin. D, Zones of alteration cut granodiorite and 

hornfels at contact at 3,540 m altitude west of Castle Creek.

wallrock and of somewhat more mafic hornblende 

granodiorite to diorite, probably derived from some 

earlier crystallized variants, perhaps from beneath the 

present level of exposure. The hornfels inclusions 

range from a centimeter to a kilometer long. (See 

geologic maps of Hayden Peak and Maroon Bells
MIDDLE TERTIARY IGNEOUS ROCKS 63

quadrangles (Bryant, 1969,1970).) The igneous rock inclusions are generally several centimeters or less in 

diameter.

Alteration in the White Rock pluton is quite spotty. 

Chloritization of hornblende and biotite and 

saussuritization of plagioclase are widespread. 

Albitization, sericitization, and silicification are locally 

found in the areas of disseminated sulfides (Bryant, 

1971c). Sills in and adjacent to the Conundrum Creek 

window are albitized in many places.

The granodiorite is a white to gray rock containing 

various amounts of mafic minerals, which are principally biotite but include hornblende, monoclinic 

pyroxene, and magnetite. Grain size ranges from a few 

tenths to several millimeters. Most of the granodiorite 

mesoscopically appears to be equigranular, but locally 

larger grains of potassic feldspar occur. In those places 

the granodiorite resembles the porphyritic 

granodiorite, but no well-defined contacts of rock containing the larger potassic feldspar grains were seen.

The granodiorite contains euhedral to subhedral 

plagioclase grains (fig. 39) that locally are alined by 

flowage. The crystals usually display oscillatory zoning in a normal trend. Cores of the plagioclase grains 

have compositions of An 55_65 , and the rims generally 

have a composition of An 20-32. In a few rocks the 

plagioclase has albite rims. In many rocks the 

plagioclase crystals are 0.5-1.5 mm long, but in a few 

they reach 5 mm. Albite locally occurs in the interstices of the well-shaped plagioclase grains.

Quartz typically is interstitial to the plagioclase and 

has a grain size of 0.1-0.5 mm (figs. 39A, B, C, E). 

However, a significant minority of the samples contain 

quartz as euhedral to anhedral phenocrysts as much as 

6 mm in diameter (fig. 39F). The phenocrysts locally include oligoclase and biotite. Some have embayed 

margins, suggesting incipient resorption in the magma 

after crystallization. Quartz also occurs in granophyric 

intergrowths with potassic feldspar.

Potassic feldspar is generally interstitial to 

plagioclase and ranges from 0.05 mm to several 

millimeters in grain size (figs. 39A, B, Q. The larger 

grains engulf plagioclase, biotite, hornblende, quartz, 

and opaque minerals. Potassic feldspar also occurs in 

granophyric intergrowths with quartz.

Brown biotite ranges from anhedral to subhedral 

grains that are later than the plagioclase to euhedral 

phenocrysts that are apparently contemporaneous 

with the plagioclase (figs. 39C, E). Grain size ranges 

from 0.3 to 4 mm but is mostly in the range of 0.5- 

2 mm. Some is intergrown with hornblende.

Amphibole ranges from greenish brown, olive green, 

and green to very pale green. The lighter green amphibole is locally derived by alteration of the darker

green type, although the two amphiboles have the 

same extinction angle. Most of the amphibole is 

anhedral to subhedral, and it ranges from 0.5 to 5 mm 

in grain size. Some of it is interstitial to the 

plagioclase, and in some rocks it appears to have formed quite late. Locally, the amphibole is derived from 

pyroxene.

Monoclinic pyroxene occurs as anhedral to subhedral 

grains 0.1-2.5 mm in diameter. It is generally colorless 

or light tan and nonpleochroic and has zAC ranging 

from 42° to 47°. Hypersthene was found with 

monoclinic pyroxene in one locality.

Chlorite forms pseudomorphs after biotite and hornblende grains. In some rocks it contains inclusions of 

carbonate and sphene. Both Mg/Fe and Fe/Mg 

varieties occur, and much of the chlorite has a very low 

birefringence; it is on the border between the two 

types.

Opaque minerals are principally magnetite except in 

areas of pyritized rock where pyrite is a major constituent and chalcopyrite a minor one. Molybdenite occurs in certain areas of highly altered rock (Bryant, 

1971c).

Common accessory minerals are zircon, apatite, and 

sphene. Allanite and rutile occur sparingly.

ALTERATION

Alteration of the granodiorite is widespread but 

quite spotty and irregular in distribution. The most 

common variety is propylitic and involves 

saussuritization of plagioclase and chloritization of 

mafic minerals. Plagioclase is also altered to sericite 

and carbonate and to clay mineral(?) and carbonate. 

Sphene or carbonate is often associated with secondary chlorite.

Less common varieties of alteration generally but 

not always associated with mineralized areas (Bryant, 

1971c) are albitization—concentrated in the Cataract 

Creek area and the west side of the Conundrum Creek 

valley by the mouth of Cataract Creek—and silicification and sericitization—locally concentrated in the 

East Maroon Creek disseminated sulfide deposits.

Fluid inclusion studies of altered rocks in areas of 

disseminated sulfides made in reconnaissance fashion 

by J. T. Nash and C. G. Cunningham, Jr., show that 

alteration took place under postmagmatic conditions 

at temperatures of 200°-375 °C. Some of the inclusions 

in the area of albitization contain as much as 35 weight 

percent NaCl. Some of the inclusions indicate boiling 

at temperatures of 225°-350 °C. The higher boiling 

temperature would indicate a maximum pressure of 

120 bars or a depth of about 1,200 m if PH o=^Total> 

but less deep if PH& <P Total- This compares with a
64 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO
MIDDLE TERTIARY IGNEOUS ROCKS 65

higher temperature (385 °C) and a greater maximum 

depth (2,700 m) for hydrothermal activity in the 

Italian Mountain Intrusive Complex just south of the 

Aspen area, based on detailed study (Cunningham, 

1976).

Alteration probably took place over the whole 

temperature range, since many samples contain inclusions with different filling temperatures. Quartz carbonate veins from other parts of the pluton have fluid 

inclusions with filling temperatures in the range 

200°-350 °C; so vein formation and alteration took 

place at similar temperatures.

Another type of alteration is very local but quite interesting. White altered-appearing rock occurs as irregular dikes and blobs in normal-appearing 

granodiorite. This type of alteration is generally near 

or at the contacts of the granodiorite and in places extends across the contact into adjacent hornfels (fig. 

3&D). It is found west and north of the head of Castle 

Creek 0.8-1.6 km north of the south edge of the 

Hayden Peak quadrangle. The white altered rock lacks 

biotite and hornblende but is rich in pyroxene or 

sphene. In one sample the pyroxene appears to have 

formed from hornblende. In one area a green zone, beFIGURE 39 (facing page).—Photomicrographs of granodiorite of the 

White Rock pluton showing textural variations. All views 

through crossed nicols; all at same scale. A, Amphibole augite 

biotite granodiorite from apophysis northwest of the Montezuma 

mine, Hayden Peak quadrangle. Euhedral to subhedral normally 

zoned plagioclase, subhedral to anhedral augite partly altered to 

amphibole, anhedral biotite, opaque minerals, and interstitial 

quartz and potassic feldspar. Analyzed sample 1, table 6. B, 

Hornblende biotite granodiorite from southeast part of Maroon 

Bells quadrangle at 3,505 m altitude east of Conundrum Creek. 

Euhedral to subhedral plagioclase with oscillatory zoning in a normal trend, subhedral to anhedral biotite, anhedral hornblende, 

opaque minerals, and interstitial quartz and potassic feldspar. C, 

Biotite granodiorite from southeast of the Montezuma mine, 

Hayden Peak quadrangle. Euhedral to subhedral normally zoned 

plagioclase with some oscillatory zoning, anhedral to subhedral 

biotite, and interstitial quartz and potassic feldspar. D, Porphyritic granodiorite from southeast of the Montezuma mine, 

Hayden Peak quadrangle. Euhedral plagioclase having some 

oscillatory zoning in a normal trend; quartz, biotite, and hornblende, in a very fine grained matrix of quartz, feldspar, and opaque mineral. Analyzed sample 7, table 6. E, Biotite granodiorite 

from 3,170 m altitude east of Hunter Peak, Maroon Bells 

quadrangle. Subhedral to euhedral plagioclase with normal and 

oscillatory zoning and subhedral to anhedral biotite in a mosaictextured groundmass of quartz and potassic feldspar. Rock locally has phenocrysts of quartz and late large potassium feldspar 

grains not shown here. F, Biotite quartz monzonite from 3,110 m 

altitude east of Conundrum Creek, Maroon Bells quadrangle. 

Large anhedral quartz includes plagioclase and biotite; subhedral 

plagioclase and anhedral biotite. Scattered late porphyroblasts of 

potassic feldspar not shown. Analyzed sample 12, table 6.

tween normal gray unaltered biotite-hornblende 

granodiorite and the white rock, contains monoclinic 

pyroxene after hornblende; the white altered rock lacks 

pyroxene but contains several percent of sphene. 

Where the white zones cut into hornfels, sphene is also 

the principal dark mineral in the altered hornfels. This 

alteration appears to have occurred in a hot dry environment, perhaps through sudden decline in 

PHZO through fracturing of the roof at an early stage in 

the cooling of the granodiorite. During the alteration 

process Fe, Mg, and H2O were lost, and Ti may have 

been introduced.

PORPHYRITIC GRANODIORITE

Porphyritic granodiorite occurs as sills, dikes, and 

laccoliths in the Maroon Bells and the western parts of 

the Highland Peak and Hayden Peak quadrangles (fig. 

40). The dikes occupy an east-west fracture system 

connecting the Snowmass pluton with the White Rock 

pluton and cutting both plutons. North of North 

Maroon Peak, dikes of this system reach 100 m in 

thickness. These dikes probably feed sills and laccoliths in the Maroon Formation. Sills are well exposed 

north of Maroon Lake, Maroon Bells quadrangle (fig. 

19). A laccolith at least 200 m thick forms the 4,065-m 

peak north of Willow Lake (fig. 40C). The largest body 

of this rock type is a sill along the east side of the 

canyon of Snowmass Creek (figs. 40A, B). Parts of that 

sill crop out in the northwestern part of the Maroon 

Bells quadrangle and in the southwestern part of the 

Highland Peak quadrangle. The sill is about 600 m 

thick and at least 9 km long. Biotite from this sill has a 

K-Ar age of 31 m.y., about 3 m.y. younger than the 

biotite age obtained on rocks from the White Rock 

pluton, and this is in agreement with the field relations 

between porphyritic granodiorite dikes and 

granodiorite of that pluton.

The porphyritic granodiorite is a light-gray rock containing conspicuous rare to moderately abundant 

grayish-orange pink-weathering phenocrysts of 

potassic feldspar that reach a length of 7 cm but are 

commonly 2-5 cm. The phenocrysts contain numerous 

inclusions of mafic minerals. White to light-yellowishgray plagioclase phenocrysts are generally 2-5 mm 

long. Light-gray quartz forms phenocrysts as much as 

1 cm in diameter but often is about 5 mm in diameter. 

Hornblende or chloritized hornblende forms 

phenocrysts 25 mm long but locally is as much as 1 cm 

long. Biotite or chloritized biotite forms phenocrysts 

as much as 3 mm in diameter.

Most of the intrusives of porphyritic granodiorite
66 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

-13% <

y •*.'-'

have chilled margins which resemble the hornblende 

porphyry dikes. In these margins the plagioclase and 

hornblende, and in some rocks the quartz, phenocrysts 

are the same size as those in the interior of the body, 

but the potassic feldspar phenocrysts are lacking. The 

groundmass is darker gray and finer grained in the 

chilled margin than in the interior of the body. The 

chilled margins range from 2 cm in smaller intrusives 

to 0.6-0.9 m thick in the larger ones. They occur even 

along some of the dikes intruding the granodiorite of

FIGURE 40.—Porphyritic granodiorite intrusives. A, Sill about 600 m 

thick in Maroon Formation on east side of Snowmass Creek, 

Capitol Peak quadrangle. Biotite from this sill dated as 31.2±1.1 

m.y. (Obradovich and others, 1969). B, Upper contact of sill 

shown in A locally cross cuts bedding in Maroon Formation, west 

edge of Highland Peak quadrangle. View encompasses about 100 

m. C, Laccolith forming 4,065-m peak north of Willow Lake, 

Maroon Bells quadrangle. Bottom contact partly concordant and 

partly discordant with red beds of the Maroon Formation. The 

rock glacier 0.8 km long is composed mostly of igneous rock from 

the cirque headwall and not of red beds from the valley sides.

the White Rock pluton, showing that the pluton had 

substantially cooled at the level now exposed before 

fracturing and emplacement of the dikes. In thin section the porphyritic granodiorite bodies have an unexpectedly fine grained groundmass with a grain size of 

0.01-0.04 mm (fig. 41). In the chilled zone the groundmass is cryptocrystalline and may contain some glass.

Quartz phenocrysts are locally euhedral, but many 

have numerous embayments (fig. 41), suggesting that 

the phenocrysts were partly resorbed by the remaining 

magma that crystallized to become the groundmass. 

Plagioclase phenocrysts are euhedral to subhedral and 

commonly altered (fig. 41). Compositions of An4o-55 

were found in unaltered grains. Where fresh, euhedral 

hornblende phenocrysts are dark olive green. They are 

mostly altered to Mg/Fe or Fe/Mg chlorite. Euhedral 

biotite phenocrysts show some evidence of partial 

resorption.

The potassic feldspar phenocrysts include quartz, 

plagioclase, sphene, and chlorite. They are generally 

euhedral. One dike had a potassic feldspar phenocryst
MIDDLE TERTIARY IGNEOUS ROCKS 67

showing well-developed orientation of included 

plagioclase grains parallel with the margin of the 

euhedral phenocryst (Hibbard, 1965).

Carbonate and sphene are often associated with 

chlorite from alteration of the mafic minerals. Apatite, 

zircon, and magnetite are accessory minerals. Allanite 

was noted in one sample. Epidote is a secondary 

mineral in some of the porphyritic granodiorite.

HORNBLENDE PORPHYRY

Hornblende porphyry dikes and sills were mapped in 

the western part of the Maroon Bells quadrangle and 

the southwestern part of the Highland Peak 

quadrangle. They generally range from 0.6 to 6 m thick 

and are various shades of green and gray. They contain 

hornblende phenocrysts, in places pseudomorphosed 

by chlorite, mostly 3-5 mm but as much as 1 cm long; 

plagioclase phenocrysts 3-5 mm long; and, in some 

rocks, quartz phenocrysts or xenocrysts as much as 5 

mm in diameter. Phenocrysts or xenocrysts of potassic 

feldspar were observed in one sill.

Plagioclase phenocrysts have compositions ranging 

from An65-An2 7 and display normal and oscillatory 

zoning. Plagioclase also occurs as 0.05-1-mm-long

FIGURE 41.—Photomicrograph of porphyritic granodiorite from sill 

60 m thick north of Maroon Lake, Maroon Bells quadrangle. 

Saussuritized euhedral phenocrysts of plagioclase and partly 

resorbed quartz phenocrysts in a matrix of fine-grained quartz 

and plagioclase. Rock also contains euhedral phenocrysts of 

potassic feldspar and chloritized biotite and hornblende (not 

shown). Analyzed sample 11, table 6.

FIGURE 42.—Photomicrograph of hornblende porphyry from a 6-mthick sill on ridge between East Maroon and West Maroon 

Creeks, Maroon Bells quadrangle. Euhedral to subhedral hornblende phenocrysts and partly resorbed quartz phenocrysts in a 

fine-grained plagioclase-rich groundmass. Analyzed sample 3, 

table 6.

laths in the groundmass. In many of the hornblende 

porphyry bodies, the plagioclase is completely altered 

to albite, sericite, and carbonate.

Quartz occurs as resorbed phenocrysts or xenocrysts 

(fig. 42) as much as 4 mm in diameter, some of which 

have reaction rims of devitrified glass(?). Quartz also 

occurs in the groundmass, where it has a grain size of 

about 0.02 mm.

Hornblende occurs as euhedral to subhedral 

phenocrysts (fig. 42) and is brown or greenish brown to 

olive green. Most hornblende is pseudomorphosed by 

green chlorite having almost no birefringence.

A small amount of monoclinic pyroxene occurs in 

subhedral grains 0.1-0.5 mm in diameter, and it is 

partly altered to hornblende. Biotite occurs as 

subhedral phenocrysts up to 0.5 mm in diameter and in 

the fine-grained groundmass.

Opaque minerals are magnetite and pyrite. Apatite 

and sphene are accessory minerals, and carbonate, 

sericite, and epidote are common secondary minerals.

ALASKITE

A few dikes of alaskite only several centimeters 

thick cut the granodiorite. No pegmatites were noted. 

The alaskites studied have a xenomorphic or 

hypidiomorphic-granular texture and a grain size of
68 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

0.2-2 mm. They are composed of anhedral to subhedral 

quartz (35 percent), anhedral to subhedral albite (20 

percent), and anhedral perthitic potassic feldspar (45 

percent). The feldspars are somewhat altered. Dikes of 

similar texture and composition are numerous in the 

Paradise stock southwest of the Aspen area (F. E. 

Mutschler, written commun., 1970).

COMPOSITION

A plot of modal proportions of quartz, plagioclase, 

and potassic feldspar of fresh to partly altered rock 

lacking fine-grained groundmass shows a range from 

diorite to quartz monzonite (fig. 43). The two alaskite 

samples plot in the granite field, but there is a significant gap between them and the quartz monzonite 

samples, which plot very close to the granodiorite field. 

Color index ranges from 0 in the alaskite to about 40 in 

one quartz diorite.

Chemical analyses (table 6) show that the middle 

Tertiary igneous rocks of the Aspen area have SiO2 

contents ranging from 55 to 70 percent when 

recalculated to 100 percent after deduction of volatiles. 

The hornblende porphyry dikes and sills (analyses 2, 3, 

5) fall in the more mafic end of the series in the 58-62 

percent SiO2 range, but mafic phases of the White 

Rock pluton (analyses 1, 4) have similar compositions. 

Similarly, analyses of porphyritic granodiorite 

(analyses 11, 13) with SiO2 of about 70 percent are not 

much different from analysis 13 of quartz monzonite 

from the White Rock pluton (SiO2 , 68.2 percent), but

Quartz EXPLANATION 

Color index 

• 0-10 percent 

+ 10-20 percent 

® 20-30 percent

* Analyzed sample

© Alaskite

50

Plagioclase 10 Potassic 

feldspar

FIGURE 43.—Proportions of quartz, plagioclase, and potassic feldspar in unaltered phaneritic middle Tertiary igneous rocks. 

Numbers of analyzed samples refer to columns in table 6.

the porphyritic granodiorite is higher in Na2O and 

poorer in K2O, CaO, and total Fe than the quartz monzonite. Accurate modes are difficult to obtain on the 

porphyritic granodiorite because of the fine grain size 

of the groundmass, the very large size of the potassic 

feldspar, and the alteration. However, the norms indicate that analyzed sample 13 falls well within the 

granodiorite field and analyzed sample 11 is just 

within it.

Analyses from the Lincoln Gulch stock and the ashflow tuffs south of Independence Pass (Candee, 1971; 

Cruson, 1972) are included because we dated the stock 

as middle Tertiary (Obradovich and others, 1969), and 

the ash-flow tuffs probably represent a more differentiated facies of the same magma. Their SiO2 content is 

within the range of similar tuffs in the San Juan Mountains (Lipman and others, 1969). Various oxides plotted against SiO2 show that FeO, MgO, TiO2, and CaO 

decrease markedly with increasing SiO2; but relatively 

little change occurs in the alkalis except in the case of 

the ash-flow tuffs and, we may infer, the alaskite.

Well-developed variation trends are shown in 

triangular plots based on the chemical analyses (fig. 

44). On the AKF diagram the trend is from the F corner to the AK line, the ash-flow tuffs being the only 

points lying beyond the center of the diagram (fig. 

44Q. In the ACF diagram the trend is away from the F 

corner to the AC line (fig. 44B), and in the AlkFM 

diagram the trend is from the middle of the diagram 

toward the Alk corner (fig. 44A).

Semiquantitative minor element analyses suggest 

that the more silicic (>65 percent SiO2) rocks of middle 

Tertiary suite are richer in minor elements than the 

silicic Laramide porphyries (tables 5, 6) and contain 

more Co, La, Mo, Sc, V, Zr, Ce, Yb, and Nd. These differences are not obvious in the more mafic rocks.

Plots of normative Pl-Qz-Or (fig. 44D) define a trend 

similar to that shown by Bateman and others (1963) 

for Laramide stocks of Colorado and New Mexico except for a change in trend towards the Qz-Or line defined by the analyses of the ash-flow tuffs of Independence Pass. The trend extends away from the PlOr line, as do many other suites from inside the continent plotted by Bateman and others. The suites from 

near the continental margin have trends extending 

away from the PI corner. These differences are due to 

differences in composition of the initial melts, which 

may be related to origin of the stocks from different 

crustal levels. Lipman and others (1972) suggested 

that two imbricate gently east dipping Benioff zones 

may have controlled the formation of early and middle 

Tertiary magmas. This suggestion, based on K2O content of igneous suites of 60 percent SiO2 (Hatherton 

and Dickinson, 1969), would put the depth of forma-
MIDDLE TERTIARY IGNEOUS ROCKS 69

K20 + N320 MgO MgO+FeO+MnO

(Na20+K20)

AI2 03+Fe203- 

(CaO+Na2 0+K2 0)

MgO+FeO+MnO Plagioclase D

Orthoclase

FIGURE 44.—Chemical and normative diagrams of analyzed middle Tertiary igneous rocks. A, AlkFM diagram; B, ACF diagram; C, AKF 

diagram; D, Normative plagioclase (Ab+An)-quartz-orthoclase diagram; line is variation trend of analysis of table 6. Dot, Aspen area; 

circled dot, Independence-Lincoln Gulch area.

tion of the Elk Mountain igneous rocks at about 230 

km on the east end of the two Benioff zones postulated 

by Lipman and others (1972). Some objections to this 

hypothesis have been stated by Gilluly (1971).

Dickinson (1975) pointed out that the relation between K2O content of magmas erupted in volcanic arcs 

and depth to associated seismic zone differs depending 

on whether the arc is on a continental margin or on 

oceanic crust. Continental margin arcs have magmas 

with higher K2O contents for a given SiO2 content than 

oceanic arcs. At seismic zone depths of 80-160 km, for 

which many data are available, K2O content at 57.5

percent SiO2 reaches 0.6 percent for continental 

margin arcs alone (Dickinson, 1975, fig. 2). Stated 

another way, depth to the seismic zone below arcs having a given K2O content covers a range of ±20 km for 

continental margin volcanic arcs. Few data are 

available from active volcanic arcs over seismic zones 

more than 200 km deep. The depth of formation of the 

magmas of the Elk Mountains would be in the range of 

230-270 km according to reasonable projections of the 

data of Dickinson (1975, fig. 2), assuming a mechanism 

of formation along a Benioff zone. 

Doe (1968) summarized evidence from lead and
70 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO 

TABLE 6.—Chemical analysis, modes, and norms of middle Tertiary igneous rocks

[Major oxides determined by rapid methods by P. L. D. Elmore, G. W. Chloe, Hezekiah Smith, Lowell Artis, J. L. Glenn, James Kelsey, and Samuel Botts, U.S. 

Geological Survey, 1967 and 1969. Minor elements determined by semiquantitative spectrographlc methods by Joseph L. Harris, U.S. Geological Survey, 

1967, and William B. Crandell, U.S. Geological Survey, 1968. Mode by point count; P, present but not intersected in counting; leaders (——), not 

present; (1), mode not available. Major oxides and C1PW norms in weight percent; mode in volume percent. C1PW norms calculated on the basis of analyses 

recalculated to 100 percent after deduction of volatiles. Results for minor elements are reported in parts per million to the nearest number in the 

series 100, 70, 50, 30, 20, 15, and 10, ,.., which represent approximate midpoints of interval data on a geometric scale. The assigned interval for 

semiquantitative results will include the quantitative value about 30 percent of the time. N, not detected. Looked for but not found: Ag, As, Au, B, 

Bi, Cd, Ge, Hf, Hg, In, Li, Pd, Pt, Re, Sb, Sn, Ta, Te, Ti, U, W, Zn]

White R

1 

W171454 

735

2 

W171460 

1970

3 

W168733 

87

4 

W171452 

WR-1

ock

5 

W168735 

75a

pluton

6 

W168730 

DA-1

and associE

7 

W171

Major o

P2 0 5 —— ——— ———— ——

a

54.6 

16.7 

2.5 

5.7

3.9 

6.9

2.8 

3.0

.12 

1.4 

1.1 

.38

.14 

.59 

2.76 

2.84

1,500 

N 

30 

20

20 

100 

5 

7

15

15 

30 

1,500

200 

30 

150 

100

20 

3 

100

57 

16 

3 

3

2 

3 

5

1

2

1 

2 

2

1,500 

15 

3 

15

100 

3 

5

N

N 

5 

15 

1,000

70 

30 

200 

150

20 

3

N

.0 

.8 

.6 

.2

.9

.1 

.7 

.8

.34 

.5 

.71 

.55

.20 

.6 

.63 

.76

57.4 

15.8 

3.8 

3.7

4.2 

4.7 

4.3 

2.8

.32 

1.2 

.88 

.00

.05 

.38 

2.69 

2.76

1,000 

N 

20 

30

5 

N 

3 

7

30 

2 

15 

500

150 

30 

70 

N

15 

2

59.

15. 

4. 

4.

2. 

5. 

2. 

2.

1.

2. 

2.

2,000 

N 

20 

30

10 

100 

5 

7

15 

5 

15 

1,500

200 

20 

300 

150

20 

2 

N

4 

3 

9 

0

7 

8 

9

4

03 

71 

0 

57

11 

05 

72 

74

59. 

16. 

3. 

3.

2. 

4. 

3. 

2.

1.

1. 

2. 

2.

1,000 

N 

10 

10

30 

70 

3

N

<30 

3 

10 

700

70 

30 

100

N

15 

2 

N

5 

5 

4 

2

3 

3 

5 

8

32 

9 

76 

00

03 

2 

70 

75

62,6 

15.8 

3.6

2.4

1.3 

4.4 

3.7 

3.0

.29 

1.0 

.52 

.24

.06 

.84 

2.71 

2.72

Mi

1,000 

N 

N 

N

1 

100 

3 

N

N 

2 

7 

500

30 

50 

150 

200

15 

3

N

64 

16 

3 

2

4 

3 

2

2 

2

453 

2

xides

1 

8 

5 

0

90 

7 

4 

7

10 

70 

48 

43

02 

09 

61 

75

ted dikes and sills

8 

W169478 

462 (M)

66.1 

16.2 

2.2 

1.8

1.2 

3.1 

3.8 

3.7

.20 

.69 

.51 

.26

.03 

.19

——

9 

W169479 

463 (M)

66.7 

16.1 

2.2 

1.8

1.2 

3.2 

3.6 

3.6

.10 

.39 

.53 

.26

.04 

.18

10 

WI69477 

461 (M)

67. 

15. 

2. 

1.

3. 

3. 

3.

6 

6

0 

7

90 

0 

4 

8

15 

85 

41 

29

03 

18

11 

W168731 

280

67.9

14.4 

2.0 

1.6

.68 

2.7 

3.7 

3.6

.35 

.85

.44 

.00

.03 

.93 

2.66 

2.68

12 13 

W168729 W167853 

501 389 (M)

68.1 69.5 

15.2 15.7 

2.6 1.5 

1.5 1.2

.68 .65 

3.6 3.3 

3.1 3.8 

3.9 2.7

.10 .07 

.49 .43 

.42 .37 

.14 .27

.07 .00 

<.05 <.05

2.69

Lincoln Gulch stock and 

volcanics of Grizzly Peak

14 15 

W171451 W171

61.2 73 

16.8 14 

3.2 1 

1.7

.96 

4.6 2 

3.0 2 

3.0 3

.63 

1.5 

.62 

.32

.08 

2.0 

2.63 2 

2.68 2

449 

10

.2 

.8 

.2 

.40

.23 

.0 

.7 

.9

.23 

.87 

.24 

.09

.06 

.05 

.59 

.66

16 

W171450

74.8 

13.8 

.78 

.20

.00 

.84 

3.4 

5.0

.20 

.63 

.15 

.10

.03 

<.05 

2.37 

2.76

nor elements

2,000 

1 

5 

3

20 

70 

3 

7

N 

7 

10 

1,500

30 

30 

200 

100

20 

3

N

1 ,000

1

N 

N

5 

200 

7 

10

N 

2 

10 

500

50 

70 

200 

700

10 

7 

200

1,000 

1 

70

N

3 

100 

3 

10

<30 

1 

10 

700

50 

50 

300 

500

10 

5 

N

1,000 

1

N 

5

2 

100 

N 

10

N 

3 

7 

500

50 

50 

200 

500

10 

5

N

700 

1 

N 

<3

3 

50 

3 

10

N 

2 

5 

300

30 

30 

150 

100

10 

2 

N

1,000 1,500 

1 3 

7 5 

7 N

50 3 

100 150 

N 3 

10 10

N N 

5 30 

5 7 

700 1,000

30 30 

50 20 

500 100 

200 200

15 20 

3 2 

N 150

2,000 2,000 

1 2 

10 N 

15 3

5 5 

200 150 

3 N 

7 10

5 N 

15 20 

10 7 

1,500 1,000

70 15 

30 30 

500 200 

300 150

20 15 

3 3 

200 100

1,000 

1.5

N 

N

1 

70 

N 

15

N 

30 

5 

300

7 

30 

50 

100

15 

3

N

Modes

Potassic feldspar —

Opaque mineral — ——

Points counted — ——

Q ___________ __ ______

8.1 

49.6 

9.4

1.2

3.5

654

5.97

18. 14 

24.25

24.70

9.94 

7.01

3.71

2.14 

,92

6.37

13.83 

8.11 

5.72

1 

81

13

4 

P

729

6

1 

11 

50

12

7 

2

5

1

1

9

7 

2

.4 

.6

.7

.3

.89 

.21 

.13 

.47

.33

.56 

.20

.46

.41 

.36

.76 

.56 

.20

1.7 

8.7

24.0

57.0 

2.0

.6 

3.8 

1.5

.6

.1

710

6.12

16.95 

37.27

15.92 

3.33 

10.71 

2.35

5.64

1.71

6.33 

3.33 

2.46 

.54

10.06 

8,25 

1.81

12. 

53. 

18. 

2.

11.

——

2.

___

P

765

18.

14. 

24.

21. 

1. 

6. 

1.

7.

1.

1.

2. 

L. 

1.

7. 

5. 

1.

5 

5 

0 

6

3

7 

3

1

57

31 

77

84 

44 

79 

87

17

92 

36

74 

44 

03 

28

35 

76 

59

0. 

22.

1.

56.

12.

3.

P

P 

3.

680

15.

17. 

30.

21.

5. 

1.

5.

1.

7. 

5. 

1.

1 

0

5

4 

2

3

4

4 

8

57

18 

76

85 

13 

95 

94

12

50

24 

13 

09 

03

77 

86 

91

17.0 

54.0 

13.0 

6.7

4.5 

5.1

P

672

19.63

18.16 

32.07

18.07 

1.12 

3.32 

.70

5.35

1.01 

.58

2.13

1.12 

.83 

.18

3.01 

2.48 

.53

4 

31

4

56

1

P

809

CIPW N

24

16 

29

20

2 

5

1

2 

2

2 

8

6

7

8 

5

2 

2

orms

.01 

.78 

.11 

.05

.71

.26 

.03

. 12

.92 

.03

.29 

.26 

.03

23.4 

48.3 

20.6 

4.4

1.4 

1.4

. 1 

,3

.1

708

22.06 

.94 

22.11 

32.51

13.83

3.02 

.71 

3.23

.98 

.62

3.73 

3.02 

.71

27.0 

42.4 

22.2 

3.3

1.3

.3

2.4

P 

.9

P

673

23.94 

1.09 

21.44 

30.70

14.29

3.01 

.69

3.21

1.01 

.62

3.70 

3.01 

.69

22. 

41. 

25. 

5.

1. 

2.

P

P

1. 

710

26. 

1. 

22. 

29.

13.

2. 

2.

3. 

2.

& 

5

8

1

6

3 

0

1

1

0

28 

15 

74 

14

16

27 

86

94

79 

70

13 

27 

86

13.8 

12.1 

38.8

26.0 

6.5

,1 

1.2 

.7

.2

1.7

——

990

26.60

21.92 

32.26

12.42 

.58 

1.75 

.63

2.99

.86

1.11 

.58 

.39 

.14

1,85 

1.35 

.49

30.7 (1)

20.0 

8.4

. 1 

1.0

.1 

.5

P ——

658

27.17 29.93 

1.19

23.21 16.12 

26.41 32.48

16.15 14.76

1.71 1.64 

.04 .36

3.80 2.20

.80 .71 

.33 .65

1.42 1.99 

1.38 1.64 

.04 .36

18.9 13 

46.7 5 

20.6 6 

5.5 2

72

3,6 

—— P

.1 

2.7

1.9 P

631 618

22.92 38 

1.07 2 

18.57 23 

26.59 23

21.71 9

2.50

4.13 

.50 

1.23 

.79

2,50 

2.50

.4 

.0 

.7 

.3

.6

-

_

.65 

.75 

.32 

.12

.45

.58

.80 

.66 

.46 

.22

.58 

.58

24.1 

5.6 

.4 

.7

68.7

.7

__

P

755

34.68 

1.52 

29.82 

29.03

3.55

.31

.57 

.29

.24

——
MIDDLE TERTIARY IGNEOUS ROCKS 

TABLE 6.—Chemical analysis, modes, and norms of middle Teritary igneous rocks —Continued

71

White Pock plu

Field No. —— —— —— -

1 

W171454 

735

55. 

17. 

2. 

5.

3. 

7. 

2. 

3.

1.

87 

09 

56 

83

99 

06 

87 

07

13

39

14

2 3 

W171460 W168733 

1970 87

59. 

17. 

3. 

3.

3. 

3. 

5.

1.

65 

58 

77 

35

03 

24 

96 

88

74 

58

21

58.79 

16.18 

3.89 

3.79

4.30 

4.81 

4.40 

2.87

.90

.05

4 

V171452 

VR-1

Recalci

59.95 

15.44 

4.95 

4,04

2.73 

5.85 

2.93 

2.42

1.01 

.58 

. 11

5 

W16R734 

75a

ilated to

61.79 

17.14 

3.53 

3.32

2.39 

4.47 

3.63 

2.91

.79

.03

ton and associated

6 7 

W16R730 W171453 

PA-1 WR-2

100 perce

64. 

16, 

3 

2.

1. 

4. 

3 

3

13 

19 

69 

46

33 

51 

79

07

53 

25 

06

nt after

64.73 

16.96 

3.53 

'..02

.91 

4.75 

3.43 

2.73

.48 

.43 

.02

dikes and sills — Continued

P 9 

W169478 H16«479 

462 W 4P3(M)

dedu

66 

16

2

1

1 

3 

3 

3

ction

.84 

.38 

.22 

.82

.21 

.13 

,84 

.74

.52 

.26 

.03

of vol<

67.22 

16.22 

2.22 

1.81

1.21 

3.22 

3.63 

3.63

.53 

.26 

.04

10 11 12 

W169477 W168431 W168729 

4P1 (M) 280 501

itiles

68.47 69.96 68.59 

15.80 14.84 15.21 

2.03 2.06 2,62 

1.72 1.65 1.51

.91 .70 .68 

3.04 2.78 3.63 

3.44 3.81 3,12 

3.85 3.71 3,93

.42 .45 .42

.03 .03 .07

Lincoln Gulch stock and vo 

of Grizzly Peak— Contir

13 14 15 

W167853 W171451 W171449 

3R9(M) NY-S IP-10

70.21 64.10 74 

15.86 17.60 14 

1.52 3.35 1 

1.21 1.78

.66 1.01 

3.33 4.82 2 

3.84 3.14 2 

2.73 3.14 3

.37 .65 

.27 .34

07 

98 

21 

40

23 

02 

73 

95

24 

09 

06

Icanics 

ued

IP 

W171450 

NY -4

75.48 

13,93 

.79

.20 

.85 

3.43 

5.05

.15 

.10 

.03

SAMPLE DESCRIPTIONS

1. Dark-gray amphibole-pyroxene biotite granodiorite. Euhedral to subhedral normally zoned plagioclase as much as 1 mm long (An55_3o), quartz and potassic 

feldspar interstitial to plagioclase, anhedral biotite as much as 0.5 mm in diameter; subhedral to anhedral augite as much as 1 mm in diameter partly 

altered to fine-grained amphibole. Apophysis of White Rock pluton. From adit at 3,901 m altitude 300 m N. 65° W. from the Montezuma mine, Hayden Peak 

quadrangle.

2. G reenish-gray dike rock mapped as hornblende porphyry. Euhedral plagioclase grains as much as 1 mm long altered to serl 

uartz, and chlorite after hornblende. From 3,700 m altitude 640 m N. 70° E. from spot elevation 3,987 m on Willow C 

ear uthern margin of Highland Peak quadrangle.

nd carbonate, interstitial 

Creek-East Snowmass Creek divide

3. Gray porphyritic rock mapped as hornblende porphyry. Euhedral to subhedral alined hornblende needles as much as 1 cm long, partly altered to chlorite; 

euhedral to subhedral plagioclase as much as 1.5 cm in diameter, saussuritized; quartz in resorbed phenocrysts or xenocrysts as much as 2 mm in 

diameter. Fine-grained groundmass is rich in plagioclase. From a sill about 6 m thick at 3,926 m altitude on the ridge between East Maroon and West 

Maroon Creeks 732 m north of where Rustler Gulch drainage joins the two drainages of the two branches of Maroon Creek, Maroon Bells quadrangle.

4. Gray biotite-hornblende granodiorite, Euhedral to subhedral plagioclase as long as 1,5 mm, oscillatory zoning in a normal trend Ango_2o; interstitial 

potassic feldspar as much as 0,6 mm long, interstitial quartz as much as 0.4 mm in diameter, subhedral to anhedral hornblende as much as 3 mm long; 

anhedral biotite to 3 mm in diameter. From apophysis of the White Rock pluton at 3,426 m altitude on Pearl Pass jeep trail about 152 m southwest of 

junction with the Montezuma mine roi.d, Hayden Peak quadrangle.

5. Gray porphyritic dike ro 

phenocrysts of hornblende 

groundmass of plagioclase,

k mapped as hornblende porphyry. Euhedral to subhedral plagioclase phenocrysts as much as 2 mm long ranging Ant) 5-27, 

as much as 7 mm long pseudomorphosed by chlorite, euhedral phenocrysts of biotite as much as 0.5 mm in diameter in a 

carbonate, sericite, and quartz. From dike at the summit of Maroon Peak, Maroon Bells quadrangle.

6. Light-gray biotite granodiorite. Euhedral to subhedral plagioclase 0.5-2 mm long with oscillatory zoning and 

partly altered; interstitial quartz 0.5-0.1 mm in diameter, interstitial potassic feldspar; subhedral biotite a 

chloritized. From margin of White Rock pluton at 3,633 m altitude at portal of adit 90 m east of labeled mine on 

S. 70° W. from 4,012-m peak on East Maroon-Rustler Gulch divide, Maroon Bells quadrangle.

ning. An52-32, sodic rims, 

0.5 mm in diameter, partly 

side of Rustler Gulch 640 m

7. Light-gray porphyritic biotite granodiorite. Euhedral plagioclase phenocrysts as much as 1.8 mm long An55-30 with o 

euhedral phenocrysts of quartz as much as 1 mm in diameter, euhedral phenocrysts of biotite as much as 2.4 mm in 

hornblende as much as 0.6 mm long in a very fine grained groundmass of quartz, feldspar, and opaque minerals. Con 

diorite 3 mm in diameter. White Rock pluton. From roadcut at 3,499 m altitude on road to Montezuma mine, Hay

33.9+1.0 TT.Q-M.fl m.v. m.y. fOKrarln\H (Obradovich r*h and anH others, rtt-horc 1969). IQAtn

as much as 1.8 mm long An55-30 with oscillatory zoning in a normal trend;

diameter; subhedral phenocrysts of 

tains inclusion of hornblende quartz 

Hayden Peak quadrangle, Biotite dated

8. Porphyritic biotite granodiorite. Subhedral to euhedral plagioclase phenocrysts and glomerophenocrysts as much as 3 mm in diameter with oscillatory 

zoning in a normal trend An^g-jg; subhedral partly resorbed phenocrysts of quartz as much as 1.8 mm in diameter; subhedral to euhedral phenocrysts of 

biotite as much as 1.5 mm in diameter in a groundmass of potassic feldspar and quartz with a grain size of 0.04-0.1 mm. White Rock pluton. From cut in 

trail 457 m north of Copper Lake; Maroon Bells quadrangle. Collected by F. E. Mutschler.

9. Porphyritic biotite quartz monzonite. Euhedral to subhedral phenocrysts and glomerophenocrysts of plagioclase as much as 3 mm in diameter with 

oscillatory zoning in a normal trend Ango_3 8 ; subhedral resorbed phenocrysts of quartz as much as 3 mm in diameter; subhedral to euhedral phenocrysts of 

biotite as much as 1.5 mm in diameter, in a groundmass of quartz, potassic feldspar and albite with a grain size of 0.05-0.2 mm. White Rock pluton. 

From cut along trail 460 m north of Copper Lake, Maroon Bells quadrangle. Collected by F. E. Mutschler.

10. Porphyritic biotite quartz monzonite. Euhedral to subhedral phenocrysts and glomerophenocrysts of plagioclase as much as 3 mm in diameter. Anss, 

locally saussuritized; subhedral, partly resorbed phenocrysts of quartz as much as 1.8 mm in diameter; subhedral to euhedral phenocrysts of biotite as 

much as 1.5 mm in diameter in a groundmass of quartz, plagioclase, and potassic feldspar with a grain size of 0.05-0.1 mm. White Rock pluton. From cut 

along trail 460 m north of Copper Lake, Maroon Bells quadrangle. Numbers 8, 9, and 10 were collected by F. E. Mutschler to illustrate the variations 

within an 100-meter-square area.

11. Porphyritic granodiorite. Euhedral phenocrysts of plagioclase as much as 1 cm long, saussuritized; subhedral phenocrysts of quartz as much as 7 mm in 

diameter, partly resorbed; euhedral phenocrysts of potassic feldspar as much as 2.5 cm long have numerous inclusions of quartz, plagioclase, sphene, and 

chlorite. Chloritized hornblende phenocrysts as much as 5 mm long; chloritized biotite phenocrysts; groundmass of quartz and plagioclase. From sill at 

3,377 m altitude 1,130 m N. 47° W. of outlet of Maroon Lake, Maroon Bells quadrangle.

12. Gray biotite quartz monzonite. Subhedral plagioclase as much as 4 mm long with some oscillatory and normal zoning Ani,;_38, partly saussuritized; 

generally anhedral quartz to 6 mm in diameter; potassic feldspar as much as 6 mm long includes quartz, plagioclase, and biotite; biotite as much as 

1.5 mm in diameter. From White Rock pluton; dump of prospect at 3,109 m altitude on east side of Conundrum Creek valley 1,220 m N. 17° E. of Silver 

Dollar Pond, Maroon Bells quadrangle.

13. Porphyritic granodiorite. Carlsbad-twinned phenocrysts of potassic feldspar as much as 3 cm long and of quartz, plagioclase, and biotite as much as 

1 cm in diameter in a very fine grained groundmass. Snowmass Creek sill NE 1/4 NW 1/4 sec. 17, T. 10 S, R. 86 W. at bottom SE side Snowmass Creek 

valley, Capitol Peak quadrangle. About 1.6 km west of the center of the west edge of the Highland Peak quadrangle and in the same sill that crops out 

in the southwestern part of the Highland Peak quadrangle and the northwestern corner of the Maroon Bells quadrangle. Collected by F. E. Mutschler. 

Biotite dated at 31.2+1.1 m.y. (Obradovich and others, 1969).

14. Porphyritic granodiorite. Euhedral to subhedral plagioclase phenocrysts as much as 5 mm long, Ani^, partly altered; subhedral partly resorbed 

phenocrysts of quartz as much as 5 mm in diameter, subhedral phenocrysts of biotite as much as 3.5 mm in diameter; in a groundmass of potassic feldspar 

and quartz with a grain size of 0.01-0.04 mm. From chilled margin of Lincoln Gulch stock. From New York collection canal J80 m east of Tabor Creek, 

New York Peak quadrangle. Biotite dated 33.9+1.1 m.y, (Obradovich and others, 1969).

15, Quartz latitic welded tuff. Anhedral to subhedral fr 

phenocrysts of biotite as much as 1.5

Quartz latitic welded tuff. Anhedral to subhedral fragments tsof quartz, quartz, sanidine, sanidine, and plagioclase plagioclase as much as 2 mm in diameter diameter and euedra 

phenocrysts of biotite as much as 1.5 mm in diameter in a very fine grained to glassy matrix. A few xenocrysts of microcline. Col 

fragments. Grizzly Mountain Rhyolite of Stark and Barnes (1935). From outcrop on jeep trail to Green Mountain 850 m N. 75° W. from 

Colorado Highway 82 west of Independence; Independence Pass quadrangle.

uhedral to subhedral 

Collapsed pumice 

BM 10,831 on

16. Rhyolitic Rhyoliti welded tuff. Anhedral to subhedral quartz, sanidine, and plagioclase as much as 2.5 mm in diameter and euhedral to subhedral biotite as mu 

as 0.4 mm in diameter in a very fine grained to glassy matrix. Collapsed pumice fragments. Grizzly Mountain Rhyolite of Stark and Barnes (1935). Fr 

New York collection canal 140 m east of labor Creek, New York Peak quadrangle.
72 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

strontium isotopes bearing on the origin of the 

Cretaceous and Tertiary igneous rocks of the Rocky 

Mountain region. They cannot have been derived by 

melting of Precambrian rocks of the upper crust such 

as are now exposed in uplifts, including the Sawatch 

Range, but they may be derived from the lower crust 

or the mantle.

The gravity map of Colorado (Behrendt and Bajwa, 

1972) shows a well-defined low along the Colorado 

mineral belt extending as far west as the White Rock 

pluton. Analysis of this low (Case, 1965; Isaacson and 

Smithson, 1976) indicates that it is caused by a 

batholithic mass. The intrusives visible at the surface 

may be offshoots of this mass emplaced at somewhat 

different times and having different cooling histories, 

related to their depth of origin in the batholith, their 

size, the temperature of the wallrock into which they 

were emplaced, and whether or not they vented.

TERTIARY AND QUATERNARY 

DIABASE DIKES

Two dikes of fine-grained diabase were found in the 

western part of the Aspen quadrangle. One dike is 

about 2.4 m thick and crops out on the Maroon Creek 

road just east of the bridge across the creek, and the 

other is exposed at 2,940 m altitude 1.5 km to the 

south. At the latter locality the diabase contains 

numerous rounded white inclusions of limestone and a 

few inclusions of quartz rimmed with carbonate. Alinement of plagioclase grains in that rock produces a 

trachytic texture.

The diabase consists of euhedral to subhedral laths 

of plagioclase An SQ.6S which are 0.5-1 mm long. Tan 

monoclinic pyroxene 0.1-0.3 mm in diameter occurs in 

the interstices of the plagioclase grains and locally 

forms phenocrysts as much as 1 mm long. Euhedral to 

subhedral magnetite 0.04-0.2 mm in diameter appears 

to have crystallized at the same time as or earlier than 

the plagioclase. Some biotite has formed by alteration 

of the pyroxene, and sericite and carbonate, by alteration of plagioclase. Some other very fine grained 

unidentified alteration products occur.

These diabase dikes are probably related to the upper Tertiary to Quaternary basalts of the region. The 

nearest mapped basalt flows are 16 km to the north 

near Woody Creek (Freeman, 1972a), and these have 

been dated as about 1.5 m.y. or early Pleistocene (Larson and others, 1975). That older basalts may have 

been present in the drainage of the upper Roaring Fork 

River is indicated by the presence of basalt in a highlevel gravel at the western edge of the Woody Creek 

quadrangle (Freeman, 1972a).

METAMORPHISM

The Oligocene plutons have produced contact 

metamorphism whereas the small Laramide plutons 

have not appreciably metamorphosed their wallrock. 

The dikes, sills, and laccoliths of Oligocene age generally metamorphosed their wallrocks for distances of 

several centimeters to about 20 m from their margins. 

The White Rock pluton, on the other hand, has a contact aureole ranging in map width from 100 m to 3 km. 

If my cross sections are approximately correct 

(Bryant, 1969, 1970), the greatest thickness of contact 

metamorphic rocks in a direction perpendicular to the 

contact of the pluton would be about 1 km.

The extent of contact metamorphism is very obvious 

in the field because most of the rocks affected are red 

beds of the Maroon Formation (figs. 45.A, B). Upon 

metamorphism they were converted to green and gray 

calc-silicate hornfels and calc-silicate-bearing sandstone. The transition between red beds and hornfels in 

the contact aureole of the White Rock pluton is 

gradual through a hundred to a few hundred meters. 

The sandstone beds are most easily bleached, perhaps 

because of their greater permeability allowing heat to 

be transmitted more effectively either through intergranular fluids or gases. The impermeable mudstones retain their grayish-red color; whereas the interbedded sandstones have completely lost theirs. In this 

transition zone specular hematite, epidote, and 

tremolite are found on joint planes.

In areas of contact metamorphism, mapping the contact between the Maroon and Gothic Formations is 

very difficult, as the distinctive color of the Maroon is 

lost. Both the lower part of the Maroon and the Gothic 

contain limestones; thus, the presence or absence of 

marble cannot be used to separate the two formations 

in areas of contact metamorphism. The Gothic has a 

few beds of dark-gray shale or shaly limestone that 

tend to retain color upon metamorphism; however, the 

metamorphosed Maroon has a few beds of dark-gray 

biotitic hornfels that are similar in appearance. 

Overall, the Gothic is more calcareous and less 

feldspathic than the Maroon, but it is difficult to use 

these criteria in mapping based on individual outcrops. 

However, the Maroon contains cpnglomerates in 

many, but not all, areas and conglomerates are lacking 

in the Gothic north of the crest of the Elk Mountains 

near Aspen; therefore, in many areas of contact 

metamorphism in the Aspen area, the Maroon can be 

distinguished from the Gothic by the presence of conglomerates. Nevertheless, the contact of the Maroon 

and Gothic Formations is difficult enough to 

distinguish in areas of contact metamorphism so that 

there may be errors in the position of this contact, such
METAMORPHISM 73

FIGURE 45.—Areas of thermal metamorphism. A, Gradation from red beds (dark rocks on right) to hornfels (light rocks on left) of the Maroon 

Formation on the east side of the head of Conundrum Creek, Gothic and Maroon Bells quadrangles. Samples 6-13, table 3, collected in 

a traverse across this contact. B, Pyritized and fractured hornfels of the Maroon Formation west of Electric Pass (Hayden Peak 

quadrangle) forms abundant talus.

as at the south contact of the White Rock pluton in the 

Maroon Bells quadrangle and in the Conundrum Creek 

window area.

In general, the contact-metamorphosed Belden Formation retains its thin-bedded and carbonaceous 

aspects (fig. 16). However, locally the limestone-rich 

parts are made more obvious by metamorphism and 

consequently are mappable as a unit separate from the 

carbonaceous shale and siltstones, as in the Conundrum Creek window and near Cooper Fork. The marble 

unit is apparently stratigraphically below the hornfels 

unit. The marble and hornfels are in most places highly 

deformed (fig. 38A) so that stratigraphic thicknesses 

are unknown. On the east side of Conundrum Creek at 

about 3,010 m altitude some marble beds are as much 

as 10 m thick, and quarrying of them was once attempted. Such thick beds of marble have led some to 

map this unit as metamorphosed Leadville Limestone. 

Beds of that thickness are exceptional in the marble 

unit. Most of the marble beds are several centimeters 

to a few meters thick and contain beds of carbonaceous 

hornfels between them.

The typical metamorphic minerals of the hornfels 

and associated rocks are diopsidic pyroxene, amphibole (generally tremolite but ranging in composition to actinolite and hornblende in some rocks), 

garnet, idocrase, scapolite, and biotite.

In the metamorphic aureole sedimentary textures of 

the sandstones and siltstones are commonly well 

preserved. The metamorphic minerals developed from

the matrix between the clastic grains. Mudstones, 

shales, and limestones are completely recrystallized. 

In most of the metamorphosed rocks pyroxene, amphibole, and biotite have a grain size of 0.1 mm or less, 

but in a few those minerals have recrystallized more 

coarsely.

In many of the conglomerates, pebbles of dolomite 

have been converted to masses of green pyroxene and 

pebbles of limestone to yellowish-brown garnet. 

Limestone pods and lenses in siltstone beds have 

similarly been metamorphosed.

In many of the metamorphosed sandstones the 

plagioclase is albite, but in a few it has been 

reconstituted to form oligoclase or andesine. 

Microcline grains are locally recrystallized to a perthitic potassic feldspar, especially near igneous contacts. 

Recrystallized quartz having a mosaic texture is found 

in places close to the contact of the pluton.

Yellowish- to reddish-brown garnet (colorless to 

yellowish brown in thin section) tends to form porphyroblasts that in places give the hornfels a spotted 

aspect. Idocrase occurs as small porphyroblasts. In 

one rock idocrase surrounds a large garnet and appears 

to have formed by alteration of the garnet. 

Wollastonite was found in hornfels adjacent to the 

west contact of the granodiorite body on upper West 

Maroon Creek and in the Conundrum Creek area.

Aluminum-excess rocks are rare in the deposits of 

the Eagle basin. Only one sample of those studied contained an aluminum silicate mineral. That rock was
74 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

from the Belden Formation and contains andalusite, 

corundum, and potassic feldspar. The feldspar is heavily sericitized.

Magnetite and pyrite are the common opaque 

minerals. Pyrite in the Maroon Formation is concentrated in the zone of pyritized rock surrounding the 

centers of alteration in the granodiorite. Pyrite is abundant in unmetamorphosed rocks of the Belden Formation, and it is difficult to tell whether any additional 

pyrite has been introduced where Belden rocks are converted to hornfels. The hematite of the red beds is converted to magnetite upon metamorphism. Locally, 

magnetite is found in segregations several centimeters 

thick in places associated with garnet. In some of these 

segregations it has a bladed habit suggesting that it 

may be pseudomorphing specular hematite.

The most widespread porphyroblastic mineral in the 

hornfels is scapolite. It forms euhedral to anhedral 

commonly sieve textured porphyroblasts as much as 

several millimeters long. The porphyroblasts include 

metamorphic pyroxene as well as other minerals, so 

they formed late during the metamorphism. Optical 

study shows that most of the scapolite is sodic and has 

a meionite content of about 20 percent. In some rocks 

the scapolite has been altered to an isotopic mineral 

with n—1.484, which is probably sodalite.

In places the hornfels is vuggy and has open fractures; and euhedral crystals a few millimeters to a centimeter or two in size fill the vugs or line the fractures. 

These features are best developed at the head of Castle 

Creek on the south side of the granodiorite plug south 

of the Montezuma mine. Honey-yellow to reddishbrown garnet and dark-green pyroxene crystals are 

common minerals growing in the open spaces. In 

places magnetite accompanies the garnet. Locally, 

epidote grows in the vugs. Albite coats some fractures 

and occurs in some vugs. In one place scapolite (Me20) 

crystals 2 cm long are associated with coarse-grained 

calcite on a joint. Zeolites including chabazite, stilbite, 

natrolite, and thomsonite fill fractures and coat the 

other minerals.

I detected no well-developed zoning based on silicate 

minerals in the contact metamorphic aureole. The 

diverse mineral assemblages are distributed in the 

various beds throughout the area of contact metamorphism. One possible zoning that might be mapped is an 

outer aureole in which mudstones retain their grayishred color and interbedded sandstones are bleached.

CHEMISTRY OF METAMORPHISM

A suite of samples across a steep contact between 

red beds of the Maroon Formation and rocks metamorphosed by the White Rock pluton was collected on the

east side of the basin at the head of Conundrum Creek 

in the Gothic and Maroon Bells quadrangles, at about 

3,750 m altitude (fig. 45^4). I was not able to walk out 

individual beds, as I had hoped to; and if I had been 

able to, it is doubtful that individual beds in the 

Maroon Formation would have maintained a constant 

original chemical composition over the distance of 

1 km covered by the sampling traverse. At each sample site a sandstone bed and a siltstone bed were collected.

The rocks at sample site 1704 (table 3) showed no 

sign of metamorphism; rocks at 1705 were a little 

bleached but definitely unmetamorphosed; at 1706 

they were mostly gray and greenish gray but a few 

beds maintained grayish-red hues; rocks at 1707 were 

fully metamorphosed. The contact between red beds 

and hornfels as shown on the quadrangle maps was 

usually determined in the alpine country from a distant view of the complete section exposed rather than 

on detailed observation of individual outcrops. That 

explains why sample 1706, which is definitely 

metamorphosed, lies just outside the area shown as 

metamorphosed on the geologic quadrangle map.

Since I cannot claim to have sampled a single bed of 

uniform composition passing from metamorphosed to 

unmetamorphosed rocks, I must consider what 

changes took place in the suite of samples between the 

sample sites rather than individual samples. Three 

changes are most obvious. The first was loss of H2O. 

This process started before the red color of the 

sedimentary rocks was affected (table 3; fig. 46). The 

second change was reduction of iron: Fe2O3 (hematite) 

was reduced to Fe3O4 (magnetite), and the big change 

in Fe2O3/FeO ratio came at the big change in color from 

predominantly red rocks to predominantly gray rocks. 

This change in mineralogy is confirmed by polished 

section study of the analyzed rocks. However, in the 

metamorphosed rocks the Fe2O3/FeO ratio is less than 

that of magnetite (fig. 46) showing that FeO occurs in 

other minerals. Much of the FeO is in pyroxene, hornblende, and possibly garnet. The third change is loss of 

CO2 . For unmetamorphosed rocks of the Maroon Formation most of the Ca is in the mineral calcite, which 

occurs as clastic grains and cement. As the rocks 

become metamorphosed CO2 was driven off, and the Ca 

present became part of Ca and CaAl silicate minerals, 

such as pyroxene, garnet, and plagioclase (table 3). 

Consequently, the Ca/CO2 ratio is an indicator of the 

extent to which CO2 has been expelled. The unmetamorphosed red beds have a ratio Ca/CO2 of 1.3, which 

is the ratio in the mineral calcite. This indicates that 

practically all the Ca in those rocks is in calcite. As CO2 

was driven off with increasing metamorphism, the ratio increased (fig. 46).
METAMORPHISM 75

1704 1705

UNMETAMORPHOSED 

RED

SAMPLES

1706

METAMORPHOSED 

GRAY AND GREEN

1707

FIGURE 46.—Chemical changes in red beds of the Maroon Formation during contact metamorphism. Loss of CO2 , H2O+, and O2 , 

as indicated by reduction of Fe, occurs with metamorphism.

Sandstones lost H2O and CO2 proportionately more 

than did the siltstones. These volatiles lost during contact metamorphism may have contributed to formation of veins in overlying rocks and to deposition of 

minerals in fractures in adjacent unmetamorphosed 

rocks.

CONDITIONS OF METAMORPHISM

The mineral assemblages of the contact metamorphic rocks related to the White Rock pluton are those 

of the hornblende hornfels facies. Turner (1968, p. 17) 

pointed out that aureoles in calcareous rocks, such as 

those of the Eagle basin, show great mineralogic variation and that zoning in them is usually obscure. The

chemical composition and permeability of the individual beds determine the mineral assemblage. 

These factors will cause variations in Pn2O and PcO2, 

which in turn will cause variations in mineral 

assemblages containing calcium and magnesium 

silicates. These variables have a greater effect on 

mineral paragenesis than PTotal- (See diagrams by 

Winkler (1967, p. 34, 35, 73).)

Fluid inclusion studies mentioned earlier (p. 63) indicate that the late hydrothermal alteration of the 

White Rock pluton occurred perhaps a million to a few 

million years after emplacement of the pluton at the 

level of present exposure took place at about 120 bars 

pressure or a depth of 1,200 m assuming hydrostatic 

conditions. The present altitude of most of the samples 

studied was 3,350-3,540 m, which would suggest that 

the land surface at the time of hydrothermal alteration 

was not far above the highest of the present peaks. 

Even assuming considerable erosion during the interval between magma consolidation and hydrothermal 

alteration, pressure during contact metamorphism was 

probably 1,000 bars or less. The topographic relief now 

existing on the White Rock pluton is great enough to 

account for a pressure difference of 300-400 bars 

assuming lithostatic conditions. According to Winkler 

(1967, p. 79), temperatures of the hornblende hornfels 

facies at 1,000 bars range approximately from 535° to 

600°C.

Reactions of matrix minerals in the sedimentary 

rocks in the contact aureole were relatively complete, 

but the framework silicates were not affected in many 

rocks, as shown by unaltered clastic grains of albite inherited from the sandstones. In relatively few rocks 

did the plagioclase recrystallize to oligoclase or 

andesine.

No well-defined aureole of albite-epidote-hornfelsfacies rocks surrounds the hornblende-hornfels-facies 

rocks. The albite-epidote-hornfels-facies rocks may be 

represented by the epidote and epidote-tremolite 

veinlets found in the partly bleached rocks at the outer 

edge of the metamorphic aureole. Apparently, in that 

area reactions were even less complete than in the 

hornblende-hornfels-facies rocks.

The source of the Na and Cl for the late scapolite porphyroblasts is not known. We do not know of any NaCl 

in the evaporite beds in the immediate vicinity; the 

beds we saw were composed of CaSO4 . The 

granodiorite magma is also a possible source of Nabearing solutions. The occurrence of high-salinity fluid 

inclusions in some of the altered granodiorite suggests 

that the magma could have been a source for the Na 

and Cl in the scapolite. The apparent uniformity of the 

composition of the scapolite in beds of otherwise differing composition indicates that scapolite formation was
76 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

externally controlled, probably by the Na and Cl, 

whatever the source. Late alteration of scapolite to 

sodalite probably took place under the influence of 

solutions similar to those that introduced the Na and 

Cl for the scapolite.

STRUCTURE

The decipherable structural history of the Aspen 

region extends from at least 1,650 to 1,750 m.y. ago, 

the time of the metamorphism of the Precambrian 

sedimentary and metamorphic rocks, to the middle 

Tertiary, or about 30 m.y. ago. Younger structures 

have not been definitely dated in the Aspen area, 

although they are widespread in Colorado. The structural history of the area indicates that older structures 

tended to affect the locations and configuration of 

younger structures.

PRECAMBRIAN STRUCTURES

At the time that the Precambrian rocks were 

metamorphosed they were isoclinally folded, and foliation and lineation formed. The Aspen area includes 

only a small area of layered rocks of Precambrian age; 

so larger scale structures cannot be delineated, nor are 

the relations known between structures observed on 

the scale of an outcrop and larger structures.

Foliation, which is generally defined by micas alined 

parallel with compositional layering, generally trends 

northeast and dips steeply to moderately either northwest or southeast. However, some foliation trends 

northwest and dips northeast. Many of the statistical 

highs on the diagram summarizing foliation (fig. 41 A) 

lie on a girdle that has an axis trending N. 40° E. and 

plunging 30° NE. Other girdles having axes trending 

N. 87° W. plunging 10° SE. and N. 72° E. plunging 

55 ° NE. can also be drawn through other combinations 

of statistical highs.

Lineation formed by alined minerals or aggregates of 

minerals, such as sillimanite, trends east to northeast 

parallel with the poles of the three statistical girdles of 

the foliation (fig. 41B). In many outcrops the mineral 

lineation is parallel with axes of minor folds.

Minor folds were classified as isoclinal or open, but 

no well-defined age relations between the types were 

detected, nor do their respective axial planes or axes 

have orientations distinctive to the fold type (figs. 44, 

4B, 41Q. Fold axes have a distribution similar to that

of the mineral lineation. The distribution of axial 

planes resembles that of the foliation. The sparse information on minor structures in the Aspen area suggests 

that the Precambrian rocks have a complex structural 

history, but the data from this small area of Precambrian metamorphic rock are insufficient to reveal that 

history.

Few data have been published on structural trends 

in the Precambrian rocks of the Sawatch Range near 

Aspen. A generalized map by Stark and Barnes (1935) 

suggested that north and east of Aspen trends are 

generally northeast whereas to the southwest of Aspen 

the trends are northwest. Even their generalized data 

suggested some fairly large scale swirls in the trends of 

both regions. Detailed mapping in the Mt. Harvard 

quadrangle (Brock and Barker, 1972) southeast of 

Aspen showed general north-northwest trends on the 

northern half of the quadrangle and east-northeast 

trends in the southern half with numerous local variations.

A number of northeast-trending shear zones of 

Precambrian age are found in the Sawatch Range; the 

best studied one is the Homestake shear zone, which 

trends southwest from Homestake Creek and Mt. Holy 

Cross towards Aspen (Tweto and Sims, 1963). The 

sheared Precambrian rocks mapped north of Hunter 

Creek at the east margin of the Aspen quadrangle are 

probably related to these northeast-trending zones. Recent reconnaissance mapping suggests that the 

sheared rocks in the Aspen area do not extend far to 

the northeast; faults on trend with the sheared rocks 

and the Homestake shear zone do (fig. 48; Tweto and 

others, 1978).

The sheared rocks in the Aspen quadrangle range 

from mortar gneiss to phyllonite, and those rock types 

are associated with slightly sheared or unsheared 

granitic rock. An accessible exposure of sheared rock is 

on the trail east of the gully at the east margin of the 

Aspen quadrangle north of Hunter Creek. There 

chlorite-sericite phyllonite and blastomylonite occur in 

two zones 5-7 cm thick in an interval of 45 cm in an exposure of sheared granitic rock that is about 30 m 

wide.

A few zones of cataclastic foliation occur outside the 

area mapped as shear rock, both east of the mapped 

area and south of Hunter Creek. In the latter area, the 

rock is quite fractured; much of this fracturing may be 

associated with the north-south faults shown on the 

geologic map, which are probably of Laramide age,
STRUCTURE 77

EXPLANATION 

3-percent contour interval 

6-percent contour interval 

9-percent contour interval

Axis Pole to axial plane 

A Isoclinal fold • 

A Open fold o

EXPLANATION

5-percent contour interval 

10-percent contour interval

FIGURE 47.—Structures in Precambrian mica schist and gneiss in 

the Hayden Peak quadrangle. All diagrams are equal-area projections in the lower hemisphere with planes of projection 

horizontal and north at top. Contours show percentages of 

points falling within 1 percent of the area of the diagram. A, 

Poles to foliation; dashed lines indicate girdles; x, poles to girdle 

(n=31). B, Mineral lineation; x's from A (n=19). C, Axes and 

poles to axial planes of minor folds.

although some may have formed earlier and may have 

been reactivated in the Laramide orogeny.

The few samples of sheared granitic rock studied in 

thin section show cataclasis and alteration effects 

without much recrystallization. Quartz is strongly 

strained and broken down into mortar. Feldspars are 

bent and broken. Plagioclase is altered, and biotite is 

bent and partly altered. These features indicate 

cataclasis under conditions of relatively low 

temperature and pressure. The phyllonite, however, 

would indicate cataclasis and recrystallization under 

conditions of somewhat higher temperature and PH o.

The Homestake shear zone has a great variety of 

shear products formed in a variety of temperature and
78 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

107°

ASPEN 

MOUNTAIN 

SYNCLINE

EXPLANATION

Oligocene plutons

Precambrian basement 

in Sawatch uplift

Fault

Thrust fault; teeth on upper plate

Syncline; arrow at end indicates 

direction of plunge

H-M

-f

Anticline

Dome

Mnnnrline

FIGUKE 48.-Some structural features in the Aspen region. Modified from Tweto Moench, and Reed (1978). Most of the primary

sources are listed in the references at the end oi this report.
STRUCTURE 79

pressure conditions during a long period of time in the 

Precambrian (Tweto and Sims, 1963). The sheared rock 

in the Aspen area is so poorly exposed and occupies 

such a small area that close comparisons are futile. 

Early movements on the Homestake shear zone may 

have been shortly after the main metamorphism 

1,650-1,750 m.y. ago. In a northeast-trending zone in 

the Front Range, Abbott (1972) demonstrated that a 

fine-grained blastomylonite formed about 1,200 m.y. 

ago. It is probable that most of the cataclastic effects 

in the zone in the Aspen quadrangle, except possibly 

the phyllonite zones, are younger and fall between 

1,200 m.y. ago and deposition of the Sawatch Quartzite. All the cataclastic effects are younger than the 

quartz monzonite they cut, which is about 1,450 m.y. 

old. The general trend of the shear zone is followed by 

faults of Laramide age, which intersect the margin of 

the Sawatch uplift at Aspen.

PHANEROZOIC STRUCTURE

The Aspen region lies on the west side of one of the 

more westerly major uplifts exposing substantial 

areas of Precambrian rocks in the southern Rocky 

Mountains about 145 km west of the edge of the Great 

Plains (fig. 3). West of Aspen the Phanerozoic rocks are 

generally flat lying although they are locally deformed 

in sharp monoclines or broken by faults. Except for the 

Uinta Mountains in northeastern Utah and the Needle 

Mountains uplift in southwestern Colorado, most of 

the mountains from Aspen west to central Utah are 

composed of Tertiary volcanic or intrusive rocks or are 

deeply dissected high plateaus underlain by littledeformed Phanerozoic rocks.

Structural events during the late Paleozoic and early 

Mesozoic affected the pattern of sedimentation in the 

Eagle basin, where the Aspen area was then located. 

Most of the major structural features of the Aspen 

area were formed during the Laramide orogeny in the 

Late Cretaceous and Paleocene about 65-70 m.y. ago 

when the Sawatch Range was uplifted and during the 

middle Tertiary episode of intrusion. Late Tertiary 

fault movements have occurred in the southern Rocky 

Mountains, but no proof for major faulting at this time 

has been found in the Aspen area.

PRE-LARAMIDE STRUCTURAL EVENTS

During Cambrian through Mississippian time the 

Aspen region was alternately inundated by the sea and 

slightly uplifted and eroded. No evidence was found for 

any local folding or faulting during this time span. In a 

later detailed stratigraphic study, Maslyn (1976) advocated faulting in the Late Mississippian to explain 

thickness variations in the Leadville Limestone. By 

Middle Pennsylvanian time uplift occurred southwest 

of Aspen, and thousands of meters of detritus from 

this uplift were deposited on the Aspen area during the 

late Paleozoic and early Mesozoic.

Northwest of Woody Creek a sharp fold trending 

northwest parallel to the edge of the Uncompahgre 

uplift formed in Permian time (Freeman, 1971b, 

1972a). Erosion of this fold beveled the Maroon Formation, and differential structural movement across the 

fold may account for the substantial differences in the 

thicknesses of the State Bridge Formation on either 

side. The presence of evaporites of the Eagle Valley 

Formation in a fault slice in the Castle Creek fault zone 

of Late Cretaceous age near the fold suggests that the 

fold could have been a salt anticline (Freeman, 1971b). 

Tweto (1977) discussed the possible role of evaporites 

in the structure and stratigraphy of the region. At 

about the same time, the Uncompahgre uplift probably 

expanded northeast so that the edge of the uplift was 

closer to Aspen during the Late Permian and Early 

Triassic than during the Pennsylvanian and Early Permian. Angular unconformities below the Chinle of Late 

Triassic age and the Morrison of Late Jurassic age (fig. 

32/4) show that the Uncompahgre highland was 

moderately uplifted in the Triassic and the Jurassic.

Stratigraphic variations across north-south-trending 

faults in the Highland Peak quadrangle indicate that 

those faults must have been active before deposition of 

the Morrison Formation in the Upper Jurassic. Those 

faults were reactivated in the middle Tertiary, or, 

perhaps, the Laramide.

The fault having the greatest pre-Morrison displacement crosses Eagle Mountain at the west margin of 

the Highland Peak quadrangle. The southwest extension of that fault was apparently followed by the 

younger Snowmass Creek fault zone. East and south of 

the fault on Eagle Mountain the Entrada, Chinle, and 

State Bridge Formations are present; whereas they are 

absent on the west and north sides of the fault. Also, 

the siltstone at the top of the Maroon Formation, 

which is at least 300 m thick, is present on the south 

and east and absent on the west and north side of the 

fault. Whether the fault was active in the Permian, 

Triassic, or Early Jurassic cannot be demonstrated; 

but the stratigraphic relations of the State Bridge Formation suggest a northeastward expansion of the Uncompahgre highland in the Late Permian or Early 

Triassic. Perhaps the fault on Eagle Mountain was a 

range-front fault at that time and was reactivated in 

Jurassic time after deposition of the Chinle Formation.
80 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

The Chinle in the eastern block resembles the Chinle 

elsewhere and does not suggest that the fault was active during deposition of that formation. This fault 

separates a region to the east where 3,000 m or more of 

Maroon Formation is preserved from one to the west 

where the Maroon is less than 2,000 m thick. Reconnaissance mapping by R. H. Moench (oral commun., 

1975) showed that the State Bridge and Chinle Formations are present on the east and absent on the west 

side of a north-trending fault just south of Basalt. 

That fault might have been a part of the fault system 

of pre-Late Jurassic age exposed near Eagle Mountain. 

Stratigraphic thicknesses differ across a north-south 

fault on the east side of Burnt Mountain in the 

Highland Peak quadrangle. On the east side of the 

fault the State Bridge is about 200 m thick, and on the 

west side about 60 m thick. West of Burnt Mountain 

the State Bridge outcrop abruptly ends at another 

north-south fault. Movement on those two faults took 

place before deposition of the Chinle Formation in Late 

Triassic. These faults were also reactivated in the middle Tertiary and (or) the Laramide.

LATE CRETACEOUS AND PALEOCENE(P) EVENTS

The Laramide orogeny began in the Aspen area after 

deposition of rocks containing Exiteloceras jenneyi, a 

widespread ammonite zone found in the Mancos Shale. 

On the north flank of the San Juan Mountains sanidine 

from a tuff in this zone has been dated at 75.2 ±2.3 

m.y. and biotite at 72.7±2.2 m.y. (Dickinson and 

others, 1968). In the Woody Creek quadrangle about 

150 m of rocks overlie the Exiteloceras jenneyi zone, 

and they contain fossils suggestive of the Baculites 

compressus or a younger zone (Freeman, 1972a). The 

Baculites compressus zone has been dated at about 

71.5 m.y. (Obradovich and Cobban, 1975). Muscovite 

from a porphyry sill cut by the Castle Creek fault zone 

dates at 72.2±2.2 m.y. Biotite from an aplite intrusion 

occupying a branch of the Castle Creek fault zone has 

been dated at 70.0±2.3 m.y. and 67.4±2.2 m.y. (two 

separates from the same specimen; Obradovich and 

others, 1969). These dates bracket the commencement 

of Laramide orogeny on the west flank of the Sawatch 

Range.

The first Stratigraphic evidence for Laramide 

Orogeny in the region is found in the Ohio Creek Formation, the nearest exposure of which is about 40 km 

southwest of Aspen in the Oh-Be-Joyful 7.5-minute 

quadrangle (Gaskill and others, 1967). There, a conglomeratic sandstone sequence 115-130 m thick is in 

gradational contact with the Mesaverde Formation of 

Late Cretaceous age. The Ohio Creek Formation contains a Paleocene flora 6 m above its base (Gaskill and

Godwin, 1963). Perhaps detritus from the initial uplift 

of the Sawatch anticline was derived from fine-grained 

Cretaceous rocks and deposited in the upper 

Mesaverde (Tweto, 1975). A small discrepancy exists 

between the Late Cretaceous ages of the porphyries of 

the Aspen district, which are believed to bracket 

Laramide structural events, and the first appearance 

of conglomerate beds to the west. K-Ar ages of micas, 

when erroneous, are usually too young rather than too 

old.

LARAMIDE STRUCTURES

FAULTS

The Aspen region has a large number of faults, which 

for purposes of discussion are divided into (1) Elk 

Range thrust zone, (2) Castle Creek fault zone, and (3) 

other faults along the margin of the Sawatch uplift.

THE ELK RANGE THRUST ZONE

"The great fold fault of the Elk Range" has been 

described by Hayden (1874, p. 65-68), Holmes (1876, 

p. 67-71), and Vanderwilt (1935b, p. 231-233; 1937, p. 

83-92). Exposures of this structure are most spectacular in the southeastern part of the Snowmass 

Mountain quadrangle (Mutschler, 1970). I advanced 

the hypothesis that the Elk Range thrust might represent the base of a thick sheet of upper Paleozoic and 

Mesozoic sedimentary rock that slid southwest off the 

Sawatch uplift in latest Cretaceous or early Paleocene 

time (Bryant, 1966). This hypothesis was used to explain a structural discontinuity between rocks with 

simple structure above and rocks with more complex 

structure below in a small area on Conundrum Creek in 

the Maroon Bells and Hayden Peak quadrangles (fig. 

49). That area was interpreted as a window in the 

thrust sheet, thus extending the fault 11 km northeast 

of its nearest previously known outcrop in the 

southwest corner of the Maroon Bells quadrangle.

Very few new firm data concerning the Elk Range 

thrust sheet have been found since 1966. In the Pearl 

Pass quadrangle just south of the Aspen area, Zoerner 

(1973) mapped one branch of the thrust system—the 

Hunters Hill thrust (fig. 48)—and interpreted the heel 

of the lower branch of that fault to be at the BeldenGothic contact or in the upper part of the Belden. His 

mapping showed that the lower plate was deformed 

into a steep fold before the Hunters Hill thrust reached 

its present position. His proposed position of the heel 

of the Elk Range thrust could be the site of a highangle fault related to numerous ones shown on detailed 

maps to the east (Zoerner, 1973; Cunningham, 1976). I, 

also, have made similar interpretations along the 

margin of the Sawatch uplift in the Aspen and Hayden 

Peak quadrangles.
STRUCTURE 81

FIGURE 49.—View of west side of Conundrum Creek window from 

3,650 m altitude on west side of Hayden Peak. Granodiorite has 

intruded along structural discontinuity between hornfels of the 

Maroon Formation with uniform dip above and deformed marble 

and carbonaceous hornfels of the Belden Formation below. Tg, 

granodiorite; PPm, Maroon Formation; IPbh, hornfels of the 

Belden Formation; Pbm, marble of the Belden Formation. Total 

topographic relief of valley side about 1,000 m.

The Doubletop thrust (fig. 48), which is the other 

known southeastern branch of the Elk Range thrust 

zone after it bifurcates at the site of the White Rock 

pluton, has not been traced east of Cement Creek (Cement Mountain quadrangle) to a possible intersection 

with structures at the margin of the Sawatch Range.

At the northwest end of the Elk Range thrust zone, 

as known at present, stratigraphic displacement on the 

Elk Range fault decreases to the northwest as the 

units become younger in the upper block (Gaskill and 

Godwin, 1966a). Detailed mapping is needed north and 

northwest of that area. Could there be another split of 

the fault heading north from the northwest corner of 

the Snowmass pluton in the Avalanche Creek area? 

Unpublished reconnaissance mapping by V. L. 

Freeman and me (1974) north and west of the 

Snowmass pluton, however, shows that the Gothic 

Formation wraps around the west and north sides of 

the Snowmass pluton; so that hypothesis apparently is 

invalid. Present knowledge suggests that the fault dies 

out in the Mancos Shale west of the Crystal River.

Even when detailed geologic maps become available 

for the whole region, it will be difficult to separate fact 

from interpretation concerning the Elk Range thrust. 

For instance, variations in the apparent thicknesses of

upper Paleozoic sedimentary rocks are subject to 

stratigraphic as well as structural interpretations. In 

the northern part of the Ruedi quadrangle (Freeman, 

1972b) south of the Fryingpan River the Maroon Formation ranges from 1,000 to 2,000 m in thickness in a 

distance of 3 km. Although a fault is mapped, it is 

within the Gothic Formation. Are there other faults 

subparallel with the bedding representing the Elk 

Range thrust, or is this a real stratigraphic thinning? 

The dying out of stratigraphic displacement on the Elk 

Range fault in the Marble quadrangle and to the 

northwest indicates that the toe of the thrust sheet did 

not break entirely free from the surrounding rocks and 

that thrusting does die out in that direction. Perhaps 

some of the movement was taken up by folding and 

less continuous faults in the lower Crystal River valley 

west of Mt. Sopris (fig. 48) (Poole, 1954). There, the 

situation is complicated by structures that may have 

been a result of salt tectonics (Mallory, 1966).

The bedding-plane faults mapped by Spurr (1898) in 

the Aspen mining district might represent the heel of 

the Elk Range thrust. However, the Contact fault, 

which he mapped between dolomite and limestone 

within the Leadville, does not represent a throughgoing fault. That stratigraphic horizon was the site of 

solution and cave formation during the erosion interval between the deposition of the Leadville Limestone 

and the Belden Formation, and perhaps during the 

Quaternary. I think that many of the breccias described by Spurr from mine workings formed by those 

processes. The Silver fault at the Leadville-Belden contact is more difficult to evaluate. In many areas of the 

mining district high-angle faults are along that contact, and in many places they are parallel with steeply 

dipping bedding. Natural exposures of the contact are 

almost nonexistent. In the southern part of the 

Hayden Peak quadrangle no fault is at that horizon; 

there, the heel of the Elk Range thrust is believed to be 

near the Belden-Gothic contact, as suggested by 

unusually thin Belden near lower Express Creek. 

North of there the Belden becomes increasingly involved in the high-angle faults of the Castle Creek fault 

zone. In the Highland tunnel, which transects the Castle Creek fault zone near the south margin of the Aspen 

quadrangle, 30 shears and faults parallel with steeply 

dipping bedding in the Maroon, Gothic, and Belden 

Formations were detected in addition to about 20 that 

cut the bedding. Some of these faults contain wide 

zones of breccia.

In the northern part of the Aspen quadrangle I interpreted a fault parallel with the bedding well up in the 

Belden to be the heel of the Elk Range thrust. Actually, it is likely that movement along the bedding planes 

associated with gravity sliding was not concentrated
82 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

everywhere on one thin stratigraphic horizon; it may 

have been at a number of stratigraphic levels in the 

Belden and the Gothic near the heel of the sheet. Farther south and west, movement became concentrated 

in the Gothic Formation as demonstrated by the 

widespread occurrence of a hundred meters or more of 

Gothic just above the thrust.

The Belden seems to form a structural front between 

the brittle much-faulted lower Paleozoic rocks and the 

less faulted younger rocks. Very few east-west faults 

penetrate the Belden to its upper contact. Exceptions 

are at Lenado Gulch and at Queens Gulch (the Butte 

fault of Spurr, 1898). This difference in structure 

across the Belden outcrop belt could be due to difference in competence between Belden and adjacent 

formations or might be caused by a structural uncoupling between the Elk Range thrust sheet and the 

margin of the Sawatch Range during early phases of 

the Laramide deformation.

The salient structural characteristics of the Elk 

Range thrust where it has been mapped along its toe 

are general concordance between the fault plane and 

gentle to moderate northeast dips in the thrust sheet, 

and discordance with beds in the underblock (Gaskill 

and Godwin, 1966a; Mutschler, 1970; Bryant, 1969, 

cross sees.). The relations between thrust sheet and 

underblock are similar farther east in the Gothic and 

Pearl Pass quadrangles east of the White Rock pluton 

where the thrust bifurcates into two faults 

(Langenheim, 1952; Prather, 1964; Zoerner, 1973). The 

thrust sheet is locally parallel with the overturned limb 

of the Schofield syncline of the underblock in the 

Snowmass Mountain quadrangle (Mutschler, 1970). 

Over most of its outcrop length a hundred to a few hundred meters of Gothic Formation occupy the base of 

the thrust sheet. Similar relations between simple 

structure in the thrust sheet and folds in the 

underblock occur in the Conundrum Creek window 

(Bryant, 1966), but there the effects of contact 

metamorphism make it uncertain whether any Gothic 

Formation is preserved below the Maroon Formation 

of the thrust sheet.

If the Elk Range thrust sheet moved off the Sawatch uplift in latest Cretaceous, it must have started 

at an early stage of uplift of the Sawatch because its 

heel is steeply folded and faulted along the margin of 

the Sawatch uplift. Movement along the thrust probably ceased before formation of the steep faults along 

the margin of the Sawatch uplift, and before intrusion 

of the 67-m.y.-old aplite of the Castle Creek fault zone 

in the Aspen district.

The Elk Range thrust zone disappears near the

south end of the Grand Hogback monocline (fig. 48). 

Movement on these features has been assumed to be 

synchronous (Hills, 1891; Vanderwilt, 1937). Holmes 

(1876), Hills (1891), and Murray (1966) thought that intrusion of the plutons in the Elk Mountains was essentially contemporaneous with thrusting. Vanderwilt 

and subsequent workers recognized that granodiorite 

cuts the fault zone and is entirely younger.

Most of the deformation producing the Grand 

Hogback monocline northwest of Glenwood Springs 

took place after deposition of the Green River Formation of Eocene age immediately to the west, for it is involved in the Grand Hogback fold (Hills, 1891). Yet the 

relations between the Elk Range thrust zone and the 

steeply dipping structures at the margin of the 

Sawatch Range in the Pearl Pass quadrangle (Prather, 

1964; Zoerner, 1973) show that movement on the 

thrust fault must have been early Laramide. However, 

the lack of angular discordance between Upper 

Cretaceous and Paleocene or Eocene rocks in the 

Grand Hogback region or in the West Elk Mountains to the south about 20 km from the present outcrop of the Elk Range thrust show that deformation 

associated with the thrusting did not extend to those 

areas. Murray (1966, p. 120-122) pointed out that the 

thrusting does not seem to be related to the development of the Grand Hogback monocline.

Estimation of distance of transport of the rocks 

above the Elk Range thrust depends upon assumptions concerning the distances required for thickness 

and facies changes in the Maroon and Gothic Formations. Evidence discussed in previous sections indicates that substantial changes may occur in short 

distances across the trend of the Eagle basin. Since 

younger rocks are thrust over older ones in the Conundrum Creek window, that feature does not put any constraints on the amount of movement of the thrust 

sheet above it. Minimum displacements are given by 

stratigraphic separations between thrust sheet and 

underblock along the south side of the Elk Mountains, 

where Gothic Formation overrides Mesaverde Formation in the Marble quadrangle and Mancos Shale at 

various localities in the Snowmass Mountain, Maroon 

Bells, Gothic, and Pearl Pass quadrangles. If one 

assumes minimal thicknesses for the Maroon Formation, stratigraphic displacements are about 2 km. Actual displacements must be at least several times that 

figure because bedding in the thrust sheet and 

underblock is nearly parallel along much of the trace of 

the thrust, and it is reasonable to assume that the 

thickness of the Maroon Formation cut by the fault 

lies somewhere between the 363 m measured by
STRUCTURE 83

Langenheim in the underblock just south of the Aspen 

area and the measured and calculated section in the 

thrust sheet in the Aspen area of 3.2 km. (See 

stratigraphic section 2, p. 131-137.)

THE CASTLE CREEK FAULT ZONE

The Castle Creek fault zone bisects the Aspen area 

and is the main structure bounding this part of the 

Sawatch uplift (figs. 48, 5(L4). This zone of faulting extends south of the Aspen area into the Pearl Pass and 

Taylor Park quadrangles (Cunningham, 1976; Zoerner, 

1973), but the south end of the main fault as indicated 

by the amount of stratigraphic displacement is in the 

northern part of the Hayden Peak quadrangle. 

Displacement reaches a maximum on the west side of 

Aspen Mountain where Precambrian rocks are against 

the upper part of the Maroon Formation and at Red 

Butte where Gothic Formation is against State Bridge 

Formation (Bryant, 1971a). In those places 

stratigraphic displacement is about 4,300 m and 

3,600 m, respectively. These figures differ from those 

given by Spurr (1898, p. 59) because he did not know 

the thickness of the Maroon Formation nor did he 

recognize the basal beds of the Maroon (which are now 

called Gothic) east of Red Butte. To the northwest 

apparent displacement diminishes until at Snowmass 

in the Woody Creek quadrangle, Freeman (1972a) 

mapped no through-going fault. However, attitudes of 

the beds and altitudes of the contacts indicate that the 

Roaring Fork valley is the site of a fold, and (or) fault. 

Here, relative displacement on the fault changes. West 

of Snowmass the fault splits and encloses a graben. 

The southern split swings north at Basalt (Welder, 

1954; Tweto and others, 1978) and joins the Red Table 

Mountain fault (fig. 48) of Bass and Northrop (1963) 

rather than following the Roaring Fork valley to Glenwood Springs as Vanderwilt (1935b) indicated. North 

of Basalt the east block moved down relative to the 

western block, opposite to the displacement in the 

Aspen area.

The change in trend of the Castle Creek fault zone 

from N. 10° W. along the margin of the Sawatch uplift 

south of Aspen to N. 70° W., about perpendicular to 

the uplift, near Snowmass occurs rather abruptly 

north of its intersection with the Roaring Fork 

syncline (fig. 48). North of Aspen the fault zone may 

follow a northwest-trending zone of weakness in the 

Precambrian basement rocks. This zone of weakness 

might have controlled the location of the fold of Permian age mapped by Freeman (1971b, 1972a). The 

magnetic map of Colorado (Zietz and Kirby, 1972) has

FIGURE 50.—Features of the Castle Creek fault zone. A, General 

view of zone south of Aspen from Highland Peak. Castle Creek 

valley in the foreground. Queens Gulch is valley with road in it 

left of center. Sharply upturned to overturned Pennsylvanian 

rocks faulted against more gently dipping Cambrian to Mississippian and Precambrian rocks. pC, Precambrian rocks; €s, 

Sawatch Quartzite; Om, Manitou, Dolomite; DC, Chaffee Group; 

Ml, Leadville Limestone; Pb, Belden Formation; IPg, Gothic Formation; PFm, Maroon Formation; TKap, aplite porphyry; Is, landslide. Dip slope consists of Manitou Dolomite, Chaffee Group, and 

a sill of hornblende quartz diorite. Heavy lines, faults; light lines, 

contacts; dotted lines, concealed faults. B, One of many unmapped faults subparallel to bedding planes in the Maroon Formation 

west of the main break; roadcut in Queens Gulch. Some beds 

wedge out along faults.
84 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

a prominent northwest-trending grain in that area, 

suggesting control of the trend of the Castle Creek 

fault zone by Precambrian or Paleozoic structures in 

the basement rocks.

One of the major folds of the Aspen area occurs at 

the point where the margin of the Sawatch uplift 

diverges from the Castle Creek fault zone. South of the 

town of Aspen a north-plunging faulted syncline contains Paleozoic sedimentary rocks (fig. 6) as young as 

the Belden Formation. The syncline is separated from 

the fault zone by the ridge of Precambrian rock south 

of West Aspen Mountain. North of Aspen the syncline 

widens abruptly and directly abuts the Castle Creek 

fault zone. The syncline becomes shallow along plunge 

both north and south of Aspen; so its total length is 

only 8 km although at its deepest point it has a 

minimum structural relief of 1,300 m. In a northeasterly direction its east limb becomes the monocline that 

flanks the Sawatch uplift.

The Castle Creek fault zone tends to dip subparallel 

with bedding in the underblock south of Aspen where 

some data are available. One of the best exposures of 

the fault is in Keno Gulch where the fault dips 75° to 

the east. There, the fault has about 5 cm of carbonaceous shaly gouge derived from the Belden Formation along it, and Precambrian quartz monzonite is 

argillized for a distance of about 3 m from the fault. 

The 4 m of Maroon Formation exposed in the 

downthrown side is laced with faults along which is 

material from the Belden Formation. Spurr (1898, p. 

93) reported that in tunnels in the Keno Gulch area the 

fault appears to dip 45°-50° E., which is similar to the 

dips in the Maroon Formation west of .the exposure of 

the fault in Keno Gulch. Spurr reported several 

exposures of the Castle Creek fault in tunnels, but 

natural exposures are rare.

Part of the fault zone west of the main break is exposed along the Queens Gulch road where some details 

of bedding-plane slip in the Gothic and Maroon Formations are well shown (fig. 50£?).

The most complete cross section of the fault zone 

was exposed in the Highland tunnel driven during the 

middle 1960's east from about 2,650 m altitude in the 

Castle Creek valley in the southern part of the Aspen 

quadrangle. There, about 50 faults were detected west 

of the Castle Creek fault proper, which at that level 

separates Precambrian quartz monzonite form Belden 

Formation. The most common type of fault observed 

in the tunnel was parallel to bedding in the Belden, 

Gothic, or Maroon Formation and had one to several 

centimeters of clayey gouge along it. Thick zones of 

breccia and gouge were common and occupied a total 

of about 230 m of the tunnel. The great number of 

faults visible in the tunnel shows that at the surface

relatively few of the faults existing in this zone have 

been mapped. Those shown on the geologic map are 

generally based on map relations between formations, 

which are relatively thick and lack marker horizons, or 

by sudden changes in dip or strike of bedding.

One strand of the Castle Creek fault zone is exposed 

in a roadcut northwest of Red Butte. Moderately north 

dipping Mancos Shale in the west part of the cut is 

fractured along north-south trends, and in the east 

part of the cut the bedding is vertical and parallel with 

the zone of fracturing.

A number of cross faults of small displacement 

associated with the Castle Creek fault zone cut the 

rocks of Red Butte, which form an overturned fault 

slice along the zone. If marker horizons were available 

in the Pennsylvanian and Permian rocks along the 

fault south of Aspen, a similar pattern of cross faults 

might be detected.

OTHER FAULTS

East of the Castle Creek fault zone along the margin 

of the Sawatch uplift are a number of faults trending 

parallel to that margin along which the downthrown 

side is on the east towards the center of the Sawatch 

uplift, in contrast to the Castle Creek fault zone, which 

is clearly related to relative upward movement on the 

east, or Sawatch uplift side. These faults extend to 

south of the Taylor River (Zoerner, 1973; Prather, 

1964) and north beyond the Fryingpan River (Mackay, 

1953). Some of these faults were described by Stark 

(1934); he drew them as dipping more gently at depth 

and called them thrust faults. The one in the northeast 

part of the Aspen area, which he called the Bigelow 

thrust, was incorrectly mapped, and faults that may be 

related to the one he mapped are apparently all high 

angle. In the Aspen area faults of this system have 

displacements of as much as 450 m depending on what 

assumption is made concerning the change in dip of 

the sedimentary rocks on the block to the west 

(Bryant, 1970, 1971a, cross sees.). Many faults of this 

system on the east side of Aspen Mountain and Richmond Hill have displacements as small as 10 m. In 

that area the situation is complicated by north-south 

faults associated with the east limb of the syncline on 

Aspen Mountain, and their sense of movement is up on 

the east. Fractures and faults believed to be related to 

the faults in the margin of the Sawatch uplift with 

down movement on the east cut through Precambrian 

rocks on the corner of the Sawatch uplift on Smuggler 

Mountain east of Aspen. No well-defined age relations 

between these faults and the Castle Creek fault zone 

were found. This fault system may be a complex zone 

marginal to the Sawatch uplift in which some exten-
STRUCTURE 85

sion occurred in the area of maximum apparent 

displacement along the Castle Creek fault zone and at 

the sharp change in trend of the margin of the uplift 

that occurs at Aspen. 

A number of cross faults of 10-100 m displacement

cut the lower Paleozoic rocks from Richmond Hill 

north to Lenado. On Aspen and Smuggler Mountains 

the north sides moved up on the average. There, faults 

are reported to dip steeply in the mines on Aspen 

Mountain and to dip 35°-40° to the south on Smuggler Mountain (Spurr, 1898). The intersections of these 

faults with favorable stratigraphic or tectonic horizons 

were the loci of the richest ore bodies in the Aspen mining district. A few of the faults of the system offset the 

Castle Creek fault, but most are apparently confined to 

east of that fault. Many of these faults appear to be 

older than the longitudinal faults, but apparently considerable overlap in ages occurs between the two 

systems.

FOLDS

The most prominent fold in the Aspen area is the 

moderately to steeply dipping faulted monocline along 

the western flank of the Sawatch uplift. At Aspen this 

monocline changes trend from N. 10°-20° W. to N. 30° 

E., and it is there that the most complicated structures 

of the region are found. This change in trend of the 

margin of the Sawatch uplift occurs at its intersection 

with the projected trend of the Homestake shear zone 

(fig. 48). Just south of Aspen a north-plunging fault 

block or much faulted anticline and a north-plunging 

syncline border the main Sawatch uplift. The syncline 

widens and flattens dramatically northward so that at 

the north edge of the Aspen quadrangle several 

thousands of meters of Maroon Formation are in it 

(Bryant, 1971a, sec. A-A\ B-E\ C-C"). The axis of 

the syncline swings westward and is cut by the Castle 

Creek fault zone, but it reappears in the Woody Creek 

and Ruedi quadrangles on the northeast side of the 

fault zone (Freeman, 1972a, b).

The second major fold in the Aspen area is the Roaring Fork syncline (fig. 48). The syncline is a broad fold 

about 32 km long, trending northwest, parallel with 

the late Paleozoic structural trends of the Eagle basin 

and the Uncompahgre uplift and with the Castle Creek 

fault zone and several folds and faults to the northeast 

of late Paleozoic and Laramide age (Freeman, 197Ib; 

Tweto and others, 1978). The southwest limb of the 

syncline occupies much of the Highland Peak 

quadrangle and dips about 20°. The northeast limb 

dips at about 40° in the west central part of the Woody 

Creek quadrangle (Freeman, 1972a). The trough of the 

syncline passes through the southern part of the

Woody Creek quadrangle about a mile north of the 

Highland Peak quadrangle and is cut by the Castle 

Creek fault zone in the southwest corner of the Ruedi 

quadrangle. The northeast limb of the syncline is cut 

by the Castle Creek fault zone in the Ruedi and Woody 

Creek quadrangles. Available data indicate that the 

structurally deepest part of the syncline is in the 

southeastern part of the Woody Creek quadrangle 

(Freeman, 1972a) where over 1,500 m of Mancos Shale 

is present and where Precambrian rocks must lie at 

depths of between 4.5 and 6 km below the land surface 

or about 3 km below sea level. The trough of the sycline 

is sharper to the east and broader to the west 

(Freeman, 1972a, b).

The Roaring Fork syncline is determined to be of 

Laramide age because it contains Upper Cretaceous 

rocks and is truncated by the Castle Creek fault zone, 

which is of Laramide age.

The Mancos Shale in the Highland Peak and Woody 

Creek quadrangles has several short folds of various 

trends (Freeman, 1972a). In the south-central part of 

the Woody Creek quadrangle, two rather sharp 

southwest-plunging folds differ from most of the structures in the area and continue northeast to the Castle 

Creek fault. V. L. Freeman (written commuri., 1975) 

suggested that they may be crumples related to local 

left-lateral movement on the Castle Creek fault.

OLIGOCENE INTRUSION AND DEFORMATION

The major plutons of the Elk Mountains—the 

Snowmass and White Rock plutons—apparently were 

emplaced to a large extent along the Elk Range thrust. 

In the Maroon Bells, Snowmass Mountain (Mutschler, 

1970), and Marble quadrangles (Gaskill and Godwin, 

1966a), granodiorite cuts and intrudes along the thrust 

fault. In the Conundrum Creek window granodiorite 

similarly occurs along the fault. East of the window I 

have interpreted similar relations between the intrusives and thrust along the margin of the Sawatch 

uplift (Bryant, 1970, sees. B-B' to E-E1). As more and 

more magma intruded along the fault, the overlying 

rocks were broken and warped. One of these breaks, 

along the east side of the White Rock pluton in the 

Hayden Peak quadrangle, probably acted as a barrier 

for the intruding magma, locally defining the edge of 

the pluton. Cataclastic textures in the granodiorite on 

that fault indicate that movement along it continued 

after consolidation of the granodiorite at the level of 

exposure. Farther west, in the Maroon Bells 

quadrangle, a branch of this fault cuts a sill of porphyritic granodiorite. As mentioned earlier, field relations and isotopic ages show that that rock type is 

younger than the granodiorite of the larger plutons.
86 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

The fault is marked by a well-defined topographic 

break in the Hayden Peak quadrangle (figs. 36, 5L4), 

that is probably due to differential erosion rather than 

late Tertiary or Pleistocene movements.

A number of faults with offsets of a few to tens of 

meters were mapped in the Maroon Formation overlying the White Rock pluton in the Cathedral 

Lake-Montezuma mine area (Hayden Peak 

quadrangle). Dikes of porphyritic granodiorite and 

granodiorite mark other fractures, some of which may 

have had displacement along them.

Another major fault zone associated with Oligocene 

intrusive activity is along Snowmass Creek. At the 

west edge of the Highland Peak quadrangle the extensive dip slope of Dakota Sandstone of Baldy Mountain 

is offset about 2 km north to Eagle Mountain across 

the fault zone and the Snowmass Creek valley. The 

fault zone feathers out into the Mancos Shale to the 

north in the Woody Creek quadrangle.

Reconnaissance mapping in the Capitol Peak 

quadrangle to the west shows that Gothic Formation 

is brought up along the fault zone as a thin slice which 

is interpreted as terminating just inside the west edge 

of the Highland Peak quadrangle (Bryant, 1972a, sec.

C-C"). To the west the slice of Gothic appears to 

The fault on the north side of the Gothic crosses the 

Capitol Creek-Snowmass Creek divide 1.2 km north of 

Mount Daly, and the southern fault bounding the slice 

is cut by the Snowmass pluton in the valley of West 

Snowmass Creek northwest of Mount Daly. Porphyritic granodiorite of the Snowmass Creek sill (or 

pluton of Pollock, 1969) appears to cut at least the 

southern branch of the fault. Whether any postintrusive movement occurred on the fault might be difficult to establish because of surficial deposits in the 

Snowmass Creek valley.

Farther west in the Capitol Creek valley, reconnaissance suggests at least two high-angle northtrending faults cut by the Snowmass pluton and probably formed during its emplacement.

Numerous faults cut the Roaring Fork syncline in 

the Highland Peak (fig. 51B) and Woody Creek 

quadrangles. Reconnaissance map information (Tweto 

and others, 1978) shows that the western part of the 

syncline is similarly faulted. At least some of the faults 

may be related to emplacement of the plutons of the 

Elk Mountains during the Oligocene. The north-southtrending faults have displacements ranging from 10 to

FIGURE 51.—Middle Tertiary faults. A, View southwest across fault 

contact of White Rock pluton at head of Sawyer Creek from east 

of 3,510-m altitude on Castle Creek-Conundrum Creek ridge, 

Hayden Peak quadrangle. Topographic break at base of steep 

rocky slope beyond the trees may be only a faultline scarp between more resistant granodiorite and less resistant sedimentary 

rock, or it may be a fault scarp due to renewed movement along 

the fault in the late Tertiary. B, View up West Willow Creek from 

3,290 m altitude on south side of Willow Creek valley. Graben 

forms low part of ridge through which glaciers from West Willow

Creek spilled into the Brush Creek drainage at least four times 

during the Pleistocene. Kml, lower shale member of Mancos 

Shale; Kdb, Burro Canyon Formation and Dakota Sandstone; Jm, 

Morrison Formation (includes thin Entrada Sandstone in middle 

and Chinle Formation at base except where Chinle shown 

separately); "fie, Chinle Formation; PFm, Maroon Formation (thin 

State Bridge Formation included at top except left of graben 

where State Bridge is absent); heavy lines, faults, dashed where 

concealed; light lines, contact.
STRUCTURE 87

as much as 700 m. The sense of displacement on most 

of them is uplift on the west towards the Snowmass 

pluton, the same sense of displacement as on the northsouth-trending part of the Snowmass fault zone. A few 

of the faults had late Paleozoic or early Mesozoic movement in the same relative direction. The east-westtrending faults have displacements ranging from 10 to 

300 m and an overall movement sense of down on the 

south side. This is the sense of movement on the easttrending part of the Snowmass fault zone, and it is difficult to explain their relation to intrusion unless 

through some complicated mechanism of subsidence of 

the uplift of the Elk Mountains after intrusion. Alternatively, some of these high-angle faults may have 

been initiated by jostling of the sheet of rocks that slid 

along the Elk Range thrust in Late Cretaceous time. 

The entire southwest limb of the Roaring Fork 

syncline contains a number of intrusions, which at 

depth may merge with a batholith 50 km long, or even 

much longer if a continuous body extended beneath 

the entire mineral belt. The emplacement of this 

batholith may have accentuated the syncline by uplifting the southwest limb, so that the present extent 

and structural relief of the syncline may differ some 

from that before the emplacement of igneous bodies 

during the Oligocene. Perhaps, like the Sawatch 

Range, the Elk Mountains underwent some slight collapse after uplift during the Oligocene to account for 

the south-side-down sense of movement on the eastwest faults.

In the upper part of West Maroon Creek valley some 

faults formed after emplacement of hornblende porphyry sills and before consolidation of granodiorite at 

its present level of exposure. These faults are cut by 

dikes of porphyritic granodiorite. Most of the faults 

trend northwest, and the dikes trend southwest into 

the Snowmass Mountain quadrangle where some small 

faults were mapped parallel with them (Mutschler, 

1970).

In the northern part of the Maroon Bells quadrangle 

a well-developed east-west dike swarm (fig. 20) extends 

between the Snowmass and White Rock plutons 

(Mutschler, 1970). Some of these dikes cut the pluton. 

The most spectacular exposures of the swarm are in 

the upper part of the Snowmass Creek drainage 

southeast of Snowmass Lake where the white dike 

rocks contrast with the red beds of the Maroon Formation.

The Oligocene intrusions have caused gentle folds 

and broad warps in the adjacent and overlying beds. In 

the northwestern corner of the Maroon Bells 

quadrangle and in the southwestern part of the 

Highland Peak quadrangle, beds generally dipping 

north toward the Roaring Fork syncline are tipped to

the east by intrusion of the Snowmass pluton about 3 

km to the west.

Broad gentle warps trending west to N. 60° W. occur 

in the Maroon Formation overlying the White Rock 

pluton. They are truncated by crosscutting parts of the 

pluton, indicating that they formed before intrusion 

reached that level. They may have formed much earlier 

by minor rumpling of the Elk Range thrust sheet during its emplacement in the early part of Laramide 

deformation.

A moderately tight syncline closely follows the east 

boundary of the White Rock pluton in the Hayden 

Peak quadrangle and diverges from the margin of the 

Sawatch uplift. These spatial relations indicate that intrusion of the pluton was responsible for that fold.

Another syncline may occur in the Castle Creek 

valley in the Aspen and Hayden Peak quadrangles between steep and overturned north-south-trending 

Maroon Formation along the Castle Creek fault zone 

and north-dipping Maroon and younger rocks on the 

west side of the valley. Spurr (1898) interpreted the 

structure there as a syncline overturned to the west, 

but I postulate several faults in the Castle Creek valley 

(Bryant, 1971a, sees. D-D', E-E'; 1970, sec. A-A 1). 

This structure could be interpreted as being due mainly to faulting. However, 1 km S. 70° W. of the Newman 

tunnel no break was detected where dips change from 

north to west.

LATE TERTIARY MOVEMENTS

The whole region was uplifted during the late Tertiary. Basalts 10-24 m.y. old laid down on a surface of 

low relief (Larson and others, 1975) now cap the tops of 

the highest ridges 50-100 km north and west of the 

Elk Mountains. In addition to uplift, broad warping 

occurred in the late Tertiary. The basalts slope from 

about 3,350 m altitude on the Flattops to 2,740 m near 

Cottonwood Pass and back up to 3,350 m on Battlement Mesa and Grand Mesa 80 km west of Aspen (Larson and others, 1975). Murray (1969) interpreted a 

series of long narrow tilted blocks of basalt between 

3,050 and 2,255 m in altitude south of Glenwood 

Springs as being due to unfolding movements on the 

Grand Hogback monocline. The regional age and 

physiographic relations of the basalt support that 

hypothesis.

In the Pliocene the eastern part of the Sawatch uplift 

was sundered by a rift to form what is now the upper 

Arkansas Valley. Taylor Park, a valley in the Sawatch 

uplift southeast of Aspen, may have been downfaulted 

by late Tertiary movements.

In the Elk Mountains the Treasure Mountain dome 

was formed in the Miocene by intrusion of a sodic
88 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

granite (Obradovich and others, 1969).

In the Sawatch Range east of Aspen a few faults cut 

the Grizzly Peak cauldron (Cruson, 1972), but it is 

unknown whether they are much younger or about the 

same age as the Oligocene volcanic rock they cut.

None of these features or the structural effects 

related to them have been traced into the immediate 

vicinity of Aspen. The broad accordant ridge crests of 

the drainage divides east and north of Castle Creek 

(fig. 36) suggest the presence of an erosion surface at 

least as old as middle Tertiary. The high country of the 

Elk Mountains suggests middle Tertiary uplift of that 

region associated with the widespread emplacement of 

igneous rocks. The scarp on the east side of the White 

Rock pluton in the Hayden Peak quadrangle may be 

related to some late Tertiary reactivation of the fault 

there, or it may be a faultline scarp caused by middle 

Tertiary uplift of the more resistant rock of the west 

side.

Spurr (1898, p. 101-105) believed that several faults 

near Tourtelotte Park had moved recently. The scarps 

he described are best interpreted as faultline scarps 

between easily credible Belden Formation and resistant Leadville Limestone.

JOINTS, SHEARS, FAULTS, AND VEINS

Joints, shears, faults, and veins measured in the field 

are summarized on stereographic projections (fig. 52). 

They are summarized according to the rock unit in 

which they occur, for the age of each unit sets different 

constraints on the age of the structures. Possible relations to larger scale structural elements are suggested

in table 7. The table can be considered a matrix, for 

earlier joints may have an effect on the orientation of 

later major structural features as well as the reverse. 

Consequently, the oldest possible age of the larger 

elements is given.

The table does not clarify all the relationships between minor and major structures, but it does suggest 

that the joints measured in the Pennsylvanian and Permian clastic rocks, which alone can be dated only as 

younger than the rocks they are in, formed during intrusion and uplift during Oligocene time, for the joints 

in those rocks are parallel to the dike swarm connecting the Snowmass and White Rock plutons, and 

parallel or perpendicular to faults of Oligocene and 

probable Oligocene age.

Oligocene or younger joint sets are parallel to the 

margin of the Sawatch Range north of Aspen and to 

joints in Precambrian rocks. That joint set in the 

Precambrian rocks may have helped determine the 

trend of the Sawatch uplift, but it is not possible to

know which came first. Orientation of joints in the 

Precambrian rocks parallel with the trend of the Colorado mineral belt and the Homestake shear zone is 

not surprising, but joints of similar trend in Oligocene 

granodiorite are more difficult to explain. Perhaps 

those joints are related to inflation or deflation of the 

postulated batholith subjacent to the mineral belt. In 

the table no consideration is given to diagonal or shear 

joints. These may occur, and they would make the relations between minor and major structures even more 

complex.

QUATERNARY SURFICIAL DEPOSITS

The surficial deposits of the Aspen area are typical 

of high, steep, often-glaciated mountain country having a variety of rock types. Most of the deposits are the 

result of geologic processes active during the 

Pleistocene and the Holocene; some are the sites of 

deposition at the present.

GLACIAL GEOLOGY

The glacial deposits of the Aspen area are the result 

of numerous fluctuations in the climate over the past 2 

m.y., although most of the deposits mapped are probably younger than 200,000 years old. The available 

details of the glacial history are more numerous for the 

younger events.

Spurr (1898) postulated two main periods of glaciation. During the older one an ice sheet overrode all the 

hills, including the summit of Richmond Hill and 

Aspen Mountain. This was followed by a glaciation 

restricted to the valleys. He believed that the welldeveloped terraces down valley from Aspen were 

deposited in a glacial lake. Knopf (1926) recognized 

that Richmond Hill bore no evidence of glaciation. He 

postulated two main episodes of valley glaciation, the 

earlier one much more extensive than the later. Ray 

(1940) briefly discussed glacial deposits along the 

Roaring Fork from Aspen to the Continental Divide 

and correlated moraines with his "Wisconsin II 

through IV substages."

Valley glaciers during middle and late Pleistocene 

time originating in the high mountain country of the

FIGURE 52 (facing page).—Orientation of faults, shears, joints, and 

veins. All diagrams are equal-area projections in the lower 

hemisphere with planes of projection horizontal and north at 

the top. Contours show percentage of points falling within 1 

percent of the area of the diagram. A, Joints in Precambrian 

rocks. B, Faults and shears in middle and lower Paleozoic rocks. 

C, Joints in Maroon and Gothic Formations and hornfels. D, 

Joints in granodiorite. E, Shears and veins in hornfels. F, 

Shears and veins in granodiorite.
QUATERNARY SURFICIAL DEPOSITS 89

n = 89

CONTOURS 1, 3, 5 PERCENT

CONTOURS 1, 2, 4, 8 PERCENT

n = 82

CONTOURS 1, 3, 5, 7 PERCENT

CONTOURS 1, 3, 5, 7 PERCENT

CONTOURS 1, 3, 5 PERCENT

•

5

CONTOURS 2, 5, 10, 15 PERCENT 

2 5 10 15
90 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO 

TABLE 7.—Relation between mapped structural elements and measured joints, shears, and veins

[ _L, perpendicular to; leaders (———), not applicable]

Measured structures

Mapped or

inferred

structures

Orientation Age

Joints in

Precambrian

rocks

Faults and 

shears in 

lower 

Paleozoic 

rocks

Joints in 

Maroon and 

Gothic 

Formations

Joints in Shears and Shears and

granodiorite veins in veins in

hornfels granodiorite

Colorado mineral belt——— N50E 

and Precambrian shear 

zones.

Eagle Basin-Uncompahgre— N35-55W 

uplift trend.

West margin of Sawatch—— N10W 

Range south of Aspen.

West margin of Sawatch—— N30E 

Range north of Aspen.

Mapped faults in Highland N-S to N20W 

Peak quadrangle- N45E to EW

Dikes in Maroon Bells——— N80W-N60E 

and western part of 

Hayden quadrangle.

Faulted east margin of—— N30W 

White Rock pluton.

Precambrian——

Pennsylvanian— 

Late Cretaceous 

Late Cretaceous 

Oligocene(?)—— 

Oligocene————

Oligocene————

N50E vert

N10W65NE

N20E70NW 

J_N60W60NE

N45E60NW

J_N45E60NW

N25E vert 

J_N55W75NE ——— JN60W80NE

——— JN70E vert 

E-W70N

E-W65S N70E vert 

J_N10E vert E-W70N

E-W80N 

E-W80N

N15W vert 

N20E vert 

N5W vert 

N80W vert

JN34E

N35E vert 

N20E vert

_[_N60E to SE

Ages of measured structures

Younger 

than 

^1,700 m.y.

Late 

Cretaceous 

or younger

Younger Oligocene 

than or younger 

Pennsylvanian

Oligocene 

or younger

Oligocene 

or younger

Elk Mountains and the Sawatch Range descended the 

major valleys to altitudes of 2,440 m or below. In the 

Roaring Fork valley, glaciers from Castle, Maroon, and 

Hunter Creeks joined that of the Roaring Fork during 

at least two episodes of glaciation (figs. 53, 54). During 

the last major episode, however, the glaciers from 

those valleys did not coalesce. Lateral moraines 

preserved along the valleys in the mountains indicate 

that the ice was 300-400 m thick during the glaciations of the late Pleistocene.

Moraine and terrace deposits were mapped and correlated by several criteria. Physiographic expression 

and position were the two main criteria used. The 

younger moraines and terraces are at lower altitudes 

than the older ones. Of course, this means that the 

record of glacial events furnished by the deposits is incomplete, for it is likely that the evidence for glacial 

events of magnitudes smaller than the largest succeeding event was destroyed. The older moraines and 

terraces are more dissected and have more subdued 

topography than the younger ones. The differences in 

the size of gullies cutting terraces D, C, and B in the 

northwest corner of the Aspen quadrangle furnishes an 

excellent example of these differences in dissection. 

Valleys cutting the oldest surface, terrace D, are 30- 

45 m deep and extend all the way through the terrace. 

Valleys cutting terrace C are 15-18 m deep and do not 

extend all the way through the terrace. Valleys cutting 

terrace B are generally less than 12m deep and extend

less than 60 m into the terrace. Even a relatively major 

stream such as Owl Creek notches into the B terrace 

for a distance of only 90 m. The surface of moraine D 

and terrace D has many fewer boulders on it than the 

younger moraines and terraces.

The differences in physiographic expression of the 

moraines are best shown north of Burnt Mountain in 

the Highland Peak quadrangle. There, no morainal 

ridges are mapped on the D moraine, which is subdued 

in topographic expression and deeply dissected by 

stream valleys. Moraine C has some well-defined 

stream gullies in it, as well as well-defined morainal 

ridges. Moraines A and B contain undrained depressions.

MORAINE DEPOSITS

The moraine deposits are composed of poorly sorted 

material ranging in grain size from silt to boulders as 

much as 6 m in diameter. The larger fragments are 

usually rounded to various degrees. However, in some 

localities, such as the northeast side of the Roaring 

Fork valley upstream from Aspen and the upper part 

of the moraine on the end of the ridge between Castle 

and Conundrum Creeks, most of the large fragments 

are angular to subangular and of local derivation. The 

moraine deposits in the cirques also contain angular 

boulders.

Many of the larger moraine deposits show crude bed-
QUATERNARY SURFICIAL DEPOSITS 91

ding of sand and gravel in large cuts (fig. 55B). Such is 

the case in a gully on the west nose of Smuggler Mountain and in the large exposure at the head of the landslide on the west side of the Snowmass Creek valley. In 

general, one cannot tell moraine deposits and terrace 

deposits apart in drill logs of water wells because of 

general similarity of the materials. Presumably the 

moraine deposits contain more silt than the terrace 

deposits. On the basis of the logs, both deposits are 

composed of sand, gravel, and boulders, with scattered 

beds of clay. Thickness of moraine deposits is difficult 

to determine since in most places they are plastered 

against valley sides. On the ridge of Smuggler Mountain Spurr (1898) reported that the Bushwacker and 

Park Regent shafts passed through 120 m of morainal 

material. That is likely to be a maximum thickness of 

the moraine deposits. Knopf's description of highly 

weathered till in the Boulder shaft suggests that the 

thick section of morainal material there may be formed 

by moraines of more than one age, for the surface in 

that area has numerous boulders and was correlated 

with moraine deposit C by me.

Moraine deposit E, the oldest moraine mapped in the 

Aspen area, is best developed on top of Red Mountain 

at altitudes of from 2,925 to 3,170 m (fig. 53). On the 

steep south side of Red Mountain near the trail, 

boulders of Precambrian rock several meters in 

diameter are numerous. In some places smaller 

fragments of Sawatch Quartzite are common. On the 

northeast shoulder of Red Mountain subdued morainal 

topography is present. Closed depressions are almost 

filled with sediment but are still swampy. The preservation of these features is due to their position on a 

broad ridge crest away from much drainage. The lobes 

of moraine deposit E, mapped as extending down the 

mountain slopes are probably lag-and-slump deposits 

from the eroded moraine. Nevertheless, it appears that 

the moraine deposit may be 100-200 m thick in places. 

Patches mapped as moraine deposit E to the west are 

at lower altitude and may be younger. A moraine 

deposit mapped at 3,230-3,320 m altitude south of 

Warren Lake was correlated with that on Red Mountain. It is difficult to tell moraine deposit from colluvium on the Precambrian quartz monzonite, which 

tends to form spheroidal boulders upon weathering. 

Subtle differences in physiographic expression were 

the criteria used.

Moraine deposit D is best developed on the hills surrounding the lower part of the drainage of Owl Creek 

(fig. 53). The glacier depositing that moraine apparently spread well up into the Owl Creek valley, for a small 

patch in the eastern part of sec. 5, T. 10 S., R. 85 W.

contains Precambrian rocks that must have been 

brought in by the Roaring Fork glacier. The terminus 

of the main valley glacier must have spread out to a 

width of about 4 km. The glacial lobe spilling over the 

east shoulder of Burnt Mountain from Willow Creek 

must have nearly joined the Roaring Fork-Castle 

Creek-Maroon Creek glacier in the Owl Creek valley. 

Similarly, the Snowmass Creek glacier must have 

joined or almost joined the lobe spilling over the west 

shoulder of Burnt Mountain from West Willow Creek. 

The terminus of the Snowmass Creek glacier spread 

into the Wildcat Creek valley on the east and the 

Hunter Creek valley on the west. The erratics scattered over a considerable area in that drainage may be 

remains of moraine deposit D. Moraine D of the main 

valley glaciers has some morainal crests preserved 

either on drainage divides or in areas of gentle slope. 

These ridges are subdued features compared to those 

of the younger moraines.

Moraine deposit C was confined to the present 

valleys at the lower ends of the major glaciers (figs. 53, 

55). It formed rather massive morainal ridges, such as 

those south of the Aspen airport (fig. 55v4), on 

Snowmass Creek (fig. 55.B), on the west end of Smuggler Mountain, and near McFarlane Creek. It reached 

the farthest downvalley of any of the recorded valley 

glaciers on Snowmass Creek and the Roaring Fork. On 

Snowmass Creek it diverted the mouth of Wildcat 

Creek some 2 km north by damming its valley. It spilled 

over into the Brush Creek drainage from Snowmass 

Creek. The spillover glaciers on either side of Burnt 

Mountain were about 2 km shorter than during deposition of moraine D.

End moraine B (fig. 56) was deposited well upvalley 

from end moraine C. The Maroon and Castle Creek 

glaciers did not join the Roaring Fork glacier. The 

Roaring Fork glacier was 6 km shorter and the 

Snowmass Creek glacier 2 km shorter than during Bull 

Lake(?) time.

The moraine deposit just east of Aspen differs from 

other end moraines of the area because it contains 

much more well-bedded and well-sorted material of 

glaciofluvial origin. This material has been exposed in 

sand and gravel pits, and the few water well logs 

available on that moraine reflect the better sorted 

nature of the material. In the gravel pit, crossbedded 

fine- to coarse-grained sand and fine-grained gravel 

were exposed in 1967. The sand was at least 3 m thick 

and overlain by 1.2-1.7 m of gravel, and it was folded 

and faulted (fig. 56.B). As much as 6 m of sand were 

recorded in water wells nearby. The faults were closely 

spaced and had displacements of less than 2 cm.
92 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

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J

FIGURE 53 (above and right). Map of upper Tertiary and Quaternary rocks of the Aspen area. Simplified 

from Bryant (1969, 1970, 1971a, 1972a) and from Freeman (1972a, b). Base from A.M.S. 1:250,000, 

Leadville, 1957.
QUATERNARY SURFICIAL DEPOSITS 

EXPLANATION

[ al | 

Alluvium

93

ImrT

Neoglacial 

moraine 

deposit Terrace deposit 

A and alluvium

Rock-glacier Inactive rock 

glacier

Moraine Outwash 

deposit A, terrace 

B,C,D deposits 

B,C, and D

Basalt

Moraine 

deposit E

miu

? 7 

Moraine 

undivided

Neoglaciation

Pinedale 

Glaciation

Bull Lake(?) 

Glaciation

Sacagewea

Ridge(?)

Glaciation

About 

1.5 m.y. old

HOLOCENE

UPPER 

PLEISTOCENE

I MIDDLE 

C PLEISTOCENE

J

Gravel

LOWER 

'PLEISTOCENE

V TERTIARY

Whether the deformation of the sand was caused by 

overriding ice or by collapse due to melting of underlying ice is unknown, but I favor the latter hypothesis.

Moraine A is another 7 km up the Roaring Fork, 

about 4 km up Snowmass Creek and Maroon Creek, 

and 2 km up Castle Creek from moraine B (fig. 53). On 

Snowmass Creek are four closely spaced end moraines 

in the moraine A complex. Glaciers at this time did not 

spill from the Willow Creek drainage over the saddle 

east of Burnt Mountain, but they did spill over the saddle west of it.

In the mountain valleys, which are deep glacial 

troughs repeatedly filled with ice 300-400 m thick (fig. 

54), geomorphic processes during the Holocene have 

tended to destroy end moraines. A few do survive. On 

East Maroon Creek two end moraines are well preserved at 2,895 and 2,926 m altitudes. Richmond, 

Malde, and Tweto (1965) recognized an end moraine 

near the junction of West Maroon and East Maroon 

Creeks, which has been mostly destroyed by deposition of outwash and alluvial material and by erosion. 

However, a lateral moraine just west of the postglacial 

gorge of East Maroon Creek suggests that an end 

moraine existed near the stream junction.

The natural dam of Crater Lake on West Maroon 

Creek is a large landslide believed by Richmond, 

Malde, and Tweto (1965) to overlie a moraine. That 

may be so, but as far as I could determine, all the 

evidence is concealed under the landslide.

At 2,895 m altitude on West Willow Creek is another 

recognizable end moraine. A succession of at least 

three end moraines is found in the Roaring Fork valley 

east of the Aspen quadrangle (Richmond and others, 

1965; G. M. Richmond, unpub. mapping, 1963).

In the Castle Creek valley no well-preserved end 

moraine was mapped upstream from the complex at 

2,438-2,499 m altitude. One may have existed at 

2,680 m altitude near the Hope mine, but if so, it lacks 

well-defined geomorphic expression. Perhaps Holocene 

fan or landslide deposits conceal other end moraines.

End moraines are abundant in many cirques 

tributary to the main valleys (figs. 57, 6L4). In some 

cirques as many as four recognizable morainal loops 

are present. The end moraines are generally less than 

7 m high and do not show on the topographic maps, 

but they are well defined when viewed on the ground. 

These end moraines are located at 3,109-4,084 m in 

altitude and are of different ages.

OUTWASH TERRACE DEPOSITS

The outwash terrace deposits (fig. 58) are composed 

of bedded gravel, sand, and rare discontinuous lenses 

of clay. Boulders are widespread and are as much as 

3 m in diameter. Numerous logs from water wells in 

these deposits are all quite similar. Because these 

valley-fill deposits contain abundant water, many 

wells are not drilled entirely through them, and corise-
94 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

sS^tn •'* j ^l&$A Mi ^ i J1; :^ i ^

CONTOUR INTERVAL 200 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 ft = 0.3048m

FIGURE 54.—Glaciers in the Aspen area during maximum extent of Bull Lake(?) Glaciation. 

Drawn on basis of moraines and topography in Bryant (1969, 1970, 1971a, 1972a) and 

Freeman (1972a). Base from A.M.S. 1:250,000, Leadville, 1957.
QUATERNARY SURFICIAL DEPOSITS 95

FIGURE 55 (left and above).—Moraine deposit C and associated features. A, View of remains of inner end moraine C from 2,590 m 

altitude on slope of Red Mountain, Aspen quadrangle. Terrace B 

surrounds moraine in center of photograph and is deeply incised 

by the Roaring Fork River. Hill in foreground is composed of Fort 

Hays Member of Mancos Shale. Beyond terrace is lateral moraine 

of outer loop of moraine C. Lateral moraine D is behind it. Gap in 

left part of skyline ridge allowed glaciers from Willow Creek to 

spill over into Owl Creek drainage at least three times during the 

Pleistocene. B, Scarp at head of landslide on west side of 

Snowmass Creek valley north of Eagle Mountain reveals that 

lateral moraine deposit C -is composed of crudely bedded, poorly 

sorted gravels dipping away from the former glacier margin. 

Larger boulders arje 1-2 m in diameter. C, Alluviated valley 

behind lateral moraine of Difficult Creek north of Fall Creek, 

Hayden Peak quadrangle. Bare slope beyond valley bottom is 

side of lateral moraine mapped as C in figure 53, but could be partly D. View to northeast.
96 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

FIGURE 56.—Moraine deposit B. A, View northwest down the Roaring Fork River valley showing undissected alluviated valley 

behind moraine B, from 2,745 m altitude on the northeast side of 

the Roaring Fork River valley southeast of Aspen. City of Aspen 

in background is on outwash terrace B graded to moraine.

Moraine B occupies wooded area between meadows and town. 

Light-colored area is sand and gravel pit in moraine. Cutoff 

meanders in middle view. B, Bedded sands tilted and cut by faults 

of small displacement probably formed by collapse after melting 

of subj acent stagnant ice. Exposure in sand pit visible in A.

quently we do not know much about their total 

thickness. Some wells as much as 50 m deep do not 

reach bedrock. Most of the records are from wells 

drilled on terrace B, and few well records from the upper terraces are available. Mine workings running 

beneath the Roaring Fork between East Aspen Mountain and Smuggler Mountain are at 200 m below the 

surface; that figure places an upper limit on the 

thickness of the valley fill there. v

The outwash terraces and their deposits are correlated to the moraines and their deposits by their 

physiographic relations (figs. 53, 59). The most obvious relation between moraine and terrace is between 

the moraine just east of Aspen and the terrace on 

which the city is built. That terrace is graded to the 

moraine. Similarly, the moraine at the mouths of Castle and Maroon Creeks is graded to the same terrace. 

That moraine was considerably modified by melt water 

so that its contact with the outwash is quite gradational. Terrace C is found only downvalley from 

moraine C, and terrace D is downvalley from moraine 

D. No terrace appears to be associated with the remains of the end moraine in the middle of the valley 

west of Red Butte; that moraine was mapped as part of 

moraine deposit C. Terrace A is not continuous, and in 

the Woody Creek quadrangle (Freeman, 1972a) it occupies the present meander belt of the Roaring Fork. 

This suggests that it formed under hydrologic conditions similar to the present, rather than being formed 

under the different conditions of the Pleistocene, as 

suggested by correlation to moraine A shown on the 

geologic map of the Aspen quadrangle (Bryant, 197la). 

Terrace A is about 5 m above the Roaring Fork at the 

city of Aspen, but downstream in the Woody Creek 

quadrangle it is about 12 m above the stream.

On the other hand, terrace A in the Snowmass Creek 

drainage of the Woody Creek quadrangle (Freeman, 

1972a) occupies a good portion of the width of the 

valley and is probably an outwash terrace related to an 

end moraine. The main part of terrace A starts about 

1.5 km north of the north edge of the Highland Peak 

quadrangle at an end moraine, which I designate as 

moraine B. A small area of terrace is found on 

Snowmass Creek just north of the Highland Peak 

quadrangle. Freeman (1972a) chose to map the end 

moraine near the quadrangle boundary as the outermost moraine deposit A; whereas I mapped the outer-, 

most moraine deposit A at 2 km upstream. I suggest 

that terrace deposit B of the Roaring Fork drainage in 

the Aspen and Woody Creek quadrangles correlates 

with Freeman's terrace deposit A of the Snowmass 

Creek drainage because both are the youngest widely
QUATERNARY SURFICIAL DEPOSITS 97

developed terraces of their respective valleys (fig. 53), 

although the terrace in the Snowmass Creek drainage 

is only 9-12 m above the creek compared to 24-36 m 

above the Roaring Fork in the Woody Creek 

quadrangle. It follows that terrace B of Freeman 

(1972a), about 40 m above Snowmass Creek, must correlate with terrace C of the Roaring Fork valley, as indicated in figure 53. Snowmass Creek is a considerably 

smaller stream than the Roaring Fork River.

AGE AND CORRELATION OF GLACIAL DEPOSITS

Few detailed studies of glacial geology have been 

made in the immediate region. However, I would like 

to suggest some correlations with the glacial sequence 

studied by Nelson (1954) in the Fryingpan River 

drainage and by Miller (1971) on Lake Creek and East 

Brush Creek on the northwest margin of the Sawatch 

Range. Unfortunately, I have not made the quantitative studies of weathering, boulder frequency, and 

soil development that Nelson did. Correlations are 

made on physiographic expression and position. The 

relative ages of deposits in the local sequence are most 

certain; correlation with the sequence on the Fryingpan River is less certain; and correlation to some 

absolute time scale is quite speculative.

The oldest glacial deposits of moraine deposit E may 

be correlative with thick gravels on the ridges 

southwest and southeast of Snowmass in and west of 

the Woody Creek quadrangle (Freeman, 1972a; 

Welder, 1954). The gravels lie higher than the 

1.5-m.y.-old basalt near Woody Creek (Freeman, 

1972a; Larson and others, 1975). The gravels in Kobey 

Park in the Ruedi quadrangle (Freeman, 1972b) lie at

S^'-TS ~ ' "*3tfe •j^-*.." •• - ii •*• K •- "•- r

. .

FIGURE 57.—Cirque moraines at 3,260-3,290 m altitude northeast 

of Hunter Peak, Maroon Bells quadrangle.

FIGURE 58.—Outwash terraces in the Roaring Fork River valley below Aspen viewed north from 2,915 m altitude on ridge southwest 

of Sterner Gulch, Aspen quadrangle. Lowest extensive terrace B 

heads at moraine east of the city of Aspen and is about 40 m 

above the river. Hill on it at right is part of inner end moraine C. 

Middle terrace C heads at right middle of view and is about 90 m 

above the river. Highest terrace D heads at right margin of 

photograph and is about 120 m above river.

much higher altitudes and may be related to gravels 

associated with 10-20-m.y.-old basalts on the high 

plateaus and interstream divides, such as Grand Mesa 

and the Flattops (fig. 59). The Roaring Fork River has 

cut down about 480 m since the high gravels 

southwest of Snowmass were deposited. If they are 

glacial outwash gravels, this cutting occurred during 

the Pleistocene. Since extrusion of the 1.5-m.y.-old 

basalt some 10 km upstream, the bed of the Roaring 

Fork River has been lowered 380 m; so if the 

Pleistocene is 2 million years or longer in duration, the 

high-level gravels may be of Pleistocene age. No such 

high-level deposits are described by Nelson (1954) from 

the Fryingpan River drainage, but V. L. Freeman (oral 

commun., 1975) found small remnants of gravel 360 m 

above the Fryingpan River south of Ruedi, and farther 

north Miller (1971) found till 460 m above the valley 

bottom on a ridge north of East Brush Creek. Near 

Durango, Richmond (1965b) correlated gravels as 

much as 600 m above the Animas River with early 

Pleistocene glaciations.

Other deposits perhaps correlative with an early 

Pleistocene glaciation occur between 2,605 and 3,110 

m altitude on the divide between Capitol and 

Snowmass Creeks, in the Capitol Peak quadrangle.

Moraine and terrace deposits D were correlated with 

the Sacagawea Ridge Glaciation of the Wind River 

Mountains by Richmond, Malde, and Tweto (1965).
98 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

NORTHWEST 

3400

WOODY CREEK 

QUADRANGLE

HIGHLAND PEAK 

QUADRANGLE

ASPEN 

QUADRANGLE

SOUTHEAST

Kobey Park

Moraine E, 

Mountain

^Moraine D

3300

3200

3100

3000

2900

2800

2700 -

2600

2500

2400

2300 -

2200

2100

\=^^________________________________________

16

KILOMETERS

FIGURE 59.—Profile along the Roaring Fork River valley showing present stream grade, terrace levels, moraines, and older gravels. Modified from Bryant (1971a, 1972a) and Freeman (197-2a, b). Dashed line indicates surface of moraine.

High level gravels 

south of Snowmass Moraine A 

and landslide

Basalt west 

of Red Canyon

Moraine \ 

/~~~ Moraine C /

/y-lnner moraine 

Terrace B

Terrace A

Present bed of 

the Roaring Fork

Those deposits may correlate (table 8) with the "Lime 

Creek stage" of Nelson (1954). Apparently, at the time 

when the gravels of terrace D were deposited, the Roaring Fork flowed over Watson Divide (Freeman, 1972a) 

and probably down the valley now occupied by East 

Sopris Creek. Gravels at Watson Divide contain 

material from the basalt flow on the north side of the 

Roaring Fork. This does not mean that all the canyon 

between Snowmass and the basalt flow was carved between the Sacagawea Ridge Glaciation and the present, for headward erosion along the Castle Creek fault 

zone had already almost captured the Roaring Fork by

this time. Although Richmond (1965a) correlated the 

Sacagawea Ridge Glaciation with the Illinoian of the 

midcontinent, recent work has indicated that some 

deposits assigned to the Bull Lake are late Illinoian 

(Pierce and others, 1976). Consequently, the D terrace 

deposit is early or pre-Illinoian; thus, more time is indicated for diversion of the Roaring Fork and some 

150 m of valley-cutting between deposition of terrace 

D and the present. Yet 240 m of valley-cutting apparently took place between the eruption of the 

1.5-m.y.-old basalt and the deposition of terrace unit D 

(fig. 59).
QUATERNARY SURFICIAL DEPOSITS 99 

TABLE 8.—Correlation of Pleistocene and Holocene events, Roaring Fork and Fryingpan drainages

Roaring Fork drainage Fryingpan drainage 

(Nelson, 1954)

Glaciation

Rock glaciers and a few cirque moraines Neoglaciation Holocene.

upvalley from moraine A.

headward erosion along Castle 

Creek fault zone.

of Snowmajis.

"Thomasville substage"

Late Pleistocene.

• Middle Pleistocene( ?) .

• Early Pleistocene(?) .

Gravels at Kobey Park- Pliocene(?).

Terrace and moraine deposits C are correlated with 

the "Bigelow and Thomasville substages" of Nelson 

(1954) (table 8). The terminal moraines of those 

"substages" are breached a similar amount, although 

the quantitative weathering data suggests some age 

difference between the Thomasville and Bigelow 

moraines. A distinct terrace was related to each of 

them by Nelson, but as mapped, the terrace related to 

the Bigelow cuts the Bigelow moraine. This terrace 

does not bear the same relation to the end moraines as 

the terraces and moraines I have correlated. Possibly 

Nelson's Bigelow terrace is related to a "washed-out" 

end moraine south of Norrie. The streambed of the Fryingpan River upstream from Norrie appears less well 

established than below Norrie, for it contains swampy 

stretches of undissected alluvium. Therefore, I would 

correlate Nelson's Thomasville terrace, which is about 

60 m above the Fryingpan River near Thomasville, 

with both the Thomasville and Bigelow moraines. That 

terrace height is greater than the 27-36 m reported by 

Nelson (1954) and is obtained from a terrace remnant 

just west of Thomasville shown on the 1970 Meredith 

7.5-minute topographic quadrangle, which was not 

available at the time of Nelson's work. Miller (1971) on 

Lake Creek found two moraines which he correlated 

with the Thomasville and Bigelow of Nelson. The 

Roaring Fork valley must have contained at least two 

end moraines in the material mapped as moraine 

deposit C, for a remnant of the younger one forms a hill 

60 m high, engulfed by terrace B in the middle of the 

valley wesit of Red Butte. The moraine C of the Aspen 

area forms massive moraines as do the Bigelow 

moraines of the Fryingpan River drainage. However,

terrace C is somewhat higher above the Roaring Fork 

River than the Thomasville terrace is above the Fryingpan River. These deposits are tentatively correlated 

with the Bull Lake.

Moraine and terrace B and younger moraines are 

probably of Pinedale or Wisconsin age. The Roaring 

Fork River has scarcely breached the moraine above 

Aspen, and the same relation holds for the end 

moraines of the "Ivanhoe substage" in the Fryingpan 

River drainage (Nelson, 1954). The B end moraine in 

the Roaring Fork valley was probably underlain by 

stagnant ice after retreat of the glacier front so that a 

number of depressions formed where sand was first 

deposited from melt water and then disturbed by collapse by further melting of underlying ice. The 

presence of an untrenched alluviated valley behind this 

moraine suggests a Pinedale age for it.

Moraine A and moraines farther upvalley may correlate with Nelson's (1954) "Hell Gate substage and 

Chapman Gulch glaciation." In the Fryingpan River 

drainage these moraines are above 3,050 m altitude, 

but in the Aspen area they are as low as 2,560 m 

altitude on the Roaring Fork, 2,470 m on Castle Creek, 

2,500 m on Maroon Creek, and 2,440 m on Snowmass 

Creek. On the Fryingpan River a lower four- to fiveloop end-morainal complex and an upper single-end 

moraine are mapped as "Hell Gate substage" (Nelson, 

1954). In the Aspen area at least four or five end 

moraines are found upstream from moraine A, and as 

many as four end moraines are found in cirques 

tributary to the main valleys.

Probably very few of the cirque end moraines are 

Neoglacial. Birkeland and Miller (1973) and Currey
100 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

(1974) gave evidence supporting the idea of Moss 

(1951) that the type Temple Lake moraine in the Wind 

River Mountains of Wyoming is pre-Altithermal in age 

and perhaps late Pinedale rather than early Neoglacial, 

as that term has been used in recent reports. They 

found that the Neoglacial moraines are much farther 

upvalley than the moraine at Temple Lake, and that 

the youngest moraine fronts an existing glacier. Carrara and Andrews (1973) concluded that the cirque 

moraines of the San Juan Mountains are late Pinedale, 

and Miller (1973) found only one Neoglacial cirque 

moraine in the northern Sawatch Range. Benedict 

(1973) found moraines near timberline in the northern 

Front Range to date from about 10,000-11,000 years 

old.

Very fresh high-altitude moraines 'in sheltered 

localities near Castle Peak are probably Neoglacial (fig. 

60). The principal mass of the moraine in the cirque 

heading on the north face of Castle Peak has three endmoraine ridges, which might be correlated with the Triple Lakes, Anderson, and Arapaho Peak advances of 

Benedict (1973) in the northern Front Range. The 

deposit of angular rock fragments between the lower 

end moraine and the upper two may be an incipient 

rock glacier; however, it lacks the characteristic ridged 

topography indicative of flow, and the end moraines 

are preserved. This mass of rock may constitute icecored moraine. A depression usually partly filled with 

snow occurs behind the innermost end moraine and 

marks the site of former ice (fig. 60.4). Two 

small moraines at 3,900 m altitude on north-facing 

slopes northwest and north of Cathedral Peak and one 

north of Hayden Peak may also be Neoglacial. 

Moraines of Neoglacial and even late Pinedale age are 

lacking in many cirques. Rock glaciers substitute for 

moraines in those cirques.

Glaciers and a full set of Neoglacial deposits occur in 

the northern Front Range only about a degree in 

latitude north of the Elk Mountains (Benedict, 1968, 

1973; Madole, 1972). The Elk Mountains probably 

have more precipitation in the winter than the Front 

Range, but the Front Range is much windier. Consequently, the snow there accumulates in sheltered 

localities in north- and east-facing cirques, and small 

variation in regional snowfall may lead to greater fluctuation in glaciation (Benedict, 1973).

The Fryingpan River drainage had a different 

regimen in the Pleistocene than the Roaring Fork 

drainage. Pinedale or late Pleistocene glaciers descended to a lower altitude in relation to Bull Lake(?) 

glaciers in the Roaring Fork than in the Fryingpan. 

The outwash terraces related to the various glacial 

episodes are more widely separated in the upper RoarFIGURE 60.—Neoglacial moraines in Hayden Peak quadrangle. A, 

Two morainal loops and closed depression behind them at 4,084 m 

altitude in cirque on north side of Castle Peak, viewed from above 

on slope of Conundrum Peak. Larger blocks in moraine about 1 m 

in diameter. B, Moraine at 4,053 m altitude in small cirque north 

of Conundrum Peak, viewed from below. Larger boulders are 1-2 

m in diameter. Typical exposures of granodiorite.

ing Fork than in the Fryingpan River drainage. This 

difference may be related to the capture of the upper 

course of the Roaring Fork River leading to increased 

rate of downcutting in the late Pleistocene. In addition, the outwash deposited during each glacial episode 

in the Roaring Fork drainage was probably thicker 

than that deposited in the Fryingpan River drainage. 

Glacial debris eroded from the Elk Mountains south of
QUATERNARY SURFICIAL DEPOSITS 101

Aspen was brought into the valley west of Aspen in addition to that brought in by the Roaring Fork and 

Hunter Creek from the Sawatch Range.

ROCK GLACIERS

Many tens of rock glaciers occur in the high country 

of the Elk Mountains southwest of Aspen (fig. 53). 

Most of them are at altitudes between 3,770 and 

3,350 m, but a few extend to below 3,050 m. Both cirque and valley wall types (Outcalt and Benedict, 1965) 

occur, but cirque types dominate. The largest rock 

glaciers are 1.8 km long and 0.45 km wide and the 

smallest less than 100 m long. The smallest rock 

glaciers, such as those in the large area of talus north 

of Buckskin Mountain, Maroon Bells quadrangle, consist of convex masses of talus that appears to have 

flowed. These small bodies may have quite a different 

origin than the larger bodies; these latter have the 

shape of ice glaciers and exhibit a complicated but 

orderly ridge-and-furrow surface topography whose 

pattern suggests viscous flow superposed on their convex cross profile (fig. 6L4). The rock glaciers are 20-40 

m thick and have steep fronts and margins (figs. 40C, 

61JB). The surfaces are composed of angular blocks 0.1 

m to several meters in diameter. Silt and sand are commonly exposed in their fronts, which are at steeper 

than the angle of repose and shed talus. Their fronts 

are the source of streams in places during the summer. 

A small pond occurs on the rock glacier north of 

Pyramid Peak, and on one rock glacier a stream was 

flowing on the surface of the rock glacier face when I 

crossed it.

Where rock glaciers and end moraines are found in 

the same cirque, the rock glaciers always cut the end 

moraines (fig. 6 LA). In other places, such as the cirque 

southwest of American Lake, Hayden Peak 

quadrangle, rock glaciers appear to have formed from 

morainal material augmented by talus from the valley 

wall.

Old rock glaciers, as mapped, are rock glaciers which 

have a well-developed cover of tundra vegetation, or, at 

lower altitudes, a forest cover. These rock glaciers are 

apparently inactive, for they do not have the steep 

fronts and sharply defined lobes of the rock glaciers 

lacking a full vegetative cover. The old rock glaciers 

have the convex cross profile of the active rock glaciers 

and they lack the terminal ridge of the cirque moraine 

complexes. The old rock glaciers are usually adjacent 

to or below active-appearing rock glaciers (fig. 61C). In 

places the active rock glacier overrides the old rock 

glacier. This relation is well illustrated in the cirque 

southwest of Cathedral Peak (fig. 61C). Some of the

active rock glaciers have scattered patches of tundra 

vegetation and even a few trees where their surface is 

below timberline. Lichen-covered rocks are common on 

their lower parts, and rocks lacking lichen are common 

at their heads; but no detailed studies of lichen 

distribution were made.

At the head of the rock glaciers in the higher more 

sheltered cirques snowbanks often remain all year. At 

the upper margin of the rock glacier in the cirque 

southeast of Pyramid Peak in 1963 glacial ice having 

small crevasses was visible. It is likely that ice occurs 

at the heads of other rock glaciers, although it may 

seldom be visible because of the snow cover.

Measurements over a 4-year period of the rock 

glacier north of Pyramid Peak and the one north of 

North Maroon Peak showed a maximum movement 

rate of 68 cm a year (Bryant, 1971b). The glaciers 

measured should be moving about as fast as any in the 

Aspen area judging by their aspect and setting. This 

movement rate exceeds maximum rates measured on 

other Colorado rock glaciers (12.5 cm/yr, Miller, 1973; 

20 cm/yr, Outcalt and Benedict, 1965; 22.5 cm/yr, 

White, 1971). Comparable movement rates have been 

measured on a rock glacier in Wyoming (Potter, 1972) 

and in Alaska (Wahrhaftig and Cox, 1959; Foster and 

Holmes, 1956).

AGE OF ROCK GLACIERS

A number of studies have recently been done on rock 

glaciers in Colorado (Benedict, 1967, 1968, 1973; 

White, 1971; Mahaney, 1973; Birkeland, 1973; Carrara 

and Andrews, 1973; Miller, 1973). In the Front Range 

at about lat 40° N. (a degree north of the Elk Mountains), lichen growth curves have been established and 

used with success (Benedict, 1967, 1968; Mahaney, 

1973). There, moraines and ice glaciers are about 

100-300 years old (Arapaho Peak advance, Benedict, 

1973); moraines and ice-cored and presently active rock 

glaciers date from 1,000 to 1,900 years ago (Audubon 

advance, Mahaney, 1972; Benedict, 1973), and 

vegetated moraines as much as 1.3 km downvalley 

date from 5,000 to 3,000 years ago (Triple Lakes advance, Benedict, 1973). Recent studies in the San Juan 

Mountains (Carrara and Andrews, 1973) and on Mount 

Sopris in the northwestern Elk Mountains (Birkeland, 

1973) point out the difficulty of using lichen growth 

curves established elsewhere on different rock types. 

They found that many rock glaciers date from late 

Pinedale time, but that these rock glaciers either continued to be active, or were reactivated numerous 

times, during the Holocene. In the northern Sawatch 

Range, Miller (1973) found rock glaciers that formed
102 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

je*..- «" '

B

JSOtli.;*

FIGURE 61.—Rock glaciers. A, Rock glacier at the head of West 

Maroon Creek viewed from ridge above. Main part of rock glacier 

is at altitude 3,660-3,536 m altitude. Typical pattern of ridges 

and swales suggesting flow. It cuts cirque moraine of late 

Pinedale(?) age on right (light-colored ridge). An outer cirque 

moraine extends down valley to right bend in stream gully 

(altitude 3,475 m). B, Steep active front of south part of rock 

glacier in cirque west of Hunter Peak, Maroon Bells quadrangle 

(altitude 3,475 m). Note silt and sand exposed in steep front, 

whereas no matrix is visible between large blocks on upper surface of glacier. C, Rock glaciers in cirque southwest of Cathedral 

Peak extending from 3,932 to 3,566 m altitude. Rock glacier at 

right has had some fine-grained as well as coarse rock on surface, 

and tundra vegetation is developed. This was mapped as old rock

glacier. Rock glacier at upper left lacks interstitial fines on upper 

surface and vegetation; and it has high, steep margins, suggesting that it is active at present.
QUATERNARY SURFICIAL DEPOSITS 103

during two earlier Neoglacial advances and are active 

now, but he found no rock glaciers that formed during 

the Gannett Peak Stade and only one Neoglacial 

moraine, which dated from the Temple Lake Stade. 

Yount (1970) found no rock glacier or moraine from the 

Gannett Peak Stade on the crest of the Sawatch Range 

east of Aspen.

Unfortunately, I had completed mapping of the high 

country of the Aspen area by the time the lichenometric studies of Benedict (1967) appeared; so I do 

not have information comparable with that of recent 

rock-glacier studies. However, in view of the studies 

cited previously it seems likely that the rock glaciers of 

the Aspen area originated at a number of times, 

although most of them are presently active. The old 

rock glaciers are inactive, but they may have formed at 

about the same time as parts of adjacent active rock 

glaciers. The rock glaciers formed after retreat of the 

Pinedale glaciers, and they are younger than most of 

the cirque moraines. Many of the rock glaciers at lower 

altitudes, such as below 3,505 m, may have formed 

during the late Pinedale; and the large ones which extend from the cirque headwalls down to that altitude 

may have a long complex history similar to that deduced by Birkeland (1973) for similar large rock glaciers 

on Mt. Sopris. Others may have formed during one or 

more advanced of the Neoglaciation, but judging from 

Miller's (1973) work perhaps very few formed in the 

Arapaho Peak advance. However, many of the rock glaciers are currently being supplied by talus and rockfall 

from adjacent peaks, and they have active advancing 

lobes or fronts.

LANDSLIDE DEPOSITS

Landslide deposits are numerous in the Aspen area, 

and they are composed of a variety of materials ranging from rock-glacier and morainal deposits to sandstone and quartz monzonite. They are most numerous 

on dip slopes of sedimentary rocks, but they also occur 

on steep terrane in fractured rock of any type. An 

outstanding example of a dip-slope landslide is in the 

cirque at the head of the West Fork of Brush Creek 

where glacial erosion cut through a dip slope on 

Dakota Sandstone, and after deglaciation, the unsupported Dakota slid down bedding in the underlying 

Morrison Formation. Landslides occur on steep 

moraine- or shale-covered slopes. In places steep talus 

or rock glacier may break loose and form landslides. 

On Conundrum Creek south of Silver Dollar Pond are 

two examples of this type of landslide (Bryant, 1969).

Landslides are composed of blocks as much as 10 m 

in diameter mixed with smaller ones, and commonly

with silt and sand. Landslides in Mancos Shale mostly 

lack large blocks; some are mudflows.

Landslides characteristically have irregular hummocky topography that lacks either the orderly ridges 

of moraine or the flow lines and convex cross profile of 

rock glaciers (fig. 62). The largest landslides are 2 km 

across. Thicknesses vary greatly. Some of the dip-slope 

varieties must be only one to a few meters thick in 

places. For instance, a dike was traceable through a 

landslide on the west side of East Snowmass Creek. 

Such landslides may grade to colluvium, which lacks 

morphological evidence for mass movement. Others, 

such as the one on the south side of the Brush 

Creek-Snowmass Creek divide, must be at least 10 or 

more meters thick. Two unusually thick landslides are 

the one damming Crater Lake on West Maroon Creek 

and the one on the Roaring Fork at the southeast corner of the Aspen quadrangle. The one at Crater Lake 

may be as much as 50 m thick, and even thicker if no 

moraine underlies it. Most of the material came from 

the steep north end of the northwest ridge of Pyramid 

Peak, and glacial oversteepening combined with bedding dips toward the valley produced conditions 

favorable for landsliding. The landslide on the Roaring 

Fork was mapped as moraine on the geologic maps 

(Bryant, 1970, 1971a), but further study indicates that 

it is a landslide, having a cusp-shaped breakaway area 

in the northeast corner of the Hayden Peak quadrangle 

(fig. 62A). It contains in places angular pieces of 

Precambrian quartz monzonite as much as 7 m long. 

The landslide apparently overlies a moraine of late 

Pleistocene age exposed on Colorado State Highway 

82. That landslide may be as much as 100 m thick.

The landslides are of various ages. Some are as old as 

Pleistocene. One landslide near Rustler Gulch is apparently older than a Pinedale lateral moraine. 

However, map relations indicate that much landsliding 

took place after retreat of the main valley glaciers so 

that most of the landslides are of very late Pleistocene 

and Holocene ages (fig. 62jB). Some landslides are probably intermittently active at present. On Snowmass 

Creek a large landslide-mudflow took place in 1956 

because of excessive soaking through a break in an irrigation ditch on a lateral moraine overlying Mancos 

Shale.

Harden (1976) gave a detailed analysis of landslides 

in the northwestern part of the Aspen area and pointed 

out their significance in land use planning.

SOLIFLUCTION DEPOSITS

Solifluction deposits were mapped above timberline 

more than at lower altitudes. Large areas of solifluction debris on the gentle surface on Precambrian rocks
104 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

FIGURE 62.—Landslides. A, Landslide of Precambrian quartz monzonite south of Tagerts Lake, Aspen quadrangle (mapped as 

moraine on geologic quadrangle map (Bryant, 197la)) viewed 

from 2,800 m altitude south across valley of the Roaring Fork 

River. Amphitheater at head. Very large angular blocks in lower 

right are as much as 6 m across. B, Landslide on south slope of 

ridge south of Rustler Gulch, Maroon Bells quadrangle. Maroon 

Formation above the Elk Range thrust slid down over Mancos 

Shale of the underblock after glacial erosion steepened the valley 

and glacier melted away.

south of McArthur Mountain were not mapped (fig. 

36). On the east side of the north end of Richmond Hill 

a large area of solifluction deposits was mapped where 

no outcrops or prospect pits showed how to map the 

many faults leading into the area.

Solifluction deposits consist of angular rock 

fragments in a matrix of silt and sand, all which have 

moved down a slope, which in many places is relatively 

gentle. They trade to talus or colluvium. Lobes or terraces are common surface features of these deposits, 

but they are subdued to absent in places.

The solifluction deposits are of Holocene and 

Pleistocene age. Most of the mapped deposits are between 3,660 and 3,960 m altitude and are probably now 

active, for permafrost is reported in the iron mine between altitudes 3,660 and 3,900 m at the head of 

Cooper Fork. The deposit on the east side of Richmond 

Hill is between 3,170 and 3,292 m altitude and probably not presently active.

COLLUVIUM

Colluvium is widespread in the Aspen area, and it 

was mapped above timberline much more thoroughly 

than at lower altitudes. It is well developed on dip 

slopes where it may grade to landslide. In gullies and 

cirques it grades to talus. In areas of complex geology, 

such as south of Aspen, it was mapped principally 

where it obscured the bedrock geology and where no 

prospect pits penetrated it. No colluvium was mapped 

on Smuggler Mountain or Red Mountain and very little 

in the northern part of the Highland Peak quadrangle.

Colluvium consists of poorly sorted material ranging 

from silt to boulders. The finer grained fractions usually are dominant, and the deposit supports vegetation. 

Although much of it is probably of Holocene age, some 

may be Pleistocene.

TALUS

Talus occupies large areas in the alpine country of 

the Aspen region. It occurs below steep cliffs on the 

sides of ridges and fringing the headwalls of cirques. 

Especially large aprons of talus are found on the west 

side of East Maroon Creek east of Pyramid Peak and in 

the cirques on the east side of that valley.

Talus is most widespread in areas underlain by hornfels (fig. 45.B). The hornfels is highly jointed and readily breaks down through frost action, but the in-
QUATERNARY SURFICIAL DEPOSITS 105

dividual fracture-bounded fragments are themselves 

quite resistant to weathering. Granodiorite is also an 

excellent source of talus, especially where fractured 

and silicified, as it is in some of the areas in which 

sulfides are disseminated.

Talus consists of angular rock fragments ranging 

from 1 cm to a meter in diameter. It is locally well 

sorted. It forms sheetlike deposits that are generally 

near or at the angle of repose. With increase in sand 

and silt content talus grades to colluvium; it also 

grades to alluvial fans and rock glaciers.

Talus as mapped has a variety of ages. Most of the 

talus in the high mountain country is probably late 

Pleistocene and Holocene in age. Many areas of talus 

are presently active, especially adjacent to the high 

peaks, such as Cathedral Peak and the Maroon Peaks; 

on almost any summer day one can hear rockfalls 

feeding those deposits. Other areas, such as that along 

the lower part of the talus apron on the west side of 

East Maroon Creek, are partly vegetated with young 

aspen trees. Those areas are presently inactive, 

although they are subject to snow avalanches in the 

winter. The talus there formed after the retreat of the 

glacier which occupied the valley of East Maroon 

Creek in the late Pleistocene.

ALLUVIAL AND 

DEBRIS FAN DEPOSITS

Alluvial and debris fan deposits are common 

features of the landscape of the Aspen region. They are 

best developed where short steep tributary valleys join 

a major valley. One area in which individual fans are 

most obvious is in the Roaring Fork valley 3-6 km 

upstream from Aspen. There, the fan-shaped deposits 

of each tributary encroach on the level alluvium on the 

floodplain of the main stream. In many places fans 

along a valley side coalesce so that the whole slope consists of fan deposits. This occurs on the north side of 

West Maroon Creek from Maroon Lake to the junction 

with East Maroon Creek and downstream along 

Maroon Creek.

The alluvial and debris fan deposits generally consist 

of poorly sorted material ranging from silt to boulders. 

They grade to talus or colluvium on their upslope 

margins and to alluvium on their downslope margins. 

Many of the deposits in the deep mountain valleys consist to a large extent of morainal material that was 

washed down from the valley sides. Judging from their 

topographic expression many of the larger fans may be

thicker than 30 m. The size of the fans is not necessarily related to the size of the streams which 

deposited them but is more related to the supply of 

easily erodible bedrock or unconsolidated material, 

such as moraine or talus, in their drainage. Good examples of relatively larger fans derived from small 

drainages are on Castle Creek at Monument Gulch, on 

the gulch to the south, and on the gulch west of Elk 

Mountain Lodge. This relation is also shown by the 

development of fans where small valleys draining Mancos Shale join major valleys. One particularly bulky 

fan heading in a short tributary valley apparently 

dams West Maroon Creek to form Maroon Lake.

Many of the fans are active and could have debris 

deposited on them during floods. In the glaciated 

valleys they started to form after withdrawal of the 

glaciers in the late Pleistocene. It is likely that fan formation was quite vigorous immediately after deglaciation because of the abundant supply of unvegetated 

moraine on the valley sides. Fans in the northern part 

of the Highland Peak quadrangle outside the glaciated 

area probably have a longer history, and it is difficult 

to know when they started to form.

A few fans are dissected and no longer active. In the 

northwestern part of the Highland Peak quadrangle 

several older fans have their surfaces about 20 m above 

younger fans. The map relations suggest that these 

fans are younger than moraines but older than the fans 

of late Pleistocene and Holocene age. Fans overlying 

terraces in the Roaring Fork River valley are younger 

than the terrace they rest on. The one in the 

northwestern part of the Aspen quadrangle is not 

much dissected and could still be the site of deposition 

given the right conditions. However, fans in a similar 

position north of Woody Creek are deeply dissected 

(Freeman, 1972a) because their drainage goes across 

rather than along terrace.

Other older, not presently active, fans are what 

might be termed "hanging fans," which formed 

against the glacier margin at tributary valleys during 

late Pleistocene time. These occur along gullies 

tributary to Maroon and West Maroon Creeks north 

and northwest of Maroon Lake.

ALLUVIUM

Alluvium occurs in small areas on valley floors 

throughout the Aspen region. The largest alluvial 

deposit is along the Roaring Fork River valley 

upstream from Aspen and is as much as 0.7 km wide.
106 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

There, the stream meanders and oxbow lakes and scars 

of abandoned meanders are well displayed (fig. 56-4). 

This deposit is probably underlain by outwash gravel, 

and it formed behind an only partially breached end 

moraine of Pinedale age. Other areas of alluvium 

formed behind postglacial alluvial fans. Areas in 

Hunter Creek and in Castle Creek near Ashcroft are excellent examples of places where this occurred. In 

other places alluvium formed in tributary valleys 

behind lateral moraines (fig. 55(7). Examples of such 

deposits are southeast of Richmond Hill and in the 

northwest corner of the Highland Peak quadrangle.

Alluvium consists of sand, silt, gravel, and peaty 

material on valley bottoms, and it includes some 

swamp deposits.

Smuggler Mountain is crossed by valleys believed to 

be controlled by fractures. Near the crest of the mountain, alluvium occurs locally in these valleys. The most 

notable area of alluvium is at Warren Lake where peat 

has been dug on a commercial basis. There, the peat is 

at least 1.8-2.1 m thick and contains scattered 

boulders of Precambrian rocks.

Logs of water wells in the alluvium along the Roaring Fork above Aspen are quite similar to those in the 

outwash deposits at and downstream from Aspen except that some have an interval at depths of 15 m and 

below that lacks boulders and gravel. All the logs show 

boulders in the upper part, although the surface of the 

alluvium appears to consist of fine-grained material. 

Perhaps at one time the valley was occupied by a lake 

dammed by the moraine B. Then, when the ice formed 

moraine A below Tagert Lake, outwash was deposited 

above the finer material. Relatively thin deposits of 

sand and silt deposited in Holocene time blanket the 

present surface.

In the unglaciated valley of Woody Creek, some 

patches of older gravel were mapped, which probably 

correlate with the gravel C mapped in the lower Woody 

Creek valley by Freeman (1972b). In glacial episode C 

outwash dammed the Woody Creek valley and caused 

alluviation. Remnants of this gravel of late Pleistocene 

age occur as much as 10 km upstream from the mouth 

of Woody Creek.

Most of the alluvium is on active floodplains and 

thus is of late Holocene age at the surface. Alluviation 

began when the streams were dammed by fans, 

moraines, or landslides; so the bottom of the alluvium 

would be as old as the formation of the deposit forming 

the dam.

ECONOMIC GEOLOGY 

DEPOSITS OF LARAMIDE AGE

Aspen and the satellitic districts of Lenado and 

Richmond Hill (fig. 63) constitute a famous silver-leadzinc mining district in which the greatest activity was 

in the 1880's and 1890's. Total production has been 

101,000,000 ounces of silver, 294,000 tons of lead, and 

11,000 tons of zinc (Henderson, 1926; Mineral Yearbooks of the U.S. Bureau of Mines. 2 Much has been 

written about the ore deposits, especially during their 

period of high production (Brunton, 1888; Emmons, 

1888; Henrich, 1889; Newberry, 1890; Sivyer, 1888; 

Spurr, 1898, 1909; Bastin, 1924; Knopf, 1926; Rohlfing, 1938; Vanderwilt, 1935b). Results of more recent 

commodity and exploration studies have been given by 

Boyd and Bromley (1953), Volin and Hild (1950), and 

Heyl (1964). The classic study was that of Spurr (1898), 

and his descriptions of presently inaccessible mines are 

of great interest to anyone studying the area.

Levering (1972) described some jasperoid samples 

from the Aspen district. Distribution of prospects and 

productive areas in the Aspen quadrangle was shown 

by Bryant (1972e).

Aspen is located in the belt of Tertiary intrusives 

and ore deposits apparently controlled by shear zones 

formed in Precambrian time (fig. 3; Tweto and Sims, 

1963). The projection of the Homestake shear zone intersects the margin of the Sawatch uplift at Aspen (fig. 

48), and both Laramide intrusive rocks and much 

faulting occur at this intersection.

I agree with earlier studies that faults controlled the 

ore deposition in the Aspen district. The principal 

disagreement among various geologists has been 

whether part of the control was by regional beddingplane faults at the Belden-Leadville contact (the Silver 

fault) and at the contact between limestone and 

dolomite in the Leadville (fig. 6; the Contact fault, 

Spurr, 1898) or whether these favorable horizons for 

ore are sedimentary contacts (Emmons, 1888; Vanderwilt, 1935b; Rohlfing, 1938; Maslyn, 1976). As I have 

pointed out, breccias along the limestone-dolomite contact in the Leadville do exist on Aspen Mountain, but I 

interpret them as being the result of solution collapse 

and filling during the Leadville-Belden erosional interval. These breccias are mineralized on Aspen Mountain, and, in that area, control of some ore bodies was 

stratigraphic, although access of the ore-bearing solutions was probably through the numerous faults there.

'Production figures are cited here in units of the original reports.
ECONOMIC GEOLOGY 107

" : ?Ji ^.i13l'H^-i^^^ '•^: to H^'J^v^S^*^--^-•''"-'' •*-'•• ">:-.-: yj^^l_m^^^n:.ix:r. ;•••. .-,- '^v, f, i - :

10 KILOMETERS

i i I i I

CONTOUR INTERVAL 200 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 

1 ft = 0.3048m

EXPLANATION

Oligocene igneous rocks

Paleocene(?) and Cretaceous 

igneous rocks

Areas of disseminated sulfides 

Mostly pyrite, some chalcopyrite 

and molybdenite

Areas of significant metal production

Areas of numerous prospects or 

significant metal occurrence

FIGURE 63.—Areas of production, prospects, disseminated sulfides, and Oligocene and Laramide igneous rocks. Base from A.M.S. 1:250,000, Leadville, 1957.
108 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Relations along the Leadville-Belden contact are 

more complicated. Much faulting occurs at the contact 

and in the Belden Formation along the margin of the 

Sawatch Range. Most of the faults are not apparent on 

the surface because neither the Belden nor its contact 

with the Leadville is well exposed. Mine workings furnish the main exposures, and disagreement exists 

among those who have looked at many mine workings.

High-angle faults parallel or nearly parallel with bedding occur in the workings on Aspen Mountain and 

Smuggler Mountain (Spurr, 1898; D. P. Rohlfing, unpub. mapping). In the Smuggler Mine, Spurr (1898, 

p. 77) had difficulty separating his Silver fault from 

the Clark fault, a high-angle fault about parallel with 

bedding. Tailings from prospect dumps north of 

Hunter Creek and near Lenado indicate a fault along 

the Belden-Leadville contact. It is in this region that 

the limestone in the upper part of the Leadville is cut 

out. The limestone reappears on Larkspur Mountain 

where Belden is in stratigraphic contact with the Leadville (Freeman, 1972b). North of Hunter Creek the 

fault at the east contact of the Belden apparently cuts 

down to the Sawatch Quartzite in one fault block. 

Spurr (1898, p. 132) reported that the fault truncates 

beds in mine workings just south of Hunter Creek.

Another complication in the faulting in the Aspen 

mining district is the possibility of early Laramide 

bedding-plane slip in the Belden Formation associated 

with the formation of the Elk Range thrust. Many 

faults, folds, and breccias occur in the Belden, and it 

has not yet been possible to be certain of their relations. Some of the breccias may even be of sedimentary 

origin. In the northern part of the Aspen quadrangle I 

have interpreted a structural discontinuity in the 

Belden as being along the heel of that thrust. To the 

south, at the head of Castle Creek, I believe the heel of 

the thrust to be stratigraphically higher and at the 

Belden-Gothic contact.

I think that much faulting occurs along the 

Leadville-Belden contact, but that locally some contacts are unfaulted, such as south of Express Creek. 

However, on the surface from Aspen Mountain north 

to the edge of the Aspen quadrangle, I believe that the 

contact is a fault subparallel to steeply to moderately 

dipping bedding.

The ores in the Aspen district, including Richmond 

Hill and Lenado, are believed to be of Laramide age. 

They were emplaced along and adjacent to faults believed to have formed during the later stages of uplift 

of the Sawatch Range, and they are also older than 

some of the movement along those faults. They bear a 

similar relation to that faulting as do dated Laramide

igneous rocks, which occur at Aspen and Richmond 

Hill but not at Lenado.

Since the Midnight mine in the Richmond Hill area 

closed in 1952, no significant amount of metal has been 

produced in the vicinity of Aspen. In 1966-68 Aspen 

Consolidated Mining Co., a subsidiary of McCulloch 

Oil Co., milled dump material from the Smuggler and 

Mollie Gibson mines. Utah Mining and Construction 

Co. conducted some exploration on a contract basis in 

the Smuggler mine in 1968 and 1969.

In 1968 Silver Dawn Mining Company did some 

trenching and sampling in Annie Basin. They obtained 

some high values from near-surface samples, which led 

to considerable publicity. Probably, those values were 

enhanced by secondary enrichment, as no mining 

resulted.

Analyses of samples of rock of possible commercial 

interest that I found on the surface in the Aspen mining district and available from drill holes in Lenado are 

given in table 9. These have from 1 to more than 10 

percent zinc, a few tenths to more than 10 percent lead, 

and 5-10 ounces of silver per ton. Cadmium in 

amounts as much as several tenths of a percent is probably associated with the zinc in sphalerite. Copper contents are < 0.1 percent. High Fe and Mn contents are 

related to near-surface secondary enrichment in those 

elements. The samples from underground are generally 

poorer in those elements and richer in Ca and Mg, except where highly silicified. In all samples the original 

rock was a limestone or dolomite, from either the Leadville or the Belden. Analyses of oxidized lead-zinc ore 

listed by Heyl (1964) are similar, although perhaps 

richer in lead and zinc. Arsenic and antimony were 

found in a sample he collected and arsenic in 3 and antimony in 1 of the 11 samples of table 9. High barium 

values are from barite, a common gangue mineral.

As pointed out by Heyl (1964, p. C60), oxidized leadzinc ore containing silver remains in the old workings 

and in dumps in the Aspen and Lenado districts. Drilling by the U.S. Bureau of Mines in the Smuggler mine 

revealed intervals of as much as 10 m of rock containing 2-5 percent combined lead and zinc and, locally, a 

few ounces of silver. Flotation concentration tests of 

this ore were unsuccessful because of fine grain size, 

friability, and oxidation of the ore minerals (Volin and 

Hild, 1950). Some richer ore may exist below old workings that beneath Smuggler Mountain extended down 

to 2,012 m altitude or 730 m below the surface. 

However, maps of the workings show fewer stopes on 

the deeper levels, probably indicating that rich ore was 

not as widespread there as closer to the surface. The 

data, if they exist, needed to estimate the amount and
TABLE 9.— Analyses, of some ore samples, Aspen, Richmond Hill, and Lenado districts

[N, element looked for but not found; leaders ( — ), element not determined. Looked for but not found: Au, Bi, La, Nb, W. Semiquantitative spectrographic analyses by D. J. Grimes, J. W. Sears, Jr., and

J. L, Finley, U.S. Geological Survey. >, greater than]

Sample 

Number

561

562

1458a

Percent

Fe

5

7

5

Mg

33

.5

Ca

5

10

3

Ti

0.03

N

.005

Mn

1,500

200

30,000

Ag

200

700

70

As

700

500

N

B

2020

N

Ba

100

100

50

Be

N

NN

Cd

>500>500

1,500

Co

30

N

10

Cr

30

10

7

Parts per million

Cu

500

300

30

Ga

NNN

Ge Mo

— 7

7

N N

Ni

30

20

10

Pb

>5,000>5,00030,000

Sb

NN

N

Sc

N

NN

Sn

NN

15

Sr

NN

7

V

30

10

7

Y

NN

50

Yb——3 Zn >10,000 >10,000 >100,000 Zr50NN Location Breccia from dump above Mollie Gibson; collected by J. H. McCarthy. Gossan on Smuggler Mountain; collected by J. H. McCarthy. Carbonate containing galena, sphalerite,

cerussite, and Fe and Mn oxides — portal of Smuggler #1 tunnel. O

.002 50,000 300 N N 7,000 1 700 3 5 150 5 N N 5 30,000 N N 15 70 N 30 1 >100,000 N Carbonate containing galena, sphalerite, O

cerussite, barite, and Fe and Mn 25 

oxides — portal of Smuggler #1 tunnel. O

.0015 50,000 100 N N 700 1 1,000 35 70 5 N N 7 >100,000 N N 20 15 N 50 3 >100,000 N Carbonate containing galena, sphalerite, §J

cerussite and Fe and Mn oxides — £H 

portal of Smuggler #1 tunnel. ^J

.0015 2,000 150 N N 30 N 2,000 33 300 N N N 3 >100,000 N N N 20 N N N >100,000 N Jasperoid and coarse carbonate with Q

euhedral quartz. Galena, sphalerite, f-J 

and cerussite, from dump at 2,718 m O altitude east side of West Aspen Mtn. f1

.003 20,000 200 N N 7,000 N 1,000 N 3 300 N 20 N 7 15,000 N N N 100 N N 1 >100,000 N Carbonate containing veinlets of O

sphalerite, galena, calcite, barite, Q 

quartz, and chalcopyrite. Ore pile H^ 

near tunnel of Little Annie.

.03 5,000 700 N N 30,000 N 7,000 N 5 1,000 N 70 N N 10,000 200 N N 70 10 N 1 >100,000 N Fragments of limestone partly replaced

by quartz and cemented by sphalerite 

galena, calcite, barite, and quartz. 

Ore pile near tunnel of Little Annie.

.003 500 150 N N 300 N 100 3 10 150 ~ ~ N 10 7,000 N N N 70 10 N — 30,000 N Fractured altered dolomitic limestone

with veinlets of sphalerite and 

galena, from drill cuttings, 

Lenado district.

L-163L-186

1.5 >10 >10 .007 3,000 200 N N 300 N 200 30 15 150 — — N 70 20,000 N N N 70 10 1 L — 100,000 N Do.

sulphide; drill core, Lenado district.

L, element present but below sensitivity level.

O 

CO
110 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

grade of ore available in the mining district are not 

available to me.

Two major exploration projects conducted during 

the 1960's were partly financed by loans from the Office of Mineral Exploration. They were an extension of 

the Highland tunnel beneath Richmond Hill and core 

drilling in the Lenado area. These projects revealed 

new information on the geology and consequently are 

discussed here. I thank Messrs. Henry L. Stein and 

Edwin J. Smart for permission to publish data 

obtained during those projects.

THE HIGHLAND TUNNEL

The Highland tunnel is located at about 2,667 m 

altitude east of Castle Creek near the south edge of the 

Aspen 7.5-minute quadrangle. During the 1930's it 

was driven 355 m N. 75° E. into the valley side. In 

1963 through 1966 Richmond Hill Exploration Co., 

Inc., extended it to a length of 1.7 km and did about 

425 m of drifting and 275 m of drilling. The goal of the 

project was to cut a downward extension of the ore 

body of the Midnight mine. No ore body was encountered, and small chalcopyrite-bearing veins were 

less numerous in the tunnel than on the surface, as indicated by prospects. The main point of interest in the 

Highland tunnel is additional control at depth of the 

geology of the Castle Creek fault zone. Some of the 

features can be correlated with those seen in the Hope 

tunnel driven in the 1920's at a similar altitude about 

1,220 m to the south. John P. Kellogg and Richard 

Beck supervised the work in the Highland tunnel and 

prepared detailed maps that were of great use to me 

when I examined the tunnel.

The Highland tunnel (fig. 64) passed through steeply 

dipping Maroon Formation for about 660 m. The 

Maroon is composed of grayish-red fine-grained 

calcareous sandstone and siltstone and has a few beds 

of silty limestone and limestone. However, some intervals of gray rock are present only 213-274 m from the 

portal and a few red beds are found as much as 487 m 

from the portal. Brown and gray sandy and silty 

limestone and calcareous siltstone and sandstone of 

the Gothic Formation of Langenheim (1952) occur for 

another 450 m along the tunnel. From 914 to 1,001 m

METERS 

3400 —|

3300 — 

3200 — 

3100 — 

3000 — 

2900 

2800 — 

2700 — 

2600.— 

2500 —

Om€p

€s€p

Not shown on 

geologic map

Not shown on 

geologic map

•'Highland" tunnel

0 1 KILOMETER'

TKp

PPm

;pg;

EXPLANATION 

Tertiary!?) and Cretaceous porphyry

Permian and Pennsylvanian Maroon Formation 

Pennsylvanian Gothic Formation of Langenheim (1952) 

Pennsylvanian Belden Formation, e=evaporite 

Mississippian Leadville Limestone

° Mississippian{?) and Devonian Dyer Dolomite

• OP Devonian Parting Formation

Ordovician Manitou Dolomite 

Cp | Cambrian Peerless Formation 

Cambrian Sawatch Quartzite

Precambrian quartz monzonite

FAULT—Arrows show relative 

movement

FIGURE 64.—Cross section along the Highland tunnel.
ECONOMIC GEOLOGY 111

from the portal, gouge and breccia of mixed limestone 

and carbonaceous shale mark the Gothic-Belden contact. Folded, faulted, and brecciated dark-gray pyritebearihg limestone, dolomitic limestone, and carbonaceous shale make up the Belden. A major zone of 

breccia and gouge occurs between 1,108 and 1,192 m 

from the portal. From 1,483 to 1,554 m is another wide 

breccia zone. At about 1,525 m the breccia contains 

numerous lenses of anhydrite. The tunnel passed 

through several intervals of white porphyry containing 

quartz and muscovite phenocrysts. The porphyry is 

highly fractured and has sheared contacts; some is 

very altered. At 1,706 m the tunnel passed through 3.4 

m of fractured and brecciated dolomite and into 

Precambrian quartz monzonite. The dolomite, probably derived from the Leadville, occurs as a fault slice 

and pinches out 60 m to the south along the fault, as 

revealed by drifting.

About 50 fault zones are exposed in the tunnel. Some 

fault zones consist of 2-5 cm of gouge parallel with 

steeply dipping bedding planes, and others consist of 

breccia as much as 90 m thick. The lack of 

stratigraphic markers in the Pennsylvanian rocks 

makes it difficult to evaluate each fault, and all cross 

sections (fig. 64) are partly interpretive, despite the 

rather complete exposures in the tunnel. Especially difficult to evaluate is the Gothic-Maroon contact in the 

tunnel. Is the gradation through as much distance as 

the exposures suggest or is there repetition of the 

gradational zone by faulting? Drifting and drilling to 

the south of a point 1,554 m from the portal revealed at 

least 30 m stratigraphically of white anhydrite and 

associated gypsum, containing some interbeds of lightgray limestone and shale. At the tunnel and just south 

of it, the evaporite apparently interfingers with normal 

Belden rocks, so that only about 12 m of breccia containing blocks of anhydrite is exposed in the tunnel. 

The fault between the Belden and the Precambrian 

rocks trends N. 45°-60° W. This trend was somewhat 

unexpected, since trends at the surface at about 3,261 

m altitude are N. 10° E. to N. 30° W. However, on the 

10th level of the Midnight mine at 2,987 m altitude 

(unpublished mapping, 1940-44, by John W Vanderwilt, supplied by Frank Willoughby) the Castle Creek 

fault trends N. 30°-45° W. This change of trend with 

depth may indicate increased displacement on the 

Winnie fault with depth.

Information available from the Hope tunnel is not as 

detailed. According to Knopf (1926), "Maroon sandstones" (Maroon and Gothic Formations of this report) 

occur over a distance of about 1,280 m as measured in 

a straight line from the portal. This does not fit with 

the geology Knopf (1926) or I mapped on the surface. 

However, 30 m of evaporite occurs about the same

distance from the porphyry sill as in the Highland tunnel. If one can project strikes from the two tunnels, the 

evaporite in the Belden and the Tertiary-Cretaceous 

porphyry are offset 120-150 m in map plan by the 

Hope fault, which must pass through the 300 m of rock 

separating the two tunnels.

The Winnie fault, which forms the north boundary of 

the fault block in which the face of the Highland tunnel 

is located, apparently dips about 55° to the north judging from information in the Midnight tunnel. Even if 

its strike swings to the south as shown on the geologic 

map, that fault would not intersect the tunnel but 

would join a higher angle fault to the west.

LENADO

The Lenado project was carried on by Rheco-Lenado 

in the period 1967-69. John P. Kellogg and James N. 

Babcock supervised the work and logged the core, and 

I am indebted to them for much of my information and 

for the opportunity to examine some of the core. My interpretation of the geology differs somewhat from 

theirs. About 1,370 m of diamond drilling was done, 

based on existing information and on a geologic map 

and geochemical study (McCarthy and Gott, 1966). 

Very low core recovery in many holes made evaluation 

of the geology and mineralization somewhat unsatisfactory. However, the drilling did reveal a number 

of faults concealed beneath the surficial deposits of the 

Woody Creek valley. Those deposits averaged about 

30 m thick at the drill sites. Faults, fractures, and breccia zones were encountered in so many places that it is 

likely that the details of the structure are much more 

complex than can be interpreted based on available 

data. One possible interpretation is given in figure 65 

based on the hypothesis that dips in the rocks below 

the Belden are steep and that strikes are similar to 

those in the exposed fault blocks on the valley sides. 

Other interpretations can be made assuming shallow 

dips. Not enough data are available to determine the 

strike of the formations in each fault block. Another 

uncertainty is the possible stratigraphic significance 

of breccias. Kellogg and Babcock interpreted many of 

the breccias in the Belden as sedimentary. I question 

that interpretation, because I have not seen nearly as 

much breccia in the Belden outside thoroughly faulted 

areas as was encountered in the drill holes at Lenado. 

However, the Belden is seldom very well exposed; so it 

is possible that such breccias might be present and not 

detected on the surface. Subsequent underground exploration revealed features interpreted as fossil caves 

and sinkholes in the Lenado area (Maslyn, 1976).

Another controversial point of the geologic interpretation is the identification of the lone outcrop of
112 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

106°45'

EXPLANATION 

Pennsylvanian Gothic Formation of Langenheim (1952)

Pennsylvanian Belden Formation 

Mississippian Leadville Limestone

Mississippian(?) and Devonian Dyer Dolomite 

Devonian Parting Formation

Ordovician Manitou Dolomite

DDH-4

Cambrian Peerless Formation 

Cambrian Sawatch Quartzite 

Precambrian quartz monzonite

STRIKE AND DIP OF BEDS

- CONTACT

- FAULT

_ CONTACT OF SURFICIAL DEPOSITS—

Hachures on bedrock side 

H HORIZONTAL PROJECTION OF DIAMOND

DRILL HOLE 

ADIT

FIGURE 65.—Geologic map of the Lenado area, showing an interpretation of the geology under Woody Creek valley based on

drill data and adjacent outcrops.

Manitou Dolomite in the fault zone south of Woody 

Creek. The rock in that outcrop is very fine grained, 

white, and siliceous, and it has an irregular bedding

resembling that in the Manitou. In thin section it is 

composed principally of cryptocrystalline quartz. It 

resembles neither typical Manitou nor Parting, but I
ECONOMIC GEOLOGY 113

interpret it as silicified Manitou. If that outcrop were 

Parting rather than Manitou, the map would be 

somewhat different. In some of the cores carbonate 

rocks are bleached, making formation identifications 

uncertain.

Interpretation of the data from core drilling leads to 

the conclusion that the whole Woody Creek valley is 

underlain by a complex fault zone containing a number 

of fault blocks intermediate between the blocks north 

and south of the fault zone and containing one highly 

downfaulted block of Gothic Formation (fig. 65).

Drill holes 5, 7, and 8 suggest that the northern 

strand of the Lenado fault zone dips about 60° to the 

south. None of the holes penetrated the southern 

strand. Apparently, no widespread mineralization of 

ore grade in the Belden Formation exists beneath the 

Woody Creek valley, although low-grade (0.1-1 percent) zinc mineralization occurs through considerable 

thicknesses of rock. Mineralization of ore grade is 

generally in fault zones, as in the Aspen Contact mine 

(Spurr, 1898). Only part of the fault zones have been 

penetrated, and in very few places did drilling encounter the Belden-Leadville contact adjacent to a 

fault, which is a favorable horizon for ore deposition.

ORE DEPOSITS OF 

MIDDLE TERTIARY AGE

A variety of ore deposits are associated with the 

granodiorite plutons of the Elk Mountains and 

therefore are of middle Tertiary age. They include contact metasomatic iron, disseminated sulfide, and 

fissure-vein lead-zinc-silver deposits and copper-silver 

sandstone deposits (fig. 63).

IRON DEPOSIT AT THE HEAD OF 

COOPER CREEK

Iron deposits have been known in the Aspen area at 

the head of Cooper Creek near the southeast corner of 

the Hayden Peak quadrangle since the early 1880's 

(Devereaux, 1884, Leith, 1906; Harder, 1909; Harrer 

and Tesch, 1959), but they have only recently been exploited. In 1961 the first substantial mine operation 

started, but production really got underway a few 

years later. According to the May 11, 1967, Aspen 

Times, ore is mined at the rate of 3,175 t (metric tons) a 

day. Since the mine is at 3,660 m altitude, operations 

are limited to summer and early fall, and a full year's 

production might be about 317,000 t of ore.

The iron deposits occur 500-800 m horizontally from 

the margin of the White Rock pluton in highly 

recrystallized limestones of the Belden Formation (fig. 

66), which have generally steep and irregular dips 

mostly towards the pluton. The ore appears to have

been emplaced along steeply dipping northwesttrending fracture zones that probably formed during 

intrusion of the pluton. Although fractures furnished 

channels for the ore-bearing solutions, the bulk of the 

ore bodies formed by replacement of Belden limestone, 

now converted to marble. Ore occurs from altitudes of 

3,600 to 3,840 m, and I saw ore zones as much as 30 m 

thick. Assuming a blanketlike deposit parallel with the 

slope of the mountain and about 20 m thick, judging 

from the length of a few prospect tunnels that had 

penetrated the ore, Harder (1909) estimated 3-4 

million tons of ore, but he also thought that more was 

actually likely to be available because of probable 

greater thickness of the deposit. On the surface, as 

suggested by exposures in mine roads in the middle 

1960's, the deposit certainly is irregular; and since it 

appears to be controlled by steeply dipping faults and 

bedding, parts of it may go to several times the depth 

assumed by Harder. However, granodiorite might be 

encountered a hundred meters or so below the surface. 

The ore is composed of massive magnetite and minor 

amounts of pyrite. The country rock is dark-gray to 

white coarse- to fine-grained marble derived from 

Belden limestones. Most of the magnetite is granular 

but locally octahedral faces on magnetite grains inFIGURE 66.—Setting of Pitkin Iron Corp. mine, Hayden Peak quadrangle, viewed east from 3,666-m knob on ridge south of Castle 

Creek. Photograph taken in 1964 before much mining took place. 

Tg, granodiorite; Pg, Gothic Formation hornfels; Pbm, Belden 

Formation marble; Ml, Leadville Limestone. Heavy lines, faults; 

light lines, contacts. IM, iron mine.
114 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

dicate the presence of some open spaces during ore 

deposition. On the surface the ore has considerable 

limonite and hematite due to weathering of pyrite and 

magnetite. Along some fractures weathering 

penetrates to depths of as much as 10 m.

The most common gangue mineral is calcite. Barite 

and siderite also occur. Rarer gangue minerals are 

quartz and garnet. Monoclinic pyroxene and colorless 

amphibole were found in one inclusion in the ore. Some 

beds in the Belden are pyroxene hornfels, a rock type 

that is more typical of the metamorphosed Gothic Formation, which lies between the Belden outcrops and 

the White Rock pluton. One inclusion found in the ore 

contains an unusual blue-green chlorite having 

2 V<10° and Y=1.565. X-ray study shows that it is rich 

in Mg, and spectrographic analyses do not suggest 

that the color is related to the presence of unusual 

amounts of any minor element.

Some of the coarsely crystallized calcite that is 

found in the upper part of the Leadville north of the ore 

zone is dark gray. Semiquantitative spectrographic 

analysis revealed no good reason for the color. 

Analyses of two ore samples showed no metal values 

except for iron. Harrer and Tesch (1959) reported 

chemical analysis of a sample: 67.2 percent Fe, 0.02 

percent P, 0.63 percent S, 1.8 percent Si; and 0.3 percent Mn. Spectrographic analysis showed 0.001-0.01 

percent Ga, Pb, Zr, Cr; 0.01-0.1 percent Cu, Ti, Co; and 

0.1-1.0 percent Mn.

DISSEMINATED SULFIDE DEPOSITS

Disseminated sulfides occur in granodiorite of the 

White Rock pluton and adjacent hornfels in areas as 

great as 9 km2 in the Maroon Bells and Hayden Peak 

quadrangles (fig. 63; Bryant, 1971c). The deposits are

composed principally of pyrite, but locally copper and 

molybdenum occur in quantitites as great as several 

tenths of a percent. Molybdenite is most abundant in 

locally numerous quartz veins in highly altered 

granodiorite. At the surface large areas of pervasive 

alteration are absent, an unfavorable indication for 

large volumes of ore (Guilbert and Lowell, 1974, p. 

103). However, no deep drilling has been done. Alteration and pyritization is younger than the bodies of porphyritic granodiorite which cut the White Rock pluton.

FISSURE VEINS

Pyritization has occurred along fractures and faults 

in hornfels and red beds of the Maroon Formation and 

in other formations throughout considerable areas 

above and adjacent to the White Rock pluton. Quartzpyrite and quartz-carbonate-pyrite veins have been 

deposited in some of the fractures. Locally, sphalerite, 

argentiferous galena, and very rare gold occur in these 

veins.

Production from these veins has been small. The 

best known productive locality has been the 

Montezuma mine just northeast of Castle Peak. There, 

according to an unpublished Bureau of Mines report 

by M. E. Volin, dated 1948, argentiferous galena, 

sphalerite, and pyrite in a gangue of quartz, calcite, 

and barite form a vein less than a meter thick. In the 

period 1897-1915 about 6,700 tons of ore were mined. 

An unknown amount of very rich near-surface ore was 

mined shortly after the discovery of the vein in 1881. 

Small amounts of ore have been shipped as recently as 

the early 1960's from the Montezuma mine.

Most of the veins observed during mapping in and 

around the White Rock pluton contain pyrite, and consist of carbonate, quartz, and barite. Traces of

TABLE 10.—Analysis, of fissure veins

[Semiquantitative spectrographic analyses by J. M. Motooka and E, L. Hosier, Gold analyses by J. G. Frisken, T. 

R. Miller, U.S. Geological Survey. N; element looked for and not found; L, element is present in undetermined 

for but not found: Cd, W, Sn]

Semiquantitative spectrographic analyses

Sample Percent Parts per million (ppm)

Number Fe Mg Ca Ti Mn Ag As Au B Ba Be Bi Co Cr Cu La Mo Nb Ni Pb Sb Sc Sr V 1t Zn Zr

Conundrum Basin

L700 2 0.5

1701 3 .2

1702 5 .5

1703 3 .5

0.2 0.5

.1 .2

5 .02

300 3 N

300 1 N

5,000 1.5 500

500

500

20

N 5

N N

L 7

30 20

10 N

100 N

N 200

5 200

7 20

N 200 15 N 200

N 100 10 N 100

N 15 20 N N

2,000 50 200 N N >5,000 N L 7 10 1,000 50 5 10 2,000 200 L 2,000 20 15

2064 15 N

2065 10 N

2066a 15 .2

2066b 5 .3

2067 10 L

2068 5 .3

2069a 7 1

2069b 5 1

2069c 3 .02

L 0.001

.07 N

L .1

.3 .15

20 300 N N

100 7 300 N

30 10 N N

30 500 L 10

500 N

	L

10 70

.15 .007 150 10 500 N N

.1 .15 20 1 N N 700

15 .007 5,000 .5 N N N

5 .05 1,000 N N N L

.3 .003 100 N N N L

100 

N 

20 

20 

20 N

L L 30

N L 5 N

N N 50 10

N N 30 N

70 N

30 N

30 N

30 20

150 N

7 L

50 L

10 50

L 20

L 100

L 150

20 100

20 1,000

L 150

20

5

150

20

100 

20 

L

N

N 5

N 20

N 5

N N

N 

150

200 N

700 N

N 150

10 1,000 300

300 N

50 L N 100

15 50 N N

20 30 N 50

N 15 N N
ECONOMIC GEOLOGY 115

malachite, azurite, galena, and sphalerite occur locally. 

Galena and sphalerite are most common in the veins 

south and east of Castle Peak.

Spectrographic analyses of samples of veins from 

some prospects at the head of Conundrum Creek and in 

the Pine Creek drainage, Hayden Peak quadrangle, 

show that anomalous amounts of silver, gold, lead, 

zinc, and copper occur; but some samples lack 

anomalous amounts of any of those metals. A few 

samples showed anomalous amounts of arsenic, antimony, or bismuth (table 10). The two best samples 

(2,066 and 2,064) would be ore if enough material of 

that tenor were available.

Fluid inclusions in samples of some of the veins 

associated with the White Rock pluton are small, two 

phase, secondary, and have filling temperatures of 

200°-325°C, according to C. G. Cunningham, Jr. (written commun., 1973). These temperatures are comparable to those found for inclusions in altered rock 

and veins in the area of disseminated sulfides.

COPPER-SILVER DEPOSITS IN DAKOTA SANDSTONE

On Snowmass Creek on the east side of Eagle Mountain and on either side of the western edge of the 

Highland Peak quadrangle are several prospects in 

Dakota Sandstone. All these prospects are near or adjacent to the Snowmass Creek fault zone, and the ones 

in the Capitol Peak quadrangle are in a fault slice between two strands of the zone (fig. 67).

The most interesting prospect (loc. 2001, called 

Snowmass Creek mine on the geologic quadrangle map 

(Bryant, 1972a)) is the one east of Eagle Mountain 

where sulfides replace plant fragments in a bed rich in 

plant debris and are also disseminated in adjacent 

quartz sandstone where carbonaceous material is

associated with the White Rock pluton

Roemer, S. Rickard, W. Campbell, and 

amount below sensitivity limit. Looked

Chemical 

analyses

(ppm) Conundrum Basin—Continued

0.2 

.02 

.3 

1.3

Pyritized and altered arkose along fracture; from prospect. 

Pyritized arkose from vein along fracture or shear; from prospect. 

Pyritized arkose by vein; from prospect. 

Pyrite and galena in vein in arkose; from prospect.

Pine Creek Area — Continued

1.80 

.06

Quartz pyrite vein. 

Quartz pyrite vein in fault.

.08 Quartz pyrite vein parallel bedding; from prospect.

26.0 Galena pyrite quartz vein.

.50 Quartz pyrite vein.

.30 Pyrite cemented breccia in hornfels.

.10 Pyrite calcite vein from prospect.

-20 Pyrite quartz vein.

.10 Pyrite quartz vein.

scarcer. U.S. Bureau of Mines records indicate that a 

small amount of copper-silver ore has been shipped 

from that prospect.

The Dakota Sandstone containing disseminated 

sulfides ranges from well-sorted fine-grained sandstone with a grain size of 0.1-0.2 mm to rather poorly 

sorted coarse-grained chert quartz sandstone with a 

grain size of several millimeters. Quartz overgrowths 

and interstitial silica occur locally. Clastic grains of zircon and tourmaline are common accessory minerals. 

Grains of feldspar and muscovite occur rarely in these 

sandstones. A light-brown micaceous mineral occurs 

between the clastic grains in many samples from the 

mineralized rock. Opaque minerals are interstitial to 

the clastic grains. In the most heavily mineralized 

sample studied in thin section, the opaque minerals 

partly replace the clastic grains. Except for the brown 

micaceous mineral mentioned, no other gangue mineral 

is associated with the sulfides, and the texture of the 

sandstones has not been affected by the mineralization. At locality 1907 some barite occurs on the joint 

planes.

Modal data (table 11) indicate that the sandstones 

from the Snowmass Creek mine containing 

disseminated sulfides have about 2-5 percent opaque 

minerals. Polished-section study indicates that the 

opaque minerals are predominantly bornite with lesser 

amounts of chalcocite, chalcopyrite, pyrite, and 

magnetite. Semiquantitative spectrographic analyses 

(table 12) show that these rocks contain 2-7 percent 

Cu, 0-0.3 percent Zn, 200-1,500 ppm (parts per 

million) Ag, and as much as 3,000 ppm Mo. The 

substantial amount of Ag is confirmed by chemical 

methods, although the values are somewhat lower, 

ranging from 75 to 325 ppm. Elements that are often 

associated with Ag, Cu, and Zn, such as Au, As, Bi, Cd, 

and Sb, are absent. The Pb content is low. Major 

elements are dominated by Si, reflecting the quartzrich nature of the Dakota Sandstone. Aluminum 

reaches 1 percent in rocks having interstitial 

micaceous mineral. Alkalies were not detected or occur 

in very small amounts; Ca and Mg contents are low.

Sample 2001e is above average in opaque-mineral 

content. In addition to containing more Cu and Ag, it 

contains more iron in pyrite and magnetite, as well as 

anomalous amounts of Co, Ni, and Pb.

Samples 1907, 2005, and 2086 from the other prospects contain 0.15-1 percent Cu, 0-7 ppm Ag, and 

5-700 ppm Mo, but they lack anomalous contents of 

other elements.

The suite of samples from the prospects in the fault 

slice of Dakota Sandstone in the Capitol Peak 

quadrangle contains 0.12-0.38 percent organic carbon. 

Other samples were not analyzed for that element.
Sample

No.

1907d

2001a

2001b

2001c

2001d

2001e

2001f

2001g

TABLE 11.— Estimated modes, in volume percent, of mineralized Dakota Sandstone from near the Snowmass

area, Pitkin County, Colo.

[Leaders ( —— ), mineral is absent; P, mineral is present but in quantities less than 1 percent; underlined

Quartz Chert Quartzite Tourmaline Zircon Feldspar Muscovite Light-brown Opaque Minerals

Mica(?)

95 p —— p —— —— —— —— 5 Bornite

78 22 2 —— P —— —— P P Bornite, chalcopyrite,

pyrite.

96 2 —— —— P —— —— —— 2 Bornite, chalcocite

93 2 —— —— —— —— —— P 5 Bornite, chalcopyrite,

chalcocite.

83 15 —— —— P —— —— P 2 Bornite, chalcopyrite,

magnetite.

92 P —— P P —— —— —— 8 Bornite, pyrite,

chalcopyrite, magnetite.

36 —— —— P P P P 5 59 Marcasite, chalcopyrite.

pyrite.

—— —— —— —— —— —— —— —— 100 Bornite, pyrite(?)

Creek fault zone, E

mineral is relative

Dark-gray sandst

Coarse-grained l

granules; irre

Light-gray sands

Light-gray sands

Light-gray fairl

chert; dissemi

Gray sandstone; 

Nodule in sandst

fragments .

Wood fragment re
ECONOMIC GEOLOGY 117

107°00'

1 KILOMETER

EXPLANATION

<

x^ 

xy

s

'fy

Qs

, . S\ • s\

0-g^

/ ////

,Kmu/, '/////

r//Kn\W/, Kml'-

Jm . : ;

"Re

Quaternary surficial deposits 

Tertiary porphyritic granodiorite 

Tertiary granodiorite

Cretaceous Mancos Shale 

Kmu, upper member 

Kmf, Fort Hays Limestone Member 

Kml, lower member

Cretaceous Dakota Sandstone and Burro Canyon Formation

Jurassic Morrison Formation (locally thin Entrada Sandstone 

included at base)

Triassic Chinle Formation

Triassic and Permian State Bridge Formation

Permian and Pennsylvanian Maroon Formation

Pennsylvanian Gothic Formation of Langenheim (1952),

CONTACT—Dashed where concealed

—— FAULT—Dashed under surficial deposits 

STRIKE AND DIP OF BEDDING

I001

ADIT—Number is analyzed sample

SHAFT

PROSPECT PIT

FIGURE 67.—Geologic map of the Eagle Mountain area showing location of prospects along the Snowmass Creek fault zone.
TABLE 12.— Analysis of mineralized Dakota Sandstone from near the Snowmass Creek fault zone, Eagle Mountain, Pitkin 

[All analyses by U.S. Geological Survey. Semiquantitative spectrographic analyses by R. H. Heidel and L. A. Bradley; gold analyses b

absorption by 0- M- Parker, J. A- Thomas, Claude Huffman, Jr., and W. P. Goss; silver calculated by fire assay difference method by 

determined on residue after digestion with 10 percent HC1 by I- C. Frost. N, element looked for but not found; leaders ( —— ), elem

L, element present in undetermined amount below sensitivity limit; G, element present in amount greater than 10 percent; <, less th

not found: K, P, As, Au, Bi, Cd, Ga, Ge, Hf , In, Li, Pd, Pt, Re, Sb , Sc, Sn, Ta, Te, Th, Tl, U, and W]

Semiquantitative

Sample

Number

1907a

1907b

1907c

200 la

2001b

2001c

200 Id

2001e

2085a

2085b

2086b

spectrographic

Percent

Si Al

G 0.7

G .3

G .15

G 1

G -5

G 1

G 1

G .5

G -5

G .7

G .5

Fe

0.7

1-5

.5

.5

-2

1

.7

7

.2

.2

.2 

.15

Mg

0.07

.015

.007

.1

.02

.05

.05

.05

.02

-05

.01 

.02

Ca

0.07

.02

.02

.02

.02

-02

.02

.05

.05

.2

-05 

.05

Na

0.05

L

L

N

N

N

N

N

L

L

L

Tl

0.15

.07

-03

.05

.02

.05

.02

-05

.05

-05

.015

Mn

7

7

20

N

20

50

N

20

5

50

50

Ag

1-5

7

5

100

500

500

500

1,500

3

N

N

B

70

20

N

30

N

?0

70

50

30

50

50 

L

Ba

150

70

50

200

50

70

100

50

200

100

200 

70

Be

N

N

N

N

N

N

N

N

N

2

1

Ce

N

N

N

150

N

N

N

N

N

N

N

Co

N

N

N

N

3

5

N

200

N

10

10

analyses

Parts per

Cr

30

15

2

5

1

1

?

2

5

5

2

Cu

3,000

10,000

2,000

20,000

20,000

70,000

30,000

G

7,000

10,000

2,000 

1,500

million (ppm)

La

N

N

N

150

N

50

50

N

70

N

100

N

Mo

7

30

700

5

200

500

5

3,000

15

20

10

Mb

N

N

N

L

N

N

N

N

N

N

N

Ni

L

N

N

N

L

5

N

50

10

10

N

Pb

N

10

10

N

20

20

N

500

20

20

10

Sr

L

N

N

20

N

N

10

N

70

10 

10

N

V

15

/

N

10

N

N

N

7

7

7 

10

N

Y

15

N

N

50

20

150

10

50

10

15 

50

N

Yb

2

1

N

2

2

7

?

5

1

2 

5

N

Zn

2,00

3,00

2,00

2,00
ECONOMIC GEOLOGY 119

Thus, although mineralization occurs at various 

places in the Dakota Sandstone through a distance of 

3.2 km along the Snowmass Creek fault zone near 

Eagle Mountain, the only place where the rock is 

known to have high silver values is at the Snowmass 

Creek mine.

The mineralization at the Snowmass Creek mine appears to be controlled by the carbonaceous beds in the 

Dakota Sandstone and by proximity to the fault zone. 

Nowhere else in the region has any such mineralization 

been reported from carbonaceous horizons in the 

Dakota.

SILVER CONTENT OF THE COPPER SULFIDES

Microprobe analyses of bornite, chalcocite, and 

chalcopyrite from the Snowmass Creek mine show 

silver in amounts as great as 1.9 weight percent in bornite and 2.7 weight percent for chalcocite in the four 

samples examined (table 13). No discrete silver mineral 

was recognized with the electron microprobe.

The quantities of silver in bornite and chalcocite 

compare with a maximum content of 0.1 percent silver 

in those minerals according to a compilation by Boyle 

(1968). He did state, however, that silver contents as 

high as 1 percent have been recorded for chalcocite 

from samples of uncertain purity. Thus, the silver content of selected grains of bornite from Snowmass 

Creek is about 20 times greater than that of previously 

reported bornite, and the silver content of the 

chalcocite is at least 2.7 times, and perhaps more, that 

of previously studied samples of that mineral.

TABLE 13.— Microprobe analyses of silver-bearing copper sulfides 

from the Snowmass Creek mine

[Analyses by G. A. Desborough, U.S. Geological Survey, 1972; 

analyses for separate areas of polished section]

Range in 

weight percent Ag

Mineral Number of 

observations

Sample 2001b

0.

0.

7-1.

7-1.

5

3

6

bornite

chalcocite

bornite

4

3

1

Sample 2001c

1.

0.

0.

1.

0

1

7-0.

1-2.

9

7

bornite

chalcopyrite

bornite

chalcocite

1

2

2

2

Sample 2001d

0.

0.

0.

7-0.

1

8

8 bornite

chalcopyrite

chalcocite

3

1

2

Sample 2001e

0.

0.

<0,

0.

0.

0.

1

6-0.

05

6-1.

5-1.

6-0.

8

9

0

8

bornite

—do ——

pyrite

bornite

—do ——

—do ——

2

3

2

4

2

3

CONCLUSIONS

The mineralization at Eagle Mountain differs 

somewhat from other known deposits in sandstone. 

Copper deposits occur in Triassic and Jurassic sandstones in Pennsylvania and New Jersey adjacent to 

basalt sills and in Precambrian conglomerates interbedded with mafic volcanics in Michigan. At Silver 

Reef in Utah silver is associated with copper in a 

Triassic sandstone containing plant remains. 

Vanadium and uranium also occur in this deposit. No 

general agreement on the source of the metals at Silver 

Reef has been reached, although Proctor (1953) believed that the elements were derived from volcanic 

ash beds in the Chinle Formation and redeposited in 

the sandstone bed where they were concentrated by 

solutions in circulating ground water and precipitation 

through contact with entombed plant debris in 

permeable buried stream channels. In many respects 

the deposit at Silver Reef is similar to the U-V-Cu 

deposits so widespread on the Colorado Plateau, which 

have no direct relation to igneous rocks and hydrothermal processes. Although the prospects at Eagle Mountain appear to have a similar type of stratigraphic control—precipitation of ore minerals by carbonaceous 

material in sandstone—they also have clear spatial 

relation to a fault zone, which had post-Dakota movement during emplacement of a granodiorite pluton. 

Thus, they appear to be low-temperature hydrothermal 

deposits related to the middle Tertiary igneous rocks.

The silver content of the copper sulfide minerals in 

the Snowmass Creek mine is high enough to make a 

significant contribution to the values in any ore there. 

However, since the silver is in the lattice of the copper 

sulfides, it cannot be separated form the copper before 

destruction of the sulfide minerals during smelting of 

the ore.

We do not have the information to allow us to judge 

what volume of rock might contain the values shown 

by samples 2001a-2001e (table 12). The prospect tunnel being driven in 1968 showed at least 10 m perpendicular to the strike of the bedding containing 

disseminated copper sulfides. The presence of other 

prospects containing as much as 1 percent Cu in the 

Dakota Sandstone along the Snowmass Creek fault 

zone indicates the presence of some mineralization 

associated with the fault zone over a distance of 3 km, 

unless that mineralization occurred before most of the 

displacement along the fault. If the mineralization is 

fault controlled, as it appears to be, the intersection of 

the Fort Hays Limestone Member of the Mancos Shale 

with the fault zone might be another favorable place to 

prospect. The Fort Hays contains some small Ag-Pb-
120 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

Cu-Zn ore deposits in Lead King Basin south of the 

Snowmass pluton (Vanderwilt, 1937).

The geology of the fault zone beneath the surficial 

deposits of the Snowmass Creek valley may be much 

more complicated than suggested here, and that probability makes exploration by drill holes through the 

surficial deposits a venture of decidedly high risk.

ORE DEPOSITS OF UNKNOWN AGE

GOLD HILL

The Gold Hill area adjoins the Hay den Peak 

quadrangle on the east. It was extensively prospected, 

and in 1967 a small mill was still standing, although it 

did not appear to have been much used. The country

TABLE 14.—Analyses of rock

[All analyses by U.S. Geological Survey. Semiquantitative spectrographlc analyses by K. J. Curry and E. L. Hosier. Gold determinations by 

and by combined fire assay and atomic absorption method by F. W. Brown, J. A. Thomas, Claude Huffman, Jr., and 0. M. Parker. N, element 

below sensitivity limit; leaders (——), not determined; >, greater than. Looked for but not found: As, Bi, Cd, Sb]

Sample 

Numbers

1024

1654a

1654b

1654c 

1654d

1654e

1654f

1655a

1655b

1655c

1656a

1656b

1658a

1658b

1658c

1659a

1659b

1663a

1663b

1665a

1665b

Semiquantitati

Fe

>10

, L

5

1 

7

1.5

5

.3

2

7

10

10

10

10

10

2

2

10

3

5

10

Percent

Mg Ca

0.015 0.03

L L

.05 L

,1 L 

L L

.2 .05

.07 L

.1 L

,15 L

L L

.02 L

,02 L

.1 L

.2 L

.05 L

,1 L

.15 L

.03 L

.1 L

,07 L

.03 L

Ti

0.05

,002

,03

,2 

.015

.15

.05

.07

.15

.01

.005

.02

.1

.3

,03

,2

,15

.02

.07

.1

,015

Mn

1,500

20

100

100 

700

200

150

100

700

700

1,600

5,000

700

5,000

1,500

300

300

500

700

1,000

3,000

Ag

N

N

1

N 

L

L

1.5

N

1.5

1.5

N

N

N

N

N

N

N

7

1.5

5

1.5

Au

N

N

L

N 

L

N

10

N

L

L

N

N

N

N

N

N

N

15

L

N

N

B

N

N

N

N 

30

L

15

N

L

30

N

15

N

N

20

N

10

N

10

N

10

Ba Be

100 N

L N

100 1

100 L 

30 L

150 1

70 1.5

100 L

150 1

L N

20 N

150 L

100 L

200 L

100 L

70 L

150 1

L L

15 L

300 N

L L

ve spectn

Co

15

N

50

5

15

7

10

L

15

15

70

200

150

70

20

N

L

100

15

5

15

Cr

5

N

5

20 

L

30

20

10

20

L

L

L

L

50

10

20

30

L

5

10

5

ographic

Parts

Cu

1,000

10

200

30 

700

150

700

10

150

7,000

70

2,000

150

200

70

10

70

2,000

3,000

20,000

10,000

analyses

per

La

30

N

N

20 

L

30

20

L

20

L

N

L

30

N

L

20

20

N

30

N

L

million

Mo

N

N

10

N 

15

L

15

N

N

15

N

10

N

N

10

N

L

N

7

20

10

Nb

N

N

N

10 

L

10

L

L

L

L

L

L

L

10

L

L

L

L

10

L

L

Ni

15

N

10

L 

30

10

20

5

7

15

50

100

20

10

50

5

10

150

5

5

30

Pb

15

L

15

L 

10

L

L

N

N

L

L

L

N

N

L

N

N

20

L

N

L

Sc

N

N

N

7

L

5

L

L

5

L

N

L

N

10

5

5

5

N

5

N

L

Sr

10

N

N

N 

N

N

15

15

N

N

N

50

N

N

L

N

N

N

N

N

L

V

15

N

10

50 

20

30

N

N

20

15

15

10

15

50

10

20

20

10

30

10

L

W Y

N 70

N N

N L

N 20 

N 30

N 15

15 50

15 15

N 5

N 10

N 100

L 20

N 15

N 20

N 15

N 15

N 10

N 20

N 30

N 15

L 15

Zn Zr

N N

N N

N 50

N 300 

L 50

N 100

N 100

N 10

N 100

L L

N N

L 20

N 100

N 100

L 30

N 150

N 70

N 10

L 15

N 50

L 20

.07 3,000 1.5 N 10 L N 10 10 7,000 L 10 L 30 L 5 N 15 L 20 L 50

1666a 5 .2 L .01 1,500 N N N 10 N 

1666b 10 .02 L ,02 500 N N 15 L N

1666c 15 .02 L .01 3,000 N N 20 L L 

1668a 10 L L .005 700 1 N N 50 N

10 L 20 N N L 10 L N N L N 30 N 10 

10 L 150 L 7 L 30 L 7 N 10 N 20 L N

20 L 300 L 20 L 30 L L N 15 N 30 200 N 

15 L 5,000 N N N 10 10 N N L N 30 N N

.015 300 7 N L L N

.015 500 .7 N L 50 L

50 10 2,000 L 7 L 30 L I, N L N 20 N 30 

50 L 1,500 L5L30NLNLN15 N L

Not shown in figure 68; precise location lost.
REFERENCES CITED 121

rock consists of Precambrian muscovite-biotite schist 

and gneiss containing lenses and bodies of quartz monzonite and pegmatite. The prospects are on northeasttrending and steeply dipping shear zones. The individual shear zones are a meter or two thick and have 

considerable lateral continuity. They have Fe and Mn 

oxides in surface exposures, and chalcopyrite and

samples from the Gold hill area

atomic absorption by T. Roemer, S. Rickard, W. Campbell, and R. Miller 

looked for but not found; L, element present in undetermined amount

Chemical analyses

Atomic Combined fire 

absorption assay and atomic 

(ppm) absorption 

(ppm)

Sample description

Au Au

6.0

.2

33 24.4

N

13.0 9, 9, 9

.3

34.0 17, 18, 18

.08

.4

24.0 13, 14, 16

.3

.5

N — -

.5

.2

N

.08

280 140

36.0 15, 18, 16

7.2 5.2

1.0

1.0

.1

N ——

.1

200 98.3

1.5

.06

Vein in gneiss containing quartz, Fe and Mn

oxides and malachite. Prospect.

Fractured quartz vein. Prospect on shear

zone.

Sheared and mineralized gneiss. Prospect

on shear zone.

Sheared gneiss. Prospect on shear zone.

Quartz-carbonate vein in sheared gneiss.

Prospect on shear zone.

Sheared and somewhat mineralized gneiss.

Prospect on shear zone.

Sheared gneiss and margin of quartz-carbonate

vein. Prospect on shear zone.

Sheared quartz vein with some pyrite and

malachite. Shear zone in quartzitic gneiss.

Sheared gneiss with vuggy quartz vein.

Shear zone in quartzitic gneiss.

Quartz-carbonate-chalcopyrite vein. Shear

zone in quartzitic gneiss.

Vein in shear in gneiss. Shaft in sheared

and mineralized schist, gneiss, and

granitic rock.

Gossan on quartz carbonate vein in sheared

gneiss. Shaft in sheared and mineralized

schist, gneiss, and granitic rock.

Mineralized shear zone in gneiss. Adit on

irregular shear.

Mineralized shear zone in gneiss. Adit on

irregular shear.

Gossan on sheared and mineralized zone in

gneiss. Adit on irregular shear.

Sheared gneiss. Adit in sheared and

brecciated quartzitic gneiss.

Fractured silicified gneiss. Adit in

sheared and brecciated quartzitic gneiss.

Vein in sheared gneiss. Adit on vein in

sheared gneiss.

Sheared and altered gneiss. Adit on vein

in sheared gneiss.

Vein in gneiss containing pyrite and

malachite. Shear zone in schist layers

between 2 layers of granitic rock.

Interior of Cu-bearing quartz-carbonate

vein. Shear zone in schist layers between

2 layers of granitic rock.

Margin of Cu-bearing quartz-carbonate vein.

Shear zone in schist layers between 2

layers of granitic rock.

Vein in shear in gneiss. Dump of caved adit.

Vuggy quartz vein in shear in gneiss; Fe and

Mn oxides. Dump of caved adit.

Vein in gneiss with carbonate and Fe oxides.

Dump of caved adit.

Vein in sheared gneiss containing pyrite and

malachite. Prospect on vein in sheared

gneiss.

Vein in gneiss containing Cu minerals, quartz,

and carbonate. Prospect on vein in sheared

gneiss.

Vein in sheared gneiss containing Cu minerals,

quartz, and carbonate. Prospect on vein in

sheared gneiss.

malachite are visible in some prospects. Silicified rock 

and thin quartz veins and chalcopyrite-bearing quartzcarbonate veins occur in some of the shear zones.

I sampled some of the prospects in connection with 

the Heavy Metals Program of the U.S. Geological 

Survey in the middle 1960's; some of the gold values 

obtained at that time were as high as 280 ppm (table 

14; fig. 68). The better values were obtained from vein 

samples, but even some of them were barren. The 

analyses show the expected high Fe and Mn contents, 

some very low silver values, scattered copper values as 

high as 2 percent, and almost no lead or zinc. The high 

gold values may be caused by enrichment near the surface, and they may be spotty and die out at depth. 

Some adits, such as numbers 1657,1663, and 1666 (fig. 

68), tested the shear zones 30-120 m below the surface. 

Perhaps unfavorable results in those prospects caused 

abandonment of the area. Probably little modern study has been made at Gold Hill.

Similar analytical data on samples from prospects in 

the Independence gold district 10 km to the northeast 

of Gold Hill shows that the gold is in quartz-pyritechalcopyrite veins locally accompanied by argentiferous galena. Those veins also have as much as 0.1 

percent zinc, which is generally absent from the Gold 

Hill samples.

Whether these deposits are Precambrian or Tertiary 

is uncertain. The close association of the veins at Independence with the Grizzly Peak cauldron (Cruson, 

1972) suggest that there the veins may be of middle 

Tertiary age. The veins at Gold Hill are several 

kilometers from either than igneous center or the 

White Rock pluton. Certainly the shear zones containing the veins may be of Precambrian age, and perhaps 

the veins themselves are of that age.

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122 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

1 KILOMETER

CONTOUR INTERVAL 40 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

1 ft =0.3048m

EXPLANATION

1663

SHAFT 

1668 X PROSPECT PIT—

Number is analyzed sample

FIGURE 68.—Locations of prospects and localities of analyzed samples, Gold Hill area. Numbers indicate analyzed samples, 

table 14. Base from U.S. Geological survey 1:24,000, New York Peak, 1960.
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124 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

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128 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

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STRATIGRAPHIC SECTIONS 1-4
130 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

SECTION 1.—Manitou Dolomite, Peerless Formation, and Sawatch 

Quartzite

[Measured on east side of ridge between two southerly forks of Express Creek; base of 

section at 3,590 m altitude, Hayden Peak quadrangle. Measured by Bruce Bryant and 

D. G. Morse]

Thickness 

(meters)

Manitou Dolomite:

48. Dolomite, poorly exposed at top of formation....... 15.1

47. Dolomite, light-greenish-gray to light-gray; weathers

yellowish gray ................................. 3.1

46. Dolomite, medium-light-gray ..................... 1.2

45. Dolomite, light-gray; weathers yellowish gray ...... .9

44. Dolomite, medium-gray .......................... 1.2

43. Dolomite, medium-light-gray; weathers medium light 

gray to yellowish gray; beds 15 cm to 1.8 m; 

stringers of white to light-gray chert over a 0.6-m 

interval 0.3 m from base and a few lenses of white 

chert 20.7 m above base ........................ 30.0

42. Dolomite, medium-light-gray, medium-dark-gray, and 

yellowish-gray; weathers yellowish gray and 

medium light gray; beds 2.5-30 cm, a few shaly 

partings; lenses and stringers of medium-darkgray chert 2.5-5 cm thick ....................... 9.1

41. Dolomite, medium-light-gray; weathers medium gray 

to yellowish gray; beds 0.6-2.4 m, a few shaly partings .......................................... 4.2

40. Sandstone, dolomitic and sandy dolomite; grades to 

medium-light-gray dolomite; beds 0.6-1.2 m; some 

medium-gray shale partings .................... 1.4

39. Dolomite, pale-yellowish-brown, grades to mediumlight-gray dolomite ............................ 1.7

38. Dolomite, pale-yellowish-brown; contains numerous 

grayish-brown to medium-gray shale partings in 

lower 0.3 m .................................... 1.2

37. Sandstone, dolomitic, pale-yellowish-brown ........ .1

36. Dolomite, medium-light-gray to yellowish-gray; beds

1-10 cm; a few gray shaly partings .............. 6.1

Thickness of Manitou Dolomite ............... 75.6

Peerless Formation:

35. Dolomite and sandy dolomite, grayish-orange ...... 1.2

34. Sandstone, dolomitic, grayish-orange to pale-yellowish-brown; contains 0.3-m bed grayish-orangepink dolomitic quartzite ........................ 3.9

33. Dolomite and sandy dolomite, grayish-orange; contains yellowish-brown dolomitic sandstone beds 

2.5-30 cm ..................................... 3.0

32. Dolomite, sandy, grayish-orange, and dolomitic

sandstone ..................................... 5.0

31. Dolomite, dusky-red to grayish-red (5R YI2), sandy, 

shaly, and somewhat calcareous; becomes interbedded with grayish-orange sandy dolomite at top ... 3.8 

30. Dolomite and sandy dolomite, grayish-orange to 

moderate-yellowish-brown, thin-bedded; contains 

light-olive-gray to medium-light-gray shale and 

dolomitic shale beds and a 0.3-m bed of sandy 

dolomite containing dusky-red shaly dolomite 

fragments..................................... 2.3

29. Shale, light-brownish-gray to medium-light-gray ... .3

28. Dolomite, sandy, thinly laminated ................. .3

27. Shale and dolomitic shale, light-olive-gray; a few

thin beds of dolomite ........................... .7

SECTION 1.—Manitou Dolomite, Peerless Formation, and Sawatch 

Quartzite—Continued

Thickness 

(meters)

Peerless Formation—Continued

26. Dolomite, sandy dolomite, dolomitic sandstone, 

shaly dolomite, and dolomitic shale, grayishorange to moderate-yellowish-brown, thin-bedded; 

shaly beds light-olive-gray; beds 0.3-7.6 cm ...... 1.6

25. Shale, somewhat dolomitic, greenish-gray to lightolive-gray; a few thin beds of shaly dolomite ...... 1.0

24. Dolomite, calcareous, grayish-orange, coarse-grained; 

contains generally flat dusky-red shaly dolomite 

fragments to 1.2 cm long ....................... .8

23. Sandstone, dolomitic, yellowish-gray; weathers grayish orange to light brown ....................... .6

22. Limestone, shaly and somewhat sandy, grayishorange to light-brown-weathering; shale in irregular partings ............................... .1

21. Shale, light-olive-gray ............................ .9

20. Limestone, sandy, and limy sandstone; grayish 

orange; weathers pale yellowish brown, dark 

yellowish brown, and moderate yellowish brown; 

beds 2.5-15 cm ................................ 7.1

Thickness of Peerless Formation .............. 32.7

Sawatch Quartzite:

19. Quartzite and sandstone, calcareous, white to grayish-orange-pink ................................ 1.1

18. Quartzite and sandstone, calcareous, grayish-orangepink to dark-yellowish-brown ................... 1.2

17. Quartzite, white, vitreous; contains a few 2.5-30-cm

beds of calcareous sandstone.................... 4.8

16. Sandstone, calcareous, light-brown; weathers moderate yellowish brown ............................ 1.0

15. Quartzite, white ................................. 1.9

14. Sandstone, calcareous; weathers moderate yellowish 

brown to dark yellowish brown; irregular, lumpy 

bedding ....................................... .03

13. Quartzite, white; contains a few dark-yellowishbrown lenses of calcareous sandstone ............ 1.1

12. Quartzite, light-gray, and dolomitic quartzite weathering pale yellowish brown to moderate yellowish 

brown; beds 5-60 cm ............................. 2.4

11. Quartzite, white, vitreous ........................ 2.4

10. Quartzite, dolomitic, light-brownish-gray to pale 

red; weathers dark yellowish brown (10 YR 4/2) to 

moderate yellowish brown (10 YR 5/4)........... .6

9. Quartzite, very light gray to white................. 3.0

8. Quartzite, light-brownish-gray (5 YR 6/1), finegrained ....................................... .8

7. Quartzite, white; grain size 0.5 mm and less; small

scale crossbedding ............................. 7.7

6. Quartzite, white; weathers to a somewhat rusty

tinge.......................................... .5

5. Quartzite, pale-yellowish-brown (10 YR 6/2)........ 1.2

4. Quartzite, light-grayish-orange (10 YR 7/4); crossbedded; scattered quartz pebble and granule beds 3.0 

3. Quartzite, white, crossbedded, grain size 0.5-2 mm;

scattered quartz-granule beds ................... 6.4

2. Quartzite, white, crossbedded; quartz pebbles to 1 

cm and granules in basal 60 cm and 12-cm bed 2 m 

above base .................................... 3.3

Thickness of Sawatch Quartzite ............... 42.8

1. Granite, coarse-grained.
STBATIGBAPHIC SECTIONS 131

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)

[This section is probably as close to a type section of the Maroon Formation as can be obtained. Its base is in Rustler Gulch in the southwest corner of the Maroon Bells 

quadrangle, and it runs approximately along the divide between Snowmass and Maroon 

Creeks to the 4,036-m peak south of Baldy Mountain in the Highland Peak quadrangle. 

No complete section of the Maroon Formation is available on Maroon Creek because of 

surficial deposits. The section is a composite of individually measured sections. The sections are connected by tracing beds from the top of one to the bottom of the next higher 

section in the field or on aerial photographs. Some faults cross the section, but their 

displacements are in a few tens of meters, and beds can usually be matched across them. 

I estimate that the accuracy of the thickness of the entire section is approximately ±150 

m. Bottom of the highest section at 3,603 m altitude in gully west of 4,036-m peak on the 

East Snowmass-Willow Creek divide; section south side of gully on a N. 70° E. trend. 

Section measured by Bruce Bryant and E. S. Grew]

Thickness 

(meters)

Morrison Formation (part):

316. Limestone, gray; contains charophytes ........... 2.7

315. Sandstone, light-greenish-gray, calcareous; contains lenses of gray limestone and of gray and 

mottled red, white, and gray chert.............. 1.2

314. Siltstone, calcareous; weathers yellowish gray .... 2.4

313. Limestone, medium-gray; clastic grains of gray

limestone and a few of quartz .................. 2.1

312. Sandstone, calcareous; weathers light gray (Nl) 

and yellowish gray (5 Y 8/1); bedding irregular; 

grades upward to limestone ................... 1.5

311. Sandstone, medium-bluish-gray (5 B 5/1), calcareous; weathers yellowish gray; beds 0.3 m and less 

separated by medium-gray (JV5) to medium-darkgray (JV4) shaly siltstone, silty shale, and shaly 

limestone beds 8-20 cm thick .................. 6.4

Partial thickness of Morrison Formation...... 16.3

Entrada Sandstone:

310. Sandstone, yellowish-gray, crossbedded, poorly

sorted; grain size 4 mm in coarser beds ......... 1.8

309. Quartzite, very light gray (JV9) to yellowish-gray 

(5 Y 8/1), crossbedded, thinly laminated; contains 

rounded grains of quartz, gray and moderate-red 

chert, and rare grains of feldspar as much as 1 cm 

in diameter................................... 5.2

Thickness of Entrada Sandstone ............. 7.0

Unconformity 

Chinle Formation:

308. Covered; grayish-brown (5 YR 3/2) siltstone and

calcareous siltstone float...................... 5.8

307. Conglomerate, limestone-pebble, mottled pale-red 

(5 R 6/2) to grayish-red (10 R 4/2) and lightgreenish-gray (5 GY 8/1); pebbles as much as 2.5 

cm in diameter ............................... 1.3

306. Covered; bench ................................. 17.7

Thickness of Chinle Formation ............... 24.8

Unconformity 

Maroon Formation:

305. Siltstone, thin-bedded; fine-grained sandstone .... 4.3

304. Sandstone, pale-reddish-brown, fine-grained, thinbedded; carbonized plant fragments ............ 9.4

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

303. Sandstone, fine-grained, siltstone, calcareous; beds

2.5-30 cm.................................... 46.6

302. Arkose, dark-reddish-brown, calcareous, micaceous; 

contains fragments of siltstone as much as 2 cm 

in diameter................................... .6

301. Arkose and siltstone, moderate-reddish-brown .... 23.8

300. Arkose, calcareous, micaceous, crossbedded, thinly

laminated, fine- to medium-grained ............. 21.6

299. Siltstone and fine-grained arkose, moderate to

dark-reddish-brown, calcareous, micaceous ..... 25.3

298. Arkose, dark-reddish-brown to moderate-reddishbrown, calcareous, micaceous, crossbedded; contains heavy-mineral partings, fragments of 

siltstone to 2.5 cm length ...................... 14.6

297. Siltstone and fine- to medium-grained calcareous 

and micaceous arkose; graded bedding; some 

sandstones thinly laminated; some crossbedded; 

chippy-weathering siltstone forms benches. 

Analyzed sample MA440 of sandy siltstone from 

this interval (table 3, col. 5).................... 143.3

296. Arkose, moderate-reddish-brown to dark-reddishbrown, calcareous, micaceous, fine- to mediumgrained; beds 2.5-90 cm thick; a few interbeds of 

siltstone ..................................... 24.1

295. Arkose, dark-reddish-brown, coarse-grained, 

calcareous; contains beds of crossbedded pebble 

and cobble conglomerate as much as 0.6 m thick; 

cobbles of Precambrian rock as much as 15 cm in 

diameter; grades to overlying unit.............. 16.5

294. Arkose, dark-reddish-brown, calcareous, micaceous, 

fine- to medium-grained; graded bedding; scattered beds of dark-reddish-brown to moderatereddish-brown calcareous, chippy-weathering 

siltstone ..................................... 22.6

293. Arkose, dark-reddish-brown to moderate-reddishbrown (10 R 4/6), calcareous, micaceous; a few 

thin beds contain pebbles of Precambrian rock .. 13.4

292. Conglomerate and arkose; conglomerate in beds 

2.5-25 cm thick; pebbles of Precambrian rock as 

much as 11 cm in diameter; irregular light-gray to 

white bleached zone ........................... 4.3

291. Arkose, dark-reddish-brown (10 R 3/4), calcareous, 

micaceous, medium- to coarse-grained; beds 

1.2-20 cm, crossbedded; a few thin beds containing pebbles of Precambrian rock ............... 14.3

[This bed approximately projected to top of section measured starting at gully at about 

3,710 m altitude northwest of saddle south of 3,986-m peak on Willow Creek-East 

Snowmass Creek ridge just north of the south boundary of the Maroon Bells quadrangle; 

trend of section about N. 35° E., top of section is at gully to north where small unmapped 

fault is located]

290. Arkose, moderate-reddish-brown, crossbedded,

thin-bedded, coarse- to fine-grained ............ 17.4

289. Siltstone, moderate-reddish-brown, calcareous .... 6.1

288. Arkose, pale-red, thin-bedded, crossbedded; some

pebble beds and lenses of conglomerate......... 22.3

287. Arkose, moderate-reddish-brown, silty, fine- to

medium-grained .............................. 3.0

286. Arkose, moderate-red, massive-weathering; scattered pebbles 2.5 cm in diameter ............... 2.1
132 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation —Continued

285. Arkose, moderate-red (5 R 5/4), medium- to finegrained, thin-bedded, crossbedded; some muddy 

partings and siltstone beds .................... 25.6

284. Arkose, pale-reddish-brown to moderate-red (5 R

5/4), coarse-grained ........................... 4.3

283. Arkose, moderate-reddish-brown, fine-grained .... .3

282. Arkose, moderate-red (5 R 5/4), coarse-grained;

2.5-5-cm bleached lenses and stringers ......... 1.8

281. Arkose, medium-grained ........................ 3.7

280. Arkose, dark-reddish-brown, fine-grained, silty,

calcareous, micaceous......................... 2.4

279. Arkose, moderate-reddish-brown, coarse-grained,

crossbedded ................................. .6

[This bed projected to top of section with top at contact of Tertiary porphyritic granodiorite body at 3,992 m altitude east of pass at head of East Snowmass Creek and bottom at 

about 3,880 m east side of pass, Maroon Bells quadrangle. Trend of section N. 70° E. Colors may be slightly modified by contact metamorphism near porphyritic granodiorite 

contact]

278. Arkose, coarse-grained, crossbedded; scattered

pebbles; a few siltstone beds ................... 26.5

277. Arkose, pale-reddish-brown, thin-bedded, calcareous, fine- to medium-grained ................... 10.1

276. Arkose, coarse-grained, crossbedded; conglomerate-filled channels as much as 1 m thick contain 

cobbles of Precambrian rock and of siltstone as 

much as 23 cm in diameter..................... 7.6

275. Arkose, medium- to fine-grained, silty ............ 1.8

274. Arkose, grayish-red (5 R 4/2), coarse-grained, crossbedded; contains channel conglomerates as much 

as 0.6 m thick containing cobbles of Precambrian 

rock as much as 13 cm in diameter.............. 35.9

273. Arkose, moderate- to dark-reddish-brown, finegrained, thin-bedded.......................... 1.5

272. Arkose, dusky-red, medium- to coarse-grained,

calcareous, micaceous ......................... 15.5

[This bed projected to the top of the 3,959-m peak on the divide between Snowmass Creek 

and East Snowmass Creek in the Highland Peak quadrangle where top of this part of section is located; bottom is in the Maroon Bells quadrangle at 3,627 m on northeast side of 

main gully southwest of the 3,959-m peak]

271. Arkose, thin-bedded, fine-grained ................ 5.8

270. Arkose, grayish-red, coarse-grained, crossbedded . .9 

269. Siltstone and fine-grained thin-bedded arkose, 

dark- to moderate-reddish-brown; locally 

moderate reddish orange and dusky red; grades 

to unit below ................................. 130.7

268. Arkose, moderate- to pale-reddish-brown finegrained to pale-red coarse-grained, calcareous, 

micaceous, some thin moderate-reddish-brown 

calcareous siltstone beds. Analyzed sample 

MB 1042 (table 3, col. 4) is arkose from this 

material ..................................... 21.6

267. Arkose, pale-reddish-brown, fine-grained, calcareous .......................................... 1.5

266. Arkose, pale-red, coarse-grained, crossbedded; contains channel conglomerates and scattered pebbles and cobbles as much as 15 cm in diameter... 19.2

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

265. Arkose, moderate-reddish-orange, fine-grained,

calcareous, thin-bedded, crossbedded........... 8.2

264. Arkose, coarse- to medium-grained, thin-bedded,

crossbedded, calcareous, micaceous ............ 15.2

263. Arkose and siltstone, pale-reddish-brown, finegrained, calcareous ........................... 1.8

262. Arkose, coarse-grained; contains scattered pebbles 

of Precambrian rock; quartz, chert, and brown 

siltstone as much as 5 cm in diameter; massiveweathering bedding units have internal beds 

1.2-5 cm thick and crossbedding; channel conglomerates as much as 1 m thick ............... 32.6

261. Arkose, medium-grained ........................ 3.0

260. Arkose, pale-reddish-brown (10 R 5/4), fine- to

medium-grained, calcareous ................... 8.8

259. Arkose, coarse-grained, calcareous, micaceous;

pebble conglomerate in channels 15 cm thick .... 10.1

258. Siltstone, moderate-reddish-orange, calcareous;

lenses out to southeast........................ .4

257. Arkose, dusky-red, crossbedded, coarse-grained; 

channel conglomerate at base cuts into underlying unit...................................... 21.0

256. Arkose, pale-reddish-brown, silty, calcareous, finegrained; irregular beds; grayish-red thinly 

laminated arkose .............................. 12.2

255. Arkose and conglomerate, pale-red to moderatered; conglomerate beds generally 15-30 cm thick 

and contain pebbles 2.5-5 cm in diameter; 

crossbedded; pebbles and cobbles locally scattered in arkose; maximum cobble size 15 cm ..... 32.6

254. Arkose, silty ................................... .9

253. Arkose, medium- to coarse-grained, calcareous, 

micaceous, crossbedded; beds of conglomerate 

containing pebbles and cobbles of Precambrian 

rock as much as 15 cm in diameter; a few duskyred muddy partings........................... 75.6

252. Arkose, thin-bedded, silty ....................... .9

251. Arkose, dusky-red and dark-reddish-brown, calcareous, micaceous; contains lenses of pebble conglomerate as much as 0.6 m thick and 3.8 m long; 

some arkose beds thin bedded and crossbedded . 5.8

250. Arkose, medium- to fine-grained, crossbedded, 

micaceous, thin-bedded; grades to underlying 

unit ......................................... 6.4

249. Arkose, silty, fine-grained, crossbedded .......... 6.1

248. Arkose, medium-grained, thin-bedded, crossbedded; contains a 0.6-m bed of coarse-grained 

arkose and conglomerate containing pebbles of 

Precambrian rock............................. 1.8

247. Arkose, moderate-reddish-brown, silty, calcareous;

bedding obscure .............................. 6.1

246. Arkose, dusky-red (5 R 3/4), moderate-reddishbrown (10 R 4/6) to dusky-brown, calcareous, 

micaceous, medium- to coarse-grained, thinly 

laminated, crossbedded; contains a few pebble 

beds, beds of silty arkose, and muddy partings .. 22.9

245. Arkose, silty, fine-grained ....................... 3.7

244. Arkose, dusky-red (5 R 3/4), calcareous, micaceous;

irregular bedding ............................. .9
STRATIGRAPHIC SECTIONS 133

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

[Following interval calculated by tracing beds in the field and on aerial and ground photographs and by dip. Much of this interval is exposed on the Maroon Bells and the rugged 

ridge to the north where measurement on the ground would be difficult]

Thickness 

(meters)

Maroon Formation—Continued 

243. Calculated interval .. 1,280

[Top of section is on peak at 4,026 m altitude in the Snowmass Mountain quadrangle southwest of Belleview Mountain; bottom is at about 3,841 m altitude on ridge northwest of 

West Maroon Pass, Maroon Bells quadrangle. Section is on or near ridge crest]

242. Arkose, pale-red, coarse-grained; channel conglomerate and scattered pebbles; pebbles to 10 cm in 

diameter; gray limestone makes up 50 percent of 

the pebbles................................... 4.9

241. Arkose, dark-reddish-brown, fine-grained ......... 1.5

240. Siltstone, moderate-red to dusky-red, calcareous,

micaceous ................................... 3.4

239. Arkose and channel conglomerate; gray limestone

pebbles predominant.......................... 1.2

238. Siltstone and arkose, dark-reddish-brown; grades

to underlying unit ............................ 5.7

237. Arkose, pale-red, coarse-grained, pebbly .......... 3.0

236. Arkose and siltstone ............................ 4.6

235. Arkose ........................................ .9

234. Conglomerate; cobbles to 15 cm in diameter; gray

limestone makes up 50 percent of cobbles ....... .9

233. Arkose ........................................ 2.4

232. Siltstone and silty limestone ..................... .9

231. Arkose, grayish-red (10 R 4/2), coarse-grained,

crossbedded ................................. 4.3

230. Siltstone and silty limestone ..................... 1.5

229. Arkose, coarse-grained; contains 0.3-m bed of channel conglomerate containing limestone (50 percent) and dolomite (20 percent) pebbles ......... 6.1

228. Siltstone, calcareous ............................ 1.2

227. Arkose ........................................ 1.2

226. Siltstone, very calcareous ....................... 1.2

225. Limestone, grayish-red to gray, silty and sandy ... 1.2 

224. Siltstone, calcareous ............................ .6

223. Arkose ........................................ .3

222. Siltstone, calcareous ............................ 1.5

221. Arkose, pale-reddish-brown to moderate-reddishbrown, fine-grained; contains 0.3 m gray 

limestone pebble bed .......................... 14.3

220. Arkose, coarse-grained .......................... 2.1

219. Siltstone, pale-brown, calcareous ................. .9

218. Limestone, light-brownish-gray to light-olive-gray,

sandy and silty ............................... 1.5

217. Arkose, fine-grained; siltstone interbeds .......... 6.1

216. Siltstone containing beds of silty limestone as

much as 8 cm thick; grades to underlying unit... 6.7 

215. Arkose, grayish-red to moderate-reddish-brown,

calcareous, micaceous, silty ................... 1.8

214. Siltstone, grayish-red (5 R 4/2), calcareous; contains 

0.3-m beds of light-greenish-gray-weathering 

olive-gray silty limestone ...................... 2.7

213. Arkose, fairly coarse grained; contains scattered 

pebble beds as much as 15 cm thick; 50 percent of 

the pebbles are gray limestone ................. 7.6

212. Siltstone and fine-grained arkose; calcareous ...... 4.3

211. Arkose, grayish-red, fairly coarse grained ......... 2.1

SECTION 2.— Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

210. Covered ....................................... .6

209. Arkose, coarse-grained .......................... 2.7

208. Arkose and siltstone ............................ .6

207. Arkose, pale-red to light-gray; grades to grayish

red and dark reddish brown .................... 1.5

206. Siltstone, calcareous ............................ .3

205. Arkose, light-red, coarse-grained; scattered pebbles and a 0.3-m pebble bed containing Precambrian rock (40 percent) and gray and white 

limestone (40 percent)......................... 5.8

204. Covered ....................................... 5.2

203. Siltstone, calcareous ............................ .3

202. Arkose, pale-red, grayish-red to dark-reddishbrown; contains a few pebbles ................. 5.5

201. Siltstone, moderate-reddish-brown, calcareous .... 1.8

200. Arkose, pale-red; a few pebbles near base; upper 

part dusky red, calcareous, and micaceous; contains a 10-cm pebble bed in which 40 percent of 

the pebbles are gray limestone ................. 6.4

199. Siltstone, moderate-reddish-brown, calcareous .... 1.8

198. Arkose, pale-red (10 R 6/2), coarse-grained ........ 6.1

197. Arkose and siltstone, grayish-red, moderate-reddish-orange to moderate-reddish-brown; 

calcareous ................................... 6.7

196. Arkose, fine-grained ............................ 2.1

195. Siltstone, calcareous ............................ .6

194. Arkose, grayish-red, calcareous .................. 2.1

193. Siltstone, gray, calcareous ....................... .6

192. Limestone, medium-gray, light-gray- to yellowishgray-weathering, silty......................... .6

191. Siltstone and fine-grained arkose; calcareous ...... 2.1

190. Siltstone, calcareous ............................ .3

189. Arkose, fine-grained, crossbedded ................ .6

188. Siltstone, calcareous; contains a few lenses of

silty limestone ............................... .3

187. Arkose, fine-grained, crossbedded ................ .6

186. Siltstone, grayish-red, hackly weathering, calcareous .......................................... 1.5

185. Arkose ........................................ 3.4

184. Siltstone, calcareous ............................ .9

183. Arkose, grayish-red, fine-grained, crossbedded .... 1.2

182. Siltstone ....................................... .3

181. Limestone, light-brownish-gray to light-olive-gray,

silty, grades downward to siltstone............. .3

180. Siltstone ....................................... .6

179. Arkose, grayish-red (5 R 4/2), crossbedded; few 

siltstone beds and partings of dark-reddishbrown mudstone ............................. 3.7

178. Arkose, pale- to moderate-reddish-brown, calcareous, micaceous, fine-grained, silty and muddy ... 1.5 

177. Siltstone and limestone, light-olive-gray, sandy,

grades downward to siltstone .................. .6

176. Arkose, grayish-red, crossbedded ................ 1.8

175. Limestone, light-brownish-gray, silty; grades downward to siltstone ............................. 2.1

174. Siltstone, calcareous ............................ 1.5

173. Arkose, light-brownish-gray to moderately reddish

brown, calcareous ............................ .6
134 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

172. Arkose, pale-red, grayish-red to moderate-reddishbrown, crossbedded, medium- to coarse-grained; 

contains channel conglomerates as much as 30 

cm thick; pebbles of Precambrian rock dominant 

in one bed; gray limestone in others ............ 17.1

171. Siltstone, moderate-reddish-brown, calcareous .... 1.5

170. Conglomerate; gray limestone pebbles dominant.. .9 

169. Arkose, grayish-red ............................. 1.2

168. Limestone and calcareous siltstone ............... 2.7

167. Arkose ........................................ .3

166. Limestone, grayish-red to light-brownish-gray,

silty ......................................... .6

165. Arkose, pale-red to light-gray; channel conglomerate as much as 30 cm thick at base composed of 

pebbles of gray limestone (60 percent), Precambrian rock (15 percent), and sandstone, dolomite, 

and quartz (25 percent) as much as 13 cm in 

diameter ..................................... .9

164. Siltstone, grayish-red, calcareous; grades downward to limestone............................. 1.2

163. Limestone, light-gray; contains lenses and beds of

pale-red silty limestone........................ 1.5

162. Arkose, grayish-red, coarse-grained, crossbedded . 3.4 

161. Arkose, pale-red, calcareous ..................... .9

160. Limestone, grayish-red- and light-olive-gray-mottled, silty .................................... 1.2

159. Siltstone, grayish-red, calcareous; grades to calcareous medium- to coarse-grained arkose ...... 4.0

[Bottom of section at about 3,841 m altitude on ridge south of Belleview Peak on projection of beds from top of peak south of West Maroon Pass. Section is continued from top 

of that peak S. 60° W. along ridge to east end of 3,853-m knob in Snowmass Mountain 

quadrangle, south down slope]

158. Arkose, grayish-red and pale-red; contains pebbles 

and beds of pebble conglomerate in which gray 

limestone is the dominant rock type ............ 14.6

157. Covered; probably siltstone ..................... 3.0

156. Arkose, grayish-red, calcareous, micaceous, thinly

laminated .................................... 2.7

155. Siltstone, moderate-reddish-brown, calcareous, 

micaceous; contains lenses and beds of grayishred to brownish-gray to light-olive-gray 

calcareous siltstone and one bed of arkose ...... 8.8

154. Arkose, grayish-red ............................. 4.3

153. Limestone, light-brownish-gray, silty; a mediumgray limestone bed 2.5-8 cm thick at top ........ .6

152. Siltstone ....................................... .3

151. Arkose, grayish-red to pale-red, crossbedded; contains local channel conglomerates as much as 1 m 

thick composed of gray limestone (60 percent), 

Precambrian rocks (15 percent), quartz, chert, 

and dolomite (25 percent), and a few beds of 

siltstone ..................................... 24.1

150. Siltstone and fine-grained arkose, moderate-reddish-brown, calcareous, micaceous ............. 4.0

149. Arkose, pale-red ................................ .6

148. Siltstone, moderate-reddish-brown, calcareous,

micaceous; contains lenses of arkose ............ 4.0

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

147. Arkose, grayish-red, fairly coarse grained, micaceous, calcareous; contains channel conglomerate 

with cobbles of gray limestone as much as 20 cm 

long ......................................... 1.8

146. Limestone, medium-gray, thin-bedded, silty, and

grayish-red calcareous micaceous siltstone...... 6.7

145. Siltstone, grayish-red to dark-reddish-brown, micaceous, calcareous; contains lenses and irregular 

beds of medium-gray silty limestone ............ .6

144. Limestone, gray, silty; grades downward to arkose .3

143. Arkose, grayish-red and pale-red, coarse-grained, 

calcareous, micaceous; contains a few pebbles of 

gray limestone ............................... 9.1

142. Siltstone ....................................... .6

141. Limestone, medium-gray; grades downward to

siltstone ..................................... .3

140. Siltstone, calcareous ............................ .3

139. Arkose, grayish-red ............................. .6

138. Siltstone, moderate-reddish-brown to dark-reddishbrown, calcareous; grades downward to arkose .. 1.5 

137. Arkose, grayish-red, coarse-grained, crossbedded; 

contains channel conglomerate in which 30 percent of the pebbles are gray limestone .......... 4.6

136. Limestone, medium- to dark-gray; grades downward to siltstone ............................. .3

135. Siltstone, grayish-red, calcareous ................ 2.1

134. Arkose, grayish-red, crossbedded; contains 15-cm 

pebble bed; 50 percent of pebbles are gray 

limestone .................................... 7.1

133. Siltstone, grayish-red and dark-reddish-brown, 

calcareous; contains a thin bed of pale-olive sandstone ........................................ 5.2

132. Siltstone; calcareous and sandy limestone, lightolive-gray; weathers pale yellowish orange ...... 1.8

131. Arkose, gray to grayish-red and pale-red, coarsegrained ...................................... 6.4

130. Limestone, light-gray to medium-gray, silty ....... .3

129. Siltstone, calcareous; a few beds of arkose ........ 14.3

128. Limestone, pale-red (5 R 6/2), silty; grades to siltstone, sandstone, and gray limestone-pebble 

channel conglomerate ......................... 2.1

127. Arkose, grayish-red, coarse-grained .............. .3

126. Limestone, medium-gray, pale-yellowish-brownweathering, thin-bedded, silty ................. .3

125. Arkose and conglomerate, grayish-red; crossbedded 

conglomerate beds 15 cm thick composed of pebbles of gray limestone (50 percent); a few beds of 

grayish-red to moderate-reddish-brown 

calcareous siltstone........................... 14.9

124. Arkose and siltstone containing lenses of limestone ........................................ 2.1

123. Siltstone containing 2.5-5 cm lenses of brownishgray to medium-gray silty limestone ........... .3

122. Siltstone; grades to coarse-grained, crossbedded, 

grayish-red, calcareous, micaceous arkose. 

Analyzed sample ME 1978 of arkose (table 3, col. 

2) ........................................... 9.4
STRATIGRAPHIC SECTIONS 135

SECTION 2.— Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

121. Siltstone, calcareous; contains scattered lenses of

limestone .................................... .6

120. Limestone, medium-dark-gray ................... .3

119. Siltstone, pale-red to grayish-red (10 R 4/2) grades 

to conglomerate containing 2.5-cm pebbles of 

gray limestone ............................... 2.4

118. Arkose, grayish-red; silty arkose ................. .6

117. Limestone, greenish-gray (5 GY 6/1), thin-bedded;

grades to grayish-red calcareous siltstone....... .9

116. Arkose, grayish-red ............................. .6

115. Siltstone, grayish-red, calcareous, micaceous; contains a few beds of sandstone .................. 2.1

114. Arkose, grayish-red, crossbedded ................ 2.7

113. Siltstone, dark-reddish-brown, calcareous, muddy; 

contains round globs of grayish-red very 

calcareous siltstone as much as 30 cm long ...... .9

112. Siltstone, grayish-red, calcareous ................ 1.2

111. Arkose, grayish-red, coarse-grained, crossbedded; 

contains local channel conglomerates as much as 

1 m thick containing cobbles as much as 13 cm in 

diameter; cobbles composed of gray limestone (50 

percent), dolomite (15 percent), Precambrian 

metamorphic and plutonic rocks (15 percent); 

quartz, sandstone, and chert (20 percent)....... 71.8

110. Limestone, medium-light-gray, silty, lenticular .... .3

109. Arkose, grayish-red, micaceous, calcareous, finegrained ...................................... 3.4

108. Arkose, light-gray, crossbedded, coarse-grained ... .3

107. Arkose and siltstone, grayish-red to reddishbrown, calcareous. Separated from underlying 

siltstone by a Tertiary porphyritic granodiorite 

dike (14.3 m thick) ............................ 17.1

106. Siltstone, grayish-red (10 R 4/2) to dark-reddishbrown; contains a few beds of grayish-red arkose 11.6 

105. Arkose, light-gray to pale-red and grayish-red,

coarse-grained ............................... 19.5

104. Siltstone, dark-reddish-brown, calcareous, micaceous ........................................ .9

103. Siltstone, light-greenish-gray (5 G 8/1), calcareous;

to medium-gray silty limestone ................ .9

102. Arkose, light-olive-gray, calcareous; contains beds

of sandy and silty limestone 1.2-10 cm thick .... 3.4

101. Limestone, siltstone, and light-olive-gray calcareous arkose; beds of sandy and silty limestone 

1.2-10 cm thick ............................... .9

100. Limestone, medium-dark-gray; contains sandy and

silty lenses ................................... 1.5

99. Siltstone, calcareous; grades downward to limestone ........................................ 1.5

98. Limestone, medium-dark-gray, silty .............. 2.7

97. Arkose, pale-red to medium-light-gray, coarsegrained; grades downward to arkose............ 4.9

96. Arkose, moderate-reddish-brown; grades downward to grayish-red arkose .................... 1.5

95. Arkose, grayish-red; grades downward to siltstone .9 

94. Siltstone, grayish-red (10 R 4/2), muddy, calcare-

.6

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

93. Arkose, grayish-red (5 R 4/2), crossbedded, finegrained ...................................... 8.8

92. Arkose, light-olive-gray to pale-red; contains a 

0.3-cm thick pebble bed in which gray limestone 

is the predominant rock type .................. 5.2

91. Arkose, grayish-red ............................. .9

90. Siltstone, dark-reddish-brown, calcareous ......... .6

89. Arkose, grayish-red to medium-gray, calcareous,

crossbedded ................................. 4.6

88. Siltstone, light-olive-gray, thin-bedded, calcareous 1.5 

87. Arkose, grayish-red, silty, calcareous ............. 1.2

86. Arkose, medium-gray to brownish-gray, moderateyellowish-brown-weathering, thin-bedded, 

calcareous ................................... 5.7

85. Siltstone and limestone, medium-gray, thinbedded ...................................... .9

84. Siltstone and mudstone, grayish-red and darkreddish-brown, calcareous..................... 6.1

83. Arkose, pale-red ................................ 2.1

82. Siltstone, grayish-red (5 R 4/2), calcareous ........ 4.0

81. Arkose, pale-red, fine-grained .................... .6

80. Arkose, light-olive-gray, pale-yellowish-orangeweathering, calcareous........................ 1.5

79. Covered ....................................... 2.7

78. Arkose, medium-light-gray to medium-dark-gray .. 1.2 

77. Siltstone, medium-light-gray to medium-dark-gray, 

pale-yellowish-orange-weathering, thin-bedded, 

calcareous ................................... 2.4

76. Covered; probably mainly siltstone; a bed of lightgray silty limestone crops out.................. 8.2

75. Arkose, pale-red to light-gray, pale-grayish-orangeweathering, calcareous, micaceous. Analyzed 

sample ME1198 (table 3, col. 3)................ 7.3

74. Arkose, calcareous; grades to light-olive-gray,

pale-yellowish-orange-weathering silty limestone 7.0

73. Arkose, pale-yellowish-brown, calcareous; some

finer grained grayish-red beds ................. 3.4

72. Arkose, grayish-orange, calcareous ............... 2.7

71. Limestone, grayish-orange, silty ................. 1.2

70. Arkose, moderate-yellowish-brown (10 GR 5/4),

calcareous ................................... -3

69. Siltstone and shaly siltstone, dark-reddish-brown,

thin-bedded .................................. 3.0

68. Hornfels from siltstone. Separated from underlying sandstone by 16.8-m-thick Tertiary porphyritic granodiorite dike ..................... .3

67. Somewhat hornfelsized fine-grained calcareous

sandstone.................................... .3

66. Arkose, pale-red to grayish-red, coarse-grained, 

thinly laminated, micaceous, calcareous; contains 

gray limestone pebbles in some beds ............ 10.1

65. Arkose, grayish-yellow, coarse-grained ........... 1.2

64. Arkose, grayish-red, thin-bedded, calcareous, micaceous ........................................ 7.9

63. Arkose, coarse-grained; scattered pebbles ........ 5.5

62. Arkose, grayish-red, calcareous; grades downward

to limestone .................................. 11.9
136 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

61. Limestone, medium-light-gray and light-brownishgray, thinly laminated ........................ 3.0

60. Arkose and siltstone, grayish-red (10 R 4/2), calcareous, micaceous ........................... 5.5

59. Limestone, silty ................................ .3

58. Arkose and siltstone, grayish-red ................ 2.7

57. Arkose, grayish-red, coarse-grained; contains a

pebble bed ................................... 6.7

56. Siltstone, grayish-red to dark-reddish-brown ...... 1.5

55. Arkose, grayish-red, fairly coarse grained ......... 1.2

54. Siltstone, grayish-red to dark-reddish-brown, calcareous ...................................... 3.4

53. Arkose, grayish-red, fairly coarse grained ......... 2.1

52. Conglomerate; maximum pebble size 5 cm; pebbles 

gray limestone (50 percent), brown dolomite (15 

percent); Precambrian metamorphic and plutonic 

rocks (20 percent) quartz, chert and others (15 

percent) ..................................... 1.5

51. Siltstone, grayish-red, calcareous ................ 2.4

50. Arkose, pale-red (5 R 6/2); and grayish-red (5 R

4/2) fine-grained arkose and siltstone ........... 5.5

49. Limestone and calcareous siltstone, mediumgray, pale-yellowish-orange-(10 YR 8/6)-weathering .......................................... .6

48. Arkose, grayish-red, coarse-grained, thinly laminated ....................................... 1.5

47. Siltstone, grayish-red (10 R 4/2), micaceous, calcareous ...................................... 3.0

46. Covered ....................................... 22.3

45. Arkose, pale-red, coarse-grained, crossbedded; contains a few beds of grayish-red (10 R 4/2) 

calcareous siltstone........................... 10.7

44. Covered ....................................... 3.4

43. Arkose, pale-red, coarse-grained; contains scattered pebbles................................. 3.7

42. Arkose, grayish-red, fine-grained; a few mudstone

partings ..................................... .6

41. Arkose, grayish-red (10 R 4/2) and light-gray,

medium-grained .............................. 1.8

40. Arkose; contains limestone-pebble conglomerate 

with lesser amounts of Precambrian rock, quartz, 

chert, jasper, and dolomite; pebbles to 5 cm in 

diameter ..................................... .6

39. Siltstone, grayish-red (5 R 4/2), pale-red- to lightred-weathering, calcareous; grades downward to 

limestone .................................... 1.2

38. Limestone, medium-light-gray; chemical analysis

sample ME642 (table 3, col. 1).................. .3

37. Arkose, moderately light gray to grayish-red

(bR 4/2), thin-bedded.......................... 7.6

36. Arkose, grayish-red, fine-grained, calcareous ...... 4.9

35. Siltstone, dark-reddish-brown to grayish-red

(10 .R 4/2), thin-bedded, calcareous .............. .6

34. Covered ....................................... 2.4

33. Arkose, pale-olive (10 Y 6/2), coarse-grained ....... .3

32. Arkose, dusky-yellow (5 Y 6/4)................... .9

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters)

Maroon Formation—Continued

31. Limestone, silty; lenses ......................... .3

30. Arkose, pale-red (10 .R 6/2) to grayish-red (10 .R 4/2); 

contains a few beds of dark-reddish-brown muddy 

arkose beds and numerous conglomerate beds 30 

cm or less thick composed predominantly of gray 

limestone but also dolomite, quartzite, and 

Precambrian rock............................. 41.7

29. Mudstone, grayish-red; dark-reddish-brown partings; lenses of calcareous siltstone, thin-bedded 1.5 

28. Arkose, grayish-red, thin-bedded, coarse-grained .. .9 

27. Siltstone; contains lenses of limestone ............ 2.1

26. Arkose, pale-red (10 .R 6/2)....................... 4.3

25. Conglomerate; contains medium-gray limestone, 

white limestone, quartz, dolomite, and granitic 

pebbles as much as 10 cm in diameter ........... .3

24. Arkose, thin-bedded, fine-grained to mediumgrained; muddy partings ...................... 5.5

23. Siltstone, thin-bedded, calcareous ................ .6

22. Arkose, pale-red (5 R 6/2), coarse-grained, calcareous, micaceous ............................... 1.2

21. Siltstone, grayish-red, calcareous; contains lenses

of limestone.................................. 1.2

20. Arkose and siltstone, pale-red (5 R 6/2), pale-reddish-brown-weathering, calcareous ............. 3.0

19. Arkose, grayish-red, pale-reddish-brown-weathering, medium-grained, calcareous, micaceous ..... 5.5

18. Arkose, light-greenish-gray to greenish-gray, grayish-yellow weathering, coarse- to medium-grained 1.5

17. Siltstone and arkose, grayish-red, thin-bedded .... .3

16. Limestone and silty limestone, moderate-light-gray

to reddish-gray; lensy beds .................... .6

15. Arkose and siltstone with limestone lenses, grayish-red, pale-reddish-brown-weathering......... 1.8

14. Arkose, light-brownish-gray and brownish-gray, 

grayish-orange-weathering, fine-grained; lightbrownish-gray thin-bedded silty limestone ...... 2.1

13. Siltstone, pale-reddish-brown, light-brownish-grayweathering, calcareous; contains grayish-red 

limestone lenses 0.5-8 cm thick and as much as 

15 cm long, and medium-gray, grayish-orangeweathering calcareous siltstone containing lenses 

of limestone.................................. 1.5

12. Covered (contact estimated by float)............. 29.9

Thickness of Maroon Formation (rounded to 

nearest 10 meters) ........................ 3,170

Gothic Formation of Langenheim (1952) (part):

11. Covered ....................................... 23.8

10. Arkose, light-greenish-gray to grayish-green,

coarse-grained, calcareous; massive bed ........ 3.0

9. Arkose, light-gray to olive-gray, calcareous, micaceous, coarse-grained, thin-bedded ............. .6

8. Limestone, dark-gray ........................... 2.4

7. Limestone, medium-gray, sandy ................. .6
STRATIGRAPHIC SECTIONS 137

SECTION 2.—Morrison Formation (part), Entrada Sandstone, Chinle 

Formation, Maroon Formation, and Gothic Formation (part) of 

Langenheim (1952)—Continued

Thickness 

(meters}

Gothic Formation of Langenheim (1952) (part)—Continued

6. Arkose, light-olive-gray, coarse-grained, calcareous, micaceous; contains a 10-cin bed of darkgray siltstone ................................

5. Arkose, medium-gray, fine-grained to coarsegrained, calcareous; beds 0.5-20 cm ............

4. Covered .......................................

3. Shale, dark-gray, silty; contains beds of dark-gray 

light-gray-weathering shaly siltstone 

0.3-1.2 m thick; only locally calcareous .........

2. Limestone, dark-gray; beds 1-15 cm thick; beds 

of micaceous silty shale .......................

1. Shale and siltstone, dark-gray, thin-bedded; some 

beds of micaceous arkose 5 cm thick and some silty limestone..................................

6.7

2.1 

5.8

[Section ends at 3,383 m altitude in gully northwest of Rustler Gulch at east margin of 

Snowmass Mountain quadrangle]

SECTION 3.—Dakota Sandstone (part), Burro Canyon Formation, 

Morrison Formation, Entrada Sandstone, Chinle Formation, and 

State Bridge Formation

[Base of section at 3,191 m altitude and south of fault in steep gully on east side of East 

Snowmass Creek SEVi sec. 15, T. 10 S., R. 86 W., Highland Peak quadrangle. Azimuth of 

section about S. 55° E.; all of section is south of fault. Section measured by Bruce Bryant

and S. G. Pollock]

Thickness 

T» , , -. , , , ,. (meters) Dakota Sandstone (part):

78. Siltstone, medium-gray, sandy; light-bluish-gray

silty sandstone, and dark-gray shale ............. 4.3

77. Sandstone, white to yellowish-gray, crossbedded; 

containing some lenses of quartz, chert, and 

feldspar granules .............................. 12.8

76. Sandstone, light-bluish-gray, silty ................. .3

75. Shale, dark-gray, silty ............................ .3

74. Sandstone, white to yellowish-gray, medium-grained, 

grayish-orange-weathering, crossbedded; grains of 

dark-gray chert and pink feldspar; a few thin 

granule beds .................................. 9.8

Partial thickness of Dakota Sandstone ......... 27.5

Burro Canyon Formation:

73. Sandstone, very light gray, fairly fine grained, 

grayish-orange-weathering; scattered granules of 

quartz, chert, and feldspar; contains thin intervals 

of light-bluish-gray silty shale; sandstone beds 

15-60 cm thick ................................ 5.2

72. Sandstone, very light gray, fairly fine grained, 

grayish-orange-weathering; scattered granules of 

quartz, chert, and feldspar ...................... 4.6

71. Sandstone; same as above but contains thin interbeds of light-bluish-gray silty shale; sandstone 

beds 15-60 cm thick ............................ 2.7

70. Sandstone, very light gray, fairly fine grained; 

grayish-orange-weathering; scattered granules of 

quartz, chert, and feldspar ...................... 4.6

SECTION 3.—Dakota Sandstone (part), Burro Canyon Formation, 

Morrison Formation, Entrada Sandstone, Chinle Formation, and 

State Bridge Formation—Continued

Thickness 

(meters)

Burro Canyon Formation—Continued

69. Conglomerate in white sandstone matrix; pebbles of 

quartz and chert to 1 cm in diameter; weathers 

grayish orange ................................ 3.4

Thickness of Burro Canyon Formation ......... 20.5

Morrison Formation:

68. Claystone and siltstone, interbedded and intergrading greenish-gray, grayish-red, to dusky-red and 

medium-bluish-gray; greens predominate; a few 

thin beds of fine-grained sandstone .............. 9.4

67. Siltstone, interbedded greenish-gray and grayishred ........................................... 4.3

66. Claystone or siltstone, locally calcareous, mediumbluish-gray to grayish-red-mottled and interbedded; beds 0.3-1 m thick......................... 20.4

65. Siltstone, calcareous, interbedded grayish-red and 

greenish-gray; grades to grayish red locally mottled with green ................................ 7.0

64. Sandstone, greenish-gray to medium-light-gray,

fine-grained, calcareous ........................ 2.4

63. Siltstone, interbedded greenish-gray and grayishred; beds 2.5 cm to 1 m thick .................... 7.0

62. Sandstone, greenish-gray (5 G 6/1), fine-grained,

calcareous..................................... .3

61. Claystone, grayish-red with greenish-gray mottling . .3

60. Siltstone, grayish-red, calcareous ................. .3

59. Siltstone, grayish-red with some greenish-gray interbeds and mottling ............................. 3.0

58. Limestone, grayish-yellow-green, silty ............. .3

57. Siltstone, predominantly grayish red .............. 2.1

56. Siltstone and fine-grained sandstone, grayishgreen (10 GY 5/2) .............................. 3.4

55. Limestone, gray, silty ............................ .3

54. Siltstone, greenish-gray (5 G 6/1) to grayish-red,

calcareous, locally mottled ...................... 8.8

53. Sandstone, medium-dark-gray, calcareous; grades to

gray limestone at top ........................... .9

52. Siltstone, greenish-gray, calcareous (5 GY 6/1 and

5 G 6/1)....................................... 3.7

51. Limestone, gray, and calcareous siltstone .......... .3

50. Siltstone, greenish-gray, calcareous (5 GY 6/1 and

5 G 6/1)....................................... 4.6

49. Siltstone, greenish-gray to medium-bluish-gray and

grayish-red, very calcareous .................... 3.4

48. Limestone, grayish-yellow-green, medium-grayweathering; contains charophytes............... 2.1

47. Sandstone, medium-light-gray, medium-grained; 

calcareous, crossbedded; some coarser beds have 

dark-gray siltstone(?) fragments to 1 cm in 

diameter ...................................... 2.7

46. Siltstone, limy, and silty limestone; grayish yellow 

green with lenses of grayish red; grades to 

limestone at top ............................... 2.1

45. Limestone, medium-gray, yellowish-gray-weathering .9

44. Siltstone, grayish-red (5 R 4/2), very calcareous ..... 3.0

43. Sandstone, greenish-gray, calcareous, lenticular .... .3
138 ASPEN 15-MINUTE QUADRANGLE, PITKIN AND GUNNISON COUNTIES, COLORADO

SECTION 3.—Dakota Sandstone (part), Burro Canyon Formation, 

Morrison Formation, Entrada Sandstone, Chinle Formation, and 

State Bridge Formation—Continued

Thickness 

(meters)

Morrison Formation—Continued

42. Limestone and calcareous siltstone, interbedded and

intergrading grayish-red and grayish-yellow-green 1.8

41. Sandstone, greenish-gray, calcareous .............. .9

40. Siltstone, grayish-yellow, calcareous ............... .3

39. Siltstone, grayish-red, calcareous ................. .3

38. Siltstone, grayish-yellow-green, calcareous ......... .3

37. Limestone, grayish-red; silty and calcareous siltstone ......................................... 1.2

36. Limestone, medium-gray; beds 8 cm to 1 m thick; 

joint-controlled calcite veinlets; becomes silty upward; light bluish gray weathering .............. 10.1

35. Limestone, grayish-yellow green, silty; contains less

silty lenses .................................... 3.4

34. Limestone, medium-gray; silty, light-bluish-grayweathering; has limestone fragments near base ... 2.4

33. Siltstone, greenish-gray, calcareous; one 10-cm bed

of gray limestone .............................. 2.4

32. Limestone, medium-gray ......................... .9

31. Sandstone, pale-yellowish-green; somewhat calcareous ........................................... 1.2

30. Limestone; weathers light olive gray............... 1.5

29. Limestone, medium-gray; contains small mediumgray to medium-bluish-gray chert stringers and 

lenses 1-5 cm thick ............................ .9

28. Limestone, medium-gray, silty, yellowish-grayweathering .................................... .3

27. Sandstone, bluish-white, medium-grained, calcareous, crossbedded; beds 0.9-1.5 m thick; weathers 

pinkish gray; grades to moderate-orange-pink- to 

yellowish-gray-weathering, light-brown sandy 

limestone at top ............................... 5.8

26. Sandstone, light-bluish-gray, medium- to finegrained calcareous, yellowish-gray-weathering; 

contains some beds of light-greenish-gray siltstone .9

25. Limestone, medium-light-gray to medium-dark-gray, 

shaly, yellowish-gray-weathering; very calcareous 

sandstone and sandy limestone; laminae 0.5-5 mm 2.1

24. Sandstone, yellowish-gray (5 Y 7/2), calcareous; contains some coarser grains at base; some shaly 

limestone interbedded along strike .............. 2.7

Thickness of Morrison Formation ............. 132.7

Entrada Sandstone:

23. Sandstone, pinkish-gray, crossbedded; contains 

laminae and scattered 1-2-mm quartz and chert 

grains ........................................ 7.0

22. Sandstone, yellowish-gray (5 Y 8/1), crossbedded, 

medium- to coarse-grained; has 1-2-mm quartz and 

rare grains of chert in a finer grained matrix; larger 

bedding units 0.9-1.2 m thick; laminae 3 mm to 

1 cm .......................................... 7.0

Thickness of Entrada Sandstone .............. 14.0

SECTION 3.—Dakota Sandstone (part), Burro Canyon Formation, 

Morrison Formation, Entrada Sandstone, Chinle Formation, and 

State Bridge Formation—Continued

Thickness 

(meters)

Unconformity

Chinle Formation:

21. Siltstone, dark-reddish-brown (10 R 3/4), calcareous ........................................... 2.7

20. Conglomerate, limestone-pebble, pale-red to grayishred (5 R 6/2 to 5 R 4/2); pebbles 1-10 mm; some 

beds of moderate-red to moderate-reddish-brown 

siltstone ...................................... 4.3

19. Siltstone, moderate-reddish-brown to moderate-red 

(5 R 4/6), very calcareous; hackly fracture; beds 

0.3-1.2 m; a few irregular lenses of pinkish-gray 

limestone ..................................... 6.4

18. Limestone, mottled moderate-red (5 R 5/4) to pinkish-gray (5 YR 8/1), silty ........................ .9

17. Siltstone, moderate-red to moderate-reddishbrown, calcareous; hackly weathering; some calcite 

along fractures ................................ 2.1

16. Limestone, mottled moderate-red (5 R 5/4) to pinkish-gray (5 YR 8/1), silty; a few lenses of limestonegranule-bearing siltstone at the top .............. 1.5

15. Siltstone, moderate-red to moderate-reddish-brown, 

calcareous; hackly weathering; some calcite along 

fractures ...................................... 1.5

14. Limestone, pinkish-gray (5 YR 8/1); grades to red

calcareous sandstone........................... .3

13. Sandstone, moderate-reddish-brown (10 R 4/6), very

calcareous..................................... 1.5

12. Siltstone, moderate-reddish-brown, very calcareous; 

some thin lenses and beds of very light gray 

limestone ..................................... 1.5

11. Limestone, grayish-red (5R 4/2), silty .............. .9

10. Limestone, pinkish-gray to very light gray ......... .3

9. Siltstone and shale, very dark red (5 R 2/6) to duskyred (5 R 3/4); very pale orange (10 YR 8/2) mottling 

in lenses and stringers; forms bench ............. 5.8

8. Sandstone and pebble conglomerate; contains 

quartz, pink to gray chert, and blackish-red (5 R 

2/2) siltstone as much as 2.5 cm in diameter but 

commonly 1-2 cm in diameter; locally mottled very 

dusky red purple (5 RP 2/2). Thickness varies along 

strike; is 3.6 m thick just 3 m from line of section 

mainly due to irregularities in bottom contact.... 1.5

Thickness of Chinle Formation ................ 31.2

Unconformity

State Bridge Formation:

7. Siltstone, very light gray to yellowish-gray, mottled dusky-red (5R3I4) ......................... .9

6. Siltstone, dark-reddish-brown (10 R 3/4), massive,

calcareous; hackly fracture ..................... 10.7
STRATIGRAPHIC SECTIONS 139

SECTION 3.—Dakota Sandstone (part), Burro Canyon Formation, 

Morrison Formation, Entrada Sandstone, Chinle Formation, and 

State Bridge Formation—Continued

Thickness 

(meters)

State Bridge Formation:—Continued

5. Conglomerate containing lenses of sandstone ...... 6.1

4. Sandstone and conglomerate, moderate-reddishbrown; contains pebbles and cobbles ............. 1.5

3. Conglomerate; largest pebbles 10 cm; composed of 

quartz, fine-grained granitic rock; low-grade 

metamorphic rock, and minor amounts of lightgray limestone................................. .9

2. Sandstone, moderate-orange-pink (10 R 7/4) to moderate-reddish-orange (10 R 6/6); pebbles of quartz, 

quartzite, and siltstone and lenses of conglomerate 

as much as 2.5 cm thick ......................... 4.0

Thickness of State Bridge Formation .......... 24.1

Unconformity

Maroon Formation (part):

Siltstone, moderate-red (5 R 4/6) to grayish-red 

(10 R 4/2), calcareous; beds 5-60 cm.............. 3.4

SECTION 4.—Dakota Sandstone and Burro Canyon Formation

[This section is on the northwest side of West Willow Creek valley; bottom of section is at 

3,377 m altitude N. 42° W. of hill 12015 on Willow-West Willow ridge, Highland Peak 

quadrangle. Section measured by Bruce Bryant and S. G. Pollock]

Mancos Shale (part):

29. Shale, dark-gray to medium-gray.

Dakota Sandstone:

28. Sandstone, light-olive-gray, fine-grained; beds 2.5-55

cm thick; partings of carbonaceous shale ......... 2.4

27. Shale and siltstone, medium-dark-gray to dark-gray 2.1

26. Sandstone, medium-gray, fine-grained, light-olivegray- to grayish-black-weathering............... 1.8

25. Shale and siltstone, medium-dark-gray ............ 9.1

24. Sandstone, dark-gray, carbonaceous; mottled light

and dark gray on weathered surface ............. 3.0

23. Shale and siltstone, dark-gray; a few sandstone

beds and lenses ................................ 1.2

22. Sandstone, light-gray to yellowish-gray, locally 

dark gray and carbonaceous; grain size 0.2-0.4 

mm; beds 15-60 cm ............................ 2.4

21. Shale, siltstone, and sandstone, dark-gray to 

medium-gray; sandstone beds as much as 15 cm 

thick.......................................... 3.7

20. Sandstone, light-olive-gray, fine-grained, dark-yellowish-orange- to light-brown-weathering ........ .6

19. Siltstone, medium-dark-gray ...................... .3

SECTION 4.—Dakota Sandstone and Burro Canyon Formation 

—Continued

Thickness 

(meters)

Dakota Sandstone—Continued

18. Shale and siltstone, medium-dark-gray to darkgray .......................................... 1-2

17. Sandstone, medium-grained, fine-grained .......... .9

16. Shale, medium-dark-gray to dark-gray, somewhat

carbonaceous.................................. 1.2

15. Sandstone, coarse-grained; pebbles of gray chert

to 1 cm in diameter; some feldspar ............... 2.1

14. Sandstone, medium-light-gray, fine-grained; a few 

fragments of carbonaceous material; light gray to 

grayish orange and dark yellowish orange weathering ........................................... 1-5

13. Clay stone, medium-gray to medium-light-gray ..... 1.8

12. Sandstone; quartz and some feldspar with a grain 

size of 0.2 mm; a few thin beds with a grain size of 

2-3 mm; weathers grayish orange, light brown, 

moderate brown, and grayish red; forms cliff ..... 10.7

Thickness of Dakota Sandstone ............... 46.0

Burro Canyon Formation:

11. Clay stone, medium-gray and light-gray ............ 2.1

10. Sandstone, siltstone, and clay stone, light-gray to

medium-light-gray; sandstone beds 0.3-1 m thick . 2.4

9. Claystone and siltstone, light-bluish-gray to greenish-gray (5 G 6/1); a few beds of sandstone 2.5-15 

cm thick ...................................... 3.7

8. Sandstone, grain size 0.2 mm; weathers light gray, 

grayish orange to light brown; some specks of Fe 

oxide ......................................... 1.5

7. Claystone, greenish-gray (5 GY 6/1)................ 1.5

6. Sandstone, very light gray; grain size 0.2 mm; scattered grains of feldspar and opaque minerals; beds 

0.3-1 m; some thin partings of very light gray 

siltstone; weathers pale yellowish orange to 

moderate brown ............................... 3.0

5. Conglomerate, gray to white; pebbles of quartz

chert and quartzite as much as 2 cm in diameter .. 1.8

4. Sandstone, similar to bed 6 ....................... 7.9

Thickness of Burro Canyon Formation......... 23.9

Morrison Formation (part):

3. Claystone, light-bluish-gray ...................... 1.5

2. Siltstone and claystone, grayish-red (5 R 4/2)....... 3.0

1. Siltstone and claystone; greenish-gray (5 G 6/1) to

light-bluish-gray (5 B 7/1)....................... __2/7

Partial thickness of Morrison Formation ....... 7.2

INDEX

Access..................................... 3

Age determinations ......................... 11, 101

Alaskite ................................... 67

Albite.................................... 33, 73, 74

alaskite porphyry ....................... 54

aplite porphyry ......................... 54

hornblende porphyry .................... 67

hornblende quartz diorite ................ 54

quartz porphyry ........................ 56

Albitization ................................ 63

Allanite.................................... 8

aplite .................................. 56

granodiorite ............................ 63

hornblende quartz diorite ................ 54

Alluvium .................................. 105

Altitude ................................... 2

American Lake ............................. 101

Andalusite ................................. 74

Anhydrite, Belden Formation ................ 25

Gothic Formation....................... 26

Highland tunnel ........................ 25

Maroon Formation...................... 33

Monument Gulch ....................... 26

Annie Basin................................ 108

Belden Formation....................... 23

Anthracospirifer rockymontanus............. 27

Antimony .................................. 108, 115

Apishapa uplift............................. 25

Aplite ..................................... 8, 54, 56

Castle Creek fault zone .................. 82

porphyry ............................... 54, 56

Arcs, continental margin .................... 69

volcanic................................ 69

Arkansas Valley............................ 87

Arsenic .................................... 108, 115

Ashcroft ................................... 15, 18

Ashcroft Mountain, Precambrian rocks ....... 8

Ashflow tuffs .............................. 68

Aspen, history ............................. 4

Aspen Contact mine ........................ 113

Aspen Mountain............................ 3,9

aplite porphyry ......................... 56

faults .................................. 84

glaciation .............................. 88

hornblende quartz diorite ................ 54

stratigraphic displacement .............. 83

Aspen 7.5-minute quadrangle, Maroon Formation ............................ 29

State Bridge Formation ................. 40

Atokan fusulinds, Elk Range thrust.......... 28

Aviculopecten sp............................ 27

Azurite .................................... 115

B

Baculites compressus zone .................. 80

Baculites perplexus zone .................... 52, 53

Baculites scotti zone ........................ 53

Baldy Mountain ............................ 86

Barite ..................................... 114

Barium .................................... 108

Basalt flows................................ 72, 98

Battlement Mesa ........................... 87

Bedding-plane features ...................... 26, 41

Gothic Formation....................... 26

Maroon Formation ...................... 29

State Bridge Formation ................. 41

[Italic page numbers indicate major references]

	Page 

Belden Formation ................... 21, 23, 25, 73, 82

aplite porphyry ......................... 56

brachiopods ............................ 25

hornblende quartz diorite ................ 54

map relations........................... 22

marble ................................. 59

quartz porphyry ........................ 55

red beds................................ 74

Belden-Gothic contact ...................... 80, 108

Belden-Leadville contact, faulting............ 108

Bellerophon (Pharkidonotus) sp. ............. 27

Bellerophontid gastropod. East Maroon Creek 28

Benioff zones............................... 68

Bigelow moraine............................ 99

Bigelow thrust ............................. 84

Biomicrites ................................ 48

Biotite quartz diorite ....................... 8, 12

Bismuth ................................... 115

Black Canyon .............................. 33

Blastomylonite............................. 76

Block faulting.............................. 5

Boulder shaft .............................. 91

Brachiopods, Belden Formation.............. 25

Gothic Formation....................... 27

Richmond Hill.......................... 22

Broken Rib Member, Dyer Dolomite.......... 20

Brush Creek................................ 3

drainage ............................... 51, 91

Brush Creek-Snowmass Creek divide, landslides .......................... 103

Brushy Basin Member ...................... 49

Bryozoa, Richmond Hill..................... 22

Burnt Mountain ............................ 80, 91

Morrison Formation .................... 47

Burro Canyon Formation ............. 49, 50, 137, 139

Bushwacker shaft .......................... 91

Butte fault................................. 82

Cadmium ...........................

Calcisphaera laevis ..................

pachysphaeriea ..................

Calc-silicate minerals ................

Cambrian rock ......................

Capitol Creek, Entrada Sandstone ....

Capitol Creek-Snowmass Creek divide . 

Capitol Creek valley .................

. 108

. 22

. 22

. 34

. 16

. 46

. 86

. 86

Capitol Peak quadrangle .................... 86, 97

Dakota Sandstone ...................... 115

Carlsbad twinned microcline ................. 9

Castle Butte ............................... 12

Manitou Dolomite ...................... 15

Parting Formation ...................... 19

Castle Butte Member ....................... 21

Castle Creek ............................... 3, 12, 74

alluvium ............................... 106

aplite .................................. 56

Belden Formation ....................... 24

Dyer Dolomite .......................... 19

fan deposits ............................ 105

glaciers ................................ 90

Gothic Formation ....................... 25

gypsum ................................ 25

Hope tunnel ............................ 25, 110

hornblende quartz diorite ................ 54

Manitou Dolomite ...................... 15

moraines ............................... 96, 99

	Page 

Castle Creek—Continued

White Rock pluton ...................... 59

Castle Creek fault zone............... 23, 3"3, 80,83, 98

Eagle Valley Formation ................. 79

Maroon Formation...................... 87

quartz porphyry ........................ 55

State Bridge Formation ................. 43

Castle Creek valley ........................ 24, 84, 87

Belden Formation....................... 23

Castle Peak ................................ 2

Cataract Creek ............................. 63

Cathedral Lake-Montezuma mine area ....... 86

Cedar Mountain Formation.................. 49

Central Colorado trough..................... 22

See also Eagle basin.

Cephalopods, Gothic Formation.............. 27

Chabazite .................................. 74

Chaffee Group .............................. 77, 54

Charophyte remains, Morrison Formation .... 48

Chernyshinella ............................. 22

Chert ...................................... 35, 41

Belden Formation....................... 25

Chinle Formation ....................... 44

Burro Canyon Formation ................ 49

Entrada Sandstone ................... s . 46

Leadville Limestone..................... 22

Morrison Formation .................... 47

Treasure Mountain dome ................ 23

Chinle Formation .................... 41, 44, 131, 137

depocenter ............................. 45

deposition.............................. 23

fossils ................................. 46

red beds................................ 45

Clark fault ................................. 35, 108

Climate .................................... 2

Clinetop Member, Dotsero Formation ........ 17

Coffee Pot Member, Dyer Dolomite........... 20

Colluvium ................................. 104

Colorado Formation ........................ 51

Colorado mineral belt....................... 7, 72, 88

Colorado Plateau ........................... 49, 50

Colorado River drainage..................... 2

Colpites kansasensis ........................ 27

Commodity reports ......................... 4

Composite ovata ........................... 27

Contact fault............................... 81

Conundrum Creek ................... 29, 41, 44, 73, 79

Belden Formation....................... 23

Gothic Formation....................... 25

landslide deposits ....................... 103

limestone .............................. 33

metals, anomalous ...................... 115

Conundrum Creek valley .................... 34, 63

Conundrum Creek window........... 63, 73, 74, 82, 85

Conundrum Peak ........................... 59

Cooper Creek ............................... 39, 59

iron deposit ............................ 113

Cooper Fork................................ 9

garnet ................................. 9

Copper ................................ 108, 114,115

Copper-silver deposits, Dakota Sandstone .... 115

Copper sulfides, silver content............... 119

Corals ..................................... 22

Correlations, Cambrian and Ordovician rocks . 16

Cozy Point................................. 52

Cozzette Member ........................... 52

Crater Lake, landslides ...................... 103

natural dam ............................ 93

141
142 INDEX

Page 

Crested Butte 15-minute quadrangle, Belden

Formation ...................... 25, 29

fossils ................................. 22, 28

Crinoid stem fragments ..................... 22

Cross Creek Granite ........................ 11

Crystal River............................... 2, 81

Curtis Formation, marine fossils ............. 49

D

Dakota Group.............................. 49

Dakota Sandstone ..................... 49, 50, 86,119

copper-silver deposits ................... 115

landslide deposit........................ 103

Deadmans Gulch Formation ................. 15, 16

Deposition, Gothic Formation ............... 40

Maroon Formation ...................... 40

Difficult Creek valley, quartz monzonite ...... 9

Dikes, Quaternary diabase .................. 72

Tertiary diabase ........................ 72

Dinosaurs, Morrison Formation .............. 49

Diorite, biotite quartz ....................... 8, 12

hornblende quartz ...................... 54

quartz ................................. 7, 58

Disseminated sulfides ....................... 63

Domatoceras sp. ............................ 27, 28

Dotsero Formation ......................... 17

Doubletop thrust ........................... 81

Durango, gravels ........................... 97

Dyer Dolomite ............................ 18, 19, 20

E

Eagle basin .......................... 5, 22, 73, 79, 85

Elk Range thrust zone................... 82

evaporite deposits ...................... 40

fusulinid sequence ...................... 28

Gothic Formation....................... 25

Maroon Formation ...................... 36

Pennsylvanian sequence................. 22

Triassic sequence ....................... 22

Eagle Mountain ........................... 49, 79, 86

Dakota Sandstone ...................... 115

Maroon Formation ...................... 35

Eagle Valley Evaporite ...................... 28

Eagle Valley Formation .................... 28, 29, 79

Earlandia .................................. 22

East Aspen Mountain ....................... 12

fossils ................................. 18

East Brush Creek ........................... 97

East Maroon-Conundrum ridge.............. 33

East Maroon Creek........................ 59, 63, 93

rock composition ....................... 28

talus................................... 104

Trepospira sp. .......................... 28

East Snowmass Creek, Chinle Formation ..... 44

State Bridge Formation ................. 40

East Snowmass Creek valley, Entrada Sandstone ........................... 46

East Snowmass Creek-Willow Creek divide ... 45

Eastern Uinta Mountains ................... 41

Economic geology .......................... 108

Elbert Formation, Upper Devonian........... 19

Elk Mountains ............................. 2

Elk Range thrust zone................... 82

glaciers ................................ 90, 100

Mancos Shale .......................... 52

Maroon Formation ...................... 29

Morrison Formation .................... 49

rock composition ....................... 58, 69

Snowmass pluton ....................... 85

Treasure Mountain dome ................ 87

White Rock pluton ...................... 85

Elk Range, geologic map .................... 4

Elk Range thrust fault zone .......... 39,59,80, 81, 87

Atokan fusulinds ....................... 28

formation .............................. 108

Entrada Formation ......................... 79

Entrada Sandstone ...................... 46, 131, 137

Eotubertina ................................ 22

Page 

Erosion, glacial............................. 7

Euphemites sp. ............................. 27

Evaporites ................... 25, 26, 27, 33, 34, 40, 79

Exiteloceras jenneyi zone ................... 52, 53, 80

Express Creek .......................... 4, 12, 21, 81

Parting Formation ...................... 19

plant fragments ........................ 28

Express Creek-Castle Creek ridge............ 28

Express Creek valley, Leadville Limestone.... 21

Precambrian rocks ...................... 7

Fall Creek.................................. 19

Fans, alluvial and debris .................... 105

hanging................................ 105

Faults ................................ 80, 84, 88, 106

Aspen Mountain area ................... 54

bedding-plane, Aspen mining district..... 81, 85

Bigelow thrust ......................... 84

block faulting .......................... 5

Butte .................................. 82

Castle Creek fault zone .......... 23, 33, 43, 55, 79,

80, 82, 83, 84, 87, 98 

Clark .................................. 35, 108

Contact fault ........................... 81

Doubletop thrust ....................... 81

Elk Range thrust..... 28, 39, 59, 80, 81, 82, 87,108

Highland tunnel zone ................... Ill

Homestead shear zone .............. 76, 77, 85, 88

Hope .................................. Ill

Hunters Hill thrust..................... 80

Lenado ................................ Ill

Red Table Mountain .................... 83

Silver................................ 22, 81, 108

Smuggler Mountain ..................... 85, 108

Snowmass Creek ........................ 79, 87

Winnie................................. Ill

Woody Creek valley ..................... Ill

See also Shears. 

Fault zones, Highland tunnel................ Ill

Felsic porphyries, composition ............... 56

Fibrolite ................................... 7

Fish plates ................................. 18

Flattops ................................... 87

Fluid inclusions ............................ 63, 75

Folds ...................................... 85

Foliation, Precambrian rocks ................ 76

Foraminifera, Leadville Limestone ........... 22

Formations, basal Paleozoic ................. 12

Belden ......................... 21, 22, 23, 25, 54,

55, 56, 59, 73, 74, 82 

Burro Canyon .................... 49, 50, 137,139

carbonate .............................. 5, 12

Cedar Mountain ........................ 49

Chinle ................. 23, 41, 44, 45, 46, 131, 137

Colorado ............................... 51

Cross Creek Granite ..................... 11

Curtis ................................. 49

Dakota Sandstone ........ 49, 50, 86, 103, 115, 119

Deadmans Gulch ....................... 15, 16

Dotsero ................................ 17

Dyer Dolomite ......................... 18,79, 20

Eagle Valley ........................... 28, 29, 79

Elbert ................................. 19

Entrada................................ 79

Entrada Sandstone .................. 46, 131, 137

Fremont Limestone ..................... 18

Oilman Sandstone ...................... 18, 20

Gothic ................... 23,25, 27, 28, 31, 34, 36,

40,72,81,82,83,86,110,131 

Green River ............................ 82

Gunnison .............................. 46

See also Entrada Sandstone. 

Harding Quartzite ...................... 18

Hell Gate Porphyry ..................... 11

Kroenke Granodiorite ................... 11

Leadville Limestone ... 18, 20, 21, 22, 23, 73, 79, 81 

Mancos Shale .................. 50, 51, 52, 80, 81,

82, 84,86,103,105,139

Page 

Formations—Continued

Manitou Dolomite ...... 15, 16, 17, 18, 54, 112, 130

Maroon ........... 23, 25, 28, 29, 46, 47, 79, 84,131

See also subheadings under Maroon

Formation. 

Maroon Conglomerate................... 29

Mesaverde ............................ 52, 80, 82

Minturn ............................... 25, 28

Moenkopi .............................. 24, 41

Molas.................................. 23

Montana ............................... 51

Morrison .................. 34, 41, 46, 47, 131, 137

Mount Garfield ......................... 53

Mount Princeton Quartz Monzonite ...... 58

Mount Zion Porphyry ................... 57

Naturita ............................... 49

Ohio Creek ............................. 80

Pando Porphyry ........................ 57

Parting ............................... 17, 18, 19

Peerless ....................... 14, 15, 16, 17, 130

Pikes Peak'Granite ..................... 7, 11

Price River ............................. 52

St. Kevin Granite ....................... 7, 11

Sawatch ............................... 14

Sawatch Quartzite ....... 12, 14,16,17, 54, 91, 130

Shinarump ............................. 46

Silver Plume Granite .................... 7

State Bridge ......... 23, 29, 40, 41, 43, 79, 83, 137

Wasatch ............................... 59

Weber ................................. 23, 25

Weber Limestone ....................... 29

Weber Sandstone ....................... 36

Weber Shale............................ 23

White Porphyry ........................ 57

Fort Hays Limestone Member ............... 57, 119

Fossil collections ........................... 18, 52

Fossil plants, Express Creek................. 28

Mancos Shale .......................... 52

Maroon Formation ...................... 37

Fossils..................................... 17

Anthracospirifer rockymontanus ......... 27

Atokan ................................ 28

Aviculopecten sp. ....................... 27

Baculites compressus zone ............... 80

perplexus zone ...................... 52, 53

scotti zone.......................... 53

Belden Formation....................... 25

Bellerophon (Pharkidonotus) sp. .......... 27

Bellerophontid gastropod................ 28

Calcisphaera laevis ...................... 22

pachysphaeriea ..................... 22

Chemyshinella ......................... 22

Chinle Formation ....................... 46

Colpites kansasensis .................... 27

Composita ovata........................ 27

Des Moinesian, Gothic Formation ........ 36

Maroon Formation .................. 31

Domatoceras sp ........................ 27, 28

Earlandia .............................. 22

East Aspen Mountain ................... 18

Eotubertina ............................ 22

Euphemites sp. ......................... 27

Exiteloceras jenneyi zone ............... 52, 53, 80

Gothic Formation ....................... 27

Hypselentoma sp........................ 27

lanthinopsis (Soleniscus) sp. ............. 27

Inocermus deformis ..................... 51

Knishtites (Retispira) nodocostatus ....... 27

Latiendothyra sp. ....................... 22

Leadville Limestone..................... 22

Linoproductus prattenianus ............. 27

Liroceras sp ............................ 27

marine ................................. 27, 37

Curtis Formation ................... 49

Maroon Formation ...................... 28, 37

Mooreoceras normals.................... 27

Myalina sp. ............................ 27

Naticopsid Trachydomia sp .............. 27

Naticopsis (Naticopsis) sp ............... 27

Palaeoneilo sp........................... 27
INDEX 143

Fossil—Continued

Palaeospiroplectammina ................

Paracaligella ...........................

Parathurammina .......................

Pharkidonotus ..........................

Plagioglypta sp. ........................

Radiosphaera...........................

Schizodus sp............................

Septabrunsiina sp. ......................

Septaglomospiranella dainae.............

Shansiella carbonaria ...................

Soleniscus .............................

South Canyon Creek Member ............

Spinoendothyria spinosa ................

Stegocoelia sp ..........................

Trachydomia sp. ........................

Trempealeauan .........................

Trepospira sp. ..........................

Tuberendothyra ........................

Fremont Limestone .........................

Front Range uplift..........................

Fryingpan River ....................... 28,1

Belden Formation.......................

Gothic Formation.......................

Fusulinid sequence, Eagle basin..............

Page

22

22

22

27

27

22

27

22

22

27

27

41

22

27

27

17

28

22

18

25

, 97, 99

25

40

28

G

Galena..................................... 59

argentiferous ........................... 114, 121

Garfield 15-minute quadrangle............... 16, 57

Garnet .................................... 9, 73, 74

Gartra Member, Chinle Formation ........... 44

Gastropods, Gothic Formation............... 27

Oilman Sandstone .......................... 18, 20

Glacial deposits, age ........................ 97

Glacial geology ............................. 88

Glaciation ................................. 2, 88, 90

Glaciers, Bigelow moraine ................... 99

Castle Creek............................ 90

Castle Creek moraine .................... 96, 99

Elk Mountains ......................... 90, 100

erosion................................. 7

Roaring Fork ........................... 91

Roaring Fork-Castle Creek-Maroon Creek 91 

rock glaciers .......................... 2,100, 101

Sacagawea Ridge Glaciation ............. 97

Sawatch Range glaciers ................. 90, 97

Snowmass Creek........................ 91

Thomasville moraine .................... 99

Glauconite, Peerless Formation .............. 15

Glenwood Canyon, correlation, Peerless Dolomite ............................ 17

Glenwood Springs .......................... 17

fossil collections ........................ 25

Maroon Formation...................... 36

Gneiss ..................................... 7,11, 41

quartz feldspar ......................... 31

Gold ................................... 59, 114, 115

Gold Hill, ore deposits ...................... 120

Gordon, quoted............................. 28

Gothic-Belden contact, Highland tunnel...... Ill

Gothic Formation............... 23,25, 34, 72, 82,131

brachiopods ............................ 27

cephalopods ............................ 27

deposition.............................. 23, 40

Des Moinesian fossils ................... 28, 36

fault ................................... 81, 86

gastropods ............................. 27

Highland tunnel ........................ 110

pelecypods ............................. 27

Red Butte.............................. 83

rock composition ...................... 23, 25, 27

scaphopods ............................ 27

Snowmass pluton ....................... 81

thickness .......................... 23, 25, 28, 31

Gothic Formation of Langenheim. See Gothic

Formation. 

Gothic-Maroon contact, Highland tunnel..... Ill

Page 

Gothic quadrangle......................... 58, 74, 82

Grand Hogback monocline .................. 82, 87

Grand Mesa ................................ 87

Granite .................................... 7

Granodiorite ............................... 7, 61, 85

alteration .............................. 63

porphyritic......................... 65, 68, 86, 87

source of talus .......................... 105

texture ................................ 61

Green River Formation...................... 82

Grizzly Peak cauldron....................... 88

Gunnison .................................. 33

Gunnison Formation ........................ 46

See also Entrada Sandstone. 

Gunnison River ............................ 50

Gypsum, Belden Formation ................. 25

Gothic Formation....................... 26

Highland tunnel ........................ 25

Hope tunnel............................ 25

Maroon Formation...................... 33

Monument Gulch ....................... 26

H

Hanging fans .............................. 105

Harding Quartzite .......................... 18

Hardscrabble Mountain, State Bridge Formation ............................ 43

Hayden Peak quadrangle.................... 2, 28, 85

anhydrite .............................. 26

aplite .................................. 56

Belden Formation....................... 23

Castle Creek fault zone .................. 83

Chinle Formation ....................... 45

Elk Range thrust fault zone .............. 80

gypsum ................................ 26

hornblende quartz diorite................ 54

iron deposits ........................... 113

landslide ............................... 103

Precambrian rocks ...................... 7

quartz porphyry ........................ 55

rock glaciers ............................ 101

sulfide deposits ......................... 114

White Rock pluton ...................... 59

Hayden Survey ............................. 4, 58

Haymaker Gulch, Gothic Formation ......... 28

Hell Gate porphyry ......................... 11

Hematite ................................. 40, 72, 74

Maroon Formation...................... 35

Peerless Formation ..................... 15

Smuggler Mountain..................... 11

State Bridge Formation ................. 41

Highland Peak quadrangle .............. 85, 86, 90,96

Burro Canyon Formation ................ 49

Dakota Sandstone ...................... 49, 115

hornblende porphyry ....

Mancos Shale ..........

Maroon Formation......

Morrison Formation ....

State Bridge Formation .

.......... 67

.......... 51

.......... 34

.......... 47

.......... 40

Highland tunnel............... 25, 56, 81, 84, 110, 111

Highlands Ski area ......................... 41

Chinle Formation ....................... 45

History, economic, Aspen ................... 4

geologic, general........................ 5

Holy Cross quadrangle, fossils ............... 17

Homestead shear zone.................. 76, 77, 85, 88

Hope fault ................................. Ill

Hope mine ................................. 54, 55

Gothic Formation....................... 25

Hope tunnel................................ 25, 110

Hornblende ............................ 61, 63, 65, 67

Hornblende porphyry ....................... 67

Hornblende quartz diorite ................... 54

chemical analyses ....................... 54

Hornfels, Belden Formation ................. 23

source of talus .......................... 104

Hunter Creek ............................. 11, 54, 90

alluvium ............................... 106

Page 

Hunter Creek valley ........................ 91

quartz monzonite ....................... 9

Hunters Hill thrust fault.................... 80

Hydrothermal alteration .................... 65, 75

Hypselentoma sp. .......................... 27

I

lanthinopsis (Soleniscus) sp. .............

Idocrase ...............................

Igneous rocks, chemical analysis .........

Illite, Morrison Formation...............

Ilmenite ...............................

Independence Pass .....................

Independent gold district, ore deposits ... 

Inoceramus deformis....................

Iron ...................................

Iron deposits ............................... 59, 113

Copper Creek........................... 113

Italian Mountain Intrusive Complex, Oligocene ............................ 58, 65

27

73

68

48

54

3,68

121

51

7, 108

J, K

Jacque Mountain Limestone Member. 

Joints .............................

Juana Lopez Member, Mancos Shale .

Keefe Peak.......................

Keno Gulch ......................

Knishtites (Retispira) nodocostatus 

Kobey Park, gravels ..............

Kroenke Granodiorite.............

28

88

51

59

84

27

97

11

Lake Creek................................. 97, 99

Landslides, age............................. 103

Larkspur Mountain, Leadville Limestone ..... 22

rock composition ....................... 108

Latiendothyra sp. ........................... 22

Lead .................................... 7, 108, 115

production ............................. 106

Leadville-Belden contact .................... 23, 81

Leadville district ........................... 19, 57

Leadville Limestone.............. 18, 20, 21, 73, 79, 81

Belden Formation....................... 23

hornblende quartz diorite................ 54

petrology .............................. 22

Lenado Gulch .............................. 25, 82

Lenado mining district..........

Lenado project .................

Lincoln Gulch, quartz monzonite . 

Lincoln Gulch stock, analysis ....

Lineation, alined minerals .......

Linoproductus prattenianus .....

Liroceras sp. ...................

106

111

12

68

76

27

27

M

Mafic minerals ............................. 3, 65

Magnesium ................................ 108

Magnetite ................................. 72, 74

granodiorite ............................ 63, 65

hornblende quartz diorite ................ 54

Maroon Formation...................... 35

Malachite.................................. 115

Mancos Shale ......................... 50, 51, 86, 139

................... 80

................... 51

ammonite zone ........

bentonite beds .........

Castle Creek fault zone . 

Elk Range fault........

fan deposits ...........

fossiliferous calcerenite. 

Gothic Formation......

landslides .............

plant fragments .......

Manganese

........... 81

........... 105

........... 51

........... 82

........... 103

........... 52

........... 108

Manitou Dolomite .............. 15, 16, 17, 18, 54,130
144 INDEX

age ...................

Castle Creek fault zone . 

chemical analyses......

deposition .............

Des Moinesian fossils .. 

Elk Range thrust zone.. 

Highland tunnel .......

marine fossils..........

plant fragments .......

pyrite .................

Page 

Manitou Dolomite—Continued

Woody Creek ........................... 112

Manitou Park, fossils ....................... 17

Maps ...................................... 4

Marble, Belden Formation.................. 23, 59, 73

Marble quadrangle ......................... 81, 82

Marine fossils .............................. 27, 37

Maroon Bells............................... 65

Maroon Bells quadrangle ............ 29, 33, 74, 80, 87

Belden Formation....................... 23

Chinle Formation ....................... 45

Dakota Sandstone ...................... 50

Entrada Sandstone ..................... 46

Gothic Formation....................... 25

granodiorite ............................ 59

hornblende porphyry .................... 59, 67

Morrison Formation .................... 47

sulfide deposits ......................... 114

Maroon Bells-Snowmass Wilderness ......... 4

Maroon Conglomerate ...................... 29

Maroon Creek........................ 3, 29, 90, 96, 99

Chinle Formation ....................... 44

fan deposits ............................ 105

lower ................................. 47, 49, 51

Maroon Formation ...................... 41

Maroon Creek valley ........................ 41

Maroon Formation.................. 25, 28, 29, 41, 46,

47,79,84,86, 131 

........... 36

........... 83

........... 35

........... 40

........... 31

........... 81

........... 110

........... 37

........... 37

........... 74

red beds .......................... 35, 40, 72, 114

rock composition ...................... 23, 29, 34

thickness .............................. 37

White Rock pluton ...................... 59

Maroon sandstones ......................... Ill

McArthur Mountain area, Precambrian rocks . 7 

Members, Broken Rib ....................... 20

Brushy Basin........................... 49

Castle Butte............................ 21

Clinetop ............................... 17

Cozzette ............................... 52

Fort Hays Limestone ................... 51, 119

Gartra ................................ 44, 45, 46

Jacque Mountain Limestone ............. 28

Juana Lopez............................ 51

Peerless Shale .......................... 12, 16

Redcliff ................................ 21

Rollins Sandstone....................... 53

Salt Wash.............................. 49

Shinarump ............................. 46

Sloane Peak ............................ 41

South Canyon Creek .................... 41, 44

White Quail ............................ 28

Mesaverde Formation ..................... 52, 80, 82

Paleocene flora ......................... 80

Mesaverde Group........................... 53

Metamorphism......................... 23, 50, 59, 72

chemistry .............................. 74

conditions.............................. 75

Mica schist ................................ 7, 11

Midnight mine ............................. 108, 110

Midnight shaft ............................. 54

Midnight tunnel............................ 55

Migmatite ................................. 12

Mineralization ............................. 7

Minerals, alaskite........................... 67

allanite ............................. 8, 54, 56, 63

andalusite.............................. 74

anhydrite ............................. 25, 26, 33

antimony .............................. 108, 115

apatite.................... 7, 35, 46, 48, 54, 63, 67

arsenic ................................. 108, 115

Page 

Minerals—Continued

azurite ................................. 115

barite .................................. 114

biotite........................... 7, 33, 72, 73, 77

bismuth................................ 115

chabazite .............................. 74

chalcopyrite ............................ 59, 63

copper............................. 108,114,115

corundum .............................. 74

epidote ....................... 8, 54, 56, 67, 72, 74

fibrolite ................................ 7

galena .................................. 59

garnet......................... 9,73, 74,100, 101

gold ................................ 59, 114, 115

gypsum ............................... 25, 26, 33

hematite ................. 11, 15, 35, 40, 41, 72, 74

idocrase................................ 73

illite ................................... 48

ilmenite ................................ 54

iron deposits........................ 59, 108, 113

lead ............................. 7, 106, 108, 115

magnetite ................... 35, 54, 63, 65, 72, 74

malachite .............................. 115

molybdenite.......................... 59, 63, 114

natrolite ............................... 74

phyllonite .............................. 76, 77

pyrite...................... 25, 56, 59, 63, 74, 114

rutile ................................. 35, 41, 63

scaptolite ............................. 73, 74, 75

sillimanite.............................. 7

silver ........................ 7, 59, 106, 108, 115

sphalerite ........................... 59, 108, 114

stilbite................................. 74

thomsonite............................. 74

tourmaline ............................. 115

tremolite ............................... 72

wollastonite ............................ 73

zeolites ................................ 74

zinc ............................. 7, 106, 108, 115

zircon.................................. 115

Mines, Aspen Contact....................... 113

Boulder shaft........................... 91

Bushwacker shaft....................... 91

Castle Creek, Hope tunnel............... 25

Cathedral Lake-Montezuma mine area ... 86 

Cooper Creek ........................... 113

Gold Hill area .......................... 120

Highland tunnel........... 25, 56, 81, 84, 110, 111

Hope ................................. 25, 54, 55

Hope tunnel............................ 25, 110

Leadville mining district ................ 19, 57

Lenado mining district.................. 106

Midnight............................... 108, 110

Midnight shaft ......................... 54

Midnight tunnel ........................ 55

Montezuma ............................ 74, 114

Park Regent shaft ...................... 91

Richmond Hill mining district............ 106

Ruby beds ............................. 59

Smuggler .............................. 108

Snowmass Creek........................ 115

Spar Gulch workings .................... 21

Minturn Formation ......................... 25, 28

Minturn quadrangle ........................ 28

fossils ................................. 17

Moenkopi Formation ....................... 24, 41

Molas Formation ........................... 23

Molybdenite ............................. 59, 63, 114

Molybdenum ............................... 114

Montana Formation ........................ 51

Montezuma mine ........................... 74, 114

Monument Gulch ........................... 26

fan deposits ............................ 105

Monzonite, Mount Princeton Quartz ......... 58

muscovite-biotite quartz................. 8

porphyritic quartz ...................... 8, 11

quartz ......................... 7, 9, 11, 12, 68, 84

Mooreoceras normale ....................... 27

Moraines .................................. 90

Morrison Formation ........... 34, 41, 46, 47, 131, 137

Morrision Formation—Continued 

landslide deposit...............

Mosquito Range...................

Mount Garfield Formation .........

Mt. Harvard quadrangle ...........

Mount Princeton batholith .........

Mount Princeton Quartz Monzonite . 

Mt. Sopis .........................

Mount Zion Porphyry..............

Murchisonid gastropod ............

Muscovite-biotite granite ..........

Muscovite-biotite quartz monzonite . 

Myalina sp. .......................

Myrmekite ........................

N, O

Naticopsid Trachydomia sp. 

Naticopsis (Naticopsis) sp... 

Natrolite..................

Naturita Formation........

North Thompson Creek ....

Northern Sawatch Range...

Page

103

17

53

76

57

58

81

57

27

9

8

27

27

27

74

49

54

7

Oh-Be-Joyful 7.5-minute quadrangle ......... 80

Ohio Creek Formation....................... 80

Ostracod remains, Morrison Formation ....... 48

Owl Creek .............................. 3, 51, 90, 91

Owl Creek drainage ......................... 51

Owl Creek valley............................ 91

Palaeoneilo sp. ............................. 27

Palaeospiroplectammina .................... 22

Pando Porphyry............................ 57

Paracaligella ............................... 22

Paradise stock ............................. 68

Parathurammina ........................... 22

Park Regent shaft.......................... 91

Parting Formation .......................... 17, 18

fossils ................................. 18

rock composition ....................... 19

Parting quartzite series ..................... 18

Pearl Pass quadrangle ..................... 80, 82, 83

Maroon Formation ...................... 39

Pectenoid pelecypod ........................ 27

Pedernal uplift ............................. 25

Peerless Formation ................. 14, 15, 16, 17, 130

Peerless Shale Member...................... 16

Sawatch Quartzite ...................... 12, 16

Pegmatite ................................. 7, II

quartz-potassic feldspar-muscovite ....... 8

Perthite ................................... 11

Phenocrysts ........................ 54, 56, 63, 65,67

euhedral ............................... 33

euhedral hornblende .................... 66

hornblende ............................. 67

plagioclase ............................. 66, 67

potassic feldspar........................ 66

quartz ................................. 66, 67

Phyllonite ................................. 76, 77

Pikes Peak Granite ......................... 7, 11

Pine Creek drainage, anomalous metals....... 115

Pitkin County .............................. 19, 20

Plagioglypta sp. ............................ 27

Plant fragments, Express Creek ............. 28

Mancos Shale .......................... 52

Maroon Formation...................... 37

Pleurotomacean Hypselentoma sp. ........... 27

Pluton, White Rock. See White Rock pluton.

Plutonism ................................. 5

Porphyries, felsic, composition ............... 56

Porphyritic granodiorite .................... 65

Porphyritic quartz monzonite................ 8, 11

chemical analysis ....................... 9

Prophyry, albite alaskite .................... 54

albite aplite ............................ 54

aplite .................................. 54, 56

diorite ................................. 54
INDEX 145

Porphyry—Continued 

hornblende ......

Mount Zion .....

Pando ..........

	Page

................. 67

................. 57

................. 57

quartz ................................. 54, 55

Tertiary Tincup Quartz Monzonite ....... 58

White.................................. 57

Precambrian rocks.......................... 7

chronology ............................. 11

Precipitation ............................... 2

Price River Formation ...................... 52

Pseudozygopleurid gastropod ............... 27

Pyramid Peak, landslides.................... 103

Pyrite ..................................... 63, 114

Belden Formation....................... 25

Maroon Formation...................... 74

quartz porphyry ........................ 56

White Rock pluton ...................... 59

Q

Quadrangles, Aspen 7.5-minute .............. 29, 40

Capitol Peak ......................... 86, 97, 115

Crested Butte .......................... 29

Crested Butte 15-minute ............... 22, 25, 28

Garfield................................ 57

Garfield 15-minute ...................... 16

Gothic ................................ 58, 74, 82

Hayden Peak ........ 2, 7, 23, 26, 28, 45, 54, 55, 56,

59,80, 83, 85,101, 103,113,114 

Highland Peak .............. 34, 40, 47, 49, 51, 67,

85,86,90,96,115 

Marble ................................. 81, 82

Maroon Bells ............ 23, 25, 29, 33, 45, 46, 47,

50,59,67,74,80,87,114 

Minturn ............................... 17, 28

Mt. Harvard ........................... 76

Oh-Be-Joyful 7.5-minute................. 80

Pearl Pass ......................... 39, 80, 82, 83

Ruedi.................... 23, 29, 31, 39, 81, 85, 97

Snowmass Mountain................... 80, 82, 87

Taylor Park ............................ 83

Woody Creek ............ 34, 37, 41, 44, 45, 49, 52,

72, 80, 83, 85,86,96,97 

Quartz diorite .............................. 7, 58

Quartz feldspar gneiss ...................... 31

Quartz monzonite ......................... 7, 9, 11, 12

chemical analysis ....................... 11

Quartz porphyry ........................... 54, 55

Quartz-potassic feldspar-muscovite pegmatite 8 

Quartzite .................................. 14, 16

Chinle Formation ........

Maroon Formation .......

Parting Formation .......

Treasure Mountain dome . 

Queens Gulch ...............

fossils ..................

......... 45

......... 29

......... 19

......... 23

......... 82

......... 18

Queens Gulch road.......................... 54, 84

R

Radiosphaera .............................. 22

Red beds, Belden Formation ................. 74

Chinle Formation ....................... 45

Maroon Formation ..................... 35, 40, 72

Red Butte.................................. 41, 83

Chinle Formation ....................... 44

Entrada Sandstone ..................... 46

Mancos Shale .......................... 51

Morrison Formation .................... 47

Red Mountain .............................. 29, 31

deposits................................ 91

Red Table Mountain fault................... 83

Redcliff Member............................ 21

Richmond Hill.................... 4, 10, 54, 56,84, 88

brachiopods ............................ 22

bryozoa ................................ 22

fossils ................................. 18

mining district ......................... 106

solifluction deposits..................... 104

Page 

Rio Grande trough.......................... 5

Roaring Fork-Castle Creek-Maroon Creek

glacier.......................... 91

Roaring Fork drainage ...................... 2

Roaring Fork glacier ........................ 91

Roaring Fork River ................. 52, 88, 96, 98, 99

alluvium ............................... 106

gravel ................................. 97

landslide ............................... 103

Roaring Fork syncline .................. 83, 85, 86, 87

Roaring Fork valley .............. 51, 56, 83, 90,93, 99

climate ................................ 2

fans ................................... 105

quartz monzonite ....................... 9

Rock glaciers............................... 101

Rollins Sandstone Member .................. 53

Rubidium-strontium determinations ......... 11

Ruby beds ................................. 59

Ruedi quadrangle ...................... 31, 81, 85, 97

Belden Formation....................... 23

Eagle Valley Formation ................. 29

Maroon Formation...................... 39

Rutile ..................................... 63

Maroon Formation...................... 35

State Bridge Formation ................. 41

Sacagawea Ridge Glaciation................. 97

St. Kevin Granite, Precambrian Y............ 7,11

Salt Wash Member ......................... 49

San Juan Basin............................. 51

San Juan Mountains ........................ 80

Molas Formation ....................... 23

San Juan region ............................ 19

Sawatch anticline........................... 80

Sawatch Formation ......................... 14

Sawatch migmatite ......................... 7

Sawatch Quartzite .............. 12, 16, 17, 54, 91, 130

fossils ................................. 14

Sawatch Range ................. 2, 7, 17, 20, 28, 54, 80

Belden Formation....................... 23

Elk Range thrust fault zone.............. 82

faulting ................................ 108

glaciers ................................ 90, 97

Gothic Formation....................... 25

Grizzly Peak cauldron ................... 88

Mount Princeton batholith .............. 57

rubidium-strontium determinations ...... 11

shear zones............................. 76

Sawatch Range uplift....................... 5, 7

Sawatch schist ............................. 7

Scaphopods, Gothic Formation .............. 27

Scapolite .................................. 73

Schist, mica ................................ 7, 11

Schizodus sp. .............................. 27

Schofield syncline .......................... 82

Septabrunsiina sp. .......................... 22

Septaglomospiranella dainae ................ 22

Shansiella carbonaria ....................... 27

Shears..................................... 88

Shelf sequence, Cambrian through Mississippian............................ 12

Shell fragments ............................ 22

Shinarump Member......................... 46

Silicification ............................... 63

Sillimanite ................................. 7

Silver .................................... 7, 59, 108

production ............................. 106

Snowmass Creek mine................... 115

Silver fault.............................. 22, 81, 108

Silver Plume Granite........................ 7

Sloane Peak Member, Lower Triassic ......... 41

Smuggler mine, faults....................... 108

Smuggler Mountain ........................ 3, 91

alluvium ............................... 106

aplite .................................. 56

faults .................................. 85, 108

hornblende quartz diorite ................ 54

quartz monzonite ....................... 10

Page 

Smuggler Mountain—Continued

Sawatch uplift.......................... 84

Snowmass ................................. 83

Snowmass Creek.................. 2, 49, 86,91, 93,99

Dakota Sandstone ...................... 115

outwash landforms...................... 2

Snowmass Creek canyon .................... 48, 65

Snowmass Creek drainage ................... 96

Snowmass Creek fault ...................... 79, 87

Snowmass Creek glacier ..................... 91

Snowmass Creek mine ...................... 115

Snowmass Creek valley ..................... 86, 91

Snowmass Mountain quadrangle ............. 80, 87

Mancos Shale .......................... 82

Snowmass pluton ............. 28, 58, 59, 81, 85,86,87

Soleniscus ................................. 27

South Canyon Creek Member, Permian fossils ............................. 41

ripple marks............................ 44

Spar Gulch, Leadville Limestone ............. 21

mine workings .......................... 21

Sphalerite .............................. 59, 108, 114

Spinoendothyra spinosa..................... 22

State Bridge Formation ............ 29, 40, 79, 83, 137

depocenter ............................. 43

deposition.............................. 23

ripple marks............................ 41

rock composition ....................... 23, 41

StegocoeKa sp. ............................. 27

Sterner Gulch .............................. 51

Stilbite .................................... 74

Stratigraphic relations, Cambrian and Ordovician rocks..................... 16

Structural features, Apishapa uplift.......... 25

central Colorado trough ................. 5, 22

Conundrum Creek window ....... 63, 73, 74, 82, 85

Front Range uplift...................... 25

Grand Hogback monocline............... 82, 87

Pedernal uplift ......................... 25

Rio Grande trough ...................... 5

Roaring Fork syncline .............. 83, 85, 86, 87

Sawatch Range uplift ................... 5, 7

Schofield syncline....................... 82

Uncompahgre highland ................ 23, 31, 79

Uncompahgre uplift........ 5, 23, 25, 31, 40, 79, 85

White River uplift ...................... 16, 17

Structure .................................. 76

Phanerozoic ............................ 79

Studies, previous ........................... 4

Sulfides, disseminated ...................... 63, 114

Surficial deposits ........................... 23

Talus ...................................... 104

Taylor Lake, Precambrian rocks ............. 9

Taylor Park ............................... 15, 16, 87

Deadmans Gulch Formation ............. 15

porphyritic quartz monzonite ............ 9, 12

Taylor Park quadrangle ..................... 83

Taylor Pass, Precambrian rocks.............. 9

Sawatch Quartzite ...................... 12

Taylor Pass road ........................... 23

Taylor Peak, Belden Formation .............. 24

Taylor River ............................... 2, 84

Tertiary Tincup Quartz Monzonite Porphyry .. 58

Thompson Creek ........................... 52

Thomasville moraine........................ 99

Thomsonite ................................ 74

Tourmaline ................................ 115

Tourtelotte Park, Parting Formation ......... 19

Trachydomia sp............................. 27

Trails...................................... 4

Transgressive sea .......................... 21

Treasure Mountain dome .................... 25, 87

Belden Formation....................... 23

Entrada Sandstone ..................... 46

Trempealeauan fossils ...................... 17

Tremolite .................................. 72

Trepospira sp. .............................. 28
146 INDEX

Trough, central Colorado. 

Tuberendothyra .........

Page

5

22

Uncompahgre highland .................... 23, 31, 79

Uncompahgre uplift..................... 5, 40, 79, 85

Belden Formation........

Chinle Formation, source . 

Gothic Formation........

Maroon Formation.......

Parting Formation.......

State Bridge Formation .. 

Upper Arkansas River valley .

Van Horn Park, Belden Formation........... 23

Veins ...................................... 88, 114

Volcanic ash ............................... 48, 49

Volcanics, ashflow tuffs ..................... 68

basalt flows ............................ 72, 98

diabase dikes ........................... 72

felsic .................................. 31

Grizzly Peak cauldron................... 88

volcanic arcs ........................... 69

Volcanism ................................. 5

W

Warren Lake .......

alluvium .......

Wasatch Formation.

3

106

59

	Page

Waterfall Gulch ............................ 23

Watson Divide ............................. 98

Weber Formation........................... 23

fossil collections ........................ 25

Weber Limestone........................... 29

Weber Sandstone........................... 36

Weber Shale ............................... 23

West Aspen Mountain ...................... 12

Manitou Dolomite ...................... 15

West Elk Mountains ........................ 82

ruby beds .............................. 59

West Fork of Brush Creek, dip-slope landslide. 103 

West Maroon Creek........................ 29, 73,93

West Maroon Creek valley................... 87

West Snowmass Creek valley ................ 86

West Village of Snowmass-at-Aspen.......... 4

West Willow Creek ......................... 93

White Porphyry ............................ 57

White Quail Member........................ 28

White River Plateau, Broken Rib Member .... 20

fossils ................................. 20

Sawatch Quartzite ...................... 17

White River uplift.......................... 16, 17

White Rock pluton ..................... 34, 58, 59, 72

alteration .............................. 63

Elk Mountains ......................... 85

Elk Range thrust zone................... 81

fossils ................................. 28

fracture zones .......................... 59

Gothic Formation....................... 26

granodiorite........................... 61,62, 66

hydrothermal alteration ................. 75

Maroon Formation...................... 86

Wildcat Creek .............................. 52, 91

Wildcat Creek valley ........................

Willow Creek, Chinle Formation .............

Entrada Sandstone .....................

Morrison Formation ....................

Willow Creek valley.........................

State Bridge Formation.................

Wind River Mountains ......................

Winnie fault................................

Wollastonite ...............................

Woody Creek ........................ 3, 16, 29

alluvium ...............................

Manitou Dolomite ......................

Sawatch Quartzite ......................

Woody Creek quadrangle ............ 41, 72, 85,

Castle Creek fault zone ..................

Chinle Formation .......................

Curtis Formation .......................

fossils .................................

Mancos Shale ..........................

Maroon Formation..................... 34

Woody Creek valley.........................

faults ..................................

X, Y, Z

Xenocrysts .......

Yochelson, quoted .

Page 

91 

44

46

47

49

40

97

111

73

31, 36

106

112

12

96,97

83

45

49

80

52, 86

37, 44

25

111

67

28

Zeolites .................................... 74

Zinc ..................................... 7,108, 115

production ............................. 106

Zircon ..................................... 115

ft U.S. GOVERNMENT PRINTING OFFICE: 1979-677-129/18