The Paleozoic and Mesozoic Rocks; 

A Discussion to Accompany the 

Geologic Map of the 

United States

GEOLOGICAL SURVEY PROFESSIONAL PAPER 9«3

The Paleozoic and Mesozoic Rocks; 

A Discussion to Accompany the 

Geologic Map of the 

United States

By PHILIP B. KING and HELEN M. BEIKMAN

GEOLOGICAL SURVEY PROFESSIONAL PAPER 903

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First printing 1976 

Second printing 1981

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract _____________________

Introduction _______________

Cambrian _________________

Marine stratified rocks ( )

Page 

1 

1

1

2

Eugeosynclinal deposits (- e) __________________________ 4

Ordovician and Cambrian___________-____________________ 7

Marine stratified rocks (O-G) ______-______----____-___ 7

Ordovician ________________________________________ 7

Marine stratified rocks (O) __________________________ 7

Eugeosynclinal deposits (Oe) __________________________ 9

Silurian and Ordovician ___________________________ 10

Eugeosynclinal deposits (SOe) ____________________ 10

Silurian __________________________________ 10

Marine stratified rocks (S) _______________________ 10

Eugeosynclinal deposits (Se) ________________ 12

Devonian and Silurian _____________________-___ 13

Marine stratified rocks (DS) _--____-_______-_-_--__--_-_ 13

Eugeosynclinal deposits (DSe) ___________________ 13

Devonian __________________________________________ 14

Marine stratified rocks (D) ______________________ 14

Continental deposits (D2c, Dac) ______________________ 16

Eugeosynclinal deposits (De) ________________________ 16

Lower Paleozoic _______________________________ 17

Marine stratified rocks (IP) __________________________ 17

Cratonic deposits __________________________ 17

Miogeosynclinal deposits _______________________ 20

Rocks of outlying areas ______________________________ 20

Eugeosynclinal deposits (IPe) ______________________ 21

Mississippian ___________________________________ 22

Pennsylvanian __________________________________________ 24

Permian _________________________________ 27

Appalachian Region __________________________ 28

Southwestern United States__________________________ 28

Midcontinent Region __________________________ 29

New Mexico ________________________________ 32

Northern Arizona ______________________________ 32

Cordilleran Region _____________________________ 33

Eugeosynclinal deposits (Pe) __________________________ 33

Upper Paleozoic ______-_-_--_--____-__________________ 33

Marine stratified rocks (uP) __________________________ 33

Mississippian ____________________________________ 33

Pennsylvanian ______________________________________ 36

Permian ___________________________________ 36

Outlying miogeosynclinal rocks ________________ 37

Eugeosynclinal deposits (upe) ______________________ 37

Paleozoic plutonic rocks___________________________ 38

Cambrian granitic rocks (-Gg) ---_______-_____---___ 38

Page

Paleozoic plutonic rocks Continued

Lower Paleozoic granitic rocks (Pgi) ___________ 38

Middle Paleozoic granitic rocks (Pg2) ___________ 39

Upper Paleozoic granitic rocks (Pg3) __________________ 39

Paleozoic mafic intrusives (Pmi) __ _________________ 42

Metamorphic complexes.-______________________ 42

Triassic and Permian _______________________ 42

Eugeosynclinal deposits (r~fc) __________ _________ 42

Triassic ______________________________ 43

Colorado Plateau __________ ______ _______________ 43

Great Plains ____________________________ 46

Appalachian Region _______________________ 46

Jurassic and Triassic (J~fc) _______________________ __ 47

Jurassic _______________________________ _________ 47

Rocky Mountain Region ______________________________ 47

Pacific coastal area ____________________-___ 48

Continental deposits(Jc) _____--___--_-_-____----_---__ 48

Lower Mesozoic ____-________________________ 48

Marine stratified rocks (1 IVfe) ______________________ 48

Eugeosynclinal deposits (1 IVfe e) ______________________ 49

Volcanic rocks (llVfev) __________________________________ 51

Cretaceous ____________________________---______-__------__ 51

Lower Cretaceous _______________________ 51

Texas ____________________________ 51

Atlantic Coastal Plain __________________ 54

Rocky Mountains _________________________ __ 55

California and Oregon ______________________ 55

Washington _________________________ 55

Upper Cretaceous __---_____--______--____---_------__ 56

Western Gulf Coastal Plain ________________ 56

Eastern Gulf Coastal Plain _______________ 56

Atlantic Coastal Plain _________________ 57

Great Plains and Rocky Mountains __-______---__ 57

Pacific coastal area ____________________________ 59

Continental deposits (Kc) _______________________ 59

Eugeosynclinal deposits (Ke) _____-_______-__-___ 60

Volcanic rocks (Kv)________________________ 60

Upper Mesozoic -_-_---__________________ ________ 61

Upper Mesozoic eugeosynclinal deposits (ul\^e) ______ 61

Cretaceous eugeosynclinal deposits (Ke) ____________ 62

Mesozoic plutonic and intrusive rocks _________.____ 62

Triassic granitic rocks (Tig) __________-__________ 62

Triassic mafic intrusives (Td) __________________________ 63

Jurassic granitic rocks (Jg) ____________________ 63

Jurassic mafic intrusives (Jmi) ______________________ 68

Cretaceous granitic rocks (Kg) __________________ 68

Cretaceous intrusive rocks (Ki) ____________________ 71

References cited ______________________________________ 72

ILLUSTRATIONS

FIGURES 1-19. Maps of:

1. Eastern United States, showing areas mapped as Cambrian on Geologic Map of United States

2. New England, showing positions of tectonic features----__-_____________-___-____

Page

3

5

III
IV CONTENTS

FIGURES 1-19. Maps of Continued Pase

3. Eastern United States, showing areas mapped as Ordovician on Geologic Map of United States--_____ 8

4. Eastern United States, showing areas mapped as Silurian on Geologic Map of United States ____________ 11

5. Eastern United States, showing areas mapped as Devonian on Geologic Map of United States -_________ 15

6. Western United States, showing areas mapped as lower Paleozoic on Geologic Map of United States ______ 18

7. Eastern Cordilleran Region, showing surface and subsurface extent of lower Paleozoic on Geologic Map of

United States _____________________________________________________ 19

8. Eastern United States, showing areas mapped as Mississippian on Geologic Map of United States ________ 23

9. Eastern United States, showing areas mapped as Pennsylvanian on Geologic Map of United States __-___ 26

10. United States, showing areas mapped as Permian on Geologic Map of United States-------_______--__-_ 30

11. Western United States, showing areas mapped as upper Paleozoic on Geologic Map of United States __ 34

12. Eastern part of Cordilleran Region, showing surface and subsurface extent of upper Paleozoic on Geologic

Map of United States _________________________________________________ 35

13. United States, showing areas mapped as Paleozoic plutonic rocks on Geologic Map of United States ______ 40

14. United States, showing areas mapped as lower Mesozoic stratified rocks on Geologic Map of United

States ____________________________________________________________ 44

15. United States, showing areas mapped as Cretaceous stratified rocks on Geologic Map of United States ____ 52

16. United States, showing areas mapped as Mesozoic and Cenozoic plutonic and intrusive rocks on Geologic Map

of United States _______._.______________________________________________ 64

17. Western United States, showing areas of Triassic granitic rocks as mapped on Geologic Map of United

States _______________________________________________________________ 66

18. Western United States, showing areas mapped as Jurassic granitic rocks and mafic intrusives on Geologic Map

of United States ______________________________________________._______ 67

19. Western United States, showing areas mapped as Cretaceous grantitic rocks and Cretaceous intrusive rocks

on Geologic Map of United States _________________ _________________________________ 69
THE PALEOZOIC AND MESOZOIC ROCKS; 

A DISCUSSION TO ACCOMPANY THE GEOLOGIC MAP OF

THE UNITED STATES

By PHILIP B. KING and HELEN M. BEIKMAN

ABSTRACT

This report deals with the Paleozoic and Mesozoic rocks that are 

exposed within the area covered by the Geologic Map of the United 

States and treats them in terms of the map legend. They are therefore discussed in chronological order, from oldest to youngest. Under 

each age, the stratified rocks in the most complete sequences are 

described first, followed by combinations such as Devonian and Silurian, Jurassic and Triassic, and finally by lower Paleozoic, upper 

Paleozoic, and lower Mesozoic. Next, special lithologic types of each 

age are taken up, the continental deposits, eugeosynclinal deposits, 

and volcanic rocks. At the end of the discussions of both Paleozoic 

and Mesozoic Eras are summaries of the plutonic rocks formed during those eras.

The better known and most extensively exposed rocks the 

Paleozoic sequence of the Central Interior and the Cretaceous of the 

Coastal Plains and Rocky Mountains are summarized briefly, as 

these are well covered in an extensive literature. More details are 

given for rocks of each age in the Appalachian and Cordilleran 

mountain belts, especially the eugeosynclinal deposits, because they 

have been poorly understood until recently.

Although the text is designed primarily to justify the representation of the units shown on the map, it also amplifies the necessarily 

brief descriptions of the units in the legend, and sufficient data are 

given to indicate the general lithologies of the units as exposed in the 

different areas, which on the map are shown primarily as timestratigraphic rather than rock-stratigraphic entities.

INTRODUCTION

The following text is a discussion and exposition of 

the different units of Paleozoic and Mesozoic rocks that 

are represented on the Geologic Map of the United 

States. It is thus partly an expansion and justification 

of the legend of the Geologic Map, but something is 

also said regarding the nature and origin of the rocks 

involved and the stratigraphy of the stratified rocks. 

On the other hand, it is not a complete treatise on these 

rocks, for such a treatise would far exceed the objectives of the report.

In the text, the better known and most extensively 

exposed sequences are summarized briefly and with 

little documentation: the Paleozoic of the Central 

Interior and the Cretaceous of the Coastal Plains and 

Rocky Mountains. Many accounts of these rocks have 

been published, and they are well described in the 

more detailed textbooks of historical geology and

stratigraphy for example, those of Dunbar (1969), 

Kay and Colbert (1965), and Kummel (1970). Correlation of the formations of the Paleozoic and Mesozoic 

systems, with useful annotations, may be found in the 

charts prepared by committees of the National Research Council that were published between 1940 and 

1960 by the Geological Society of America. In addition, 

some regional surveys are available, the most notable 

of which is the Geologic Atlas of the Rocky Mountain 

region (Mallory, 1972).

More details and more documentation are given of 

the complex Paleozoic and Mesozoic rocks in the Appalachian and Cordilleran mountain belts in the east 

and west, understanding of which is now increasing as 

a result of geologic work during the last few decades. 

These rocks are partly eugeosynclinal, partly crystalline, are generally poorly fossiliferous, and contain 

embedded plutonic rocks. Besides new fossil discoveries, much light has been thrown on the ages and 

relations of these rocks by radiometric dating, as indicated by citations in the text.

The Geologic Map represents only the surface 

geologic features of the country, and the text accordingly deals only with the exposed rocks; little or no 

mention is made of the concealed rocks that have become known from subsurface studies.

The text is illustrated primarily by small-scale maps 

of the United States, which show the extent of the different Paleozoic and Mesozoic systems or other gross 

units represented on the Geologic Map.

CAMBRIAN

The Cambrian System, the lowest unit of the 

Phanerozoic Eon, is represented on the Geologic Map of 

the United States by marine stratified rocks (-£), with 

basal Lower Cambrian clastic rocks (-6q) differentiated 

in places; by eugeosynclinal deposits (-Ge) and associated volcanic rocks (-Gv); and by a few Cambrian 

granitic rocks (- g) described with the other Paleozoic 

plutonic rocks in a later section.
PALEOZOIC AND MESOZOIC ROCKS

MARINE STRATIFIED ROCKS (- )

Cambrian strata are most extensively exposed in the 

cratonic area of the Central Interior Region (fig. 1). 

Here they form a wide band along the southern margin 

of the Precambrian rocks of the Lake Superior Region, 

from Minnesota across Wisconsin into the Upper 

Peninsula of Michigan; they also form an area surrounding the Precambrian rocks in the Ozark dome in 

Missouri. Smaller bands of outcrop adjoin the Precambrian of the Adirondack dome in New York State, the 

Arbuckle and Wichita Mountains in southern Oklahoma, and the Llano uplift in central Texas. Cambrian rocks also form long narrow bands in the folded 

and faulted miogeosynclinal belt of the Appalachian 

Region east of the Central Interior Region.

The Cambrian System is extensive as well in the 

Cordilleran Region west of the Central Interior, but for 

the most part its outcrops and those of succeeding 

lower Paleozoic systems are so narrow and discontinuous that they are all merged on the Geologic Map into a 

single unit (IP). In a few areas, however, Cambrian 

outcrops are sufficiently extensive for representation, 

notably in the Great Basin and in the thrust belt extending northward from southeastern Idaho into 

northwestern Montana. The Cambrian is also shown 

separately in Arizona, from the Grand Canyon to the 

southeastern part of the State; the thin overlying Devonian System, elsewhere classed as lower Paleozoic, is 

here merged with the upper Paleozoic (uP).

The Cambrian is the oldest system containing shelly 

fossils suitable for stratigraphic analysis, and it has 

been elaborately zoned and correlated. At least 10 fossil zones are recognized. The Cambrian has been divided into a Lower, Middle, and Upper Series 

sometimes termed the Waucoban, Albertan, and 

Croixan (or St. Croixan) (Lochman-Balk, 1972, p. 61). 

These are not represented on the Geologic Map, for the 

large outcrops in the Central Interior are all Upper 

Cambrian, and the Middle and Lower Cambrian Series 

appear only in the mountain belts to the east and west, 

where the outcrop bands are too narrow to be subdivided.

In most places the Cambrian System overlies the 

Precambrian with a large hiatus and profound unconformity. Throughout the Central Interior, Upper Cambrian lies on deformed rocks 500 to 2,000 m.y. (million 

years) older (Precambrian Y, X, and W). In contrast, in 

some of the geosynclinal sequences in the mountain 

belts to the east and west, where both Lower Cambrian 

and Precambrian Z stratified rocks are present, the 

stratigraphic break between them is slight or absent, 

creating problems in classification. These problems are 

most acute in the southwestern part of the Great Basin, between Las Vegas, Nev., and the Inyo Mountains,

Calif., where fine-grained Precambrian Z and Lower 

Cambrian strata form a conformable sequence as much 

as 21,000 ft (6,400 m) thick, with diagnostic Cambrian 

fossils only in the upper third. Here and elsewhere, 

however, the problematical rocks form outcrops so 

small on the scale of the Geologic Map that for our 

purposes they can be disregarded.

The top of the Cambrian System is generally conformable with the Ordovician but is definable by 

paleontological means. The "Ozarkian System" which 

was proposed by E. O. Ulrich in the early part of the 

century for a unit between the Cambrian and the Ordovician has now been discredited, and its proposed 

components have been assigned to one system or the 

other. The only difficulties in mapping the boundary 

are in parts of the Appalachian miogeosyncline where 

Upper Cambrian and Lower Ordovician rocks are parts 

of a thick mass of carbonates, as in such units as the 

Knox Group (Knox Dolomite of older reports) in the 

Southern Appalachians. By detailed stratigraphic 

work the Cambrian part of the Knox (Copper Ridge 

Dolomite or Conococheague Limestone) can be separated from the Ordovician part, but components of the 

two ages are not everywhere shown on the source 

maps; in such places they have been divided arbitrarily 

on the Geologic Map. From New Jersey northward, 

however, the outcrop belts ar,e narrower and more 

complex, and so it has been necessary to merge the 

Cambrian and Lower Ordovician carbonates into a 

single unit (O-C), even though data for their separation 

are available in places (see section "Ordovician and 

Cambrian").

The miogeosynclinal sequence in the Appalachian 

Region begins with Lower Cambrian clastic deposits 

(- q), which are separately shown on the Geologic Map 

where their outcrop belts are sufficiently wide. They 

are typified by the Chilhowee Group of Tennessee, Virginia, and Maryland, 3,000 ft (900m) or more thick, 

with conglomerates and arkoses in the lower part and 

prominent quartzite layers separated by shale and 

siltstone above. Similar rocks flank the western edges 

of the Green Mountains and Berkshire Hills uplifts in 

western New England, with the thick Cheshire 

Quartzite at the top. Shelly fossils of the Olenellus zone 

occur only in the upper formations of the clastic deposits, although trace fossils such as Scolithus are 

found much lower. The lowest parts are unfossiliferous, hence are classed as Cambrian (?). Throughout the 

length of the Appalachian miogeosyncline, the clastic 

deposits are succeeded abruptly by a great carbonate 

sequence more than 10,000 ft (3,000 m) thick that includes the remainder of the Lower Cambrian, the Middle and Upper Cambrian, and the Lower Ordovician 

Series.
CAMBRIAN

GULF OF MEXICO

, > *

FIGURE 1. Eastern United States, showing areas mapped as Cambrian on Geologic Map of United States. Includes units 

of marine stratified rocks (-G), basal Lower Cambrian clastic rocks (-Gq), Lower Ordovician and Cambrian carbonate 

rocks (O-G), eugeosynclinal deposits (-Ge), and volcanic rocks (Cv).
PALEOZOIC AND MESOZOIC ROCKS

The miogeosynclinal sequence of the Cordilleran 

Region, in the Great Basin west of the "Wasatch line," 

also begins with Lower Cambrian clastic deposits 

known from place to place as the Brigham, Tintic, and 

Prospect Mountain Quartzites. For consistency, it 

would have been interesting to have separated these 

on the Geologic Map, but although they are as thick 

and prominent as those in the Appalachian 

miogeosyncline, their outcrops are more discontinuous 

and patchy and too small for representation. In this 

region, as in the Appalachian miogeosyncline, the clastic deposits are succeeded by carbonate rocks, in this 

case of Middle and Late Cambrian age, with thicknesses in the classic sections in the House Range, western 

Utah, and the Eureka district, east-central Nevada, of 

8,200 and 5,400 ft (2,500 and 1,600 m), respectively.

Throughout the Central Interior Region, the basal 

Cambrian deposits are also sandstone, but they are the 

basal deposits of the cratonic sequence and are all of 

Late Cambrian age except in the extreme west (parts 

of the Rocky Mountains and Colorado Plateau), where 

some of them are as old as Middle Cambrian. In northeastern New York State the Potsdam Sandstone on the 

flanks of the Adirondack dome is closely adjacent to the 

Cheshire Quartzite of the basal Lower Cambrian clastic deposits across Lake Champlain to the east, but it is 

of Late Cambrian age like the other basal cratonic deposits farther west in the Central Interior. The type 

Croixan Series in the border region of Minnesota and 

Wisconsin is nearly all different varieties of sandstone 

and about 1,000 ft (300 m) thick; fossils throughout it 

permit its division into three stages and eight zones 

(Bell and others, 1956).

Cambrian (-G) is shown on the Geologic Map in the 

two core areas of the Ouachita Mountains foldbelt 

the Broken Bow uplift of southeastern Oklahoma and 

the Benton uplift of southwestern Arkansas. The unit 

so shown is the Collier Shale, traditionally classed as 

Cambrian but from which early Ordovician (Tremadocian) conodonts have recently been collected (Repetski 

and Ethington, 1973). The age designation on the map 

is therefore erroneous, but it at least illustrates the 

structurally highest parts of the foldbelt.

EUGEOSYNCLINAL DEPOSITS ( e)

Cambrian eugeosynclinal deposits are shown on the 

Geologic Map throughout much of the length of the 

Appalachian Region, east of the miogeosynclinal belt. 

In the Cordilleran Region, where present, they are 

merged with the other lower Paleozoic eugeosynclinal 

deposits (1 Pe). Characteristic components of the 

eugeosynclinal deposits are volcanic rocks (-Gv), which 

are differentiated on the Geologic Map where they 

underlie sufficiently large areas.

Cambrian rocks of eugeosynclinal facies form an 

outcrop belt in western New England that extends 

along the east flank of the Green Mountains and other 

Precambrian uplifts from the Canadian border 

through Vermont and Massachusetts to Connecticut 

(fig. 2). In Vermont, north of the plunging end of the 

Precambrian, Cambrian rocks extend across the Green 

Mountains anticlinorium to adjoin the Cambrian 

miogeosynclinal rocks, and so a transition between 

them can be worked out (Cady, 1960, p. 539-543). The 

dominant carbonate rocks of the latter change eastward into argillaceous and coarser clastic rocks (now 

schistose), with lenticular volcanic units, and the sequence thickens dramatically to more than 20,000 ft 

(6,000 m). Fossils are virtually absent, but correlations 

can be reasonably established with the fossiliferous 

miogeosynclinal sequence to the west and with dated 

units along the strike in Canada to the north, suggesting that Lower, Middle, and Upper Cambrian Series 

are all represented.

Cambrian eugeosynclinal deposits are also mapped 

in the Taconic area west of the Green Mountains in 

western New England and eastern New York State, 

where they are allochthonous on the Cambrian and 

Ordovician miogeosynclinal rocks. Here again, the sequence is dominantly argillaceous or silty, with minor 

coarser layers; however, it is no more than a few 

thousand feet (300-600 m) thick, and interbedded volcanic rocks are rare (Zen, 1967, p. 14-22). Parts of the 

sequence are fossiliferous and indicate that much of it 

is of Lower Cambrian age, although Middle and Upper 

Cambrian fossils have been found in places. According 

to present beliefs, the rocks of the Taconic sequence 

formed in the transition zone between the miogeosynclinal carbonates and the thick eugeosynclinal rocks at 

a site a little east of the present Green Mountains axis 

and were transported as one or more slices, largely by 

gravity sliding, onto a sea floor where Middle Ordovician deposits were still accumulating.

Small areas of Cambrian rocks are mapped much 

farther east in New England, in eastern Maine and 

Massachusetts; however, most of these rocks are poorly 

defined paleontologically, and their age is suggested 

mainly by their relations to overlying beds or by 

radiometric dating. The Grand Pitch Formation at the 

base of the eugeosynclinal sequence in northeastern 

Maine is a red slate containing the trace fossil Oldhamia, which occurs elsewhere in Cambrian rocks. 

South of Boston in Massachusetts are the much better 

dated Hoppin Slate and Weymouth Formation with 

Lower Cambrian fossils and the Braintree Slate with 

Middle Cambrian Paradoxides, the faunas being of Atlantic facies, unlike the North American faunas in the 

Appalachian miogeosyncline to the west. Unfortu-
CAMBRIAN

Narragansett 

co Basin

100 150 KILOMETRES

FIGURE 2. New England, showing positions of tectonic features referred to in the discussions of the Cambrian, Ordovician, Silurian,

and Devonian eugeosynclinal rocks.
PALEOZOIC AND MESOZOIC ROCKS

nately, all these occurrences are very small some 

little more than specimen localities and so it is impractical to mark them on the Geologic Map. The map 

indicates as Cambrian the somewhat larger outcrops of 

the Westboro Quartzite northwest of Boston, but its 

age is uncertain; it may be of late Precambrian age, as 

was deduced by Emerson (1917, p. 24).

In the Central Appalachians, the principal 

eugeosynclinal deposit of Cambrian age is the 

Glenarm Series of the Piedmont province in Maryland 

and adjacent States (Hopson, 1964, p. 54-128; Higgins, 

1972). At the base, next to the domes of Precambrian 

Baltimore Gneiss, are the Setters Formation (quartz 

schist and quartzite) and the Cockeysville Marble, followed by the great clastic mass of the Wissahickon 

Formation, more than 20,000 ft (6,000 m) thick. Although the Wissahickon consists of high-grade 

metamorphic rocks, it contains abundant relict 

sedimentary structures which indicate that it is a 

flysch1 or turbidite deposit. Locally interbedded in the 

Wissahickon is the thick lens of the Sykesville Formation, once interpreted as a granitic intrusive but actually a coarse submarine slide derived from an eastern 

source, containing heterogeneous clasts and blocks of 

sedimentary and metamorphic rocks. Toward the 

southeast, next to the Coastal Plain, the Wissahickon 

intern"ngers with and is overlain by the James Run 

Formation of volcanic and volcaniclastic rocks. In the 

western Piedmont of Maryland, the Wissahickon is replaced by and partly overlain by shallow-water phyllites, marbles, and basalt flows (Ijamsville Phyllite, 

etc.) which are represented on the Geologic Map as 

Cambrian stratified rocks ( ).

The age of the Glenarm Series has long been disputed, being placed in late Precambrian or early 

Paleozoic time. Hopson (1964, p. 203-207) proposed a 

late Precambrian age because enclosed granitic plutons have ages of 470 to 550 m.y. and of about 425 m.y. 

The events separating the oldest of these plutons from 

the formation of the Glenarm Series were inferred to 

have been sufficiently prolonged that the series must 

have formed before Cambrian time. However, Higgins 

(1972, p. 1008-1009) demonstrated that the older 

group of supposed grantitic intrusives (470-550 m.y.) 

is actually an assemblage of metamorphosed sediments; hence the dates define the maximum age of the 

Glenarm, or perhaps its true age that is, Cambrian 

rather than Precambrian.

 The term "flysch" is derived from the original Flysch of the European Alps and is used 

consistently in this report for a peculiar assemblage of thinly interbedded sandy and shaly 

rocks; the sandy rocks commonly show grading and other structures which suggest that 

they were derived from turbid flows, and the shaly rocks represent interludes of pelagic 

sedimentation. Flysch is a deep-water deposit that commonly accumulated in linear troughs 

in mobile belts. Not included are other synorogenic deposits (to which the term has sometimes been misapplied), many of which formed in quite different environments.

Southwestward in central Virginia, in apparent continuity with the Glenarm, is the Evington Group of the 

Lynchburg area (Espenshade, 1954, p. 14-21; Brown, 

1958, p. 28-38). On the flank of the Blue Ridge, resting 

on the Precambrian Z Lynchburg Formation (or the 

Catoctin Greenstone, where present), is the Candler 

Phyllite, about 5,000 ft (1,500 m) thick, which is followed by several thousand feet of more varied 

strata schist, marble, and quartzite with about 

1,000 ft (300 m) of greenstone metavolcanic rocks at 

the top. The Evington Group contains no fossils, and no 

radiometric dates are available; from its relations to 

the Lynchburg and by comparison with the Glenarm, it 

is presumably early Paleozoic (Cambrian?) and is so 

represented on the Geologic Map (-Ce).

Still farther southwestward, near the VirginiaNorth Carolina border, and more or less in continuity 

with the Evington Group, is the Alligator Back Formation (Rankin and others, 1973, p. 17-19). It overlies the 

Precambrian Z Ashe Formation (= Lynchburg Formation) and consists of laminated graywacke and pelite 

with "pinstripe" structure; volcanic rocks interfinger 

northeastward. Although there is no clear evidence of 

age, it is likewise indicated as Cambrian (-Ce) on the 

Geologic Map.

A much larger area of Cambrian eugeosynclinal deposits (- e) forms the Carolina Slate Belt of the Southern Appalachians, extending as a wide band across the 

Piedmont province from southern Virginia through 

North and South Carolina to eastern Georgia (Conley 

and Bain, 1965; Sundelius, 1970). In central North 

Carolina the band is 130 mi (210 km) wide, but it narrows to the northeast and southwest. The Slate Belt 

contains a sequence of gently deformed low-grade 

metamorphic clastic and volcanic rocks more than 

30,000 ft (9,100 m) thick. These are adjoined to the 

northwest and southeast by higher grade metamorphic 

rocks, in part the metamorphosed equivalents of the 

Slate Belt rocks, but probably mainly older. They are 

intruded here and there by granitic and mafic plutons 

with ages of 520 to 595 m.y. (Fullagar, 1971, p. 2852- 

2854). Areas of low-grade clastic and volcanic rocks 

like those in the Slate Belt also occur farther southeast 

and are encountered in many drill holes beneath the 

Atlantic Coastal Plain.

In North Carolina, a rather consistent stratigraphy 

can be worked out and mapped in the rocks of the Slate 

Belt. Below (with the base not exposed) is the thick 

Uwharrie Formation of rhyolitic and rhyodacitic flows 

and pyroclastics. Above are thinner units of laminated 

shales, mudstones, and siltstones, alternating with 

volcanic-rich units. The subdivisions are not shown on 

the Geologic Map, but the volcanic rocks (-Cv) are separated where they underlie sufficiently large areas. The
ORDOVICIAN

age of the rocks in the Slate Belt can be indicated only 

in a general way. At one locality in southern North 

Carolina, are a few Paradoxides of probably Middle 

Cambrian age (St. Jean, 1973). From rocks in the same 

general area, an Ordovician age of 440 to 470 m.y. was 

obtained by the lead-alpha (Pb/alpha) method, but this 

age seems to be unreliable and too young. A considerable part of the sequence is probably late Precambrian 

(Z). In northern North Carolina, on the Little River 12 

miles (20 km) north of Durham, Lynn Glover III and 

his associates have found Ediacaran (= Vendian) type 

fossils, which are the imprints of primitive wormlike 

animals on the bedding surfaces of volcaniclastic 

strata. At the north end of the belt in Virginia, 

radiometric ages in excess of 600 m.y. have been obtained from the slate belt rocks (Glover and Sinha, 

1973).

In the Cordilleran Region, the only authentic Cambrian eugeosynclinal deposits known to us are the 

Scott Canyon Formation which forms a small area in 

the south part of Battle Mountain, north-central 

Nevada. Limestones interbedded in its cherts, shales, 

and greenstones contain archeocyathids of Lower or 

Middle Cambrian age. It is mapped as part of the lower 

Paleozoic eugeosynclinal deposits (IPe). The Cambrian (-G) shown in Battle Mountain and nearby ranges 

is the Harmony Formation, an arkosic turbidite of Late 

Cambrian age, which is classed as a "transitional" 

facies rather than eugeosynclinal (Roberts and others, 

1958, p. 2829-2830), hence is included in the normal 

stratified sequence on the Geologic Map.

ORDOVICIAN AND CAMBRIAN

MARINE STRATIFIED ROCKS (O-G)

As indicated above, the Appalachian miogeosynclinal rocks include a thick sequence of carbonates that 

extends from the Lower Cambrian into the Lower Ordovician System. In the Central and Southern Appalachians, it has been possible to separate the Cambrian and Ordovician components on the Geologic 

Map, but from New Jersey northward the outcrop belts 

are narrower and more complex and have been merged 

into a single unit (O-G).

In New Jersey and southern New York, the subdivisions are apparent in places but have not been worked 

out regionally, and they are shown as undivided units 

on the source maps the Kittatinny and Wappinger 

Limestones of the northwestern belts and the Inwood 

and Stockbridge Marbles of the metamorphic belt east 

and southeast of the Hudson Highlands. Farther north, 

in the Champlain Lowlands of Vermont and adjacent 

New York, the subdivisions of the Cambrian and 

Lower Ordovician carbonates have been worked out

and mapped in detail (see Geologic Map of Vermont, 

1961) but cannot be represented on the scale of the 

Geologic Map.

In the eugeosynclinal area of New England, the 

source maps likewise indicate some of the units as 

"Ordovician-Cambrian" (meaning Ordovician or Cambrian), but these have been arbitrarily assigned to one 

system or the other on the Geologic Map.

ORDOVICIAN

The Ordovician System is represented on the 

Geologic Map of the United States by marine stratified 

rocks (O), divided in part into Lower, Middle, and 

Upper Series (Oi, O2, Os), and by eugeosynclinal deposits (Oe) with associated volcanic rocks (Ov). The 

Ordovician is shown separately in the eastern twothirds of the country; from the Rocky Mountains westward it is merged with the other lower Paleozoic systems (IP, IPe).

MARINE STRATIFIED ROCKS

The most extensive exposures of Ordovician strata 

are in the cratonic area of the Central Interior Region, 

where they are nearly flat lying or are gently tilted on 

the flanks of the broad domical uplifts (fig. 3). In the 

north, they form a broad band of outcrop between the 

Cambrian and Silurian sequences around the west, 

south, and east flanks of the Wisconsin dome, extending from Minnesota to the Upper Peninsula of Michigan and as far south as Illinois. Farther south, they 

form much of the surface of the Ozark dome in Missouri and Arkansas largely Lower Ordovician with 

the Middle and Upper Ordovician Series in narrow 

bands around the edges. Middle and Upper Ordovician 

rocks form the crestal areas of the Cincinnati and 

Nashville domes farther east, where the Lower Ordovician rocks is not exposed. The three series also 

encircle the Adirondack dome in New York State, and 

smaller Ordovician outcrops occur in the Arbuckle and 

Wichita Mountains and the Llano uplift in Oklahoma 

and Texas. Intervening areas in the Central Interior 

are covered by younger strata, but the presence of Ordovician rocks beneath them is known from drilling.

Ordovician rocks also crop out in long bands 

throughout the length of the miogeosynclinal belt of 

the Appalachian Region east of the Central Interior 

Region and emerge in the core areas of the Ouachita 

Mountains foldbelt to the south.

The three series of the Ordovician System are shown 

separately throughout the Central Interior, as well as 

in parts of the Appalachian miogeosynclinal belt in 

New York and Pennsylvania. In the remainder of the 

miogeosynclinal belt, the Ordovician outcrop bands are
PALEOZOIC AND MESOZOIC ROCKS

GULF OF MEXICO

FIGURE 3. Eastern United States, showing areas mapped as Ordovician on Geologic Map of United States. Includes units 

of marine stratified rocks (O) and their subdivisions (Oi, O2, Oa), eugeosynclinal deposits (Oe), volcanic rocks (Ov), 

and a few of lower Paleozoic (IP) in the Southern Appalachians.
ORDOVICIAN

too narrow for subdivision and are shown as a single 

unit (O). On the other hand, the Lower Ordovician 

Series in the Ozark dome forms such an extensive area 

that further separation is needed to illustrate the geology. Here, the series is divided at the base of the Jefferson City Dolomite into units Oia and Oib, although 

there are no fundamental paleontological or sedimentological differences between the two parts. (Somewhat 

similar subdivisions were made on the Geologic Map of 

1932).

The Lower Ordovician or Canadian Series (Oi) is 

largely carbonate, much of it dolomite, which succeeds 

similar carbonates of the Upper Cambrian Series (as 

noted above). In many places the two components were 

not separated in early reports, resulting in units such 

as the Knox of the Southern Appalachians, the Arbuckle of the Arbuckle and Wichita Mountains, Oklahoma, and the Ellenburger of the Llano area, Texas; 

relations are now clarified, and these and similar units 

are classed as groups. Throughout the Appalachian 

miogeosyncline, the series is about 2,000 ft (600 m) 

thick, but it has a maximum thickness of about 5,000 ft 

(1,500 m) in the deep trough adjoining the Arbuckle 

and Wichita Mountains. It thins in the cratonic area, 

being about 1,000 ft (300 m) thick in the Ozark dome 

and no more than a few hundred feet thick on the 

flanks of the Wisconsin dome (Prairie du Chien Group).

The Middle Ordovician or Mohawkian Series (O2)2 is 

more varied, largely limestone and shale but with notable units of sandstone in the lower part. Its abundantly fossiliferous strata have been the field of labor 

of many paleontologists and stratigraphers, and it has 

been minutely subdivided and correlated from place to 

place. The details do not concern us here, but a few 

general items are worth noting.

In the northern Midwestern States, the basal Middle 

Ordovician unit is the Saint Peter Sandstone, a clastic 

sheet of vast extent probably derived from the Canadian Shield (Dake, 1921, p. 221-224). To the north it 

lies on a rough surface eroded on Lower Ordovician 

rocks, but southward it fingers out into carbonate 

formations.

Eastward in the Appalachian miogeosyncline, the 

Middle Ordovician limestones and shales with shelly 

fossils give way to shales and coarser clastic rocks in 

which the principal fossils are graptolites. These shaly 

rocks include some important belts of flysch, such as 

the Normanskill Formation of New York State and the 

Martinsburg Formation of the Central Appalachians

2Termmology ol the Middle Ordovician Series is confused. According to some usage (for 

example. Twenhofel and others, 19541. the Middle Ordo\ ician is the Champlamian, which is 

subdivided into the Cha/yan below and the Mohawkian (Black Ri\er and Trenton Limestones and equivalents! abo\ e. All these names are derived from the classic sequence in 

New York.

(which continues upward into the Upper Ordovician, 

although it is all marked as O2 on the Geologic Map) 

(McBride, 1962, p. 39-43). These shaly rocks (02) form 

the autochthonous substratum of the allochthonous 

Taconic rocks in eastern New York and western New 

England. Southeast of the Hudson Highlands, they 

pass into a higher grade metamorphic facies (Manhattan and Berkshire Schists) which extends eastward in 

Connecticut to an obscure boundary ("Cameron's 

line"), possibly a thrust, which separates them from 

the eugeosynclinal Hartland Schist (Oe).

Graptolite shales with some interbedded sandstone 

also compose the Lower and Middle Ordovician Series 

in the Ouachita Mountains foldbelt (Mazarn Shale, 

Blakeley Sandstone, and Womble Shale).

The Upper Ordovician or Cincinnatian Series (Oa) is 

typified by exposures in the Cincinnati dome of Ohio 

and adjacent States, where its shales and limestones 

(Eden, Maysville, and Richmond Groups) are richly 

fossiliferous. Other shales, the Maquoketa, form the 

Upper Ordovician in the northern Midwestern States. 

Farther southwest, the Upper Ordovician sequence 

(and part of the Middle Ordovician) is mainly cherty 

limestone, the Viola of the Arbuckle and Wichita 

Mountains, which passes into the Bigfork Chert in the 

nearby Ouachita Mountains. Eastward toward the Appalachian belt, the uppermost Upper Ordovician passes into redbeds, in part continental the Queenston 

Shale of northwestern New York and the Juniata and 

Sequatchie Formations farther south which are postorogenic to the Taconian deformation in the foldbelt 

itself.

EUGEOSYNCLINAL DEPOSITS

Ordovician eugeosynclinal deposits and associated 

volcanic rocks (Ov) are mapped in numerous areas in 

all the New England States, but especially on the west 

and east flanks of the Connecticut Valley 

synclinorium, with smaller less continuous areas to 

the east and southeast. Their stratigraphy and ages are 

best deciphered in the north, where the rocks are least 

metamorphosed and fossils obtainable in places. The 

stratigraphy of the higher grade metamorphic rocks 

farther south, as in Massachusetts and Connecticut, is 

indicated by comparisons with or by actual tracing 

from these better known rocks.

The eugeosynclinal rocks of the Connecticut Valley 

are in a homoclinal belt that extends south from the 

Canadian border through Vermont and Massachusetts 

into Connecticut, between the Cambrian rocks of the 

Green Mountains and other uplifts on the west and the 

Silurian and Devonian rocks of the synclinorium; they 

are mainly phyllites and schists but include lenticular 

bodies of sandstone and volcanic rocks; in northern
10 PALEOZOIC AND MESOZOIC ROCKS

Vermont and adjacent Canada some of the units contain graptolites.

The eugeosynclinal rocks east of the Connecticut 

Valley from New Hampshire southward are in the 

Bronson Hill anticlinorium actually a highly irregular chain of domes much entangled with plutons and 

with pronounced nappe structures. Volcanic components are much greater here than to the west; most of 

the sediments are tuffaceous, and the thick Ammonoosuc Volcanics occur near the middle (Billings, 

1956, p. 12-21). The anticlinorium may have originated as a volcanic arc in the eugeosyncline. Above the 

Ammonoosuc is the Partridge Formation, a black 

sulfidic argillaceous rock that continues southward 

into the Brimfield Schist of Massachusetts. The Ordovician sequence in the Bronson Hill anticlinorium is 

more than 20,000 ft (6,000 m) thick, but its base is not 

exposed; it is overlain unconformably by the fossiliferous Silurian Clough Quartzite which also truncates 

the Highlandcroft Plutonic Series (P gi), intrusive into 

the Ordovician.

Along the continuation of the belt across northern 

Maine, Middle Ordovician graptolites occur at various 

places in shales above the volcanic rocks. An interesting variant in northeastern Maine is the Shin Brook 

Formation of tuffaceous sediments and volcanic breccias that contains a large assemblage of Middle Ordovician shelly fossils (brachiopods, trilobites, and so 

forth), probably formed in a shoal area in the 

eugeosyncline (Neuman, 1964).

Small areas of Ordovician eugeosynclinal rocks are 

shown on the Geologic Map as overlying the Cambrian 

sequence in the allochthonous Taconic area of eastern 

New York. They are graptolite-bearing shales and 

graywackes like the Normanskill Formation of the 

surrounding autochthon but include strata of Early as 

well as Middle Ordovician age.

The only authentic Ordovician formation known to 

us in the eugeosynclinal area south of New England is 

the Arvonia Slate of central Virginia that contains 

abundant (though much deformed) shelly fossils of 

Middle or Late Ordovician age (Brown, 1969, p. 25-26); 

it lies unconformably on metamorphic and plutonic 

rocks of earlier Paleozoic age. Also shown as Ordovician (Oe) on the Geologic Map are the Quantico Slate 

south of Washington, B.C., and the Peach Bottom Slate 

of southern Pennsylvania, but although traditionally 

they have been correlated with the Arvonia, their age 

is less certain and they may be older (Higgins, 1972, 

p. 972). The existence of Ordovician rocks elsewhere in 

the Piedmont of the Central and Southern Appalachians is a possibility, but no evidence for them is 

available; present indications are that most of the 

Piedmont supracrustal rocks are older.

SILURIAN AND ORDOVICIAN

EUGEOSYNCLINAL DEPOSITS (SOe)

In northeastern Maine (Aroostook County) the hybrid category "Silurian and Ordovician" is used on the 

Geologic Map to designate the Carys Mills Formation 

and some related deposits. These form a sequence 

about 12,000 ft (4,000 m) thick of calcareous silty and 

sandy rocks that extends from the Middle Ordovician 

into the Lower Silurian Series (Caradoc to Llandovery), as indicated by graptolites and other fossils (Pavlides, 1968, p. 8-13). Use of the hybrid term in this area 

demonstrates the lack of any Taconian erogenic activity and consequent structural discordance, such as occurs between the Ordovician and Silurian Systems in 

much of the remainder of New England. Although the 

Carys Mills is calcareous and lacks any volcanic material, it is not miogeosynclinal. It is, instead, a calcareous flysch laid down in the depths of a part of the 

eugeosyncline that was far from any tectonic or volcanic areas.

SILURIAN

The Silurian System is represented on the Geologic 

Map of the United States by marine stratified rocks (S), 

divided in part into the Lower, Middle, and Upper 

Silurian Series (Si, 82, Sa), and by eugeosynclinal deposits (Se) with associated volcanic rocks (Sv). The 

Silurian is shown separately in the northeastern third 

of the country; in the Southern Appalachians and the 

Ouachita Mountains it is combined with the Devonian 

(DS), and from the Rocky Mountains westward it is 

combined, where present, with the other lower 

Paleozoic systems (IP, IPe).

MARINE STRATIFIED ROCKS (S)

As with the systems below and above, the Silurian is 

most extensively exposed in the cratonic area of the 

Central Interior Region, where the strata are gently 

tilted off the flanks of the broad domical uplifts (fig. 4). 

Silurian outcrops are less extensive, however, than 

those of the adjoining systems, and in places they are 

truncated, or nearly so, by the systems above. The most 

prominent band of outcrops encircles the Michigan basin, from northern Ohio, Indiana, and Illinois, through 

eastern Wisconsin, the Upper Peninsula of Michigan, 

and southern Ontario (see Geologic Map of Canada), 

and across the Niagara Gorge in western New York. A 

large detached area lies southwest of the Wisconsin 

dome in eastern Iowa and northwestern Illinois. Small 

remnants are preserved on the flanks of the Nashville 

and Ozark domes farther south. The New York outcrops bend around the northeastern end of the Allegheny synclinorium into the miogeosynclinal belt of
SILURIAN 11

I r "" V "<J

GULF OF MEXICO

FIGURE 4. Eastern United States, showing areas mapped as Silurian on Geologic Map of United States. Includes units of 

marine stratified rocks (S) and their subdivisions (Si, 82, Sa), eugeosynclinal deposits (Se), volcanic rocks (Sv), and 

Silurian and Ordovician eugeosynclinal deposits (SOe).
12 PALEOZOIC AND MESOZOIC ROCKS

the Appalachians, where Silurian rocks form numerous bands encircling the folds of the Valley and Ridge 

province from Pennsylvania into Virginia, but from 

southern Virginia southwestward the bands become so 

narrow that they are merged with the Devonian (DS), 

as in the Ouachita Mountains to the west.

According to North American usage, the Silurian is 

divided into the Lower Silurian or Alexandrian Series 

(Si) (Oswegan or Albion of earlier usage), the Middle 

Silurian or Niagaran Series (82), and the Upper Silurian or Cayugan Series (Ss). This practice conflicts with 

usage in Great Britain and elsewhere in western 

Europe, where only a Lower Silurian (Llandovery) and 

an Upper Silurian (Wenlock and Ludlow) Series are 

recognized, the boundary between them being in the 

middle of the North American Middle Silurian (Berry 

and Boucot, 1970, p. 13-16). Nevertheless, the subdivisions of the Silurian System, where shown on the 

Geologic Map, follow the conventional North American 

usage.

The three series of the Silurian are separated on the 

Geologic Map throughout the broad outcrop areas in 

the north, but the Middle Silurian (82) is the most extensive and accounts for a large part of the outcrop 

area, partly because of its resistant carbonate formations. The Lower Silurian Series (Si) is inconsequential 

except in New York and the Iowa-Illinois area, and the 

Upper Silurian is only prominent at the surface in 

northern Indiana and Ohio. It is best developed 

downdip, in subsurface beneath the Michigan basin 

and Allegheny synclinorium. The Silurian is not subdivided on the map in the smaller areas in the southern part of the Central Interior, or in the Appalachian 

miogeosynclinal belt.

Throughout the Central Interior the Silurian sequence is accordant with the Ordovician below and the 

Devonian above, although separated from them at 

many places by a hiatus of greater or lesser magnitude. 

At the Falls of the Ohio near Louisville, for example, 

Middle Silurian carbonates (Niagaran) are overlain 

without discordance by Middle Devonian carbonates 

(Onondaga equivalent), the contact actually being 

within a single layer. Nevertheless, the Geologic Map 

indicates a low-angle truncation of the Silurian by the 

Devonian in Indiana, Illinois, and Iowa.

Eastward along the edge of the folded rocks of the 

northern Appalachians, a structural unconformity develops at the base of the Silurian sequence, reflecting 

the Taconian orogeny in this part of the foldbelt. The 

discordance is prominent west of the Hudson River in 

southern New York, northern New Jersey, and eastern 

Pennsylvania but fades out to the south. Northwesttilted Lower Silurian sandstones and conglomerates 

(Shawangunk and Tuscarora) overlie highly disturbed

Middle and Upper Ordovician flysch (Normanskill and 

Martinsburg). A significant outlier of the unconformity 

occurs at Becraft Mountain east of the Hudson, where 

Lower Devonian rocks overstep the Silurian and lie 

directly on deformed and faulted Cambrian and Ordovician rocks of the Taconic allochthon.

Carbonate rocks dominate the cratonic area of the 

Central Interior. The Niagaran Series in particular,- in 

the middle of the sequence, is a sheet of dolomite that 

extends westward from New York State to Iowa. One of 

its stronger layers, the Lockport Dolomite, forms the 

rimrock of Niagara Falls. The Niagaran carbonates 

are studded wtih mound reefs, some of them 400 ft (120 

m) thick, which grew on a sea floor of marked relief. In 

New York, however, the lower part of the Niagaran 

(Clinton Group) is shaly and contains beds of red iron 

ore.

Limestones and shales, rather than dolomite, are 

more prominent in the Lower and Upper Silurian Series, and the latter, or Cayugan, contains large volumes 

of evaporites, especially rock salt. These have mostly 

been leached back from the outcrops and are known 

mainly in subsurface. Cayugan time marked a climax 

in the sinking of the Michigan basin, and the series is 

4,000 ft (1,200 m) or more thick in its center (beneath 

the cover of younger Paleozoic strata); nearly half of it 

is salt (Cohee, 1965, p. 217-218). The Cayugan salt 

deposits have long been exploited commercially, both 

in Michigan and the Appalachian Region; drilling for 

salt long preceded drilling for oil in the same areas.

In the Appalachian miogeosyncline the Silurian carbonates give place to clastic deposits, in part nonmarine, related to the Taconian orogeny and its aftermaths. Especially prominent are the ridge-making 

sandstones of the Lower Silurian, known from place to 

place as the Shawangunk, Tuscarora, Clinch, and 

other local terms. In Pennsylvania the Upper Silurian 

rocks include the Bloomsburg Redbeds, as much as 

5,000 ft (1,500 m) thick, which thin westward and intertongue with gray shales and limestones.

EUGEOSYNCLINAL DEPOSITS (Se)

Silurian eugeosynclinal deposits and associated volcanic rocks (Sv) are mapped in the New England States 

north of Connecticut and Rhode Island. None are 

known in the Piedmont province of the Central and 

Southern Appalachians, and probably they do not exist 

there. As in the adjacent systems, the stratigraphy is 

plainest to the north and northeast, where the metamorphic grade is low and fossils are relatively 

abundant especially in northern and eastern Maine, 

but also in a peculiar narrow belt of low-grade metamorphism along the Connecticut River in western New 

Hampshire.
DEVONIAN AND SILURIAN 13

In the northwestern belts flanking the Connecticut 

Valley synclinorium, the Silurian sequence lies with 

structural discordance on the Ordovician, and at a few 

places in northern Maine it is also moderately unconformable below the Devonian. It is thin generally little more than 1,000 ft (300 m) thick and includes important bodies of quartz sandstone and carbonate 

(Shaw Mountain Formation and Northfield Slate on 

the west flank in Vermont, Clough Quartzite and Fitch 

Formation on the east flank in New Hampshire, all 

Middle Silurian). The units in New Hampshire form 

such narrow outcrop belts and the structure is so complex that they are indicated only in places on the 

Geologic Map, although they are actually fairly continuous. The Silurian rocks of these belts are more of 

miogeosynclinal than of eugeosynclinal facies, and 

their inclusion with "Se" on the Geologic Map-is misleading. The deposits represent an interlude in northwestern New England, following the Taconian 

orogeny, between the eugeosynclinal regimes that 

dominated Ordovician and Devonian time.

Southeastward into the Merrimack synclinorium in 

New Hampshire and Maine, the Silurian sequence itself becomes eugeosynclinal and thickens dramatically 

to more than 15,000 ft (4,600 m), as shown by its wide 

outcrop bands on the^ Geologic Map. The Merrimack 

Group in the south is mainly slates and schists with a 

few more sandy units and minor volcanic rocks. A band 

of Silurian slates more than 30 mi (50 km) broad extends northeastward through central Maine, nearly 

across the State.

Nearer the coast, volcanic rocks (Sv) dominate. They 

are well displayed in the Eastport area at the eastern 

tip of Maine, where they are virtually unmetamorphosed and contain fossils at numerous levels that indicate Middle and Late Silurian ages. Farther southwest, metamorphism is greater and fossil control is 

sparse. Brachiopods and ostracodes in the Ames Knob 

Formation of the Penobscot Bay area are of Late Silurian age. A few Late Silurian ostracodes have been 

found in the Newberry Volcanics north of Boston in 

eastern Massachusetts, and the barren Lynn and Mattapan Volcanic Complexes south of it may be of 

roughly the same age.

DEVONIAN AND SILURIAN

MARINE STRATIFIED ROCKS (DS)

On the Geologic Map, the Devonian and Silurian 

Systems are combined into a single unit in the 

miogeosynclinal belt of the Southern Appalachians 

and in the Ouachita Mountains.

The outcrop belts of both systems become narrow in 

southwestern Virginia, partly from steepening of the

structure, more from thinning of the sequences. The 

great wedge of Devonian clastic rocks that is prominent in New York and Pennsylvania thins and becomes inconsequential in Virginia and beyond. In 

Tennessee the combined thickness of the two systems 

is about 1,000 ft (300 m), and in Alabama no more than 

a few hundred feet. In Alabama the units are the Silurian Red Mountain Formation, with red iron ores like 

those of the Clinton in New York that are exploited 

commercially at Birmingham, and the Devonian Frog 

Mountain Sandstone.

Mapped also as undivided Devonian and Silurian in 

Alabama are the upper rocks of the sequence in the 

Talladega belt, southeast of the area of miogeosynclinal rocks. These include the persistent Cheaha (= Butting Ram) Sandstone, followed in the south by the 

Jemison Chert with abundant shelly fossils of Early 

Devonian age, and farther north by the Erin Slate from 

which fossil plants of supposed Carboniferous age have 

been collected. The slates and phyllites in the Talladega sequence beneath the Cheaha Sandstone are 

indicated on the map as lower Paleozoic (IP) (see below).

In the Ouachita Mountains, the Devonian and Silurian (DS) of the Geologic Map comprise the Blaylock 

Sandstone with Lower Silurian monograptids; the 

Missouri Mountain Slate, unfossiliferous but probably 

also Silurian; and the Arkansas Novaculite. The first 

two are prominent in the southern outcrop belts but 

insignificant farther north, whereas the Arkansas is 

persistent and forms mountain ridges that encircle the 

older Paleozoic rocks of the core areas. It is a few 

hundred to a thousand feet (80-300 m) thick and is a 

condensed sequence with conodonts indicating that it 

embraces all the Devonian Period and the lower part 

of the Mississippian (Kinderhookian) as well (Hass, 

1951).

EUGEOSYNCLINAL DEPOSITS (DSe)

Many of the source geologic maps in New England 

designate units by the hybrid term "Devonian and 

Silurian" or "Devonian or Silurian," indicating either 

that they contain rocks of both ages (as indicated by 

fossils or other less direct evidence) or that there is 

uncertainty as to which system they should be assigned. The Geologic Map of Maine (1967) lists more 

than 20 such hybrid map units. For the most part, we 

have arbitrarily assigned such units to one of the two 

systems on the Geologic Map, on the basis of the preponderance of evidence for one or the other, but this 

has not been possible in the Merrimack synclinorium 

that extends from southeastern Maine and New 

Hampshire to Massachusetts.

The broad band of Silurian eugeosynclinal rocks that
14 PALEOZOIC AND MESOZOIC ROCKS

extends across southeastern Maine is separated from 

the Devonian eugeosynclinal rocks by bands nearly as 

broad labeled on all the source maps as "Devonian or 

Silurian," which it would be presumptuous on our part 

to attempt to classify. Those on the northwestern flank 

are calcareous silty and sandy rocks (Madrid and Fall 

Brook Formations) which overlie strata with Upper 

Silurian (Ludlow) fossils and underlie equivalents of 

the Devonian Seboomook Formation; those on the 

southeastern flank (Vassalboro and Berwick Formations) similarly overlie fossiliferous Upper Silurian but 

lack Devonian rocks at the top (Osberg and others, 

1968; Ludlum and Griffin, 1974). The Geologic Map 

also extends the southeastern belt of "Devonian or 

Silurian" to the better dated rocks near Penobscot Bay 

to include rocks that were poorly understood at the 

time of compilation. Surveys now available indicate 

that this area is more heterogeneous than realized and 

includes not only Devonian and Silurian but also older 

Paleozoic and possibly even Precambrian rocks (Osberg, 1974).

On the west flank of the Merrimack synclinorium 

next to the Bronson Hill anticlinorium, the Geologic 

Map indicates a broad band of "Devonian or Silurian" 

in southern New Hampshire, Massachusetts, and Connecticut. On the Geologic Map of New Hampshire 

(1955), the part in that State was mapped as Devonian 

Littleton Formation, but there and in Massachusetts 

later work has indicated the existence of complex 

nappe structures that involve not only the Devonian 

but rocks as old as the Ordovician Partridge Formation 

(Thompson and others, 1968). Not all these complexities are yet resolved, and the noncommittal designation of DSe was recommended by geologists of the 

Survey who are working in New England. The eastern 

boundary of the area, as shown on the Geologic Map, is 

unsatisfactory and probably does not express the true 

relations.

DEVONIAN

The Devonian System is represented on the Geologic 

Map of the United States by marine stratified rocks 

(D), divided in part into Lower, Middle, and Upper Devonian Series (Di, D2, Da); by Middle and Upper Devonian continental deposits (Dac, Dac); and by eugeosynclinal deposits (De), with associated volcanic rocks 

(Dv). The Devonian, like the Silurian, is shown separately only in the northeastern third of the country; in 

the Southern Appalachians and the Ouachita Mountains it is combined with the Silurian (DS), and from 

the Rocky Mountains westward it is combined with the 

other lower Paleozoic systems (IP, IPe).

MARINE STRATIFIED ROCKS (D)

As with the systems below and above, the Devonian

is most extensively exposed in the cratonic area of the 

Central Interior Region (fig. 5), where the strata are 

gently tilted on the flanks of the broad domes and basins. The most prominent area of outcrop is that in 

southwestern New York and adjacent Pennsylvania 

and Ohio, where the strata dip gently southwestward 

into the Allegheny synclinorium. It displays the classic 

Devonian sequence, known since the early days of the 

New York Survey a century and a half ago, to which 

much of the system in the rest of the country has frequently been compared. Somewhat narrower bands of 

outcrops encircle the Cincinnati dome and Michigan 

basin in Ohio, Indiana, and Michigan, and a large outlying area of southwest-tilted Devonian strata occurs 

in Iowa and adjacent Minnesota and Illinois. The New 

York outcrops bend around the northeastern end of the 

Allegheny synclinorium into the miogeosynclinal belt 

of the Appalachians, where the Devonian rocks form 

numerous bands among the folds of the Valley and 

Ridge province through Pennsylvania into Virginia. 

Beyond Virginia they thin to such an extent that they 

are combined with the Silurian (DS).

According to North American usage, the Devonian is 

divided into the Lower Devonian Series (Helderberg 

and Oriskany) (Di), the Middle Devonian Series 

(Onondaga, Hamilton, and Tully) (D2), and the Upper 

Devonian Series (consisting of the remainder of the 

system) (Da). The resulting subdivisions are quite unequal, the Lower Devonian being thin and inconstant 

and the Upper Devonian very thick as is well illustrated on the Geologic Map by the relative widths of 

the three series in New York. As a substitute, five 

named series have been proposed: Ulsterian, Erian, 

Senecan, Chautauquan, and Bradfordian, the last 

three in the Upper Devonian (Cooper and others, 1942, 

p. 1732). In addition, the European stage names 

(Gedinnian, Coblenzian, Eifelian, Gevetian, Frasnian, 

and Famennian) have come into increasing use in 

North America.

The three series of the Devonian System are differentiated on the Geologic map throughout the broad 

areas in the north, which for the most part consist 

about equally of Middle and Upper Devonian rocks. 

The Lower Devonian Series appears only as a narrow 

band in New York State; where present elsewhere, it is 

combined with the Middle Devonian. The Devonian is 

not subdivided on the map in the smaller outcrop areas 

in the Central Interior, nor in the Appalachian 

miogeosynclinal belt.

The Devonian sequence of the eastern outcrops in 

New York and Pennsylvania is 12,000 to 15,000 ft 

(3,600-4,500 m) thick, forming the apex of a great clastic wedge of Middle and Upper Devonian deposits, 

partly continental (see below), that is frequently referred to as the "Catskill delta." The wedge is a product
DEVONIAN 15

> M

,,_/____»

?' !,

GULF OF MEXICO

FIGURE 5. Eastern United States, showing areas mapped as Devonian on Geologic Map of United States. Includes units of 

of marine stratified rocks (D) and their subdivisions (Di, D2, Da), Devonian and Silurian marine stratified rocks (DS), 

continental deposits (D2c, Dac), eugeosynclinal deposits (De), volcanic rocks (Dv), Devonian and Silurian eugeosynclinal deposits (DSe), and volcanic rocks (DSv).
16 PALEOZOIC AND MESOZOIC ROCKS

of the Acadian orogeny that was in progress in the 

Appalachian foldbelt to the east, and it thins westward 

into the Central Interior Region, as well as southwestward along the strike in the Appalachian miogeosynclinal belt.

The Lower Devonian strata and the Onondaga 

Limestone precede the development of the clastic 

wedge and consist of thin persistent shale and limestone units, as well as one prominent sandstone layer, 

the Oriskany. The Onondaga and its equivalents 

extend far westward into the Central Interior, for example, to the Falls of the Ohio at Louisville, Ky., mentioned earlier.

In New York, the Middle and Upper Devonian coarse 

clastic rocks intertongue westward with finer grained 

sandstones, gray shales, and thin limestones; these intertongue in turn with black shales (Cooper, 1933; 

Chadwick, 1935), the transitions in each successive 

part being displaced a little farther west. West of New 

York the Middle and Upper Devonian sequences are no 

more than a few thousand feet (300-600 m) thick. 

There are important limestone units in the Middle Devonian, with coral and stromatoporoid reefs and banks, 

but the Upper Devonian is shaly as far west as the 

Mississippi River.

Upper Devonian black shales with various spans of 

age are extensive in the Midwestern States, including 

such units as the Ohio, New Albany, and Antrim 

Shales. They are followed in places by similar shales of 

Early Mississippian (Kinderhookian) age, creating 

problems in placing the Devonian-Mississippi an boundary on the Geologic Map. This is the case, for example, in the heavily drift covered border region between 

Indiana and Michigan, underlain by the Antrim, New 

Albany, Ellsworth, Sunbury, and Coldwater Shales, 

the middle three being indicated on the State Maps as 

"Devonian and Mississippian"; on the Geologic Map 

the systemic boundary is arbitrarily located between 

the New Albany and the Ellsworth.

South of Virginia the sole representative of the black 

shales is the thin very persistent Chattanooga Shale 

(included in DS on the Geologic Map), mostly Late Devonian but including Early Mississippian beds in 

places, that is notable not only for its conodont fauna 

(Hass, 1956) but also for radiometric dating of its 

uraniferous shales at 350 m.y. It lies with a hiatus on 

earlier Devonian rocks, which it oversteps to rest on 

stratf as old as Ordovician, producing a low-angle regional unconformity traceable as far west as 

Oklahoma.

CONTINENTAL DEPOSITS (D2C, Dsc)

As indicated above, the proximal part of the Middle 

and Upper Devonian clastic wedge in New York and

Pennsylvania is formed of continental deposits 

conglomerates, coarse sandstones, and redbeds that include a fossil forest at Gilboa, N.Y., of late Middle Devonian (Tully) age. The continental deposits project in 

the heights of the plateaulike Catskill Mountains that 

overlook the Hudson Valley. The extent of the continental deposits indicated on the Geologic Map is based 

on the Geologic Maps of New York and Pennsylvania. 

Small patches of little-deformed red continental deposits, in part plant bearing, also occur within the Appalachian foldbelt in eastern Maine, which are 

younger than the Acadian orogeny and lie on upended 

Devonian and earlier Paleozoic eugeosynclinal rocks. 

The Mapleton Sandstone of Aroostook County is late 

Middle Devonian, and the Perry Formation of the 

Eastport district at the eastern tip of Maine is Late 

Devonian; the ages of other occurrences are less certain. For convenience, all are grouped as Middle Devonian (D2C) on the Geologic Map.

EUGEOSYNCLINCAL DEPOSITS (De)

Devonian eugeosynclinal deposits and associated 

volcanic rocks (Dv) are prominent in New England, but 

none are known in the Piedmont province of the Central and Southern Appalachians. All the eugeosynclinal Devonian rocks in New England are Lower Devonian (or no younger than low Middle Devonian) in 

contrast to the scanty Lower Devonian Series in the 

miogeosyncline and craton to the west and mark the 

final climax of eugeosynclinal subsidence and 

sedimentation prior to the Acadian orogeny. Devonian 

deposits, well dated by fossils, form broad outcrop belts 

in Vermont, New Hampshire, and Maine; narrower 

more metamorphosed extensions have been traced into 

Massachusetts and Connecticut.

Devonian phyllites and schists form a belt 25 mi 

(40 km) wide in the trough of the Connecticut Valley 

synclinorium in eastern Vermont and are as much as 

15,000 to 20,000 ft (4,500-6,000 m) thick. The western 

part (Waits River Formation) is calcareous and the 

eastern part (Gile Mountain Formation) is quartzose; 

they have complex mutual relations, partly from intergradation, partly from major nappe structures. 

Metavolcanic rocks are rare, except for an amphibolite 

layer in the eastern part and fossils are rather sparse. 

Equivalents extend southward in a narrower belt into 

Massachusetts, where they include the long-known 

fossil locality at Bernardston. A small patch 

(Wepawaug Schist=upper part of Orange Phyllite of 

earlier usage) emerges from beneath the Triassic cover 

west of New Haven, Conn. (Fritts, 1962).

The northeastern part of the Connecticut Valley 

synclinorium in northern Maine is dominated by the 

Seboomook Formation, a mass of deep-water shaly and
LOWER PALEOZOIC 17

sandy turbidites as much as 20,000 ft (6,000 m) thick 

(Boucot, 1961, p. 169-171). However, a shoal-water 

belt extending from Moosehead Lake northeastward 

for 90 mi (140 km) to north of Mount Katahdin received sandy deposits (Tarratine and Matagamon 

Formations). Early Devonian (Oriskany and early 

Onondaga) fossils are abundant in the shoal-water deposits and sparse in the deep-water deposits. Lying on 

the shoal-water deposits at Mount Kineo, Traveler 

Mountain, and elsewhere are thick masses of rhyolitic 

volcanic rocks (Dv), probably erupted from calderas in 

an island arc (Rankin, 1968). They are succeeded by 

several thousand feet (300-600 m) of additional 

sediments the shallow-water Tomhegan Formation 

at Moosehead Lake and the brackish-water or terrestrial Trout Valley Formation at Traveler Mountain. 

The former contains Oriskany shelly fossils, the latter 

fossil plants of early Middle Devonian age. The Trout 

Valley Formation is unmetamorphosed and little deformed and is either a late orogenic or a postorogenic 

deposit.

In the Bronson Hill anticlinorium of New Hampshire, east of the Connecticut Valley synclinorium, the 

Silurian is overlain by the Littleton Formation of shaly 

and sandy rocks with a volcanic member near the middle (mainly tuffs and breccias); it has been variably 

metamorphosed to phyllite or to garnet and sillimanite 

schists and gneisses (Billings, 1956, p. 27-35). Lower 

Devonian (Oriskany) fossils are well preserved in the 

low-grade belt along the Connecticut River, and a few 

have been recovered even in the high-grade rocks 

farther east. The Littleton is about 4,500 ft (1,400 m) 

thick in the Bronson Hill anticlinorium, but like the 

preceding Silurian it thickens eastward into the Merrimack synclinorium to 16,000 ft (4,900 m).

The Devonian and Silurian rocks of the Merrimack 

synclinorium in New Hampshire continue southward 

into east-central Massachusetts, but relations here 

have been clouded by the occurrence of Carboniferous 

(Pennsylvanian?) plants at the "coal mine" near Worcester, and so for many years the whole complex of 

deposits was assigned to the Carboniferous system as 

it was on the Geologic Map of the United States of 

1932, following Emerson (1917). The main body of the 

rocks is, however, lithically very different from the authentic Pennsylvanian of the Narragansett and Boston 

basins to the southeast, and it was involved in the 

mid-Paleozoic (Acadian) deformation. Present judgment is that the rocks at the "coal mine" are merely a 

remnant of a younger formation enclosed technically 

in much older rocks. On the Geologic Map, Pennsylvanian (IP) is marked in a small patch at the fossil locality, and the surrounding rocks are classed as Silurian 

and Devonian (Se, De).

LOWER PALEOZOIC

From the Rocky Mountains westward, the Cambrian, Ordovician, Silurian, and Devonian Systems 

form such small outcrops, either singly or together, 

that they are combined on the Geologic Map of the 

United States into a unit of lower Paleozoic, with a 

distinction between marine stratified rocks (IP) and 

eugeosynclinal deposits (IPe). In a few areas the Cambrian sequence (-G) forms outcrops sufficiently extensive for representation; elsewhere it is merged with the 

other systems. The same designation is also used for 

some small outcrops of lower Paleozoic rocks farther 

east in the United States, as explained below.

MARINE STRATIFIED ROCKS (IP)

The lower Paleozoic marine stratified rocks are of 

several different kinds cratonic deposits, miogeosynclinal deposits, and miscellaneous rocks of outlying 

areas which it is appropriate to describe separately.

CRATONIC DEPOSITS

Lower Paleozoic cratonic deposits similar to those in 

the Central Interior Region extend across the Central 

and Southern Rocky Mountains, the Colorado Plateau, 

and the Basin and Range Province of New Mexico and 

Arizona; there they are exposed in narrow bands of 

tilted strata along the edges of the uplifts of Precambrian rocks (fig. 6). The Cambrian is shown separately 

in Arizona, but in some parts of the Basin and Range 

Province all the lower Paleozoic is missing.

The lower Paleozoic cratonic deposits are no more 

than a few hundred or few thousand feet (60-600 m) 

thick in any of the outcrops, and each system has its 

own pattern of distribution and thickness independent 

of the others, reflecting in part the shifting through 

time of the epicontinental seas (fig. 7). Sequences at 

any locality are thus incomplete, lacking one or more 

systems or parts of systems. Details of distribution and 

thickness of the systems in outcrop and subsurface are 

illustrated in the "Geologic Atlas of the Rocky Mountain Region" (Mallory, 1972), to which the reader is 

referred. Especially notable is the complete absence of 

Silurian rocks from any outcrop area except in southern New Mexico, although its former presence in 

places is suggested by remnants preserved in diatremes in northern Colorado.

Equally interesting are the areas where the lower 

Paleozoic rocks are missing entirely and upper 

Paleozoic or lower Mesozoic rocks lie directly on Precambrian. Some of these areas, such as the Front 

Range in Colorado and Wyoming and the Uncompahgre Plateau in western Colorado, were the sites of 

geanticlines that were raised in later Paleozoic time,
18 PALEOZOIC AND MESOZOIC ROCKS

T

FIGURE 6. Western United States, showing areas mapped as lower Paleozoic on Geologic Map of United States. Includes units 

of marine stratified rocks (IP), eugeosynclinal deposits (IPe), Cambrian (-6), and Lower Ordovician (Oi).
LOWER PALEOZOIC 19

C SILURIAN DEVONIAN

FIGURE 7. Eastern part of Cordilleran Region, showing surface and subsurface extent of the different systems 

grouped as lower Paleozoic on Geologic Map of United States: A Cambrian (lines L, M, and U indicate 

maximum extent of Lower, Middle, and Upper Cambrian Series), B Ordovician, C Silurian, 

D Devonian. Compiled from Geologic Atlas of Rocky Mountain Region (Mallory, 1972) and other sources. '
20 PALEOZOIC AND MESOZOIC ROCKS

when older deposits, if they had ever existed, were 

eroded. Other areas, such as the block ranges of northern New Mexico, are parts of the "Transcontinental 

arch," a paleotectonic feature that extended southwestward from the Lake Superior Region, upon which 

many of the Paleozoic systems either never deposited 

or were laid down so thinly that they were removed 

later. Relations are well illustrated in the New Mexico 

ranges, where representatives of all the lower 

Paleozoic systems occur in the south but with each one 

thinning and wedging out northward toward the site of 

the arch until none remain.

Westward from southern New Mexico, only Cambrian rocks and a thin Devonian formation (Martin 

Limestone) persist into Arizona. Here, it is appropriate 

on the Geologic Map to represent the Cambrian(-e)but 

to merge the Devonian with the Mississippian and 

Pennsylvanian into a unit of upper Paleozoic (uP).

MIOGEOSYNCLINAL DEPOSITS

The miogeosynclinal lower Paleozoic rocks form all 

or large parts of many of the ranges in the Northern 

Rocky Mountains and the eastern Great Basin. Here, 

the Cambrian System can be separated at many places 

on the Geologic Map.

The miogeosynclinal deposits do not differ in either 

lithology or origin from the cratonic deposits, but they 

are greatly thicker and have a more complete sequence. In Utah, the change from craton to 

miogeosyncline takes place near the present western 

edge of the Colorado Plateau along the "Wasatch 

line" a tectonic boundary with ancient antecedents. 

West of it in the Great Basin, Lower Cambrian strata 

wedge in at the base of the sequence, and all the succeeding lower Paleozoic deposits thicken; Silurian 

rocks, so notably missing from the craton to the east, 

make their appearance. Contrasts between the two 

lower Paleozoic sequences have been further emphasized during the Cretaceous Sevier orogeny, when 

the two were telescoped along great thrusts along the "line."

In the Great Basin of western Utah and eastern 

Nevada, Middle Cambrian through Devonian sequences are characteristically 10,000 to 15,000 ft 

(3,000-4,500 m) thick and overlie 3,000 ft (900 m) or 

more of Lower Cambrjan clastic deposits. The longknown sequence at Eureka, east-central Nevada, is 

14,500 ft (4,300 m) thick and is composed of 60 percent 

limestone, 30 percent dolomite, 8 percent shale, and 2 

percent quartzite (Nolan and others, 1956). A notable 

sandy unit above the basal elastics is the Middle Ordovician Eureka Quartzite (= Swan Peak Quartzite) 

about 300 ft (100 m) thick, which spreads across most 

of the eastern half of the Great Basin, like the nearly

contemporaneous Saint Peter Sandstone of the Central 

Interior, and like the Saint Peter is derived from areas 

of crystalline rocks in the craton. Not only are the 

Cambrian and Lower Ordovician rocks of carbonate 

facies, as in the Appalachian miogeosyncline, but also 

so are the younger lower Paleozoic systems; the strata 

above the Eureka Quartzite in the Eureka district 

(Upper Ordovician, Silurian, and Devonian) are about 

6,000 ft (1,000 m) of limestone and dolomite.

Some of the lower Paleozoic miogeosynclinal strata 

are involved in a highly metamorphosed plastically deformed infrastructure ( and IP, with metamorphic 

overprint) which emerges in windows from beneath the 

less altered Paleozoic rocks in the Ruby Range, northeastern Nevada, and in tectonically similar situations 

to the east and north.

Mapped with the lower Paleozoic miogeosynclinal 

rocks is the so-called "transitional assemblage" of formations (-G and IP) which are tectonically entangled 

with rocks of the eugeosynclinal or "western assemblage" east and northeast of Winnemucca, northcentral Nevada (Roberts and others, 1958, p. 2817), but 

which have features not entirely like either the 

miogeosynclinal or the eugeosynclinal deposits. They 

include a thick quartzite like the Lower Cambrian 

clastic deposits farther east, various overlying graptolite shales, and the remarkable Upper Cambrian Harmony Formation, an arkosic turbidite of unknown provenance which is interleaved tectonically with quite 

different Paleozoic and Mesozoic rocks in many ranges.

ROCKS OF OUTLYING AREAS

Besides the cratonic and miogeosynclinal rocks of 

the Cordilleran Region, some rocks in small areas 

farther east, in Texas, Oklahoma, and the Southern 

Appalachians are mapped as undivided lower 

Paleozoic.

The Marathon region of western Texas is a smallscale replica of the Ouachita Mountains foldbelt, and 

its Cambrian, Ordovician, and Devonian rocks are 

broadly similar to the lower Paleozoic of the Ouachitas. 

They are exposed in several anticlinoria which are too 

small on the scale of the map to permit subdivision.

In the Arbuckle and Wichita Mountains of southern 

Oklahoma, the Upper Cambrian Reagan Sandstone 

and overlying carbonates are separately mapped (-6), 

but the succeeding lower Paleozoic sequence is not 

subdivided (IP). Much of the latter is Ordovician, 

which includes the very thick carbonates of the Lower 

Ordovician Arbuckle Group, but thin Silurian and Devonian units (Hunton Group and Woodford Chert) 

occur at the top.

The designation lower Paleozoic (IP) is used for 

rocks in a few outcrop belts in the Piedmont province of
LOWER PALEOZOIC 21

the Southern Appalachians; their precise ages are uncertain, but they appear to be younger than the rocks 

adjacent to them. Their occurrence is as follows:

(1) Slates and phyllites in the Talladega belt, 

Alabama, below the Cheaha Sandstone and associated 

fossiliferous rocks (DS); they may be Ordovician, but 

both older and younger ages have been claimed.

(2) The Wedowee Formation and Ashland Schist in 

the next belt to the southeast, composed of rocks like 

those in the lower part of the Talladega sequence but 

more metamorphosed and more involved with plutonic 

rocks.

(3) Rocks of the Wacoochee (= Pine Mountain) belt 

of the southern Piedmont in Georgia and Alabama 

(Hollis Quartzite, Chewacla Marble, and Manchester 

Schist), which lie with apparent unconformity on orthogneisses (Ygn) that have yielded 1,000-m.y. 

radiometric dates.

(4) Rocks of the Murphy Marble Belt in northeastern Georgia and southwestern North Carolina. They 

overlie and are synclinally downfolded into the Ocoee 

Supergroup (Z) and include the Murphy Marble near 

the top. The marble has been correlated with the 

Lower Cambrian Shady Dolomite on physical resemblance, and a few poorly preserved Paleozoic fossils 

have been recovered from it.

(5) Low-grade schists of the Chauga belt on the 

southeastern side of the Brevard fault zone in northwestern North Carolina.

(6) Rocks of the Kings Mountain belt on the South 

Carolina-North Carolina border east of the Brevard 

fault zone, which include distinctive units of schist and 

quartzite, and the Gaffney Marble.

EUGEOSYNCLINAL DEPOSITS (IPe)

Lower Paleozoic eugeosynclinal deposits occur 

mostly in Nevada and California, ttie only exceptions 

shown on the Geologic Map being a few small areas in 

south-central Idaho. In contrast to the carbonatequartzite facies of the miogeosynclinal belt to the east, 

they are a facies of elastics, cherts, and volcanics. All of 

them are either continental margin or "off the continent" deposits, but they probably formed in diverse 

environments, which are difficult to reconstruct because of the wide separation of the different groups of 

exposures.

The eugeosynclinal rocks in Nevada crop out in the 

ranges of the Great Basin in a belt 80 mi (130 km) wide 

that extends south-southwest across the center of the 

State from the Idaho border to the California border. 

For more than half this breadth, the eugeosynclinal 

rocks are allochthonous on the contemporaneous 

miogeosynclinal rocks along a major surface of movement, the Roberts thrust, which formed during the Antler orogeny of late Devonian and early Mississippian 

time. The miogeosynclinal rocks appear as windows in 

the different ranges, partly or wholly surrounded by 

the overlying eugeosynclinal rocks. The lower 

Paleozoic sequence is overlain by postorogenic Mississippian and Pennsylvanian deposits (Diamond Peak 

Formation, Battle Formation, and others).

The lower Paleozoic eugeosynclinal sequence in 

Nevada is probably as much as 50,000 ft (15,000 m) 

thick, although not all of it is preserved in any one area 

(Roberts and others, 1958, p. 2816-2817). On the average, shale constitutes 20 to 40 percent; sandstone, 

graywacke, and quartzite 10 to 30 percent; and volcanic rocks from a few percent to 30 percent. The volcanic rocks are andesitic and basaltic pillow lavas and 

associated pyroclastics. The most extensive component 

is of Ordovician age (the shaly Vinini Formation and 

the equivalent sandy Valmy Formation farther northwest). Rocks of other ages occur only in smaller areas. 

The Cambrian (Scott Canyon Formation) is represented only in a small outcrop in the south part of 

Battle Mountain. The Silurian and Devonian rocks 

(Elder Sandstone and Slaven Chert) are typically developed in the northern Shoshone Range, although 

they occur sporadically elsewhere. Graptolites are 

common in the Ordovician and Silurian and are the 

chief means of zonation and correlation; other fossils 

are more sparse, both here and in the lower and higher beds.

In the Sierra Nevada of eastern California, lower 

Paleozoic eugeosynclinal rocks occur both east and 

west of the Jurassic and Cretaceous granitic rocks of 

the Sierra Nevada batholith in the core of the range. 

Those on the eastern side, west and northwest of 

Owens Valley, are preserved only in roof pendants up 

to 19,000 ft (5,800 m) thick and are mostly Ordovician 

(Rinehart and others, 1959, p. 941-944). Those on the 

west side extend southward along the foothills for more 

than 100 mi (160 km) from Taylorsville past Placerville and are collectively termed the Shoo Fly Formation (although other names have been given to different parts in the past) (Clark and others, 1962; McMath, 

1966, p. 178-179). Fossils of Silurian age have been 

obtained in one area near Taylorsville; other parts are 

barren but might include Ordovician as well as Silurian rocks. The Shoo Fly attains a thickness as great as 

50,000 ft (16,000 m), with no base visible, and consists 

of weakly metamorphosed phyllite and minor chert, 

siltstone, and quartzose sandstone, and some tuff and 

greenstone. It is overlain unconformably toward the 

northeast by the dacitic volcanics of the Sierra Buttes 

Formation which contain Devonian ammonoids in 

quartzite lenses (Anderson and others, 1974). On the 

west it is adjoined by the Calaveras Formation which
22 PALEOZOIC AND MESOZOIC ROCKS

is generally considered to be upper Paleozoic, but the 

contact is mostly faulted.

In northern California, lower Paleozoic eugeosynclinal rocks occur in the eastern subprovince of the 

Klamath Mountains, where they are so entangled with 

ultramafic rocks and Jurassic granitic plutons that 

only fragments are preserved in any area and no complete sequence is known. In the north, west of Mount 

Shasta, are the Duzel and Gazelle Formations (Hotz, 

1971, p. 7-8) of shale, volcanic graywacke, chert, and 

lenticular limestone, the one in thrust contact with the 

other. Shelly fossils (corals, brachiopods, and trilobites) in the limestone lenses indicate that the Duzel is 

Late Ordovician(?) and the Gazelle is Silurian. No 

younger Paleozoic rocks are known in the area. 

Farther south, in the Redding area, is a mass of Devonian volcanics the Copley Greenstone that includes 

andesitic pillow lavas, and the local overlying body of 

Balaklala Rhyolite. They are capped by cherty shales 

and local limestone banks of the Kennett Formation 

with Middle Devonian fossils, shoal water deposits 

that probably accumulated on the crest of a volanic 

island arc. The Devonian rocks are overlain by the 

Mississippian Bragdon Formation, but the contact is 

seemingly tectonic.

The central metamorphic belt of the Klamath Mountains is a thrust slice west of and tectonically beneath 

the rocks of the eastern subprovince. It is formed of 

Salmon Hornblende Schist and Abrams Mica Schist, 

which have yielded K/Ar radiometric ages of 270-329 

m.y. and Rb/Sr ages of 380 m.y., indicating that the 

original rocks are Devonian and older and had a protracted mid-Paleozoic metamorphic history (Hotz, 

1971, p. 10-11). They might be metamorphic equivalents of some of the lower Paleozoic eugeosynclinal 

rocks farther east, and they are indicated on the 

Geologic Map as IPe, with metamorphic overprint.

MISSISSIPPIAN

The Mississippian System (M) is portrayed on the 

Geologic Map of the United States in the eastern twothirds of the country; from the Rocky Mountains westward it is merged with the other upper Paleozoic systems into a single unit (uP, uPe). Within the region 

where it is mapped, the Mississippian is classed as 

marine stratified rocks; some continental deposits 

occur in the northeastern part of the Central Appalachians but are not separated. No eugeosynclinal deposits are distinguished; they are missing in the Appalachian Region, where eugeosynclinal conditions 

were terminated by the Devonian Acadian orogeny. 

Eugeosynclinal deposits are known only in the extreme western part of the United States, where those of

Mississippian age are merged with the rest of the 

upper Paleozoic (uPe).

The Mississippian System is essentially the same as 

the Lower Carboniferous of Europe, and the differences 

are not fundamental; the top of the Lower Carboniferous is placed between the Visean and Namurian 

Stages, at a level within the American Chesterian 

Series (Weller and others, 1948, p. 107-109). The U.S. 

Geological Survey defers to European usage by designating the Mississippian and Pennsylvanian as "the 

Carboniferous Systems." The American Mississippian 

is divided into the Kinderhookian, Osageian, Meramecian, and Chesterian Series, the first two being combined on the Geologic Map as Mi, the other two being 

labeled M2 and Ma. The first two have also been called 

Lower Mississippian and the second two Upper Mississippian, but these are not useful for purposes of the 

Geologic Map.

As with the other Paleozoic systems, the Mississippian is most extensively exposed in the cratonic area of 

the Central Interior Region, where it forms broad 

bands of gently tilted strata between the crests of the 

domes and the depths of the basins (fig. 8). From the 

type region along the upper Mississippi River in Iowa 

and Illinois, outcrops are nearly continuous southwestward around the Ozark dome, southeastward 

around the Illinois basin around the Nashville and 

Cincinnati domes, and on the flank of the Allegheny 

synclinorium. A large detached area in the lower 

peninsula of Michigan surrounds the Michigan basin. 

Where the outcrop belts are sufficiently wide, the system is divided into the three parts (Mi, M2, Ms).

Mississippian rocks also form narrow outcrop bands 

throughout the miogeosynclinal belt of the Central and 

Southern Appalachians and occur farther southwest in 

the deformed areas of the Ouachita and Arbuckle 

Mountains of Arkansas and Oklahoma and the 

Marathon region of western Texas.

The Mississippian strata are generally conformable 

with the Devonian; the black shales in the upper part 

of the latter are followed by black shales in the Kinderhookian series. Throughout much of the Interior 

Region, Mississippian rocks are unconformable beneath the Pennsylvanian, the discordance being most 

prominent toward the north, where the Chesterian 

series is missing at the top of the Mississippian and the 

Morrowan and Atokan series are missing at the base of 

the Pennsylvanian. In Iowa and northern Illinois, the 

Pennsylvanian sequence bevels the Mississippian at a 

low angle, to the west lying on different subdivisions of 

the Mississippian sequence and farther east on rocks of 

earlier ages down to the Ordovician.

The Mississippian sequence is 6,000 ft (1,800 m) or
MISSISSIPPIAN 23

GULF OF M

FIGURE 8. Eastern United States, showing areas mapped as Mississippian on Geologic Map of United States. Includes 

units of marine stratified rocks (M) and their subdivisions (Mi, M2, Ma).
24 PALEOZOIC AND MESOZOIC ROCKS

more thick in the Appalachian miogeosyncline. To the 

northeast, in eastern Pennsylvania, it is largely continental (Pocono and Mauch Chunk Formations), with a 

few thin coal beds in the lower part. Southwestward 

along the strike it is marine, with much limestone in 

the middle (in the Meramecian series) and with shales 

and sandstones below and above. In Alabama, the 

lower part of the Chesterian series is the Floyd Shale, 

which oversteps eastward in the Valley and Ridge 

province onto rocks as old as Early Ordovician. The 

upper part is the Parkwood Formation, a sandy unit 

which may extend conformably upward into the basal 

Pennsylvanian.

Westward into the Central Interior, the Mississippian rocks thin to little more than a few thousand feet 

(300-600 m) and are mainly limestone. Oolitic limestones in the Meramecian series near Bedford, Ind., are 

extensively quarried for building stone. The Kinderhookian and Osageian limestones of Iowa and Illinois are famous for their crinoids. Farther south, the 

Osageian carbonates are siliceous in such units as the 

Fort Payne Chert near the Nashville dome and the 

Boone Chert on the south flank of the Ozark dome. The 

Chesterian Series at the top is more heterogeneous 

than the rest, with much sandstone and many little 

units of shale and limestone.

Near the common corners of Missouri, Oklahoma, 

and Arkansas, there are discrepancies between the 

State Maps in representation of the subdivisions of the 

Mississippian System. In southwestern Missouri the 

map shows Osageian (cherty limestones) (Mi), 

Meramecian (limestones) (M2), but no Chesterian (Ma). 

In northeastern Oklahoma the map shows Boone Chert 

(presumably Osageian), followed by Chesterian rocks; 

much the same units continue eastward into Arkansas. 

The Chesterian series wedges out northeastward near 

the northeastern corner of Oklahoma; the Meramecian 

of Missouri must continue southwestward into the 

upper part of the Boone Chert, but its extent is uncertain. On the Geologic Map, the Meramecian series is 

shown as wedging out southwestward in Oklahoma, 

beyond which Osageian rocks (Mi) are shown in contact with Chesterian (Ma); rocks of Meramecian age 

may be present also but are not represented on the 

Geologic Map.

A very different facies of the Mississippian sequence 

from those considered so far occurs in the foldbelt of the 

Ouachita Mountains in Arkansas and Oklahoma and 

in its outlier in the Marathon region of western Texas. 

Here, novaculite is succeeded abruptly by a great 

flysch sequence. The novaculite is a condensed sequence; conodonts in the Arkansas Novaculite indicate 

that it includes all the Devonian Period and the early 

Mississippian (Kinderhookian) as well. The flysch, by

contrast, is a very thick body of clastic turbidites, laid 

down in a rapidly subsiding trough. In the Ouachita 

Mountains the lower two formations, the Stanley 

Shale and Jackfork Sandstone, are as thick as 22,000 ft 

(6,700 m), thinning to the north. In the northern part 

of the mountains, the succeeding Johns Valley Shale 

and Atoka Formation are nearly as thick. Equivalent 

formations in the Marathon region are thinner, although still of impressive proportions.

The age of the lower part of the flysch sequence has 

long been disputed. At one time or another, the whole 

has been called Pennsylvanian by some, and all the 

sequence into the lower part of the Johns Valley Shale 

has been called Mississippian by others. Fossils are 

scarce and have been variously interpreted. Plant remains, broken and transported, have yielded ambiguous testimony (Miser and Hendricks, 1960, p. 1831). 

However, conodonts in the lower part of the Stanley 

are clearly Meramecian (Hass, 1950), and other fossils 

higher up are Chesterian; goniatites in the Jackfork 

are Morrowan (Gordon and Stone, 1969). The Mississippian fossils in the Johns Valley Shale are in transported blocks, derived from the Caney Shale in the 

foreland to the northwest.

On the Geologic Map, the Stanley Shale is labeled 

Mississippian (M) on the assumption that it is of 

Meramecian and Chesterian age, and the Jackfork 

Sandstone is labeled earliest Pennsylvanian (IP ia), or 

of Morrowan age. The Tesnus Formation of the 

Marathon region includes equivalents of both the 

Stanley and Jackfork; it contains Mississipian conodonts in the lower part and Pennsylvanian plants in 

the upper part. For convenience, all the Tesnus is 

labeled Mississippian (M) on the Geologic Map.

PENNSYLVANIAN

The Pennsylvanian System (IP) is portrayed on the 

Geologic Map of the United States in the eastern twothirds of the country; from the Rocky Mountains westward it is merged with the other upper Paleozoic systems into a single unit (uP, uPe). Within the region 

where it is mapped, the Pennsylvanian is classed as 

marine stratified rocks, even though it includes coal 

measures and other land-laid deposits, especially toward the east. No eugeosynclinal deposits are distinguished; they are known only in the extreme western 

part of the country, where those of Pennsylvanian age 

are merged with the rest of the upper Paleozoic (uPe).

The Pennsylvanian Period is essentially the same 

as the Upper Carboniferous of Europe; the Namurian 

Stage extends a little lower than the Pennsylvanian, 

into the American Chesterian Series. The Namurian, 

Westphalian, and Stephanian Stages, and others of 

European usage, are not widely referred to in North
PENNSYLVANIAN 25

America. Instead, classification is based on two sets of 

subdivisions the coal measures sequence in the east 

consisting of the Pottsville, Allegheny, Conemaugh, 

and Monongahela Groups and the marine sequence in 

the west comprising the Morrowan, Atokan, Desmoinesian, Missourian, and Virgilian Series. Correlations between the two sets of units are known. The 

Morrowan and Atokan are approximately Pottsville; 

and the Desmoinesian, Missourian, and Virgilian are 

approximately Allegheny, Conemaugh, and Monongahela, respectively, except that the Conemaugh embraces somewhat more strata above and below than 

the Missourian. On the Geologic Map the two sets of 

names are used interchangeably; the Morrowan, Atokan, and Pottsville are indicated as IP i, the succeeding 

units as IP 2, IP 3, and IP 4. The Pennsylvanian has also 

been subdivided into Lower, Middle, and Upper 

Pennsylvanian, the Lower Pennsylvanian being the 

Morrowan, the Middle Pennsylvanian the Atokan and 

Desmoinesian, and the Upper Pennsylvanian the Missourian and Virgilian; this is not useful for purposes of 

the Geologic Map.

Exposures of the Pennsylvanian sequence are 

mainly in the cratonic area, where it forms the centers 

of the basins and is the youngest Paleozoic rock preserved (fig. 9). Along the front of the Appalachian 

foldbelt, it forms the trough of the Allegheny 

synclinorium in Pennsylvania, West Virginia, Kentucky, and Alabama. A little of the Permian Dunkard 

Group overlies it in the north, and some outliers of 

Pennsylvanian rocks are preserved to the east in the 

folds of the Valley and Ridge province. Farther west, 

the Pennsylvanian forms a large area in the Illinois 

basin of Illinois, Indiana, and Kentucky, and a smaller 

area in the center of the Michigan basin. West of the 

Mississippi River it underlies an even larger area that 

extends from Iowa to Texas, with an extension eastward in the Arkoma basin of Oklahoma and Arkansas 

between the Ozark dome and the Ouachita Mountains 

foldbelt; the strata are mostly tilted westward beneath 

Permian rocks in the Prairie Plains homocline. All 

these areas were originally more nearly continuous; 

some of them are still almost connected by intervening 

outliers, and the sequences in the different areas can 

be closely matched with each other.

The Pennsylvanian System has been intensively 

studied, partly for its great deposits of coal toward the 

east and for its importance in petroleum exploration 

farther west. Its continental and marine deposits have 

an unprecedented multiplicity of lateral and vertical 

variations, accompanied by extreme persistence of 

many thin sedimentary units and a remarkable cyclic 

pattern of deposition in many regions, all suggestive of 

extreme tectonic stability. The record of land plants

and marine animals of the time is voluminous (Moore 

and others, 1944, p. 659).

The Pennsylvanian sequence is generally unconformable on the underlying Mississippian, especially in 

the northern Midwestern States, where it truncates 

the various subdivisions of the Mississippian at a low 

angle and extends downward across strata as old as 

Ordovician. Conformity with the Mississippian rocks 

occurs only along the edges of the Appalachian and 

Ouachita foldbelts. The Pennsylvanian is generally 

conformable with the Permian above in the Allegheny 

synclinorium and Prairie Plains homocline, and there 

have been considerable differences through the years 

in the placement of the boundary between the two systems.

The Pennsylvanian sequence is about 3,000 ft 

(900 m) thick in the Allegheny synclinorium, thinnest 

in the north and thicker southwestward mainly because of increase in the Pottsville Group at the base. In 

Alabama, Pennsylvanian rocks are 10,000 ft (3,000 m) 

thick, with the Pottsville alone represented. In most of 

the Illinois basin to the west the sequence is less than 

2,000 ft (600 m) thick, but it thickens to the south .to 

about 3,000 ft (900 m). West of the Mississippi River, 

the Pennsylvanian rocks are less than 2,000 ft (600 m) 

thick in the northern part of the Prairie Plains homocline, but they thicken southward in Oklahoma and 

Arkansas, mainly by wedging in of the Morrowan and 

Atokan Series at the base. Along the front of the 

Ouachita Mountains in Arkansas, the Atoka Formation alone is 18,000 ft (5,500 m) thick, but it thins 

rapidly northward across the Arkoma basin to a featheredge along the front of the Ozark dome 80 mi (130 

km) to the north.

The Pennsylvanian rocks are dominantly nonmarine 

and coal bearing to the east and dominantly marine to 

the west, but the two types of deposits are complexly 

interfingered in very thin units. Through much of the 

Interior Region, the sequence consists of cyclical units, 

or cyclothems. In the Illinois basin, a typical cyclothem 

begins with sandstone, resting on a channeled surface, 

followed by sandy shale, freshwater limestone, underclay, and coal, which is overlain by gray and black 

shale and marine limestone (Wanless, 1962, p. 49-50). 

In the Allegheny synclinorium, nonmarine elements 

dominate and the coals are thicker, but fossiliferous 

marine limestones persist into Ohio and western 

Pennsylvania (Branson, 1962, p. 199). In Kansas, each 

cyclothem is dominantly marine shale and various 

kinds of limestone, but a coal bed sometimes occurs in 

the lower part (Merriam, 1963, p. 103-108). Each cyclothem and its subdivisions are traceable for scores or 

hundreds of miles. The ultimate cause of the cyclical 

sedimentation is uncertain, but the rock record indi-
26 PALEOZOIC AND MESOZOIC ROCKS

GULF OF

FIGURE 9. Eastern United States, showing areas mapped as Pennsylvanian on Geologic Map of United States. Includes 

units of marine and nonmarine stratified rocks (P) and their subdivisions (Pi, Pia, P2, Pa, P4).
PERMIAN 27

cates fluctuating deposition of level surfaces near sea 

level under conditions of great stability.

In the northern Midwestern States, sedimentological 

studies indicate that the sandstone beds were derived 

from the north, from crystalline areas in the Canadian 

Shield, and some of the channels in which they were 

transported have been traced. Farther south and 

southeast, a greater proportion of the elastics was derived from the Appalachian and Ouachita foldbelts.

In Alabama, the thick Pottsville sequence is all 

shallow-water, with coal beds at intervals throughout. 

It joins in the subsurface beneath the Mississippi Embayment with the equally thick or thicker Atoka Formation of the Arkoma basin, which is a deep-water 

flysch or turbidite that filled a rapidly subsiding 

trough along the northern edge of the Ouachita 

foldbelt. A depth change between the two is indicated 

and is duplicated in the Atoka itself, which changes 

northward toward the Ozark uplift into a shallowwater deposit more like the Pottsville.

The underlying Jackfork Sandstone ( P ia) and Johns 

Valley Shale, of Morrowan age, are also flysch facies. A 

remarkable feature of the Johns Valley in the frontal 

belts of the Ouachita Mountains is the occurrence of 

wildflysch, or beds containing blocks and slabs a 

hundred feet or more (30-60 m) across of older formations of the cratonic sequence to the north and northwest. Similar block beds occur in the Marathon region 

of western Texas, but at a somewhat higher level, in 

the Haymond Formation of Atokan age.

In the Arkoma and Ardmore basins north and west 

of the Ouachita Mountains, an important discontinuity 

is indicated at the top of the Atokan Series by a moderate unconformity at the base of the Desmoinesian and 

by the occurrence in the Desmoinesian of conglomerates derived from the Ouachita Mountains. These 

suggest that much of the deformation in the Ouachita 

foldbelt had been completed by the end of Atokan time. 

This is confirmed by drilling in the Coastal Plain south 

of the Ouachita Mountains, where fossiliferous Desmoinesian carbonates lie undeformed on steeply folded 

Ouachita rocks.

The Pennsylvanian sequence in north-central Texas 

is separated from that in the remainder in the Prairie 

Plains homocline to the north by overlapping Cretaceous deposits but is broadly similar. It has long been 

divided into the Bend, Strawn, Canyon, and Cisco 

Groups. The Bend Group (including the prominent 

Marble Falls Limestone) is of Morrowan and Atokan 

age and is unconformable beneath the Strawn Group 

which overlaps it from the east. The Strawn and Canyon Groups are broadly equivalent to the Desmoinesian and Missourian Series, but the original Cisco

Group has now been partitioned between the Virgilian 

Series and the basal Permian Wolfcampian Series.

Most of the outcrops of Pennsylvanian age have been 

divided on the Geologic Map into the four divisions 

(FPi, IP2, IPs, and B*4), but for various reasons the 

Pennsylvanian is not divided in the smaller outlying 

areas. In the Marathon region of western Texas, the 

undivided Pennsylvanian (IP) includes the Dimple 

Limestone and Haymond Formation, which are flysch 

deposits of Morrowan and Atokan age, and the thinner 

shallow-water Gaptank Formation that embraces the 

rest of the Pennsylvanian System into the Virgilian 

Series. In Pennsylvania and Maryland the Pennsylvanian is undivided in the outlying downfolds in the Valley and Ridge province the Anthracite basins, the 

Broadtop basin, and the Georges Creek basin. The 

subdivisions of the main Pennsylvanian area are recognizable in these outliers, but their outcrops are too 

narrow for representation on the scale of the Geologic 

Map.

Also shown as undivided Pennsylvanian (IP) are the 

rocks of the Narragansett and Boston basins in southeastern New England, whose precise correlation with 

the standard sequences is less certain. They lie unconformably on earlier Paleozoic and Precambrian 

metamorphic and plutonic rocks and are postorogenic 

to their deformation (Quinn and Oliver, 1962). However, they themselves are steeply folded and along the 

south coast are intruded by the late Paleozoic Narragansett Pier Granite (Pgs). The rocks of the Narragansett basin are at least 10,000 ft (3,000 m) thick 

and consist largely of sandstone and shale of gray to 

black color, with some redbeds in the lower part in the 

north. Conglomerates composed of pebbles and boulders of the surrounding rocks occur at the base and at 

intervals higher up in the sequence, and there are a 

few beds of coal, largely altered to graphite. Fossil 

plants are fairly abundant and indicate a middle 

Pennsylvanian age. The rocks of the Boston basin to 

the northeast are 5,000 ft (1,500 m) or more thick and 

differ somewhat from those of the Narragansett basin. 

A few late Paleozoic plants have been collected but do 

not furnish data for firm correlation. The rocks of the 

Boston basin may be older than those of the Narragansett basin. A younger, or Permian, age has been 

claimed for them on the basis of an alleged tillite 

(Squantum Tillite Member of Roxbury Conglomerate), 

but this deposit is probably neither Permian nor a 

tillite.

PERMIAN

The Permian System (P) is shown on the Geologic 

Map of the United States in a small area in the Ap-
28 PALEOZOIC AND MESOZOIC ROCKS

palachian Region, in extensive areas in the Midcontinent Region, and in parts of the Cordilleran Region. 

Elsewhere in the Cordilleran Region, from the Rocky 

Mountains westward, it is merged with the remainder 

of the upper Paleozoic (uP, uPe). Most of it is classed 

as marine stratified rocks, although some of it is actually of continental origin. In a few areas in California, 

Permian eugeosynclinal deposits (Pe) are separated 

from the remainder of the upper Paleozoic. In the Midcontinent Region and Colorado Plateau, the Permian is 

divided into its four series (Pi, Pz, Pa, ?4), and some of 

these are divided into smaller subdivisions (Pza, Pzb, 

Paa, Pab). the meaning of which is explained below.

The Permian is the youngest system of the Paleozoic 

Era. Its type area is in European Russia, where it was 

proposed by Murchison in 1841 to supplant older terms 

such as Dyas that had been in use in western Europe 

for Paleozoic strata above the Carboniferous (Dunbar 

and others, 1960, p. 1764-1767). In the United States, 

the type area is in the richly fossiliferous marine strata 

of western Texas, where exposures in various mountain ranges (especially the Glass and Guadalupe 

Mountains) are divided into the Wolfcampian, Leonardian, Guadalupian, and Ochoan Series (Pi, Pz, Pa, ?4) 

(Adams and others, 1939). The Permian has also been 

divided into Lower and Upper Permian (with a somewhat uncertain boundary between them in the United 

States), but these are not useful for the Geologic Map.

The position of the base of the Permian has fluctuated through the years, the common tendency being 

to move it downward. As originally defined by Murchison, the lower division was the Kungurian Series, and 

the underlying Artinskian Series was included in the 

Carboniferous System. Later it was realized that the 

Artinskian was, in fact, younger than any Carboniferous rocks in western Europe, and so it was accordingly 

added to the Permian System. At present, the base of 

the Permian is placed below the Sakmarian Series 

(originally a lower division of the Artinskian), which is 

equivalent to the Wolfcampian in the United States. 

This basal unit is especially characterized by the zone 

of the fusulinid Pseudoschwagerina.

At most places the Permian sequence is conformable 

on the underlying Pennsylvanian. The contact is unconformable in parts of west Texas, as a result of 

orogenies in the adjacent Ouachita foldbelt. Throughout the United States the Permian is everywhere unconformable with the overlying Mesozoic rocks and 

especially with the Triassic System where it is present. 

In the west Texas standard area, the overlying strata 

are Upper Triassic. In the Cordilleran Region farther 

west, Lower Triassic rocks are common, but the highest Permian stage is Leonardian or Guadalupian; so, a 

considerable gap exists, representing latest Permian

time. Commonly, the unconformity at the top is not 

structural, and in parts of the Cordilleran Region the 

Permian and Triassic Systems are rigidly parallel for 

long distances, even though a hiatus exists between 

them.

APPALACHIAN REGION

The Dunkard Group (Pi) occupies the deeper part of 

the Allegheny synclinorium in Pennsylvania, Ohio, 

and West Virginia as erosional remnants on top of the 

Monongahela Group (IP 4) (fig. 10). Its preserved thickness is about 1,200 ft (400 m), but it was probably 

originally much thicker. The Dunkard is dominantly 

nonmarine. A few limestone beds in the northwestern 

exposures suggest vague marine connections, but the 

remainder is nonmarine shale, partly red, and 

sandstone. A few coal beds occur but not as abundantly 

as in the Pennsylvanian System, prompting the early 

term "Upper Barren Measures" for the group. As in the 

Pennsylvanian strata beneath, the different rock types 

are arranged in cyclical order.

Fossils in the Dunkard Group are sparse (Berryhill, 

1967; Barlow, 1972). They include vertebrates, insects, 

freshwater invertebrates, and plants. Only the plants 

offer much indication of precise age; most of them are 

characteristic Pennsylvanian types, but they also include Callipteris conferta, which is commonly considered to be a Permian index fossil and is the chief basis 

for the traditional classification of the group as early 

Permian. Nevertheless, this assignment has been 

questioned by some who would prefer to place the 

group in the late Pennsylvanian. On the Geologic Map, 

the Dunkard Group is classed as early Permian (Pi), or 

approximately equivalent to the Wolfcampian Series 

farther west.

SOUTHWESTERN UNITED STATES

Permian rocks extend continuously in a wide band in 

the Midcontinent Region from Nebraska to central 

Texas, where they are part of the west-tilted Prairie 

Plains homocline (fig. 10). They are also exposed in 

west Texas and eastern New Mexico on the opposite 

flank of the intervening West Texas Permian basin. 

The central part of the basin is covered by Mesozoic 

and Cenozbic rocks, but the connection between the 

Permian rocks on the two sides has been traced in detail by closely spaced drilling. An outlying area to the 

south, in the Glass Mountains, duplicates the formations of northern west Texas. Farther west in New 

Mexico, Permian rocks are exposed on the flanks of 

many of the block mountains, and still farther west in 

northern Arizona they spread across the southern part 

of the Colorado Plateau to the western end of the 

Grand Canyon. In all these areas, dips are sufficiently
PERMIAN 29

low and outcrops correspondingly so broad that the 

Permian rocks are subdivided in detail on the Geologic 

Map.

The thickness of the Permian sequence is about 

1,000 to 3,000 ft (300-900 m) in Kansas, thinning 

northward to disappearance in Nebraska and thickening southward to 10,000 ft (3,000 m) in surface sections 

in west Texas and to as much as 18,000 ft (5,500 m) in 

the subsurface. In the Grand Canyon area to the west, 

it is about 2,000 ft (600 m) thick.

Classification of the Permian sequence in the region 

is based on standard sections in west Texas, where the 

Wolfcampian, Leonardian, Guadalupian, and Ochoan 

Series (Pi, Pz, Pa, P-0 are named and defined. All the 

series except the Ochoan at the top are open-sea 

marine deposits and are richly fossilferous. The Ochoan is an evaporite deposit, the thick Castile Anhydrite and Salado Halite, with the thin Rustler Dolomite at the top. The Rustler contains the only fossils, a 

scanty fauna of brachiopods and mollusks that are 

unmistakably Paleozoic.

Difficulties are encountered in extending the west 

Texas subdivisions outside the type area; assignment 

of beds near the middle to either the Leonardian or the 

Guadalupian Series is especially controversial, as 

explained below. The west Texas strata formed in a 

circumscribed area, the Delaware basin, that was 

nearly surrounded by carbonate barriers, culminating 

in the great Capitan Limestone reef in the upper part 

of the Guadalupian. Outside the basin, deposits were 

laid down in shallower water, in evaporite pans, or on 

low surfaces that received redbed deposits. Large 

numbers of the fossils characteristic of the basin fail to 

extend into these outside areas. Many problems thus 

exist regarding the proper classification of the much 

larger area of Permian rocks to the north, resulting in 

some uneasy compromises on the Geologic Map. These 

will probably satisfy no dedicated stratigrapher but 

are explained in the correlation diagram included at 

the bottom of the legend. In each general area, a feasible subdivision is possible, but correlations between 

the areas that are implied on the map are not necessarily realistic.

MIDCONTINENT REGION

In the Midcontinent .Region, from central Texas to 

Nebraska, Permian outcrops are continuous, and equivalent units are traceable from one end to the other. 

The only complication is near the Arbuckle and 

Wichita Mountains in southern Oklahoma and northern Texas, where the Wolfcampian and lower part of 

the Leonardian Series (upper Cisco Group and Wichita 

Group) pass into a red arkosic continental facies (Pic, 

P2ac). On the map, the southern edge of these deposits 

in Texas is drawn to include localities where vertebrate fossils are abundant. Subdivisions are difficult to 

trace through the red continental deposits. The Coleman Junction Limestone, which is the dividing marker 

between the Wolfcampian and Leonardian Series 

farther south, disappears, but a uraniferous sandstone 

near its level persists and is identifiable by radiometry 

(Chase, 1954). South of the continental deposits the 

upper part of the Cisco Group and the Wichita Group 

(here originally called the "Albany Formation") are 

marine fossiliferous shales and limestones. North of 

the continental deposits, the Wolfcampian Series (Admire, Council Grove, and Chase Groups of Kansas) is 

likewise marine shales and limestones, but the lower 

Leonardian (Sumner Group) consists of red deposits 

and includes in subsurface the thick salt beds of the 

Wellington Formation. The higher Leonardian (P2b) 

is red throughout and includes the Clear Fork Group 

in Texas and the Nippewalla Group in Kansas. These 

red deposits, and those higher up in the Permian, are 

classed with the marine deposits on the map. They consist of persistent layers of red shale, gypsum, and thin 

carbonates that are widely traceable; probably they 

were laid down on broad level surfaces which maintained at least tenuous connections with the sea.

Above these strata in the Midcontinent Region are 

two well-defined bodies of rocks, the El Reno (= Pease 

River) Group and the Whitehorse Group (Psa, Pab), 

each marking a distinct cycle of sedimentation and 

each at least locally having a disconformity at the 

base. At the base of the El Reno is sandstone (San 

Angelo or Duncan), followed by red shales (Flowerpot 

and Dog Creek) which contain variable thicknesses of 

gypsum and dolomite (Blaine). In Texas, the Blaine 

contains a sparse invertebrate fauna including the 

ammonoid Perrinites hilli. Correlation with the west 

Texas standard section is controversial. The Perrinites 

occurs no higher than the Leonardian Series, but 

downdip to the west, in the subsurface, beds equivalent 

to the Blaine contain lower Guadalupian 

parafusulinids (Parafusulina rothi). On the Geologic 

Map, the group is classed as lower Guadalupian (Psa), 

but with misgivings. The succeeding Whitehorse 

Group (Pab) is a more uniform deposit than the beds 

below, with a lighter red color, and is divided into the 

Marlow Formation (shaly) and the Rush Springs 

Sandstone. Above the Rush Springs is the Cloud Chief 

Gypsum, generally excluded from the group but part of 

the same depositional cycle. Channel sandstones in the 

Marlow Formation contain a small fauna of marine 

invertebrates (Newell, 1940) identical with those in 

the backreef upper Guadalupian rocks (Artesia Group) 

in southeastern New Mexico, and correlation with 

these beds is further assured by subsurface tracing 

across the West Texas basin.
30 PALEOZOIC AND MESOZOIC ROCKS

FIGURE 10. United States, showing areas mapped as Permian on Geologic Map of United States. Includes units of marine and 

nonmarine stratified rocks (P) and their subdivisions (Pi, P2, P2a, P2b, Pa, Paa, Pab, P4),continental deposits(Pic, P2ac),and eugeosynclinal deposits (Pe). Does not include many small outcrops in Cordilleran Region which are merged with the remainder of 

the upper Paleozoic (uP, uPe).
PERMIAN 31

GULF OF MEXICO

FIGURE 10. Continued.
32 PALEOZOIC AND MESOZOIC ROCKS

The uppermost Permian deposits of the Midcontinent Region are the redbeds of the Quartermaster 

Formation, which are discontinuously exposed in 

Texas and Oklahoma beneath Mesozoic and Cenozoic 

deposits. The basal layer in Texas, the Claytonville 

Dolomite, is traceable westward in the subsurface into 

the Salado Halite and Rustler Dolomite of the Ochoan 

Series, and the Quartermaster must be the shoreward 

edge of the great evaporite sequence of the West Texas 

Permian basin; it is shown as ?4 on the Geologic Map.

NEW MEXICO

In eastern New Mexico, rocks equivalent to those in 

the Midcontinent Region reappear on the western 

margin of the West Texas basin, where they are represented by the Abo Sandstone, Yeso Formation, 

Glorieta Sandstone, San Andres Limestone, and Artesia Group.

The Abo Sandstone is a red continental deposit containing plant and vertebrate fossils that interfingers 

southward near the Texas border with the marine 

Wolfcampian Hueco Limestone and hence is classed as 

Pic. Some marine Wolfcampian rocks persist locally at 

the base (Bursum Formation) but are not separated on 

the Geologic Map from the Pennsylvanian (uP).

The Yeso Formation is a redbed sequence, with beds 

of carbonate and evaporite, of lower Leonardian age 

(P2a). The Glorieta Sandstone is a thin deposit that 

interfingers laterally with both the Yeso and San 

Andres units. The San Andres Limestone has by far 

the largest surface area of the Permian rocks of eastern 

New Mexico and is the caprock of many mountain 

ranges and plateaus. Like the El Reno Group of the 

Midcontinent Region, to which it appears to be broadly 

equivalent, its age with respect to the west Texas 

standard sequence is controversial. Its brachiopods and 

other invertebrate fossils, including Perrinites hilli, 

are Leonardian forms, but Guadalupian parafusulinids 

occur, especially in the eastern exposures. The limestone is not divided on the source map (Geologic Map of 

New Mexico of 1965), and it is all indicated on the 

Geologic Map as Leonardian (P2b). Very likely, however, it is a composite unit. During recent mapping in 

the southeastern part of the State, Kelley (1971, p. 7- 

14) divided it into the Rio Bonito, Bonney Canyon, and 

Fourmile Draw Members. The Rio Bonito, or thickbedded lower part, is the only member represented in 

the type San Andres Mountains to the west and is the 

most likely candidate for a Leonardian age. The two 

higher members, and especially the evaporitic Fourmile Draw Member at the top, are more likely 

Guadalupian, and the Fourmile Draw seems to be 

traceable into the middle Guadalupian beds in the 

Guadalupe Mountians.

The succeeding Artesia Group (Pab) is clearly equivalent to the upper Guadalupian of the Guadalupe 

Mountains and has been traced in the subsurface into 

the Whitehorse Group of the Midcontinent Region. It is 

partitioned horizontally into five formations (Grayburg, Queen, Seven Rivers, Yates, and Tansill), but 

each of these changes facies northward from backreef 

carbonates in the Guadalupe Mountains into evaporites and redbeds in the Pecos Valley.

The overlying Ochoan evaporitic formations (P<0 in 

the southeastern corner of the State are part of the 

type Ochoan Series.

NORTHERN ARIZONA

The Permian of northern Arizona is divided into the 

Supai Formation, Hermit Shale, Coconino Sandstone, 

and the Toroweap and Kaibab Limestones.

The Kaibab and Toroweap form by far the greatest 

area of exposure, as they are the limestone caprock of 

this part of the Colorado Plateau. The Toroweap is a 

minor local unit, formed in a separate sedimentary cycle, but it is not greatly different in age from the 

Kaibab. Rather abundant brachiopods and other invertebrate fossils indicate that both are Leonardian (Pzb); 

they are seemingly equivalent to the San Andres 

Limestone in New Mexico to the east, although the 

surface continuity is broken by cover of Mesozoic 

deposits.

The three Permian formations beneath are more 

heterogeneous and have a wider range in age, but they 

occupy much smaller outcrop areas. The white 

Coconino Sandstone is a continental dune deposit 

without fossils except for vertebrate tracks. The red 

Hermit Shale below contains an abundant flora, probably of early Leonardian age. The thicker red Supai 

Formation is largely Wolfcampian. It interfingers with 

marine Wolfcampian rocks (Pakoon Formation) at the 

western edge of the plateau, but at a few places 

elsewhere the Supai includes some strata of Pennsylvanian age in the lower part. Because of the small 

outcrop area of these formations, they are all marked 

on the Geologic Map as lower Leonardian (?2a).

Northward in the Colorado Plateau in Utah, the 

Permian System emerges in small to large areas in the 

higher uplifts but is not subdivided (P). The largest 

area is in the Monument unwarp in the south part of 

Utah, where the rocks consist of units of sandstone like 

the Coconino and units of redbeds; these units are 

classed as members of the Cutler Formation, which has 

an age range nearly equal to the whole Grand Canyon 

sequence. A little limestone, classed as Kaibab but 

probably younger, appears at the top of the Permian in 

the San Rafael Swell but in areas too small to be differentiated.
UPPER PALEOZOIC 33

CORDILLERAN REGION

Marine Permian rocks, with age ranges from 

Wolfcampian to Guadalupian (= Phosphoria), are 

about 5,000 ft (1,500 m) thick in the eastern Great 

Basin of Nevada and Utah and in many places form 

sufficiently large areas to be differentiated on the 

Geologic Map (P); elsewhere, the Permian is merged 

with the remainder of the upper Paleozoic (uP).

EUGEOSYNCLINAL DEPOSITS

Permian eugeosynclinal deposits in the western part 

of the Cordilleran Region are mostly merged with the 

other upper Paleozoic deposits (uPe) or are included in 

the Triassic and Permian eugeosynclinal deposits 

(~fi Pe), but they are separately shown in a few areas in 

northern California.

In the Taylorsville area of the northern Sierra 

Nevada are the volcanic and volcaniclastic Arlington, 

Reeve, and Robinson Formations, which in a few places 

contain large parafusulinids of Permian age and in 

others the Permian fish Helicoprion (McMath, 1966, 

p. 179-180).

In the eastern part of the Klamath Mountains farther north are the McCloud Limestone, Nosoni Formation (mudstone and tuff), and the Dekkas andesite, 

about 6,000 ft (1,800 m) thick. All are fossiliferous; 

fusulinids and corals of Wolfcampian and Leonardian 

age occur in the McCloud, and fossils in the Dekkas are 

of Guadalupian (Capitan) age (Albers and Robertson, 

1961, p. 21-30).

UPPER PALEOZOIC

From the Rocky Mountains westward, the Mississippian, Pennsylvanian, and Permian Systems form 

such small outcrops, either singly or together, that 

they are combined on the Geologic Map of the United 

States into a unit of upper Paleozoic, with a distinction 

between marine stratified rocks (uP) and eugeosynclinal deposits (uPe). In some areas, as explained 

above, the Permian (P) forms outcrops sufficiently extensive for representation and partial subdivision; 

elsewhere, it is merged with the other systems.

MARINE STRATIFIED ROCKS (uP)

As in the lower Paleozoic systems, the marine 

stratified rocks of the upper Paleozoic systems of the 

Cordilleran Region formed partly in a cratonic area 

and partly in a miogeosynclinal area, the geographic 

positions in each being essentially the same (fig. 11). 

However, the deposits in the two areas are much more 

varied than before, reflecting, in large part, the greater 

crustal unrest of later Paleozoic time.

The shifting patterns of distribution of the different 

systems in the cratonic area are illustrated in the 

"Geologic Atlas of the Rocky Mountain Region" (Mallory, 1972; see also fig. 12). The three systems were 

originally nearly continuous in the miogeosynclinal 

area to the west, although their continuity is now broken by subsequent erosion or by burial beneath 

younger deposits.

MISSISSIPPIAN

Mississippian rocks are widely distributed in the 

cratonic area, the only exception being in parts of Colorado and northern New Mexico. In Colorado, they 

may originally have been more extensive but were removed by erosion during the disturbances of Pennsylvanian and later times; however, in much of northern 

New Mexico they were probably never deposited.

In the exposures in the mountain ranges of the 

cratonic area, the Mississippian sequence is from 500 

to 2,000 ft (150-600 m) thick. A striking feature is the 

great sheet of carbonate rocks of middle Mississippian 

age (mainly Osageian, partly Meramecian, but with 

variable upper and lower limits from place'.to place), 

typified by the Madison Limestone of the Northern 

Rocky Mountains but including the Rundle of the 

Canadian Rocky Mountains, the Pahasapa of the Black 

Hills, the Redwall of the Grand Canyon, the Escabrosa 

of southern Arizona, and other named units in local 

areas. Higher Mississippian rocks, including Chesterian, are of more local extent, and lowest Mississippian 

rocks, or Kinderhookian, are scantily developed.

In the miogeosynclinal area to the west, the Mississippian sequence is generally thicker, but in variable 

amounts, indicating mild tectonic activity. It is 7,000 ft 

(2,100 m) thick in an irregular area in northwestern 

Utah, where it represents the initial deposits of the 

Oquirrh basin, a paleotectonic feature characterized by 

thick shallow-water deposits laid down in an area of 

unusual subsidence that culminated during Pennsylvanian time. The Mississippian deposits are mainly 

limestone but include some shale units; they are notable for including an unusually complete sequence from 

Devonian to Pennsylvanian, or from Kinderhookian 

through Chesterian time. Another maximum of 7,000 

ft (2,100 m) of Mississippian strata occupies a linear 

trough farther west in central Nevada, along the edge 

of the Antler erogenic belt, whose deposits are of quite 

different character. The lower Mississippian unit 

(Joana Limestone) is inconsequential, and most of the 

sequence is middle or upper Mississippian clastic 

deposits the Chainman Shale and the overlying or 

interfingering quartzites and conglomerates of the 

Diamond Peak Formation. They are part of a clastic 

wedge derived from the erogenic belt to the west and
34 PALEOZOIC AND MESOZOIC ROCKS

FIGURE 11. Western United States, showing areas mapped as upper Paleozoic on Geologic Map of United States. Includes 

units of marine stratified rocks (uP), Mississippian rocks (M), Pennsylvanian rocks (P, Pi, Pz, IPs, P4), Permian rocks (P, 

P i, Pic, P2, P2a, P2ac, P2b, Ps, Paa, Psb, P4), clastic wedge, ge deposits (uPc), and eugeosynclinal deposits (uPe, Pe).
UPPER PALEOZOIC 35

C PERMIAN D PALEOTECTONICS

FIGURE 12. Eastern part of Cordilleran Region, showing surface and subsurface extent of the systems grouped 

as upper Paleozoic on Geologic Map and some of the paleotectonic features of the time: A, Mississippian. B, 

Pennsylvanian. C, Permian. D, Paleozoic tectonic features. Compiled from Geologic Atlas of Rocky Mountain Region (Mallory, 1972) and other sources.
36 PALEOZOIC AND MESOZOIC ROCKS

are separately shown on the Geologic Map as a clastic 

wedge facies (uPc). Also so mapped are the Mississippian Milligen Formation and the Pennsylvanian Wood 

River Formation of south-central Idaho, which are 

clastic wedge deposits derived from erogenic belts 

farther west.

PENNSYLVANIAN

Pennsylvanian deposits in the cratonic area are 

much more varied than in any of the preceding 

Paleozoic systems, owing mainly to intracratonic 

orogenic activity on the site of the Southern Rocky 

Mountains, which produced the Colorado system of 

structures, or "Ancestral Rocky Mountains" (Mallory, 

1972). Two geanticlines were raised: the Front Range 

geanticline on the site of the present Front Range and 

some ranges to the west, and the Uncompahgre geanticline that extended southeastward from the Uncompahgre Plateau of western Colorado across the site of 

the San Juan Mountains into northern New Mexico. 

The northeastern geanticline was raised somewhat 

earlier than the southwestern, although clastic deposits were shed off both of them through Pennsylvanian into Permian time. Between the two geanticlines 

was a deeply subsiding linear basin, the Central Colorado trough, which received as much as 12,000 ft 

(3,700 m) of clastic sediments during Pennsylvanian 

time.

Few structures attributable to the Pennsylvanian 

orogeny can be identified, and their record is mainly 

derived from the red clastic deposits along the flanks, 

including such units as the Fountain Formation on the 

eastern edge of the Front Range and the Sangre de 

Cristo Formation of the Sangre de Cristo Mountains 

(derived from the southeastern end of the Uncompahgre geanticline). Southwest of the Uncompahgre 

geanticline in southwestern Colorado and eastern 

Utah is the Paradox basin, another area where 

Pennsylvanian deposits are over 7,000 ft (2,100 m) 

thick; here, however, the greater part of the sequence 

is evaporite, mainly halite, but includes layers of 

potash salt.

North and south of the Colorado system in the 

cratonic area, the Pennsylvanian is no more than 1,000 

ft (300 m) or so thick and largely carbonate; it includes 

the Amsden and Tensleep Formations of Wyoming, the 

main part of the Naco Group in southern Arizona, and 

the Magdalena Group in New Mexico. In central New 

Mexico, the Magdalena rests directly on Precambrian 

rocks in most places; Mississippian rocks are sporadic 

and the lower Paleozoic sequence wedges out northward in the southern part of the State.

The Pennsylvanian sequence is not more than 2,000 

or 3,000 ft (600-900 m) thick in most of the

miogeosynclinal area, but it thickens to as much as 

13,000 ft (4,000 m) in the Oquirrh basin of northwestern Utah. Most of this sequence is the Oquirrh Formation, whose base is of Morrowan age and whose upper 

part extends into Wolfcampian time (not divided on the 

Geologic Map). The formation consists of interbedded 

limestones and sandstones, the latter made up of finegrained clean quartz sands. The sands could not have 

been derived from any orogenic area; apparently they 

came from the craton (where similar sandstones of 

lesser thickness occur) and were trapped in a unique 

restricted area of exceptional subsidence in the 

miogeosyncline.

Farther west in the miogeosyncline, in the Ely and 

Eureka areas, Nevada, the Pennsylvanian is represented by the Ely Limestone, which succeeds the 

clastic wedge deposit of the Diamond Peak Formation. 

Pennsylvanian rocks also occur in the Winnemucca 

area still farther west, where they are unconformable 

on "transitional" and eugeosynclinal lower Paleozoic 

rocks of the Antler orogenic belt. Some is limestone, 

but the conglomeratic middle Pennsylvanian Battle 

Formation is included. Also included in the upper 

Paleozoic in this area on the Geologic Map are Permian 

units, the upper part of the Antler Peak Limestone and 

the Edna Mountain Formation, the latter with high 

Permian (Phosphoria) fossils (Roberts and others, 

1958, p. 2841-2844).

PERMIAN

As indicated above, the more extensive areas of 

Permian rocks in the Cordilleran Region are separately mapped and in part subdivided; however, narrower belts of outcrop elsewhere are merged with the 

remainder of the upper Paleozoic.

Through most of the cratonic area the Permian sequence is no more than 1,000 or 2,000 ft (300-600 m) 

thick. Near the previously formed uplifts in Colorado, 

it is mostly red clastic deposits, an upward continuation of those of the Pennsylvanian, but finer grained. 

Southwestward and southward, carbonate rocks appear in the Leonardian Series (separately mapped as 

the Kaibab, Toroweap, and San Andres Limestones, 

?2b). Northward, an important component is the complex of marine deposits that includes the Park City and 

Phosphoria Formations, the first shallow-water limestone, the second deep-water phosphatic shale and 

chert. Although the older part of the complex is 

Leonardian, the main body is Guadalupian and contains many of the same marine fossils as are found in 

the standard Guadalupian section in west Texas. 

Eastward across Wyoming, the complex of marine deposits intertongues with redbeds of the Chugwater and 

Goose Egg Formations.
UPPER PALEOZOIC 37

In the miogeosynclinal belt to the west, thicknesses 

of Permian rocks as great as 10,000 ft (3,000 m) are 

recorded in parts of northwestern Utah, mostly an upward continuation of the Oquirrh Formation, and are 

included in the upper Paleozoic (uP) on the Geologic 

Map. Somewhat thinner sequences elsewhere in western Utah and in eastern Nevada form prominent areas 

of outcrop and are shown as undivided Permian (P). 

The westernmost of these, the Carbon Ridge and Garden Valley Formations of the Eureka district, are dominantly clastic deposits that lie unconformably on the 

rocks beneath the Carbon Ridge on miogeosynclinal 

rocks, the Garden Valley on lower Paleozoic eugeosynclinal rocks (Vinini Formation) (Nolan and others, 

1956, p. 65-68).

OUTLYING MIOGEOSYNCLINAL ROCKS

Fragments of upper Paleozoic miogeosynclinal rocks, 

mingled with other rocks of varied kinds and ages, crop 

out in various parts of southern California. In the 

Mojave Desert are the Orogrande Series and Fairview 

Valley Formation, mainly carbonates, which contain a 

few upper Paleozoic fossils. To the south in the San 

Bernardino Mountains are the Saragossa Quartzite 

and Furnace Limestone, forming roof pendants in the 

Mesozoic plutons. Mississippian fossils are found in the 

Furnace, but a wider age range is suggested by recent 

studies (Steward and Poole, 1974), which indicate the 

presence of strata as old as Lower Cambrian. Smaller 

remnants of similar rocks occur in the San Gabriel 

Mountains to the west. More enigmatic is the Sur 

Series of the Coast Ranges, which forms part of the 

basement of the Santa Lucia Mountains south of Monterey. It is a body of high-grade schists and gneisses, 

with some large masses of marble. Its age is undetermined, but the series is indicated on the Geologic Map 

as upper Paleozoic miogeosynclinal rocks (uP), with 

metamorphic overprint.

EUGEOSYNCLINAL DEPOSITS (uPe)

In north-central Nevada, upper Paleozoic eugeosynclinal deposits closely adjoin the upper Paleozoic 

miogeosynclinal deposits (Battle Formation, and so 

forth) already referred to, but they lie in the upper 

plate of the Golconda thrust that was emplaced during 

the Sonoma orogeny of late Permian and early Triassic 

time. The principal rocks are of Permian and possible 

Pennsylvanian age, but units of Mississippian age 

occur in outlying areas (Silberling and Roberts, 1962, 

p. 16-18).

In the Sonoma Range near Winnemucca and some 

nearby ranges are the Pumpernickel and Havallah

Formations, with a total thickness of more than 15,000 

ft (4,500 m). The Pumpernickel is mostly dark bedded 

chert with minor volcanic rocks, and the Havallah is 

mainly fine-grained sandstone, with some interbedded 

chert and argillite. The Havallah contains Wolfcampian or early Leonardian fusulinids; it also contains reworked Atokan fusulinids in the lower part. A few 

Pennsylvanian conodonts have been recovered from 

the otherwise barren Pumpernickel Formation, 

suggesting that it is Pennsylvanian. Recent observations (John M. Stewart, oral commun., 1973) suggest 

that thick units of Havallah and Pumpernickel lithology are interbedded, and so the two may be not far 

apart in age.

In the East Range farther west and the Independence Range to the north are the Inskip and Schoonover 

Formations of chert, argillite, limestone, and volcanic 

rocks, from both of which a few Mississippian fossils 

have been collected (Fagan, 1962).

In California, upper Paleozoic eugeosynclinal deposits occur both east and west of the Jurassic and 

Cretaceous batholith that forms the core of the Sierra 

Nevada. On the eastern flank near Owens Valley, the 

Ordovician rocks referred to earlier are succeeded by 

7,500 ft (2,300 m) of siliceous hornfels with upper 

Paleozoic fossils (Rinehart and others, 1959, p. 941- 

945). On the western flank is the thick mass of the 

Calaveras Formation, cherts, slates, and volcanics, 

largely unfossiliferous but generally classed as upper 

Paleozoic (Clark, 1964). In the Taylorsville area at the 

north end of the Sierra Nevada is an array of formations, dominantly pyroclastic, composed of dacite, 

andesite, and basalt (McMath, 1966, p. 179-180). The 

lower beds contain Mississippian fossils, and the 

higher are Permian (separately mapped as Pe).

In the southeastern part of the Klamath Mountains, 

the Devonian sequence is succeeded by the Bragdon 

Formation, a mass of shale and sandstone, with minor 

volcanics and some conspicuous layers of conglomerate. It is overlain by the dominantly volcanic Baird 

Formation. The Bragdon and Baird contain Mississippian and Pennsylvanian fossils. The succeeding rocks, 

beginning with the McCloud Limestone, are separately 

mapped as Permian eugeosynclinal deposits (Pe).

In the Blue Mountains uplift of north-central Oregon, upper Paleozoic eugeosynclinal rocks are exposed 

in small areas beneath the more extensive lower 

Mesozoic eugeosynclinal rocks. Relations are best 

shown in the Suplee area at the western end of the 

uplift, where the rocks are least altered and are abundantly fossiliferous (Merriam and Berthiaume, 1943; 

Dickinson and Vigrass, 1965, p. 14-16). The lowest 

unit contains Mississippian' and Devonian invertebrates, the middle Pennsylvanian plants, and the
38 PALEOZOIC AND MESOZOIC ROCKS

upper early Permian fusulinids and other invertebrates. The lower unit is limestone and sandy limestone; the middle is mudstone, conglomerate, and 

chert; and the upper is largely volcanic, with irregular 

fossiliferous limestone lenses. The Paleozoic rocks are 

structurally unconformable below the Triassic. Much 

farther northeast, near Baker and Sparta, the Elkhorn 

Ridge Argillite and Clover Creek Greenstone contain 

both later Permian fusulinids and Triassic fossils 

(Bostwick and Koch, 1962) and are shown on the 

Geologic Map as part of the Triassic and Permian 

eugeosynclinal deposits (~EPe).

In many of the mountain ranges in northern 

Washington, immediately south of the Canadian border, upper Paleozoic eugeosynclinal deposits, in part 

metamorphosed, are associated with lower Mesozoic 

eugeosynclinal deposits but are much disrupted by 

Mesozoic plutons. They include the Chilliwack Group 

on the west flank of the northern Cascade Range, the 

Hozameen Group on the east flank, and still farther 

east the Anarchist Series. All these units are 

graywackes, cherts, and volcanics, with occasional 

lenses of fossiliferous limestone. For the most part, 

they are of Permian, or Permian and Triassic age, but 

the Chilliwack Group has yielded Devonian and lower 

Pennsylvanian as well as Permian fossils (Misch, 1966, 

p. 115-117).

PALEOZOIC PLUTONIC ROCKS

Plutonic rocks of Paleozoic age are extensive in the 

crystalline part of the Appalachian Region (New England and Piedmont provinces), and smaller bodies 

occur in Oklahoma and parts of the Cordilleran Region 

(fig. 13). They include both felsic (granitic) rocks and 

mafic rocks. The granitic rocks are divided into four 

categories according to age Cambrian, lower 

Paleozoic, middle Paleozoic, and upper Paleozoic; the 

mafic rocks are not subdivided, as they form much 

smaller areas which are not feasible to separate and 

not all their relative ages are known.

CAMBRIAN GRANITIC ROCKS ( g)

Plutonic rocks of Cambrian age occur only in a 

unique tectonic and igneous province that extends 

northwestward from the Wichita Mountains area of 

southern Oklahoma into the Rocky Mountains of 

southern Colorado. Although the rocks are of varied 

compositions, all are shown as granitic on the Geologic 

Map.

The most extensive plutonic rock is the Wichita 

Granite, which crops out along the axis of the Wichita 

Mountains for 65 mi (97 km). Its Cambrian age is attested by dates of 535 to 550 m.y. obtained by U/Pb,

Rb/Sr, and K/Ar methods (Ham and others, 1964, p. 

60-79). It is a floored body which underlies and intrudes the congeneric effusives of the Carlton Rhyolite 

and which overlies the somewhat older Raggedy 

Mountain Gabbro, itself a floored intrusive. Extensions 

of these rocks into surrounding areas are known from 

drilling, and the Carlton Rhyolite reappears to the east 

in two small outcrops in the Timbered Hills of the 

western Arbuckle Mountains. Drilling also indicates 

that the units mentioned overlie supracrustal rocks 

not exposed at the surface the Navajoe Mountain 

Basalt and the Tillman Metasedimentary Group, the 

latter a sequence of graywackes at least 15,000 ft 

(4,500 m) thick that fills a deep trough bordered to the 

north and south by older Precambrian crystalline 

rocks. The Wichita Mountains rocks contrast with the 

Tishomingo Granite and other basement rocks of the 

eastern Arbuckle Mountains (Ygi), which have yielded 

radiometric ages of 1,320-1,400 m.y.

Although of Cambrian age, the plutonic and extrusive rocks of the Wichita Mountains are part of the 

basement of the Midcontinent Region. Like the older 

Tishomingo Granite to the east, they are overlain with 

rough erosion surface by the Upper Cambrian Reagan 

Sandstone at the base of the cratonic sequence. In the 

Wichita Mountains, both the granitic rocks and the 

adjacent lower Paleozoic sedimentary rocks project as 

peaks and knobs that have been partly exhumed from 

the surrounding Permian redbeds that formerly buried 

them.

In Colorado, three small alkalic complexes lie in terranes of Precambrian crystalline rocks, two in the Wet 

Mountains at the south end of the Front Range and 

another at Iron Hill north of the San Juan Mountains. 

These have yielded radiometric ages of 520-580 m.y. 

and are likewise Cambrian. Probably they lie on a 

northwestern prolongation of the Wichita Mountains 

plutonic province (Parker and Sharp, 1970, p. 3; Olson 

and Marvin, 1971).

LOWER PALEOZOIC GRANITIC ROCKS (Pgi)

Lower Paleozoic granitic rocks, with ages of about 

400 to 500 m.y. (Ordovician and Cambrian), occur 

throughout the length of the crystalline part of the 

Appalachians. In New England, they include several 

different plutonic series.

In the Connecticut Valley of western New Hampshire, five small bodies of the Highlandcroft Plutonic 

Series intrude the Ordovician rocks, and one of them is 

truncated and overlain unconformably by the Clough 

Quartzite, proving its pre-Silurian age. The plutonic 

rocks were originally quartz monzonite, but they are 

much sheared and chloritized.

In the Bronson Hill anticlinorium to the east is the
PALEOZOIC PLUTONIC ROCKS 39

more extensive Oliverian Plutonic Series, which extends southward from New Hampshire into Massachusetts and Connecticut (Naylor, 1968). The Oliverian characteristically is a series of elongate domes, 

whose foliation is accordant with that of the surrounding and overlying Ammonoosuc Volcanics. The outer 

part of each body is weakly foliated gneiss, but commonly there is a core of massive granitic rock, which 

partly crosscuts the mantling gneisses. Both gneiss 

and granite have yielded Pb/Pb and Rb/Sr ages of 440 

to 450 m.y. Present belief is that the gneissic rock was 

metasomatized from felsic volcanics underlying the 

Ammonoosuc, into which the more massive core rocks 

were intruded as magmas. The Oliverian plutonic 

rocks formed at a deeper crustal level than the Highlandcroft plutonic rocks, in an environment of plastic 

deformation.

Another group of lower Paleozoic granitic rocks is in 

southeastern New England, represented by the alkalic 

Cape Ann, Peabody, and Quincy Granites, north and 

south of Boston. Formerly it was believed that they 

were late Paleozoic, an account of their fresh appearance and their apparent affinities with other young 

alkalic granitic rocks farther northwest, but Pb/Pb determinations on zircons yielded ages of 435 to 452 m.y. 

Evidently this region was outside of and southeast of 

the region of Acadian orogeny, where only earlier influences prevailed (Zartman and Marvin, 1971).

In the Piedmont province of the Central and Southern Appalachians, the existence of many bodies of 

lower Paleozoic granitic rocks is indicated by field relations, supported in many places by radiometric dating. 

All of them precede the regional metamorphism, which 

occurred about 380 to 420 m.y. ago (Butler, 1972). 

Many dates have been obtained by Fullagar (1971) by 

Rb/Sr whole-rock methods on plutonic rocks in the 

Piedmont province, and especially in North and South 

Carolina; these dates were extrapolated on the 

Geologic Map to related plutons in surrounding areas. 

The oldest group of plutons has ages of 520 to 595 m.y., 

hence are Cambrian and Ordovician. One of the plutons, the Farrington igneous complex near Chapel 

Hill, N.C., intrudes the early Paleozoic rocks of the 

Carolina Slate Belt and resembles other plutons in the 

Slate Belt rocks; they are probably congeneric with the 

volcanic effusives of the belt. Another, the Hatcher 

complex in central Virginia, is unconformable beneath 

the Ordovician Arvonia Slate. In Maryland, various 

small granitic plutons (not shown on the Geologic Map) 

have ages of about 440 m.y. (Hopson, 1964, p. 199- 

201).

MIDDLE PALEOZOIC GRANITIC ROCKS (Pgz)

Middle Paleozoic granitic rocks with ages of 350 to 

400 m.y. (Devonian) are more plentiful than the lower

Paleozoic granitic rocks, especially in New England. 

Their principal representative in New England is the 

New Hampshire Plutonic Series (Billings, 1956, p. 

125-129), which forms many plutons in and east of the 

Bronson Hill anticlinorium, and its extensions southward into Massachusetts, northeastward into Maine as 

far as Mount Katahdin, and into northeastern Vermont. The earlier members of the sequence are concordant gneissic bodies, probably synorogenic to the Acadian orogeny, such as the Bethlehem Gneiss and 

Kinsman Quartz Monzonite, which form thick sheets 

in the lower part of the Littleton Formation. Later 

members are cross-cutting plutons of binary granite. 

The later granites of New Hampshire have been dated 

at 380 m.y., and the Mount Katahdin pluton in Maine 

at 358 m.y. The coastal plutons of eastern Maine may 

belong to a distinctly younger, late Devonian plutonic 

series (Chapman, 1968, p. 386-388); they are nearly 

circular cross-cutting granitic bodies, embedded in 

somewhat earlier mafic intrusives.

In the Piedmont province of North and South 

Carolina granitic plutons with ages of 385 to 415 m.y. 

are common in the plutonic complex of the Charlotte 

belt, which adjoins the Carolina Slate Belt on thenorthwest (Fullagar, 1971, p. 2854-2856).

In the western United States, a single granite body 

of middle Paleozoic age has been identified in the 

Beaverhead Range along the eastern border of Idaho. 

It has been dated by K/Ar methods at 441 m.y. (Scholten and Ramspott, 1968, p. 18-21), but there is some 

reason to suspect that the granite might actually be 

Precambrian (Armstrong, 1975, p. 447-448).

UPPER PALEOZOIC GRANITIC ROCKS (Pg3)

Upper Paleozoic granitic rocks, with ages of 250 to 

300 m.y. (Pennsylvanian and Permian) occur chiefly in 

the Piedmont province of the Southern Appalachians.

The only known occurrence in New England is the 

Narragansett Pier Granite (and the minor associated 

Westerly Granite) on the south coast of Rhode Island, 

which intrudes the Pennsylvanian rocks of the Narragansett basin. It has yielded ages of 240 m.y. by K/Ar 

methods, 259 m.y. by Rb/Sr methods on biotite, and 

299 m.y. by Rb/Sr whole-rock methods (Quinn, 1971, p. 

51). The White Mountain Plutonic Series of northern 

New England was formerly assumed to be of late 

Paleozoic age, but it is now known from radiometric 

dating to be Jurassic (Jg).

The upper Paleozoic granitic rocks in the Piedmont 

province occur as discrete plutons in the Carolina Slate 

Belt and eastern edge of the Charlotte belt in North 

and South Carolina and Georgia. They are crosscutting postmetamorphic bodies with ages of about 300 

m.y. (Fullagar, 1971, p. 2856-2857). The oldest of the
40 PALEOZOIC AND MESOZOIC ROCKS

FIGURE 13. United States, showing areas mapped as Paleozoic pi utonic rocks on Geologic Map of United States. Includes units of 

Cambrian granitic rocks (- g), lower, middle, and upper Paleozoic granitic rocks (Pgi, Pg2, Pg3), and mafic intrusives ( Pmi).
PALEOZOIC PLUTONIC ROCKS 41

GULF OF MEXICO

FIGURE 13. Continued.
42 PALEOZOIC AND MESOZOIC ROCKS

group is the Petersburg Granite next to the Coastal 

Plain overlap in eastern Virginia, which is postmetamorphic like the rest and which has been dated at 

330 m.y. by U/Pb determinations on zircon (Wright 

and others, 1975). (It is shown on the Geologic Map 

with the middle group of granites, Pg2>. The youngest 

of the group is the Siloam Granite in the southeastern 

Piedmont of Georgia, which has yielded a Rb/Sr 

whole-rock age of 269 m.y. (Jones and Walker, 1973). 

Northwest of the other plutons, near Atlanta, is the 

Stone Mountain Granite, which has yielded ages of 280 

m.y. by Rb/Sr methods and 294 m.y. by K/Ar methods 

(Smith and others, 1968; Whitney and Jones, 1974).

The only recorded upper Paleozoic granitic pluton in 

the western United States is the Mount Lowe 

Granodiorite of southern California, part of the 

plutonic complex in the San Gabriel Mountains north 

of Los Angeles which includes rocks ranging in age 

.from Precambrian to Cretaceous. The Mount Lowe 

body has a radiometric age of 220-m.y., near the 

Permian-Triassic boundary (Silver, 1971, p. 194).

PALEOZOIC MAFIC INTRUSIVES (P/mi)

Mafic rocks, mainly gabbro and diorite, intrude the 

crystalline rocks throughout the length of the Appalachian Region, but in smaller masses than the 

granitic rocks. They have various Paleozoic ages and 

various relations to the adjacent granitic rocks and to 

the regional metamorphism, but they are not subdivided on the Geologic Map because of their small 

dimensions and because the relations of many of them 

are not known with certainty. Many of the mafic intrusives are phases of the same plutonic series as the 

granitic rocks with which they are associated, but 

some are certainly older.

METAMORPHIC COMPLEXES

The rocks in a few areas in the United States are so 

strongly metamorphosed and complexly deformed, and 

have yielded so little indications of their original ages, 

that they are mapped as metamorphic complexes (m), 

without any age designation. They form extensive 

tracts in the Piedmont province of the Southern Appalachians and smaller areas in northern Washington, 

in the Cascade Range and ranges east of it. Here, instead of classifying the rocks as to age or sequence, 

they are divided into schist and phyllite (ms), felsic 

paragneiss and schist (mi), mafic paragneiss (hornblendite and amphibolite) (1112), migmatite (ma), and 

felsic orthogneiss (granite gneiss) (IIM).

The metamorphic complexes of the Piedmont province are probably of Precambrian Z (late Precambrian) and early Cambrian age. In many places they seem

to lie stratigraphically beneath the Cambrian eugeosynclinal deposits (- v) of the Carolina Slate Belt and 

other areas, but other parts seem to be merely the more 

highly metamorphosed equivalents of the Cambrian 

rocks. Much more field investigation will be needed to 

untangle the true relations of the metamorphic complexes and the Cambrian sequence from one area to 

another. In the Virgilina area along the VirginiaNorth Carolina border, radiometric dating indicates 

rocks with ranges in age from 575 m.y. to 740 m.y. that 

were deformed between 575 and 620 m.y. ago (Glover 

and Sinha, 1973). The older rocks are more 

metamorphosed than the younger ones in the Virgilina 

synclinorium; however, there is no clear break between them, and they are much alike, suggesting that 

there was no significant depositional break between 

the Cambrian eugeosynclinal deposits (-Ge) and the adjacent metamorphic complex (m).

The metamorphic complexes in the northern Cascade Range are of undertermined age but appear most 

likely to be middle and upper Paleozoic. They are older 

than Paleocene and Cretaceous sediments on the 

flanks of the range and are probably older than earlier 

Mesozoic rocks of the area; they are involved in 

orogenic deformation that preceded the formation of 

these rocks. A few radiometric ages have been obtained 

of about 250 m.y., suggesting that the rocks themselves are Permian and older (Misch, 1966, p. 107- 

115).

The rocks in the western part of the range make up 

the Shuksan metamorphic suite, with dominant 

blueschist metamorphism, which overlies less 

metamorphosed Paleozoic and Mesozoic metamorphic 

rocks along a major low-angle thrust. The Shuksan is 

separated from the Skagit metamorphic suite farther 

east, with greenschist metamorphism, by a major 

high-angle fault. The Skagit suite includes the Cascade River Schist (ms) and the higher grade migmatized Skagit Gneiss (mi). Both sets of metamorphic 

rocks were derived from eugeosynclinal sediments and 

volcanics but otherwise have little in common; so their 

original relations to each other are unknown.

East of the Cascade Range is the so-called "Colville 

batholith," actually a metamorphic complex much like 

the more famous Shuswap Complex in British Columbia to the north. It consists of a core of paragneiss (mi), 

overlain by an outward-dipping body of granitic orthogneiss (im) (Fox and Rinehard, 1973).

TRIASSIC AND PERMIAN

EUGEOSYNCLINAL DEPOSITS T< Pe)

A rather diverse assemblage of eugeosynclinal deposits is present in Oregon, California, and Nevada, in
TRIASSIC 43

which volcanic rocks are an important component and 

which contain both Permian and Triassic fossils.

In the Blue and Wallowa Mountains of northeastern 

Oregon are various units of argillite and greenstone 

(Elkhorn Ridge Argillite and Clover Creek 

Greenstone; Gilluly, 1937, p. 14-26), which contain occasional limestone lenses. Some of these lenses contain 

Leonardian and younger Permian fusulinids, others 

Triassic fossils (Bostwick and Koch, 1962). A little 

farther east, along the Snake and Salmon Rivers in 

western Idaho, are the Seven Devils Volcanics, which 

likewise contain both Permian and Triassic fossils. All 

these units are very thick eugeosynclinal accumulations, structurally complex and partly metamorphosed. 

Whether the Permian and Triassic parts represent 

continuous sequences, or whether several units are 

represented, is undetermined. Farther west in the Blue 

Mountains area, the Triassic sequence is unconformable on the Permian and is grouped with the Jurassic as 

lower Mesozoic (1 N/fee) on the Geologic Map.

Extending the length of the Klamath Mountains in 

southwestern Oregon and northern California is the 

wide band of the Western Paleozoic and Triassic belt 

(Irwin, 1966, p. 21-24; Hotz, 1971, p. 11-13). It forms a 

structural block tectonically beneath that of the Central Metamorphic belt and tectonically above that of the 

Western Jurassic belt, and it is entangled with Jurassic granitic plutons and masses of ultramafic rocks. 

The rocks themselves are fine-grained clastic sediments, bedded cherts, mafic volcanic rocks, and lenses 

of crystalline limestone. These have been given various local names, such as Applegate Group and Chanchelulla Formation, but the limits of such units are 

uncertain. Permian ammonoids and fusulinids have 

been collected in some places, and Triassic fossils in 

others; fossils of older Paleozoic ages have been reported but lack modern verification. Most of the rocks 

have been subjected to low greenschist-grade metamorphism, but there are some areas of higher grade 

metamorphic rocks along the eastern side, such as the 

Stuart Fork Formation (shown by overprint on the 

Geologic Map). An exceptional area of low-grade 

schists at Condrey Mountain on the Oregon-California 

border forms a subcircular window, surrounded by 

higher grade schists; on the Geologic Map, these rocks 

are doubtfully correlated with those of the Western 

Jurassic belt (1 N/fe e) but may be older.

In northwestern Nevada the Koipato Group of 

mainly nonmarine rhyolitic and andesitic lavas and 

associated volcaniclastic rocks unconformably underlies Middle Triassic sediments. It is more than 10,000 

ft (3,000 m) thick in its type area in the Humboldt 

Range, with no base visible, but it thins to disappearance about 40 mi (65 km) to the east, where it overlies

upper Paleozoic eugeosynclinal rocks (Havallah and 

Pumpernickel Formations, uPe) that were deformed 

by the Sonoma orogeny. Its upper part contains Lower 

Triassic ammonoids. The Permian fish Helicoprion has 

also been reported, but its occurrence is suspect; Pb/ 

alpha dates of 230 to 290 m.y. (Permian) have been 

obtained but require verification (Silberling, 1973, p. 

349-351).

Farther south is the volcanic Pablo Formation and 

the associated sedimentary Diablo and Excelsior Formations, which contain Permian and Triassic fossils 

and are apparently broadly correlative with the 

Koipato. The Diablo lies unconformably on deformed 

Cambrian and Ordovician eugeosynclinal rocks (1 Pe) 

(Silberling and Roberts, 1962, p. 26-28).

TRIASSIC

The Triassic, the lowest system of the Mesozoic Era, 

is scantily represented on the Geologic Map of the 

United States. It is separately shown in the Colorado 

Plateau, in a few of the ranges of the eastern Rocky 

Mountains, on the western and eastern borders of the 

Great Plains in New Mexico and Texas, and in a series 

of fault troughs in the Appalachian Region from New 

England to South Carolina. In most of the Cordilleran 

Region, it is merged, where present, with the Jurassic, 

as a unit of lower Mesozoic (IN/fe , 1 N/fee). Elsewhere in 

the United States, and especially in the Central Interior Region, Triassic rocks are missing and were probably never deposited (fig. 14).

The Triassic System is divided into the Lower Middle, and Upper Triassic Series, but these terms are not 

used on the Geologic Map. The stage terms in the Alps 

of Europe (Scythian, Anisian, Ladinian, Karnian, Norian, and Rhaetian) are frequently used in discussions 

of the stratigraphy of the marine Triassic rocks of the 

west (Reeside and others, 1957, p. 1455-1456).

Most of the rocks shown as Triassic on the Geologic 

Map are nonmarine and dominantly red colored, but 

these rocks are not specifically designated as continental deposits. Except in the Colorado Plateau, all of 

them are Upper Triassic. The Triassic marine deposits 

of the Cordilleran Region are grouped with Jurassic in 

the lower Mesozoic unit.

COLORADO PLATEAU

In the Colorado Plateau, the Triassic system is represented by the Moenkopi and Chinle Formations the 

first Lower and low Middle Triassic, the second Upper 

Triassic with a combined thickness of about 2,000 ft 

(600 m). At the top is the Glen Canyon Group, also 

partly of Triassic age, which is shown on the Geologic 

Map as a unit of Jurassic and Triassic
44 PALEOZOIC AND MESOZOIC ROCKS

-y-v..-

m

«* *'

FIGURE 14. United States, showing areas mapped as lower Mesozoic stratified rocks on Geologic Map of the United States. 

Includes units of Triassic and Permain eugeosynclinal deposits ("6 Pe), Triassic ("fc ), Jurassic and Triassic (J "fc ), Jurassic (J, 

(Jc), and undivided lower Mesozoic (1M, IMe,
TRIASSIC 45

GULF OF MEXICO

FIGURE 14. Continued.
46 PALEOZOIC AND MESOZOIC ROCKS

The Moenkopi Formation is a red fine-grained 

evenly bedded deposit, with thin beds of gypsum, and a 

few of limestone toward the west. Most of it is nonmarine, but it grades westward into marine deposits, 

where it contains the Early Triassic ammonoid 

Meekoceras in the lower part. Elsewhere, the only fossils are vertebrate tracks and bones.

The overlying Chinle Formation is more sandy and 

brilliantly colored. The persistent Shinarump Conglomerate at the base was laid on the eroded surface of 

the underlying Moenkopi Formation. The Chinle is entirely nonmarine and contains vertebrates, fossil wood, 

and a few freshwater invertebrate shells. The Chinle 

extends eastward beyond the edge of the Moenkopi, 

into northwestern New Mexico.

GREAT PLAINS

The Dockum Group, another red Upper Triassic 

unit, forms extensive outcrops in eastern New Mexico 

and northwestern Texas, on each side of the Tertiary 

caprock of th/e Great Plains, and is continuous in the 

subsurface within the intervening area, where it is the 

final deposit of the West Texas Permian basin. In the 

center of the basin, it is about 2,000 ft (600 m) thick, 

but it is thinner in the outcrops. Various local names 

have been given to subdivisions, such as Santa Rosa 

Sandstone in New Mexico and Tecovas and Trujillo 

Formations in Texas, but they are inconstant and have 

no general significance (E.D. McKee, in McKee and 

others, 1959, p. 13-14). The Dockum is entirely nonmarine; vertebrates have been found at many places 

and indicate an approximate correlation with the 

Chinle Formation farther west.

APPALACHIAN REGION

Rocks of the Upper Triassic Newark Group form a 

series of faulted troughs in the southeastern part of the 

Appalachian Region from southern New England to 

South Carolina. Another fault trough of Newark rocks 

is in Nova Scotia to the northeast, and rocks like the 

Newark Group, probably also in fault troughs, have 

been penetrated by drilling in the Atlantic and Gulf 

Coastal Plains in Georgia, Alabama, and Florida, and 

as far west as southern Arkansas (Eagle Mills Formation).

In the Appalachian Region, the Newark rocks lie on 

the deeply eroded edges of the Paleozoic rocks and the 

crystalline rocks of the Piedmont. The fault troughs 

broadly parallel the trends of the earlier structures, 

however, and in North Carolina, at least, are symmetrically placed on each side of the metamorphic climax 

in the Piedmont rocks. Commonly, the troughs have

the form of half-grabens, with the beds tilted toward a 

master fault on one side or the other, which was active 

during sedimentation. The half-grabens are symmetrically placed; thus, the Connecticut Valley Triassic has 

its master fault on the east, and the New JerseyPennsylvania Triassic, en echelon to the west, has its 

master fault on the northwest. The Deep RiverWadesboro basin and the Dan River basin in North 

Carolina have a similar arrangement. In places, there 

is some evidence that the Triassic deposits were originally continuous between the opposing troughs; in 

western Connecticut a small downfaulted outlier in the 

Pomperaug Valley duplicates the sequence in the 

larger Triassic areas to the east and west. In addition 

to the master faults, the Triassie rocks are displaced by 

many other faults that mostly formed after the close of 

sedimentation. The faults are tensional (taphrogenic) 

features and were probably produced by rifting associated with the opening of the Atlantic Ocean during 

Mesozoic time.

Along part of the northwestern border in Pennsylvania, the highest Triassic beds overlap the Paleozoic 

rocks without surface rupture, but the master fault 

probably lies beneath and had ceased its activity before 

the end of Triassic deposition. On the Geologic Map, 

the fault is represented as continuous.

The rocks of the Newark Group are all nonmarine 

conglomerates, sandstones, siltstones, and shales, 

complexly interfingered; many of them are conspicuously red, but some are gray or black. In the Connecticut Valley, the Newark Group is about 12,000 ft 

(3,700 m) thick, and in the New Jersey-Pennsylvania 

area at least 20,000 ft (6,000 m); lesser thicknesses 

occur in the narrower basins farther south.

Most of the coarser beds are arkosic. In New Jersey 

and Pensylvania, the first sediments are arkoses derived from crystalline highlands to the southeast, and 

coarse debris from across the border fault to the northwest appears only above these arkoses. In both the 

Connecticut Valley and the New Jersey-Pennsylvania 

area, the deposits next to the border faults are coarse 

fanglomerates; some fanglomerates in Pennsylvania 

are formed of carbonate clasts ("Potomac marble"), derived from Paleozoic formations to the northwest. Near 

the middle, in both the Connecticut Valley and New 

Jersey-Pennsylvania areas, are dark-gray to black 

lacustrine shales (for example, the Lockatong Formation), which finger out into the red coarse sediments 

(D. B. McLaughlin, in Reeside and others, 1957, p. 

1491-1494). In the southern basins there are more 

gray strata, and important beds of coal; the latter have 

been mined in the Richmond basin, Virginia, and the 

Deep River basin, North Carolina.
JURASSIC 47

In the Connecticut Valley and in New Jersey, three 

basalt flows are interbedded in the upper part (~fcv). 

Both here and farther south, the Newark rocks contain 

thick sills of diabase ("fci), the most prominent of which 

is the Palisades sill of eastern New Jersey that overlooks the Hudson, River and which has been dated 

radiometrically at 190-200 m.y. (Erikson and Kulp, 

1961). In addition, numerous vertical diabase dikes 

(shown on the Geologic Map) in the Triassic rocks 

extend far out into the older rocks of the Piedmont 

province (King, 1971).

Various nonmarine (freshwater or continental) fossils are abundant in parts of the Newark Group, including fishes and plants in the dark shaly beds and 

vertebrate bones and tracks in the coarser red sediments. They indicate roughly a Late Triassic age, although opinions differ as to precise correlations with 

standard sections elsewhere.

JURASSIC AND TRIASSIC (Jl)

The designation Jurassic and Triassic (J"fc) is used in 

the western part of the Colorado Plateau for the rocks 

of the Glen Canyon Group the Wingate Sandstone, 

the Moenave and Kayenta Formations, and the Navajo 

Sandstone (Baker and others, 1936, p. 4-6). Their age 

is uncertain between Triassic and Jurassic, as fossils 

are very sparse and relations to better dated rocks in 

surrounding areas are ambiguous; present judgment is 

that the Wingate is Upper Triassic and the Navajo 

Lower Jurassic. However, the group is a cohesive unit, 

characterized by great cliff-forming sandstones that 

are a very prominent geomorphic feature in the western part of the plateau, and it has not been subdivided 

on some of the source maps (Geologic Map of Arizona, 

1969); so, it seems best to portray the group on the 

Geologic Map as a separate unit (J"fc). Eastward in Colorado and New Mexico, the group thins out and loses 

its distinctive character, and thus the separate designation is dropped.

The Glen Canyon Group is commonly 1,000 to 2,000 

ft (300-600 m) thick, but the different components vary 

from place to place; the Navajo Sandstone is thickest 

toward the west where the Wingate Sandstone is thin 

or absent, and the Wingate is thickest toward the 

southeast and extends beyond the featheredge of the 

Navajo. Both the Wingate and Navajo are massive 

cliff-forming sandstones, with prominent festoon 

crossbedding, and are probably largely of eolian origin; 

the Wingate is characteristically red, and the Navajo 

white or buff. The intervening Moenave and Kayenta 

Formations are thinner bedded sandstones that form a 

topographic bench between the Wingate and Navajo

Sandstones, and the Kayenta contains lenses of 

mudstone and impure limestone.

JURASSIC

The Jurassic System is represented on the Geologic 

Map of the United States by marine stratified rocks (J) 

in the Southern Rocky Mountains, the Colorado 

Plateau, and the Pacific coastal area of California and 

Oregon and by continental deposits (Jc) in a few areas 

in the northern part of the Interior Region. Elsewhere 

in the Cordilleran Region, it is merged with the Triassic System into a unit of lower Mesozoic (IMz, IMze). In 

the remainder of the country, and especially in the 

Interior and Appalachian Regions, Jurassic rocks are 

absent and were probably never deposited (fig. 14). 

Jurassic rocks are well developed beneath the Gulf 

Coastal Plain, where they have been extensively 

explored by drilling, but none of them are exposed.

The Jurassic is divided into the Lower, Middle, and 

Upper Jurassic Series. In addition, the western European stage terms (Lias, Bajocian, Callovian, Oxfordian, Kimmeridgian, Portlaridian or Tithonian, and 

others) are commonly used in discussions of Jurassic 

stratigraphy in North America. None of these -subdivisions are shown on the Geologic Map of the United 

States, because of the narrow outcrop belts.

ROCKY MOUNTAIN REGION

Along the western edge of the Rocky Mountain-Region, the Jurassic rocks are miogeosyclinal and 

marine. The Arapien Shale and evaporites of central 

Utah are 5,000 to as much as 10,000 ft (1,500-3,000 m) 

thick, and farther north, from the northern Wasatch 

Mountains into the thrust belt of southeastern Idaho, a 

more varied assemblage of formations is about 5,000 ft 

(1,500 m) thick (mostly shown as IK/fe on the Geologic 

Map), including the Nugget Sandstone, Twin Creek 

Limestone, Preuss Sandstone, and Stump Sandstone, 

which range from Lower Jurassic into Upper Jurassic.

In the Colorado Plateau, the equivalent to the 

Nugget is probably the Navajo Sandstone, and the 

higher strata have equivalents in the San Rafael 

Group, 1,000 to 2,000 ft (300-600 m) thick (Baker and 

others, 1936, p. 6-9): the Carmel Formation, Entrada 

Sandstone, Curtis Formation, and Summerville Formations. Most of the group is nonmarine, but the Carmel and Curtis contain fossiliferous marine tongues, 

by which they can be linked with the miogeosynclinal 

strata to the northwest. To the north, in the Central 

and Northern Rocky Mountains, equivalent beds are 

all marine and form the Sundance Formation in the 

south and the more comprehensive Ellis Group farther 

north (mostly shown as 1 Mz on the Geologic Map).
48 PALEOZOIC AND MESOZOIC ROCKS

Above these deposits throughout the Rocky Mountain Region is the latest Jurassic Morrison Formation, 

about 250 to 750 ft (75-230 m) thick, nonmarine 

throughout and probably a flood-plain and lacustrine 

deposit, and composed largely of variegated 

mudstones, but with sandstone units locally, especially 

in the southwest part. It spreads over an area of 

655,000 mi2 (1,680,000 km2) in the Rocky Mountains 

and Great Plains to the east and was produced during a 

remarkable period of quiescence in a region that was 

tectonically restless both before and after. The Morrison is notable for its abundant remains of dinosaurs 

and other reptiles, but it also contains fossil freshwater 

pelecypods and plants.

The Colorado Plateau formations extend eastward 

across the Southern Rocky Mountains and form narrow bands of outcrop where they are turned up along 

the eastern flanks of the Front Range and Sangre de 

Cristo Mountains; they thin to a featheredge in the 

Great Plains beyond.

Mention should also be made of the Upper Jurassic 

Malone Formation, exposed in a single area close to the 

Rio Grande in western Texas (Albritton and Smith, 

1965, p. 25-38), which is the sole representative in the 

United States of the Jurassic miogeosynclinal rocks 

that are extensive in the Sierra Madre Occidental to 

the south in Mexico.

PACIFIC COASTAL AREA

Along the western side of the Sacramento Valley in 

northern California, the Upper Jurassic (Portlandian 

or Tithonian) Knoxville Formation is exposed for 120 

mi (190 km) and forms the base of the "Great Valley 

sequence" of upper Mesozoic strata. The rocks of this 

sequence dip homoclinally eastward beneath the valley and are up to 50,000 ft (15,000 m) thick; the Knoxville part alone is 16,000 ft (5,000 m) thick (Bailey and 

others, 1964, p. 124-130), but it ends abruptly near the 

40th parallel at a transverse fault, north of which the 

Lower Cretaceous rocks (Paskenta) form the base of 

the sequence. The Knoxville is thin-bedded shale and 

sandstone (flysch), with some layers of pebbly 

mudstone, that contains a sparse fauna of Buchia 

piochii, Inoceramus, belemnites, and various ammonoids. This fauna is Tithonian, but Buchia rugosa, a 

Kimmeridgian fossil, has recently been discovered in 

the basal layers (Jones, 1975). Paleocurrents indicate 

that the Knoxville sediments were derived from the 

northeast, probably from the Sierra Nevada and 

Klamath Mountains, whose Jurassic and older rocks 

had been deformed by the Nevadan orogeny of midJurassic time.

The Knoxville was deposited on an ophiolitic or 

oceanic crust, now preserved along its western edge as

a great sheet of serpentinite, followed by diabase, pillow lava, and radiolarian chert, which pass upward 

into the normal Knoxville sequence (Bailey and others, 

1970). The serpentinite and overlying Knoxville are 

thrust westward over the partly coeval Franciscan 

Formation.

Other Knoxville-type Upper Jurassic rocks crop out 

in places at the base of the Great Valley sequence 

farther south along the edge of the valley, but they are 

too small to separate from the Cretaceous on the 

Geologic Map.

Jurassic rocks are also shown on the Geologic Map in 

the structurally complex coastal area of southwestern 

Oregon, where they are juxtaposed against other 

Mesozoic rocks Upper and Lower Cretaceous (uK, 

IK), the Dothan Formation (Franciscan equivalent, 

uM:e), the Galice Formation (!M:e), and masses of serpentinite. The largest Jurassic unit is the Otter Point 

Formation (Koch, 1966, p. 36-43; Coleman, 1972, p. 

12-14), a thick graywacke-shale unit, including much 

volcanic material (pillow lavas and pyroclastic debris), 

that contains Buchia piochii and other Tithonian fossils.

CONTINENTAL DEPOSITS (Jc)

In the northern part of the Interior Region, the 

Paleozoic rocks are capped in places by red continental 

deposits, which palynological studies demonstrate are 

of Early Jurassic age (Cross, 1967), although they have 

erroneously been assigned to the Pennsylvanian or the 

Permian System in the past. The largest area is in the 

center of the Michigan basin, where 300 to 400 ft (90- 

120) m) of red shale and sandstone, with minor gypsum, overlie the Pennsylvanian sequence (Cohee, 

1965, p. 220). The Fort Dodge Gypsum and associated 

redbeds similarly overlie Pennsylvanian rocks in a 

smaller area in north-central Iowa. Another area of 

Jurassic continental deposits is mapped in northwestern Minnesota next to the Canadian border but is apparently known mainly from drilling beneath the glacial cover; it is the south end of an extensive belt in 

Manitoba, lying between the Cretaceous sequence on 

the west and the Paleozoic on the east.

LOWER MESOZOIC

In extensive areas of the Cordilleran Region, the 

Triassic and Jurassic rocks are combined on the 

Geologic Map into a unit of lower Mesozoic, which includes marine stratified rocks (llVb), eugeosynclinal 

deposits (!M:e), and volcanic rocks (!M:v).

MARINE STRATIFIED ROCKS (IMz)

The lower Mesozoic marine stratified rocks are relatively thin cratonic deposits in Wyoming and Montana
LOWER MESOZOIC 49

but are thicker miogeosynclinal deposits in southeastern Idaho and westward in the Great Basin of Utah 

and Nevada. They also change westward from dominantly continental deposits to dominantly marine 

deposits.

The cratonic deposits, as exposed on the flanks of the 

mountain uplifts, are 500 to 1,500 ft (150-500 m) thick 

and comprise the redbeds of the Chugwater Formation 

(Upper Triassic) below, followed by the marine Sundance or Ellis Formations, and topped by the continental Morrison Formation (Jurassic). The Triassic redbeds wedge out northward in southern Montana, and 

only the Jurassic units persist beyond.

In the miogeosynclinal belt of southeastern Idaho, 

the Triassic sequence thickens to nearly 7,000 ft (2,100 

m) and the Jurassic to 5,000 ft (1,500 m), and there is a 

more complex array of formations the Dinwoody, 

Thaynes, and Ankareh Formations in the Triassic, and 

the Nugget Sandstone, Twin Creek Limestone, and 

Preuss and Stump Sandstones in the Jurassic. The 

Idaho Triassic section is notable for containing the 

thickest and most complete sequence of Lower Triassic 

in the world, and it contains ammonoids at many 

levels, including Meekoceras (Kummel, 1954, p. 165). 

The basal Dinwoody Formation lies with nearly parallel bedding and no indication of erosion on the Permian 

Phosphoria Formation, although separated from it by a 

considerable hiatus. The thicker, overlying Thaynes 

Formation is dominantly limestone, although with 

silty and sandy members, and intertongues with redbeds toward the craton. Jurassic rocks do not extend 

beyond western Utah, but the Triassic System is well 

represented in northeastern Nevada (Elko County), 

where the sequence is mostly limestone and as much as 

3,000 ft (900 m) thick (Clark, 1957, p. 2200-2209).

A quite different set of lower Mesozoic miogeosynclinal deposits occurs in west-central and southwestern 

Nevada. It is separated from those just described by a 

100-mile (160 km) gap, which was probably a land barrier. In the Sonoma Range and elsewhere near Winnemucca in north-central Nevada are two sequences, 

the Augusta to the east and the Winnemucca to the 

west, that were juxtaposed by moderate westward 

thrusting during later Mesozoic time (Silberling and 

Roberts, 1962, p. 19-25). Both lie in part on the 

Koipato Formation fKPe), but they have no formations 

in common. The Augusta sequence was deposited 

nearer the shore to the east, and its lower part passes 

eastward into conglomerates and coarse elastics 

(China Mountain and Panther Canyon Formations); 

much of the rest is limestone. Where fully developed, 

the sequence is about 8,000 ft (2,400 m) thick. The 

Winnemucca sequence formed farther from shore and 

passes westward into deep-water turbidites and eventually into eugeosynclinal deposits. In its miogeosynclinal phase to the east, its lower part (Prida and 

Natchez Pass Formations) is largely limestone, but the 

higher parts are shaly and sandy; the Winnemucca sequence reaches a thickness of 10,000 ft (3,000 m). The 

Augusta sequence is of Middle and Upper Triassic age, 

but the Winnemucca sequence includes Lower Jurassic 

rocks at the top.

About 60 mi (100 km) south of the exposures of these 

sequences is the Luning sequence of latest Triassic and 

Early to Middle Jurassic age (Silberling and Roberts, 

1962, p. 28-33), which also was deposited west of a 

shoreline. The greater part of the sequence is the Luning Formation, as much as 8,000 ft (2,500 m) thick, 

largely of carbonate rocks, including coral reefs. Later 

on, local folding and thrusting occurred, and so the 

coarse elastics and fanglomerates of the Dunlap Formation at the top rest on different older parts of the 

sequence from' place to place, and even on pre-Triassic 

rocks.

EUGEOSYNCLINAL DEPOSITS

Small areas of lower Mesozoic rocks, intruded by 

Mesozoic granitic rocks, occur in western Nevada and 

to the west in the Sierra Nevada of California. They ' 

change westward from miogeosynclinal to eugeosynclinal deposits; on the Geologic Map the line of separation is made between unit J"fc (shale, mudstone, 

siltstone, and sandstone) and unit J"fc (shale, 

sandstone, volcanogenic clastic rocks, andesite, and 

rhyolite) as represented on the compilation for the new 

Geologic Map of Nevada.

Lower Mesozoic eugeosynclinal deposits, again associated with Mesozoic granitic rocks, are more extensive in the Sierra Nevada. In the eastern Sierra the 

thickest sequence is in the Ritter Range roof pendant 

west of the head of Owens Valley, where there are 

30,000 ft (9,000 m) of intermediate to felsic pyroclastic 

rocks (Rinehart and others, 1959, p. 945). Early Jurassic fossils occur about a third of the way up in the 

sequence, and so there is a possibility that unrecognized Triassic and later Jurassic rocks may be present.

The largest area of lower Mesozoic eugeosynclinal 

deposits in the Sierra Nevada, however, is in the western foothills from the Mother Lode belt westward, 

where there is about 15,000 ft (4,500 m) of volcanic 

rocks and volcaniclastic sediments (Clark, 1964, p. 

6-8). These include the "Mariposa Slate" of the Gold 

Belt folios, as well as "porphyrite," "diabase", and 

"amphibolite" that were once considered to be intrusive but are now known to be supracrustal volcanic 

deposits. (They were erroneously grouped with the 

Sierra Nevada granitic rocks on the Geologic Map of
50 PALEOZOIC AND MESOZOIC ROCKS

the United States of 1932.) The rocks have been subjected to low-grade greenschist metamorphism, but 

fossils occur at various places which indicate ages 

ranging from Middle to Late Jurassic (Bajocian to 

Kimmeridgian). The base of the sequence is generally 

juxtaposed against serpentinite which was probably 

original oceanic crust, and no Triassic is present. The 

sequence is also largely older than the Knoxville Formation at the base of the Great Valley sequence to the 

west, which is largely Portlandian OTithonian), although it includes some Kimmeridgian at the base 

(Jones, 1975).

Modern work indicates that these Jurassic rocks actually consist of several sequences of unlike formations. The eastern sequence includes the original 

Mariposa Formation at the top, of slate, tuff, and 

graywacke, underlain by the Logtown Ridge Formation of andesitic volcanic breccia, pillow lava, .tuff, and 

sandstone. The western sequence begins with the 

Gopher Ridge Volcanics of basaltic, andesitic, and 

rhyolitic pyroclastic rocks and lavas, followed by the 

Salt Spring Slate (possibly equivalent to the 

Mariposa), the Merced Falls Slate, and the Copper Hill 

Volcanics which resemble those of the Gopher Ridge. 

Both sequences are island-arc deposits which originally formed far apart, but which are now closely adjoined as a result of faulting and subduction (Schweickert and Cowan, 1975, p. 1329-1331). Toward the 

north the two sequences are separated by the 20-mile 

(32-km) Smartville terrane, which is ophiolitic ocean 

floor facies, with pillow basalts underlain by sheeted 

dike complexes and intrusive gabbro. This terrane 

wedges out southward between the sequences, leaving 

a narrow belt of melange containing a great variety of 

tectonic blocks, including Permian fusulinid-bearing 

limestone of unknown original provenance (Duffield 

and Sharp, 1975).

In the southern part of the Sierra Nevada, the prevailing granitic rocks contain many small roof pendants of supracrustal rocks, most of which are undated 

but all of which are shown on the Geologic Map as 

lower Mesozoic eugeosynclinal deposits (llVfee).

In the Taylorsville area at the north end of the 

Sierra Nevada, the Triassic System is represented by 

the Hosselkus Limestone and Swearinger Slate, two 

relatively thin nonvolcanic abundantly fossiliferous 

units of Late Triassic age. They are followed on Mount 

Jura by a 13,000-ft (4,000-m) sequence of clastic and 

volcaniclastic rocks, subdivided into many units whose 

fossils indicate a nearly complete sequence from base 

to top of the Jurassic System (McMath, 1966, p. 181- 

182). In a somewhat similar sequence along the strike 

to the north in the easternmost belt of the Klamath 

Mountains, 9,000 ft (2,700 m) of Triassic rocks overlie

the Permian sequence and are succeeded by 7,000 ft 

(2,100 m) of Jurassic rocks.

A very different sequence of lower Mesozoic 

eugeosynclinal deposits forms the Western Jurassic 

belt of the Klamath Mountains for its entire length, 

from southwestern Oregon into northern California 

(Irwin, 1966, p. 24-25; Hotz, 1971, p. 13-14). It is 

faulted against the Western Paleozoic and Triassic belt 

(~fiPe) on the east and the Franciscan Formation 

(=Dothan Formation) on the west. Its rocks constitute 

the Galice Formation, volcanic below and clastic 

above, that is as much as 15,000 ft (4,600 m) thick and 

that is dated by fossils as of Oxfordian and Kimmeridgian age; it has commonly been compared with the 

Mariposa Formation of the western Sierra Nevada.

Northeast of the Klamath Mountains, in the Blue 

Mountains uplift of north-central Oregon, lower 

Mesozoic and Paleozoic rocks are again exposed. Those 

in the eastern part are mainly mapped as Triassic and 

Permian eugeosynclinal deposits (~fiPe), but the 

stratigraphy in the western part is clearer and the 

rocks less metamorphosed; a typical sequence occurs in 

the Suplee-Izee area (llVfee) (Dickinson and Vigrass, 

1965, p. 17-67). Lying unconformably on the Paleozoic 

is a sequence about 25,000 ft (7,600 m) thick of Late 

Triassic (Karnian) to early Late Jurassic (Callovian) 

age, the different parts themselves separated by angular unconformities. Its sediments are largely argillites 

and siltstones, with interbedded sandstones and minor 

limestones, but lavas and volcaniclastic rocks occur in 

nearly all the units and dominate the upper third of 

the sequence.

Far to the south, in the Peninsular Range of southern California, small to large bodies of supracrustal 

rocks are invaded by the Cretaceous Peninsular 

batholith. Most of these rocks are lower Mesozoic, although Paleozoic (uPe) rocks may occur in the San 

Jacinto Range to the east. The rocks are least 

metamorphosed and the sequence is plainest at the 

northwestern end, in the Santa Ana Mountains 

(Yerkes and others, 1965, p. 23). Below is the Bedford 

Canyon Formation, at least 20,000 ft (6,000 m) thick of 

argillite and slate, with some sandstone and limestone; 

it is overlain with angular unconformity by the Santiago Peak Volcanics, several thousand feet thick. Fossils in the Bedford Canyon indicate an early Late 

Jurassic (mainly Callovian) age (Imlay, 1962, p. 98- 

100). (Similar fossils occur in the Santa Monica Slate, 

exposed in an inlier northwest of the Los Angeles basin.) The Santiago Peak Volcanics have not been dated; 

they are overlain unconformably by Upper Cretaceous 

rocks and may be of latest Jurassic or even Early Cretaceous age, like similar rocks farther south in Baja 

California. The Julian Schist in the core of the Penin-
CRETACEOUS 51

sular Range has not been dated, but it may be a 

metamorphic phase of these formations.

In the Mojave Desert region, between the Peninsular 

Range and the Sierra Nevada, many small areas are 

represented on the Geologic Map as lower Mesozoic 

eugeosynclinal deposits (llVfee). They include the Sidewinder Volcanics and Ord Mountain Group, of andesite 

and rhyolite flows and volcaniclastic rocks, generally 

believed to be Triassic. In the Clark Mountains farther 

east, however, volcanics overlie the Jurassic Aztec 

Sandstone. Similar rocks of less certain ages occur 

southeastward as far as the Colorado River and are 

classed as lower Mesozoic on the Geologic Map. In 

southwestern Arizona the State Map shows units of 

"Mesozoic gneiss" and "Mesozoic schist"; for purposes 

of the Geologic Map, the first is assumed to be Precambrian reworked by Mesozoic orogenies and the second 

to be lower Mesozoic eugeosynclinal deposits.

VOLCANIC ROCKS (IMzv)

Terrestrial volcanic rocks (as contrasted with the 

eugeosynclinal volcanics already discussed) occur in 

several small areas, unrelated to each other, in widely 

scattered parts of the United States.

The Moat Volcanics of New Hampshire (Billings, 

1956, p. 35-37) are associated with and probably congeneric with the Jurassic White Mountain Plutonic 

Series and are preserved in downdropped blocks surrounded by ring dikes of the plutonic rocks. They consist of rhyolite flows and breccias up to 11,000 ft (3,300 

m) thick and lie unconformably on Paleozoic rocks that 

were deformed and metamorphosed during the Acadian orogeny. They contrast surprisingly with the 

Upper Triassic sedimentary and volcanic rocks of the 

Newark Group not far to the south in New England.

In the northeastern part of the Cortez Range in 

north-central Nevada is the Pony Trail Group of volcanic wacke, tuff, and rhyolite flows about 10,000 ft 

(3,000 m) thick (Muffler, 1964, p. 20-39). It is not in 

contact with the adjacent Paleozoic rocks, but it is 

overlain in part by the Cretaceous Newark Canyon 

Formation (Kc) and is intruded by Jurassic granitic 

plutons. These volcanic rocks contrast with the lower 

Mesozoic miogeosynclinal rocks not far to the east and 

west but probably accumulated on the land barrier 

which separated the two groups of miogeosynclinal deposits.

In some of the ranges close to the Mexican border in 

southern Arizona is another assemblage of lower 

Mesozoic volcanic rocks (Hayes and Drewes, 1968, p. 

51-54), which lie unconformably on Paleozoic rocks 

and are overlain unconformably by the Lower Cretaceous Bisbee Group. They consist of a lower volcanic 

unit as much as 10,000 ft (3,000 m) thick of rhyodacite

and andesite flows and tuffs, a middle redbed unit 

about 2,000 ft (600 m) thick, and an upper volcanic 

unit about 7,000 ft (2,100 m) thick of silicic flows and 

tuffs. Fossils are rare and not diagnostic, but Pb/alpha 

and K/Ar determinations indicate that the sequence 

ranges in age from Early Triassic to Early Jurassic. 

The volcanics are, further, intruded by Jurassic granitic rocks that have yielded a Pb/alpha date of 184 m.y.

CRETACEOUS

In terms of the Geologic Map of the United States, 

the Cretaceous System, or uppermost division of the 

Mesozoic, by far overshadows the Jurassic and Triassic 

Systems, as well as many of the Paleozoic systems, not 

only in the wide extent of its exposures but in the variety and complexity of its formations.

The Cretaceous sequence forms a nearly continuous 

band of outcrop in the Atlantic and Gulf Coastal Plains 

and expands in Texas into the hill and plateau country 

to the northwest (fig. 15). It is even more extensive in 

the central and northern Great Plains and westward in 

the Rocky Mountains. Other outcrops occur in the 

Pacific coastal area through California into southwestern Oregon. Most of the Cretaceous rocks so represented are marine, but the marine deposits in the 

Rocky Mountains change westward in the Great Basin 

into continental deposits (Kc).

The Cretaceous is divided into the Lower Cretaceous 

and Upper Cretaceous Series (IK, uK), whose local representatives in the Gulf Coastal Plain are called the 

Comanche and Gulf Series. The Lower Cretaceous 

comprises the Neocomian, Aptian, and Albian Stages, 

and the Upper Cretaceous the Cenomanian, Turonian, 

Coniacian, Santonian, Campanian, and Maestrichtian 

Stages of the European classification.

LOWER CRETACEOUS

TEXAS

The Comanche Series of the Lower Cretaceous is 

mainly of Albian age but may include some Aptian 

rocks at the base. The remainder of the Aptian stage, 

and the Neocomian, are unrepresented in Texas, although they are well displayed in Mexico to the south, 

where they have been called the Coahuila Series (Innlay, 1944, p. 1005-1007). The Comanche Series departs 

from the European classification by including some 

Cenomanian units at the top (Del Rio Clay and Buda 

Limestone); the discrepancy is not fundamental, as 

these are very thin units.

The Comanche Series is divided into the Trinity, 

Fredericksburg, and Washita Groups (IKi, 1K2, IKs), 

which are separately represented on the Geologic Map 

in the broad outcrop area that extends across central
52 PALEOZOIC AND MESOZOIC ROCKS

*n

n

FIGURE 15. The United States, showing areas mapped as Cretaceous stratified rocks on the Geologic Map of the United States; 

Lower and Upper Cretaceous are separately shaded. Includes units of Lower Cretaceous (IK, IKi, 1K2, u Nk e) and of Upper 

Cretaceous (uK, uKi, uKa, uKa, uK4, Kc, Ke, Kv) age.
CRETACEOUS 53

GULF OF MEXICO

FIGURE 15. Continued.
54 PALEOZOIC AND MESOZOIC ROCKS

and northern Texas. (For a summary of the Comanche 

Series, see the useful but now somewhat outdated presentation by Adkins (1932, p. 272-400).) In west Texas, 

where the outcrop bands are narrow and the structure 

more complex, the separation is not made, nor is it 

made in the small outlying areas of the Comanche 

Series in southern Arizona, Oklahoma, and southern 

Kansas.

The Trinity Group (IKi) is an irregular basal deposit 

that lies on the eroded surface of Paleozoic and Triassic 

rocks. Where best developed near Austin, it is 800 ft 

(240 m) or more thick and consists of the Travis Peak 

Formation below of sands, clays, and thin limestones 

and the overlying Glen Rose Formation of ledgemaking limestones alternating with softer marls. Toward the north and west, the group thins and finally 

disappears; sandstone tongues increase in prominence 

and finally dominate altogether. The Fredericksburg 

Group (1K2), 500 ft (150 m) or more thick, contains the 

thick widespread rudistid-bearing Edwards Limestone, with the more marly Walnut Clay and Comanche Peak Limestone below and the Kiamichi Formation above. The Washita Group (IKa) in north Texas 

consists of marls and clays with thin interbedded limestones, divided into an array of formations. Southward 

toward Austin it thins into a more condensed sequence, 

the Georgetown Limestone, with the thin well-marked 

Del Rio Clay and Buda Limestone at the top. The units 

in western Texas are similar but have some local variations.

In a broad area from Austin to the Pecos River and 

beyond, the Comanche Series lies nearly flat and forms 

the Edwards Plateau. Much of the surface of the 

plateau is not formed of the Edwards Limestone, however, but of similar limestones of the Washita Group, 

and lower formations are penetrated only in canyons 

along the edges. The extent of the Washita is much 

greater than has been shown on previous maps 

(Geologic Map of United States of 1932; Geologic Map 

of Texas of 1937), where the base of the Washita had 

been placed erroneously 100 to 200 ft (30-60 m) too 

high, resulting in a notable difference in map pattern 

in this flat-lying terrane.

In the southern part of the plateau, the simple 

stratigraphy farther north breaks down, and the Fredericksburg and Washita Groups merge into a massive 

reef deposit, the Devils River Limestone (Smith, 1970, 

p. 43-44). (On the Geologic Map, this facies change is 

ignored and the Fredericksburg-Washita boundary is 

sketched arbitrarily through the deposit.) The Devils 

River Limestone reef is the only surface exposure in 

the United States of a regional feature a south-facing 

shelf break and barrier reef that extends far eastward 

in the subsurface beneath the Gulf Coastal Plain and

at the surface southwestward across the Rio Grande 

into Mexico.

From northern Texas, the outcrop belt of the Comanche Series extends eastward across southern Oklahoma into southwestern Arkansas, but its top is 

progressively truncated by the unconformity at the 

base of the Upper Cretaceous Woodbine Sand (uKi) 

until none remains, although it continues in full development in the subsurface to the south.

Westward from western Texas, small outcrops of 

Comanche Series in southern New Mexico and in 

southeastern Arizona, where it is represented by the 

Bisbee Group, are as much as 10,000 ft (3,000 m) thick 

(Hayes and Drewes, 1968, p. 55-56); they are typically 

developed in the Mule Mountains near Bisbee. Here, 

the middle part is the Mural Limestone, with Trinity 

and possibly Fredericksburg marine fossils, and the 

Morita Formation below and Cintura Formation above 

are pinkish sandstones and siltstones, largely terrestrial. The Glance Conglomerate at the base lies on a 

rough surface eroded on the Paleozoic rocks and the 

lower Mesozoic volcanics and likewise contains some 

interbedded lava. Northwestward, the Mural Limestone wedges out, and the terrestrial deposits alone 

remain.

North of Texas, small outliers of Comanche rocks are 

scattered over the Permian rocks in western Oklahoma, and a larger remnant occurs at the edge of the 

Great Plains in southern Kansas, where the series is 

represented by the Cheyenne Sandstone and Kiowa 

Shale (Merriam, 1963, p. 60-61). The latter contains 

thin limestones and shell beds whose fossils are of 

Washita age, the older units having disappeared by 

overlap. Farther north, the Comanche rocks largely 

wedge out, although their thinned equivalents may be 

represented in the Dakota Sandstone (uKi).

ATLANTIC COASTAL PLAIN

East of Arkansas, Lower Cretaceous rocks are missing for a long distance at the edge of the Coastal Plain, 

and their next appearance is in northern Virginia, 

Maryland, and New Jersey, where they form the 

Potomac Group, typically developed near Washington, 

D.C. (Spangler and Peterson, 1950, p. 62-69). The 

Potomac Group consists of terrestrial sandstones, 

sandy shales, and clays, with local beds of gravel and 

lignite; it is as much as 800 ft (240 m) thick and has 

been divided into the Patuxent, Arundel, and Patapsco 

Formations. Equivalents in New Jersey are in the 

Raritan Formation which is partly Upper Cretaceous. 

Some of thenonmarine Tuscaloosa Formation of North 

Carolina, generally classed as Upper Cretaceous, may 

also be of Potomac age. The Potomac group contains
CRETACEOUS 55

fossil plants at many levels, and the Arundel Formation contains reptiles like those of the Morrison Formation of the Rocky Mountains. According to commonly accepted correlations, the group is of Neocomian 

age at the base but extends into the Aptian Stage.

ROCKY MOUNTAINS

Lower Cretaceous rocks, mostly a few hundred feet 

(60-100 m) thick, are extensive in the Rocky Mountains and in the subsurface in the Great Plains to the 

east, but on the Geologic Map they are for the most 

part merged with the Dakota Group (uKi) of the Upper 

Cretaceous on the Geologic Map. The Dakota itself is a 

problematical unit: In its original area in eastern Nebraska it is all Upper Cretaceous; in other areas the 

so-called Dakota is Lower Cretaceous; in still others 

the Dakota Group includes formations of both ages. In 

the Black Hills of South Dakota and Wyoming, for 

example, rocks mapped as uKi include the Inyan Kara 

Group (Lakota Sandstone, Fuson Shale, and Fall River 

Sandstone, the latter the Dakota Sandstone of original 

reports), the Skull Creek Shale, Newcastle Sandstone, 

and Mowry Shale, the last of late Albian age in other 

words, all Lower Cretaceous. In northeastern New 

Mexico and southwestern Colorado, the thin marine 

Purgatoire Formation, a tongue of the Comanche 

Series, is included in uKi. Similar combinations, or 

others with varied terminologies, occur in other areas.

On the Geologic Map, the only Lower Cretaceous 

rocks represented as such are shown in the west and 

northwest, where the Lower Cretaceous rocks are 

thicker and not involved in the "Dakota problem."

In the thrust belt of southeastern Idaho and western 

Wyoming, the Lower Cretaceous (IK) includes the 

Gannett Group, 3,500 to 5,000 ft (1,000-1,500 m) thick 

(Eyer, 1969); the Bear River Formation, 500 ft (150 m) 

thick; and the Wayan Formation, 3,000-4,000 ft (900- 

1,200 m) thick, which range in age from Neocomian 

through Albian. Only the thin Bear River Formation 

contains marine elements; the Gannett and Wayan are 

continental tectonic deposits related to the growth of 

the thrust belt, with much conglomerate and coarsegrained sandstone and interbedded red and purple 

mudstone. Pauses in tectonic activity during deposition of the Gannett Group are indicated by two units of 

lacustrine limestone.

In west-central Montana the Lower Cretaceous 

series is represented by the Kootenai Formation, about 

1,000 ft (300 m) thick of nonmarine conglomerate and 

purplish or greenish shale and mudstone, probably of 

Aptian age. It lies unconformably on other nonmarine 

deposits, probably equivalent to the Jurassic Morrison 

Formation and is succeeded by marine shales that are 

themselves of high Lower Cretaceous age at their base.

CALIFORNIA AND OREGON

The Lower Cretaceous rocks on the western side of 

the Sacramento Valley in northern California are part 

of the "Great Valley sequence" and form the Shasta 

Series, which has traditionally been divided into the 

Paskenta and Horsetown Formations, although other 

stratigraphic names are now used. It overlies the 

Upper Jurassic Knoxville Formation and is more than 

17,000 ft (5,100 m) thick, ranging in age from Neocomian to Albian (Bailey and others, 1964, p. 130-133). 

At the north end of the valley, it oversteps the Knoxville and lies directly on the eroded surface of the deformed Jurassic and older rocks of the Klamath Mountains and their embedded plutons. The lowest beds are 

nearly identical with those of the Knoxville beneath 

and are distinguished mainly by a different species of 

Buchia. Higher up, they are somewhat more varied, 

with layers of graywacke, conglomerate, and 

mudstone, and thin limestone interbeds, the whole still 

a turbidite (flysch) deposit like the preceding Knoxville. To the west within the Coast Ranges are long 

narrow outliers of Lower Cretaceous rocks, which are 

in fault contact with the adjacent Franciscan assemblage and differ notably from it in their lack of 

metamorphic rocks, greenstone, and serpentinite even 

though sparse fossils in the Franciscan prove that part 

of it is younger. These outlying areas of Lower Cretaceous are evidently klippen of the Coast Range 

thrust sheet which forms the base of the main Great 

Valley sequence to the east.

Lower Cretaceous rocks are also exposed in the 

southern Coast Ranges and along the west side of the 

San Joaquin Valley, but their occurrence here is more 

sporadic than farther north.

In the structurally complex area of southwestern 

Oregon, small areas of Lower Cretaceous rocks are 

part of the Myrtle Group. The Myrtle lies unconformably on the Jurassic rocks of the Galice Formation and 

is faulted against the Dothan Formation (= Franciscan). Above its basal conglomerates are rhythmically 

bedded sandstones and mudstones with interbedded 

shelly layers, probably mainly shallow-water deposits 

with only a few deep-water turbidites.

WASHINGTON

In the northern part of Washington State, in the 

Methow River valley east of the Cascade Range, Cretaceous rocks occupy a downfaulted trough 20 mi (32 

km) wide that extends southward from the Canadian 

border nearly to the Columbia River and for an even 

greater distance northward into British Columbia 

(Barksdale, 1975, p. 24-50). The rocks are thick and 

steeply folded but not metamorphosed like those that
56 PALEOZOIC AND MESOZOIC ROCKS

flank them on the east and west. The rocks are all 

clastic and include lithic sandstone, arkose, black 

shale, and chert- and granite-pebble conglomerate, of 

marine origin (in the Buck Mountain, Goat Creek, 

Panther Creek, Harts Pass, and Virginian Ridge Formations), followed by continental arkose (Winthrop 

Sandstone), and topped by andesite tuff, breccia, and 

flows (Midnight Peak Formation). Total thickness is as 

much as 50,000 ft (15,000 m). The sequence is fossiliferous, and ranges in age from Neocomian to 

Cenomanian, with possible Jurassic at the base; the 

volcanics at the top (Kv) may be Turonian. All the 

rocks are represented as Lower Cretaceous (IK) on the 

Geologic Map.

UPPER CRETACEOUS

WESTERN GULF COASTAL PLAIN

The well-developed Upper Cretaceous sequence in 

Texas, the Gulf Series, provides a standard of reference 

for sequences elsewhere in the Coastal Plain. (For a 

useful summary, now somewhat outdated, see Adkins 

(1932, p. 400-516). Correlations are presented by 

Stephenson and others (1942).) On the Geologic Map, it 

is divided into the Woodbine Sand (uKi) of Cenomanian age; the Eagle Ford Shale and Austin Chalk (uK2) 

of Turonian, Coniacian, and Santonian age; the Taylor 

Marl (uKs) of Campanian age; and the Navarro Group 

(uK4) of Maestrichtian age. Most of the exposed Upper, 

Cretaceous is normal neritic fossiliferous shales, 

marls, and chalks; however, marginal basal clastic 

rocks occur in the Woodbine, and terrestrial coalbearing beds occur in the Navarro near the Rio 

Grande.

The Gulf Series is everywhere unconformable on the 

Comanche Series in surface outcrops. The Woodbine 

Sand truncates all the groups of the Comanche eastward into Arkansas. South westward, the Woodbine itself wedges out on the unconformity and disappears 

near Waco. Beyond, in west Texas, the Eagle Ford 

Shale (uK2) lies with a hiatus on the Washita Group 

(IKa). The Gulf Series is followed at all places by a 

disconformity, which separates it from the overlying 

Midway Group of Paleocene age.

The Woodbine Sand (uKi) is mainly poorly consolidated sand, in part leaf bearing and probably nonmarine, which intertongues with clays, some of them 

lignitic but some of them oyster-bearing nearshore 

brackish water deposits. In places it contains much volcanic material and in Arkansas includes interbeds of 

gravel.

The Eagle Ford Shale (uKa) in its typical area is 

marine black shale but changes laterally into marls, 

and in much of west Texas it is calcareous flagstone

(Boquillas Flags). The Austin Chalk is a solid body of 

white chalk in its type area, but with a tongue of marl 

and clay (Bonham) in the middle toward the northeast. 

The typical chalk weathers to a rich black soil, extensively planted in cotton.

The Taylor Marl or Group (uKa) is more varied than 

the units that precede it. In its typical area it is largely 

marl, but northeastward in Texas and Arkansas it contains many traceable units of sand and chalk that are 

separately named, and for some distance west of San 

Antonio the marls are partly or wholly replaced by the 

reef deposit of the Anacacho Limestone. The sand units 

are marginal deposits that indicate the ephemeral existence of nearby shorelines.

The Navarro Group (uK4) is equally varied and has 

been divided into many named formations. North of 

Austin, for example, it consists of the Neylandville 

Marl, Nacatosh Sand, Corsicana Marl, and Kemp Clay. 

Near the Rio Grande, the lower half of the group passes 

into terrestrial coal-bearing deposits (Olmos Formation), which are more important in the Sabinas basin in 

Mexico to the south.

EASTERN GULF COASTAL PLAIN

Upper Cretaceous rocks reappear in the eastern part 

of the Gulf Coastal Plain, beyond the wide gap of the 

Mississippi Embayment where they are covered by 

Cenozoic deposits, and exhibit the same lithologies as in 

Texas, but arranged in a different order. (For a summary, now somewhat outdated, see Stephenson (1926, 

p. 231-245). Some of the later developments are presented by Eargle (1953).) Lower Cretaceous does not 

emerge in the region; the basal deposit is Upper Cretaceous throughout.

The sequence is best developed in Alabama, where it 

comprises the Tuscaloosa Formation (uKi), approximately equivalent to the Woodbine Sand; the Eutaw 

Formation (uK2), approximately equivalent to the 

Eagle Ford and Austin Formations; the Selma Chalk 

(uKa), approximately equivalent to the Taylor Marl; 

and the Ripley Formation (uK4), approximately equivalent to the Navarro Group.

The Tuscaloosa Formation consists of irregularly 

bedded nonmarine sands, clays, and gravels, in places 

lignitic and with fossil plants at many levels. The 

Eutaw Formation is a marine deposit, mainly 

glauconitic and micaceous sand with some interbedded 

clay. The Selma is a thin-bedded to massive chalk, 

much like the Austin although one stage younger (uKa 

rather than uKa), but it fingers out into sands and clays 

northwestward in Mississippi and Tennessee and 

eastward in Georgia. The Ripley, like the Eutaw, is 

marine sands and clays, in part glauconitic. Toward 

the head of the Mississippi Embayment, in Kentucky
CRETACEOUS 57

and Illinois, all the lower part of the Upper Cretaceous 

sequence wedges out by overlap, and so the Ripley lies 

directly on Paleozoic rocks.

Eastward in Georgia, South Carolina, and North 

Carolina, the Eutaw Formation wedges out, and so the 

higher units (uKa and uKU) lie with a hiatus on the 

Tuscaloosa Formation (uKi). In eastern Georgia and in 

South Carolina, the outcrop belt of the Upper Cretaceous rocks is much interrupted by overlapping Tertiary deposits, and the next large outcrop area is in 

southern North Carolina, where the Upper Cretaceous 

extends nearly to the coast along the broad upwarp of 

the Cape Fear arch. Here, the Tuscaloosa is a nonmarine sandy and gravelly deposit as it is in Alabama, 

but the higher Black Creek and Pee Dee Formations 

(uKa, uKu) differ from the Alabama units, being 

marine clays, sands, and marls.

ATLANTIC COASTAL PLAIN

The Upper Cretaceous sequence is concealed by overlapping Tertiary deposits in northern North Carolina 

and throughout Virginia, but it reappears in Maryland 

and extends through New Jersey into the New York 

City area, resting on the Lower Cretaceous Potomac 

Group, which it bevels to the northeast (Spangler and 

Peterson, 1950, p. 15-52).

The Upper Cretaceous comprises the Raritan, 

Magothy, Matawan, and Monmouth Formations (or 

Groups), which span the whole period from Cenomanian to Maestrichtian, but they are all thin units and 

their outcrop bands are so narrow that the whole is 

represented on the Geologic map as an undivided unit 

(uK). The Upper Cretaceous is marine, in contrast to the 

Potomac Group, and is composed of sands, clays, and 

marls, including many beds of highly glauconitic greensand.

GREAT PLAINS AND ROCKY MOUNTAINS

By far the largest area of outcrop of Upper Cretaceous 

rocks in the United States is in the central and northern Great Plains and the contiguous Rocky Mountains 

(fig. 15). These rocks are the product of a single great 

seaway that connected the Gulf of Mexico on the south 

and the Arctic Ocean on the north. The eastern featheredg'e of the deposit is in eastern Kansas and western 

Iowa and Minnesota, whence it extends 800 mi (1,300 

km) westward through the Rocky Mountains to the 

front of the Cordilleran thrust belt in the Northern 

Rocky Mountains and the eastern Great Basin. 

Throughout this area the Upper Cretaceous was originally a continuous deposit; it is now interrupted in the 

Rocky Mountains where it has been eroded from the 

uplifts, and it is covered in many areas in the Great 

Plains by Tertiary deposits.

There is a vast literature on the Upper Cretaceous in 

the region which it would be fruitless to attempt to 

document on its stratigraphy, paleontology, sedimentology, and economic potential but most of the publications deal with special areas or problems. General 

syntheses are few; Cobban and Reeside (1962) have 

presented the correlations, and a general survey of the 

rocks and their problems appears in the "Geologic 

Atlas of the Rocky Mountain Region" (McGookey and 

others, in Mallory, 1972, p. 190-228).

The gross units of the Upper Cretaceous in the region 

are the Dakota Group (uKi), the Colorado Group (uK2), 

the Montana Group (uKa), and the Laramie and associated formations (uKU). These are approximately 

equivalent to the Woodbine Sand, the Eagle Ford Shale 

and Austin Chalk, the Taylor Marl, and Navarro 

Group, respectively, of the western Gulf Coastal Plain, 

and the symbols are used interchangeably between the 

two regions, although they are not precisely correlative 

in detail. Thus, the Dakota Group (uKi) toward the east 

is basal Upper Cretaceous, but as mapped farther west 

it includes Lower Cretaceous and in places is entirely 

Lower Cretaceous; the upper part of the Montana Group 

(uKa) includes equivalents of the lower part of the 

Navarro Group.

The Upper Cretaceous sequence is 2,000 ft (600 m) 

thick or less in the eastern Great Plains, but it thickens 

to 20,000 ft (6,000 m) at the front of the Cordilleran 

thrust belt on the west. As these thicknesses imply, the 

dominant sediment source was in the Cordilleran region to the west, where erogenic deformation was in 

progress during much of the period and was accompanied by thrusting, volcanism, and batholithic intrusion, whose erosion products were shed eastward into 

the Cretaceous seaway. The eastern side of the seaway 

provided only minor sediment sources; only the basal 

Dakota Sandstone along the eastern margin appears to 

have been derived from the craton.

The Upper Cretaceous sequence in the Great Plains 

is relatively simple. The Dakota Sandstone (uKi) at 

the base is a terrestrial marginal deposit. The succeeding Colorado Group (uK2) includes dark shale and 

widespread carbonate deposits the thin Greenhorn 

Limestone in the Benton Group below, and the thicker 

Niobrara Chalk above, which is much like the Austin 

Chalk of the western Gulf Coastal Plain and of about 

the same age. The dark marine Pierre Shale dominates 

the Montana Group (uKa) above, but it is topped by 

thinner marginal deposits of the Fox Hills Sandstone. 

This is succeeded by terrestrial coal-bearing deposits 

known from place to place as the Laramie, Lance, or 

Hell Creek Formations (uKU), which are overlain, 

mostly conformably, by the similar Paleocene terrestrial deposits of the Fort Union Formation.
58 PALEOZOIC AND MESOZOIC ROCKS

Along the eastern margin of the Upper Cretaceous 

outcrop its subdivisions are thin and heavily drift 

covered; so, they are not divided on the Geologic Maps 

of Iowa or Minnesota, nor on the Geologic Map of the 

United States. In considerable areas in northern Minnesota, the thin unconsolidated Coleraine Formation 

of Upper Cretaceous age lies between the glacial drift 

and the Precambrian basement (Sloan, 1964) but is not 

shown on the Geologic Map (King and Beikman, 1974, 

fig. 13).

Complications develop westward in the Rocky Mountain Region. Carbonate deposits, such as the Niobrara, 

fade out in the marine shales, and in the shales appear 

westward-thickening wedges of coarser elastics 

shallow-water sandstones at their proximal ends 

changing distally into coal-bearing terrestrial deposits. A minor wedge, the Frontier Formation, occurs 

low in the Colorado Group in western Wyoming, but 

the main wedges, which are higher up, near the middle 

of the Montana Group, are known in the Southern 

Rocky Mountains and Colorado Plateau as the 

Mesaverde Formation and in Montana as the Judith 

River Formation. The Mesaverde wedge partitions the 

marine shales into the Mancos Shale below and Lewis 

Shale above. The Mesaverde wedges have irregular 

distal ends, and so a unit referred to as Mesaverde at 

one place may be higher or lower stratigraphically 

than the Mesaverde at another and the ages of the 

enclosing marine shales may also differ accordingly.

These intertonguing relations pose problems for representation on the Geologic Map. It would be instructive 

to be able to differentiate on the map between the 

marine deposits and the clastic wedges with their terrestrial deposits, but the various wedges produce so 

complex a pattern that it is not feasible on the scale of 

the map. On the Geologic Map the divisions shown the 

Dakota, Colorado, Montana, and Laramie are therefore solely time-stratigraphic, regardless of facies at 

any particular place. The Montana Group covers an 

enormous area in the northern Great Plains, occupying 

nearly half of North and South Dakota, and its vast 

Pierre Shale has no regionally distinguishable subdivisions. In the plains of Montana, however, clastic wedges 

such as the Judith River Formation make subdivision 

possible. Here, in order to clarify the geology and bring 

out the structure, the Montana Group is divided into 

two parts, uKaa and uKab, using the base of the Judith 

River Formation as the line of separation.

In northeastern Utah, along the south edge of the 

Uinta basin, the Castlegate Sandstone in the clastic 

wedge of the Mesaverde Group is traceable westward 

into the coarser Price River Formation. The Price 

River becomes a red bouldery piedmont deposit on the 

edge of the Wasatch Mountains, where it lies unconformably on a rough erosion surface of strongly deformed older Mesozoic and Paleozoic rocks (Spieker, 

1946, p. 130-132). Moreover, beneath the unconformity is an older coarse piedmont deposit, the Indianola 

Formation, of Colorado age. There is no unconformity 

at the base of the Indianola, but it is obviously related 

to a newly deformed terrane not far to the west. The 

Mesaverde clastic wedge in this segment can be directly related to erogenic activity in the Cordilleran 

belt to the west, and relations are probably similar for 

most of the Upper Cretaceous clastic wedges of the 

Rocky Mountain Region, although the actual connections are not so clearly preserved as in Utah. The different pulses of erogenic activity produced a succession 

of transgressions and regressions in the Upper Cretaceous deposits, of which there are four principal ones; 

transgressions are produced when the marine shales 

spread westward, and regressions when the clastic 

wedges advanced eastward (McGookey and others, in 

Mallory, 1972, p. 206).

The piedmont deposits in Utah of Colorado and Montana age are manifestations of the Sevier orogeny 

(Armstrong, 1968, p. 444-449), in the Cordilleran 

miogeosyncline in Utah, southeastern Idaho, and 

southwestern Wyoming, that resulted in eastward 

transport of thick miogeosynclinal Paleozoic rocks over 

thinner cratonic sequences on a series of gently dipping thrust faults. These episodes of thrusting began 

during the Jurassic and culminated during Late Cretaceous time, although there were minor episodes as 

late as the Eocene Epoch (Armstrong and Oriel, 1965, 

p. 1857-1861).

Different in style and time from the Sevier orogeny 

is the type Laramide orogeny in the eastern ranges of 

the Central and Southern Rocky Mountains, generally 

assumed to have terminated the Cretaceous Period in 

those areas. The Laramide orogeny resulted mainly in 

upthrusting of the ranges and depression of the intervening basins. These movements began in Late Cretaceous time, as indicated by thickness variations in 

the higher Cretaceous basin deposits in the Central 

and Southern Rocky Mountains, but the principal unconformities resulting from the orogeny are between 

the Paleocene and Eocene deposits, rather than at the 

top of the Cretaceous sequence. In the Northern Rocky 

Mountains, however, from Montana northward into 

Alberta, thrusting like that produced during the 

Sevier orogeny continued later and reached its climax 

during Laramide time (latest Cretaceous and 

Paleocene).

The true top boundary of the Cretaceous System in 

the Rocky Mountains was long controversial the 

"Laramie question" which was debated for many years 

following the work of the Hayden Survey a century ago
CRETACEOUS 59

(Merrill, 1904, p. 647-658). It was observed that 

Triceratops, the last of the dinosaurs, was found in the 

Laramie beds, whereas Tertiary mammals were abundant in the succeeding Fort Union Formation; 

nevertheless, it was claimed that the fossil plants in 

both the Laramie and Fort Union were of Tertiary aspect. During Laramie and Fort Union time, the Rocky 

Mountains were rising, as shown by erosional debris in 

these formations, and it was assumed that an immense 

unconformity lay concealed in these deposits in the 

plains, which would presumably mark the top of the 

Cretaceous. Much futile effort was expended in a 

search for this unconformity. These questions are now 

largely resolved, and the vertebrate and paleobotanical evidence reconciled. Concepts have been further 

clarified by recognition of the Paleocene as a separate 

series of the Tertiary Period and by classification of the 

Fort Union as Paleocene rather than Eocene.

PACIFIC COASTAL AREA

Upper Cretaceous rocks of the Pacific coastal area 

occur mainly in California, and in a few minor extensions in southwestern Oregon. In northern California 

they are the upper part of the "Great Valley sequence," 

and have sometimes been called the Chico Series. This 

term is inappropriate, as the type Chico is a thin nearshore deposit of Campanian age that overlaps the 

basement rocks of the Sierra Nevada, whereas the 

main body on the west side of the valley is a more 

complete sequence of deeper water deposits more than 

15,000 ft (4,500 m) thick, divisible into a number of 

formations (Bailey and others, 1964, p. 133-135). In 

one segment it consists of the Venado, Yolo, Sikes, 

Funks, Guinda, and Forbes Formations, which are of 

Cenomanian to Campanian age. All of them are interbedded sandstone, siltstone, and shale a typical turbidite or flysch sequence the different formations 

being distinguished mainly by varying proportions of 

the coarse and fine clastic components. In places the 

lower part contains lenses of slumped material, including large boulders of quartz diorite, evidently derived 

from Sierra Nevada basement, against which the deposit probably overlaps in subsurface not far to the 

east.

Upper Cretaceous rocks extend southward along the 

west side of the San Joaquin Valley and are well displayed along the eastern flank of the Diablo Range, 

where they are 25,000 to 30,000 ft (7,600-9,000 m) 

thick, with the thick Panoche Formation below and the 

thin Moreno Shale above. The Panoche is again an 

alternation of sandy and shaly beds, a turbidite or 

flysch sequence, but it locally contains thick lenses of 

conglomerate that includes clasts of porphyry and 

granitic rocks. The Panoche lies in places on thin remnants of Lower Cretaceous and Jurassic rocks, but its 

base is mainly faulted against the Franciscan rocks of 

the Diablo Range along a segment of the Coast Range 

thrust. The Upper Cretaceous sediments were derived 

mainly from the Sierra Nevada to the east, and there is 

no detritus that can be attributed to the crystalline 

basement of the Salinian block, across the San Andreas fault immediately to the west. Within the Salinian block itself, late Upper Cretaceous rocks (Campanian and Maestrichtian) of the Asuncion Group, of a 

different facies, lie on the crystalline rocks.

Other thick sequences of Upper Cretaceous rocks are 

preserved in places farther south in the Coast Ranges 

and in the western part of the Transverse Ranges, and 

Upper Cretaceous deposits lie unconformably on the 

Jurassic sequence (Bedford Canyon and Santiago Peak 

Formations) in the Santa Ana Mountains, at the 

northwestern end of the Peninsular Range of southern 

California. These last range in age from Cenomanian 

to Campanian and are divided into the Trabuco, Ladd, 

and Williams Formations.

At the eastern edge of the Klamath Mountains near 

the California-Oregon border, deformed and metamorphosed Mesozoic and Paleozoic eugeosynclinal rocks 

are overlain by the Upper Cretaceous Hornbrook Formation, which dips gently eastward beneath the 

Eocene volcanic rocks of the Cascade Range. It is a 

body of sandstone, siltstone, and mudstone about 2,500 

ft (760 m) thick, with marine fossils of Cenomanian 

and Campanian age at several levels (Peck and others, 

1956).

CONTINENTAL DEPOSITS (Kc)

Continental deposits of Cretaceous age (Kc) are 

separately mapped in parts of the Cordilleran Region. 

The designation is used especially for the coarser 

poorly stratified deposits. Excluded are the finer 

grained stratified deposits more intimately associated 

with the normal marine sequence, such as the Lower 

Cretaceous Gannett Group and Wayan and Kootenai 

Formations, and the Upper Cretaceous coal-bearing 

terrestrial wedges of the Mesaverde and other formations. It is used especially in central Utah for the 

coarse piedmont deposits next to the Sevier erogenic 

belt, such as the Indianola and Price River Formations.

Farther west, in the Great Basin, are small isolated 

areas of Cretaceous continental deposits that probably 

formed in local basins. They are typified by the 

Newark Canyon Formation of the Eureka district, 

central Nevada (Nolan and others, 1956, p. 66-70), 

which is a heterogeneous deposit about 2,000 ft (600 m) 

thick of siltstone and conglomerate, with many layers 

of freshwater limestone, that lies on the truncated and 

eroded surface of all the Paleozoic formations of the
60 PALEOZOIC AND MESOZOIC ROCKS

district. The limestone contains gastropods of Early 

Cretaceous age, as well as plant and fish fossils.

About 150 mi (250 km) farther northwest, in the 

Jackson Mountains of northwestern Nevada, are other 

small areas of Cretaceous continental deposits 

(Willden, 1958), which lie on Triassic and Permian 

eugeosynclinal rocks ffiPe). The King Lear Formation 

consists of conglomerate and siltstone with some beds 

of freshwater limestone that contain Lower Cretaceous 

gastropods like those in the Newark Canyon Formation. It is overlain in places by another conglomeratic 

deposit, the unfossiliferous Pansy Lee Formation, 

which likewise predates the Tertiary volcanic rocks of 

the area. Both units are indicated as Cretaceous continental deposits (Kc) on the Geologic Map.

In southern Nevada, east of Las Vegas, is another set 

of Cretaceous continental deposits, which lie unconformably on deformed older rocks and are themselves 

much deformed (Longwell and others, 1065, p. 41-45). 

On the eastern flank of the Muddy Mountains are the 

Willow Tank Formation and Baseline Sandstone, 

about 4,000 ft (1,200 m) thick, which contain fossil 

plants of middle Cretaceous age. The Willow Tank and 

Baseline are succeeded by the mass of the much 

coarser Overton Fanglomerate, which contains large 

blocks and slabs of the Paleozoic formations. Farther 

west, southeast of Frenchman Mountain, is the Thumb 

Formation, much like the Willow Tank and Baseline 

Formations, but containing lenses of coarse breccia 

composed of Precambrian metamorphic rocks. All 

these units are synorogenic deposits, laid down while 

deformation was in progress in the region.

In the ranges of the southwestern desert region of 

Arizona, many areas of Mesozoic sedimentary rocks 

are indicated on the State Map (1969). They are a deformed sequence of shale, sandstone, limestone, and 

conglomerate, less metamorphosed than the rocks on 

which they lie and overlain unconformably by Tertiary 

volcanic rocks. Although unfossiliferous, they are 

classed as Cretaceous continental deposits (Kc) on the 

Geologic Map.

Far to the north, in the Purcell Trench north of Pend 

Oreille Lake in northern Idaho, are small areas of the 

Sandpoint Conglomerate, formed of clasts of the Belt 

rocks, lying in a terrane of the Belt Supergroup and 

Cretaceous plutons (Harrison and others, 1972, p. 6). 

No data for its age are available; it was once assigned 

to the Paleozoic, but the preponderance of evidence 

now suggests that it is Cretaceous (Kc) and is so indicated on the Geologic Map.

EUGEOSYNCLINAL DEPOSITS (Ke)

Cretaceous eugeosynclinal deposits are dealt with at 

the close of a later section entitled "Upper Mesozoic."

VOLCANIC ROCKS (Kv)

Small areas of volcanic rocks of Cretaceous age (Kv) 

are shown in various parts of the Cordilleran Region 

on the Geologic Map.

Of these, the most significant are those surrounding 

the Boulder batholith in west-central Montana (Robinson and others, 1968, p. 563-569). The Elkhorn Mountain Volcanics which adjoin the batholith and form 

part of its roof are remnants of a volcanic accumulation 

that was probably originally 10,000 ft (3,000 m) thick, 

the lower part mainly andesite and rhyodacite breccia 

and lava, the middle part rhyolite welded tuff, and the 

upper part erosional debris derived from the lower 

members. It lies unconformably on Upper Cretaceous 

sedimentary rocks of Santonian age and is overlain 

unconformably by middle Eocene volcanic rocks. 

Radiometric dating has yielded ages of about 73 to 78 

m.y. and suggests that the eruptive episode lasted for 

about 4 m.y. in Campanian time. The period of eruption overlapped that of emplacement of the Boulder 

batholith itself, which has been dated between 71 and 

82 m.y.

Northeast of the Elkhorn Mountain Volcanics and 

the Boulder batholith, in the outer thrust zone of the 

Northern Rocky Mountains, are the Adel Mountain 

Volcanics, which are somewhat younger, more alkalic, 

and petrographically different.

Farther southeast in Montana, between the Crazy 

Mountains and Bearpaw Mountains, the volcanic rocks 

of the Grey Cliff field lie on various Upper Cretaceous 

units as young as the Hell Creek Formation (uKO but 

are indicated on the source maps as Cretaceous. The 

Late Cretaceous and Paleocene rocks of the Crazy 

Mountains basin themselves contain large quantities 

of andesitic debris but are included in the stratified 

sequence on the Geologic Map.

Other areas of Cretaceous volcanic rocks (Kv) are 

shown on the Geologic Map in southwestern New 

Mexico and southeastern Arizona. Some of them are 

interbedded with fossiliferous sedimentary rocks as old 

as Lower Cretaceous, and others follow conformably on 

the highest Cretaceous sediments. The Geologic Map of 

Arizona (1969) likewise indicates as Cretaceous the 

older volcanic rocks of many of the ranges in the 

southwestern part of the State. Evidence for their age, 

however, is inconclusive, and the Cretaceous designation is based mainly on structural evidence; they are 

cut by intrusives of supposed "Laramide" age and are 

unconformably overlain by undoubted Tertiary volcanic rocks. Some of these rocks may indeed be Cretaceous, but an early Tertiary age for most of them 

seems to accord better with the volcanic sequence in 

adjoining States; they are therefore marked as lower 

Tertiary volcanics (ITv) on the Geologic Map.
UPPER MESOZOIC 61

UPPER MESOZOIC

UPPER MESOZOIC EUGEOSYNCLINAL DEPOSITS (ulVfee)

The upper Mesozoic eugeosynclinal deposits are 

primarily represented by the Franciscan Formation, or 

assemblage, which dominates the coastal region of 

California, with extensions northward into southwestern Oregon and southward into Baja California (Bailey 

and others, 1964). The Franciscan forms much of the 

surface of the Coast Ranges north of San Francisco Bay 

and extends into Oregon as the Dothan Formation. 

Similar rocks extend westward to the coast, but part of 

them in this segment are treated separately as "Cretaceous eugeosynclinal deposits" (Ke). South of San 

Francisco Bay the Franciscan forms the basement 

northeast of the San Andreas fault as far as the south 

end of the San Joaquin Valley. Southwest of the San 

Andreas fault in this latitude is the different basement 

terrane of the structural block of Salinia, composed of 

the metamorphic Sur Series (uF2) and intrusive Cretaceous granite (Kg), but the Franciscan reappears 

along the coast in the southern Coast Ranges, beyond 

the Nacimiento fault. South of the Transverse Ranges 

in southern California is a large area of Franciscan 

rocks, mainly offshore, whose presence is indicated by 

small outcrops on Santa Catalina Island and in the 

Palos Verdes Hills and by abundant blueschist and 

other Franciscan debris in the San Onofre Breccia of 

Miocene age along the coast.

The Franciscan is a chaotic partly metamorphosed 

assemblage of graywacke and shale, with interbedded 

pillow basalt, radiolarian chert, and minor limestone, 

in which masses of serpentinite and other ultramafic 

rocks are embedded; its thickness is indeterminate but 

is on the order of 30,000 ft (9,000 m) or more. Fossils 

are rare, but enough have now been discovered to indicate that it includes rocks of Late Jurassic (Tithonian) 

to Late Cretaceous (Turonian and even Campanian) 

age, approximately coeval to the "Great Valley sequence" to the east and younger than the mid-Jurassic 

Nevadan orogeny. No base of the Franciscan is known, 

but it probably lies on an oceanic crust. The assemblage is a submarine trench and ocean-floor deposit 

that has been crowded and subducted against the continent to the east and added to it during late Mesozoic 

and early Tertiary time. The next strata in depositional contact above the Franciscan are Tertiary and 

commonly Miocene; older Tertiary and Cretaceous 

rocks which adjoin it are commonly faulted against it.

Parts of the Franciscan are a melange of tectonically 

disordered blocks and slabs of all sizes, formed of rocks 

of heterogeneous lithologies, origins, and ages (Hsu, 

1968). Other parts are straight-forward sequences, or 

"broken formations" at most. In those parts of the

northern Coast Ranges where the structure has been 

studied in most detail, thick units of melange alternate 

with thick units of more straightforward sequences. In 

these areas, the Franciscan is found to be divided into a 

succession of east-dipping tectonic slices, the higher 

slices to the east containing the oldest rocks of Jurassic 

age and the lower slices to the west containing rocks of 

successively younger Cretaceous ages. Further complications and disorder are produced by north-northwesttrending strike-slip faults of the San Andreas fault 

family, which further sheared and displaced the rocks 

during Tertiary time.

Strike-slip faulting of large displacement is probably 

responsible for the introduction of the crystalline 

basement mass of Salinia between the two Franciscan 

areas in the southern Coast Ranges. It is a reasonable 

assumption that all the now-separated bodies of Franciscan rocks originally formed a continuous body in a 

deep-water offshore regime and that Salinia is a sliver 

of continental rocks, detached from some area farther 

south and moved into a position foreign to it. The granites of Salinia are shown by radiometric dating to be of 

Cretaceous age, as young as or younger than the Franciscan rocks that adjoin them; yet, their only contacts 

are faults, and there is no indication of any contact 

metamorphism in the Franciscan or any debris in the 

Franciscan derived from Salinia.

The potassium feldspar content of the graywackes in 

the Franciscan and the Great Valley sequence is of 

interest (Bailey and others, 1964, p. 139-147). The 

graywackes in the Great Valley sequence show a progressive increase in potassium feldspar from Jurassic to 

Upper Cretaceous time, suggesting that during this 

interval the granitic plutons of the Sierra Nevada and 

Klamath Mountains were becoming more and more 

unroofed and subject to erosion. Remarkably, 

graywackes in the coeval Franciscan immediately to 

the west contain little or no potassium feldspar, except 

in the coastal belt (Cretaceous eugeosynclinal deposits, 

Ke). Possibly the source of the Franciscan graywackes 

was different from that of the Great Valley sequence.

The Franciscan rocks have been rather pervasively 

metamorphosed to the blueschist (high-pressure lowtemperature) facies (Bailey and others, 1964, p. 89- 

112). For the most part, this is barely perceptible; a 

zeolite facies with laumontite is near the coast, and 

farther inland the graywackes have been jadeitized, 

without altering their primary sedimentary structures. Higher grade blueschist with pumpellyite, 

glaucophane, and lawsonite occurs in a band along the 

eastern side of the Franciscan area, close to the Coast 

Range thrust, and its extreme phase has been named 

the South Fork Mountain Schist (Blake and others, 

1967); it is marked by a metamorphic overprint on the
62 PALEOZOIC AND MESOZOIC ROCKS

Geologic Map. A large outlying area, the Colebrooke 

Schist, occurs in southwestern Oregon (Coleman, 1972, 

p. 27-58). The schists decrease in grade and intensity 

downward from the sole of the fault, and so rocks on 

the ridgetops are more metamorphosed than those in 

the intervening valleys. Radiometric dating by the 

K/Ar method indicates that the metamorphism of the 

Franciscan rocks has a range in age from 70 to 150 

m.y., or about the same time span as the age of the 

assemblage itself as indicated by fossils, showing that 

metamorphism went on hand in hand with the 

sedimentation (Suppe and Armstrong, 1972). The oldest dates are in the South Fork Mountain Schist on the 

east, and younger dates are farther west.

Also shown on the Geologic Map as upper Mesozoic 

eugeosynclinal deposits (with a metamorphic overprint) are an assortment of metamorphic rocks of 

greenschist facies east of the Franciscan area in southern California. They include the Pelona Schist, close to 

the San Andreas fault in the San Gabriel and San Bernardino Mountains, the Orocopia Schist in the desert 

ranges east of the Salton Trough farther southeast, 

and the Rand Schist in the Mojave Desert to the northeast (Ehlig, 1968). Although these have been called 

Precambrian, they are everywhere in fault contact 

with the true Precambrian as well as with the late 

Paleozoic Mount Lowe Granodiorite, and they lack the 

pervasive plutonism of the Precambrian rocks. Various 

lines of indirect evidence suggest that the schists are 

70 to 100 m.y. old. The original rocks were interbedded 

graywackes, siltstones, and shales; they may be more 

or less equivalent to the Franciscan rocks to the west, 

but in a greenschist, rather than a blueschist facies.

In the northern part of Washington State, on the 

western flank of the Cascade Range, is the Nooksack 

Group, which contains fossils of Late Jurassic and 

Early Cretaceous age (Kimmeridgian to Valanginian) 

(Misch, 1966, p. 118) and is therefore shown as upper 

Mesozoic eugeosynclinal deposits (ulVfee) on the 

Geologic Map. The Nooksack is a flyschlike sequence of 

graywackes and siltstones containing volcanic detritus 

and a few lenses of conglomerate, the whole a deepwater deposit laid down under conditions of tectonic 

unrest. As described, the Nooksack somewhat resembles the Franciscan but does not have its chaotic 

structure.

CRETACEOUS EUGEOSYNCLINAL DEPOSITS (Ke)

In the northern Coast Ranges, a western belt of 

Franciscan-type rocks differs significantly from the 

main body of the Franciscan farther east; the belt extends for 150 mi (250 km) along the coast south of Cape 

Mendocino and for as much as 30 mi (50 km) inland. Its 

rocks have less structural disorder than those farther

east and are mainly graywackes; interbedded lava and 

chert are rare. The graywackes contain appreciable 

quantities of potassium feldspar, in contrast to the 

Franciscan farther east (Bailey and others, 1964, p. 

140); graywackes containing potassium feldspar in the 

San Francisco Peninsula might be correlative. The 

rocks of the coastal belt are the youngest part of the 

Franciscan assemblage; at least part of the belt is of 

Late Cretaceous age, as indicated by occasional fossils, 

and it is accordingly mapped as Ke. However, dinoflagellates and angiosperm pollen from many localities 

in the belt are definitely Eocene (Evitt and Pierce, 

1975), thus greatly extending the time during which 

rocks of the Franciscan assemblage accumulated.

MESOZOIC PLUTONIC AND INTRUSIVE ROCKS

Plutonic and intrusive rocks of Mesozoic age are a 

significant component of the western part of the Cordilleran Region, where they form about 10 percent of the 

surface (fig. 16). They are less abundant in the eastern 

part of the Cordillera (where the intrusives are mainly 

Tertiary) and are minor in the eastern United States, 

except for the granitic White Mountain Series in 

northern New England. All the Mesozoic periods are 

represented, although plutonic rocks of Cretaceous age 

are by far the most abundant. The ages of the plutonic 

and intrusive rocks are indicated by their relations to 

enclosing rocks and more often by radiometric dating. 

Radiometric dates are now available for most of the 

important plutonic bodies in the United States, and 

such ages can be extrapolated further to undated adjacent bodies of similar character.

TRIASSIC GRANITIC ROCKS (Tig)

Granitic rocks of Triassic age occur in a few places in 

the western part of the Cordilleran Region (fig. 17).

One group of quartz monzonite and granodiorite plutons runs along the eastern edge of the Sierra Nevada 

in eastern California, from Bishop northwestward past 

Mono Lake, and into some of the ranges of western 

Nevada. The intrusive episode represented, designated 

as the Lee Vining epoch (Evernden and Kistler, 1970, 

p. 19), has been dated by K/Ar methods at between 195 

and 210 m.y., or Middle to Late Triassic. These rocks 

are intruded by the more prevalent Jurassic and Cretaceous plutonic rocks of the same area.

In the eastern part of the Blue Mountains of northeastern Oregon, near Sparta, a group of much-sheared 

albitized granites are distinctly older than the quartz 

diorite and granodiorite plutons of Cretaceous age 

(Gilluly, 1933, p. 66-67). Relations to adjacent supracrustal rocks and intrusives suggest that they are of 

Triassic age, which has been confirmed by a few
MESOZOIC PLUTONIC AND INTRUSIVE ROCKS 63

radiometric dates in the range of 210 to 250 m.y. (G. W. 

Walker, oral commun., 1971).

Several bodies of Triassic granitic rocks occur in the 

crystalline complex of northern Washington. In the 

core of the Cascade Range is the Marblemount Quartz 

Diorite, which predates the Cretaceous metamorphism 

of the range and is overlain unconformably by the Cascade River Schist (ms) (Misch, 1966, p. 105). Dating of 

zircons by U/Pb methods yields ages of 215 to 220 m.y., 

or Early Triassic (Mattinson, 1970), and later metamorphism has not reset the zircon ages. In the Okanogan Range to the east are the granitic rocks of the 

Toats Coulee Magma Series (Hibbard, 1971, p. 3029- 

3031), which is composed of granodiorite, tonalite, and 

tonalite porphyry and which is premetamorphic or 

synmetamorphic to a Late Triassic orogeny and older 

than the adjacent plutons of the Cretaceous Horseshoe 

Basin Magma Series. The rocks have yielded a K/Ar 

age of 194 m.y., or Late Triassic.

TRIASSIC MAFIC INTRUSIVES ("fci)

Mention has already been made of the diabase intrusives in the Newark Group of the Appalachian Region 

from New England to Virginia. The thick Palisades sill 

of eastern New Jersey has been dated radiometrically 

at 190 to 200 m.y. (Erikson and Kulp, 1961).

JURASSIC GRANITIC ROCKS (Jg)

Jurassic granitic rocks are more extensive than 

those of the Triassic in the western part of the Cordilleran Region (fig. 18). They also occur in northern New 

England (fig. 16). They are, however, by no means as 

extensive as implied on the Geologic Map of the United 

States of 1932, where all the Mesozoic plutonic rocks of 

the Cordilleran Region were assigned to the Jurassic; a 

large part of these is now known to be of Cretaceous 

age. Jurassic granitic rocks are absent, for example, in 

all the plutons of the northern part of the Cordillera in 

the United States, in Montana, Idaho, Washington, 

and northeastern Oregon.

In California, Jurassic granitic rocks occur in the 

Sierra Nevada and the Klamath Mountains. In the 

Sierra Nevada, the Jurassic granitic rocks occur east 

and west of the axis of the range, which is formed of 

Cretaceous rocks of the main Sierra Nevada batholith 

(Bateman and Wahrhaftig, 1966, p. 115-122). Field relations indicate more than one Jurassic intrusive 

epoch; some plutons in the western foothills are truncated by the Melones fault zone, whereas others are 

not. This is confirmed by K/Ar dating, which indicates 

an early episode of intrusion with ages of 160 to 180 

m.y., called the Inyo Mountains intrusive epoch and 

represented chiefly east of the Sierra Nevada, and a

later episode with ages of 132 to 148 m.y., called the 

Yosemite intrusive epoch and represented on the western slope of the Sierra Nevada (Evernden and Kistler, 

1970, p. 17-19). (These are not separated on the Geologic Map.) The Jurassic granitic rocks of the western 

flank are dominantly quartz diorites and granodiorites, less silicic than the Cretaceous granitic rocks of 

the main batholith. In the foothills the Jurassic granitic rocks form large equidimensional plutons embedded in the Jurassic eugeosynclinal rocks, but they 

merge to the east into a more continuous body which 

forms the western part of the main batholith, as at the 

lower end of Yosemite Valley (El Capitan Granite, and 

so forth). Except for a few minor Cretaceous plutons, 

all the granitic rocks of the Inyo and White Mountains 

are Jurassic of the first epoch.

The granitic rocks of the Klamath Mountains of 

northern California and southwestern Oregon are all 

Jurassic and form large diorite and granodiorite plutons elongated parallel with the trends of the country 

rocks; they are most abundant in the Central 

Metamorphic belt and the Western Paleozoic and 

Triassic belt (Hotz, 1971, p. 15-17). Three epochs of 

intrusion are recognized by radiometric dating: from 

165 to 167 m.y., from 145 to 155 m.y., and from 127 to 

140 m.y., the first and last broadly equivalent to the 

two intrusive epochs in the Sierra Nevada. (Again, 

these are not differentiated on the Geologic Map.) The 

Shasta Bally pluton at the south end of the mountains, 

of the last intrusive epoch, is overlain unconformably 

by the earliest Lower Cretaceous rocks of the Great 

Valley sequence.

Eastward in the Great Basin of Nevada, Jurassic 

granitic rocks (as dated radiometrically) seemingly 

have a random distribution with respect to the Cretaceous granitic rocks. All the plutons, as exposed at 

the surface, are smaller than those in the Sierra 

Nevada, but the true dimensions of some are obscured 

by Tertiary cover. The easternmost Jurassic pluton is 

the Panther Spring Granite, intrusive into the Cambrian strata of the House Range in western Utah and 

dated at 143 m.y.

In northern New England, far from the areas just 

discussed, is the White Mountain Plutonic Series, 

named for its prominent development in central New 

Hampshire but with outlying plutons in Vermont on 

the west and Maine on the east (Billings, 1956, p. 

129-135, 145-146). It is a set of fresh crosscutting intrusions younger than the orogenies in the Paleozoic 

rocks and principally forms ring dikes and cauldron 

subsidences, but with one large batholith (actually an 

aggregate of coalesced ring dikes). It consists of alkalic 

rocks, with some mafic end members, but mainly of 

quartz syenite and alkali granite, of which the most
64 PALEOZOIC AND MESOZOIC ROCKS

< }

o 

r>

m 

>

*,

FIGURE 16. The United States, showing in separate patterns areas mapped as Mesozoic and Cenozoic plutonic and intrusive rocks 

on Geologic Map of the United States. Mesozoic rocks include units of Triassic plutonic and intrusive rocks < f,g, f, i), Jurassic plutonic rocks (Jg, Jmi), and Cretaceous plutonic and intrusive rocks (Kg, Kgi, Kg2, Kg3, Kgn, Ki). Cenozoic rocks include 

units of Tertiary intrusive rocks (Ti).
MESOZOIC PLUTONIC AND INTRUSIVE ROCKS 65

Cenozoic plutonic rocks

Mesozoic plutonic rocks

GULF OF MEXICO

B'IGURE 16. Continued.
66 PALEOZOIC AND MESOZOIC ROCKS

FIGURE 17. Western United States, showing areas of Triassic granitic rocks (Tig) as mapped on Geologic Map of

the United States.
MESOZOIC PLUTONIC AND INTRUSIVE ROCKS 67

/

i....

o

Ci

FIGURE 18. Western United States, showing areas mapped as Jurassic granitic rocks (Jg) and mafic intrusives (Jmi) on Geologic Map of the United States.
68 PALEOZOIC AND MESOZOIC ROCKS

prominent is the Conway Granite. The age of the 

White Mountain Series is commonly quoted as 180 

m.y., or Early Jurassic, on the basis of concordant 

U/Pb, K/Ar, and Rb/Sr determinations on the Conway 

Granite (Lyons and Faul, 1968, p. 312). A wider range 

of sampling of the different White Mountain plutons 

reveals a much greater spread of ages from 110 to 185 

m.y., or from Early Jurassic into Early Cretaceous 

time (Poland and others, 1970). The White Mountain 

epoch thus overlaps that of the Monteregian intrusives 

in Canada to the north, which extend in a chain for 150 

mi (250 km) northwestward from near the border to 

Montreal and which have Cretaceous ages of 84 to 123 

m.y. The White Mountain and Monteregian intrusives 

are evidently closely related, both sequentially and 

magmatically.

JURASSIC MAFIC INTRUSIVES (Jmi)

In the Stillwater and West Humboldt Ranges of 

west-central Nevada are some areas of diorite and 

gabbro, which have been emplaced as tabular masses 

at shallow depths and are associated with basaltic 

lavas (Page, 1965). They have been dated by K/Ar 

methods at 150 m.y., or Late Jurassic.

CRETACEOUS GRANITIC ROCKS (Kg)

The dominant granitic rocks of the Cordilleran Region are of Cretaceous age. They occur throughout the 

length of California and into Oregon, with outlying 

bodies in Nevada and Arizona, and in Montana, Idaho, 

and Washington (fig. 19). Some of them form small to 

moderate-sized plutons, but in places the plutons are 

aggregated into large batholiths, such as the Peninsular Range batholith of southern California, the Sierra 

Nevada batholith farther north, and the Idaho 

batholith in the mountain area of central Idaho. In 

places, the rocks are divided on the Geologic Map according to age into Lower Cretaceous granitic rocks 

(Kgi), Upper Cretaceous granitic rocks (Kg2), and 

latest Cretaceous granitic rocks (Kgs); the gneissic 

border rocks of the Idaho batholith are also differentiated (Kgn). Elsewhere, the Cretaceous granitic 

rocks are not divided (Kg).

The best known Cretaceous granitic rocks are those 

of the Sierra Nevada, which form a continuous body 25 

mi (40 km) or more wide along the crest of the range for 

its entire length (Bateman and Wahrhaftig, 1966, p. 

116-125). Two general times of emplacement are 

represented the Huntington Lake intrusive epoch 

with ages of 104 to 121 m.y., or Lower Cretaceous 

(Kgi), and the Cathedral Range intrusive epoch with 

ages of 79 to 90 m.y., or Upper Cretaceous (Kg2), which

forms the main body (Evernden and Kistler, 1970, p. 

17). The first consists of quartz diorite, quartz monzonite, and granodiorite. The second, represented by the 

Tuolomne Intrusive Series, includes the Sentinel 

Granodiorite, Half Dome Quartz Monzonite, Cathedral 

Peak Granite, and Johnson Granite Porphyry, and is 

more siliceous than the older Cretaceous and Jurassic 

intrusives of the Sierra Nevada. The Late Cretaceous 

age of the youngest granitic rocks of the Sierra Nevada 

raises interesting questions as to their relation to 

sedimentation of the Great Valley sequence, which 

was in progress during this time to the west. During 

emplacement of the batholith, the site of the Sierra 

Nevada was probably being raised and eroded, and the 

batholith surface unroofed, to provide the vast accumulation of Cretaceous sediments in the Great Valley.

Farther south is the equally large mass of the Peninsular Range batholith ("batholith of southern California"), which extends past San Diego into Mexico, 

where it forms the backbone of Baja California as far 

south as the 29th parallel (Larsen, 1954). Unlike the 

Sierra Nevada batholith, it was intruded during a 

single epoch. In the United States it cuts Upper Jurassic rocks and in Baja California cuts Lower Cretaceous 

rocks as young as Albian; its deeply eroded surface is 

overlain by undeformed Upper Cretaceous rocks of 

Campanian and Maestrichtian age. In southern 

California it has been dated by U/Pb methods as between 109 and 120 m.y., and in BajasCalifornia as between 100 and 115 m.y. (Armstrong and Suppe, 1973, 

p. 1385). In both areas, K/Ar dates decrease in the 

easternmost exposures to as little as 80 m.y., but these 

probably reflect cooling events related to greater depth 

of erosion of this part of the batholith. Like the Sierra 

Nevada batholith, the Peninsular Range batholith is 

composed of many plutons, which vary in composition 

from gabbro, through granodiorite and tonalite, to 

granite.

In the Salinian block of the Coast Ranges west of the 

Sierra Nevada, various granitic plutons invade the Sur 

Series (uR) and form parts of the Santa Lucia, Gabilan, 

and Santa Cruz Ranges, as well as the Farallon Islands, Point Reyes, and Bodega Head farther north. 

They include quartz diorite, adamellite, granodiorite, 

and granite. Dating by K/Ar methods yields ages as 

young as 77 m.y., but the time of intrusion is clearly 

older, as the eroded surfaces of the plutons are overlain 

by the Asuncion Group of Late Cretaceous (Campanian) age (Compton, 1966, p. 288-287). Probably this is 

a "cooling date," representing the time when argon 

could be retained in the rock, after uplift from the deep 

crustal level indicated by the high amphibolite and 

granulite metamorphic facies of the enclosing Sur
MESOZOIC PLUTONIC AND INTRUSIVE ROCKS 69

FIGURE 19. Western United States, showing areas mapped as Cretaceous granitic rocks (Kg, Kgl, Kg2, Kg3, Kgn) and Cretaceous intrusive rocks (Ki) on Geologic Map of the United States.
70 PALEOZOIC AND MESOZOIC ROCKS

Series. An Rb/Sr whole-rock date from the Santa Lucia 

Range of 117 m.y., or Early Cretaceous, is probably 

closer to the actual time of intrusion.

Similar young Cretaceous ages have been obtained 

from the granitic rocks of the San Gabriel and San 

Bernardino Mountains and probably are also "cooling 

dates" (Evernden and Kistler, 1970, p. 22; Armstrong 

and Suppe, 1973, p. 1383). Dating by U/Pb methods 

suggests plutonic events between 160 and 170 m.y. and 

75 to 90 m.y. (Silver, 1971).

East of the Sierra Nevada in western and northern 

Nevada, granitic rocks form many small plutons, 

which are shown by radiometric dating to be partly 

Jurassic, partly Cretaceous, and Tertiary. The Cretaceous granites have ages of 87 to 105 m.y. and 68 to 

71 m.y., approximately the same as those of the two 

intrusive epochs in the Sierra Nevada, (Silberman and 

McKee, 1971).

In the Mojave Desert region of southern California 

and the desert ranges of southwestern Arizona are 

many small to moderate-sized bodies of Mesozoic 

granitic rocks, which mostly yield K/Ar and U/Pb ages 

of 64 to 95 m.y., or Late Cretaceous, but also yield from 

150 to 165 m.y. and from 190 to 200 m.y., or Jurassic 

and Triassic (Armstrong and Suppe, 1973, p. 1383- 

1384). However, the extent of the rocks of different 

ages is imcompletely known, and except for one pluton 

in the Clark Mountains of the eastern Mojave Desert, 

all are classed as Cretaceous (Kg) on the Geologic 

Map.

The Idaho batholith sprawls across the mountains of 

central Idaho, from the Snake River Plain to northwestern Montana, with an area of about 16,000 mi2 

(42,000 km2 ) (Ross, 1936). It plunges southward beneath the Cenozoic volcanic rocks of the Snake River 

Plain, but it may be nearly connected in this direction 

with the Sierra Nevada batholith, as numerous inliers of granitic rocks emerge from beneath the Tertiary cover in southwestern Idaho and northwestern 

Nevada.

The Idaho batholith intrudes rocks of the Belt 

Supergroup (Y) on the northeast, lower and upper 

Paleozoic miogeosynclinal rocks (IP, uP) on the 

southeast, and lower Mesozoic eugeosynclinal rocks 

(IMze) on the southwest. It is bordered, especially on 

the north, by a wide zone of regional metamorphism, 

where parts of the Belt formations reach sillimanite 

grade. Most of this regional metamorphism preceded 

the actual emplacement of the batholith, suggesting 

that its site had been subjected to a considerable period 

of prior crustal heating (Hietanen, 1962, p. 97-99). 

Large inclusions in the batholith have been 

metasomatized and converted into gneisses that fade 

out into the surrounding intrusive. The oldest supracrustal rocks which overlie it are the Casto Volcanics 

(ITv) of Eocene age, and it is intruded by many small 

plutons of early Tertiary age (Ti).

The batholith underlies a rough wilderness area, 

and while parts of it have been mapped in fair detail, 

large parts are still poorly known or even unexplored. 

The surface outline of the batholith is highly irregular, 

with projections of granitic rocks into the surrounding 

country rock and many small to large inclusions or 

pendants of country rock within the batholith. It is 

nearly bifurcated near the middle by a belt of inclusions, shown on the map as metamorphosed Belt 

supergroup (Y), older Precambrain (Xm), and border 

phase of the batholith (Kgn). In this area, an eastward 

projection of granitic gneisses, mapped as part of the 

batholith, has proved from radiometric determinations 

to be 1,500 m.y. old, or early Precambrian Y 

(Armstrong, 1975, p. 440-441). The parts north and 

south of this belt of inclusions may be respectively 

termed the Bitterroot lobe and Atlanta lobe of the 

batholith. The batholithic rocks are mainly granodiorite and quartz monzonite. So far as is known, they do 

not form many individual plutons like those that 

characterize the Sierra Nevada and Peninsular Range 

batholiths; instead, large areas are of nearly uniform 

composition, and compositional changes from one part 

to another appear to be gradational. Along parts of the 

periphery, however, is a more mafic gneissic border 

phase (Kgn).

Emplacement of the batholith may have extended 

over a considerable period, and some rocks included 

with it may be much older, such as the 1,500-m.y.- old 

granitic gneisses mentioned above. The main period of 

emplacement appears, however, to have been during 

the Cretaceous. Radiometric determinations have 

yielded equivocal results, probably due in part to contamination with Precambrian materials and to updating during Tertiary plutonism. Present evidence indicates that the Atlanta, or southern lobe, has an age of 

about 70 to 100 m.y., or comparable to the last plutonic 

event in the Sierra Nevada. The Bitterroot lobe is apparently somewhat younger, with an age of about 80 

m.y., or close to that of the Boulder batholith to the 

east (Armstrong, 1975, p. 445).

Many granitic plutons occur northwest of the Idaho 

batholith along the Canadian border from the Purcell 

Trench of northern Idaho to the northern Cascade 

Range of Washington State. They are the southward 

extensions of plutons in the western Cordillera of 

British Columbia. Near the Columbia River they 

plunge southward beneath the cover of the Miocene 

Columbia River lavas; they may not continue much 

farther, as much of the lava was probably erupted onto 

an oceanic crust. For the most part, the granitic rocks
MESOZOIC PLUTONIC AND INTRUSIVE ROCKS 71

are of Cretaceous age and have yielded radiometric 

ages close to 100 m.y., or middle Cretaceous time, although several plutons of Triassic age have already 

been noted and others of early Tertiary age (Ti) occur, 

especially toward the west. The eastern plutons invade 

the Belt Supergroup; others farther west lie in 

Paleozoic miogeosynclinal rocks of the Kootenai arc, 

and those beyond in Paleozoic and Mesozoic eugeosynclinal rocks (Yates and others, 1966, p. 55). They intrude eugeosynclinal rocks as young as Middle Jurassic and are overlain unconformably by plant-bearing 

Upper Cretaceous rocks, but abundant granitic debris 

first appears in Eocene conglomerates.

East of the Idaho batholith in western Montana is an 

array of younger Cretaceous granitic plutons, of which 

the largest and best known is the Boulder batholith. 

They are classed on the Geologic Map as "latest Cretaceous granitic rocks" (Kga) and are commonly referred to as "Laramide" intrusives. Emplacement of 

these granitic rocks overlaps in time that of the Upper 

Cretaceous granitic rocks (Kg2) of the Sierra Nevada; 

however, this emplacement continued later, and the 

associations of the two sets of rocks are quite different.

The Boulder batholith extends north-northeast 

transverse to the regional trends of the enclosing 

rocks, with an area of about 2,300 mi2 (5,700 km2) and 

a length of about 60 mi (100 km) (Robinson and others, 

1968). It invades the Belt Supergroup and the 

Paleozoic and Mesozoic rocks as young as the Elkhorn 

Mountain Volcanics (Kv) of Campanian age; it is overlain unconformably by the middle Eocene Lowland 

Creek Volcanics (ITv). The batholith is a composite 

mass of a dozen or more plutons of calc-alkalic rocks 

which range in composition from syenogabbro to alaskite but are dominantly quartz monzonite and 

granodiorite. Nearly three-fourths of the batholith was 

emplaced between 71 and 82 m.y., and thus it partly 

overlaps the eruption of the Elkhorn Mountain Volcanics (73 to 78 m.y.), as well as the thrust faulting of 

the country rocks which occurred during Campanian 

and Maestrichtian time (approximately between 66 

and 80 m.y.).

The mode of emplacement and the structure of the 

batholith remain controversial. Was it a steep-walled 

intrusive, descending into the depths from its roof of 

Elkhorn Mountain Volcanics (Klepper and others, 

1971, p. 1580)? Or was it a shallow floored body that 

was open to the sky, on whose surface the Elkhorn 

Mountain Volcanics congealed as a sort of slag (Hamilton and Myers, 1967, p. C6-C9)? Probably the truth 

lies somewhere between these two extremes.

Another group of "Laramide" plutons (Kga) of smaller individual dimensions is in the Mineral Belt of Colorado, mostly in the Front Range (Tweto, 1968, p.

564-565). They have yielded radiometric ages of about 

60 to 70 m.y., and clasts derived from them have been 

identified in the Paleocene deposits of the Denver basin 

to the east. These plutons are part of a chain of intrusive rocks that extends southwestward along the Mineral Belt past the San Juan Mountains, but most of 

those west of the Front Range are younger, of early 

Tertiary age (Ti).

Other "Laramide" granitic rocks (Kga) occur in 

southern Arizona, partly intermingled with but mostly 

to the east of the earlier Mesozoic granitic rocks (Jg, 

Kg) (Armstrong and Suppe, 1973, p. 1385).

CRETACEOUS INTRUSIVE ROCKS (Ki)

Small bodies of intrusive rock of Cretaceous age 

occur at the inner edge of the Gulf Coastal Plain in 

southwestern Arkansas and central Texas, associated 

with tuffaceous rocks interbedded in the Upper Cretaceous marine sequence.

Southeast of Little Rock in Arkansas, two sizeable 

knobs of nepheline syenite project through the early 

Tertiary deposits of the Midway and Wilcox Groups, 

which contain their weathered products, including 

commercial deposits of bauxite (Gordon and others, 

1958, p. 60-71). A little farther west, in the folded 

Paleozoic rocks of the Ouachita Mountains, are 

numerous plugs of similar rock, of which the largest 

and best known is that at Magnet Cove, and many 

satellitic dikes. About 80 mi (130 km) to the southwest, 

at the edge of the Coastal Plain near Murphreesboro, 

four small pipes of diamond-bearing peridotite cut the 

Lower Cretaceous Trinity Group (Miser and Purdue, 

1929, p. 99-117), containing the only abundant 

diamonds in the United States. Genetically related to 

the intrusive rocks are beds of volcanic tuff and tuffaceous sandstone in the lower part of the Upper Cretaceous in the same area the Woodbine Sand (uKi) 

and the Tokio Formation of Austin age (uKy) some of 

which are as much as 125 ft (40 m) thick (Ross and 

others, 1929).

In central Texas, near the Balcones fault zone at the 

edge of the Coastal Plain, another group of Cretaceous 

intrusive rocks extends from Austin 150 mi (250 km) 

westward past Uvalde (Lonsdale, 1927, p. 9-46). They 

are small plugs and laccoliths of nepheline basalt and 

phonolite, which intrude Cretaceous rocks as young as 

the Austin and Taylor Formations, or somewhat 

higher than the intrusive and volcanic rocks in Arkansas. As in Arkansas, the associated marine deposits of 

the Austin and Taylor contain much volcanic debris, as 

well as several layers of bentonite. The Pilot Knob intrusive near Austin has sometimes been referred to as 

a fossil volcano, for which there is some support in the
72 PALEOZOIC AND MESOZOIC ROCKS

abundant volcanic debris in the surrounding Austin 

and Taylor Formations. Nevertheless, the intrusive 

rocks everywhere cut these formations, indicating a 

younger age. It has been postulated that there were 

two periods of igneous activity in the district, one during Late Cretaceous and another in early Tertiary time 

(Lonsdale, 1927, p. 44-46), but the interval between 

them need not have been great; it seems more likely 

that all the igneous rocks are of Late Cretaceous age, 

as represented on the atlas sheets of the new Geologic 

Map of Texas (Austin and San Antonio sheets, 1974), 

and this assignment is followed on the Geologic Map.

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