U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2006 −1326

Molluscan Fossils and Stratigraphic Descriptions 

from the Upper Cretaceous Mancos Shale,

West-Central Colorado
Molluscan Fossils and Stratigraphic Descriptions

from the Upper Cretaceous Mancos Shale,

West-Central Colorado

By E.A. Merewether, D.A. Sawyer, and W.A. Cobban

Open-File Report 2006–1326

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Merewether, E.A., Sawyer, D.A., and Cobban, W.A., 2006, Molluscan fossils and stratigraphic descriptions from the 

Upper Cretaceous Mancos Shale, west-central Colorado: U.S. Geological Survey Open-file Report 2006-1326, 17 p,
iii

Contents

Introduction.....................................................................................................................................................1

Reference Cited..............................................................................................................................................1

Outcrop Descriptions

Section near Alkali Creek, Delta County..................................................................................6

Section near Village of Hotchkiss, Delta County....................................................................8

Section near Village of Mack, Mesa County...........................................................................9

Section near Olathe, Montrose County..................................................................................11

Section in Peach Valley, Delta County ...................................................................................12

Section North of Peach Valley, Delta County........................................................................14

Section near Red Rock Canyon, Montrose County..............................................................15

Section near Uncompahgre, Montrose County....................................................................16

Figure

1. Outcrop sections and fossil locatities in Upper Cretaceous strata in

counties of west-central Colorado.............................................................................................2

Tables

1. Members of Upper Cretaceous age in the lowest part of the Mancos

Shale of west-central Colorado ..................................................................................................2

2. Names and locations of selected outcrops of Upper Cretaceous strata and

names of associated molluscan fossils in Delta, Mesa, and Montrose

Counties, west-central Colorado ................................................................................................4

3. Molluscan fossil localities in Upper Cretaceous strata in Delta, Garfield, Mesa,

and Montrose Counties, west-central Colorado......................................................................5
Molluscan Fossils and Stratigraphic Descriptions

from the Upper Cretaceous Mancos Shale,

West-Central Colorado

By E.A. Merewether, D.A. Sawyer, and W.A. Cobban

Introduction

This report is based on lithostratigraphic and biostratigraphic data derived from investigations of Upper Cretaceous strata in 

Delta, Garfield, Mesa, and Montrose Counties in west-central Colorado (fig. 1). The data were obtained by personnel of the U.S. 

Geological Survey in the years 1955 through 2004, during studies of the Dakota Sandstone and the overlying Mancos Shale. 

Described herein are exposed strata of marine origin in the uppermost Dakota and the lower part of the Mancos (Cenomanian, 

Turonian, and Coniacian Stages) and related and associated collections of molluscan fossils (tables 1, 2, and 3).

The stratigraphic nomenclature used in this report for the lower part of the Mancos follows that of Molenaar and others (2002), who divided the lower Mancos into six members (table 1), from oldest to youngest, the Graneros, Bridge Creek 

Limestone, Blue Hill, Juana Lopez, Montezuma Valley, and Niobrara. The strata consist mainly of shale but can include chalk, 

calcarenite, siltstone, sandstone, and bentonite; commonly, they enclose a variety of concretions. In outcrops, the members are 

distinguished essentially by differences in their content of calcium carbonate (table 1) supplemented by the identity of constituent fossils.

Thicknesses of members from the measured outcrops vary in the region, which probably reflects lateral changes in facies 

and possibly the effects of truncation at disconformities in the stratigraphic sequence. Disconformities might mark the bases of 

the Juana Lopez and the Niobrara Members. Furthermore, the members at several places are poorly exposed, and dips used for 

the measurements may be incorrect. The Graneros Member, measured at five localities, ranges in thickness from 35 ft at the Red 

Rock section to 85 ft at the Peach Valley section. The Bridge Creek Limestone is 42 ft thick near Mack, 50 ft thick near Uncompahgre, and 140 ft thick at the Red Rock section. All of the Blue Hill Member was described at five of the selected outcrops 

where it ranges in thickness from about 100 ft near Olathe to 217 ft near Uncompahgre. The Juana Lopez is as much as 120 ft 

thick near Mack, but at five other outcrops to the southeast it is 40 to 86 ft thick. The Montezuma Valley Member is as much as 

115 ft thick near Mack and is 52 ft thick at Alkali Creek and 100 ft thick near Olathe.

Reference Cited

Molenaar, C.M., Cobban, W.A., Merewether, E.A., Pillmore, C.L., Wolfe, D.G., and Holbrook, J.M., 2002, Regional stratigraphic cross sections of Cretaceous rocks from east-central Arizona to the Oklahoma Panhandle: U.S. Geological Survey 

Miscellaneous Field Studies Map 2382.
G A R F I E L D

M E S A

M O N T R O S E

O U R A Y

D E L T A

G U N N I S O N

U T A H

R ifle

Grand Junc tion

D elta

Montrose

R idgway

G ateway

Uravan

0

0 40 MILES

60 KILOMETERS

S A N M I G U E L

P I T K I N

R I O B L A N C O

x

x

x

x x

x

x

Mack

Alkali

Creek Hotchkiss

Peach Valley

Olathe

Red 

Rock

Uncompahgre

+ +

+ + +

+

+

+++

+

+

+

+

+ +

+

+ ++

+

+

+ +

+

++

7563

7550

7558

7557

7817

7809

8177

7794

7800

7777

8152 7788

8139

10676

10677

7833

11886

11884

14167

8259

14146

14208

10686

14209

6855

14164

26946

Figure 1. Measured outcrops (X) and fossil localities (+) in Upper Cretaceous strata in counties of west-central Colorado. Numbers 

indicated specific fossil collections.

 Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
MEMBER DOMINANT LITHOLOGY THICKNESSES (FT.) AGE (STAGES)

Niobrara shale, calcareous about 580 late Turonian and Coniacian

Montezuma Valley shale, noncalcareous 52 TO 115 late middle and early late Turonian

Juana Lopez shale and calcarenite 40 TO 120 late middle Turonian

Blue Hill shale, noncalcareous 101 TO 217 early middle Turonian

Bridge Creek Limestone shale, calcareous 42 TO 140 late Cenomanian and early Turonian

Graneros shale, noncalcareous 35 TO 101 late Cenomanian

Table 1. Members of Upper Cretaceous age in the lower part of the Mancos Shale of west-central Colorado. Names follow 

nomenclature of Molenaar and others (2002).

Table 1. 
OUTCROP NAME COUNTY SEC. TWN. RNG. USGS CATALOGUE NO. MOLLUSCAN GUIDE FOSSILS STRATIGRAPHIC UNIT

Alkali Creek Delta SE, SW 25 4S. 3E. D10682 Prionocyclus quadratus Niobrara Mbr.-Mancos Sh.

Inoceramus incertus

D10683 Prionocyclus quadratus Niobrara Mbr.-Mancos Sh.

Inoceramus incertus

D10681 Scaphites whitfieldi Montezuma Valley Mbr.-Mancos Sh.

Inoceramus perplexus

D10680 Scaphites whitfieldi Montezuma Valley Mbr.-Mancos Sh.

Prionocyclus novimexicanus

Baculites yokoyamai

Inoceramus perplexus

D10679 Scaphites warreni Juana Lopez Mbr.-Mancos Sh.

Prionocyclus wyomingensis

Inoceramus dimidius

D10685 Prionocyclus macombi Juana Lopez Mbr.-Mancos Sh.

Lopha lugubris

Inoceramus dimidius

D10684 Prionocyclus hyatti Blue Hill Mbr.-Mancos Sh.

Inoceramus howelli

Hotchkiss Delta NE, NW 31 14S. 93W. D14167 Inoceramus perplexus Montezuma Valley Mbr.-Mancos Sh.

Prionocyclus novimexicanus

Scaphites whitfieldi

SW, NE 36 14S. 94W. D8257 Prionocyclus macombi Juana Lopez Mbr.-Mancos Sh.

Inoceramus dimidius

Lopha lugubris

D14166 Inoceramus dimidius

Lopha lugubris

D14165 Pycnodonte aff. P. kellumi Bridge Creek Limestone Mbr.-Mancos Sh.

Mack Mesa SE, NW 31 2N. 3W. D10442 Scaphites whitfieldi Montezuma Valley Mbr.-Mancos Sh.

Inoceramus perplexus

Baculites yokoyamai

NW, NW 5 1N. 3W. D10446 Prionocyclus macombi Juana Lopez Mbr.-Mancos Sh.

Inoceramus dimidius

Lopha lugubris

NW, NW 31 2N. 3W. D10441 Collignoniceras woollgari Blue Hill Mbr.-Mancos Sh.

Pinna petrina

Laternula lineata

NW, NW 31 2N. 3W. D10440 Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

Olathe Montrose SW, SE 9 50N. 9W. D14168 Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

Peach Valley Delta SE 33 15S. 94W. D11894 Inoceramus dimidius Juana Lopez Mbr.-Mancos Sh.

SW 34 15S. 94W. D11893 Inoceramus dimidius Juana Lopez Mbr.-Mancos Sh.

Lopha lugubris

D11892 Prionocyclus macombi Blue Hill Mbr.-Mancos Sh.

Inoceramus dimidius

D11891 Prionocyclus hyatti Blue Hill Mbr.-Mancos Sh.

D11890 Inoceramus dimidius Blue Hill Mbr.-Mancos Sh.

Lopha lugubris

SE 34 15S. 94W. D11889 Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

D11888 Johnsonites sulcatus Graneros Mbr.-Mancos Sh.

Borissiakoceras compressum

Inoceramus macconnelli

Red Rock Montrose SE, SW 29 50N. 8W. D14150 Inoceramus dimidius Juana Lopez Mbr.-Mancos Sh.

Lopha lugubris

D14149 Pseudoperna congesta Bridge Creek Limestone Mbr.-Mancos Sh.

Inoceramus sp.

Uncompahgre Montrose NE, NE 3 47N. 9W. D11883 Prionocyclus macombi Juana Lopez Mbr.-Mancos Sh.

Scaphites warreni

Inoceramus dimidius

D11880 Prionocyclus macombi Juana Lopez Mbr.-Mancos Sh.

Inoceramus dimidius

Lopha lugubris

D11879 Prionocyclus macombi Blue Hill Mbr.-Mancos Sh.

Scaphites carlilensis

D11878 Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

D11882 Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

D11881 Plicatula sp. Graneros Mbr.-Mancos Sh. 

Table 2. Names and locations of selected outcrops of Upper Cretaceous strata and names of associated molluscan fossils in 

Delta, Mesa, and Montrose Counties, west-central Colorado. Names of members of the Mancos Shale follow nomenclature of 

Molenaar and others (2002).

 Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
FOSSIL COLLECTIONS, COUNTY SEC. TWN. RNG. MOLLUSCAN STRATIGRAPHIC UNITS

USGS CATALOGUE NO. GUIDE FOSSILS (Sh.=shale; Mbr.=member)

26946 Montrose 19 48N. 9W. Plesiacanthoceras wyomingense Dakota Sandstone

D2040 30 48N. 9W. Conlinoceras tarrantense Dakota Sandstone

D6855 NW, SW 23 49N. 8W. Baculites obtusus Mancos Shale

D14164 NW, NW 20 48N. 9W. Pycnodonte aff. P. kellumi Bridge Creek Limestone Mbr.-Mancos Sh.

D14209 NW 8 50N. 9W. Magadiceramus stantoni Mancos Shale

D7833 Delta NE, SW 10 14S. 96W. Baculites obtusus Mancos Shale

D7835 Baculites asperiformis

D7836 Baculites asperiformis

D7837 Baculites asperiformis

D8142 NE, SE 10 14S. 96W. Baculites perplexus Mancos Shale

D8143 Baculites perplexus

D8256 SW, NE 36 14S. 94W. Pycnodonte n. sp.

D8259 SE, NE 4 14S. 91W. Baculites perplexus Mancos Shale

D10677 SE, SW 8 4S. 3E. Prionocyclus macombi Juana Lopez Member-Mancos Shale

Inoceramus dimidius

Lopha lugubris

D10678 Prionocyclus hyatti Blue Hill Member-Mancos Shale

D10686 SE, NW 2 15S. 94W. Pycnodonte aff. P. kellumi Bridge Creek Limestone Mbr.-Mancos Sh.

D11884 SE, SE 7 51N. 11W. Pycnodonte newberryi Bridge Creek Limestone Mbr.-Mancos Sh.

D11885 SW 33 15S. 96W. Inoceramus howelli Blue Hill Member-Mancos Shale

Exogyra sp. 

D11886 SE 33 15S. 96W. Prionocyclus macombi Juana Lopez Member-Mancos Shale

D14146 SE, SW 7 15S. 92W. Inoceramus dakotensis Juana Lopez Member-Mancos Shale

D14167 NE, NW 31 14N. 94W. Scaphites whitfieldi Montezuma Valley Mbr.-Mancos Shale

Prionocyclus novimexicanus

. Inoceramus perplexus

D14208 SE, SE 21 15S. 94W. Pycnodonte aff. P. kellumi Mancos Shale

D7557 Mesa NE 29 8S. 105W. Haresiceras natronense Mancos Shale

D7777 NW, SE 2 10S. 100W. Baculites asperiformis Mancos Shale

D7783 SE, NE 2 10S. 100W. Baculites perplexus Mancos Shale

D7786 Baculites gilberti

D7788 NW, NW 34 9S. 100W. Baculites asperiformis Mancos Shale

D7794 NE 25 1N. 1E. Baculites sp. (weak flank ribs) Mancos Shale

D7795 Baculites obtusus

D7800 SW, NW 3 1N. 1W. Baculites perplexus Mancos Shale

D7809 SW 7 9S. 100W. Baculites sp. (weak flank ribs) Mancos Shale

D7812 Baculites asperiformis 

D7817 NE 18 8S. 104W. Baculites sp. (weak flank ribs) Mancos Shale

D7820 Baculites obtusus

D7821 Baculites mclearni

D7823 Baculites asperiformis 

D8139 SE, SE 32 12S. 2E. Prionocyclus macombi Mancos Shale

Lopha lugubris

Inoceramus dimidius

D8152 NW, SW 26 9S. 100W. Baculites perplexus Mancos Shale

D8155 SW, SW 34 9S. 100W. Baculites obtusus Mancos Shale

D8158 Baculites asperiformis 

D8167 NW, NE 29 8S. 2W. Baculites perplexus Mancos Shale

D8177 NW, SE 20 9S. 100W. Baculites asperiformis Mancos Shale

D10676 NW, NE 26 3S. 2E. Inoceramus howelli Blue Hill Member-Mancos Shale

Crassostrea sp.

D7550 Garfield SE 4 8S. 102W. Baculites perplexus Mancos Shale

D7558 NW, NE 13 8S. 105W. Baculites obtusus Mancos Shale

D7560 Baculites asperiformis

D7563 SE, NW 3 8S. 103W. Baculites perplexus Mancos Shale

Table 3. Molluscan fossil localities in Upper Cretaceous strata in Delta, Garfield, Mesa, and Montrose Counties, westcentral Colorado. Names of members of the Mancos Shale follow nomenclature of Molenaar and others (2002).

Table 3. 
OUTCROP DESCRIPTIONS

In the following descriptions of outcropping strata, the stratigraphic units are in descending order (youngest to oldest). 

 SECTION NEAR ALKALI CREEK, DELTA COUNTY

Outcrops in SE1/4 SW1/4 sec. 25, T.4 S., R.3 E. (Point Creek 7 1/2-minute quadrangle). Measured and described by E.A. 

Merewether and W.A. Cobban in 1978. 

 Thickness (feet) 

Niobrara Member (part): 

11. Chalk, light brownish gray; fossiliferous at 2 ft above 

base (D10682-Prionocyclus quadratus). 9

10. Shale, medium dark gray, calcareous; contains bentonite, 

0.5 ft thick, at 3 ft above base. Fossiliferous at 5 ft

 above base (D10683-Prionocyclus quadratus). 8

 __________ 

Thickness of Niobrara Member (part) 17 

Montezuma Valley Member: 

9. Shale, dark gray, calcareous; contains concretions. Fossiliferous 

concretions at base (D10681-Scaphites whitfieldi). 12

8. Shale, dark gray, slightly calcareous. Bentonite, 0.3 ft thick, 

at top of unit. Fossiliferous at 27 ft above base 

(D10680-Scaphites whitfieldi). 40

 ___________

Thickness of Montezuma Valley Member 52 

Juana Lopez Member:

7. Interlaminated shale, dark gray, and less siltstone, brownish 

gray, and calcarenite. Fossiliferous in uppermost 5 ft. 

(D10679-Scaphites warreni). 31

6. Interbedded shale, dark gray, and less siltstone and calcarenite. 

Fossiliferous at 11 ft above base 

(D10685-Prionocyclus macombi). 20

 ___________

Thickness of Juana Lopez Member 51

 

 Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION NEAR ALKALI CREEK, DELTA COUNTY (continued)

Blue Hill Member (part):

 

5. Interlaminated shale, dark gray, and siltstone, greenish gray 

to light brown. Fossiliferous limonitic concretions 

19 ft above base (D10684-Prionocyclus hyatti). 53

4. Shale, dark gray; calcareous concretions at base 

and top. 18

3. Shale, dark gray, noncalcareous. 18

2. Siltstone, soft; poorly exposed; concretions at base 

and in upper half. 9

1. Shale, soft; poorly exposed. 50+

___________ Thickness of Blue Hill Member (part) 148+

Section Near Alkali Creek, Delta County 
SECTION NEAR VILLAGE OF HOTCHKISS, DELTA COUNTY

Outcrops in NE1/4 NW1/4 sec. 31, T.14 S., R.93 W. (Lazear quadrangle). Measured and described by E.A. Merewether and D.A. 

Sawyer, September 22-26, 2003. 

 Thickness (feet) 

Niobrara Member (part): 

5. Limestone. 0.5 

Montezuma Valley Member: 

4. Shale, dark gray, slightly calcareous. Fossiliferous beds at 

15-20 ft below top (D14167-Scaphites whitfieldi). 75 

Juana Lopez Member: 

3. Interbedded shale, siltstone, and calcarenite. Fossiliferous in lower part

(D8257-Prionocyclus macombi)

(D14166-Inoceramus dimidius). 40

Blue Hill Member: 

2. Shale, gray; poorly exposed; includes bentonite and 

few concretions. 60+ 

Bridge Creek Limestone Member (part): 

1. Shale, gray, calcareous; poorly exposed. Fossiliferous near top 

 (D14165-Pycnodonte aff. P. kellumi).

 Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION NEAR VILLAGE OF MACK, MESA COUNTY

Outcrops in NW1/4 NW1/4 sec. 31, T.2 N., R.3 W. (Mack 71⁄2-minute quadrangle). Measured and described by E.A. Merewether 

and W.A. Cobban on September 7, 1977. 

 Thickness (feet)

Niobrara Member (part): 

14. Shale, gray, calcareous. 10

 __________

Thickness of Niobrara Member (part) 10

Montezuma Valley Member: 

13. Shale, medium to dark gray, slightly calcareous. Fossiliferous calcareous 

concretions in lowermost 50 ft and uppermost 35 ft 

(D10442-Scaphites whitfieldi). 115

 __________

Thickness of Montezuma Valley Member 115

Juana Lopez Member: 

12. Interbedded shale, siltstone, and calcarenite; poorly exposed. 

Fossiliferous calcarenites in uppermost 30 ft 

(D10446-Prionocyclus macombi). 65

11. Interbedded shale and siltstone; concretions in lowermost 

10 ft. 55

 __________

Thickness of Juana Lopez Member 120

Blue Hill Member: 

10. Shale and siltstone, poorly exposed. Concretions in siltstone 

at 12 ft above base. 54

9. Siltstone, olive gray, slightly calcareous, soft; 

contains Ophiomorpha and other burrows. 31

8. Sandstone, very fine and fine grained, silty, partly calcareous; poorly 

exposed. Concretions at 11 ft above base. 

Unit contains burrows. 34

7. Sandstone, medium gray to brownish gray, very fine grained, silty, 

calcareous; laminated with low-angle tabular cross-beds; 

contains horizontal and vertical smooth burrows. Fossiliferous 

(D10441- Collignoniceras woollgari). 8

6. Siltstone, brownish gray, soft, poorly exposed. 13 

 __________

Thickness of Blue Hill Member 140

Section near Village of Mack, Mesa County 
SECTION NEAR VILLAGE OF MACK, MESA COUNTY (continued)

Bridge Creek Limestone Member: 

5. Siltstone, brownish gray, calcareous, soft. 28

4. Shale, medium dark gray, calcareous, poorly exposed. Fossiliferous 

in basal 5 ft (D10440-Pycnodonte newberryi). 10

3. Siltstone, clayey, calcareous; bentonite at top. Fossiliferous

(Pycnodonte newberryi). 4 

 ___________ 

Thickness of Bridge Creek Limestone Member 42 

Graneros Member: 

2. Shale, dark gray, noncalcareous; silty shale, siltstone, and 

bentonite in uppermost 7 ft; poorly exposed. 50

 __________

Thickness of Graneros Member 50

Dakota Sandstone (part): 

1. Sandstone; contains scattered black pebbles as much as 0.5 in long; 

beds about 1 ft thick. Abundant, horizontal, smooth, branching burrows 

at top. 

10 Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION NEAR OLATHE, MONTROSE COUNTY

Outcrops in SW1/4 SE1/4 sec. 9, T.50 N., R.9 W. (249209E, 4276795N), (Olathe 7 1/2-minute quadrangle). Measured and 

described by E.A. Merewether and D.A. Sawyer in September, 2003. 

 

 Thickness (feet)

Niobrara Member (part): 

10. Shale, gray, calcareous; poorly exposed. 

Montezuma Valley Member: 

 9. Shale, dark gray, noncalcareous; poorly exposed. 100

 ___________

Thickness of Montezuma Valley Member 100

Juana Lopez Member: 

8. Interbedded shale, siltstone, sandstone, and calcarenite; laminated 

and thin bedded. 40

7. Interbedded dark gray shale and light brownish gray siltstone; 

laminated and thin bedded. 46

 __________

Thickness of Juana Lopez Member 86

Blue Hill Member: 

6. Shale, dark gray, noncalcareous; concretions at base. 6

5. Shale, dark gray, noncalcareous. 50

4. Shale, dark gray, noncalcareous; poorly exposed. 45

 ___________ 

Thickness of Blue Hill Member 101

Bridge Creek Limestone Member: 

3. Shale, gray, calcareous, poorly exposed; concretions at 50 ft above 

base. Fossiliferous in uppermost 5 ft (D14168-

Pycnodonte newberryi, from the upper Cenomanian zone of 

Euomphaloceras septemseriatum). 85

__________

Thickness of Bridge Creek Limestone Member 85 

 

Graneros Member: 

2. Shale, dark gray, noncalcareous; concretions 

at 15 ft and 44 ft above base; poorly exposed. 50

 __________

Thickness of Graneros Member 50

Dakota Sandstone (part): 

1. Sandstone 

Section near Olathe, Montrose County 11
SECTION IN PEACH VALLEY, DELTA COUNTY

Outcrops in SE1/4 sec. 33 and SW1/4 sec. 34, T.15 S., R.94 W. (Olathe NW 71⁄2-minute quadrangle). Measured and described by 

E.A. Merewether and W.A. Cobban on June 23, 1982. 

 Thickness (feet)

Juana Lopez Member: 

18. Interbedded shale, minor calcareous siltstone, and calcarenite; poorly 

exposed. Fossiliferous calcarenite in uppermost 5 ft

(D11894-Inoceramus dimidius). 32

17. Interbedded shale, medium gray, soft, and less siltstone, brownish gray, 

and calcarenite. Fossiliferous calcarenite at 6 ft 

above base (D11893-Inoceramus dimidius). 10 

 ___________

Thickness of Juana Lopez Member 42

Blue Hill Member: 

16. Interbedded shale, dark gray, and minor siltstone, light brownish gray, 

calcareous; fossiliferous (D11890-Prionocyclus macombi). Ferruginous 

concretions in uppermost 15 ft. 27

15. Interbedded shale, dark gray, noncalcareous, and minor siltstone, light 

brownish gray, calcareous. Calcareous septarian concretions in 

uppermost 3 ft. 20

14. Interbedded shale, dark gray, noncalcareous, well indurated, and minor 

siltstone, light brownish gray, calcareous, and calcarenite. 

Fossiliferous calcarenite near base of unit (D11890-Inoceramus dimidius). 

Fossiliferous at 4 ft. above base (D11891-Prionocyclus hyatti). 15

13. Shale, dark gray, noncalcareous; septarian concretions at top. 5

12. Shale, dark gray, noncalcareous; contains a few large 

cone-in-cone concretions. 12

11. Shale, gray, silty, noncalcareous, soft; cone-in-cone concretions 

at base. 15

10. Shale, gray, slightly calcareous; includes thin beds of siltstone in uppermost 

5 ft; poorly exposed. 73

9. Shale, gray, poorly exposed. 30 

 ___________

Thickness of Blue Hill Member 197

12  Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION IN PEACH VALLEY, DELTA COUNTY (continued)

Bridge Creek Limestone Member: 

8. Shale, gray, calcareous, soft; thin beds of bentonite in uppermost part. 8

7. Shale, gray, slightly calcareous, soft; poorly exposed. 14

6. Shale, gray, soft; bentonites at top and 2 ft below top; poorly exposed. 

Fossiliferous near top (D11889-Pycnodonte newberryi). 20

5. Concealed, probably shale, slightly calcareous; cone-in-cone concretions 

in lowermost part and calcareous concretions in uppermost part. 74 

 ___________

Thickness of Bridge Creek Limestone Member 116

Graneros Member: 

4. Shale, gray, soft; poorly exposed. 60 

3. Interbedded shale, dark gray, noncalcareous; minor siltstone, noncalcareous, 

with load casts and small burrows. 5

2. Interbedded siltstone and shale, soft; contain horizontal burrows. 

Fossiliferous calcareous concretions at top

(D11888-Johnsonites sulcatus). 20

 ____________

Thickness of Graneros Member 85

Dakota Sandstone (part): 

1. Sandstone; upper surface displays abundant small and large ripple marks. 

Section in Peach Valley, Delta County 13
SECTION NORTH OF PEACH VALLEY, DELTA COUNTY

Outcrops in S1/2 sec. 21, T.15 S., R.94 W. (Olathe NW 7 1⁄2-minute quadrangle). Measured and described by E.A. Merewether 

and D.A. Sawyer on May 14, 2004. 

 Thickness (feet)

Niobrara Member (part): 

3. Shale, gray, calcareous; poorly exposed. Fossiliferous beds 

at about 55 ft and 120 ft above base. 350+

 ____________

Thickness of Niobrara Member (part) 350+

Montezuma Valley Member: 

2. Shale, dark gray, noncalcareous; poorly exposed. 90

 ____________

Thickness of Montezuma Valley Member 90

Juana Lopez Member (part): 

1. Interbedded calcareous shale and siltstone, and calcarenite. 

14  Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION NEAR RED ROCK CANYON, MONTROSE COUNTY

Outcrops in SE1/4 SW1/4 sec. 29, T.50 N., R.8 W. (Red Rock Canyon 71⁄2-minute quadrangle). Measured and described by E.A. 

Merewether and D.A. Sawyer on September 25, 2003. 

 Thickness (feet)

Juana Lopez and Blue Hill Members, undivided: 

5. Shale, dark gray; calcarenite bed at top; poorly exposed. 

Fossiliferous near top (D14150-Inoceramus dimidius). 135

 __________

Thickness of Juana Lopez and Blue Hill Members, undivided 135

Bridge Creek Limestone Member:

4. Shale, calcareous; poorly exposed. Pycnodontes in 

upper part. 80 

3. Shale, dark gray, calcareous; calcareous concretions 

at top. 60

 __________

Thickness of Bridge Creek Limestone Member 140

Graneros Member: 

2. Shale, brownish gray, noncalcareous. 35

 ___________

Thickness of Graneros Member 35

Dakota Sandstone (part): 

1. Sandstone, well indurated. 

Section near Red Rock Canyon, Montrose County 15
SECTION NEAR UNCOMPAHGRE, MONTROSE COUNTY

Outcrops in NE1/4 NE1/4 sec. 3, T.47 N., R.9 W. (Colona 71⁄2-minute quadrangle). Measured and described by E.A. Merewether 

and W.A. Cobban in June, 1982. 

 Thickness (feet)

Juana Lopez Member: 

16. Shale, dark olive gray, calcareous, soft, and minor calcarenite. 

Fossiliferous in uppermost 15 ft (D11883-Scaphites warreni). 35

15. Shale, olive gray, slightly calcareous, soft, and minor calcarenite; 

siltstone, 1.5 ft thick, at top. Fossiliferous (D11880

-Prionocyclus macombi). 9

14. Shale, medium gray to olive gray, soft, noncalcareous in lowermost 

5 ft, calcareous in uppermost 2 ft; concretions at top. 7 

 __________

Thickness of Juana Lopez Member 51 

Blue Hill Member: 

13. Interlaminated shale, dark gray, noncalcareous, jarositic, and minor 

siltstone. Fossiliferous ferruginous concretions near top

(D11879-Scaphites carlilensis). 23

12. Shale, dark gray, silty, noncalcareous; few laminae and thin beds 

of siltstone in uppermost 5 ft; concretions 3 ft. below top. 20

11. Shale, dark gray, silty, noncalcareous; cone-in-cone concretions 

at base; septarian concretions at top. 20

10. Shale, dark gray, silty, noncalcareous; brownish gray, silty, 

calcareous concretions, 4 ft in diameter, at base. 13

9. Concealed, probably shale. 111

8. Shale, dark gray, noncalcareous, soft; poorly exposed. 30

 __________

Thickness of Blue Hill Member 217

Bridge Creek Limestone Member: 

7. Shale, gray, calcareous, and chalk, at top; bentonites in 

basal 4 ft. Fossiliferous at base (D11882-

Pycnodonte newberryi) and 5 ft below top 

(D11878- Pycnodonte newberryi). 21

6. Concealed, probably shale, calcareous. 29

 __________ 

Thickness of Bridge Creek Limestone Member 50 

16  Molluscan Fossils and Stratigraphic Descriptions from the Upper Cretaceous Mancos Shale, West-Central Colorado
SECTION NEAR UNCOMPAHGRE, MONTROSE COUNTY (continued)

Graneros Member: 

5. Concealed, probably shale; septarian concretions 3 ft below 

top. 10

4. Concealed, probably shale; septarian concretions about 5 ft 

below top. 35

3. Concealed, probably shale; septarian concretions at top. 21

2. Shale; poorly exposed; concretions near base and fossiliferous 

calcareous concretions at top (D11881-Plicatula sp.). 33

1. Bentonite, dusky orange. 2 

 _________

Thickness of Graneros Member 101

Section near Uncompahgre, Montrose County 17