The Cenozoic Rocks; 

a Discussion to Accompany 

the Geologic Map of the 

United States 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 904 

The Cenozoic Rocks; 

a Discussion to Accompany 

the Geologic Map of the 

United States 

By PHILIP B. KING and HELEN M. BEIKMAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 904 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1978 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

First printing 1978 

Second printing 1984 

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

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Abstract ----------------------------------------------------------------------------------------------

Introduction --------------------------------------------------------------------------------------

Tertiary System -----------------------------··-------------------------------------------------

Paleocene and Eocene Series ----------------------------------------------

Marine stratified rocks --------------------------------------------------

Gulf Coastal Plain --------------------------------------------------

Midway Group (Tx) ----------------------------------------

Wilcox Group (Tel) ------------------------------------------

Claiborne Group (Te2) ------------------------------------

Jackson Group (Te3) --------------------------------------

Atlantic Coastal Plain -----------------------------------------· 

Pacific Coastal Area ---------------------------------------·-····· 

California -----------------------------------------------------·------

Oregon and Washington -------------------·----------

Continental deposits -------------------------------------------·----------

Great Plains and Rocky Mountains ................. . 

Paleocene deposits (Txc) -------·------------------------

Eocene deposits (Tee) -------------------------------------- Eocene lacustrine deposits (Tel) _________________ _ 

Outlying areas ---------------------------------·------------------------

New Mexico and west Texas ------------------------

Arizona ----·-----------------------------------------------------------

Nevada and California ----------------------------------

Western Montana --------------------------------------------

Washington --------------------------------------------------------

Eugeosynclinal deposits ------------------------------------------------

Oligocene Series ------------------------------------------------------------------------

Marine stratified rocks --------------------------------------------------

Gulf Coastal Plain --------------------------------------------------

Pacific Coastal Area ----------------------------------------------

California ···---------------------------------------------------------

Oregon and Washington ------------------·-----------

Continental deposits ------------------------------------------------------

Great Plains and Rocky Mountains ------·-----------

Outlying areas ----------------------------------------------------------

Oregon ------------------------------------------------------·-----------

California ------------------------------------------------------------

Miocene Series --------------------------------------------------------------------------

Marine stratified rocks --------------------------------------·-----------

Gulf Coastal Plain --------------------------------------------------

Ailantic Coastal Plain ·········--------------------····---------

Pacific Coastal Area ----------------------------------------------

California ------------------------------------------------------------

Oregon and Washington ------------------------------

Continental deposits ------------------------------------------------------

Northern Great Plains and Rocky Mountains 

Outlying areas ----------------------------------------------·-----------

New Mexico and Arizona ------------------------------

Washington and Oregon ------------------------------

California -----------------------------------·------------------------

Pliocene Series --------------------------------------------------------------------------

Marine stratified rocks ····---------------------------: ................. . 

Atlantic and Gulf Coastal Plains ---------------------- Pacific Coastal Area in California ___________________ _ 

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Tertiary System-Continued 

Pliocene Series-Continued 

Continental deposits ------------------------------------------------------

Sheet deposits ------------------------------------------------------------

Ogallala Formation ----------------------------------------

Bidahochi Formation --···---------------------------------

Basin deposits ---------------------------------------------------------

Santa Fe Formation ----------------------------------------

Gila Formation ------------------------------------------------

Muddy Creek Formation ------------------------------

Humboldt Formation --------------------------------------

Salt Lake Formation --------------------------------------

Deposits associated with volcanic rocks ......... . 

Idaho Group ------------------------------------------------------

Other formations ----------------------------------------------

Quaternary System -----------· ····-------------------------------------------------------

Gulf Coastal Plain -----------------------------------------------------------------

Mississippj Embayment ------------------------------------------------

Other areas ---·--------------------------------------------------------------------

Atlantic Coastal Plain -------------------------------------·--------------------

Great Plains ---------------------------------------------------------·------------········ 

Pleistocene ......................................................................... . 

Undifferentiated Quaternary ..................................... . 

California ------------------------------------------------------------------------------------

Basin and Range province ................................................. . 

Cenozoic volcanic rocks ............................................................... . 

Lower Tertiary volcanic rocks (lTv) ................................. . 

Eocene marine"pillow basalt (Teb) ........................... . 

Lower Tertiary andesite (ITa) ................................... . 

Clarno Formation (lTv) of northeastern Oregon .. 

Challis Volcanics (lTv) of south-central Idaho ... . 

Western Montana ........................................................... . 

Absaroka volcanic field, Wyoming and Montana 

Eastern Great Basin -------------------------------------------··········· 

San Juan volcanic field, southwestern Colorado ... 

Other areas of lower Tertiary volcanics in southern 

Rocky Mountains ....................................................... . 

Southwestern Arizona --------·-················----------------·········· 

Mogollon Plateau, southwestern New Mexico _____ _ 

Trans-Pecos Texas ------------------------------------··········------------

Upper Tertiary volcanic rocks (Tmv, Tpv) ................... . 

Upper Tertiary andesites of Washington (uTa) ... . 

Upper Tertiary andesites of Oregon and California 

(uTa) ----------------------------------------------------------------------------------

Columbia River Group of Washington, Oregon, and 

Idaho (Tmv) ................................................................. . 

Southeastern Oregon and northeastern California. 

Snake River Plain of southern Idaho ..................... . 

Great Basin of Nevada and adjacent States ----------

Sierra Nevada and Coast Ranges of California __ 

Southern Rocky Mountains ------------------------------------------

Volcanic fields of Colorado Plateau ......................... . 

Upper Tertiary volcanic rocks of southern Basin 

and Range province in Arizona and California 

III 

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IV CONTENTS 

Cenozoic volcanic rocks-Continued Page Page 

Quaternary volcanic rocks (Qv, Qf) ................................. . 

Andesitic volcanics of Cascade Range in 

Washington, Oregon, and California ................... . 

Basalts of Snake River Plain (Qv) ........................... . 

Felsic volcanic rocks of Yellowstone National Park 

(Qf) ···················································································· Eastern border of Sierra Nevada, California ....... . 

Clear Lake area, California Coast Ranges ........... . 

Jemez Mountains, New Mexico ................................. . 

Basaltic fields of New Mexico and Arizona ......... . 

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Tertiary intrusive rocks (Ti) ....................................................... . 

Cascade Range ......................................................................... . 

Oregon Coast Range ............................................................. . 

Central Idaho ........................................................................... . 

Central Montana ..................................................................... . 

California and Great Basin ................................................. . 

Colorado Plateau ..................................................................... . 

Southern Rocky Mountains ................................................. . 

Southeastern Basin and Range province ....................... . 

References cited ............................................................................... . 

ILLUSTRATIONS 

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FIGURE 1. Surface distribution of Paleocene and Eocene stratified rocks in the United States ............................................................. 4 

2. Surface distribution of Oligocene stratified rocks in the United States ....................................................................................... 18 

3. Surface distribution of Miocene stratified rocks in the United States ......................................................................................... 22 

4. Surface distribution of Pliocene stratified rocks in the United States ......................................................................................... 30 

5. Distribution of Quaternary deposits in the United States, and limits of Pleistocene glacia.tions ..................................... 40 

6. Distribution of Cenozoic volcanic rocks in western United States ............................................................................................... 48 

7. Distribution of lower Tertiary volcanic rocks in western United States ..................................................................................... 50 

8. Distribution of volcanic rocks of successive Cenozoic ages in Great Basin ............................................................................... 55 

9. Distribution of radiometric dates on felsic volcanic rocks in Great Basin ................................................................................. 56 

10. Tertiary volcanic rocks of San Juan Mountains, southwestern Colorado ................................................................................. 57 

11. Cenozoic volcanic units of Mogollon Plateau, southwestern New Mexico .................. .............................................................. 60 

12. Distribution of Miocene volcanic rocks in western United States ................................................................................................. 62 

13. Distribution of Pliocene volcanic rocks in western United States ............................. ................................................................... 63 

14. Distribution of subdivisions of Columbia River Group in Washington, Oregon, and Idaho ............................................... 65 

15. Distribution of Quaternary volcanic rocks in western United States ........................................................................................... 72 

16. Distribution of Tertiary intrusive rocks in western United States ............................................................................................... 76 
THE CENOZOIC ROCKS: 

A DISCUSSION TO ACCOMPANY THE GEOLOGIC MAP 

OF THE UNITED STATES 

By PHILIP B. KING and HELEN M. BEIKMAN 

ABSTRACT 

The Cenozoic rocks formed during the last 65 million years of 

geologic time and, being the last to form, still cover nearly half the 

country. They are a varied and complex assemblage, including 

marine stratified rocks near the present Atlantic, Gulf, and Pacific 

coasts, continental deposits in the interior region, and volcanic 

rocks through much of the western Cordillera. 

The marine stratified rocks in the east form most of the Atlantic 

and Gulf Coastal Plains and range in age from Paleocene to 

Pliocene. They are the thin updip edges of deposits that thicken in 

subsurface toward the coasts and are largely shall~w-water 

marine deposits, with marginal brackish-water and continental 

phases, that change downdip in subsurface into deeper-water 

marine deposits. 

The marine stratified rocks in the California segment of the 

Pacific coast likewise range in age from Paleocene to Pliocene and 

Pleistocene, but they are thicker, more varied, and more complex 

and have been more heavily involved in orogenic deformation. As 

a result, they do not lie in regionally consistent belts of outcrop but 

form numerous sedimentary basins of different ages. They include 

not only shallow-water marginal deposits and fringing continental deposits, but nearer the coast deep-water deposits that were 

laid down beyond the edge of the continent of the time. 

The marine stratified rocks of the Pacific coastal segment to the 

north, in Oregon and Washington, are mainly of Eocene, 

Oligocene, and Miocene age. Like their counterparts farther south, 

they range from near-shore and continental deposits on the east, 

to deep-water deposits farther west, but they differ in their much 

greater volcanic component, including submarine pillow lavas, 

terrestrial lavas, and a large volcanic-derived content in all the 

sediments. 

Continental deposits of Cenozoic age are especially prominent 

in the Rocky Mountains and adjacent Great Plains and range in 

age from Paleocene to Pliocene. The Paleocene and Eocene 

deposits fill to great thickness basins between the newly raised 

mountain uplifts and east of the mountains. They are mostly 

flood-plain and swamp deposits but include some extensive 

deposits laid down in lakes, represented by the Paleocene 

Flagstaff Limestone and the Eocene Green River Formation. The 

Oligocene, Miocene, and Pliocene deposits are thinner, but they 

are even more extensive east of the Rocky Mountains, where they 

spread as great sheets across the Great Plains that were laid down 

by streams carrying detritus derived from erosion of the 

mountains to the west. 

The Quaternary deposits are only partly represented on the 

Geologic Map. Not shown, for example, are the glacial drift and 

associated deposits that are extensive in the northern states. 

Quaternary deposits are shown only where they are an essential 

feature of the bedrock pattern and attain sizeable thickness, as in 

the Mississippi Embayment, the Atlantic and Gulf Coastal 

Plains, and the intermountain basins in the west. 

Volcanic rocks of Cenozoic age occur throughout the western 

Cordillera, but they form an especially thick, continuous blanket 

in the northwestern States of Washington, Oregon, and Idaho. All 

ages are represented, from Eocene to Quaternary. 

The lower Tertiary (Eocene and Oligocene) volcanic rocks are 

less widely exposed than the younger ones. They are dominantly 

of andesitic or intermediate composition. They include the marine 

pillow basalts of the Coast Ranges of Oregon and Washington 

that are interbedded in the Eocene strata and the lower Tertiary 

andesites of the Cascade Range not far to the east. Farther east are 

some notably thick and extensive volcanic fields, as in the 

Absaroka Mountains of northwestern Wyoming, the San Juan 

Mountains of southwestern Colorado, and the Mogollon Plateau 

of southwestern New Mexico. 

The upper Tertiary (Miocene and Pliocene) volcanic rocks are 

more extensively exposed and of more diverse compositions. They 

include the upper Tertiary andesites of the Cascade Range and the 

great mass of tholeiitic plateau basalts of the Columbia River 

Group in the Columbia Plateau. Farther south, in the Great Basin, 

is an extensive body of felsic volcanic rocks, mainly ash -flow tuffs 

or ignimbrites that spread as broad sheets across the featureless 

terrane prior to its disruption by Basin and Range block-faulting. 

Smaller areas of upper Tertiary volcanic rocks occur throughout 

the remainder of the Cordillera, but notably along the southwestern and southern edges of the Colorado Plateau. 

Quaternary volcanic rocks have a much smaller distribution 

and are restricted to areas that remained tectonically active in 

recent times. They include the chain of andesitic volcanoes on the 

crest of the Cascade Range from Washington, through Oregon, 

into northern California; and a transverse belt of basaltic 

volcanics that extends along the Snake River Plain of southern 

Idaho and into southeastern Oregon. Basaltic flows occur in many 

other areas; in addition there are a few notable centers of felsic 

volcanism-in Yellowstone National Park, along the eastern side 

of the Sierra Nevada, and in the Jemez Mountains of New Mexico. 

Tertiary intrusive rocks form much smaller areas in the western 

Cordillera than those of Mesozoic ages and are of diverse ages and 

habits. They include many small hypabyssal stocks and laccoliths, but there are a few larger masses of deep-seated origin and 

batholithic dimensions. The latter are especially prominent in the 

northern Cascade Range of Washington, the region of the Idaho 

batholith in Idaho, and the Southern Rocky Mountains of 

Colorado. 

INTRODUCTION 

This is the last of a series of four reports dealing 

with features shown on the Geologic Map of the 

1 
2 THE CENOZOIC ROCKS 

United States (1974). Professional Paper 901 deals 

with general principles and problems relating to the 

map; Professional Paper 902 with the Precambrian 

rocks; Professional Paper 903 with the Paleozoic and 

Mesozoic rocks; and Professional Paper 904 (the 

present report) with the Cenozoic rocks. 

The Cenozoic rocks formed only during the last 65 

million years of geologic time, but, being the 

youngest, they spread over nearly half the country. 

They are a varied and complex assemblage, including marine stratified deposits along the Atlantic, 

Gulf, and Pacific coasts, continental deposits in the 

interior region, and volcanic rocks in the western 

Cordillera. The Cenozoic rocks in the eastern half of 

the country are flat-lying or gently tilted, but those in 

the western half have been mildly to strongly 

deformed by various orogenies during Cenozoic time. 

The treatment of the Cenozoic rocks in the present 

account, as in the account of the Paleozoic and 

Mesozoic rocks, closely follows the outline of the map 

legend. The rocks of successive ages are treated in 

turn, from Paleocene to Holocene. Under each 

category, the first rocks described are the marine 

strata, in geographic order from east to wes~, and 

next the continental deposits; a small body of eugeosynclinal deposits of early Tertiary age is treated 

separately. Following this, the Cenozoic volcanic 

rocks are dealt with, again by successive ages, from 

one area to another. In a final section, the Tertiary 

intrusive rocks are treated briefly. 

The report is illustrated by a series of maps 

showing the surface extent of the rocks of different ages (figs. 1-7, 12, 13, 15, 16), as they are 

represented on the Geologic Map of the United 

States. In addition, a few figures are included that 

show special features of the volcanic rocks that 

are not brought out clearly on the Geologic Map. 

The Cenozoic is conventionally divided into the 

Tertiary and Quaternary Systems, the first being 

much the greater, the second only representing the 

last 2 million years of geologic time. The Tertiary is 

in tum divided into the Paleocene, Eocene, Oligocene, Miocene, and Pljocene Series, and the Quaternary into the Pleistocene and Holocene Series. The 

Quaternary is distinguished from the Tertiary 

mainly by its glacial periods-although this climatic 

regime, in fact, actually began during the later 

Tertiary. 

The Cenozoic Era succeeded the great orogenies of 

later Mesozoic time-although orogenic deformation 

continued into the Cenozoic and continues even 

today in some areas. 'I~1e Mesozoic orogenies 

produced a major reorganization of the continent, so 

that it became more widely emergent than ever 

before. The great epicontinental seas of previous eras 

were permanently expelled from most of the continent, so that during the Cenozoic marine deposits 

were laid down only near the present coasts. 

Continental deposits continued to be laid down in 

some of the interior areas, but other areas, such as 

much of the eastern interior, became permanent land 

surfaces. The emergent regime of the Cenozoic 

resulted in quickening of erosional processes, so that 

great quantities of clastic detritus were eroded from 

the land and delivered to the areas of sedimentation 

that remained. Another result of the Mesozoic 

orogenies in the western Cordillera was the subsequent increase in volcanic activity, resulting in 

extensive fields of lavas, pyroclastic rocks, and 

volcaniclastic sediments being spread over much of 

the Western States during the Cenozoic. 

TERTIARY SYSTEM 

PALEOCENE AND EOCENE SERIES 

The Paleocene and Eocene Series formed during a 

span of about 28 million years, or the first third of 

Tertiary time. In terms of the standard European 

sequence, the Paleocene corresponds to the Danian, 

Montian, and Thanetian Stages; and the Eocene to 

the Ypresian, Cuisian, Lutetian, Auversian, Bartonian, and Ludian Stages. As explained below, 

several schemes of stages have been used for local 

purposes in the marine sequence in California, and 

another set of stages is used for the North American 

continental Tertiary. 

Although the Paleocene and Eocene Epochs 

occupied a sizeable fraction of Tertiary time, the 

rocks of these epochs are considered together in this 

account. In some places, as in the Rocky Mountain 

region, the two sets of deposits are considerably 

different, because they formed in different depositional basins. In other places, however, the distinctions are less marked, and there is controversy as to 

the boundary between the two series. 

The Paleocene and Eocene are represented on the 

Geologic Map of the United States by stratified 

sedimentary rocks, mostly marine (Tx, Te); by 

continental deposits (Txc, Tee); and by eugeosynclinal deposits (Tee). In the Gulf Coastal Plain, the 

Eocene stratified rocks are separated into three 

divisions (Tel, Te2, Te3), which correspond to the 

Wilcox, C!aibome, and Jackson Groups. In parts of 

the Rocky Mountain region, Eocene lacustrine 

deposits (Tel) are differentiated amidst the continental deposits. Eocene volcanic rocks in the 

Western States are mostly grouped in the lower 

Tertiary volcanic rocks (lTv), which are discussed 
TERTIARY SYSTEM 3 

separately later. However, in the Coast Ranges of 

Oregon and Washington, Eocene pillow basalts 

(Teb) are separately mapped. 

Paleocene and Eocene rocks cover sizeable areasperhaps as much as 10 percent of the area of the 

Forty-eight States shown on the Geologic Map 

(fig. 1). They form a broad outcrop belt along the 

inner part of the Gulf Coastal Plain, and smaller, 

more discontinuous areas in the same position in the 

Atlantic Coastal Plain. The most ex4!nsive outcrop 

areas of Paleocene and Eocene rocks are, however, in 

the Rocky Mountain region and northern Great 

Plains, where they form the centers of basins 

between the mountain uplifts. Along the Pacific 

Coast, Paleocene and Eocene rocks are recognized 

from southern California to northwestern Washington, but because of the considerable deformation 

to which they have been subjected, individual outcrops are small and discontinuous. 

MARINE STRATIFIED ROCKS 

GULF COASTAL PLAIN I 

The Paleocene and Eocene Series are exposed in 

belts around the inner half of the Gulf Coastal Plain. 

The outcropping rocks are the updip edges that 

extend in subsurface toward the Gulf, where they 

form a thick, relatively unbroken, dominantly 

argillaceous marine sequence. The exposed rocks, to 

which main attention is given here are thinner 

more interrupted, more coarsely clasti~ deposits, that 

were laid down in marine, littoral, and continental 

environments along the edge of the continent. 

MIDWAY GROUP (Tx) 

The Midway Group at the base of the Tertiary 

coastal plain sequence was named in 1887 for 

Midway Landing on the Alabama River in Alabama. 

The group crops out in a nearly continuous band 

from the Rio Grande in Texas to Georgia, with a 

notable deflection northward to southern Illinois in 

the Mississippi Embayment. Exposed sections of the 

Midway are generally less than 500 ft (150 m) thick. 

In most places the Midway can be divided into two or 

more formations. Eastward from Georgia, the 

Midway is overstepped by succeeding Eocene 

formations. 

In contrast to parts of the succeeding Eocene 

sequence, the Midway is marine throughout; in 

places it much resembles the uppermost Cretaceous 

deposits on which it lies, although with a notably 

different fauna. The Midway is a subtropical, warm1 A general review of the Paleocene and Eocene formations of the Gulf Coastal Plain io 

preoented by Murray (1961, p. 367-394). For an earlier more detailed summary of the 

Texas oegment, see Plummer (1932, p. 531-699). 

water deposit, whose fauna also differs from 

apparently correlative strata elsewhere, as in western Europe and on the Pacific Coast. It is commonly 

considered to be the sole representative of the 

Paleocene in the Gulf Coastal Plain, but accumulating paleontological evidence suggests that the top 

of the Paleocene extends somewhat higher in terms 

of the European standard sections (see below). 

In Texas the Midway is divided into the Kinkaid 

and Wills Point Formations, the first of glauconitic 

sands and clays with several lenses of coquinid 

limestone, the second largely clay and silty clay of a 

deeper water facies. In Mississippi and Alabama it is 

divided into the Clayton Formation, Porters Creek 

Clay, and Naheola Formation. The Clayton somewhat resembles the Kinkaid, being glauconitic, 

sandy, and limy, and the Porters Creek the Wills 

Point, being dark laminated clay. The Porters Creek 

extends northward beyond the remainder of the 

group into southern Illinois. The Naheola differs 

from the others in being sandy and noncalcareous. 

On the crest of the Sabine uplift, south of the main 

outcrop belt, in northwestern Louisiana and eastern 

Texas, a large circular area of outcrop was formerly 

mapped as Midway but is now interpreted as a 

marine downdip argillaceous facies of th,e Wilcox 

Group and is so represented on the Geologic Map. 

Only a small outcrop in the north part of the uplift, 

northwest of Shreveport, is now considered to be of 

Midway age. WILCOX GROUP (Tel) 

The Wilcox Group, or initial Eocene unit of the Gulf 

Coastal Plain, was named in 1906 for Wilcox County, 

Alabama, to replace the previously used lithologic 

term "lignitic." The term Sabine was proposed at 

about the same time for the same unit west of the 

Mississippi and is still used by some geologists in 

Louisiana. The Wilcox forms an interrupted band of 

outcrop in the inner part of the Gulf Coastal Plain in 

Texas eastward to Georgia, where it is cut off by 

overstep of younger Eocene units. As indicated 

above, the Wilcox is also exposed in a large area on 

the crest of the Sabine uplift in northwestern 

Louisiana and eastern Texas. On the outcrop the 

Wilcox is as much as 1,000 feet (300m) thick. In large 

parts of its outcrop (for example, in Texas and the 

Mississippi Embayment) the Wilcox is of nonmarine 

facies and is indivisible in many areas. In other 

places, howe rer, intertonguing marine strata permit 

subdivision of the group into formations, which vary 

from one region to another. 

In central Texas the Wilcox Group was divided by 

Plummer (1932, p. 574) into the thick medial nonmarine Rockdale Formation of sands, clays, and 
4 

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THE CENOZOIC ROCKS 

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--- ·---: 

FIGURE 1.-Map of the United States showing surface distribution of Paleocene and Eocene rocks as represented on the 

Geologic Map of the United States (units Tx, Txc, Te, Tee, Tel, Tee, Teoe). 
PALEOCENE AND EOCENE SERIES 5 

---W 

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EXPLANATION 

Eocene rocks 

Paleocene rocks 

'-· .... ' 

FIGURE I.-Continued. 

lignites, underlain and overlain by thin fossiliferous 

marine units, the Seguin and Sabinetown Formations. In later work (as shown on the Waco and 

Austin sheets of the Texas Geologic Atlas, 1970, 

1974) the same sequence of nonmarine beds is 

divided into the Hooper Formation below, the 

medial-sandy Simsboro Formation (classed as a 

member by Plummer), and the Calvert Bluff 
6 THE CENOZOIC ROCKS 

Formation above; the marine units are not mentioned. Farther southwest in Texas, the undivided 

Wilcox has traditionally been termed the Indio 

Formation. 

In Alabama, Mississippi, and Georgia the nonmarine beds of the Wilcox are principally in its upper 

part, and the group is divided into the Nanfalia, 

Tuscahoma, and Hatchetigbee Formations (with the 

Bashi Marl Member at the base of the latter). 

Planktonic foraminifers indicate that the PaleoceneEocene boundary, in terms of European standard 

sections, is above the boundary between the Midway 

and Wilcox Groups (Berggren, 1965, p. 280-282). On 

the basis of these foraminifers the Bashi Marl of 

Alabama is basal Eocene (Ypresian). The Nanfalia 

Formation (and the local Salt Mountain Limestone) 

are upper Paleocene (Sparnacian). The overlying 

Tuscahoma Formation and the Seguin Formation of 

Texas do not contain planktonic foraminifers 

suitable for long-range correlations but are presumed to be upper Paleocene. 

This creates problems of representation on the 

Geologic Map, where the units adopted are the 

Midway and Wilcox Groups, which are assigned to 

Paleocene and Eocene ages, respectively. It seems 

fruitless to attempt to subdivide the Wilcox on the 

basis of these new judgements, as the PaleoceneEocene boundary in the Wilcox is certainly known 

only in the relatively complete marine sequence in 

Alabama. Elsewhere, much of the Wilcox is nonmarine and is generally undivided on the source 

maps. We consider it best to leave the boundaries 

where they are, pending the receipt of other data, 

with the understanding that part of the Wilcox is 

actually Paleocene. 

At the eastern end of the Eocene outcrop belt, in 

South Carolina, equivalents of the Wilcox reappear 

in a small area from beneath the overlap of the 

younger formations and are known as the Black 

Mingo Formation. The Black Mingo contains some 

of the marine mollusks which occur in the Nanfalia 

and Tuscahoma Formations of Alabama, as well as 

some others that are more characteristic of the 

Midway Group in that area, so that the Black Mingo 

evidently contains undifferentiated strata of both 

early Eocene and Paleocene age (Cooke and MacNeil, 1952, p. 20-21). 

CLAIBORNE GROUP (Te2) 

The Claiborne Group was named in 1847 for 

Claiborne Bluff on the Alabama River in Alabama. 

It is perhaps 1,000 feet (300 m) or more thick, and 

because of its thickness and various structural 

complications it has a much larger outcrop area than 

any of the other Paleocene or Eocene units, 

especially in the Mississippi Embayment and west of 

the Mississippi River. In south Texas it forms an 

outcrop band 60 miles (95 km) or more broad in the 

Rio Grande embayment, and in northeast Texas it 

occupies most of the East Texas embayment and 

extends around the north and south sides of the 

Sabine uplift into southern Arkansas and northern 

Louisiana, where its outcrop is 120 miles (190 km) 

broad. It also underlies all the northern two-thirds of 

the Mississippi Embayment as far north as Illinois, 

although its outcrops are interrupted in part by 

downfolds of Jackson Group (Te3) and by Quaternary deposits. East of the Mississippi River it forms 

a narrower band of outcrop that extends eastward 

across Georgia and South Carolina, beyond the 

truncated edges of the Midway and Wilcox Groups. 

The Claiborne is notable for its highly fossiliferous 

marine strata, whose fossils have been extensively 

collected and investigated for years, since the days of 

the early geological explorers. Its deposits characteristically form a series of cyclical sequences, each 

beginning with transgressive glauconitic, calcareous, fossiliferous strata; passing up into dark 

argillaceous deeper-water deposits; and terminating 

with regressive carbonaceous to lignitic sandy and 

argillaceous deposits. 

In Alabama the Claiborne is divided into the 

Tallahatta Formation, the Lisbon Formation, and 

the Gosport Sand. The Tallahatta is one of the more 

resistant units of the Gulf Coastal Plain and forms a 

prominent cuesta, as its sands and clays are 

indurated on the outcrop to "buhrstone," which was 

used in the early days for millstones. The Gosport is 

the most fossiliferous of the three units; its fossils 

were first described by Conrad as early as 1833. 

In Texas the Claiborne Group is divided into the 

Carrizo Sand, the Mount Selman Formation (with 

the Reklaw, Queen City, and Weches Members), the 

Cook Mountain Formation (with the Sparta and 

Crockett Members), and the Yegua Formation. The 

units are all widely recognizable, and they or their 

equivalents have been mapped across Texas and 

into Louisiana. The Carrizo at the base is continental and is a sheet of sand that was spread widely 

along the shore. The Yegua at the top is again of 

continental or brackish water facies. The intervening units are mainly marine or littoral. The 

Weches Member is the most prominently exposed 

unit and forms escarpments and mesas. It is highly 

glauconitic and its weathered surfaces are deeply 

encrusted with brown iron ore that has been 

exploited commercially from time to time (Eckel and 

Purcell, 1934, p. 482-503). 
PALEOCENE AND EOCENE SERIES 7 

In eastern Georgia and in South Carolina the 

Claiborne Group is represented by the McBean 

Formation, 100 feet (30 m) or so thick, and 

approximately equivalent to the Lisbon and Mount 

Selman Formations of the Claiborne Group farther 

west. 

jACKSON GROUP (Te3) 

The Jackson Group, or highest Eocene unit in the 

Gulf Coastal Plain, was named in 1856 for Jackson, 

Mississippi. The group is 500 feet (150 m) or more 

thick, and forms a nearly continuous band of outcrop 

in the middle of the coastal plain from the Rio 

Grande in Texas to South Carolina. Several large 

outliers of the Jackson occur in down warps in the 

Mississippi Embayment as far north as Kentucky, 

and a large inlier emerges on the crest of the Ocala 

uplift in northwestern Florida. 

Strata of the Jackson Group were deposited during 

a separate and widespread marine transgression in 

the Gulf region. In the eastern part of the region the 

deposits are mainly calcareous, in the central part 

argillaceous, and in the western part sandy. 

In Texas the Jackson was formerly grouped as the 

Fayette Fortnation, but its sandstone and clay units 

are widely traceable, and it is now divided into the 

Caddell, Wellborn, McElroy, and Whitsett Formations, with various members (Eargle, 1959). The 

sediments are rich in volcanic detritus and contain 

many deposits of commercially exploitable bentonite 

and fuller's earth (King, 1940, p. 170-178). In south 

Texas the upper, or Whitsett Formation, also 

contains large uranium deposits that have been 

mined since 1960 (Eargle and others, 1975). 

In the central, or type area, near Jackson, 

Mississippi, the Jackson consists of the Moodys 

Branch Marl, a glauconitic, sandy and shaly deposit; 

and the Yazoo Clay, a dominantly argillaceous 

deposit. Northward in the Mississippi Embayment 

marine components fade out and the group becomes 

largely clastic and continental. Calcareous components increase southeastward, and in Florida the 

group is represented by the solid body of the Ocala 

Limestone, poorly to well indurated and crowded 

with foraminifers, bryozoans, mollusks, and echinoids. In north western Florida the Ocala is an 

important aquifer and is the source of many large 

springs. 

ATLANTIC COASTAL PLAIN 

Except for the Castle Hayne Formation of eastern 

North Carolina (of Claiborne and Jackson age) no 

Paleocene or Eocene rocks are exposed for 250 miles 

( 400 km) north of South Carolina. The next 

Paleocene and Eocene outcrops are in the segment in 

northern Virginia, whence they continue to the 

term in us of the Coastal Plain in northeastern New 

Jersey. The rocks form a narrow band of outcrop next 

to the Cretaceous, which is overstepped in places by 

Miocene deposits. 

The Paleocene and Eocene deposits in this region 

are not more than a few hundred feet thick on the 

outcrop and are all shallow-water marine deposits. 

The rocks are highly fossiliferous in places and are 

composed mostly of glauconitic sands and clays; 

however, some more calcareous units occur. The 

Paleocene is represented in Virginia and Maryland 

by the Aquia Formation, and in New Jersey by the 

Hornerstown and Vincentown Formations. The 

overlying Eocene deposits, largely of Wilcox age, are 

more discontinuous in outcrop because of Miocene 

cover. In Virginia and Maryland the Eocene 

comprises the Nanjemoy Formation, and in New 

Jersey the Manasquan Formation. 2 

PACIFIC COASTAL AREA 

Marine stratified Paleocene and Eocene rocks are 

exposed along the en tire length of the Paclfic coastal 

belt from southwestern California to northwestern 

Washington, and in places attain thicknesses of 

more than 10,000 feet (3,000 m). Because the rocks 

have been subjected to greater deformation than in 

the Gulf and Atlantic coastal areas, individual 

outcrops are smaller and more discontinuous, 

especially in California where folding and faulting 

have been greatest. The largestandmostcontinuous 

outcrop area is in the Coast Ranges of western 

Oregon and Washington, where folding is relatively 

open. 

Throughout the length of the Pacific coastal belt 

the Paleocene and Eocene deposits exhibit a 

westward transition across a zone 80 to 120 miles 

(125-190 km) wide. They grade from nonmarine 

continental deposits on the east, through shallowwater nearshore deposits, to deep-water deposits, 

mainl'y formed on deep-sea fans. However, this 

pattern varies considerably from place to place, and 

in California the original geographic pattern has 

been much disordered by large strike-slip movements along the San Andreas and other faults. In the 

Olympic Mountains of northwestern Washington, 

some deposits are differentiated as "eugeogeosynclinal" and separately mapped (Tee). 

2Many details of these units are given by Spangler and Pete.rson (1960, p. 52-61, 72-

75). Their planktonic Foraminifera are discuBSed by Loebhch and Tappan (1957, 

p. 1127-1132). 
8 THE CENOZOIC ROCKS 

Distinctions between deposits of Paleocene and 

Eocene ages are unclear in many areas, partly 

because of the scantiness of fossils, the equivocal 

nature of the faunas, and uncertainty of correlation 

with type Paleocene and Eocene. Paleocene marine 

deposits are differentiated on the map in California 

where they have been identified with certainty; 

elsewhere, both here and farther north, the Paleocene, if present, is not separated from the more 

extensive Eocene deposits. 

CALIFORNIA~ 

The paleontology of the Paleocene and Eocene 

marine rocks of California has been intensively 

studied for many years and has produced several 

sets of stages that are in common use. Using the 

larger invertebrate fossils, the Paleocene has been 

divided into the Martinez and Meganos Stages, and 

the Eocene into the Capay, Domengine, transition, 

and Tejon Stages. On the basis of benthonic 

Foraminifera, the Paleocene has been divided into 

the Ynezian and Bulitian Stages, and the Eocene 

into the Penutian, Ulatisian, and Narizian Stages 

(Mallory, 1959). Many of the first set of names were 

derived from formations in local areas, and in older 

reports they were used as formation names in wide 

areas in California. This usage is now outmoded, and 

the names are used as formations only in places in or 

near where they were first defined, other names 

being substituted in other areas where the rocks are 

of different facies although of approximately similar 

age. Correlation of the California Paleocene and 

Eocene units with units of these ages on the Gulf 

coast is rather vague, although some similarity of 

foraminiferal species in the two areas has been 

noted. 

Northeastern California, the region northeast of 

the San Andreas fault and north of the Tehachapi 

Mountains, is less disrupted tectonically than in the 

rest of the State, hence best preserves the original 

pattern of the Paleocene and Eocene deposits. 

In the western foothills of the Sierra Nevada, the 

lower Eocene lone Formation (Tee) overlaps the 

Mesozoic basement. Except for a few marine 

intercalations, the lone is a continental deposit as 

much as 1,000 feet (300 m) thick; it is composed of 

sandstone members, some beds of lignite, and 

members of remarkably pure kaolinitic clay that 

formed during a time of deep chemical weathering of 

3For a general summary, with emphasis on the sedimentology, see Nilsen and Clarke 

(1975). Data on southern California are given by Durham (1954), on the Great Valley by 

Haeckel (1966, p. 223-230), and on the Coast Ranges of central and northern California 

by Page (1966, p. 263-266). Interesting early summaries, now largely outmoded, were 

presented by Reed (1933, p. 120-146), and by Reed and Hollister (1936, p. 17-27). 

the bedrock in a tropical climate. The lone is more or 

less confluent with the ancient river channel 

deposits on the slopes of the Sierra Nevada to the 

east (the "Auriferous Gravels"). A somewhat similar 

continental deposit, the Montgomery Creek Formation, crops out along the western edge of the Cascade 

Range farther north, lying unconformably on the 

basement rocks of the Klamath Mountains to the 

west, and passing eastward beneath the Pliocene 

lavas of the Cascade Range. 

The Eocene deposits change rapidly into marine 

facies when traced in subsurface beneath the Great 

Valley. On its western side, in the Diablo Range and 

north of San Francisco Bay, the Paleocene and 

Eocene still consist of deposits that were mostly laid 

down in waters of moderate depth. Nevertheless, 

drilling in the Great Valley has revealed several deep 

gorges or submarine canyons which slope westward 

from the Sierra Nevada and are filled with deeperwater deposits. The most impressive is the Markley 

gorge in the latitude of Sacramento, formed during 

the late Eocene and Oligocene. 

On the east slope of the Diablo Range near Mount 

Diablo, the exposed sequence is about 9,000 feet 

(2,750 m) thick above the Upper Cretaceous and is 

divided into the Martinez Formation, the Meganos 

Formation, Capay Formation, Domengine Sandstone, and the Nortonville Shale Member of the 

Kreyenhagen Formation. Except for the Nortonville, 

all the units are prevailingly sandy and contain 

conglomerate; the sediments were mainly derived 

from the Sierra Nevada to the east. 

Accumulating evidence suggests that Late Cretaceous and early Tertiary movements had occurred 

along the San Andreas fault (or its ancestor), so that 

the Salinian block of basement rocks already 

projected some distance northward along the west 

side of the San Joaquin Valley, producing a shallow 

embayment. As a result, some of the sandstones of 

the southern area were derived from western sources 

and contain Franciscan and Salinian detritus. 

In the northern Coast Ranges, occasional outliers 

of Paleocene and Eocene rocks of Great Valley-type 

occur within the Franciscan terrane, but they are 

parts of klippen that have been carried westward 

along the Coast Range thrust. A quite different 

facies forms a part of the "coastal belt" of the 

Franciscan assemblage (Ke of map) and is shown by 

fossils to be of Paleocene and Eocene ages (Evitt and 

Pierce, 1975). It is less disordered than the main body 

of the Franciscan assemblage and probably formed 

during the waning stages of the Franciscan deep-sea 

trench. 

In the Eureka district along the coast farther 
PALEOCENE AND EOCENE SERIES 9 

north, the Franciscan is overlain by the Yeager 

Formation (Tx), a fossiliferous shallow-water deposit, also classed as of Paleocene age, which is 

evidently a near-shore facies of the upper part of the 

"coastal belt" Franciscan. 

The rocks southwest of the San Andreas fault were 

shifted by strike-slip movements at least 400 miles 

(650 km) northwestward between early Tertiary and 

Quaternary time, so that they are not closely related 

to rocks of the same age across the fault to the 

northeast. In the segment in the central and 

southern Coast Ranges, Paleocene and Eocene 

deposits overlap against the crystalline basement of 

the Salinian block, and include more continental 

slope and ocean floor deposits than to the northeast. 

They lie in places on Upper Cretaceous rocks, but in 

many other places they overstep directly onto the 

crystalline basement. The deposits include sequences 10,000 to as much as 20,000 feet (3,000-

6,000 m) thick of sandy flysch or turbidite that 

formed on deep-sea fans at the foot of the continental 

slope. 

The flysch sequences include the Eocene Butano 

Sandstone of the Santa Cruz Mountains and even 

thicker flysch in the southern Santa Lucia Mountains, the La Panza Mountains, and the Sierra 

Madre, which extend from Paleocene to upper 

Eocene. The latter form the largest outcrop area of 

lower Tertiary rocks in the coastal belt of California 

(Tx, Te). They are contiguous to, but in fault contact 

with, a mo'"e varied Paleocene and Eocene sequence 

in the Santa Ynez Range to the south. In the eastern 

Santa Ynez Range, the main part of the sequence is 

the thick Paleocene and Eocene Juncal Formation of 

arkosic sandstone, shale, and mudstone. Farther 

west the sequence forms the Anita Shale, Matilija 

Sandstone, Cozy Dell Shale, and Coldwater Sandstone. The thin lenticular orbitoidal Sierra Blanca 

Limestone is at the base in many places, and is the 

only conspicuous carbonate in the Tertiary of the 

California coastal belt. The Eocene marine strata in 

this area are succeeded by the red continental Sespe 

Formation (Toe), mainly Oligocene but including 

late Eocene at the base, which expresses a major 

regression of the sea. 

South of the Los Angeles Basin, Paleocene and 

Eocene deposits fringe the western sides of the Santa 

Ana Mountains and Peninsular Ranges past the 

Mexican border into Baja California; the strata are 

tilted westward but are not greatly deformed. In the 

Santa Ana Mountains, a 4,000-foot (1,200-m) sequence consists of the Paleocene Silverado Formation and the Eocene Santiago Formation, both 

shallow-water deposits with nonmarine interbeds. 

To the south in the Peninsular Range, the sequence 

thins to 1,600 feet (490 m). Here, the upper part, or 

Poway Group, contains the distinctive "Powaytype" clasts, which are well-rounded gravels of hard, 

resistant rocks, especially porphyritic rhyolite tuff, 

whose source area has been much debated but was in 

some undetermined terrane to the east. 

Near the San Andreas fault in the segment from 

the San Gabriel Mountains southeastward, slivers of 

marine Paleocene and Eocene rocks occur which 

were originally parts of extensive formations, now 

broken and displaced by fault movements. They 

include the Paleocene San Francisquito Formation 

of the Valyermo area and Cajon Pass, and the 

Eocene Maniobra Formations of the Orocopia 

Mountains northeast of the Salton Sea. 

OREGON AND WASHINGTON I 

Marine lower Tertiary rocks reappear north of the 

Klamath Mountain barrier of Mesozoic rocks in the 

coastal belt of Oregon and Washington. In northwestern Oregon, Eocene rocks form a nearly continuous belt of outcrop 40 or 50 miles (100-130 km) 

wide in the Oregon Coast Ranges west of the 

Willamette Valley, with only minor cover of younger 

Tertiary. North of a downwarp along the Columbia 

River, they reappear in smaller outcrops in the 

Coast Ranges of southwestern Washington. Farther 

north, Eocene rocks form a wide band of outcrop 

around the eastern and northern edges of the 

Olympic Mountains. Except in the Olympic area, 

the rocks are only broadly folded. 

The lower Tertiary rocks of Oregon and Washington, like those of California, are continental 

margin deposits, and marine fossils at various levels 

indicate correlations with the California sequence. 

Fossils in the lower beds in Oregon indicate 

correlation with the Capay and Domengine Stages of 

the lower Eocene of California; the Paleocene is 

seemingly unrepresented. Nevertheless, the Oregon 

and Washington Eocene is of markedly different 

facies, containing large volcanic components that 

are lacking in California. 

The Eocene overlaps the Mesozoic of the Klamath 

Mountains on the south and that of Vancouver 

Island on the north, but otherwise its base is not 

exposed. Except at the margins, much of it probably 

accumulated on oceanic crust off the edge of the 

continent. The total thickness of the Eocene here is 

10,000 to 20,000 feet (3,000-6,000 m) or more. Because 

of the associated sedimentation and volcanism, the 

oceanic basement, and other features, much of the 

4For general summaries see Snavely and Wagner (1963, 1964). 
10 THE CENOZOIC ROCKS 

Eocene of the region can be considered eugeosynclinal; on the Geologic Map, however, only the part of 

the sequence in which these features occur in 

exaggerated degree is so indicated. 

Along the margins of the depositional trough are 

shallow-water deposits, passing inland into coal 

measures, especially east of the Puget Trough on the 

flanks of the Cascade Range in Washington, and 

more locally at the north edge of the Klamath 

Mountains in Oregon. These pass westward into the 

deep-water marine deposits in the main area of the 

trough. 

Beginning in the earliest Eocene and continuing 

locally to the end of the Eocene, basaltic volcanics 

(Teb) were erupted from a series of centers along the 

axis of the trough. The volcanic material is estimated 

to have a volume of 60,000 cubic miles (250,000 km3), 

the largest of any volcanic unit in the Pacific 

Northwest. The volcanic bodies are known from 

south to north as the Umpqua, Siletz River, and 

Tillamook Volcanics in Oregon, and the Crescent 

and Metchosin Volcanics in Washington. Thick 

masses of pillow lavas alternate or interfinger 

marginally with water-laid fragmental basaltic 

debris, and at times the volcanic buildup was great 

enough that parts of the centers emerged as islands. 

The earlier volcanics are succeeded in the south by 

the middle Eocene Tyee Formation, a thick-bedded, 

graded turbidite of arkosic sandstone and siltstone. 

Paleocurrent data indicate that it was derived from 

the pre-Tertiary crystalline basement rocks of the 

Klamath Mountains to the south. Farther north, the 

equivalent of the Tyee is gray, tuffaceous siltstone 

and water-laid pyroclastics. 

Upper Eocene rocks are somewhat more restricted 

than the earlier Eocene, and in many places lie 

unconformably on them. Alkalic basalt was erupted 

locally and passes laterally into gray, thin-bedded to 

massive tuffaceous siltstone and mudstone, called 

by such names as Nestucca and Yamhill Formations 

in Oregon. At this time, nonmarine coal measures 

and other deposits spread farther into the trough. 

A tectonic situation different from the rest of the 

Oregon and Washington Eocene prevailed in the 

Olympic Peninsula of northwestern Washington. 

Here, the Eocene encircles the peninsula on the north 

and east, facing outward toward younger Tertiary 

formations, and consists of the Crescent Formation 

of basaltic volcanics 20,000 feet (6,000 m) thick and 

associated sedimentary units, all of which stand 

vertically and are in thrust contact with the sheared, 

partly metamorphosed eugeosynclinal rocks of the 

core of the Olympic Mountains (Tee, Toee, Tmoe) 

(Tabor, 1972, p. 1805-1807). 

Following the Eocene, there was a widespread 

withdrawal of the sea so that the succeeding 

Oligocene occupies much smaller areas within the 

Coast Range belt. 

CONTINENTAL DEPOSITS 

The Paleocene and Eocene continental deposits 

occur in many areas of the interior of the western 

United States; none are preserved farther east and 

were probably never laid down there. The largest 

areas of these deposits are those in the northern 

Great Plains and Rocky Mountains, which accumulated in basins between and east of mountains that 

had been newly raised during the Laramide orogenies of late Mesozoic and early Tertiary time. 

Smaller, more widely separated patches of deposits 

are preserved in various outlying areas west of the 

Rocky Mountains from the Canadian to the Mexican 

borders and were probably never much more extensive than they now are. 

The continental deposits have yielded many 

remains of plants and vertebrates, especially in the 

Great Plains and Rocky Mountains, from which a 

system of stages has been constructed that is independent of but parallel to the marine stages of the 

east and west coasts (Wood and others, 1941). The 

Paleocene has been divided into the Puercan, 

Dragonian, Torrejonian, Tiffanian, and Clarkforkian Stages, and the Eocene into theW asatchian, 

Bridgerian, Uintan, and Duchesnian Stages. All 

the stages are based on formations at localities 

within the Rocky Mountain region, but they have 

been extended far beyond the original formations 

into rocks of different facies and settings. 

GREAT PLAINS AND ROCKY MOUNTAINS 5 

Paleocene and Eocene continental deposits are 

extensive in the northern Great Plains and the intermontane basins of the Rocky Mountains. They are 

closely related to the Laramide orogeny of Late 

Cretaceous and early Tertiary time, which created 

the basins, and the intervening uplifted ranges 

which supplied much of the sediment source. The 

orogeny also permanently expelled the seas from 

this part of the continent (with the minor exception 

of the Cannonball Formation, noted below), so that 

the deposits were laid down on piedmont slopes or 

flood plains, in swamps, and lakes. In some of the 

deeper basins, the deposits attain thicknesses of 

10,000 feet (3,000 m) or more. Unconformities at 

various levels in the sequence, especially along the 

edges of the basins, attest to the progress of the 

5 For summaries, see Robinson (1972) and McDonald (1972). 
PALEOCENE AND EOCENE SERIES 11 

orogeny, but the rocks of the basins themselves were 

little disturbed, and lie nearly horizontal over wide 

areas. 

The unconformities are local, and the sequence in 

other places is conformable-from the latest Cretaceous into the Paleocene and from the Paleocene 

into the Eocene-so that there has been much controversy as to age assignments, notably as to the 

Cretaceous-Tertiary boundary (the so-called "Laramie question"). Many of the deposits are similar from 

one age to the next, but there was a gradual change 

in the deposits with time, reflecting different sediment sources, climatic conditions, and other factors; 

there were also changes in the faunas and floras, 

reflecting both changes in environment and evolutionary development. Despite the orogeny, the 

region remained at low altitude through the Paleocene and Eocene; even the mountain areas were not 

lofty but were worn down by erosion nearly as 

rapidly as they were raised. A notable change in 

depositional patterns, probably related to tectonic 

movements, occurs between the Paleocene and 

Eocene. The Paleocene forms deposits in nearly all 

the basins, where as the Eocene is thick and extensive 

only in the intermontane basins. In the Great Plains 

east of the. Rocky Mountains the Paleocene spreads 

out over great areas; the Eocene, if present, is confined to small remnant patches. 

PALEOCENE DEPOSITS (Txc) 

Both the latest Cretaceous (Laramie, Hell Creek, 

and Lance Formations) and the Paleocene deposits 

are somber-gray, lignitic sands and clays that accumulated in swamps in a humid, semitropical environment. Near the mountains, granitic detritus 

appears in the sequence. Much futile effort was once 

expended in a search for unconformities in the basin 

deposits that presumably would mark the CretaceousTertiary boundary, but the deposits are actually conformable, or nearly so. Nevertheless, the highest 

Cretaceous deposits contain Triceratops and others 

of the last of the dinosaurs, whereas the succeeding 

Paleocene contains only archaic mammals. However, part of the Laramie flora is of Tertiary aspect, 

so much adjustment has had to be made to reconcile 

the plant and animal evidence; this dilemma has 

now apparently been resolved. 

The term Fort Union Formation (or Group) has 

virtually become the generic term for the Paleocene 

deposits of North and South Dakota, Montana, and 

Wyoming; farther south other names are used. The 

Fort Union was named by Meek and Hayden in 1862, 

during the early days of geological exploration of the 

northwest, for the now-defunct Fort Union at the 

mouth of the Yellowstone River near the present 

town of Buford, North Dakota. In the Williston basin 

of North Dakota and in many other places, it is as 

much as 6,000 feet (1,800 m) thick. Because of its 

uniform character it has not been divided in many 

places, but elsewhere various components have been 

recognized and named, such as the Tullock Lake, 

Lebo, and Tongue River Members (or Formations). 

Volcanic detritus is rare in the Fort Union; westward 

in the Crazy Mountains basin of south-central Montana, however, the Fort Union passes into the Livingston Formation, a mass of andesitic tuff that 

includes highest Cretaceous below. 

Along the east edge of the outcrop area, in North 

Dakota and northern South Dakota, the Cannonball 

Formation (Tx) interfingers westward into the lower 

part of the Fort Union Formation. It consists of 

shallow-water marine shales-the last marine deposits of the continental interior, probably formed 

during a brief incursion from the Arctic. It is named 

for the Cannonball River in North Dakota, along 

which its characteristic "cannonball" concretions 

are prominent. The formation contains oyster banks 

and about 150 species of mollusks and foraminifers, 

the latter resembling those of the Midway Group of 

the Gulf Coast. 

The Paleocene deposits are preserved in two 

smaller areas along the front of the Rocky Mountains 

farther south, in the Denver and Raton basins of 

Colorado and New Mexico. In both areas they overlie 

beds of Laramie age (uK4) and are more or less 

gradational with them. The Dawson Arkose in the 

Denver basin is composed of granitic detritus from 

the adjacent Front Range and intergrades laterally 

with the finer-grained Denver Formation. In the 

Raton basin the fine-grained coal-bearing Raton 

Formation similarly intergrades northward with the 

coars~r Poison Canyon Formation which contains 

volcanic detritus. At Spanish Peaks at the north end 

of the basin, it is followed by the small remnant of the 

Huerfano Formation of Eocene age. 

A more complex set of Paleocene deposits occurs 

much farther west, in north-central Utah southwest 

of the Uinta basin whose original basinal form is 

now much disrupted by the block-faulting in the 

High Plateaus of Utah (Spieker, 1946). Below is the 

variegated shale, sandstone, and conglomerate of 

the North Horn Formation which bridges the 

Cretaceous-Paleocene boundary and contains Cretaceous dinosaurs in its lower part and Paleocene 

mammals in its upper part. It is followed by the 

lacustrine Flagstaff Limestone, whose white ledges 

form a prominent and persistent capping of the High 

Plateaus. The Flagstaff contains fresh-water mol-
12 THE CENOZOIC ROCKS 

lusks. Variegated clays and marls at the south end 

of the plateau that have traditionally been called 

Wasatch Formation are probably of Flagstaff age 

and are so represented on the Geologic Map (Txc). 

The lake in which the Flagstaff accumulated varied 

in shape and size with time, but late in the epoch 

probably covered most of central Utah although only 

fragments are now preserved. The Flagstaff is 

succeeded by the Colton Formation which is 

considered to be the local representative of the 

Wasatch Formation, of Eocene age, on the basis of 

scanty molluscan remains. 

It would have been interesting to have differentiated the Flagstaff lacustrine deposits in the same 

manner as the Eocene (Green River) lacustrine 

deposits on the Geologic Map, but the areal pattern is 

too complex for such representation, and they are 

therefore grouped with the North Horn Formation as 

Paleocene continental deposits (Txc). 

In the San Juan basin to the southeast, in northwestern New Mexico and southwestern Colorado, 

Paleocene continental deposits about 2,000 feet 

(600 m) thick lie between the highest Cretaceous 

(uK4) and the Eocene (Tee) in the center of the basins. 

In the southern part they comprise the thin, sandy 

and conglomeratic Ojo Alamo Formation and the 

thicker, finer-grained Nacimiento Formation. These 

units are replaced along the north side of the basin 

by the much coarser Animas Formation (merged 

with the Upper Cretaceous on the Geologic Map), 

which contains large quantities of andesitic debris 

derived from the early eruptions in the San Juan 

Mountains to the north. The Nacimiento and 

Animas include the Puerco and Torrejon Formations 

and "Tiffany beds" of former usage, each of which 

contains a significant Paleocene mammalian fauna 

that has served as the basis for the Puercan, 

Torrejonian, and Tiffanian Stages of the continental 

Tertiary time scale. 

EOCENE DEPOSITS (Tee) 

Eocene continental deposits succeed the Paleocene 

in all the intermontane basins of the Rocky 

Mountains, and attain sizeable volumes. They are as 

much as14,000feet(4,250m)thickin the Uinta basin 

on the south flank of the Uinta Mountains, and 

exceed 5,000 feet (1,500 m) in many of the other 

basins. By contrast, Eocene deposits are lacking or 

inconsequential east of the mountains, where only 

small remnants of the Golden Valley Formation are 

recorded above the Fort Union Formation in North 

Dakota; the Huerfano Formation occupies a small 

area at the north end of the Raton basin in Colorado. 

Eocene deposits record a marked change in 

conditions from those of the Paleocene-from 

somber-gray lignitic Paleocene deposits laid down in 

a moist swampy environment, to red-banded Eocene 

clastics formed in a drier climate on piedmont slopes 

and flood plains. Forests gave place to open 

savannas and grasslands, and the mammalian 

community changed from forest-dwelling animals to 

browsing herbivores. At the same time, evolutionary 

changes resulted in the extinction of the archaic 

Paleocene mammals and the substitution of mammals of more modern aspect. Primitive horses 

appeared, and higher up in the sequence (and 

continuing into the Oligocene) the titanotheres, 

whose gigantic bones aroused the interestofthefirst 

geological explorers, and later became the field of 

collectors and paleontologists such as Marsh and 

Cope. 

In southwestern Wyoming, northwestern Colorado, and northeastern Utah, terrestrial piedmont 

and flood-plain sedimentation was interrupted during the middle Eocene by lacustrine conditions 

whose deposits (the Green River Formation) are 

differentiated from the rest on the Geologic Map (Tel) 

and are discussed separately below. 

Although the Eocene deposits formed in a tectonic 

environment, the region remained at an altitude as 

low as that of the Paleocene; the successive uplifts 

never attained the height of lofty mountains. The 

similarity of mammalian faunas from one basin to 

the next indicates that there were no topographic 

hindrances to free migration, and in places the 

original deposits seem to have been confluent 

between basins, the connections having been removed by subsequent erosion. 

The Wasatch Formation (Tee) at the base of the 

Eocene was named by Hayden in 1868 for outcrops in 

Echo Canyon on the line of the Union Pacific Railroad on the east side of the Wasatch Mountains in 

northeastern Utah, where its red conglomeratic 

sandstones are prominent. The type Wasatch was 

subsequently divided into the Knight, Fowkes, and 

Almy Formations, but this subdivision now appears 

to be invalid. The Fowkes is a younger Tertiary 

tuffaceous deposit accidentally infaulted in the 

sequence. The lower red sandstones in Echo Canyon, 

which are unconformable below the Knight, are 

actually Cretaceous (Echo Canyon Formation); elsewhere, beds mapped as Almy are indistinguishable 

from the Knight, and the latter is the only true 

representative of the original Wasatch. 

The name Wasatch has been widely extended from 

the type area to other parts of Utah, Wyoming, 

Colorado, and New Mexico, but the lithologic 

Wasatch varies considerably in scope and age from 
PALEOCENE AND EOCENE SERIES 13 

place to place, so that stratigraphic precisionists 

prefer to substitute local names in the different 

areas. Nevertheless, it has become virtually a 

generic term for the early Eocene continental 

deposits of the region, and it is retained here in this 

sense. Its faunas are the basis for the Wasatchian 

Stage of the continental Tertiary time scale. The only 

exception to the general application of the name 

Wasatch is in the Wind River basin, where the name· 

Wind River Formation has traditionally been used. 

The name is of about the same vintage as the name 

Wasatch, and applies to about the same strata, 

although it apparently includes some younger beds. 

Marginally, the Wasatch is a red, coarse, bouldery 

piedmont deposit (as at the type locality) that in 

many places oversteps the Paleocene and Cretaceous 

to lie on older rocks in the cores of the ranges. Farther 

out in the basins it is a red-banded sandy clay and 

sandstone that was probably deposited by streams 

on flood-plains. 

In northeastern Utah, southwestern Wyoming, 

and northeastern Colorado, the Green River lake 

beds are overlain by higher Eocene continental 

deposits (shown as Tee on the Geologic Map, like the 

Wasatch). They include the Bridger of the Green 

River basin and the Uinta of the Uinta basin, the 

latter apparently somewhat younger than the former. The Bridger and Uinta much resemble the 

Wasatch Formation below but red colors are rarer, 

and some beds are chocolate brown. Minor lenses of 

lacustrine deposits occur, and the Bridger contains 

much fine-grained tuff and other volcanic detritus. 

The Duchesne River Formation overlies the Uinta 

Formation along the southern flank of the Uinta 

Mountains and has been variously classed as of 

Eocene or of Oligocene age. On the Geologic Map it is 

shown as Oligocene (Toe), and it is df'scribed below 

under that series. 

In the San Juan basin of nC'rth western New 

Mexico, the Eocene of the central part was formerly 

termed Wasatch Formation, but it has been renamed 

San Juan Formation. It is about 1,500 feet (450 m) 

thick and is divisible into various intertonguing 

members of different lithologies. In the central part 

are units of shale and fine-grained sandstone, some 

red, some gray or varicolored, and around the edges 

are coarse conglomeratic units that were spread as 

fans into the basin. 

EOCENE LACUSTRINE DEPOSITS (Tel) 

Middle Eocene lacustrine deposits, known collectively as the Green River Formation, cover an area of 

about 30,000 square miles (78,000 km2) in southwestern Wyoming, northwestern Colorado, and 

northeastern Utah, and have a thickness of 3,000-

5,000 feet (900-1,500 m). An outlying lacustrine 

deposit, the Tatman Formation, occurs in the 

Bighorn basin to the north, where it overlies the local 

representative of the Wasatch Formation (= Willwood Formation). 

The Green River Formation has been extensively 

investigated because of its economic potential-its 

deposits of oil shale and of trona (a form of sodium 

carbonate)-but much of its broader interpretation 

and philosophical rationalization is the work of 

Wilmot H. Bradley (1964 and earlier reports), who 

has devoted a large part of his scientific career to the 

subject. 

The Green River Formation is preserved in the 

Green River and Washakie basins in Wyoming, the 

Piceance basin in Colorado, and the Uinta basin in 

Utah. It accumulated in Lake Gosiute and Lake 

Uinta that lay north and south of the Uinta 

Mountains axis, and which were partly confluent 

around its eastern end. From time to time, the Rock 

Springs uplift in southwestern Wyoming formed an 

island between the Green River and Washakie 

basins. 

The Green River Formation characteristically 

forms light-colored, thin-bedded outcrops that contrast unmistakably with the adjacent Eocene terrestrial deposits. Much of it is varved, and on the basis 

of its annual layers it is estimated that the lakes 

endured for about 6,500,000 years. In detail, its 

deposits exhibit a great variety of facies, depending 

on their distance from shore, fluctuations in depth 

of water and salinity, and of the extent of the lakes 

throughout time. During much of their life, the lakes 

had exterior outlets to the north and south, but in the 

middle of the epoch, in Wyoming at least, the lake 

shrank, the outlet disappeared, and the waters 

became highly saline, causing deposition of the beds 

of trona and other evaporites. 

The oil shale (an organic-rich marlstone), one of 

the characteristic components of the Green River, is 

especially concentrated in the Parachute Creek 

Member in the Piceance basin of northwestern 

Colorado. Other facies include light gray, brown, or 

buff dolomitic marlstone and shale; dolomitic marlstone thickly studded with salt-crystal molds; oolitic 

limestone and bedded algal deposits; limy sandstone 

and sandy marls tone; anti beds of volcanic ash that 

become increasingly abundant upward. Marginally, 

sandy deposits interfinger with the finer-grained 

deposits. The formation contains fossil fishes, 

plants, pelecypods, gastropods, and fly larvae. Bird, 

mammal, and reptile bones occur in the shore facies. 

Because of fluctuations of the lake, the Green River 
14 THE CENOZOIC ROCKS 

deposits intertongue extensively with the terrestrial 

Wasatch deposits, especially in Wyoming, where a 

succession of tongues of Green River and Wasatch 

have been mapped and named. 

OUTLYING AREAS 

NEW MEXICO AND WEST TEXAS 

South of the Raton and San Juan basins of 

northern New Mexico, small, widely separated 

patches of Paleocene and Eocene continental deposits are preserved which are probably remnants of 

units that were originally more extensive but are 

now so dismembered that the outlines of their basins 

.are no longer discernible. The basins were probably 

never as extensive, nor the deposits as thick as those 

in the Rocky Mountains to the north. 

At the north end of the Sandia Mountains 

northeast of Albuquerque is the Galisteo Formation, 

about 3,000 feet (900 m) thick, which lies unconformably on the Cretaceous and consists of variegated 

sand and clay, in part tuffaceous, with some beds of 

fresh-water limestone. Vertebrate bones at various 

levels indicate Wasatchian and younger Eocene ages. 

West of the Rio Grande farther south, the Baca 

Formation fringes the north edge of the Datil 

volcanic field for about 100 miles (160 km); lying 

unconformably on the Cretaceous to the north and 

passing southward beneath the younger Tertiary 

volcanics. It is about 700 feet (210 m) thick and 

consists of a coarse basal conglomerate followed by 

red and white sandstone and clay. Indications of age 

are scanty, but the formation is presumed to be 

Eocene. 

At the north end of the Caballo Mountains just 

east of the Rio Grande the McRae Formation, about 

3,000 feet (900 m) thick, overlies the Upper Cretaceous Mesaverde Formation (Kelley and Silver, 1952, 

p. 115-120). It consists of a basal conglomerate 

containing many volcanic clasts that is overlain by 

maroon shales with some interbedded sandstone 

layers. Dinosaur bones have been collected in the 

lower part, and if not reworked, indicate a Cretaceous age for this part. Higher beds contain little but 

equivocal fossil plants. The formation appears to be 

older than those just described, and on the Geologic 

Map it is assigned to the Paleocene (Txc). 

The Cub Mountain Formation in the Sierra Blanca 

area 80 miles (130 km) to the east is similar and 

probably correlative. It lies unconformably on the 

surrounding Cretaceous and is followed unconformably by the early Tertiary volcanics that form the 

heights of the Sierra Blanca peaks. 

Much farther south, in the Big Bend region of west 

Texas and east of the Chisos Mountains, Upper 

Cretaceous continental deposits are overlain by 

continental deposits of Paleocene and Eocene ages 

(Maxwell and others, 1967, p. 98-107). The Black 

Peaks Formation, an alternation of sand and clay 

with "cannonball" concretions, is about 800 feet 

(250m) thick and contains Paleocene mammals of 

Torrejonian and Tiffanian ages. It is followed by the 

very similar Hannold Hill Formation about 500 feet 

(150m) thick that contains mammals of Wasatchian 

age. As both formations crop out in a very small area, 

they are shown together as Paleocene (Txc) on the 

Geologic Map. 

ARIZONA 

To the west in Arizona a dozen or more still smaller 

areas classed as Eocene continental deposits (Tee) 

are widely despersed in the Basin and Range 

province. For the most part they are the units 

designated as TKs on the State Map (1969). They are 

tilted and faulted, reddish-brown to gray conglomerate, sandstone, shale, and thin limestone with thicknesses of several thousand feet, which lie unconformably on the Meso.zoic and older rocks and lie 

beneath the main body of Tertiary volcanics and 

associated sediments. Aside from these structural 

relations, there is little specific evidence as to their 

ages, although the deposits in the Artillery Mountains in the western part of the State contain palm 

roots of possible Eocene age. 

One of the more interesting of these units is the 

Pantano Formation of the Tucson area, 6,500 feet 

(2,000 m) thick, of sandstone, mudstone, and graniteboulder fanglomerate (Finnell, 1970). Unlike the 

younger Tertiary continental deposits of the surrounding area, the Pantano has been steeply 

upended, strongly deformed, and thrust over the 

gneisses of the Rincon and Catalina Mountains. 

Like the other continental deposits of southern 

Arizona, there is little evidence for its precise age; 

on the Geologic Map it is classed as Eocene (Tee), but 

radiometric determinations on interbedded rhyolites 

and andesites yield ages of 25 to 36 m.y ., which 

indicates a range in age between early Oligocene and 

early Miocene. 

NEVADA AND CALIFORNIA 

In southern Nevada east of Las Vegas, the Horse 

Spring Formation consists of more than 1,000 feet 

(300m) of yellowish siltstone, whitish limestone, and 

an upper unit of coarser clastic deposits (Longwell 

and others, 1965, p. 45-48). The formation is conformable on the Cretaceous and older rocks of the 

district, and is unconformable beneath the late 

Tertiary Muddy Creek Formation. Fossils are scanty 

and the age of the Horse Spring is uncertain. It 
PALEOCENE AND EOCENE SERIES 15 

contai~s a few plants, pelecypods, and gastropods 

of possible Eocene age, but radiometric determinations at one locality suggest a Miocene age. On the 

Geologic Map it is indicated as Eocene (Tee). 

The lower Tertiary continental deposits west of the 

Sierra Nevada in California have already been noted 

(p. 7). East of the Sierra Nevada the only one 

recorded is the Goler Formation of the El 'Paso 

Mountains, a short distance north of the Barlock 

fault (Nilsen and Clarke, 1975, p. 24). It is about 

?,500 feet (2,000 m) thick, lies unconformably on an 

Igneous and metamorphic basement and is overlain 

by Pliocene continental deposits. it consists of a 

lower. conglomerate and breccia unit, overlain by 

~rkosic sandstone and red mudstone. Fossil plants 

In the Goler have been ascribed an Eocene age, but its 

vertebrates are probably Paleocene. It is indicated as 

Paleocene (Txc) on the Geologic Map. 

WESTERN MONTANA 

Within the mountains of western Montana are a 

few early Tertiary continental deposits. 

In the Flathead Valley west of the Lewis and 

Livingston Ranges, or frontal ridges of the Belt 

terrane at the Canadian border is the Kishenehn 

Formation, an early Tertiary deposit much obscured 

by Pleistocene glacial deposits (Ross, 1959, p. 67-72; 

described as "older alluvium"). It consists of wellbedded, well-indurated, fine-grained sand silt and 

clay, ~th so~e interbedded lignite. Fro~ pl~ce to 

place It contains plant remains, mollusks, and fish. 

The bulk of the material seems to be late Eocene but 

some collections may be of Miocene or other Tertiary 

ages. It is represented as Eocene (Tee) on the 

Geologic Map. 

Much farther south, in the Lima region of 

southwestern Montana, is the Beaverhead Conglo~erate (~cholten and others, 1955, p. 368-369), 

which occupies the low ground in .front of many of 

the ranges, and is overthrust in places by the 

Mesozoic and Paleozoic rocks; in other places it lies 

unconformably on them. It is as much as 9 000 feet 

(2,700 m) thick and consists of coarse red c~nglom­ erate, with interbedded sandstone and some freshwater. limestone .. It is evidently a synorogenic 

deposit, formed d unng the deformation of this part of 

the Rocky Mountains, and is probably Paleocene 

(Txc), although fossil control is sparse. At one 

locality north of Lima it is overlain unconformably 

by deposits that contain late Eocene mammals (Sage 

Creek Formation). About 40 miles (65 km) to the east 

of the Lima region, the Sphinx Conglomerate forms 

a sm~ll outlier on the summit of the Madison Range 

and Is probably correlative with the Beaverhead. 

WASHINGTON 

In central Washington, on the east side of the 

Cascade Range in the latitude of Seattle, is a complex 

array of lower Tertiary continental deposits and 

associated volcanics that were elucidated early in 

the century by the work of Smith (1904), and Smith 

and Calkins (1906); they were later reviewed by 

Foster (1960). 

At the base is the Swauk Formation (Txc) more 

than 4,000 feet (1,200 m) thick, a nonvolcanic fluviatile deposit composed largely of arkose, in part 

conglomeratic, with some interbedded shale that 

lies unconformably on pre-Tertiary metam~rphic and plutonic rocks. It contains fossil plants of 

Paleocene age. In the mountains to the west are 

smaller areas of the Guye Formation, once thought 

to be younger, but now known from paleobotanical 

evidence to be correlative. 

The Swauk is overlain unconformably southward 

by the Teneaway Basalt and the local Silver Pass 

Volcanics 1,000-5,000 feet (300-1,500 m) thick (lTv), 

which are followed conformably by another continental deposit, the Roslyn Arkose, whose leaves and 

rare vertebrate remains are middle or upper Eocene. 

The Guye Formation to the west is also foMowed by 

Eocene continental deposits and volcanics, such as 

the Naches Formation which contains fossil leaves. 

The lower Tertiary rocks of central Washington 

are overlain unconformably by the lower part of the 

great volcanic sequences of the Cascade Range and 

Columbia Plateau-on the west the Miocene Keechelus Andesite (uTa) and on the east theY akima Basalt 

at the base of the Miocene Columbia River Group 

(Tmv). 

Farther north, on the west side of the Cascade 

Range, is the Chuckanut Formation (Txc). It is 

lithically much like the Swauk, of similar origin 

similarly unconformably on the pre-Tertiary rocks' 

and with fossil plants of about the same age, but it 

attains thicknesses of 15,000 feet ( 4,500 m) or more. 

Various small to large outliers of Paleocene continental deposits occur in the Cascade Range between 

the Swauk and Chuckanut, which suggest that the 

two formations were once connected. 

EUGEOSYNCLINAL DEPOSITS6 

The only Tertiary rocks shown as eugeosynclinal 

on the Geologic Map are those in the core of the 

Olympic Mountains of northwestern Washington, 

whose sparse fossils indicate an early Eocene to 

Miocene age. However, a considerable part of the 

6Abstracted from manuscripts in preparation by W. M. Cady and R. W. Tabor. 
16 THE CENOZOIC ROCKS 

Eocene rocks of the coastal belt of Oregon and 

Washington also have many eugeosynclinal qualifications, being marine turbidites and interbedded 

submarine basaltic volcanics that accumulated on 

an oceanic crust. 

The peripheral rocks of the Olympic Mountains 

surround the core on the north, east, and southeast 

and are a thick, steeply-dipping, outward-facing 

sequence that ranges in age from Eocene to Miocene. 

The Eocene part, mainly the Crescent Formation, 

resembles that farther south in the coastal belt, 

already described (p. 8). 

The core rocks are thrust-faulted against the 

peripheral rocks. These faults, including the Hurricane Ridge fault on the north, now dip steeply, but 

probably originally dipped eastward at a much lower 

angle; they and other concentric faults within the 

core form the boundaries of crustal slabs that were 

subducted eastward from the ocean floor beneath the 

continent and its marginal deposits. Westward into 

the core, each slab consists of rocks somewhat 

younger than those in the slab to the east; hence, the 

Geologic Map distinguishes units from east to west 

as of Eocene age (Tee), Oligocene and Eocene age 

(Toee), and Miocene and Oligocene age (Tmoe). 

The core rocks are marine shales, turbidites, and 

minor basalts. To the east they are penetratively 

sheared, and metamorphosed to phyllite and semischist of prehnite-pump~llyite and low-rank greenschist facies (Tabor, 1972) (on the Geologic Map they 

are shown with a metamorphic overprint, although 

elsewhere on the map this overprint is reserved for 

rocks of amphibolite grade or higher). Beds and folds 

are commonly.sheared out and the sandstones form 

boudins. To the west, the rocks are still highly folded 

and faulted but are less penetratively sheared and 

the only metamorphic minerals are zeolites. 

Sparse microfossils and megafossils indicate that 

the core rocks to the east are Eocene (a few collections 

may be Paleocene~ but nothing older is known). 

Westward in the core more numerous fossils indicate 

late Eocene and Oligocene ages, and near the coast 

early to middle Miocene. 

Rocks in the core include pillow basalt, basaltic 

breccia and tuff, tuffaceous argillite, and associated 

limestone. The basaltic rocks are especially abundant in the eastern, or Eocene block, where they form 

pods and lenses in sheared rock but were probably 

originally autochthonous. The rarer lenses of basaltic rock in the blocks farther west might be allochthonous, as the youngest foraminifers so far 

identified in the associated limestones are all 

Eocene. 

The sediments in individual outcrops indicate 

consistent, outward-facing sequences, but no general 

stratigraphic sequence is determinable; most of the 

rock is "broken-formation," and passes in places into 

melange. 

OLIGOCENE SERIES 

The Oligocene Series, which formed during about 

11 million years of mid-Tertiary time, corresponds to 

the Tongrian, Rupelian, and Chattian Stages of the 

European marine sequence and is divided into the 

Chadronian, Orwellian, and Whitneyan Stages of 

the American continental sequence. The Oligocene is 

only meagerly represented on the Geologic Map of 

the United States (fig. 2)-among the marine 

deposits of the Gulf Coastal Plain and the Pacific 

northwest (To), among the continental deposits of 

the northern Great Plains and Rocky Mountains 

(Toe), and in smaller areas elsewhere. Some other 

deposits may be of Oligocene age but are mapped 

with the series above or below on the Geologic Map. 

Oligocene volcanic rocks in the western United 

States are grouped with others of the lower Tertiary 

(lTv). 

MARINE STRATIFIED ROCKS 

GULF COASTAL PLAIN 

Strata of Oligocene age are shown on the Geologic 

Map of the United States as a narrow band of outcrop 

in the eastern Gulf Coastal Plain extending from 

Mississippi into Georgia, and less continuously 

eastward into South Carolina and westward into 

Louisiana. They emerge again in northern and 

central Florida on the flanks of the Ocala uplift. No 

Oligocene is known in the Atlantic Coastal Plain 

north of South Carolina, at least at the surface. No 

Oligocene is mapped in the western Gulf Coastal 

Plain in Texas, although the non-marine Frio Clay of 

the southern part of the State may be of this age; it 

was grouped with the Miocene on the Geologic Map 

on the advice of D. H. Eargle (oral commun., 1971). 

The Oligocene of the outcrops in the Gulf Coastal 

Plain is only a few hundred feet thick, but it thickens 

to many thousands of feet downdip in subsurface 

where a complex stratigraphy has been worked out. 

The outcrops in the eastern half of the Gulf Coastal 

Plain are dominantly calcareous, with many limestone beds and formations, which are interbedded 

with marl and lesser sand and clay, giving rise to 

many thin stratigraphic units. 

The most prominent part of the Oligocene is the 

middle, or Vicksburg Group, named in 1848 for 

exposures near Vicksburg, Mississippi, at the 

west end of the main outcrop belt. The Vicksburg is 

divided in Mississippi and Alabama into the Mint 

Spring Marl, Marianna limestone, and Byram 
OLIGOCENE SERIES 17 

Limestone, as well as various minor units or 

members. The Marianna and Byram Limestones 

form the main part of the Oligocene in Florida. 

In the type region the Vicksburg is underlain by 

the Forest Hill Sand and Red Bluff Clay, which lie on 

the Jackson Group; and it is overlain by the 

Chickasawhay Marl and its equivalent, the Flint 

River Formation, which in Florida passes into the 

Suwannee Limestone. 

To the west, across the Mississippi River in 

Louisiana, only equivalents of the Vicksburg persist 

in the outcrops, which are here dark shale with some 

sandy and lignitic layers. They are overlapped near 

the west edge of the State by the Miocene Catahoula 

Formation and do not come to the surface in Texasunless the Frio Clay in the far southwest is of 

Oligocene age. 

PACIFIC COASTAL AREA 

CALIFORNIA 

The Oligocene of California is represented by the 

Lincoln and Blakeley Stages of marine megafossil 

zonation, and the greater part of the Refugian Stage 

of microfossils. However, there is much uncertainty 

as to the correlation of beds assigned to the 

Oligocene in California with the type Oligocene in 

Europe, so that there is controversy as to the exact 

limits of the true Oligocene. In addition, Oligocene 

was a time of widespread emergence in the coastal 

area of California, so that the supposed Oligocene 

formations are very locally distributed and rather 

thin. On the Geologic Map, the Oligocene is thus not 

shown for the most part, its strata being merged with 

the underlying Eocene or overlying Miocene. 

The largest area of Oligocene shown on the map is 

the Sespe Formation, a nonmarine unit about 

5,000 feet (1,500 m) thick that is intercalated in the 

marine sequence in the western part of the Transverse Ranges; from place to place the Sespe extends 

from the upper Eocene into the lower Miocene. It is a 

red clastic deposit largely of granitic debris that 

contains vertebrate bones at several levels. At the 

western end of the Santa Ynez Mountains it grades 

into marine beds (Alegria and Gaviota Formations)~ 

Farther north, marine formations of Oligocene age 

(not represented on the Geologic Map) include parts 

of the San Lorenzo Formation of the Santa Cruz 

Mountains and the Kreyenhagen Shale of the 

western part of the San Joaquin Valley. 

OREGON AND WASHINGTON 

In the northern coastal belt of Oregon and 

Washington, as in California, the Oligocene is 

thinner and more restricted than the Eocene, but its 

outcrops are large enough to be represented on the 

Geologic Map. Much of the anticlinorium of the 

Oregon Coast Ranges was emergent, and the 

Oligocene deposits occur along its west and east 

edges and around its north end. 

Vigorous pyroclastic volcanism in the Cascade 

Range to the east caused the Oligocene marine 

deposits to be dominantly tuffaceous; they pass 

eastward into volcanic-rich continental deposits 

along the edge of the Cascade Range, which lack the 

coal measures of the preceding Eocene. The Oligocene deposits include the Y aquina Formation of the 

Oregon coast, composed of cross-bedded arkosic and 

tuffaceous sandstone, and the Lincoln Formation of 

the Washington coast, consisting of massive tuffaceous siltstone and fine-grained sandstone. East of 

the Coast Range uplift in Oregon is the richly 

fossiliferous Eugene Formation. North of the 

Olympic Mountains along the Strait of Juan de 

Fuca, more than 10,000 feet (3,000 m) of Oligocene 

silts and clays were deposited. As this area was 

farther removed from the volcanic activity to the 

east, these deposits contain much less tuffaceous 

material than the other Oligocene units. 

CONTINENTAL DEPOSITS 

Continental deposits of Oligocene age are widely 

distributed in the northern part of the interior region 

of the Western United States, but individual outcrop 

areas are mostly small and widely dispersed, the 

largest being in the Great Plains and eastern part of 

the Rocky Mountains. In many places, especially 

toward the west, the Oligocene continental deposits 

are intermingled with contemporaneous volcanic 

rocks, which are grouped with the other lower 

Tertiary volcanics (lTv) and are described separately 

later. 

GREAT PLAINS AND ROCKY MOUNTAINS 

The White River Formation (or Group) has become 

the generic name for most of the Oligocene deposits 

in the Great Plains of South Dakota, Nebraska, and 

Colorado, and westward into the Rocky Mountains 

of Wyoming. The formation was named by Meek and 

Hayden in 1858 for the White River in southwestern 

South Dakota southeast of the Black Hills, along 

which it is spectacularly exposed in the Big 

Badlands. To the south, the White River reappears 

from beneath the Miocene tnd Pliocene deposits in 

the valleys of the North and South Platte Rivers, 

where it extends from the front of the Rocky 

Mountains in Wyoming and Colorado, eastward into 

Nebraska. Outliers of the White River to the west in 

Wyoming are mostly in intermontane basins, but 

some occur high up in the mountains, indicating the 

great original extent of this broad sheet of deposits. 
18 

_, 

I 

I ~, __ ........ ________________ ( 

f .. ~· ~) 

/ ~ I 

I' I f ------1___ .,. i 

( 

~ 

THE CENOZOIC ROCKS 

•, 

\ 

'L, 

) 

·-----------

·-, • r-·----- ,_---~-!"',j ·---

,; ·-------~---- / ----

I 

I 

I 

I 

I 

I 

I 

!. __ \._ I ----------

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\: 

\} 

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·-...... 

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··-r-·· ----------------. .----

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I 

I 

\ 

I, I - . : ... ;.. \ 

.. ~----------------------; " . ' I ' . : l 

I 

I 

~-

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, _____ "', ., 

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z 

-------: ... _; 

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FIGURE 2.-Map of the United States showing surface distribution of Oligocene stratified rocks as represented on the Geologic 

Map of the United States (units To, Toe, Toee). 
OLIGOCENE SERIES 19 

-~ · ...... _r·-,_.,"\.,_,._ .• 

\. 

---------------\ 

\ ,_\: _______ _ 

\ 

.I 

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I 

t.---------------,;' 

-·->~ \,, ~. \..: '.r-, ' 

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G U L F OF MEXICO 

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,f 

r\ 

---r----~ \ .. ( \ I : . '. I \ 

"\ ' ' \ ' 

\ -~------ :-

I -·. -"'. t--

\ 

,.,.. . -· 

FIGURE 2.-Continued. 

In its type area in the Great Plains, the White River 

is 600 to 700 feet (180-210 m) thick, but elsewhere it is 

mostly thinner. In the Great Plains it is divided into 

the Chadron Formation of clay, sandy clay, and 

river-channel sandstone, and the overlying thicker, 

finer-grained Brule Clay. A somewhat older Oligocene deposit forms Slim Buttes northwest of the 

main area in South Dakota and is locally present 

beneath the Chadron elsewhere. Both the Chadron 

and Brule abound in vertebrate bones, notably the 

gigantic titanotheres (Osborn, 1929, p. 103-112). 

The White River Formation is a terrestrial deposit 
20 THE CENOZOIC ROCKS 

that was spread across a broad plain sloping gently 

eastward from the Rocky Mountains and has been 

little deformed since. (An early idea that it was a 

subaqueous deposit laid down in a vast lake has long 

since been discredited.) It includes massive, lenticular river-channel sandstones, surrounded by 

more extensive finer-grained flood-plain or overflow 

deposits, and occasional deposits that were laid 

down in local ponds and marshes. Volcanic ash 

forms occasional layers. The river channels can be 

traced for considerable distances, and extend eastward and southeastward from the mountains. Their 

mineral content can be matched with their respective 

sources-the Black Hills, the Laramie Range, and 

the Front Range (Clark, 1975, p. 99-104). Probably 

the rivers coalesced eastward in to streams draining 

south into the Gulf of Mexico. The excess of 

deposition in the region can be attributed to a general 

change from the moist climate of Eocene time to one 

of increasing aridity. 

The vertebrate faunas of the White River reflect 

these varied environments. The fine-grained floodplain deposits contain chiefly plains and grassland 

animals, whereas the river-channel deposits contain 

chiefly forest and fluviatile animals. The lagoon and 

swamp deposits contain fish, alligators, and aquatic 

plants. 

South of the White River area, in the Great Plains 

of Colorado, the Castle Rock Conglomerate forms a 

small outlier in the center of the Denver basin south 

of Denver. It is essentially an extension of the White 

River deposits and contains many of the same 

vertebrate bones. It is underlain by a rhyolite ashflow tuff lying unconformably on the Paleocene 

deposits that has been dated radiometrically at 34.8 

m.y. and is probably an extension of the Thirtymile 

volcanic field in the Front Range to the southwest 

(Steven, 1975, p. 83). 

Within the southern Front Range itself are the 

Florissant Lake Beds, famed for their plant and 

insect remains, but with very few vertebrates 

(MacGinitie, 1953). Underlying tuff beds have a 

radiometric age of 35 to 36 m.y., and the lake beds 

cannot be greatly younger. The lake beds accumulated in a shallow basin on a surface carved in late 

Eocene time and dammed by the eruptions of the 

Thirtymile volcanic field to the west. Their floras 

indicate that the lake beds formed in a warm, 

subhumid climate at an altitude of about 3,000 feet 

(1,000 m), as compared with their present altitude of 

9,000 feet (2, 750 m). 

On the south flank of the Uinta Mountains is the 

Duchesne River formation (Toe). It lies conformably 

on the Eocene Uinta Formation, but it is a coarser 

deposit that represents a new cycle of sedimentation, 

and it overlaps northward on the pre-Tertiary rocks. 

Opinions differ as to whether the formation is of 

Eocene or Oligocene age; the Duchesnian Stage of 

the continental Tertiary is conventionally placed in 

the Eocene, but at least the uppermost members (La 

Point and Star Flat) are certainly Oligocene (Clark, 

1975, p. 110). It has yielded a K/ Ar age of 39.3 m.y. 

OUTLYING AREAS 

OREGON 

In north-central Oregon the John Day Formation 

flanks the Blue Mountains uplift on the northwest, 

where it lies unconformably on the Eocene Clarno 

Formation (lTv), and is overlain unconformably by 

the basalts of the Columbia River Group (Tmv). Both 

the Clarno and John Day are of volcanic origin, but 

the John Day is more dominantly sedimentary, 

hence is shown as continental deposits (Toe) on the 

Geologic Map. The John Day is about 2,000 feet (600 

m) thick and is composed largely of tuffaceous 

sediments-water-laid tuffaceous siltstone and rhyolitic air-fall tuff and ash-flow tuff. It has been 

famous since the turn of the century for its 

mammalian remains, which are assigned to the 

Whitneyan and Arikareean Stages of the continental Tertiary (upper Oligocene and lower Miocene). 

Farther west in Oregon, in the westel'Il part of the 

Cascade Range, are the lower Tertiary andesites of 

the Little Butte Volcanic Series (ITa) (Peck and 

others, 1964, p. 11-26). In some segments of its 

outcrops, nearly three-quarters of the series consists 

of poorly bedded andesitic tuff and breccia; andesitic 

lapilli tuff, welded tuff, and breccia; stratified tuff 

and tuffaceous sandstone and conglomerate. These 

volcaniclastic components of the Little Butte Series 

are shown separately on the Geologic Map as 

continental deposits (Toe). Fossil plants indicate 

that they are of middle Oligocene to lower Miocene 

age. 

In the Basin and Range part of southern Oregon, 

the lowest unit exposed in Steens Mountain and 

other block-faulted ranges is the Cedarville Formation (Toe), typified by exposures in the Warner Range 

of northeastern California, where as much as 

7,500 feet (1,200 m) is exposed (Russell, 1928, p. 402-

416). It consists of andesitic tuffs, agglomerates, and 

mudflow deposits, with minor lava. The lower beds 

contain a middle Oligocene flora, but the upper part 

extends into the Miocene. The Cedarville is unconformable beneath the main Miocene and Pliocene 

volcanic sequences in the ranges. 

CALIFORNIA 

In the southwestern Klamath Mountains of 
MIOCENE SERIES 21 

northern California are several small areas of the 

Weaverville Formation (Toe) (Irwin, 1966, p. 29-30), 

which is a deposit of sandstone, shale, conglomerate, 

tuff, and lignite as much as 2,000 feet (600 m) thick, 

that contains fossil plants of Oligocene age. The 

Weaverville lies unconformably on the pre-Tertiary 

basement rocks of the Klamath Mountains and was 

probably deposited in local, partly down-faulted 

depressions. 

In eastern California, in the Grapevine Mountains 

east of Death Valley, the Paleozoic rocks are overlain 

by the Titus Canyon Formation (Toe), about 3,000 

feet (900 m) thick, composed of sandstone, in part 

tuffaceous, conglomerate, mudstone, and algal 

limestone, that contains lower Oligocene (Chadronian) mammals (Stock and Bode, 1935). Farther 

south in the Death Valley region the Oligocene is 

volcanic (Artists Drive Formation, lTv). 

In the Orocopia Mountains of southeastern California, east of the Salton Sea and the San Andreas 

fault, marine Eocene rocks (Maniobra Formation, 

Te) are overlain by an unnamed unit of nonmarine 

conglomerate, sandstone, and mudstone (Toe) of 

probable Oligocene and possible lower Miocene age. 

MIOCENE SERIES 

The Miocene Series, whose rocks formed during 

about 14 million years of mid-Tertiary time, corresponds to the Aquitanian, Burdigalian, Helvetian, 

Tortonian, Samartian, and Pentian Stages of the 

European marine sequence. In North America, as in 

the Eocene and Oligocene, the marine sequence in 

California, and the continental sequence in the 

interior region, are divided into stages of local 

application. Next to the Eocene and Paleocene, the 

Miocene is the most extensively exposed Tertiary 

series in the United States (fig. 3). Marine deposits 

(Tm) occur in the Atlantic and Gulf Coastal Plains 

and the Pacific coastal areas of California and 

farther north. Continental deposits (Tmc) are extensive in the northern Great Plain1::~ and Rocky 

Mountains and in smaller areas in the interior region 

farther west. Miocene volcanic rocks (Tmv, Tmf) are 

widely distributed in the western interilor, where they 

interfinger in many places with the continental 

deposits; they are treated under a later heading. 

MARINE STRATIFIED ROCKS 

GULF COASTAL PLAIN 7 

The Miocene formations are exposed in a narrow to 

wide band of outcrop in the shoreward part of the 

'Compiled from many sources; few modem summaries are available, but earlier 

summaries by Plummer (1932, p. 710-749) and Cooke (1945, p. 190-195) are still useful. 

Coastal Plain that extends continuously from the 

Rio Grande in Texas to South Carolina, except for a 

20-mile (32-km) alluvium-covered gap along the 

Mississippi River. Miocene rocks are also extensive 

throughout the Florida peninsula. On the outcrop the 

Miocene is about 2,000 feet (600 m) thick, and most of 

it from Alabama westward is brackish-water or 

continental deposits. It changes to marine in 

subsurface downdip where it contains a sequence of 

widely traceable foraminiferal zones. It thickens 

dramatically, especially across growth faults downthrown toward the coast, so that near the Texas and 

Louisiana coast it attains 48,000 feet (14,500 m) 

(Crouch, 1959, p. 1285-1286). 

Terminology of the Miocene deposits has been 

much confused by a host of stratigraphic names that 

have been used in different senses by successive 

authors (Murray, 1961, p. 407-412). Actually, in 

surface outcrops at least, the gross subdivisions are 

fairly clear. 

At the base in the western half of the Coastal Plain 

is the Catahoula Sandstone, named for Catahoula 

Parish, Louisiana, and recognized from Alabama to 

Texas. In Texas it is generally 200-300 feet (60-90 m) 

thick and is formed of clay and sandstone. The clay 

is bentonitic and noncalcareous, and the sandstone 

fine- to medium-grained, cross-bedded and tuffaceous. In south Texas the formation thickens to 

1,000 feet (300m) and is more coarsely volcanic; this 

part was called the Gueydan Formation by Bailey 

(1926) and is now distinguished as the Catahoula 

Tuff. Its volcanic clasts include boulders, pebbles, 

and chunks of lava, porphyry, and pumice. Many of 

these can be specifically matched with contemporaneous volcanic formations in the trans-Pecos 

mountains and upstream along the Rio Grande, but 

some show so little water-rounding that they may 

have come from now-concealed volcanic centers in 

the Coastal Plain itself. The Catahoula in Texas 

contains a few brackish-water mollusks and more 

abundant plants, including palms that must have 

grown in tropical coastal swamps. 

The Catahoula Sandstone contains workable 

deposits of fuller's earth in east Texas (King, 1940, 

p. 179-182) and important deposits of uranium in 

south Texas (Eargle and others, 1975, p. 773-777). 

In south Texas the Catahoula is underlain by the 

Frio Clay 150 feet (45 m) thick, of lacustrine origin. 

Its age is uncertain and it might be Oligocene, 

although it is included with the Miocene on the 

Geologic Map. Northward, the Catahoula oversteps 

the Frio and lies unconformably on the Eocene. 

In Texas, from the Brazos River southwestward, 

the Catahoula is followed unconformably by the 
22 

\ 

' 

THE CENOZOIC ROCKS 

·----~--.r-r-------------------. .---- ~ a.~. : I 

\ I : 

·-......---.... .. -......-------~( 

,.,. ·. . 

. .._., ' <~ I ; ' : 

( ) ~' y ; ', 

(-~f/t !.____ ~------- ----------- --~ ' - ". --------- l l ---- ~-· 'j ------~ : 

I 

I 

)' 

(., 

.. - , : ------ I I : ~ 1-

~ 

I 

I 

\ 

I -------: 

,' -~-- ---r--------! --- ~ --------,_----~\ 

\ 

\. 

\ 

I L0.1 __ -d.. ~- \ ,,, . ..,.~-- ---- ..... ~ : -!~ .. l 

I \' 

I 

, ____________________ _ 

I 

.,.. /---- >\ I --------- I ! \. ~-\.: ----,-------- _______________ _1 ____________________ _ 

\ : ,: 1 _________ _ 

\} : i : 

' I 

,,"""' 

J L • 'Ill' 

... ----,...·· 

' ... ....,..., 

-..... --· •, '""-·:.r·-.... _,-

·-....... 

'- I 

-..... ---- .. ! r------c-----------.J ,..__~·----· . \ 

---· '" 

\ 

\ 

' '-, 

FIGURE 3.-Map of the United States showing surface distribution of Miocene stratified rocks as represented on the Geologic 

Map of the United States (Tm, Tmc, Tmoe). 
MIOCENE SERIES 

-~ ·-....... _r--t.. •• ""'-·-··--. 

J'\ . ./''~ ... , 

: \ I ._ : \ ( ·., 

/ 

,f 

f"\ ~- .............. ~ '. 

---r ) ' ._ I \ 

I : . \ I , "'\ . ' . , 

\_J------

1 'r __ ...... - -"\" \ 

\ \. -, 

·---------------\ ------""'\ _..,..... '-,.. ......... 

\ ~ \:,---- ) ---r--

/. I \\ --

' 

I \ \ \ • I :'· \.-------:- \ / . .P~ ,....._~'!__ 

(.-------------.;' \ ,.,: ,, ;' 

' ( \ : i' __ \·-.~ t \ . ..-v r·· . ·,, C \ .. ..,.._.\.. I I ', ,.- ·.- I '-: .r-: I / \ J 

I ( ; :"-, _'-/ .. --·· /-. t;.r·f'·· - - ' t' } ;' __ .. _... ... ---·- . -' ,.·· ---)----

1 ~~~-S -------------------- ,,/'! 

----· ~~ --' ___ .1 '---------------?__r-· - \ ~ ,/ / ___ .~.----( ,.(------------------\\ --, __ 

.,_ 

) ~ 

/ \ _.......... I ~ .\ _..,_., \ 

:_ .. -------L, 

I 

I 

. ~· \ .. , 

,.~. 

GULF OF MEXICO 

FIGURE 3.-Continued. 

23 

Oakville Sandstone 200-500 feet (60-150 m) thick, a 

coarser deposit formed during a new cycle of 

sedimentation. Drainage during its deposition was 

now from the northwest, from Cretaceous outcrops 

near and beyond the Balcones Escarpment, and the 

gravels contain notable quantities of reworked 

Cretaceous marine shells (as much as 5 percent of the 

volume). The Oakville Sandstone also contains 
24 THE CENOZOIC ROCKS 

abundant bones of horses, camels, mastodons, and 

rhinoceros that are of Arikareean and Hemingfordian (middle Miocene) age. No marine fossils are 

known on the outcrop, but occur in subsurface a short 

distance downdip. 

The Oakville is followed conformably by the 

Fleming Formation (= Lagarto Clay) and merges 

eastward with its lower part. The Fleming is 1,200 

feet (360m) thick and is similar to the Oakville but 

finer grained. It contains brackish-water mollusks 

and mammalian bones of Barstovian (latest Miocene) age. 

The nonmarine facies of the Miocene continues in 

surface outcrops eastward across Louisiana and 

Mississippi into Alabama. The lower sandy beds are 

everywhere termed Catahoula, but the upper beds 

are divided into the Hattiesburg and Pascagoula 

Clays, each about 400 feet (120 m) thick. The 

Pascagoula contains a few layers with marine 

fossils. 

In the eastern part of the Gulf Coastal Plain, the 

exposed Miocene passes into a marine, calcareous, 

highly fossiliferous facies of about the same thickness as the nonmarine deposits farther west. At the 

base in Florida and Georgia is the Tampa Limestone, 

more variable and less pure than the Oligocene and 

Eocene limestones beneath it; it is approximately 

equivalent to the Catahoula Sandstone farther west. 

It is followed by the Alum Bluff Group, comprising 

the Chipola and Shoal River Formations and the 

local Oak Grove Sand. The Chipola includes coarse 

siliceous sand and limestone, and the Shoal River is 

chiefly sand and clay. The Alum Bluff passes 

northeastward into the Hawthorne Formation, which 

is extensive in Georgia and extends as far as South 

Carolina. The Hawthorne consists of gray phosphatic sand with lenses of fuller's earth that are 

mined commercially in many places. Besides its 

invertebrate shells, it contains mammalian bones of 

Hemingfordian age. 

ATLANTIC COASTAL PLAINS 

North of a gap inN orth Carolina where Cretaceous 

formations extend nearly to the coast on the Cape 

Fear arch, Miocene deposits are extensive in the 

Atlantic Coastal Plain as far as New Jersey. The 

Miocene units are, in fact, the most extensive of the 

Tertiary deposits of this part of the Coastal Plain, 

and in places overstep the older Tertiary and Cretaceous so that they extend onto the pre-Mesozoic 

crystalline rocks of the Piedmont province. 

The Miocene consists of a number of thin shaly 

8For summary, see Spangler and Peter10n (1950, p. 61, 75-79). 

and sandy formations, all marine and very fossiliferous, all separated by unconformities. In Maryland, 

Delaware, and northern Virginia, it consists of 

the middle Miocene Calvert, Choptank, and St. 

Mary's Formation, overlai)l toward the south by the 

Yorktown Formation, traditionally classed as upper 

Miocene, which is the most extensive unit in 

southern Virginia and North Carolina. In New 

Jersey, the middle Miocene is represented by the 

Kirkwood and Cohansey Formations, and no equivalents of the Yorktown are present. 

The Calvert Formation is prominently exposed in 

the Calvert Bluffs on the west shore of Chesapeake 

Bay in Maryland, where it consists of diatomaceous 

clay with a shell bed in the middle that is a mass of 

pelecypods and gastropods; the shells weather free 

and strew the modern beaches of the bay. The 

overlying Choptank is more sandy but is also very 

fossiliferous. The St. Mary's is clay, sandy clay, and 

sand. 

The Yorktown at the top is typically exposed in 

the bluffs of the York River at Yorktown, Virginia, 

and consists of coquina, marly sand and clay, and 

glauconitic sand. Recent paleontological work indicates that it and the Duplin Marl (its extension into 

South Carolina) are lower Pliocene in age, rather 

than upper Miocene in terms of European standard 

sections (J. E. Hazel, written commun. April, 1975). 

Moreover, glauconite in the Yorktown has yielded a 

Pliocene K/ Ar age of 4.4 m.y. The Yorktown is the 

only upper Tertiary unit exposed south of the James 

River in Virginia and beyond through North Carolina; hence, all the Miocene shown on the Geologic 

Map in this segment of the Atlantic Coastal Plain 

should be transferred to the Pliocene. 

PACIFIC COASTAL AREA 

CALIFORNIA 9 

The Miocene of the coastal area in California has 

the most complex display of rocks and formations, 

and the thickest sequences in surface exposures 

(9,000 feet, 2, 700 m, or more) of any Miocene rocks in 

the United States. It occurs in numerous small 

outcrops in the Coast Ranges south of San Francisco 

bay, eastward across the San Joaquin Valley, 

south ward across the western part of the Transverse Ranges, around the Los Angeles Basin, and 

along the western flank of the Peninsular Ranges 

nearly to the Mexican border. Narrow fault-bounded 

slices of what were originally extensive Miocene 

formations extend eastward along the north side of 

9Earlier summaries are given by Reed (1"933, p. 163-226) and Reed and Hollister 

(1936, p. 32-46); more recent data appears in publications by Kleinpell (1938) and 

Bramlette (1946). 
MIOCENE SERIES 25 

the Transverse Ranges. Continental deposits of 

Miocene age are widespread in southeastern California, especially in the Mojave Desert region. 

A large body of knowledge regarding the California Miocene has accumulated, partly as a result 

of academic research, partly from petroleum exploration, and can only be summarized briefly here. 

The California Miocene has long been divided on 

the basis of marine megafossils into the Vaqueros, 

Temblor, Briones, Cierbo, and Neroly Stages, but 

these are rather vague and not entirely satisfactory, 

as they are · based on rock-stratigraphic units 

mainly in the Central Coast Ranges. Later, it was 

divided on the basis of foraminifers into the 

Zemorrian, Saucesian, Relizian, Luisian, Mohnian, 

and DelmontianStages (Kleinpell, 1938, p.103-157). 

These stages are believed to be more nearly timestratigraphic units, which cut through the complex 

array of formations and rock facies, and they have 

come in to wide use. 

North of the Transverse Ranges, the San Andreas 

fault zone extends through the Miocene formations. 

This segment of the fault has undergone more than 

300 miles (480 km) of right-lateral displacement 

during and since Miocene time, which has resulted 

in marked offsets of Miocene facies across it, such 

as the boundary between marine and continental 

deposits, and the boundary between warm-water 

and cold-water marine faunas (Addicott, 1968). 

Clearly, the geography during Miocene time was 

quite different from that at present, which greatly 

complicates paleogeographic reconstructions. 

Lower Miocene deposits (Zemorrian and Saucesian) include much fairly coarse clastic material, 

and in various places have been termed the 

Vaqueros and Temblor Sandstones. The most 

characteristic rocks of the younger Miocene are 

diatomite and associated siliceous sediments, whose 

white, thin-bedded ledges are a characteristic feature of the landscape of the Coast Ranges and other 

parts of California (Bramlette, 1946). They are 

known collectively as the Monterey Formation, but 

they are a facies that varies greatly in scope and age 

from place to place-in places only middle Miocene, 

in others both middle and upper Miocene and even 

basal Pliocene. The siliceous rocks include porcelanite or siliceous mudstone, laminated siliceous 

shale, and thin-bedded chert and cherty shale. 

Diatomite, or diatom-rich siliceous rock, occurs 

mainly in the upper parts of individual sequences; 

however, the lower siliceous rocks were also probably originally diatomaceous and have been altered 

by diagenesis. The siliceous rocks intertongue 

marginally with mudstone, siltstone, and sandstone, which in some areas replace them entirely. 

The upper Miocene in the Salinas Valley of the 

· central Coast Ranges is again a coarse clastic 

deposit, the Santa Margarita Formation. 

Volcanic rocks (Tmv), mainly andesite and basalt, 

are a locally significant component of the California 

Miocene. They occur, for example, at The Pinnacles 

in the Gabilan Range of central California, and in 

the western Santa Monica Mountains farther south, 

where they are more than 5,000 feet (1,500 m) thick. 

Tuffaceous deposits and bentonite are widely distributed in the middle and upper Miocene. There is a 

possible, although unproved relation between the 

volcanic episode and the widespread distribution of 

siliceous sediments. 

The marine Miocene deposits grade eastward and 

southeastward into continental deposits. The latter 

occur at the western foot of the Sierra Nevada along 

the eastern side of the San Joaquin Valley (Mehrten 

and Zilch Formations), which to the west toward the 

San Andreas fault in the Bakersfield area change to 

marine deposits. West of the San Andreas fault in 

the latitude of Bakersfield, the continental deposits 

are offset northward, so that the transition into 

marine deposits occurs near Caliente Mountain. To 

the south in the Transverse Ranges are the Tick 

Canyon and Tecuya Formations with Arikareean 

and Hemingfordian (lower Miocene) mammals and 

the Mint Canyon Formation with Barstovian (upper 

Miocene) mammals. There are unsolved discrepancies in age assignments between these fossiliferous 

continental deposits and the fossiliferous marine 

deposits with which they intergrade (Durham and 

others, 1954). 

Structural unconformities occur at many places 

below, above, and within the California Miocene, 

but individual unconformities are of local extent 

and are seemingly represented elsewhere by conformable sequences. There were thus no universal 

times of orogeny and def~rmation during the epoch. 

Nevertheless, there seems to have been a general 

reorganization of depositional regimes in middle 

Miocene time as a result of tectonic movements; 

middle and upper Miocene deposits appear to have 

been more extensive and varied than those of the 

lower Miocene. 

OREGON AND WASHINGTON 

In the north western coastal area of Oregon and 

Washington, Miocene marine deposits are much 

less extensive than in California; most of the 

Tertiary rocks of the Coast Ranges are Eocene and 

Oligocene. The Miocene is mostly confined to 

narrow strips along the coast, although it extends 
26 THE CENOZOIC ROCKS 

somewhat farther inland in the Grays Harbor area 

of southwestern Washington. 

In the Newport embayment of western Oregon, 

the lower Miocene is represented by 3,500 feet 

(1,000 m) of the organic-rich Nye Mudstone, but the 

most extensive formation along the coast is the 

middle Miocene Astoria Formation, which consists 

of about 1,000 feet (300 m) of sandstone and 

siltstone. 

The Astoria is interbedded with basalt flows 

erupted from local centers and is succeeded along 

the downwarp near the Columbia River by basalt of 

the Columbia River Group (Tmv) that flowed from 

the main eruptive area of flood basalts east of the 

Cascade Range. Near the coast, the normal terrestrial basalts of the Columbia River pass into 

accumulations of pillow lava and breccia. 

CONTINENTAL DEPOSITS 

NORTHERN GREAT PLAINS AND ROCKY MOUNTAINS 

In the north-central Great Plains, in the segment 

centering in western Nebraska and southeastern 

Wyoming, Miocene continental deposits are part of 

the great sheet of outwash that was spread on the 

slope east of the Rocky Mountains from Oligocene 

through Pliocene time. Much of the Miocene has 

traditionally been assigned to the Arikaree Group, 

but an overlying Hemingford Group has been 

recognized later (Lugn, 1939, p. 1251-1258). These 

groups are the types of the Arikareean and 

Hemingfordian Stages of the continental Tertiary, 

although the Hemingford apparently contains beds 

of the Barstovian Stage at the top. 

The Miocene deposits are 500-1,000 feet (150-

200 m) thick in western Nebraska, but wedge out 

eastward and southward between the underlying 

and overlying Tertiary. Channeling is characteristic, and each successive formation lies on a 

channeled surface of the one beneath. For example, 

the Gering Formation at the base of the Arikaree 

lies on a deeply channeled surface cut on the 

Oligocene Brule Clay, and the Marsland Formation 

at the base of the Hemingford lies with marked 

erosional unconformity on the Harrison Formation 

of the Arikaree. Some of the channels, such as those 

of the Gering Formation, contain coarse sand and 

gravel, but for the most part the Miocene sediments 

are fine-grained sands and silts, indicating that 

during the epoch the Rocky Mountains were not 

actively shedding material. The upper part of the 

Sheep Creek Formation at the top of the Hemingford is less channeled than the rest and consists of 

fine-grained flood-plain material and aeolian silt. 

Westward in the Rocky Mountains of Wyoming 

and Colorado, the Miocene deposits are preserved in 

separate basins, in some of which they attain 

thicknesses much greater than those in the Great 

Plains. The basins approximate the original depositional sites, but during the later Miocene, to judge 

from the occurrence of scattered outliers, sediments 

seem to have been spread widely between the basins 

and across some of the intervening ranges. 

One of the most extensive Miocene deposits is the 

Split Rock Formation, typically developed in the 

Sweetwater area in the center of Wyoming, where it 

surrounds and partly buries the Precambrian rocks 

of the Granite Mountains (Wg) (Love, 1961, p. 5-25). 

It is about 2, 700 feet (830 m) thick, and vertebrate 

evidence indicates that it includes most of the 

Arikareean and Hemingford Stages. It consists 

mainly of fine-grained silty sandstone with some 

units of coarser porous sandstone. Much of the 

sediment was derived from the northwest, and some 

of it is of aeolian origin; the Rocky Mountains were 

not actively shedding detritus during Split Rock 

time. 

In the vicinity of the eastern Uinta Mountains, 

130 miles (210 km) southwest of the Sweetwater 

area, is the Browns Park Formation. It is typically 

developed in a graben that splits the crest of the 

Uinta Mountains at their eastern end, but it extends 

farther east along the valley of the Yampa River; 

remnants also occur along the south flank of the 

Uinta Mountains and along the west flank of the 

Park Range, indicating that the deposit was 

originally much more extensive. The Browns Park 

is about 1,800 feet (550 m) thick, and most of it is 

white or gray, fine-grained, tuffaceous sandstone of 

aeolian and fluviatile origin, but at the base is a 

coarse conglomerate, composed of quartzite of the 

Uinta Mountain Group (Y). Mammalian remains 

indicate that the formation is mainly upper Miocene 

(Hemingfordian and Barstovian); radiometric ages 

range from 26 m.y. to 9 m.y. (Izett, 1975, p. 186-189). 

On the north flank of the Uinta Mountains is the 

Bishop Conglomerate, which caps a series of high 

terrace remnants. Its conglomerates resemble those 

at the base of the Browns Park Formation, with 

which it is not in contact, which has led to the 

suggestion that it is of the same age, but present 

indications are that it is earlier Miocene. To the 

east, between the Park and Front Ranges, are 

smaller areas of Miocene continental deposits, the 

North Park and Troublesome Formations, largely 

uppermost Miocene. 

In the high plains of northwestern Montana, close 

to the Canadian border and far from the formations 

just discussed, the Flaxville Gravel forms the cap of 
MIOCENE SERIES 27 

remnant plateaus on top of the Paleocene and 

high Upper Cretaceous rocks. It is a few hundred 

feet thick and consists of gravel derived from the 

Rocky Mountains far to the west intermingled with 

some sand and clay. Rather abundant vertebrates 

suggest a late Miocene or early Pliocene age; it is 

shown on the Geologic Map as Miocene (Tmc). 

A different set of deposits occurs in the basins 

within the mountains of southwestern Montana. 

They were originally termed the "Bozeman lake 

beds," but most of them are fluviatile rather than 

lacustrine in origin, and modern work indicates that 

they are heterogeneous, including deposits of many 

Tertiary ages, now collectively referred to as the 

Bozeman Group (Robinson, 1961, p. 1003-1004). 

Included in the Bozeman Group in some of the 

basins are Eocene and Oligocene strata that are 

unconformable beneath the younger Tertiary. The 

younger Tertiary forms the greater part of the 

surface areas of the basins, and its vertebrates 

indicate late Miocene and Pliocene ages; it is shown 

on the Geologic Map as Miocene (Tmc). The deposits 

include coarse conglomerate and fanglomerate 

along the edges of the ranges, and finer-grained 

sediments farther out. They were laid down in 

intermontane depressions produced by downwarping and downfaulting during various stages of 

Tertiary time. The Miocene and Pliocene deposits 

much resemble those that were laid down in 

intermontane basins farther south in the western 

interior that are shown on the Geologic Map as 

Pliocene and Quaternary (Tpc, Q), but the average 

age of the deposits in Montana seems to be 

somewhat greater. 

OUTLYING AREAS 

NEW MEXICO AND ARIZONA 

Continental deposits classed as Miocene are 

shown in many small areas in western New Mexico 

and southern Arizona. A few have been dated by 

fossils or by radiometric means, but most are 

inferred to be middle Tertiary from th~ir relation to 

underlying and overlying Tertiary sediments and 

volcanics. Most of the deposits are within or on the 

borders of mountain areas, hence are older than the 

Basin and Range faulting or contemporaneous with 

its early phases. 

In northern New Mexico, west of the Rio Grande 

Valley, is the Carson Conglomerate, a coarse 

bouldery deposit composed of clasts of volcanic and 

Precambrian rocks. It surrounds and half buries the 

Precambrian rocks of the Petaca area (Xm), and 

overlaps Mesozoic rocks to the west. It is overlapped 

in tum on the east by the Pliocene rocks of the Rio 

Grande Valley. Aside from being obviously middle 

Tertiary, its exact age is undetermined. 

Farther south, at the south end of the Caballo 

Mountains, are the Palm Park and Thurman 

Formations (Kelley and Silver, 1952, p. 120-123). 

The first is a red bouldery deposit with many 

volcanic clasts, the second a light buff tuffaceous 

clay. They overlap the Paleozoic rocks of the 

Caballo Mountains and are overlain by the Pliocene 

Santa Fe Formation of the Rio Grande Valley. The 

formations seem to be younger than the Paleocene 

McRae Formation (Tx) at the north end of the 

mountains, although not in contact with it, and are 

probably of middle Tertiary age. 

West of the Rio Grande in southwestern New 

Mexico is the extensive Mogollon-Datil volcanic 

province, which continues into southeastern Arizona. The volcanic rocks have been called collectively the Datil Formation, but recent work indicates that they include at least three major subdivisions of Oligocene and Miocene ages (Elston 

and others, 1973, p. 2261-2263). Most of the Datil 

rocks are lavas, but the New Mexico Geologic Map 

(1965) represents the northwestern part as clastic 

rocks of volcanic fragments (Tds) and boulder beds 

and coarse clastic rocks (Tdc). Their westward 

extension is shown on the Arizona Geologic Map 

(1969) as undifferentiated Tertiary sediments (Ts). 

They are shown on the Geologic Map of the United 

States as Miocene continental deposits (Tmc), but 

they may well be Oligocene. 

Farther southwest, in the Basin and Range 

province of southern Arizona, widely separated 

small areas in the ranges are shown on the Arizona 

Geologic Map (1969) as Miocene and Miocene(?) 

sedimentary rocks (Tms). They are shown on the 

Geologic Map of the United States as Miocene 

continental deposits (Tmc). They consist of conglomerate, sandstone, siltstone, limestone, tuff, and 

minor lavas. Some are known to be of Miocene age 

from fossils or radiometric dating. 

WASHINGTON AND OREGON 

In the Spokane area of northeastern Washington 

is the Latah Formation, a lacustrine deposit that 

accumulated in valleys dammed during the eruption 

of the basalts of the Columbia River Group. The 

Latah consists of clay and sand, in part tuffaceous, 

and beds of diatomite. Rather abundant fossil 

plants indicate middle and upper Miocene ages. 

Although the formation is extensive in small areas 

along the northeast edge of the basalt plateau, only 

a few of the larger areas can be indica ted on the 

Geologic Map where they are shown as Miocene 
28 THE CENOZOIC ROCKS 

continental deposits (Tmc). 

In the volcanic area of southeastern Oregon, 

Miocene lavas cover wide areas, and sediments are 

interbedded with them in many places, some 

forming areas large enough to be represented on the 

Geologic Map as Miocene continental deposits 

(Tmc). They include the Mascall Formation of the 

western part of the Blue Mountains uplift, which 

contains Barstovian vertebrates and plants, and 

the Payette Formation of eastern Oregon and 

southwestern Idaho, which contains Miocene plants. 

CALIFORNIA 

Some of the Miocene continental deposits that 

interfinger with nearby marine deposits in the 

Coast Ranges and Transverse Ranges have already 

been noted (p. 25). Miocene continental deposits also 

extend much farther east in southeastern California, 

into the Mojave Desert and thence eastward to the 

Colorado River. As in adjoining southern Arizona, 

the deposits are preserved in small patches in the 

ranges where they overlie pre-Tertiary sedimentary 

and plutonic rocks, and they are intermingled with 

volcanic rocks of Miocene age (Tmv). Many small 

areas are shown on the atlas sheets of the Geologic 

Map of California, where they are indicated as Me. 

Not all of them can be represented on the Geologic 

Map of the United States, and some of them are 

merged with the associated volcanic rocks. 

The Miocene continental deposits of the Mojave 

Desert have long been referred to as the "Rosamund 

Series," but this term has been loosely applied to 

beds with a wide range of ages, and the Miocene 

part has now been renamed the Tropico Group 

(Dibblee, 1958, p. 136). In the western part of the 

desert, it is about 2,800 feet (850 m) thick and is 

divided into an assortment of sedimentary and 

volcanic formations, none of which are of more than 

local extent. The Tropico (= Rosamond) is not very 

fossiliferous in its type area, but the probably 

equivalent Barstow Formation farther east contains abundant mammalian remains of upper 

Miocene age, which are the basis of the Barstovian 

Stage of the continental Tertiary. The Ricardo 

Formation, northwest of Barstow across the Garlock 

fault, has sometimes been compared with the 

Barstow Formation, but its mammalian fauna is 

Clarendonian, or early Pliocene. 

Along the northern border of the San Gabriel 

Mountains, at the south edge of the Mojave Desert, 

are narrow fault wedges of formerly more extensive 

middle Tertiary continental deposits; pink, massively bedded coarse sandstone and interbedded siltstone. The Punchbowl Formation lies south of the 

San Andreas fault, and the Cajon Formation 30 

miles (90 km) to the east lies north of it. The two 

formations may be parts of the same deposit, now 

separated by right-lateral displacement on the fault, 

but it has been objected that the Punchbowl 

contains Clarendonian vertebrates and the Cajon 

Barstovian vertebrates. This argument loses force 

when it is realized that both units are sparsely 

fossiliferous and very thick-the Punchbowl 6,000 

feet (1,800 m) and the Cajon 8,000 feet (2,400 m). The 

discrepancy in ages of the known faunas may well 

be due to the vagaries of collecting. 

PLIOCENE SERIES 

The Pliocene Series is commonly considered to 

have formed during the final10 million years of the 

Tertiary Period (2-10 m.y. ago) and to correspond to 

the Plaisancian and Astian Stages of the European 

marine sequence and to the Clarendonian, Hemphillian, and Blancan Stages of the American continental sequence. However, a considerable body of 

opinion now favors reducing the length of the 

Pliocene to 3 million years (2-5 m.y. ago) (Berggren 

and Van Couvering, 1974) and placing the Clarendonian and part of the Hemphillian continental 

stages in the upper Miocene. Also, the marine stages 

that have been proposed in Europe for the upper 

Miocene and lower Pliocene are suspect as time 

units; some may be lateral facies of others. There is 

difference of opinion, besides, as to the PliocenePleistocene boundary. The Pleistocene is commonly 

assumed to begin with the first glaciation (Nebraskan in North America), but in terms of the 

marine sequences farther south in Europe, the base 

is defined as the boundary between the Astian and 

Calabrian (=Villafranchian) Stages, which appears 

to be older than the first glaciation, lending support 

for the concept of a "preglacial Pleistocene" epoch. 

Pliocene marine deposits (Tp) are very scantily 

developed within the United States (fig. 4), due 

probably to a general emergence of the continent; on 

the Geologic Map they are shown in the Gulf 

Coastal Plain and Atlantic Coastal Plain and in the 

Pacific coastal belt of California. Pliocene continental deposits (Tpc) are much more extensive, 

especially in the Great Plains and the Cordilleran 

region of the western United States. In the Cordilleran region there are also many occurrences of 

Pliocene volcanic rocks (Tpv, Tpf) which are discussed under a later heading. 

MARINE STRATIFIED ROCKS 

ATLANTIC AND GULF COASTAL PLAINS 

The only truly marine Pliocene rocks shown on 
PLIOCENE SERIES 29 

the Geologic Map are in the southeastern part of the 

Atlantic Coastal Plain (although, as noted above, 

the Yorktown Formation and Duplin Marl farther 

north may also be early Pliocene). Farther west, in 

the Gulf Coastal Plain, the Pliocene deposits are 

brackish water or continental, but they are discussed here for convenience. 

In Florida the principal Pliocene formation is the 

Caloosahatchee Marl (Tp) exposed near Lake Okeechobee in the southern part of the peninsula. It 

consists of sand and beds of molluscan shells, many 

of which are indistinguishable from modern species 

(Cooke, 1945, p. 214-227). As the terrain in this part 

of Florida is low and flat, only a few feet of the 

formation is exposed at any place, but the total 

thickness is probably not great. The Caloosahatchee grades laterally into the Buckingham Marl 

and the Tamiami Formation which, although 

separately named, are merely facies of the same 

deposit. 

Farther north in Florida, remnants of Pliocene 

deposits are scattered over the older Tertiary rocks, 

but are not shown on the Geologic Map. They 

include the Alachua and Bone Valley Formations, 

which are estuarine or terrestrial deposits, sometimes filling sinkholes, with a fauna mainly of 

vertebrates. They are notable for their phosphate, 

concentrated by weathering from the underlying 

formations. 

Near the coast in eastern South Carolina is the 

Waccamaw Formation (Tp), which is approximately 

equivalent to the Caloosahatchee, and like it is a 

thin fossiliferous layer of sand and shells. 

Far to the west, in south Texas, is the Goliad 

Formation (Tp), which forms a band of outcrop on 

the downdip side of the Miocene Formations 

extending from the Rio Grande as far north as the 

Colorado River (Plummer, 1932, p. 750-760). Although shown as marine on the Geologic Map, it is 

primarily a brackish water and fluviatile deposit. 

The Goliad consists of 100-200 feet (30-60 m) of 

clays and fine to coarse sands, with much gravel in 

the lower part, cemented in places by caliche. The 

gravels are mainly chert and quartz pebbles but 

include bone fragments from the underlying Miocene. It contains a few indigenous mollusks and 

vertebrate bones, the latter being of Pliocene age 

and comparable to those of the Ogallala Formation 

of northwestern Texas. Northeast of the Colorado 

River the Goliad is overlapped by the younger Willis 

Sand (= Citronelle). 

The Citronelle Formation (Tpc) and its equivalents are the most extensive Pliocene deposits in the 

Coastal Plain and are shown on the Geologic Map 

in discontinuous outcrops from central Texas to 

Georgia, at the outer edge of the Tertiary deposits, 

and next to the Quaternary deposits along the coast. 

The Citronelle was named for a locality in 

southwestern Alabama (Matson, 1916) to replace 

the discredited term "Lafayette," which had been 

used indiscriminately for a wide variety of terrestrial deposits fringing the coastal part of the 

Coastal Plain Tertiary. In Texas equivalent deposits 

are termed the Willis Sand. In Louisiana terminology has been confused by the Pleistocene terrace 

sequence proposed by Fisk (1938, ff), who believed 

that his oldest and highest terrace surface and 

deposits, the Williana, were equivalent to the 

Citronelle, but if the Williana exists in the form 

envisioned by Fisk, it is probably younger (Doering, 

1956, .p. 1832-1837). 

The Citronelle forms a thin sheet on the uplands 

along the coastward part of the Tertiary outcrop 

and extends long distances inland as erosion 

remnants along the interfluves, truncating the 

earlier (mainly Miocene) deposits. Its basal surface 

and its deposits slope coastward at a lower angle 

than the dip of the underlying strata, but steeper 

than the nearly undisturbed Pleistocene deposits 

that border it toward the coast. The Citronelle was 

laid down after the older Tertiary had been tilted 

and peneplaned and was tilted in turn prior to the 

Pleistocene; these tiltings express a regional epeirogenic upwarp of the continental interior late in 

Tertiary time. 

The regional extent of the Citronelle has been 

reviewed by Doering (1956, 1960), who extends its 

equivalents northeastward in the the Atlantic 

Coastal Plain beyond the last outcrops shown on 

the Geologic Map, in Georgia, across South Carolina, North Carolina, into Virginia. In these States, 

as farther south, thin deposits overlie the older 

Tertiary and Cretaceous on the interfluves, but 

since they are not clearly differentiated on our 

source maps, they are omitted from the Geologic 

Map. Across the Potomac, in the Coastal Plain of 

southern Maryland is the Brandywine Formation, 

likewise an upland remnant of fluviatile deposits of 

possible late Pliocene or early Pleistocene age. The 

Brandywine may be equivalent of the Citronelle, 

although the great distance and the lack of fossil 

control renders this correlation speculative at best. 

The Brandywine is not shown on the Geologic Map 

(but see King and Beikman, fig. 15). 

The Citronelle deposits consist of a basal gravelly 

sand overlain by sandy clay and clayey sand, laid 

down by streams that were vigorously eroding the 

continental interior as a result of the late Tertiary 
30 THE CENOZOIC ROCKS 

·------ ------ . ~ ... , I 

\ _, : 

·\--~-;-----( "'I ~. / (., ~ ~~./ ) .,\ .. f :~!0. ·. - - '\.-.~~-:'.;·-----·--

-- ~ IIJ ' ' ' : ' 

---------·--- ~ • • .. / ( ! ' 

-i -----~----- ! . ~.. ! 

I ./'A '' . 

• ' \

,,., i ::~--~-l~-~tij 

I " • : i_ 

~~~: , a. , ------r--·----·---- ~-- , I : I \ f ,. I \ : 

·. • ~ : I \~l I 

\'. ~ . ' 

\ -'~;" I 

~ .> \ 9t -- ------------L _____ _ 

·: .. ') . "' -~ ~. " ·r .. "- \. ' / \", ~ .. ' •' • I '~"""" ~ ~ I 

• • 

i--------------------.----

1 

I \ 

I I 

I \ 

~----------------------; 

i '. 

: l 

I 

' 

L 

'•--" 

,-....... -, "'' (, •· -A;' ----'--;::: ____________ ' ~~~' '·"'-·:r·-'\.._,-

0 

'> 

'Z. 

"-- I "-._.. 1 : r----- -·~-·oao·-: 

\ \ 

'-, I 

l 

' I 

FIGURE 4.-Map of the United States showing surface distribution of-Pliocene stratified rocks as represented "On the Geologic 

Map of the United States (units Tp, Tpc). 

tl 
MIOCENE SERIES 31 

\. 

·---------------\ 

\ ,_\:: _______ _ 

-I 

I 

./ 

\ 

(,..--------------.;-' 

t 

'-, 

' 

I 

I 

I 

( 

' ..r-, J 

' ; A_( 

( ~;.r-·f'·' ., -' 

\~-5 ----- , ----------- 1 ;!.} ___ __, 

.. ---------------c_ -1 ~ _,../ ) ___ _,_ .. ( 

.. ~------1"-------\ --\_ ) 

/ \ 

·: ~I \ 

·----------t, > 

I 1 

I 

\ 

-~-' ) 

G U L F OF MEXICO 

j\ .. /'' ............ , 

,' \ I ._ 

: \ { ·-,, 

,f \ 

,f ,-, __ .... _.....~ \ 

---r ) ' 

', ( \ I : , 

'. I '" "'\ I 

'I ( ~ \------

1 ---.-"\- 'r--

\ ------'"'\ I 

__. ... - ' 

,.,. . . .. 

FiGURE 4.-Continued. 

epeirogenic uplifts. Toward the coast, some of the 

deposits accumulated in lagoons, estuaries, and 

marshes. In the type area in southwestern Alabama, 

the formation is as much as 300 feet (90 m) thick; 

elsewhere it is 100-150 feet (30-45 m) thick, but the 

top is commonly eroded. In the type area, the basal 

deposits contain fossil plants at one locality, and 

abundant vertebrate bones at another, both of late 

Pliocene age (Isphording and Lamb, 1971, p. 776-

777). The upper part of the formation may extend 
32 THE CENOZOIC ROCKS 

into the Pleistocene, and arguments have been 

presented that it is mainly a "pre-glacial Pleistocene" deposit (a stage not recognized by the U.S. 

Geological Survey, hence the assignment of the 

formation to the Pliocene on the Geologic Map). 

PACIFIC COASTAL AREA IN CALIFORNIA 

In the Pacific coastal area, Pliocene rocks occur 

only in California, where they are extensive. As 

shown on the Geologic Map, they are exposed from 

the north to the south ends of the State, but they are 

most abundant in the central and southern Coast 

Ranges and Transverse Ranges, and eastward to 

the border of the San Joaquin Valley. Many of the 

deposits are marine (Tp), but continental deposits 

(Tpc) are more abundant eastward, and upward in 

the sequence. Pliocene continental deposits also 

fringe the western foot of the Sierra Nevada along 

the east edge of the San Joaquin and Sacramento 

Valleys. 

The Pliocene deposits of California are notable for 

their great thickness, especially in rather local 

areas. Surface sections of marine Pliocene strata in 

the Ventura basin exceed 15,000 feet (4,500 m), and 

similar thicknesses may occur in subsurface in 

some of the other basins. Continental deposits 

nearly as thick occur in some of the enclosed basins 

in the Coast Ranges and elsewhere. 

As with the older Tertiary, the Pliocene rocks 

have been deformed, in places steeply. In some of 

the best known areas, especially in the Los Angeles 

and Ventura basins and the south part of the San 

Joaquin Valley, the sequences are conformable 

through the Pliocene into the lower Pleistocene, and 

the two are strongly unconformable with the overlying upper Pleistocene. This has given rise to a 

widely held belief that the major orogeny in the 

coastal belt was mid-Pleistocene ("the greatest since 

the Cretaceous"), but in other areas major unconformities occur within the Pliocene or at its base, so 

that the supposed mid-Pleistocene orogeny was 

merely the terminal phase of a long sequence of 

tectonic instability. During this interval, the ranges 

between the basins were in process of uplift with 

little internal deformation. 

The marine Pliocene of California is divided into 

the Jacalitos, Etchegoin, and San Joaquin megafossil stages, and into the Repettian, Venturian, and 

Wheelerian foraminiferal stages. In addition, the 

Clarendonian, Hemphillian, and Blancan Stages of 

the continental Pliocene sequence are frequently 

referred to. 

Pliocene marine deposits are exposed in many 

areas near the coast of California, but the principal 

occurrences and greatest thicknesses are in embayments where the ocean extended inland from the 

present coast-the large Los Angeles basin in the 

south, and the smaller Ventura and Santa Maria 

basins farther northwest. In addition, a thick 

sequence of marine deposits occurs in the southwest 

part of the San Joaquin Valley farther inland, 

which was formed in a tongue of the sea somehow 

connected with the ocean across the site of the 

southern Coast Ranges (the connection probably 

now offset from its original position by displacement on the San Andreas fault). Another marine 

embayment, represented by small outcrops on the 

west side of the Imperial Valley to the southeast, is 

unrelated to the rest, and it formed in a former 

northward extension of the Gulf of California. 

In the Los Angeles and Ventura basins, the 

Pliocene marine strata are divided into the Repetto 

and Pico Formations. The deposits are mostly 

sandy silts and sandy clays, some diatomaceous, a 

little coarser in the Repetto than the Pico. Sandstone and conglomerate are interbedded in the lower 

part, and interfinger marginally toward the east, 

where they pass eventually into continental deposits. Foraminiferal studies indicate that in the 

Ventura basin, and probably also in the Los 

Angeles basin, the initial Pliocene deposits were 

laid down in water 4,000-5,000 ft (1,200-1,500 m) 

deep, and that the water gradually shoaled during 

the epoch, as a result of filling of the basin with 

sediments (Natland and Kuenen, 1951, p. 80-85). 

The basin was filled by deposits brought in by 

turbidity currents, which show most of the classic 

turbidite structures. The coarse sandstones were 

brought in by submarine slumps off adjacent 

gravelly shores and beaches. Many shallow-water 

fossils and fragments of terrestrial plants were 

swept in by the turbidity currents and were redeposited in deep water. 

In the southwestern part of the San Joaquin 

Valley, in the Coalinga district and Kettleman 

Hills, the marine Pliocene is 5,000-7,000 feet (1,500-

2,100 m) thick and comprises the Jacalitos and 

Etchegoin Formations. It is followed by the brackishwater or fresh-water Tulare Formation that bridges 

the upper Pliocene and lower Pleistocene. As in the 

Los Angeles and Ventura basins, the deposits are 

rather fine-grained clastics-silts below and above 

and sands in the middle. 

In the San Francisco region farther north is a 

correspondingly thick but more complex array of 

Pliocene formations. In the Santa Cruz Mountains 

west of the San Andreas fault is the marine 

Purisima Formation about 6,000 feet (1,800 m) thick; 
PLIOCENE SERIES 33 

it is composed of volcanic sandstone, mudstone, and 

siliceous mudstone, which passes into shallowwater or littoral deposits toward the top. In the San 

Francisco Peninsula east of the San Andreas fault 

is the Merced Formation of similar origin, composition, and thicknesses, somewhat younger than the 

Purisima and including lower Pleistocene in its 

upper part. Across San Francisco Bay to the east, as 

in the Berkeley Hills, all the Pliocene is nonmarine. 

At the base is the Orinda Formation, 6,000 feet 

(1,800 m) or more thick, of fluvial, alluvial, and 

lacustrine sandstone and mudstone, in part conglomeratic. It is followed by the dominantly volcanic Berkeley Group, with several persistent lava 

units separated by clastic units. North of the bay 

the similar Sonoma Volcanics (Tpv) are extensive. 

These Pliocene deposits are succeeded in several 

parts of the central Coast Ranges by coarse continental deposits which bridge the Pliocene-Pleistocene 

boundary. They include the Livermore Gravels, 

5,000 feet (1,500 m) thick in the intermontane 

Livermore Valley east of the Berkeley Hills, and the 

equally thick but more extensive Paso Robles 

Formation farther south at the head of the Salinas 

Valley. 

Most of the northern Coast Ranges north of San 

Francisco Bay are a terrane of Franciscan rocks, 

but Pliocene marine deposits encroach on them in 

places in embayments near the coast. Not far north 

of the bay are extensions of the Merced Formation, 

and the Ohlson Ranch Formation. Much farther 

north, beyond Cape Mendocino, the Wildcat Canyon Group occupies the trough of a syncline along 

the Eel River, where its rather fine-grained clastics 

attain a thickness of 12,000 feet (3,600 m). 

East of the Coast Ranges in the Sacramento 

Valley and northern San Joaquin Valley, the 

Pliocene deposits are all continental. They are 

extensively exposed at the north end, south of 

Redding, the dominant sediments on the west side 

being the Tehama Formation and the dominant 

andesitic tuff on the east side of the Tuscan 

Formation. Farther south on the east side of the 

valley, near the latitude of Sacramento, is the 

similarly andesitic Mehrten Formation, which includes rocks of both Miocene and Pliocene ages. The 

andesitic debris was derived from eruptive centers 

in the southern part of the Cascade Range to the 

northeast and in the Sierra Nevada to the east. 

CONTINENTAL DEPOSITS 

Continental deposits of Pliocene age, which are 

extensive in the western interior of the United 

States, have several different habits: (a) sheet 

deposits of wide extent but small thickness (mainly 

in the Great Plains; (b) basin(= bolson) deposits of 

narrow dimensions but great thickness (in basins 

between the ranges in the Basin and Range 

province); and (c) deposits associated with and 

grading into contemporaneous volcanic rocks (mainly in the northwestern United States). These 

distinctions can serve as a basis for the following 

treatment of the continental deposits. 

SHEET DEPOSITS 

OGALLALA fORMATION 10 

By far the most extensive and continuous Pliocene continental deposit in the United States is that 

of the Ogallala Formation, which covers the Great 

Plains east of the Rocky Mountains (with only 

minor interruptions from later dissection) from 

southern South Dakota to western Texas, a distance 

of 800 miles (1,300 km), with a preserved width of as 

much as 400 miles (650 km). The Ogallala Formation has traditionally been classed as Pliocene and 

is so represented on the Geologic Map, but if the 

recently proposed upper age limit of the Miocene at 

5 m.y. is accepted, all the Ogallala except for the 

Blancan Stage at the top would be uppet Miocene. 

Throughout this expanse, the Ogallala lies at or 

near the surface of the plains, and as shown as 

continuous in outcrop of the Geologic Map. In actual 

fact, much of the flat upper surface of the plains is 

masked by small thicknesses of Quaternary materials, which are shown in considerable detail on 

some of the StP.te Maps-drifted sand and sand 

dunes, loess in the north, and occasional basins 

filled with various Pleistocene formations. The true 

outcrops of the Ogallala are mostly in the erosion 

scarps or "breaks" around the edges of the plains or 

in the walls of valleys and canyons that trench the 

deposit. 

The Ogallala was originally more extensive east 

and west. Westward. it extended up to the foot of the 

Rocky Mountains, from which most of its sediment 

was derived (a narrow remnant of the deposit 

extending up to the mountains, known as "The 

Gangplank," is still preserved in southeastern 

Wyoming). Eastwiird the Ogallala thinned to a 

feather edge across the Midcontinent States. Erosion after the Pliocene has cut back the original 

edges into frayed-out escarpments or "breaks"-

prominent on the east, more subdued on the west. 

In the north, the Ogallala lies nearly conformably 

on similar earlier Tertiary continental deposits 

10'fhe many publications on the Ogallala Formation include Plummer (1932, p. 763-

776; called the "Panhandle Formation"), Lugn' 1939, p. l:z55-1263), Frye and othera 

(1956), Frye and Leonard (1957, p. 10-19; 1959), and lzett (1975, p. 201-202). 
34 THE CENOZOIC ROCKS 

(Miocene and Oligocene, already described), but 

from Kansas southward it lies on the deeply eroded, 

channeled surfaces of Cretaceous, Triassic, and 

Permian strata. The formation is from 200-500 feet 

(60-150 m) thick and consists of poorly consolidated 

silt, sand, and gravel in varying proportions, with 

rare layers of volcanic ash, bentonite, diatomite, 

and fresh-water limestone. Some parts are cemented 

by silica, and others by lime. The most firmly limecemented parts are "mortar beds" or "caprock" 

which stand in ledges, the most prominent of which 

is everywhere at the top of the formation. The lime 

cement of these layers is caliche, a product of soil 

formation in a semiarid climate, and it was formed 

during or shortly after deposition of the sediments, 

as clasts of the rna terial are reworked in the 

overlying Quaternary deposits. 

Although the Ogallala deposits are heterogeneous, 

there is monotony in the heterogeneity, and there 

are no truly valid rock-stratigraphic subdivisionsmerely gross contrasts between the lower and upper 

parts. Nevertheless, in Nebraska subdivisions have 

been recognized, named, and dignified as "formations" (although they are little more than inconstant members)-the basal Valentine, followed by 

the Ash Hollow, Sidney (local), and Kimball. These 

have paleontological basis, however, and their 

equivalents can be recognized as "zones" as far 

south as Texas. In Texas, two well-marked vertebrate faunas have served as the basis for the 

Clarendonian and Hemphillian Stages of the 

American continental Cenozoic sequence. In addition, at the top of the Ogallala at a few places in 

Texas are deposits separated as the Blanco Formation, which is the basis for the Blancan continental 

stage. The age of the Blancan has been variously 

assigned; weight of present opinion is that it is 

uppermost Pliocene (C. A. Repenning, oral communication, 1975), although in the past it has 

sometimes been equated with the Nebraskan or 

even the Kansan Stages of the Pleistocene. 

Age determinations by potassium-argon and 

fission-track methods have been made on volcanic 

ash from various layers in the Ogallala Formation 

at localities from Nebraska to Texas. They range 

from 16 or 17 m.y. for layers near the base to 5 m.y. 

for a layer immediately above the type Hemphillian 

(Izett, 1975, p. 201-202). 

Mammalian bones are locally abundant in the 

Ogallala Formation and have been collected and 

studied for many years. They are useful for indieating regional correlations with other parts of 

North America or abroad, but they are of less use for 

detailed stratigraphic work because of their erratic 

occurrence. Terrestrial gastropods are preserved in 

some of the more limy beds. Far more abundant are 

plant remains-grass seeds and nuts of small 

shrubs, which are present in nearly every exposure 

and every lithology. The plant remains can be used 

for zonation, and the zones can be recognized from 

one end of the formation to the other. 

The origin of the Ogallala deposits was brilliantly 

expounded by Willard Johnson in a classic report of 

the turn of the century (Johnson, 1901, p. 612-656). 

He showed that they were formed on a long slope 

extending eastward from the Rocky Mountains, 

with about the same gradient (10-15 feet per mile; 

1.2-1.4 m per km) and in the same semi-arid climate 

as at present. Streams flowed eastward across the 

plains, where they dropped their loads of sediments 

derived from erosion of the mountains. Johnson 

supposed that the streams spread out in distributaries on a smooth surface, but modern data on the 

deposit (in part from drilling) demonstrates that the 

streams actually flowed in valleys, which they filled 

with coarse material, spreading finer material 

across the interfluves from time to time. 

With the end of the Pliocene, the aggradational 

epoch was terminated by climatic change and 

possibly by tectonic movements, so that the sheet of 

sediments was dissected and partly destroyed by 

erosion during the Pleistocene-a process which 

continues to the present. 

BIDAHOCHI FoRMATION 

On the opposite side of the Rocky Mountains, in 

northeastern Arizona and northwestern New Mexico, is the Bidahochi Formation, another sheet of 

deposits of about the same age as the Ogallala, 

although less extensive (Repenning and Irwin, 

1954). (In early reports this was correlated with the 

Eocene Wasatch Formation and is so represented on 

the Geologic Map of 1932.) The Bidahochi lies on the 

Mesozoic formations of the Colorado Plateau in the 

Hopi country southwest of the Defiance Plateau and 

has a thickness of about 450 feet (140m). It consists 

of a lower mudstone unit of lacustrine origin and an 

upper cross-bedded sandstone of fluviatile origin, 

between which are basalt flows (Tpv) in many 

places. The formation contains the remains of fossil 

beavers of middle Pliocene (Hemphillian) age. 

The fluviatile parts of the Bidahochi Formation 

were laid down by southwestward-flowing streams 

and contain much detritus derived from volcanic 

areas farther northeast, as well as from the Rocky 

Mountains. The original southwestern extent of the 

deposit is poorly known, as much of it has been 

eroded, but a few outliers occur in this direction, 
PLIOCENE SERIES 35 

including some on the rim of the Mogollon Plateau. 

Northeast of the area of Bidahochi deposits, on 

the Arizona-New Mexico border, is the unfossiliferous Ch uska Sandstone, also shown as a Pliocene 

continental deposit (Tpc) on the Geologic Map. The 

Chuska has sometimes been correlated with the 

Bidahochi but is of another character, being mainly 

a coarsely crossbedded dune deposit. Probably it is 

somewhat older, although still middle or upper 

Tertiary (Wright, 1956, p. 427-431). 

BASIN DEPOSITS 

Within the Basin and Range province of the 

southwestern United States the basins, or bolsons, 

were thickly filled during later Cenozoic time with 

erosional waste derived from the intervening ranges. 

Filling of the basins began with the initiation of the 

Basin and Range block-faulting in late Miocene or 

early Pliocene time and has continued with some 

interruptions to the present. 

The earlier basin fill is largely of Pliocene age, as 

shown by fossils, although the lower parts of some 

of the deposits extend into the upper Miocene and 

the upper parts may be lower Pleistocene; they are 

shown as Pliocene continental deposits (Tpc) on the 

Geologic Map. These deposits probably underlie 

most of the basins, but in many areas they are 

masked at the surface by similar Quaternary 

deposits (Q). They have been uncovered mainly in 

areas of through-flowing drainage, such as those of 

the Rio Grande, Gila, Colorado, and Humboldt 

Rivers. On the Geologic Map, the basin areas thus 

appear partly in colors of gray (Q) and partly of 

yellow (Tpc). 

The earlier basin fill is known variously as the 

Santa Fe Formation in New Mexico, the Gila 

Formation in southeastern Arizona, the Muddy 

Creek Formation in southern Nevada, the Humboldt Formation in northeastern Nevada, and the 

Salt Lake Formation in northern Utah. Nevertheless, despite their wide dispersal and diverse terminology, they share many common characters. 

Close to the bordering mountains the deposits are 

coarse fanglomerates, but they become finer-grained 

farther out, and near the centers of the basins they 

are fine-grained sands, silts, and clays that are 

partly of lacustrine origin. The deposits accumulated after the great eruptions of mid-Tertiary time 

and are mainly non-volcanic, although minor volcanic units occur in places, mainly in the lower 

parts. The deposits were laid down after the first 

Basin and Range faulting had blocked out the 

basins and ranges, but before renewed faulting 

during Quaternary time. Renewed faulting has 

placed them in structural contact with the bordering 

ranges and has faulted and tilted them internally, 

the deformation being greatest close to the mountains and passing into less deformed or flat-lying 

contemporaneous deposits in the centers of the 

basins. Mter the earlier deposits were deformed, 

their upper surfaces were eroded into pediments 

before the Quaternary deposits were laid over them. 

SANTA FE FORMATION 

The Pliocene continental deposits of New Mexico, 

especially near the Rio Grande in the center of the 

State, are known as the Santa Fe Formation, which 

was named by Hayden in 1869 for exposures near 

Santa Fe, New Mexico. Much of the modern 

knowledge of the formation is derived from the work 

of Kirk Bryan and his students at Harvard University, whose many publications include those of 

Bryan and McCann (1937), Smith (1938), Denny 

(1940), and Wright (1946). 

The Santa Fe Formation characterizes especially 

the series of structural depressions drained by the 

Rio Grande that extend from the San Luis Valley in 

southern Colorado, southward the length of New 

Mexico into western Texas. As shown on the 

Geologic Map, exposures of the Santa Fe are nearly 

continuous· through this distance, broken only by 

the Quaternary alluvial cover and occasional cappings of Quaternary basalt flows (Qv). In the 

southwestern part of the State, discontinuous outcrops of similar deposits occur westward along the 

south edge of the Mogollon Plateau, to connect 

vaguely with the Gila Formation of southeastern 

Arizona. 

The Santa Fe Formation is a heterogeneous body 

of continental deposits that was laid down in 

structural basins like those at present and in a 

similar climate. The basins were outlined by warping and faulting late in Miocene time and had 

somewhat different dimensions from the present 

ones, in some places larger. Along the margins are 

fanglomerates with clasts largely derived from the 

rocks of the adjacent mountains-Precambrian 

crystallines, Paleozoic and Mesozoic sediments, and 

older Tertiary volcanics. Farther out are wellbedded sands, and in a few places thinly stratified 

clays and evaporites that accumulated in playas. 

The Santa Fe is unique among the basin formations in containing traces of a through-flowing 

river-channels containing well-rounded gravels 

distantly derived from the north. The gravels have 

been traced from the latitude of Santa Fe at least as 

far south as the latitude of Socorro. Although the 

river was ancestral to the present Rio Grande and 
36 THE CENOZOIC ROCKS 

lies near its course, it diverges widely in places, and 

apparently extended across modern ranges that 

were raised after Santa Fe time. Where the river 

went farther south is conjectural; it may have 

drained in to a closed depression in north western 

Chihuahua, for through-flowing drainage to the 

Gulf of Mexico across the mountain barriers in the 

Big Bend area of Texas does not seem to have been 

established until after the Santa Fe epoch. 

The Santa Fe contains vertebrate bones at a good 

many localities. The original collections made near 

Santa Fe have been determined to be of upper 

Miocene to middle Pliocene age (Barstovian, Clarendonian, and Hemphillian). Farther south only 

Pliocene bones have been collected, and at a few 

localities fossils of the Blancan Stage-a stage 

sometimes considered to be lower Pleistocene. The 

bulk of the deposit is certainly Pliocene (Tpc) and it 

is so represented on the Geologic Map. 

Subdivisions have been recognized in the Santa 

Fe Formation in places, and the unit has been 

classed as a "group" by some authors. These are, 

however, merely local members, which from the 

very nature of the deposit could not extend far in 

any direction-considering the complex interfingering of different sorts of deposits laid down in 

contrasting environments in different parts of the 

basin. In the Santa Fe area, however, a lower unit, 

the Abiquiu, is widespread and is decidedly more 

tuffaceous than the overlying beds; it is usually 

considered to be a basal member of the Santa Fe. 

Observed surface sections of the Santa Fe range 

from 1,000 to 4,500 feet (300-1,300 m) thick but are 

uncertain because of the considerable minor faulting within the formation. Well data suggest that in 

the centers of the deeper basins the thickness of the 

formation may exceed 5,000 feet (1,500 m). 

The Santa Fe Formation has been considerably 

deformed by subsequent faulting and tilting, and 

the later movements have much modified the 

Pliocene form of the Rio Grande depression. Younger 

block-faulting has raised new mountains along its 

borders which have reduced the size of the original 

depression, and in a few places, as in the Valles 

Mountains west of Santa Fe, volcanic edifices have 

been built up in the original depression. The upper 

surface of the Santa Fe is not the original depositional surface but is a widespread planation surface 

(the Ortiz pediment) that was cut in early Pleistocene time. GILA FORMATION 

The term Gila Formation (originally called "Conglomerate") is commonly used for late Tertiary 

continental deposits in the intermontane basins of 

southeastern Arizona. It was named by Gilbert in 

1875 for exposures near the Gila River in the eastern 

part of the State and in adjacent New Mexico. In 

subsequent mapping the name has been loosely 

applied to almost any poorly consolidated late 

Cenozoic continental deposit of diverse lithology 

and age, including (improperly) Quaternary alluvial 

terrace deposits at the top. As a result, some 

geologists class the Gila as a group, and others wish 

to abandon the term entirely. Nevertheless. the 

name is, with proper restrictions, still useful as a 

regional designation. 

Like the deposits of the Santa Fe Formation, 

those of the Gila Formation include coarse fanglomerates along the margins of the basins, with 

clasts derived from the diverse rocks of the adjoining mountains; fine-grained, better bedded sandy 

deposits farther out; and fine-grained, thinly bedded 

playa deposits in the centers of the basins (Knechtel, 

1936). Unlike the Santa Fe deposits, those of the 

Gila contain no trace of a through-flowing river; 

through-flowing drainage by the Gila and other 

rivers was not established in the region until after 

the depositional epoch during Quaternary time. 

Where the Gila Formation has been studied in 

detail, it seems to be capable of more meaningful 

subdivision than the Santa Fe, and the epoch was 

broken into several depositional cycles with slightly 

different deposits and separated by unconformities. 

The Gila in the valley of the San Pedro River southward from its junction with the Gila River at 

Winkelman is now known in much detail (Heindl, 

1963; Krieger and others, 1974). The oldest unit of 

the group, the San Manuel Formation, was deposited in a longitudinal basin west of the present 

axis of the San Pedro Valley. It consists of 

fanglomerate, and alluvial and possible playa 

deposits, and attains a thickness of 5,000 feet 

(1,500 m). It is notable for its lenses of mega breccia 

composed of large masses of the hard pre-Tertiary 

rocks, which were probably carried into the basin in 

gigantic landslips. An overlying tuff unit has been 

dated radiometrically as middle Miocene, so that 

the San Manuel deposits are evidently lower Miocene. Unconformably on the San Manuel at the 

north end of the valley is the Big Dome Formation, 

a coarse alluvial deposit about 600 feet (180 m) 

thick, which was laid·down in the present structural 

depression. It is younger than the radiometrically 

dated middle Miocene tuff, and radiometric dates on 

interbedded tuffs suggest an upper Miocene age. 

Following the Big Dome epoch, the structural 

depression was further restricted by faulting, and 

the alluvial and lake-bed deposits of the Quiburis 
PLIOCENE SERIES 37 

Formation were deposited along the axis of the 

valley. They are about 1,500 feet (450 m) thick and 

contain middle Pliocene (Hemphillian) mammalian 

bones. 

Other areas of the Gila have not been studied in 

as much detail as the one just described, so that the 

regional history and extent of subdivisions of the 

Gila are still imperfectly known. Deposits with 

Pliocene bones occur southward in the San Pedro 

Valley nearly to the Mexican border and indicate 

the wide extent of the Quiburis in this direction. In 

the Safford Valley farther east, the main body of the 

Gila, or "Solomonsville beds," about 2,000 feet (600 

m) thick, also contains Pliocene fossils (Wood, 1959, 

p. 60) but is underlain unconformably by the earlier 

"Bonita beds" of massive volcanic conglomerate. 

Above are the "Frye Mesa beds" of bouldery 

conglomerate that fringe the edges of the bordering 

mountains and are clearly Pleistocene. All the Gila 

of southeastern Arizona is indicated as Pliocene 

continental deposits (Tpc) on the Geologic Map. 

Drilling in the basins of south western Arizona 

indicates the presence of evaporites in the Pliocene 

deposits, suggesting a tenuous connection with the 

sea in the Gulf of California. 

Mt:onv CREEK FoRMATION 

The late Tertiary basin deposits of southern 

Nevada are the Muddy Creek Formation (Longwell 

and others, 1965, p. 48-49), typically developed in 

Muddy Valley northeast of Las Vegas. The Muddy 

Creek occurs in all the intermontane depressions 

near the Colorado River from Las Vegas eastward 

to the Grand Wash Cliffs at the edge of the Colorado 

Plateau and has been partly submerged by the 

waters of Lake Mead. 

Like the basin deposits previously described, the 

Muddy Creek consists of fanglomerate and other 

coarse deposits near the mountain borders, passing 

basinward into well-stratified deposits of sandstone, siltstone, and clay. In the centers of some of 

the basins are evaporite deposits-chiefly gypsum, 

but with thick beds of rock salt in the Virgin Valley. 

Bedded manganese oxides occur at one place, and 

fresh-water limestone in others. One or two basalt 

flows are interbedded. Except near some of the 

bordering faults, the formation is virtually uncleformed and flat-lying. The age of the Muddy Creek 

is uncertain, as fossil remains are scarce; it is 

probably mainly Pliocene, although a Miocene age 

has been ascribed to some of the vertebrate material. 

The Muddy Creek Formation is 2,000 feet (600 m) or 

more thick. 

The Muddy Creek was deposited in an arid 

environment not unlike that of the present intermontane basins in a region of interior drainage 

which wa~ ponded in places to produce lacustrine 

deposits such as the evaporites and fresh-water 

limestones. All the sediments are locally derived, 

and there is no indication of any through-flowing 

river, such as the Colorado River that now traverses 

the area (Longwell, 1946, p. 821-826). Deposits of the 

Colorado River, including well-rounded foreign 

gravels, lie on the eroded surface of the Muddy 

Creek and were deposited during several stages, 

none of which are older than Pleistocene. 

Deposits like those of the Muddy Creek occur in 

the intermontane basins northward from the 

Colorado River into eastern Nevada and adjacent 

southwestern Utah. Part of these deposits is the 

Panaca Formation, from which Pliocene vertebrate 

bones have been collected. 

Hl'MBOLDT FORMATION 

The late Tertiary Basin deposits of northeastern 

Nevada are the Humboldt Formation (Sharp, 1939), 

which was named by Clarence King in 1878 for 

exposures in the upper drainage of the Humboldt 

River. It is well exposed near the river, as in the 

vicinity of Elko and the Ruby Mountains, and 

occurs in most of the intermontane basins northward to the edge of the State. South of the Humboldt 

River the formation is mostly concealed by Quaternary basin deposits. In some publications the term 

Humboldt has been loosely applied to almost any of 

the later Tertiary deposits of the region, but some 

earlier Tertiary deposits are not properly a part of it 

and should be excluded. Except where these occur, 

the formation lies unconformably on the preTertiary rocks of the bordering ranges. 

The Humboldt Formation is about 3,000 feet 

(900 m) thick and consists of fanglomerate and 

conglomerate near the mountains, which pass 

upward and outward into sandstone, mudstone, and 

siltstone, much of which is tuffaceous. It is of 

fluviatile and lacustrine origin. The fanglomerates 

near the Ruby Mountains consist of clasts of 

Paleozoic limestone below, passing up into clasts of 

crystalline rocks, indicating progressive unroofing 

of the range, probably ·by uplift along block faults 

during deposition of the formation. Other ranges in 

the region were probably also undergoing block 

uplift during Humboldt time. At the margins of the 

deposit, as at Palisades Canyon on the Humboldt 

River west of Elko, rhyolite flows intertongue with 

the upper part of the deposits. 

The Humboldt Formation contains rather abundant vertebrates, as well as fresh-water inverte-
38 THE CENOZOIC ROCKS 

brates and plants. The vertebrates are generally 

ascribed a late Miocene age, and there is no 

indication of any of Pliocene age. Nevertheless, it is 

shown along with the other basin deposits as a 

Pliocene continental deposit (Tpc) on the Geologic 

Map. 

SALT LAKE FORMATION 

In northeastern Utah and southeastern Idaho the 

late Tertiary basin deposits are the Salt Lake 

Formation, named by Hayden in 1869. It is exposed 

on the flanks of the Wasatch Range north of Salt 

Lake City and between other ranges farther north, 

as far as the Snake River Plain. The formation 

consists of tuff, tuffaceous sandstone, shale, and 

marl, partly of fluviatile and partly of lacustrine 

origin. Exposed sections 1,500 feet ( 450 m) thick are 

common, and the total may exceed 3,000 feet (900 

m). Subdivisions have been recognized in some 

areas, and units of several different ages may be 

present, although the bulk of the formation is 

believed to be Pliocene. The Salt Lake lies unconformably on the Wasatch and other Eocene continental deposits, and in some of the valleys it is 

overlain by sheets of basalt of Quaternary age. In 

some of the ranges the formation has been much 

disturbed and offset by later block-faulting, but in 

other areas it is little deformed. 

DEPOSITS ASSOCIATED WITH VOLCANIC ROCKS 

Pliocene continental deposits (Tpc) are shown on 

the Geologic Map in many parts of the volcanic 

province of the northwestern United States, where 

they are associated with and grade into Pliocene 

volcanic rocks and are not ordinarily basin-fill 

deposits like the group just described. In many 

areas they are merely end-members of a sequence 

ranging from lava flows, through ash-flow tuffs and 

tuffs, to strata more clearly sedimentary, and thus 

they have no special designations. In some places, 

however, they have more stratigraphic entity and 

have been given formation names. 

IDAHO GROlTP 

In the western third of the Snake River Plain of 

south western Idaho is a complex sequence of 

Pliocene and early Pleistocene continental deposits 

and interbedded volcanics, the Idaho Group, which 

accumulated in this northwest-trending segment 

during its downwarping and downfaulting (Malde 

and Powers, 1962, p. 1201-1212). 

Beneath the Idaho Group, especially on its southern border, are the largely felsic ldavada Volcanics 

(Tpf) whose sedimentary members contain plants 

and other fossils of lower Pliocene age. Above the 

group, especially toward the east, are the basalt 

flows of the Snake River Group (Qv) of Pleistocene 

and Holocene age. 

The Idaho Group itself is a body of fluviatile and 

lacustrine deposits and interbedded basalt flows 

about 3,000 feet (500 m) thick and of middle and 

upper Pliocene and lower Pleistocene age, which is 

divisible into a set of named formations. All the 

formations are lenticular and are mostly unconformable with each other because of progressive 

down warping and downfaulting of the plain during 

accumulation. All the sedimentary units are rather 

abundantly fossiliferous and contain vertebrate 

bones, plants, and fresh-water mollusks which 

provide many data on the different ages. The 

subdivisions are the Poison Creek Formation, Banbury Basalt, and Chalk Hills Formation, all of 

Pliocene age; the Glens Ferry Formation which 

bridges the Pliocene-Pleistocene boundary; and the 

T9ana Gravel, Bruneau Formation, and Black Mesa 

Gravel of Pleistocene age. 

OTHER FORMATIOi\'S 

On the eastern flank of the Cascade Range in 

central Washington is the Ellensburg Formation, a 

fine- to coarse-grained clastic deposit, largely derived from erosion of the middle Tertiary andesitic 

volcanics of the Cascades (Waters, 1955, p. 670-675). 

The Ellensburg overlies the Yakima Basalt of the 

Columbia River Group and contains in its lower 

part thin interbedded basalt flows like those beneath. Eastward it interfingers with and is apparently largely replaced by the upper basalts of the 

Columbia River Group. The Ellensburg contains 

plants, fresh-water gastropods, and vertebrates of 

early Pliocene age. 

Along the valley of the John Day River, in the 

Blue Mountains uplift of northeastern Oregon, is 

the Rattlesnake Formation, composed of fanglomerate, bouldery gravel, and welded tuff. It lies unconformably on the Miocene and earlier sedimentary 

and volcanic rocks of the region, and contains 

middle Miocene (Hemphillian) vertebrates, but its 

upper part may extend into the Pliocene. 

In western Nevada are the Truckee Formation of 

the Reno area (Thompson and White, 1964, p. 16-17) 

and the Esmeralda Formation of Esmeralda County 

farther southeast (Robinson and others, 1968). Both 

are late Tertiary fluviatile and lacustrine deposits, 

laid down in disconnected structural basins that 

correspond closely to the modern blockfaulted depressions. Both formations contain large volumes of 

volcanic debris-basaltic and andesitic in the 

Truckee, rhyolitic in the Esmeralda. This debris is 

intermingled with non-volcanic sedimentary com-
QUATERNARY SYSTEM 39 

ponents, including siltstone, diatomite, beds of coal 

(in the Esmeralda), and pebble conglomerates 

formed of clasts of the pre-Tertiary rocks. The 

Truckee Formation is at least 2,000 feet (600 m) 

thick, and the Esmeralda in places attains 9,000 feet 

(2,700 m). The Truckee Formation interfingers with 

the upper part of the andesitic Kate Peak Formation 

and is overlain by younger basalts. The Esmeralda 

interfingers with and is overlain by the volcanic 

rocks of the Silver Peak center. The Truckee 

contains fossil plants of middle Pliocene age. Fossil 

remains in the Esmeralda are more diverse, and 

include plants, mammals, fish, ostracodes, and 

mollusks of Hemingfordian, Barstovian, and Clarendonian ages; radiometric dates by the K/ Ar 

method range from 13 to 4 m.y., or from late Miocene 

to late Pliocene. 

In eastern California, east of the south end of the 

Sierra Nevada, is the Ricardo Formation of continental and lacustrine sediments and interbedded 

lavas and tuffs, which attains a thickness of 

7,000 feet (2,100 m) and contains lower Pliocene 

(Clarendonian) vertebrates. 

QUATERNARY SYSTEM 

Quaternary deposits are only partly represented 

on the Geologic Map of the United States, because 

they are primarily a surficial mantle over the preQuaternary bedrock-representation of which is the 

principal objective of the map. The appropriate 

place for a complete representation of the Quaternary deposits is on a surficial geology map, several 

of which have been published of different parts of 

the country. The U.S. Geological Survey is now 

planning a surficial geology map of the whole 

United States, as a companion to the present 

geologic map. 

The Geologic Map of the United States does not 

represent the glacial and other deposits of Pleistocene age that blanket large parts of the Northern 

· Interior States, nor the loess and drifted sand that 

are extensive in some other places. In such areas the 

bedrock may be partly or wholly concealed, and its 

representation must be by subcrop or subdrift 

mapping, based more on the results of drilling and 

geophysical data than on outcrops. In the Northern 

Interior States we have, however, marked the limits 

of the earlier and later Pleistocene glaciations, to 

suggest areas in which the bedrock is likely to be 

extensively concealed. 

The Geologic Map does, however, represent the 

Quaternary deposits along the Atlantic and Gulf 

Coastal Plains and in intermontane areas in the 

West, where they are an essential feature of the 

bedrock pattern (fig. 5). In such areas the Quaternary deposits attain sizeable thicknesses, and in 

many areas the bedrock pattern beneath them is 

poorly known or unknown. In the Coastal Plains of 

the Eastern United States, the Quaternary deposits 

are divided into the Pleistocene (Qp) and Holocene 

(Qh) Series. The Quaternary deposits of the Western 

United States are not separated on the Geologic 

Map and are represented as an undifferentiated unit 

(Q). In many of these areas, the respective outcrops 

of the Pleistocene and Holocene Series would be too 

small to show on the scale of the map, and the 

differentiation between them has not been made on 

many of the source maps. 

GULF COASTAL PLAIN 

MISSISSIPPI EMBAYMENT 

By far the largest area of Quaternary deposits in 

the Coastal Plains is that in the Mississippi 

Embayment, which extends northward from the 

Louisiana coast for 600 miles (950 km) to southern 

Illinois. Extending centrally through the embayment is the alluvial valley of the Mississippi River, 

filled largely by Holocene deposits, panked on one 

or both sides by Pleistocene terrace and other 

upland deposits. 

Much of our knowledge of the alluvial valley we 

owe to the classic memoir of Fisk (1944), in which 

the modern landforms and the processes at work on 

them are ably set forth, and an attempt made to 

reconstruct its earlier Quaternary history. Further 

investigations, and the accumulation of more data 

since publication of this memoir, have resulted in 

considerable modifications of Fisk's proposed history (Durham and others, 1967; Saucier, 1974). A 

major change that affects representation on the 

Geologic Map is the reassignment of the lower 

alluvial terraces in the valley from after the end of 

the last glacial period, 10,000 to 12,000 years ago, to 

earlier periods, some as early as the Sangamon 

Interglacial Stage about 100,000 years ago; all the 

terraces in the valley are mapped as Pleistocene 

rather than Holocene. 

The oldest continental deposits near the Mississippi River valley are the Citronelle Formation of 

probable late Plio~ene age, already described, which 

forms a coastward-dipping body of sediments in the 

uplands to the east and west. In addition, Fisk 

identified two high-level terrace deposits and surfaces on the uplands df supposed early Pleistocene 

age, the Williana and Bentley. While it is true that 

vague, deeply dissected surfaces can be recognized 

in the uplands, the surfaces were probably cut from 
40 

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THE CENOZOIC ROCKS 

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' \ 

~ 

----------------~--- !---------!"~ I 1 ~ : ~ I L 

' ~. 

',...p. 

~-- .... ·-"'"'-:.r·-.... _.-

\ 

' '-, 

FIGURE 5.-Map of the United States showing surface distribution of Quaternary deposits as represented on Geologic Map of the 

United States (units Q, Qp, Qh). Within the lines in the northern part of the map, the pre-Quaternary rocks are extensively covered 

by Pleistoce~e glacial deposits which are not mapped (solid line, limit of Wisconsin Glaciation, broken line limit of earlier glaciations). 
I 

\ 

G U L F 

QUATERNARY SYSTEM 

~. ---- ~' --~--::'_;~·" I 1.--- \ / ~"--'-!..... 

~: ,, / , (' 

.. -v ;-· 

_,? I 

\ '-._ _) 

,1'-.._.•-./ 

;' ----- -------~·:-~-~_-:T -·--- ·.' __ _;---- J 

,; _,/ ___ .!.---' ( 

-------\ ·-\ 

\ ',"\ 

\ 

\ 

OF MEXICO 

FIGURE 5.-Continued. 

,-,_,, ............. , 

: \ I '. 

\ \ \ ', '· 

,! ' 

,f 

(\ -·' ----r--- ) \ 

', ( .. t I \ 

', i "\ ' .. ( _: \-------

~--~ 

\ 

I 

41 

time to time in the Citronelle deposits. The deci- The development of the valley was controlled by 

pherable history of the valley begins with the the glacial and interglacial periods of Pleistocene 

Montgomery Terrace Formation of the Yarmouth time. During glaciations, sea-level was lowered as 

Interglacial Stage (post-Kansan). much as 400 or 450 feet (120-140 m), resulting in the 
42 THE CENOZOIC ROCKS 

cutting of a system of fairly deep valleys in the 

bedrock floor. Their existence, which was first 

suspected as early as 1881 by drilling in the 

alluvium, have been abundantly documented since 

by a closer net of drill holes. During each interglacial stage, sea-level rose to nearly its original 

level, and the valley was aggraded; the streams, at 

first heavily loaded with glacial outwash, had a 

braided pattern, and passed later into a meanderbelt pattern. During the braided stream stage, silt 

was carried eastward by the wind and deposited as 

loess on the uplands on this side of the valley. 

During last post-glacial, or Holocene cycle, the river 

has now attained the meander-belt stage throughout the alluvial valley (Qh). The earlier aggradational stages are represented by the terrace deposits 

that border the valley (Qp). 

The oldest identifiable Pleistocene terrace formation (Qp) is the Montgomery, of inferred Yarmouth 

age, represented by deeply weathered, dissected 

remnants. Its deposits attain thicknesses of 300 or 

400 feet (90-120 m), and slope seaward at a rate of 

3-4 feet per mile (0.6-0. 7 m per km), probably 

passing beneath the succeeding Prairie Terrace 

Formation at steadily increasing depths. The Prairie 

Terrace Formation, of presumed Sangamon age, is 

far more extensive, especially in the coastal belt 

of southern Louisiana, and its surface includes 

meander-9elt deposits inland, and deltaic plains, 

relict lagoons, and barrier islands near the coast. At 

levels below the Prairie Terrace are smaller terraces 

formed during the Wisconsin glacial and interglacial substages, including the fluvial Deweyville 

Terrace in upstream areas and various braidedstream terraces within the alluvial valley. 

The Holocene deposits (Qp) of the alluvial valley 

include meander belts formed at different times by 

the Mississippi River and its tributaries, as they 

shifted this way and that across the valley. Between 

the meander belts are back-swamp deposits. Downstream, the deposits form deltaic plains with branching distributaries and along the coast chenier plains 

with elongate beach ridges ("cheniers"). 

OTHER AREAS 

West of the- Mississippi Embayment, in southwestern Louisiana and in Texas, a belt of Quaternary deposits 40-80 miles (65-130 km) wide lies 

between the Tertiary outcrops and the coast. Their 

surfaces are a succession of broad plains or benches 

stepped down coastward, which express a set of 

formations. Stratigraphic terminology in Texas 

antedates that in the Mississippi alluvial valley and 

elsewhere in Louisiana; broader correlations between them are clear, but there is disagreement on 

details (compare Doering, 1956, p. 1,831-1,840; and 

Bernard and LeBlanc, 1965, p. 176-179). Traditionally, the Pleistocene has been divided in to the lissie 

and Beaumont Formations. On the Houston and 

Beaumont sheets of the Geologic Atlas of Texas 

(1968), the lissie is divided into the Bentley and 

Montgomery Formations, supposedly correlative 

with the Louisiana units, and the Beaumont is 

correlated with the Prairie Formation of Louisiana, 

with which it is physically continuous; in addition, 

the Deweyville Terrace Formation is recognized 

along the upstream parts of the major rivers, as it is 

in Louisiana. 

This segment of the Coastal Plain differs from 

that in the Mississippi Embayment in that none of 

its rivers headed in a glaciated area farther north, 

so that direct glacial control was lacking; otherwise, 

the deposits and conditions of deposition were much 

the same. Most of the plains were deltaic, and the 

surface of the Beaumont, especially, is marked by 

branching distributary channels whose natural 

levees are sandy deposits separated by areas of clay 

deposits. Toward the coast, the Beaumont passes 

into brackish-water and- near-shore deposits which 

contain many shell banks. 

Holocene deposits are confined to offshore bars 

and to broad belts of stream alluvium that extend 

far up the major river valleys. On the Geologic Map, 

these alluvial belts are arbitrarily cut off about 

100 miles (160 km) from the coast so as not to 

obscure the bedrock formations, but they actually 

extend nearly to the inner edge of the Coastal Plain. 

In south Texas, not far north of the Rio Grande, an 

extensive body of drifted sand spreads inland over 

the Pleistocene deposits and is mapped with the 

Holocene. 

East of the Mississippi Embayment, the Citronelle 

and older Tertiary formations reach close to the 

coast so that the Pleistocene deposits are mostly 

missing and the Holocene is represented by only a 

narrow coastal strip. The only exception is in northwestern Florida where the Appalachicola River has 

built a delta which includes several Pleistocene 

terraces that are seemingly correJative with those 

farther west (Doering, 1956, p. 1,854-1,858). 

ATLANTIC COASTAL PLAIN 

Representation of the Quaternary of the Atlantic 

Coastal Plain on the Geologic Map is based, for the 

most part, on the available State Geologic Maps. 

These maps show as Quaternary a belt of varying 

width along the coast where the Tertiary rocks 

farther inland are no longer exposed, and they 

ignore areas of Quaternary deposits farther inland 
QUATERNARY SYSTEM 43 

where they are thinner and more discontinuous. 

This procedure is desirable on the Geologic Map, 

whose purpose is to emphasize the pre-Quaternary 

bedrock. Nevertheless, as examination of the literature of the Quaternary of the Atlantic Coastal Plain 

will show, the results are empirical rather than 

morphological, as explained below. 

The only modifications of representation from the 

source maps were made in Maryland, Delaware, 

and New Jersey, and in northeastern Florida. In the 

first three States, the inland extent of the Quaternary deposits is shown in much greater detail on the 

State Maps than farther south, interspersed with 

outcrops of the Tertiary rocks. Here, the same 

procedure was adopted as farther south, and the 

Quaternary was mapped only beyond the limits of 

Tertiary outcrops. For the actual extent of the 

Quaternary in these States, see King and Beikman 

(1974, fig. 15, p. 37). 

In northeastern Florida, the State Map (1964) 

indicates the Quaternary as "Recent and Pleistocene; several lower marine and estuarine terrace 

deposits." In this area, further subdhjsion was 

made on the basis of MacNeil's map (1950, pl. 19), 

which represents the Quaternary terrace formations 

and their shorelines. Shown as Pleistocene were the 

higher terrace formations-the Okefenokee at about 

150 feet ( 45 m), the Wicomico at about 100 feet 

(30 m), and "the Pamlico at about 30 feet (10 m). 

Shown as Holocene were the Silver Bluff at about 

10 feet (3 m), which MacNeil classes as postWisconsin, and surfaces below it. The results are 

somewhat questionable, but no better map representation was available to us. 

On the Geologic Map, the Holocene of the Atlantic 

Coastal Plain is represented as a strip along the 

coast from Florida through Georgia into South 

Carolina; farther north all the Quaternary is mapped 

as Pleistocene. It is true that Holocene deposits form 

the offshore bars here, as well as narrow strips 

along the inlets, but in areas too small to separate. 

Moreover, the Pleistocene and Holocene are not 

separated on many of the source maps. Beyond New 

Jersey, on Long Island, Martha's Vineyard, Nantucket, and Cape Cod, the Quaternary deposits are 

clearly a product of the Pleistocene glacial periodthe massive Wisconsin terminal moraines and 

associated deposits. 

An extensive literature on the Quaternary deposits 

of the Atlantic Coastal Plain exists, and geologists 

have much debated their meaning, with confusing 

results-as to what parts are marine and what parts 

continental, as to whether they are classifiable 

according to altitude (that is, whether they formed 

on surfaces representing different stands of the sea 

during the Pleistocene), their relation to glaciation, 

and their possible relation to crustal warping. The 

publications up to 1940 were summarized by Flint 

(1940), and since then still other views have been 

expressed (for example, MacNeil, 1950; Doering, 

1960; and Alt and Brooks, 1965), some of them widely 

divergent. 

The terrace concept was championed especially 

by C. W. Cooke, who recognized as many as seven 

surfaces forming a descending flight from as high 

as 270 feet (90 m) to sea-level. All the surfaces were 

supposed to be unwarped and horizontal from one 

end of the Coastal Plain to the other and to have been 

formed by marine invasions during the Pleistocene 

glacial period, each marine submergence being less 

than the one preceding, for reasons not entirely 

clear. These interpretations have been accepted 

only in part by other geologists. 

In Virginia and North Carolina two scarps 

created by marine erosion are widely traceable-the 

Surry at 90 feet (27 m) and the Suffolk at 25 feet 

(7.5 m). Above the Surry there is no convincing 

evidence of any marine deposits (Flint, 1940, p. 777). 

All the Quaternary shown on the Geologic Map in 

these States is below the Suffolk scarp. In Florida 

and Georgia, evidence of marine submergence to 

higher levels is better documented; MacNeil (1950, 

p. 99) reports evidence of marine scarps, bars, and 

deposits at .altitudes as high as 150 feet (45 m). 

These higher features are shown as Pleistocene on 

the Geologic Map. For this area a strongly dissenting view is taken by Alt and Brooks (1965, 

p. 407-410) who recognize shorelines as high as 

250 feet (75 m), but interpret the higher ones as of 

Miocene and Pliocene age and concede as Pleistocene only those below 30 feet (10 m). 

The only marine surface and deposit which can 

definitely be correlated with the glacial events to the 

north is the Cape May Formation of coastal New 

Jersey, standing at 30 feet (10 m), which can be 

traced into Sangamon interglacial deposits (between the Illinoisan and Wisconsin glaciations). 

GREAT PLAINS 

For the most part, Quaternary deposits in the 

continental interior east of the Rocky Mountains 

are not shown on the Geologic Map, but some are 

represented in the southern part of the Great Plains, 

part of which are classed as Pleistocene, part as 

undifferentiated Quaternary, as explained below. 

Omitted from the map are the sand hills of northwestern Nebraska, of Quaternary age, which lie on 

the Pliocene Ogallala Formation (King and Beikman, 1974, fig. 14, p. 35), and the glacial deposits 
44 THE CENOZOIC ROCKS 

farther north which form a nearly complete blanket 

over the bedrock east of the Missouri River (King 

and Beikman, 1974, fig. 11, p. 31). The extent of the 

glacial deposits is suggested on the Geologic Map by 

the lines representing the limits of glaciation. 

PLEISTOCENE 

The largest area shown as Pleistocene is an 

irregularly shaped body in southeastern Colorado, 

whose deposits have commonly been shown as 

Pliocene Ogallala Formation on published geologic 

maps (although they are shown as Miocene on the 

Geologic Map of North America of 1965, for reasons 

not apparent). The unit lies on an eastward-sloping 

surface on the divide north of the Arkansas River 

that projects in low scarps above the surrounding 

lowlands. This surface is distinctly lower than that 

of the Ogallala Formation to the east-more than 

500 feet (150 m) as projected-and must be a 

younger feature. 

The deposits which underlie the western part of 

the surface are the Nussbaum Formation, named by 

Gilbert in 1897, which crops out around its edges. 

The surface itself is mantled by windblown sand, 

which extends to the east end of the area; the eastward extent of the Nussbaum beneath the surface is 

uncertain. The Nussbaum is an alluvial deposit, 

much less consolidated than the Ogallala deposits 

and more like the Quaternary alluvium on lower 

surfaces in the area (Scott, 1963). No diagnostic 

fossils have been found in the Nussbaum, but it 

seems to be the first member of the alluvial sequence 

and to be of early Pleistocene age. The area is 

accordingly marked as Pleistocene (Qp) on the 

Geologic Map. 

A smaller area of Pleistocene is indicated farther 

east, in the Great Plains of Meade County at the 

south edge of Kansas. The deposits of the area are 

the Meade Formation, a body of sand and gravel 

that fills a basinlike area to a thickness of more 

than 200 feet (60 m) (Frye, 1942, p. 103-109). It 

contains vertebrates and mollusks of early Pleistocene (Nebraskan) age, and it is overlain by one of 

the layers of the widespread Pearlette Ash of 

Kansan age. Pleistocene deposits occur on the 

Tertiary and older 'rocks in many other places in 

this part of the Great Plains, but they do not attain 

the thickness or have the basinlike form of those in 

the Meade County area. 

Another area of Pleistocene is indicated at the 

south edge of the Great Plains west of Big Spring, 

Texas. This was the site of Lake Lomax, that 

occupied a basin on the surface of the Ogallala 

Formation during late Pleistocene (Wisconsin) time 

(Frye and Leonard, 1964, p. 16-19). The basin was 

probably created by subsidence, resulting from 

solution of the underlying bedrock. Similar but 

smaller basins containing Pleistocene lake deposits, 

termed the Tahoka Formation, are scattered here 

and there over the surface of the Great Plains 

(= Llano Estacado) in Texas, but they are not 

represented on the Geologic Map. 

South of Lake Lomax a large area of Pleistocene 

is shown on the Geologic Map surrounded by Lower 

Cretaceous limestones of the Edwards Plateau. This 

area is usually shown as Ogallala Formation on 

published geologic maps, but the deposits which 

cover its surface are only a few feet thick, are locally 

derived, and are probably younger. 

A final area is indicated as Pleistocene on the 

Geologic Map in southeastern New Mexico, between 

the Pecos River and the Ogallala Formation of the 

Great Plains at the east edge of the State. It is 

shown on the New Mexico Map (1965) as "QabAlluvium and bolson deposits and other surficial 

deposits." The area is an irregular surface of intermediate height between the Roswell basin along 

the Pecos and the Great Plains to the east. Most of it 

is covered with heterogeneous surface deposits, 

including windblown sand and caliche, probably of 

variable thickness due to solution-collapse of the 

underlying bedrock. To distinguish these deposits 

from the alluvium along the Pecos River they are 

tentatively shown as Pleistocene (Qp) on the Geologic Map. 

UNDIFFERENTIATED QUATERNARY 

The Roswell and Toyah Basins along the Pecos 

River in southeastern New Mexico and western 

Texas much resemble the intermontane basins 

farther west, except that they are not defined by 

faults and other structures but have been produced 

by erosional excavation that has been accentuated 

by collapse resulting from solution of salt and other 

evaporites in the underlying Permian bedrock. 

The Roswell Basin in New Mexico, the smaller 

and better defined of the two, is bounded on the west 

by the limestone foothills of the Sacramento Mountains and on the east by lower scarps of Permian 

redbeds and Triassic sandstones. Southward, near 

Carlsbad, New Mexico, the Pecos River flows from 

the basin onto Permian and Triassic bedrock, before 

passing into the Toyah Basin in Texas. The Toyah 

Basin is much larger, more irregular, and less well 

defined; it is bounded by the trans-Pecos mountains 

on the west and by Lower Cretaceous limestones of 

the Edwards Plateau on the south and east. At its 

lower end, the Pecos again flows on bedrock in a 
QUATERNARY SYSTEM 45 

canyon in the Edwards Plateau to its junction with 

the Rio Grande. 

The surface cover of the Roswell Basin is complete, but that in the western part of the Toyah 

Basin is interrupted by many outcrops of bedrock. 

The surface materials in the two basins form a 

succession of descending terraces whose formation 

has been ascribed to events during the glacial 

period (Fiedler and Nye, 1933, p. 106-113) The 

Quaternary deposits beneath the surface have a 

variable thickness, commonly more than 100 feet 

(30 m), but are much thicker in places. Drill holes 

indicate depths of more than 300 feet (90 m) in parts 

of the Roswell Basin, and well over 500 feet (150 m) 

in parts of the Toyah Basin. The deepest fill in the 

Toyah Basin is along its northeast side, where there 

are few rock outcrops and where the underlying 

Permian salt beds were originally thickest (Maley 

and Huffington, 195"3). The thick alluvial fill was 

produced in areas of deep circulation of ground 

water and consequent salt solution and collapse. 

Collapse due to solution is evident near the Pecos 

River at the lower end of the Toyah Basin, where the 

otherwise flat-lying Lower Cretaceous limestones 

dip steeply and erratically toward the valley. 

East of the Great Plains in Kansas and Oklahoma, the source maps indicate broad, lengthy 

strips of Quaternary along the Arkansas, Cimmaron, Canadian, and other rivers. These appear to 

be chiefly windblown sand and silt derived from the 

river valleys. We have been reluctant to mark these 

surficial deposits on the Geologic Map but have 

shown the larger strips. The strip along the Arkansas River, especially, is too extensive to ignore, as it 

interrupts the continuity of the bedrock formations 

whose patterns beneath are conjectural. 

CALIFORNIA11 

The nonglacial Quaternary of California exceeds 

in variety and complexity any other in the United 

States-marine deposits in coastal embayments 

and uplifted marine terraces along the coasts, 

terrestrial deposits in the inland basins, and indications of major orogeny within the period. Moreover, the sequence of glacial deposits in the Sierra 

Nevada can be related at least tenuously with nonglacial sequences in the nearby lowlands. Nevertheless, the system is shown as an undifferentiated 

unit (Q) on the Geologic Map, and a subdivision into 

Pleistocene and Holocene Series is not attempted as 

it was in the Mississippi Embayment. Data for such 

a subdivision are available in most areas, but the 

liThe Quaternary of California is summarized by Wahrhaftig and Birman (1965). An 

earlier summary by Reed (1933, p. 255-273) is of historical interest. 

results would be too minute and complex for the 

scale of the map. Also, the marine lower Pleistocene 

of southern California, which is conformable on the 

Pliocene and forms a narrow band of outcrop 

adjacent to it, is for convenience mapped with the 

Pliocene (Tp ). 

The marine Pleistocene is best developed in the 

Ventura and Los Angeles basins of southern California. In this region, a major structural unconformity has long been known between deformed 

strata and nearly undisturbed overlying deposits, 

which Lawson (1893) first supposed to mark the 

Tertiary-Quaternary boundary. Later, however, paleontologists determined that the upper part of the 

deformed sequence was also Pleistocene, so that the 

period of orogeny indicated by the unconformity 

actually lay within the Pleistocene Epoch. Paleontological differentiation of Pliocene and Pleistocene 

in the region was based on Lyellian principles of 

relative percentages of modern genera and species, 

principally of the marine megafossils (Woodring, 

1952, p. 402), but it has since been largely confirmed 

by planktonic Foraminifera. 

The classic section in the region is that in the 

Palos Verdes Hills on the coast south west of the Los 

Angeles basin, where marine Pleistocene fossils 

were first reported by Ralph Arnold in 1903 and 

more fully documented in later reports (for example, 

Woodring and others, 1946). Here, the lower Pleistocene is about 600 feet (180 m) thick and comprises 

the Lomita Marl, Tims Point Silt, and San Pedro 

Sand. These beds have been tilted and are overlain 

unconformably by flat-lying upper Pleistocene deposits, including the Palos Verdes Sand, on uplifted 

marine terraces. Middle Pleistocene is missing. 

Northeastward in the Los Angeles basin, the 

Pleistocene thickens in subsurface to 3,000 feet 

(900 m) and the mid-Pleistocene hiatus may 

disappear. 

Most of the fossils in the lower Pleistocene of the 

Palos Verdes Hills are northern cool-water forms, a 

fact which was once thought to indicate the onset of 

glacial conditions, but which might well reflect 

depth of water and coastal upwellings. A radiometric date of three million years on glauconite 

from the Lomita Marl, obtained by J. S. Obradovich, 

is disconcertingly old for Pleistocene and is suspect 

for a number of reasons. 

In the Ventura basin to the northwest, surface 

sections of the Pleistocene are considerably thicker 

than in the Palos Verdes Hills-as much as 2,500 

feet (760 m), a sizeable figure but modest compared 

with the 15,000 feet (4,500 m) or so of the underlying 

Pliocene. The lower Pleistocene follows conform-
46 THE CENOZOIC ROCKS 

ably on the Pliocene Pico Formation and comprises 

the Santa Barbara and San Pedro Formations. 

Here, the mid-Pleistocene unconformity is even 

more pronounced than in the Los Angeles basin. 

The Pliocene and lower Pleistocene are steeply 

upended and even overturned and are overlain with 

right-angled unconformity by upper Pleistocene 

terrace deposits. As explained earlier (p. 32) the 

orogeny implied by this unconformity, although 

impressive here and at some other places, is merely 

a local and terminal event in a long sequence of 

tectonic instability. 

Farther north, in the San Francisco Peninsula, is 

the marine Merced Formation, whose upper part, 

conformable on the lower, is of early Pleistocene 

age. Both the lower and upper Merced are moderately deformed. 

The marine upper Pleistocene of the coastal area 

mostly forms the capping of flights of uplifted wavecut terraces. In the Palos Verdes Hills there are 13 

marine terraces between sea-level and 1,300 feet 

(400 m). The number and maximum height of the 

terraces varies from one coastal segment to another, 

and their altitudes vary as a result of warping; the 

cover of marine fossiliferous deposits is best preserved on the lower surfaces. Some of the sea-level 

changes implied by the terraces are probably 

attributable to subtraction and addition of sea 

water during the glacial and interglacial stages, but 

the uplift of the higher terraces is more likely 

tectonic. The lowest terrace, standing at about 

100 feet (30m) has commonly been correlated with 

the Sangamon Interglacial Stage, and near Santa 

Cruz has yielded radiometric ages of about 110,000 

years. An unsolved problem is that the terraces are 

restricted to the present coastal slopes; none of the 

nearby interior valleys show any indications of the 

deep submergence or successive marine planations 

implied by the coastal terraces. 

Most of the Pleistocene deposits just described are 

only indifferently represented on the Geologic Map. 

On the map, by far the largest area of Quaternary is 

that in the San Joaquin and Sacramento Valleys 

and consists of varied terrestrial deposits of different ages. The total thickness of Quaternary 

deposits attains 15,000 feet (4,500 m) in the southern 

part of the San Joaquin Valley but is much less 

farther north. 

On the southwest side of the San Joaquin Valley 

(as near the Kettleman Hills) is the lower Pleistocene Tulare Formation, a lacustrine or marsh 

deposit containing fresh-water mollusks. It conformably overlies the marine Pliocene Etchegoin 

and San Joaquin Formations and was deformed 

with them along the anticlines along the edge of the 

valley during mid-Pleistocene time. The deformation fades out northeastward across the valley, 

although an unconformity persists. Near the middle 

of the Tulare in subsurface is a thin, widespread, 

diatomaceous silty clay layer, the Corcoran Clay 

Member. On the east side of the valley this is 

overlain locally by pumiceous ash which has been 

dated by the K/ Ar method at 600,000 years. 

On the northeast side of the San Joaquin Valley 

are a set of alluvial formations derived mainly from 

erosion of the Sierra Nevada. Below is the Turlock 

Lake Formation, which is traceable in subsurface 

into the Tulare Formation, followed by the middle 

and late Pleistocene Riverband and Modesto Formations. The latter is essentially undeformed, but it 

stands in low benches above the Holocene alluvium 

of the present valley bottoms. 

The Quaternary deposits of the Sacramento Valley 

to the north are much thinner and contain some 

volcanic detritus. They lie unconformably on the 

eroded surfaces of the rertiary and older rocks. 

BASIN AND RANGE PROVINCE'~ 

Undifferentiated Quaternary (Q) is shown on the 

Geologic Map in most of the intermontane basins of 

the . Basin and Range province, from northern 

Nevada and Utah to southern Arizona and New 

Mexico. With the exception of some minor marine 

layers in the Salton Trough of southern California, 

all are continental deposits. Both Pleistocene and 

Holocene deposits are present but are not separated 

on the Geologic Map. 

The Quaternary deposits generally overlie similar 

intermontane deposits of Pliocene and Miocene age 

(Tpc), but these are only partly exposed, and in 

many basins the Quaternary deposits are the only 

surface formations. The thickness of the Quaternary is variable and in many areas uncertain. In 

basins with through-flowing drainage, such as 

those of the Rio Grande, Gila, and Colorado Rivers, 

the Quaternary is generally a rather thin set of 

terrace deposits on top of the much thicker Tertiary, 

but in other basins it attains considerable thicknesses in its own right. Surface sections of as much 

as 2,000 feet (600 m) occur in the Salton Trough, and 

the deposits may attain 3,000 feet (900 m) in 

subsurface. Thicknesses as great or greater probably occur in the basins with interior drainage in 

the northern part of the province. Thicknesses in 

the centers of the basins are uncertain because of 

t2Jncludes data from Morrison (1965) and Kottlowski, Cooley, and Rube (1965). 
CENOZOIC VOLCANIC ROCKS 47 

the difficulty of distinguishing in well records 

between the Quaternary deposits and the similar 

underlying Tertiary deposits. 

Much of the present topography of the Basin and 

Range province is a product of Quaternary time, 

and very little of the late Tertiary topography has 

persisted. An important part of Basin and Range 

structure is a product of early Pleistocene blockfaulting, which accentuated earlier fault blocks and 

raised or lowered new fault blocks. Subsequent 

erosion and deposition has profoundly modified the 

ranges and basins. Erosional products of the ranges 

have been carried into the adjacent basins; in 

basins with exterior drainage, large parts of these 

erosional products have been carried out of the 

region, but in basins without outlet they have 

remained as Quaternary deposits. 

A significant result of Quaternary tectonic movements has been the epeirogenic arching of the 

central part of the Great Basin with respect to its 

edges, so that even the basin floors in the central 

part stand 2,000-3,000 feet (600-900 m) higher than 

those of its east and west sides. As a result, all the 

present lakes of the region, and to an even greater 

extent the large Pleistocene lakes, lie close to the 

east and west sides rather than in the center. 

The Quaternary deposits in the closed basins 

resemble those of the late Tertiary-sloping alluvial 

fans and bajadas along the edges of the mountains, 

passing into nearly flat-lying finer-grained alluvium in the centers of the basins, with deposits of 

ephemeral lakes or playas in the lowest parts. 

Climatic variations during the Quaternary from 

arid to sub-humid are reflected in the nature of the 

successive deposits and the soils formed from them. 

During the rainy periods of Pleistocene time, the 

basins were filled with more permanent water 

bodies than now exist, the most important of which 

are those formed during the last, or Wisconsin 

Glaciation; traces of lacustrine epochs during the 

earlier glaciations also survive. 

The largest and best-known Wisconsin lakes were 

Lake Bonneville and Lake Lahontan on the east 

and west sides of the Great Basin, although smaller 

lakes existed in most of the other closed depressions 

throughout the Basin and Range province. Lake 

Bonneville at its maximum covered most of the 

western half of Utah, had a depth of 1,100 feet 

(335 m) and an area of 19,940 square miles 

(51,700 km2). Lake Lahontan was smaller and more 

irregular, with many intervening mountain ridges 

rising above it. It had a maximum depth of 700 feet 

(213 m) and an area of 8,665 square miles (22,442 

km2). The thickness of the lacustrine deposits 

produced by these lakes and their predecessors is 

uncertain, but definitely exceeds 500 feet (150 m). 

CENOZOIC VOLCANIC ROCKS 

Volcanic rocks of Cenozoic age cover at least a 

third of the surface of the Western United States, 

with the greatest areas in the Pacific Northwest 

(Washington, Oregon, and Idaho), and smaller 

areas elsewhere (fig. 6). The rocks range in age from 

Paleocene and Eocene to Quaternary, the most 

extensive being of Miocene and Pliocene ages. 

Classification of the Cenozoic volcanic rocks for 

purposes of the Geologic Map of the United States 

presents various problems, some still partly unsolved. Enough is now known, however, to apply a 

general classification throughout the map area, and 

to avoid undifferentiated categories such as "Tertiary volcanics (Tv)" and "Quaternary and Tertiary 

volcanics (QTv)" which have appeared on earlier 

maps. 

The volcanics are divided on the Geologic Map 

into those of early Tertiary (lTv), Miocene (Tmv), 

Pliocene (Tpv) and Quaternary (Qv) ages. The early 

Tertiary· volcanics include those of Paleocene, Eo- - , cene, and Oligocene ages, and they are generally 

more deformed and hydrothermally altered than the 

younger volcanics. Their exact ages are undetermined in many areas, and they crop out in such 

relatively small areas that it appeared fruitless to 

attempt a more detailed subdivision. Although the 

volcanics are mostly not divided according to 

composition, several exceptions are made. The felsic 

or rhyolitic volcanics are important in many areas, 

have somewhat different habits from the rest, and 

express a significant history. They are mapped 

separately wherever data are available (lTf, Tmf, 

Tpf, Qf). The remaining volcanics, indicated by "v," 

have compositions varying from andesitic to basaltic but include a few undifferentiated felsic 

volcanics. 

In the Coast Ranges of Oregon and Washington, 

large masses of submarine pillow basalts (Teb) are 

interbedded in the Eocene stratified sequence. In the 

Cascade Range to the east, the dominant andesitic 

eruptives are divisible into lower Tertiary andesites 

(ITa) of Eocene and Oligocene ages, unconformably 

overlain by upper Tertiary andesites (uTa) of 

Miocene and Pliocene ages. The basalts of the 

Columbia River Group east of the Cascade Range 

are mapped with the remaining non-felsic Miocene 

volcanics (Tmv). 

Ages of the volcanic rocks can be inferred to some 

extent by their stratigraphic and structural rela-
48 

0 

• 

-----

THE CENOZOIC ROCKS 

-r-r-----

.,: 1: -----------------------.. ___ _ 1'-------------------._---. • ~- \ 

'L, 

.,. 

\ 

I 

\. 

~----------------------~ 

i '. _______ o l 0 : 

I 

l 

~-

---- ,------------------'-----"'~-

\ 

'

'· . : : ""-.. ~~~~...... ' .. w, ' .:· . L------.-------- I 

.. t/ \- ,,~ ~~t 'I '" \. ! --·:·-----'- -----~ 

•• ' ; " ' • i ·~' ' , 0 

~)·; ! '\ • • i . • •• ~ ' ! --------------------

·-··~~~~:- ! -~-It t : 'II [~•~ I : 

• ,.. .---- 0 I ' ,, • \\ i• }. _____________ !____ ~~ : 

! .. :.,.~-~--'0~, .. : +"t~ j ---- ~-~-=:=:=:~----------- • ~1.t· .:11) , .:~· . o .. • ' I 1· 

,. ~· ,o, "'• j~ !" . : : 

• \ ~ •• ' .. : • ! I . !-~· -' I ' I 0 ' 

\ '<~".,., .,:,: •• , •• ~~~. L,_..,_·--_ _, 

--~----~~ •• ~~·· •• ~~ .: ;.. '-"'--·J-·-~--- --~ ' If .... • • , ·~ • 

"'~- ~-\!":~ \. ; ~ ',.~tit --- : ---._.._.._ __ • __ • :---•-\ ___________ j 

-o ·, 

\ 

\ 

,~ ' .. . 

\ . ---------, ' •/ ', It-' \ 

\ 

FIGURE 6.-Map of western United States showing distribution of Cenozoic volcanic rocks as Pepresented on Geologic Map 

of the United States. 
LOWER TERTIARY VOLCANIC ROCKS (II\·) 49 

tions and are confirmed in part by plant and 

vertebrate remains in interbedded tuffaceous layers. 

To an increasing extent, however, ages are being 

obtained by radiometric dating. Radiometric data 

are relatively complete in a few areas, but in many 

others the data are incomplete or lacking. Further 

radiometric dating will no doubt bring about important revisions of the ages of the volcanics in many 

areas. 

The known ages of the volcanic rocks indicate 

that the groupings of the volcanics do not correspond neatly to the different Tertiary epochs, so that 

there are frequently Oligo-Miocene groups, MioPliocene groups, and the like. The age designations 

by the conventional epochs as shown on the map 

are thus to some extent unrealistic, and the assignments of the volcanic groups to one epoch or 

another have had to be arbitrary. Some of these 

problems are discussed below. 

LOWER TERTIARY VOLCANIC ROCKS (lTv) 

The lower Tertiary volcanic rocks are largely of 

Eocene and Oligocene age. Paleocene volcanic rocks 

are of minor extent and belong to a different cycle of 

eruption more closely related to the immediately 

preceding Laramide orogeny. Where present, they 

are generally included with the Cretaceous volcanics (Kv). The rocks mapped as lower Tertiary 

volcanics range in age from about 55-25 m.y. 

Despite the rather lengthy period represented, 

separation of the Eocene from the Oligocene volcanic rocks is generally not feasible at the present 

state of knowledge, and many of the sequences in 

different areas include rocks of both ages. 

Lower Tertiary volcanic rocks occur throughout 

the western United States, but their present areal 

extent is much less than that of the younger 

volcanic rocks (fig. 7), due partly to subsequent 

removal by erosion and to burial beneath younger 

rocks. Although both Eocene and Oligocene volcanic rocks occur throughout the region, the lower 

Tertiary volcanic rocks average somewhat older in 

the north than in the south; north of the 41st 

parallel Eocene components dominate, farther south 

Oligocene. 

Most of the lower Tertiary volcanic rocks are 

shown on the Geologic Map as a single unit (lTv). In 

the Pacific Northwest, however, two categories are 

shown separately-the Eocene submarine basalts of 

the Coast Ranges (Teb) and the lower Tertiary 

andesites of the western part of the Cascade Range 

(ITa). In addition, the felsic volcanic rocks of the 

upper part of the lower Tertiary sequence in the San 

Juan Mountains of southwestern Colorado are 

indicated separately (ITt). 

The general composition of the lower Tertiary 

volcanic rocks has been summarized as follows 

(Lipman and others, 1972, p. 220): 

Lower and middle Cenozoic volcanic fields and associated 

intrusives *** constitute a broad petrologic association of 

predominantly intermediate compositions. Lavas are typically 

andesite, dacite, and quartz latite; the common hypabyssal 

intrusives are granodiorite, monzonite, and quartz monzonite. 

Somewhat more silicic rocks, especially ash-flow tuffs of quartz 

latite and low-silica rhyolite that are abundant in some areas, 

can plausibly be interpreted as high-level differentiates of 

intermediate-composition magmas. Very mafic and very silicic 

rocks are much less abundant; in many fields they are sparse or 

absent. Deviations from this broad outline are most conspicuous 

at the Pacific and continental interior margins of the Cenozoic 

volcanic province, where mafic rocks are more abundant. 

EOCENE MARINE PILLOW BASALT (Teb} 

The Eocene pillow basalts of the Coast Ranges of 

Oregon and Washington have already been treated 

in connection with the Eocene sedimentary rocks of 

that area (p. 10), so they will be mentioned only 

briefly here. They occur throughout the length of the 

Coast Ranges, from southwestern Oregon to northwestern Washington, and are interbedded with, or 

form thick lenses in, the lower and middle Eocene 

geosynclinal marine sediments; they are less important in the upper Eocene. The basalts are 

estimated to have a volume of 60,000 cubic miles 

(250,000 km3) and were built up in a series of 

volcanic centers along the axis of the geosynclinal 

trough, interfingering marginally with the sediments and in places built up to such a thickness that 

they emerged as islands. 

LOWER TERTIARY ANDESITE (ITa) 

The Cascade Range, in its 550-mile (890-km) 

length from northern California, through Oregon, 

to northern Washington, is a complex accumulation 

of andesitic volcanics varying in age from early 

Tertiary through Quaternary. The range is asymmetrical, with a broad, deeply dissected western 

part that slopes westward to the Puget Trough, the 

Willamette Valley, and other lowlands, and with a 

narrower higher part to the east, crowned by a 

chain of young volcanoes. The western, lower 

Cascades are formed of a mass of slightly alterP.d 

and somewhat folded· andesitic eruptives, ranging 

in age from late Eocene, through the Oligocene, into 

the early Miocene. The High Cascades to the east 

are built of little dissected andesitic eruptives of 
50 THE CENOZOIC ROCKS 

------,~--- ~ 

---------- \ 

\ 

\ 

\ 

"'f-' \~ . - ..._ 

_____ .-J----- I --. j___________________ \ -------- • . ' t. . I " : _______ _ ----- t ·a.. f.· ! ' ,. : \• I 

• , , . "' I : · .. 1 • J. ".. ' : I •• 0 , I . '··,_ ,:f.1f·•· ! : t I '... ~ I I- • : \ ------ J• 't: ~0 , I ~.. 'o 

, "I ,, I . ----- '·. ~.~0 •,fi : ~ J----------

'·., • 1-----------------\------- 1..;---------~=---------.. ·• 1

· I I \ 

'" : 0 

• • 0 

~;-,_ ~: ! I \ 

"· •. I l ··) : I '--"·---4,,---.,'J'-_.,__,' 

.... ~ I 

/ 

: • #, •• 

J .... 

~.··r.'"'i -\ 

• .. I 

I 

' •j .... ) ____ •• ---.:::._ 't ~ ._" .. 0 ... ~-------------1 

------~ , •• 1 • I ------'--

---------~~~----'-: ___ f · ...... ·,!_,., 0~ ' •,(-~--,,_ -.--4.f1_/ \, 

\ 

I 

\ 

FIGURE 7 .-Map of western United States showing distribution of lower Tertiary volcanic rocks as represented on Geologic 

Map of the United States (units lTv, lTf, ITa, Teb). 
LOWER TERTIARY \'OLCAI\:IC ROCKS (IT') 51 

Pliocene and Quaternary age. 

In Oregon, the great bulk of the western Cascades 

is formed of the Little Butte Volcanic Series (Peck 

and others, 1964, p.ll-26), between 5,000 and 10,000 

feet (1,500-3,000 m) thick in most places, but in some 

as much as 15,000 feet (4,500 m). On the west it lies 

in places on the upper Eocene Colestin Formation, a 

continental deposit with its own volcanic components (Tee), and elsewhere on the pre-Tertiary 

basement. Northwestward, the Little Butte Series 

intertongues with marine tuff and sandstone of 

Oligocene and early Miocene age. Toward the north 

it is overlain by the middle Miocene Columbia River 

basalt (Tmv) and toward the east by the upper 

Tertiary andesites of the High Cascades (uTa). 

The Little Butte Series (ITa) is formed of massive 

beds of andesitic and dacitic tuff, deposited by 

ashflows, and interbedded flows and breccias of 

olivine basalt, and of bedded tuff, welded tuff, and 

flows of dacite and rhyodacite. In some segments, 

pyroclastic and volcaniclastic rocks make up as 

much as three-quarters of the sequence and are 

shown separately on the Geologic Map as continental deposits (Toe). The rocks were erupted from 

chains of volcanic vents, of which at least 30 have 

been identified. Most of the volcanic rocks are partly 

or completely. altered by low-grade (zeolite facies) 

metamorphism, and they have been altered to 

hornfels near intrusives. 

Fossil plants of middle to late Oligocene and 

early Miocene age occur at many levels and 

localities in the Little Butte Volcanic Series. The 

upper Oligocene floras are like those in the John 

Day Formation farther east in Oregon. The older 

floras indicate a tropical or subtropical climate, and 

the youngest a somewhat cooler climate. 

The lower Tertiary andesites extend about 45 

miles (72 km) southward into California to the 

latitude of Mount Shasta, beyond which they are 

overstepped by the younger Cenozoic volcanics 

(Macdonald, 1966, p. 69-70). The California sequence has about the same thickness as in Oregon; 

the rocks are generally less altered because of lack 

of intrusives. There are some units as much as 1,000 

feet (300 m) thick of coarse andesitic tuff-breccia, 

containing angular blocks which reach 4 feet (1.2 m) 

across. 

In northern Oregon the lower Tertiary andesites 

are covered by younger volcanics for about 50 miles 

(80 km) toward the Columbia River, but they 

reappear in southern Washington, west of Mount St. 

Helens and Mount Rainier, where they form the 

Ohanapecosh Formation, named for typical exposures in Mount Rainier National Park (Fiske and 

others, 1963, p. 4-20). Exposed sections of the formation are as much as 10,000 feet (3,000 m) thick, 

and the total may exceed 15,000 feet (4,500 m). The 

Ohanapecosh consists of volcaniclastic rocks and 

basaltic andesite lava. Lava flows and coarse 

volcaniclastic rocks form thick local accumulations 

that grade laterally into finer grained clastic rocks. 

Ash-flow tuffs and rhyolites are a very minor 

constituent. Large parts of the formation were laid 

down under water, probably in lakes and coastal 

lagoons. The clastic rocks contain fossil plants of 

middle and upper Eocene age at some localities; the 

highest unfossiliferous part of the unit may extend 

into the Oligocene. 

In Washington, the boundary between the lower 

and upper Tertiary andesites of the Cascade Range 

is placed on the Geologic Map at the unconformable 

contact between the Ohanapecosh and Stevens 

Ridge Formations on the advice of C. A. Hopson 

(written commun. 1972). The Stevens Ridge is of 

upper Oligocene and Miocene age, so that the 

mapped bouudary is at a lower level than that in 

Oregon. 

West ward in the Washington Cascades, the andesitic components fade out and intertongue with 

continental deposits, such as the coal-bearing upper 

Eocene Cowlitz Formation of the Centralia-Chehalis 

district. The underlying N orthcroft Formation is 

largely andesitic, but its lower part is sedimentary 

and contains basaltic detritus derived from the 

west. The upper Eocene formations of this area lie 

on the east edge of the lower and middle Eocene 

pillow basalts of the Coast Ranges to the west. 

CLARNO FORMATION (lTv) OF NORTHEASTERN OREGON 

East of the Cascade Range, the next extensive 

body of lower Tertiary volcanic rocks is in the 

northern and western parts of the Blue Mountains 

uplift, the Clarno and John Day Formations. The 

John Day is more dominantly volcaniclastic than 

the Clarno, is shown as Oligocene continental 

deposits (Toe) on the Geologic Map, and is described 

under an earlier heading (p. 20). 

The Clarno, lying unconformably beneath the 

John Day and unconformably on the pre-Tertiary 

basement rocks of the Blue Mountains uplift, is 

more than 5,000 feet (1,500 m) thick. It is composed 

of basaltic, andesitic, and rhyolitic flows, breccia, 

tuff, and volcanic conglomerate, with thick lenses of 

of fine-grained water-laid ash; thick flows of 
52 THE CENOZOIC ROCKS 

hornblende-bearing porphyritic andesite and dacite 

and associated breccia occur locally. Silicified wood 

i~ common in the volcanic breccia, and leaf impressions are abundant in some of the silty beds. 

Vertebrate remains are rare or absent. The floras 

are lower to upper Eocene in age, and the highest 

are perhaps Oligocene. Radiometric ages of 41 to 

34 m.y. (Eocene and Oligocene) are reported from 

the Clarno. 

CII.\I.LIS \'OI.C.\:\'ICS (ITY) OF SOUTH-CENTRAL IDAHO 

In south-central Idaho east of the Idaho batholith 

the early Tertiary Challis Volcanics overlie Paleo~ zoic rocks (Ross, 1962, p. 73-102). They cover an 

ar~~ of 3,000 square miles (8,000 km2 ); they were 

onginally even more extensive but have now been 

fragmented by deformation and erosion. For the 

mos~ part they were spread out on a nearly level 

erosion surface of the Paleozoic rocks but some 

higher peaks projected, adjoined by bouidery basal 

conglomerate. The Challis Volcanics were originally more extensive to the south, across the site of 

the Snake River Plain, and the remnants of lower 

Tertiary volcanics in the ranges of northernmost 

Nevada are probably their former extensions· the 

intervening downwarp and rift of the Snake River 

Plain, thickly filled by younger volcanics did not 

begin to develop until the Miocene. ' 

The Challis Volcanics are 2,500 to more than 

5,000 feet (750-1,500 m) thick. Subdivisions can be 

recognized in most areas. The thickest and most 

persistent unit is the latite-andesite member at the 

base. Above this member are flows of basalt and 

calcic andesite, commonly interbedded with tuff the 

most conspicuous of which are in the Ge;mer 

Tuffaceous Member in the upper part. 

In the north west part of the volcanic area, a more 

al~red phase of the lavas was formerly distinguished as the Casto volcanics, thought to be of 

Permo-Triassic age, but it is now interpreted to be 

?n. altered phase of the Challis, produced in prox-

~mity to the large early Tertiary granodiorite pluton 

In that area; only one volcanic unit is thus present. 

The Challis Volcanics contain fossil plants at 

many localities and levels, assigned to the Eocene 

and to the Oligocene by various paleobotanists. A 

few vertebrate remains, reported to be of Miocene 

age, occur. Lead/alpha determinations on intrusives in the Challis Volcanics yield Eocene ages of 

40 to 50 m.y. 

The floras are a product of a sub-alpine conifer 

forest that lived at an altitude of about 4,000 feet 

(1 ,200 m), which indicates the existence of a 

highland along the central a~ds of the Cordillera ' 

between lower country to the east and west (Axelrod, 

1968). As a result of later deformation, the early 

Tertiary volcanics north of the Snake River Plain 

have been raised 5,000 feet (1,500 m) higher, to 

altitudes of 10,000 feet (3,000 m) or more. 

WESTERN MONT ANA 

Many small areas of lower Tertiary volcanics 

occur in western Montana, both within the Rocky 

Mountains and in outlying mountain groups in the 

Great Plains to the east. They are probably remnants of formerly more extensive volcanic fields 

now much reduced by erosion, whose extent i~ suggested by the even wider scatter of early Tertiary 

hypabyssal intrusions, especially in the region east 

of the Rocky Mountains. 

One of the larger bodies of the lower Tertiary 

volcanics of western Montana is the Lowland Creek 

Volcanics on the western flank of the Boulder 

batholith near Butte (Smedes, 1962), which covers 

an area of 800 square miles (2,000 km2) and has a 

thickness of nearly 6,000 feet (1,800 m). The volcanics lie on an eroded surface with 1,000-2,000 feet 

(300-600 m) of relief cut on the Boulder batholith 

and the associated Elkhorn Mountain Volcanics. 

The Lowland Creek is a sequence of quartz latitic 

volcanic rocks, divisible into as many as six units, 

each separated by unconformities, some of which 

involved block-faulting and other deformation. The 

units include ash-flow sheets, volcanic breccias, 

quartz latite lava, and quartz latite vitrophyre, and 

th~y are cut by many small contemporaneous 

intrusives. The lower part contains a few nondiagnostic plants (once thought to be of Oligocene 

age) and has yielded early Eocene K/ Ar ages of 

48-50 m.y. 

East of the Rocky Mountains intrusives occur in 

the Crazy Mountains, Castle Mountains, Little Belt 

Mountains, Highwood Mountains, Bearpaw Mountains, Little Rocky Mountains, and Sweetgrass 

Hills. These are mostly near-surface stocks and 

laccoliths, and in some of the areas, notably the 

Highwood and Bearpaw Mountains, effusive equivalents are preserved. The igneous rocks range in 

composition from ordinary rhyolites, andesites,and 

syenites, to strongly alkalic rocks rich in potash and 

soda (Larsen, 1940). The igneous rocks east of the 

Rocky Mountains are generally credited to be of 

Eocene age. 

ABSAROKA VOLCANIC FIELD, WYOMING AND MONTANA 

One of the major lower Tertiary volcanic areas in 

the western United States is the Absaroka volcanic 

field in northwestern Wyoming and southern Mon-
LOWER TERTIARY \'OI.CANIC ROCKS (IT\') 53 

tana, which forms the high, deeply dissected volcanic plateau of the Absaroka Mountains with a 

northwestward extension into the Gallatin Range. 

The volcanic rocks, called the Absaroka Volcanic 

Supergroup (Smedes and Proskta, 1972), have an 

area of 10,000 square miles (2,500 km2), and a 

volume of 7,000 cubic miles (29,000 km3 ). The 

volcanic area extends southeastward for 160 miles 

(250 km), with a width of as much as 60 miles 

(95 km). Toward the southwest, the deeply eroded 

surface of the Absaroka rocks is overlapped by the 

young~r volcanics of Yellowstone National Park, 

and toward the northeast they border the Precambrian massif of the Beartooth Mountains, which 

probably formed a barrier to the eruptions. Eastward and southeastward the volcanics rise in 

imposing erosional escarpments that overlook the 

Bighorn and Wind River basins, and the volcanics 

were originally more extensive in these directions. 

Thicknesses of the volcanic rocks in continuous 

sections are as much as 6,500 feet (2,000 m), and the 

total composite thickness probably exceeds 12,000 

feet (3,650 m). 

The Absaroka volcanic rocks are principally calcalkalic andesite, with some dacite flows and breccias and minor potassic lavas of the shonkinite 

suite. In the area east of Yellowstone Park, the 

volcanics fall into two groups, each with a lower 

acid breccia, a middle basic breccia, and an upper 

set of basalt sheets (Rouse, 1937). These have 

traditionally been called the early acid breccia, 

early basic breccia, early basalt sheets, late acid 

breccia, late basic breccia, and late basalt sheets. 

Later, the units of the sequence were formally 

named (Smedes and Proskta, 1972, p. 10-11); the 

early acid breccia is the Washburn Group, the early 

basic breccia and basalt sheets are the Sunlight 

Group, and the late acid breccia, late basic breccia, 

and the late basalt sheets are the Thorofare Creek 

Group. The acid breccias are light colored and 

consist of hornblende andesite and hornblendemica andesite. The basic breccias are dark-colored, 

owing to their higher content of ferro-magnesian 

minerals. 

The Washburn Group constitutes much of the 

sequence in the Gallatin Range, but farther southeast it has a patchy distribution owing to the 

irregular surface on which it was laid and to erosion 

of its top, so that it is no more than 1,000 feet 

(300 m) thick. The early basic breccia of the 

Sunlight Group is much more prominent, more 

widely distributed, and thicker, reaching 5,000 feet 

(1,700 m) near the eruptive centers. It is a complex of 

agglomerates, volcanic conglomerates, tuffs, and 

mudflows. The early basalt sheets, which average 

1,000 feet (300 m) thick, occur throughout the 

eruptive field. The late acid breccia of the Thorofare 

Creek Group is as much as 2,500 feet (760 m) thick, 

and the late basic breccia as much as 2,500 feet 

(825 m). The late basalt sheets are preserved only as 

remnants on the high divides. 

In the Gallatin Range to the northwest, where the 

rocks are mostly Washburn Group, flows dominate 

over breccias (Chadwick, 1970), but at the southeast 

end of the field in Wyoming, the sequence in the 

Thorofare Creek Group is dominantly sedimentary 

and volcaniclastic (Love, 1939, p. 73-85). Here, the 

group is represented by the Tepee Trail Formation 

and Wiggins Formation. 

The Absaroka volcanics were erupted from a 

series of centers extending northwestward through 

the field. The centers lie in two belts which are 15 

miles (25 km) apart in the Gallatin Range and 

diverge southeastward to 35 miles (55 km) in the 

Absaroka Mountains. The vent areas were composite stratovolcanoes and shield volcanoes composed 

of flow breccias, lava flows, mudflows, and avalanche debris, with steep primary radial dips; the 

vents were intruded by stocks, plugs, laccoliths, 

and radial swarms of dikes. Away from the vents, 

the rocks pass into low-dipping volcanic conglomerate and breccia, volcanic sandstone, and siltstone, 

and air-fall tuff. 

The Absaroka volcanic rocks lie stratigraphically 

over the non-volcanic early Eocene Wasatchian 

beds in the nearby Bighorn and Wind River basins, 

but they are themselves early Tertiary. They contain abundant plant remains and some vertebrate 

fossils and have been dated radiometrically. This 

evidence indicates that the Washburn Group is late 

Wasatchian and early Bridgerian in age, and that 

the Sunlight and Thorofare Creek Groups are of 

Bridgerian and Uintan age. Earlier interpretations 

that the upper part of the Absaroka sequence 

extends into the Oligocene have now been disproved. 

Genetically related to the volcanic activity is the 

Heart Mountain detachment thrust in the Paleozoic 

rocks beneath the volcanics along the northeast 

side of the Absaroka Range (Pierce, 1957). Here, 

Ordovician to Mississippian carbonates have become detached from the underlying Cambriun 

strata and have moved southeastward, probably by 

gravity sliding; on the ea~t edge of the mountains 

they ride up and over the Wasatch Formation, with 

remnants of the carbonate rocks overlying the 

Wasatch in Heart Mountain and McCulloch Peaks 

well out in the Bighorn Basin. The upper carbonates 
54 THE CENOZOIC ROCKS 

have moved independently as detached blocks, 

leaving voids that were filled by the early basic 

breccia at the base of the Sunlight Group. Movement of the blocks may have been nearly contemporaneous with the eruptions of the Washburn 

Group, which is scantily preserved in the area. The 

gravity sliding was somehow triggered by the 

volcanism in the Absaroka volcanic field. 

EASTERN GREAT BASIN 

Lower Tertiary volcanic rocks (lTv), mostly Oligocene, form the lower part of the Tertiary sequence in 

many of the ranges in the eastern part of the Great 

Basin, in southwestern Utah and east-central 

Nevada (figs. 8, 9). The volcanics are of several 

contrasting compositions as indicated below, but 

they are not differentiated on the map. In many 

places the lower Tertiary volcanics lie on an erosion 

surface cut on the Mesozoic and older rocks, but in 

others thin units of older Tertiary continental 

deposits intervene. The lower Tertiary volcanics 

wedge out westward and southward, so that younger 

Tertiary volcanics lie directly on the pre-Tertiary 

rocks. Eastward, however, they extend beyond the 

edge of the Colorado Plateau. 

In the western part of the region, the lower 

Tertiary volcanics are lavas of intermediate composition, mostly andesite and dacite, with a thickness 

of about 1,000 feet (300 m). Their extrusion began 

abruptly about 39 m.y. ago and ceased at about 34 

m.y. ago· (McKee and Silberman, 1970, p. 2326). 

Eastward, these lavas give place to and are overlain 

by felsic volcanics. 

The felsic volcanics are mainly rhyolitic ash-flow 

tuffs or ignimbrites, which were spread as broad 

sheets over a nearly level surface for thousands of 

square miles. Successive flows thus form a recognizable stratigraphic sequence that can be traced 

and correlated from one mountain range to another. 

Eruption of the felsic volcanics began about 34 m.y. 

ago and continued from the Oligocene into the 

Miocene. 

In the Iron Springs district of southwestern Utah 

the basal ash-flow tuff sequence, or Needles Range 

Formation, lies on the non-volcanic Claron Formation (Txc of Geologic Map) (Mackin, 1960, p. 99-100; 

Anderson and Rowley, 1975, p. 10-13). The Claron, 

about 1,500 feet (450 m) thick, is a continental 

deposit of clastics and interbedded limestone. It is 

lithically similar to and probably equivalent to the 

so-called "Wasatch Formation" at the south end of 

the High Plateaus of Utah to the east, which is of 

Paleocene and Eocene (Flagstaff and Green River) 

age. The age of the Claron itself is undetermined, 

but it probably includes Paleocene and Eocene. The 

Needles Range Formation, which 'lies on it conformably and interfingers with it laterally, has a radiometric age of 29-28 m.y. (Oligocene). It consists of 

two to five flows that are recognizable over an area 

of about 13,000 square miles (33,000 km2) in Utah 

and Nevada. To the west, the Needles Range is 

underlain by two other ash-flow units, the Stone 

Cabin and Windous Butte Formations, probably 

equivalent to the upper part of the Claron and 32 

m.y. or more old (Cook, 1965, p. 12-16). Above the 

Needles Range are other ash-flow units, one of 

which, the Isom Formation, has an age of about 

25 m.y., or late Oligocene. The higher ash-flow units 

of the Great Basin, of Miocene and later ages, are 

discussed under a later heading. 

SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO 

The San Juan Mountains of southwestern Colorado are a lofty, deeply dissected volcanic plateau 

with an area of about 9, 700 square miles (25,000 

km2). Mter discovery of precious metals in the 

mountains in the 1870's, the region was investigated by many geologists, especially by the staff of 

the U.S. Geological Survey who produced a set of 

geologic folios. The results were summarized in a 

classic report by Larsen and Cross (1956). Much 

new work has been done in the last few decades, 

resulting in many revisions and reinterpretations; 

for a summary, see especially Lipman, Steven and 

Mehnert (1970). 

The San Juan volcanic field developed over the 

Uncompahgre-San Luis geanticline, an intermittently positive axis that was raised during late 

Paleozoic time, and again in Late Cretaceou~ and 

Paleocene time as a part of the Laramide orogeny. 

During the Laramide orogeny, andesitic volcanism 

occurred along the axis. Although most of the 

andesitic rocks were eroded, their detritus occurs in 

the Paleocene and Eocene deposits of the San Juan 

basin. During the late Eocene, the eroded surface of 

the Precambrian along the axis was covered by the 

thin, non-volcanic Telluride and Blanco Basin 

Formations; the main period of volcanism was later. 

Initial eruptions from numerous volcanoes produced voluminous intermediate lavas and breccias 

(lTv), mostly of alkali andesite, rhyodacite, and 

mafic quartz latite that make up the San Juan, Lake 

Fork, and Conejos Formation. The early volcanics 

blanketed most of the field and have a volume of 

more than 10,000 cubic miles (40,000 km3 ) (fig. 10). 

These eruptions occurred between 35 and 40 m.y. 

ago, or during the early Oligocene. 

The intermediate volcanics are overlain by great 
LO\\'ER TERTIARY \'OLCANIC ROCKS (l'h) 55 

lTv - Lower Tertiary volcanics Tmv- Miocene volcanics 

l-;_----_1 ______ --,-______ _ I , i 

I I 

I I 

I I 

I I 

I I 

I I I . •i ,, J 

." I I 

-~, / i "' . I I 

, , "· r----- " I 

, 

, 

Tpv - Pliocene volcanics Qv - Quaternary volcanics 

0 100 200 MILES I I I 

0 100 200 Kl LOMETE RS 

FIGURE 8.-Maps of Great Basin in Nevada and adjacent States showing distribution of volcanic rocks of successive Cenozoic ages as 

represented on Geologic Map of the United States. Notice the apparent outward progression through time of volcanic activity 

from a center in east-central Nevada and adjacent Utah. 

sheets of quartz latite and low-silica rhyolite ashflow tuffs (lTf). More than 15 separate sheets 

derived from calderas in the western and central 

San Juan Mountains have been recognized. The 

ash-flow eruptions all occurred within 5 m.y., 

between 30 and 25 m.y. ago. Although areally 

extensive, their volume is only about half that of the 

older intermediate lavas. Four of the largest cal-
56 THE CENOZOIC ROCKS 

~-------- _l j NEVADA - ----.---~UTAH __ _ 

I : I 

I I 

I I I .... ; . . . . . . . . . . I 

i : . :. ·.·:~·~··. 

I ;: .·· .: 1

1

.·::- "' · .. · .. . ~ ... · 

'""' .. . :· . . .. -1 

c.~ I 

</' I 

~~ I ~.-}-

~"' I 

'"' ~iuzoNA --

.· 

.. 

A '"'', i 

30-40 m.y. 

' 1--·;-----·-· _ _l ___ -----------

. ! . . . i 

I I 

I .I I _._.-- -- --.. ·. .j i ).-:·· ··· ... _, 0 I • h 

I I\ . L 

" 1: 

'"'·. _/·r " I '""". . ! 

:"' ' '·. I ' .. " .. ~------ '"'. ,. 

c '"' 

10-20m.y. 0 100 

I 

~ 

-·r--·- _j . --- --------r------ ' I I I , I 

. I I I 

I .. ' ···~, 

I . · .. : I\ 

i . . .:. . ! J 

'""' ·_ .. { ""' ·=···. .·. •' , I 

""-. .. . i ' • • II ' .. , I """ L-- -----

'" I B 

20-30 '""' m.y. i 

·! 

r--~-------L _______ r-_____ _ 

j f j I I 

I . I I 

I I 

i ,' ·l . j. i) 

,""': A ... ""-., . i ""' I 

.. '"' ! 

, I """ .. ' '""" r-------

D '""" i 

200 Ml LES 

I 

0- 10 m.y. 

0 100 200 KILOMETERS 

FIGURE 9.-Maps of Great Basin in Nevada and adjacent States showing the distribution of radiometric dates on felsic volcanic rocks ot 

successive ages and illustrating the same outward progression of activity shown in figure 8. Original "core" area is outlined by 

dotted line. Note that the units used here are not exactly comparable to those of figure 8 but represent arbitrary 10 my intervals. 

(Mter Armstrong and others, 1969.) 

deras are shown on the Geologic Map but there are 

many others (fig. 10). Those in the western part of 

the field formed the San Juan volcanic depression 

about 25 miles ( 40 km) long, in which are nested the 

Silverton and Lake City calderas. Those in the 

central part formed a cauldron complex 50 miles 

(80 km) long and 15-25 miles (25-40 km) wide, 

containing six or more partly coalesced calderas. 
LOWER TERTIARY \'OI.CA:\:IC ROCKS (II\) 

390r1T09_a ________ -=G~'r_a_nd __ Ju_n_c_ti_on--~10 -o--------------------~ o------------------~--~106° 

I 

I . 

I 

I 

I 

I 

I 

• 

I I 

I 

38°r--

l 

I 

I 

I I 

I I 

I 

I 

C1) 

c 

C1) 

u 

0 

.!:!l 

0 

{ 

EXPLANATION 

Late basalt 

and rhyolite 

Lavas 

-

erupted concurrently with 

ash-flow tuffs 

Ash-flow tuffs 

Early intermediate 

lavas and breccias 

0 

Caldera 

0 10 I I I 

20 MILES 

I 

FIGURE 10.-Map of lower and middle Tertiary volcanic rocks in San Juan Mountains area, southwestern Colorado, 

showing distribution of principal units and calderas (after Lipman and others, 1970). 

The ash-flow sequence is interbedded with and 

overlain by intermediate to silicic lava flows and 

breccias, the youngest of which is the Fisher Quartz 

Latite, erupted about 25 m.y. ago (near the end of 

the Oligocene). 

Minor eruptions, which continued into the Miocene are represented by erosional remnants on the 

crests of the mountains. These, and the Pliocene 

volcanics of nearby areas, are discussed under later 

headings. 

The San Juan volcanic field was originally much 

more extensive than its present limits. An outlier 

with a nearly identical volcanic sequence occurs in 

the West Elk Mountains north of the canyon of the 

Gunnison River. The spread of the volcanics was 

limited on the northeast by the uplift of the Sa watch 

Mountains, and partially on the southeast side by 

the Precambrian massif of the Needle Mountains. 

To the northwest, west, and southwest, the volcanics spread widely. Hints as to their former extent 

are afforded by congeneric intrusions in the Mesozoic and older rocks. Eastward, the field is covered 

by later Tertiary deposits (Tpc) in the downwarp of 

the San Luis Valley, but its edge probably coalesced 

with the Oligocene volcanic fields of the southern 

Front Range and Wet Mountains (see below). 

OTHER AREAS OF LOWER TERTIARY VOLCANICS IN 

SOUTHERN ROCKY MOUNTAINS 

The San Juan volcanic field is the largest of many 

areas of lower Tertiary volcanics in the Southern 

Rocky Mountains of Colorado. At the close of the 

early Tertiary eruptions, lava flows and volcaniclastic sediments probably blanketed most of the 

present mountainous areas south of the central 

Front Range. There were outlying volcanic areas to 

the north, in the Never Summer Mountains of the 

northern Front Range and the Rabbit Ears Range 

between North and South Parks (Steven, 1975). 

The largest of the outlying areas is the Thirty-
58 THE CENOZOIC ROCKS 

nine Mile field in the southern Front Range (Epis 

and Chapin, 1968) with an area of 1,500 square 

miles (3,900 km2). The field was spread over an 

erosion surface of low relief cut on Precambrian 

rocks in Eocene time following the Laramide 

orogeny. Initial eruptions from a cauldron in the 

northern part of the field were andesite flows and 

breccias with a late Eocene age of 40 m.y ., or 

somewhat earlier than the first eruptions in the San 

Juan field to the west. Later eruptions from centers 

farther south first produced andesite flows and 

pyroclastics, followed by silicic ash-flow tuffs with 

an early Oligocene age of about 34 m.y. They were 

spread northeastward into a lake dammed by the 

eruptions in which the Florissant Lake Beds accumulated. Minor andesitic eruptions continued into 

the Miocene, from which 19 m.y. dates have been 

obtained. 

A smaller area of lower Tertiary volcanics occurs 

in the Silver Cliff and Rosita Hills districts on the 

west slope of the Wet Mountains to the south. 

Eruptions were from local cauldrons, and as in the 

other fields, include both andesite flows and rhyolitic ash-flow tuffs. The oldest are 40 m.y. old (late 

Eocene), but the bulk of the volcanics is Oligocene, 

with ages of 38-33 m.y. 

Extensive andesitic eruptions in the Mineral Belt 

of the central Front Range during the Laramide 

orogeny contributed large quantities of andesitic 

debris to the Late Cretaceous and Paleocene formations in the Denver and North Park basins to the 

east and west, but few outcrops of the andesite 

remain in the source areas. Volcanism was renewed 

during the Oligocene, but except for a few remnants 

of flows, most indications of the event are in the 

associated intrusives, which were formerly confused 

with the Laramide intrusives (Kg3) that occur in the 

same area. The large Mount Princeton pluton in the 

Sawatch Range to the west has been dated radiometrically as late Eocene or early Oligocene and 

probably was the root of a volcanic center of that 

time whose surface rocks have been eroded. 

The lower Tertiary volcanic fields of the Southern 

Rocky Mountains may have formed over a large 

subjacent batholith or chain of batholiths (Steven, 

1975, p. 88-91). A series of pronounced gravity lows 

extending northeastward from the San Juan Mountains to the Mineral Belt of the central Front Range 

is probably a geophysical expression of the batholith. 

SOUTHWESTERN ARIZONA 

Lower Tertiary volcanic rocks (lTv) are shown on 

the Geologic Map in many of the desert ranges of 

southwestern Arizona, along with the more prevalent Miocene and Pliocene volcanics (Tmv, Tpv). The 

older volcanics of the region were classed on the 

Geologic Map of Arizona (1969) as Cretaceous (Kv). 

Cretaceous volcanics occur farther east in the State, 

where they intertongue with fossiliferous strata; 

those farther west were assigned a Cretaceous age on 

the tenuous assumption that they were involved in 

the Laramide orogeny, but most of them are 

certainly Tertiary, as shown by radiometric dating 

at various localities. 

The rocks shown as lower Tertiary volcanics are 

generally more deformed and altered and less freshappearing than those of younger ages, and in many 

places they are separated by angular unconformities, but they are probably of diverse ages. 

In northwestern Arizona south of Boulder Dam, a 

17,000-foot (5,200 m) sequence of Tertiary volcanics 

overlying the Precambrian basement is divided into 

the Patsy Mine, Golden Door, and Mount Davis 

Volcanics (Longwell, 1963, p. 18-29); the lower part 

of these is represented as lower Tertiary volcanics on 

the Geologic Map. The Patsy Mine Volcanics are a 

plausible candidate for lower Tertiary, because they 

are more deformed and altered than the higher units. 

However, radiometric determinations give little 

comfort to the notion. Radiometric ages determined 

on the Patsy Mine are between 14 and 18 m.y. 

(Miocene), except for a doubtful 40 m.y. date in the 

lower part (Anderson and others, 1972, p. 275). To the 

west in the Mojave Desert region of California, no 

volcanics older than Miocene are known. 

Much farther southeast, in the Tucson Mountains 

west of Tucson, the lowermost volcanic unit (Cat 

Mountain Rhyolite), shown as lower Tertiary on the 

Geologic Map, has yielded Cretaceous ages of 65-70 

m.y. (Bikerman and Damon, 1966, p. 1228). 

In the intervening area, the designation of lower 

Tertiary for the older volcanics is generally more 

appropriate, as Oligocene dates of 26-29 m.y. have 

been reported from a number of localities, as in the 

Sierrita Mountains and the Yuma area. Compositions range from andesite to rhyolite. 

MOGOLLON PLATEAU, SOUTHWESTERN NEW MEXICO 

The Mogollon Plateau is a volcanic field covering 

more than 20,000 square miles (50,000 km2) at the 

southeast corner of the Colorado Plateau in southwestern New Mexico and adjacent Arizona. On the 

Geologic Map its rocks are shown as Miocene and 

Pliocene volcanics (Tmv, Tpv), with minor areas of 

lower Tertiary volcanics (lTv) along the southeast 

edge. This representation was based on an interpretation of units shown on the Geologic Maps of New 

Mexico (1965) and Arizona (1968). On the New 
LO\\'FR TERTIARY \'OLCANIC ROCKS (lTv) 59 

Mexico map, the older volcanics are classed as Datil 

Formation (Td), divided into various lithologic 

facies, and on the Arizona map as "intermediate 

volcanic rocks" (Tvi); these were assigned a Miocene 

age, following prevailing opinion. The younger 

basaltic rocks are shown on both maps as QTb; these 

were assigned a Pliocene age. 

Further studies of the field were in progress at the 

time of map compilation and suggested important 

revisions of age assignments, but specific details did 

not become available until the Geologic Map was in 

press. The new data, supported by radiometric 

determinations, indicate that in general the rocks 

designated as Miocene on the Geologic Map are 

actually Oligocene (lTv), and those designated as 

Pliocene are actually Miocene (Tmv) (fig. 11) (Elston 

and others, 1973; Elston and others, 1976). 

The volcanic rocks of the Mogollon field comprise 

three overlapping magma suites: (a) an early calcalkalic andesite to rhyolite suite with ages of about 

29-43 m.y.; a high-silica alkali rhyolite suite with 

ages of about 21-31 m.y.; and (c) a basaltic suite, 

contemporaneous with the other two suites, but 

continuing from about 37 m.y. to the present, but 

reaching its peak during and just after the highsilica alkali rhyolite eruptions. The volcanic rocks of 

the Mogollori field evolved in a single direction, and 

not in cycles. Calc-alkalic rocks appeared early and 

then faded out, becoming scarcer just as the highsilica alkali rhyolite became abundant; basaltic 

andesite and basalt occur at many stratigraphic 

levels. 

The rocks of the rhyolite suite are believed to have 

originated in a large subjacent batholith. They were 

erupted from a series of cauldrons and volcanictectonic depressions, mostly confined to the Mogollon Plateau, some more than 35 miles (55 km) in 

diameter. They are bordered by broad ring-fracture 

zones. Within the cauldrons ash-flow tuffs exceed 

3,000 feet (1 ,000 m) in thickness, but they thin in the 

outflow sheets. Ring-fracture zones are characterized by domes and flows of flow-banded rhyolite 

associated with moat and flank deposits of bedded 

pumice. Lower temperature pumice leaked out along 

ring fractures and filled moats around resurgent 

domes. Lake beds, landslide blocks, and conglomerate beds are common in the moats. 

During the main period of eruption, the region was 

tectonically quiet. Following the eruptions, Basin 

and Range structure, produced by regional tension, 

began to develop about 21 m.y. ago, but the blockfaulting thereby created was confined to the 

periphery of the plateau. Basalts are interbedded in 

the lower part of the Gila Formation that was 

deposited in the block-faulted depressions. Very little 

of these later basalts was erupted within the plateau 

itself. 

TRANS-PE<:OS TEXAS 

In the southwestern part of the trans-Pecos region 

of west Texas, 240 miles (380 km) southeast of the 

Mogollon field, a broad lower Tertiary volcanic field 

extends south westward across the Rio Grande into 

Mexico. The volcanic field includes the Davis 

Mountains on the northeast, extends westward into 

the Sierra Vieja or Rim Rock area, and southward 

into the Chisos Mountains of the Big Bend area. 

Large parts of the field have been studied in recent 

decades, mostly by the staff of the Bureau of 

Economic Geology and by students of the University 

of Texas, the results of which have been summarized 

and correlated by Maxwell and Dietrich (1970). A 

notable feature of the volcanic rocks is their high 

alkalic content, which is especially evident in the 

associated intrusions. 

The volcanic rocks were erupted onto an irregular 

erosion surface of Cretaceous and Paleozoic rocks, 

from several centers, each providing a succession of 

volcanic and volcaniclastic units, having a wide 

variety of local names. These sequences have been 

correlated by widespread layers of ash-flow tuff, of 

which the Mitchell Mesa or Brite Tuffs are most 

persistent, and have been used as a base-line for 

correlation by Maxwell and Dietrich (1970, pl. 1). On 

the Geologic Map this base-line has been adopted for 

separating lower Tertiary volcanics (lTv) from 

Miocene volcanics (Tmv). Although this separation 

is convenient for map purposes, it is misleading as to 

age, for the rocks below the base-line are mainly 

Eocene, and those above it mainly Oligocene. 

The oldest Tertiary rocks in the trans-Pecos region 

are in the Big Bend area surrounding the Chisos 

Mountains. Here, most of the former extent of the 

volcanics has been eroded, leaving only remnants in 

the higher mountains. Congeneric intrusive stocks 

abound in the Cretaceous of the surrounding lower 

country. East of the Chisos Mountains the Cretaceous rocks are succeeded by the nonvolcanic Black 

Peaks, Hannold Hill, and Canoe Formations, which 

contain Paleocene, lower Eocene, and middle Eocene 

vertebrates, respectively. Thin basalt flows occur in 

the Canoe Formation, but the main volcanic 

sequence consists of the conformably overlying 

Chisos and South Rim Formations, totaling 3,600 

feet (1,100 m) thick. The Chisos is mainly volcaniclastic, but it includes widely traceable basalt 

members; the South Rim is massive rhyolite flows 

and associated flow-breccia. A few late Eocene 

mammals occur in the lower part of the Chisos 
60 THE CENOZOIC ROCKS 

Q 

Q 

~ 

~Tof ~@ ~ 

@'pT miT 

~ <qJ) 

~ .c:'fE!> I 

0 10 20 30 40 MILES 

I I I I I I I I I I 

0 10 20 30 40 50 KILOMETERS 

FIGURE 11.-Map of Mogollon Plateau and vicinity, southwestern New Mexico showing distribution of.the Cenozoic volcanic units 

and calderas. The representation is a recent revision and differs in important respects from than on Geologic Map of the United 

States. (Compiled from Elston and others, 1976, supplemented by Geologic Map of New Mexico, 1965). 
llPPER TERTIARY \'OLCANIC ROCKS (Tmv. Tpv) 61 

EXPLANATION 

Alluvium 

Basaltic lava 

J{ 

j{ 

Santa 

~ Fe and 

Gila Formations 

Basaltic andesite 

Ill 

c: 

Ill 

u 

0 

.!!!> 

5 

Felsic ~ volcanic rocks 

(Rhyolite lava and ashflow tuff) 

[E] 

Volcaniclastic 

rocks 

Basaltic andesite 

Felsic volcanic rocks 

(Rhyolite lava and ashflow tuff) 

W!Yi~)J Volcaniclastic 

rocks 

Andesite 

>-

Precambrian, B Paleozoic, }~ ~ and Mesozoic rocks E-;' 

~ 

.---.. 0:: 

/ " c.. Outlines of 

cauldrons and calderas 

Contact 

Fault 

Dashed where approximate 

or concealed 

FIGURE 11.-Continued. 

Formation, and it has yielded radiometric dates of 

42-31 m.y., or late Eocene to early Oligocene. The 

South Rim Formation has been dated as 30-29 m.y. 

A different but partly equivalent volcanic sequence occurs in the sou them Davis Mountains to 

the north, where thick volcaniclastic units separated 

by thinner, widely traceable flows and ignimbrites 

project in prominent escarpments and cuestas. The 

volcanic components thicken and merge northward 

toward eruptive centers in the core of the Davis 

Mountains. The upper part of the sequence, approximately equivalent to the South Rim Formation, 

includes the Mitchell Mesa Ignimbrite, and at the top 

the Rawls Basalt, both of which are extensive. 

The succession in the Rim Rock area farther west 

is notable for its abundant vertebrate remains, 

which have been correlated with radiometric dates 

from associated volcanic layers (Wilson and others, 

1968). The lowest tuff unit contains a Uintan 

vertebrate fauna, and volcanic units directly below 

and above are 40 and 38 m.y. old respectively. The 

four higher vertebrate faunas represent different 

parts of the Duchesnian Stage, near the EoceneOligocene boundary, and associated volcanic layers 

have yielded radiometric ages of 38-34 m.y. Above 

the vertebrate-bearing beds is the base-line unit of 

Brite Ignimbrite with an early Oligocene age of 33-

29 m.y. Mafic dikes which cut the volcanic rocks of 

the area have Miocene ages of23-18m.y. (Dasch and 

others, 1969). 

One of the youngest units in the volcanic sequence 

is the Perdiz Conglomerate, which forms extensive 

gravel hills and plains east of the Rim Rock and 

south of Marfa. It is composed of clasts of a wide 

variety of volcanic rocks eroded from the older units, 

as well as clasts of Cretaceous rocks. It contains a 

few Tertiary fossils, but its precise age is undetermined. 

UPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv) 

Upper Tertiary volcanic rocks form nearly twothirds of the surface area of the Cenozoic volcanic 

fields in the western United States (figs. 12, 13) and 

cover a span extending from about 26 m.y. to about 2 

m.y. ago, or about 23 m.y. On the Geologic Map the 

volcanics are differentiated by age as Miocene and 

Pliocene volcanics (Tmv, Tpv), in places on rather 

tenuous evidence, but in this account it is convenient 

to consider them together, area by area, rather than 

by age. 

The upper Tertiary volcanic rocks form a large and 

complex assemblage. On the Geologic Map, a 

compositional distinction is made between upper 

Tertiary andesites (uTa) in the Cascade Range of the 

Northwestern States and felsic volcanics (Tmf, Tpf), 

mainly rhyolites and ash-flow tuffs, in the region to 

the southeast. The remaining undifferentiated 

volcanics (Tmv, Tpv) are dominantly basaltic but 

include mixed associations and some small areas 

of felsic volcanics. 
62 

--

-~-

THE CENOZOIC ROCKS 

-r-;------- : ' --------...... I t --------------

!, 

\ 

'L, 

) 

r-·---------------------

' I 

I \ 

I I 

\ 

r·---------------------; '\.-.-~.:'.:·------------ I ._ ---· ~ 1 

I 

I 

I 

----... I 

--,---------: 

I 

. : 

I 

I I 

I ~-

.------------------,_----,~ 

,, 

\ 

' ' \ I 

I 

L------.--- I I ·-----.... ______ -L. ______ • 

.!. ·-

.. 

I 

,---------------------

1 

L 

'---" , ___ , 

'·"-·:r·-..... _.-

\ \ 

'-, 

FIGURE 12.-Map of western United States showing distribution of Miocene volcanic rocks as represented on Geologic Map 

of the United States (units Tmv, Tmf, uTa). 
-

I 

I 

·-......·------------~-t . .. ~' 

I 

_;' 

UPPER TERTIARY VOLCANIC ROCKS (fmv, Tpv) 

-------------------------------~r------------

1 

I 

I 

-,---· 

\ 

\, 

\ 

'\ 

~ L----------------------:; C·\ : .'1 

) ! : ·- ,----------------------, \ 

t.. \-.-~-r-.: I : 

----·, U./ ·r. ! I ,\_ ..... v : l .___.. .. ! : ~ 

.,.- ,· I -----------, ·-·--,· I ~ :------- - ', 

-!.L . .- I • • l I \ -~f' -~------: \. ,. I J ·; " . I 

~ ' : I ~.,.~ .. i•l~ .• • , t ~ L-----,-----------------_._·---·-: 

-

~. f,;- I• I ~ \ ----------------- • '=-~~~ • 1 ~ t~ ~·.. r·-- ~ ,,. ! l . ,, . \ 

, .. ,......__ .. -=.-;., I 1 :s.') .,.... i 

-~~~~ ..... ~'7~ \ -~ . !----- •. ,b...-~ i ____________________ _ . • -.., ,. ~ ------------r--------- ~ ---...... - i-- ------ l ~-,_ , .... \ . i .. i---- \ 

·'·-,~~~4,. tl' .. ! ~ , : \ 

\•• 

.... •., ( . ' 

j I l .. ~. - __ ....,,_ ...... ':{--"-.·' 

{ ·~·. ~~- _____ j .... ~4! · ...... , i'li .. ~ 

··.J ' ' 

· . ....._ ' ! r··----C_------------ _______ : ___ ; ......... 

...... 

\ __ ...... _ -, 

I '. 

\ 

' \ 

' '-, 

\ 

' 

FIGURE 13.-Map of western United States showing distribution of Pliocene volcanic rocks as represented on Geologic Map 

of the United States (units Tpv, Tpf). 

63 
64 THE CENOZOIC ROCKS 

The passage from early Tertiary to late Tertiary 

time is characterized by a change from a dominantly calc-alkalic intermediate or andesitic regime 

to one of more varied volcanism (Christiansen and 

Lipman, 1972, p. 279-284). Basaltic fields are 

extensive, notably in the Columbia River Plateau to 

the northwest, but there are also bimodal basaltrhyolite fields, as in the Great Basin, as well as 

differentiated basaltic and alkalic fields, and complex fields. The change from the earlier to the later 

volcanic regimes was not simultaneous across the 

region, but appears to have begun earlier in the 

south, toward the end of the Oligocene, and to have 

progressed northward in Miocene time and later. 

Onset of the later volcanic regime was a manifestation of the dominant tensional tectonic state that 

began in the western part of the United States 

during the later Cenozoic. It was accompanied at 

least in its earlier phases, by extensional Basin and 

Range block-faulting. The earlier mixed volcanics 

spread indiscriminately across an unfaulted terrain, 

but later volcanics were closely related to the 

development of block-faulted basins and ranges. 

Ultimately, this tectonism and volcanism is related 

to plate motions on the Pacific Ocean floor to the 

west (Christiansen and Lipman, 1972, p. 279-284). 

UPPER TERTIARY ANDESITES OF WASHINGTON (uTa) 

The andesitic volcanism of the Cascade Range in 

Washington, Oregon, and California, which began 

during the early Tertiary, continued during the later 

Tertiary and the Quaternary. The later Tertiary 

andesitic eruptions built the High Cascades east of 

the lower western Cascades that had been formed 

by the older andesites. 

In Washington, the upper Tertiary andesites form 

smaller, more disconnected areas than those farther 

south. Early in the century they were designated the 

Keechelus Andesite by Smith and Calkins (1906) 

from exposures in the Snoqualmie quadrangle on 

the east side of the range. These geologists recognized, however, that their unit was apparently 

composite, with a body of older altered andesitic 

volcanics, separated from younger, much fresher 

volcanics by an obscure unconformity. Subsequently, the name Keechelus has been extended to 

other parts of the Cascade Range in Washington, 

and misapplied to a considerable variety of andesitic units, ranging in age from Eocene to Pliocene 

(Waters, 1961a, p. 43-47). 

Much later, volcanics equivalent to the lower 

Keechelus were subdivided and named in Mount 

Rainier National Park, southwest of the Snoqualmie quadrangle (Fiske and others, 1963, p. 20-

30). The lower subdivision, the Stevens Ridge 

Formation, lies unconformably on the Ohanapecosh Formation, described previously (p. 51), and is 

about 3,000 feet (900 m) thick. In consists of flows 

and volcaniclastic rocks, generally light-colored 

and quartz-bearing. Ash-flows are abundant in the 

lower part and give place upward to coarse volcaniclastic rocks. The Stevens Ridge contains a middle 

Oligocene to lower Miocene oreodont and other 

fossils which indicate about the same age. The 

overlying Fife Peak Formation, about 2,400 feet 

(730 m) thick, consists of olivine basaltic and 

andesitic lavas, darker colored than the Stevens 

Ridge, with minor interbedded rhyolite flows and 

volcaniclastic rocks. No fossils have been found in 

the Fife Peak, but it lies stratigraphically between 

the late Oligocene to early Miocene Stevens Ridge 

Formation, and the late Miocene Yakima Basalt 

that overlaps it on the east. 

The Stevens Ridge and Fife Peak are intruded by 

the Tatoosh granodiorite pluton which forms the 

foundation of the Mount Rainier volcano; this 

pluton is equivalent to the Snoqualmie Granodiorite that intrudes the Keechelus rocks of the 

Snoqualmie quadrangle. Both the Tatoosh and 

Snoqualmie plutons were explosively unroofed late 

in their history and gave rise to associated andesitic volcanics. These products have largely been 

eroded in the Mount Rainier area but are extensively preserved in the Snoqualmie area, where they 

are the upper unit of the Keechelus Andesite. Unlike 

the lower Keechelus, which underlies the Yakima 

Basalt, the upper Keechelus overlies it, and interfingers eastward with the clastics of the Ellensburg 

Formation of Pliocene age. 

lTPPER TERTIARY ANDESITES OF OREGON Ar\D 

CALIFORNIA (uTa) 

The upper Tertiary andesites of Oregon and 

California form a broader, more continuous outcrop 

band than in Washington, and they are mainly 

younger or largely of Pliocene age (Williams, 1942, 

p. 17-19; Macdonald, 1966, p. 71-74). 

The High Cascades of Oregon form a lofty plateau 

surmounted by steep-sided shield volcanoes of 

olivine basalt and olivine-bearing andesite, largely 

of Pliocene age, above which rise the still higher 

Quaternary composite andesitic cones that form a 

chain along the crest. The Qua ternary volcanics 

form only a small part of the volume of the High 

Cascades; the preceding Pliocene volcanics were far 

more voluminous. The earlier eruptions were largely 

quiet effusions of mafic lava of striking uniformity. 

Part of the lava escaped from fissures along the 

western border of the belt and flowed westward 

66 THE CENOZOIC ROCKS 

Snake River area farther east. The last two units are 

probably nearly contemporaneous but differ somewhat in composition. They are separated from the 

overlying Yakima by unconformities with slight 

angular discordances. The Yakima Basalt, in turn, 

is divisible into three unnamed members (Wright 

and others, 1973, p. 371-374), the lower two of which 

correspond to the original Yakima Basalt. The 

upper member, preserved only in the center of the 

field, is more or less equivalent to the sedimentary 

rocks of the Ellensburg Formation on the west side. 

Radiometric ages by the K/ Ar method of the 

basalts of the Columbia River Group indicate that 

they were extruded during an interval of about 9 

m.y. The Picture Gorge Basalt has been dated 

between 21 and 15 m.y., and the Yakima Basalt 

between 20 and 15 m.y. (Gray and Kittleman, 1967), 

or from early to late Miocene. The dates do not seem 

to confirm the considerable hiatus between the older 

and younger basalt formations that is suggested by 

the stratigraphic evidence. The highest member of 

the Yakima, which is equivalent to the Ellensburg 

Formation, probably extends into the early Pliocene. 

The basalts of the Columbia River Group are 

monotonously uniform tholeiites, composed of 

pigeonite, labradorite, and magnetite-rich glass, 

and virtually without olivine. They differ little in 

composition through 3,000-foot (900-m) sections. 

There are, nevertheless, subtle compositional differences between the successive stratigraphic units 

(Waters, 1961b, p. 591-604; Wright and others, 1973, 

p. 377 -383). The Picture Gorge and Imnaha flows 

contain about 5 percent olivine, 47-50 percent silica, 

and notably higher percentages of alumina, magnesia, and lime than the younger flows. The Yakima 

flows contain 53-54 percent silica, much potash and 

titania, and very little olivine. The flows of the 

highest member are again olivine-bearing. 

The Columbia River flows were erupted from 

fissures rather than from volcanoes; they were 

produced in a tensional regime. Several dike swarms 

have been identified, mostly on the southeast and 

south sides of the plateau, from which the basalt 

flowed north ward. The largest of these is the Chief 

Joseph (= Grand Ronde) swarm in northeastern 

Oregon, where there may be as many as 21,000 

north-northwest-trending dikes that extend for 165 

miles (265 km) or more (Taubeneck, 1970; Waters, 

1961b, p. 586). Another group, the Monument 

Swarm, occurs farther west along the John Day 

River on the north side of the Blue Mountains uplift. 

There are smaller swarms along the edge of the field 

to the west and northwest. The dikes are prominently exposed on canyon walls and in roadcuts but 

are difficult to trace across the intervening forested 

uplands, where they are much weathered. They are 

therefore incompletely mapped and are only inadequately represented on the Geologic Map, where a 

few of the many dikes of the Chief Joseph and 

Monument swarms are shown. 

As a result of the northward flow of the basalt, 

marginal lakes and sedimentary basins were ponded 

against the adjacent highlands, producing the 

Latah Formation on the north and Payette Formation on the east, both of Miocene age, and the 

somewhat younger Ellensburg Formation on the 

west. The basalts also flowed westward into western 

Oregon and Washington through a sag in the older 

volcanics of the Cascade Range near the site of the 

present Columbia River, where they assumed a 

pillowed subaqueous phase along the Pacific shoreline of the time and mingled with other Miocene 

basalts derived from local centers (Snaveley and 

others, 1973, p. 388-391). 

SOUTHEASTERN OREGON AND NORTHEASTERN 

CALIFORNIA 

South of the basalt field of the Columbia River 

Group, beyond the Blue Mountains uplift, Cenozoic 

volcanic rocks underlie all the surface in southeastern Oregon and in the Modoc Plateau of 

northeastern California as far as northern Nevada. 

Unlike the Columbia River basalts, these volcanics 

are much broken by Basin and Range block-faulting 

and are much more varied, both in composition and 

age. Miocene volcanic rocks emerge in the higher 

uplifts, followed by Pliocene volcanics and volcaniclastic sediments, and in the lower areas by 

Quaternary volcanics (Walker, 1970; Macdonald, 

1966, p. 89-95). 

The Miocene includes several thick bodies of 

basalt (Tmv), such as the Steens Basalt of Steens 

Mountain in southern Oregon, and the Warner 

Basalt of the Warner Range in northeastern California. These units are also identifiable in adjacent 

ranges, although it is doubtful that they formed a 

continuous sheet. In California, moreover, the term 

Warner Basalt has been used for basalts at different 

stratigraphic levels, some of which are Pliocene. 

The Steens Basalt is as much as 5,000 feet (1,500 m) 

thick in its type area, but the Warner Basalt is 

thinner. The Steens Basalt has been dated by the 

Kl Ar method as 15-16 m.y. old, or about the same 

age as the main part of the Columbia River Group, 

but it and the other basalts of the region are 

dominantly alkali olivine basalts, rather than 

tholeiitic like the Columbia River lavas; some of the 

flows are nonporphyritic high-alumina basalts. 
lJPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv) 67 

Individual flows are mostly thinner than those of 

the Columbia River, being mostly 20 feet (6 m) thick 

or less, and columnar jointing is rare. The Strawberry Volcanics on the south flank of the Blue 

Mountains uplift are mainly basaltic like the rest, 

but they pass upward into more felsic dacitic and 

rhyolitic lavas (Tmf). 

The overlying rocks, mainly Pliocene, are a more 

complex assemblage, formed while block-faulting 

was in progress. They include sheet basalts and 

basalts erupted from shield volcanoes (Tpv), some 

dated at 9 m.y., and also large volumes of felsic 

volcanic rocks (Tpf), which form tuffs, flows, and 

domes. In places these pass into volcaniclastic 

sedimentary rocks (Tpc). There is, however, a 

notable absence of andesites and other intermediate 

volcanics. For the most part, the subdivisions of the 

Pliocene rocks have not been named, but the 

Danforth and Harney Formations have been distinguished in southern Oregon, where they are 

applied to mafic and felsic tuffs and volcaniclastic 

sediments that overlie the Steens Basalt. In northeastern California is the Alturas Formation, largely 

of lacustrine origin, composed of diatomite, tuffaceous silty and sandy shale, and sandstone, which 

contains Pliocene plants and mammals. Westward, 

the Pliocene volcanics mingle with the east edges of 

the ur 'ler Tertiary andesites of the Cascade Range. 

SNAKE RIVER PLAIN OF SOUTHERN IDAHO 

East of the region just discussed is the Snake 

River Plain, which extends across southern Idaho. 

The plain is a downwarp transverse to the earlier 

Cordilleran structures, extending northwestward in 

its western part and northeastward farther east, 

with a length of nearly 350 miles (550 km). It is 

entirely a late Cenozoic feature, probably caused by 

crustal rifting, and has been filled thickly by 

Pliocene and Quaternary volcanic rocks and associated sediments, beginning with felsic volcanics 

(Tpf) and changing to mafic later (Tpv, Qv) (Malde 

and Powers, 1962). 

The initial part of the sequence is the Idavada 

Volcanics, which are exposed in a wide band in 

southwestern Idaho, sloping northward toward the 

axis of the plain. Small outliers project from beneath 

the younger lavas on the north side of the plain, and 

in smaller areas along the edges farther east, 

suggesting that the ldavada extends beneath the 

axis of the plain. In southwestern Idaho the 

Idavada is as much as 3,000 feet (900 m) thick and 

lies unconformably on older mineralized volcanics, 

largely of Miocene age. The ldavada Volcanics 

consist of welded ash-flow tuffs and rarer lavas that 

are partly silicic latite and partly rhyolite. A 

prominent component is the Cougar Point Tuff that 

forms the rim of the south-facing cuestas along the 

Nevada border, overlooking the older volcanics 

(Coats, 1964, p. 13-15). The sedimentary parts of the 

formation in the western part of the plain contain 

fossil plants, land snails, and diatoms of lower 

Pliocene age, but fossils of apparently somewhat 

younger age occur farther east. 

Overlying the ldavada Volcanics in the uplands 

south of the western part of the plain is the Banbury 

Basalt of the Idaho Group (the various subdivisions 

of the group have already been described in the 

section on "Idaho Group"). The Banbury is as much 

as 900 feet (290m) thick, but it thins southward to a 

feather edge along the Nevada border. Mammals 

and other fossils in sedimentary intercalations are 

of middle Pliocene age, and the basalt has yielded a 

radiometric age of 9 m.y. Overlying volcanic rocks 

of the Snake River Plain are of Quaternary age. 

GREAT BASIN OF NEVADA AND ADJACENT STATES 

Miocene and Pliocene volcanic rocks (Tmv, Tpv) 

form an extensive and complex assemblage in the 

Great Basin of Nevada, and extend into Utah and 

California. Important parts of the assemblage are 

felsic (Tmf, Tpf); they are shown separately on the 

Geologic Map. Felsic eruptions began in eastern 

Nevada and southwestern Utah in the late Oligocene, as described in the section on "Eastern Great 

Basin," and progressed westward and southwestward during Miocene and Pliocene time (Armstrong 

and other, 1969; McKee and Silberman, 1970; McKee 

and others, 1971; Silberman and McKee, 1972). 

The Tertiary volcanic sequence in central and 

western Nevada falls into three groups: andesitic 

and dacitic flows with ages of 36 to 34 m.y. (included 

on the Geologic Map in the lower Tertiary volcanics 

(lTv); rhyolite ash-flows with ages from 34 to 20 m.y. 

(divided on the map between lower Tertiary and 

Miocene volcanics, lTv, Tmf); and alkali basaltic 

and andesitic flows with ages less than 17 m.y. 

(mainly shown as Tpv). The first two groups were 

spread widely over a featureless terrain; the third 

group accompanied and followed the beginning of 

block-fault~ng and the development of Basin and 

Range structure. 

The sequence of volcanic rocks in Lander and 

Churchill Counties is typical of central Nevada 

(McKee and Silberman, 1970; McKee and Stewart, 

1971). The lower Tertiary andesite and dacite flows 

(lTv) are followed by Miocene ash-flow tuffs (Tmf) 

more than 1,500 feet (450 m) thick, which consist of 

a succession of ash-flows or cooling units. These are 
68 THE CENOZOIC ROCKS 

widely traceable from range to range but were 

derived from different centers and wedge out laterally in various directions, so that not all the units 

occur in single sections. The oldest formation is the 

Edwards Creek Tuff, composed of five cooling units, 

with an age of 27 m.y. The Bates Mountain Tuff, 

commonly composed of three cooling units, has an 

age of 24 m.y.; and the New Pass Tuff, a single 

cooling unit, has an age of 22 m.y. In northern 

Lander County the oldest tuff formation is the 

Caetano Tuff, which is preserved in an east-west 

belt that extends across three or four of the ranges. 

In northern Lander County, the ash-flow tuff 

sequence is followed unconformably by the upper 

volcanic sequence (Tpv); here, basalt and basaltic 

andesite flows as much as 1,000 feet (300m) thick, 

with ages of 15-10 m.y., stand in mesas and cuestas 

that overlook the older volcanics. These basalts and 

andesites are closely related to the volcanics adjoining the Snake River Plain to the north, already 

described. 

In western Nevada the Miocene ash-flow deposits 

are scantily developed, and most of the volcanic 

rocks belong to the younger sequence (Tpv). They 

were erupted from local centers, where they may 

attain thicknesses of several thousand feet, but 

their total volume is less than that of the earlier 

volcanics. Their composition is varied, with dominant calc-alkaline andesite and dacite, but including some flows of rhyolite and alkali-olivine basalt. 

They include the volcanic sequence in the Virginia 

and Carson Ranges (Thompson and White, 1964, p. 

12-18), and farther south those in the Bodie Hills, 

the Goldfield Hills, and the Silver Peak Range 

(Robinson and others, 1968). The best known 

sequence, in the Virginia and Carson Ranges, has 

at the base the Alta Formation, of soda trachyte and 

hornblende andesite, with some fossil plants of 

Oligocene or Miocene age, followed by the thicker 

and more varied mass of the Kate Peak Formation, 

composed largely of hornblende andesite flows and 

flow-breccias, with some flows of basalt and rhyolite, and much tuff-breccia. Hydrothermally altered 

parts of the Alta and Kate Peak Formations were 

the hosts of the fabulous Comstock Lode of the 

Virginia City district. The Kate Peak is apparently 

intergradational with the Alta Formation, and its 

upper part interfingers with the sedimentary 

Truckee Formation of the adjoining basins. Fossil 

plants indicate that the Kate Peak is largely of 

Pliocene age (Tpv). The Kate Peak and Truckee are 

overlain unconformably by the basalts of the 

Lousetown Formation that is largely Pleistocene 

(Qv). 

A different sequence of upper Tertiary volcanic 

rocks occurs in southern Nevada, where they form 

an extensive and nearly unbroken field centering in 

the Nevada Test Site of southern Nye County and 

attain a thickness of as much as 40,000 feet 

(12,000 m) (Ekren, 1968). Miocene ash-flow tuffs 

crop out around the edges, but the most extensive 

surface rocks are Pliocene ash-flow tuffs (Tpf) that 

were erupted from local calderas, two of which, the 

Timber Mountain and Black Mountain calderas, are 

still expressed at the surface and are shown on the 

Geologic Map. Older calderas have been covered by 

the younger volcanics and are known only from 

drilling and geophysical surveys. Radiometric ages 

by the K/ Ar method range from 27 m.y. (early 

Miocene) for the oldest ash-flow tuffs to 6 m.y. 

(Pliocene) for the youngest (Marvin and others, 

1970). Minor basalt flows (Tpv) overlie the youngest 

ash-flows and are related to the development of the 

Basin and Range structure. 

SIERRA NEVADA AND COAST RANGES OF CALIFORNIA 

On the long western slope of the Sierra Nevada, 

between its crest and the Great Valley, are many 

erosional remnants of Tertiary volcanic rocksabundant in the north, sparse in the south-which 

are parts of the "Superjacent Series" of the early 

reports which lies unconformably on the "Bedrock 

Series" of the Mesozoic and earlier rocks. The 

Superjacent Series includes accumulations that 

formed during a prolonged period, from Eocene to 

Pliocene, and in the northern Sierra, at least, was 

originally a much more extensive cover. The superfacent rocks were laid down on a surface that sloped 

westward like the present range, but much more 

gently, as much of the great tilting and uplift of the 

Sierra occurred during the late Pliocene and early 

Pleistocene, before which the surface was more or 

less confluent with that of the Great Basin to the 

east. Most of the earlier Tertiary deposits, although 

significant, are of small extent so that on the 

Geologic Map the superjacent rocks are largely 

indicated as Pliocene volcanics (Tpv), themselves 

generalized and reduced in area at the expense of 

the bedrock formations. 

The oldest Tertiary deposits, of Eocene age, occur 

only in the northern Sierra Nevada. The lone 

Formation (Tee) of the Sierra foothills, a continental 

and littoral deposit, extends upslope into the Auriferous Gravels, which were laid down in the 

channels of vanished rivers. They are followed by 

various basaltic and andesitic units of Eocene and 

Oligocene age (Durrell, 1966, p. 187-189), a few of 

which are shown on the Geologic Map as lTv. 
UPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv) 69 

Much more extensive, both in the northern and 

central Sierra Nevada, are the Miocene and Pliocene volcanic formations (Durrell, 1966, p. 189-192; 

Slemmons, 1966). In the central Sierra they include 

the rhyolitic Valley Springs Formation, the andesitic Relief Peak Formation, the latitic Stanislaus 

Formation, and the andesitic Disaster Peak Formation, the first two Miocene, the last two Pliocene, as 

indicated by radiometric dating and fossil plants. 

An interesting feature of the younger lavas are the 

"Table Mountains" which follow the sinuous courses 

of ancient river beds but now rise above the 

surrounding terrain. In the foothills the equivalent 

of the younger volcanics is the Mehrten Formation, 

composed largely of andesitic debris. 

In the southern Sierra Nevada, Tertiary volcanic 

rocks form only widely spaced outliers (Bateman 

and Wahrhaftig, 1966, p. 139-145), a few of which 

are shown on the Geologic Map (Tpv). Probably 

they were never much more extensive, as this part of 

the Sierra had much more erosional relief during the 

Tertiary than farther north, perhaps 4,000-6,000 

feet (1,200-1,800 m); the flows accumulated in 

valleys below the higher summits. The flows are 

mainly basaltic and andesitic, and different outliers 

have been dated radiometrically as between ·9 m.y. 

and 2 m.y. 

In the Coast Ranges west of the Sierra Nevada 

upper Tertiary volcanic rocks were erupted from a 

number of widely spaced centers and interfinger 

laterally with the contemporaneous Tertiary sediments, some of which themselves contain much 

volcanic debris. 

In the southern Coast Ranges and farther south, 

the volcanics are mainly of lower Miocene (preMonterey) age. In the western Santa Monica Mountains of the Transverse Ranges is a Miocene 

volcanic pile 5,000 feet (1,500 m) thick of submarine 

andesites and basalts, with extensive associated 

shallow intrusives. The volcanic field extends westward into the Channel Islands and is well represented on Santa Cruz Island. Much farther south, 

San Clemente Island is largely formed of subhorizontal Miocene lava flows. Farther north, in the 

southern Coast Ranges west of the San Andreas 

fault, are the Miocene volcanics of the Pinnacles 

area in the Gabilan Range, composed of rhyolite 

flows, tuff, and breccia; and the lower Miocene 

Mindego Formation of the Santa Cruz Mountains of 

basalt, pillow lava, and tuff, with an age of about 

23 m.y. East of the San Andreas fault in the southern 

Diablo Range (not far from the Pinnacles area) are 

the middle Miocene andesite and basalt flows and 

agglomerates of the Quien Sabe area. 

Farther north in the Coast Ranges, east of the 

San Andreas fault, the volcanics are younger. In the 

Berkeley Hills east of San Francisco Bay, the 

Berkeley Group at the top of the thick nonmarine 

Pliocene sequence consists of several persistent lava 

units, separated by clastic units; it has been dated 

by the K/ Ar method as between 12 m.y. and 5 m.y. 

old. Closely related to the Berkeley Group, although 

somewhat younger, are the more extensive Sonoma 

Volcanics north of the bay, of andesite, basalt, and 

rhyolite flows, tuffs, breccias, and agglomerates, 

with minor pumice and obsidian, and water-laid 

volcaniclastic sediments. The Sonoma Volcanics 

have yielded an age of about 3 m.y. 

SOUTHERN ROCKY MOUNTAINS 

Upper Tertiary volcanics are less extensive in the 

Southern Rocky Mountains than the more voluminous lower Tertiary volcanics already described, but 

they occur in widely separated areas. 

West of the Rocky Mountains in northwestern 

Colorado are many erosional outliers of basaltic . 

volcanics. They were labeled QTv on the Geologic 

Map of Colorado (1935), but obviously could not be 

Quaternary, as they form mesas on the "top of the 

country," such as Grand Mesa east of Grand 

Junction and the Flat Tops in the northern part of 

the White River Plateau. On the Geologic Map they 

are classed as Pliocene (Tpv), but later geologic 

work indicates that the outliers have a greater 

range of later Tertiary ages (Larson and others, 

1975). 

The rocks shown as Tpv on the map are mainly of 

two age groups. The older volcanics are of Miocene 

age, with radiometric ages between 24 and 20 m.y., 

and interfinger laterally with the Browns Park 

Formation. A younger group is of late Miocene and 

early Pliocene age, with radiometric ages between 

15 and 9 m.y. Both groups are represented in the 

Flat Tops outlier in the northern White River 

Plateau, but only the second occurs in Grand Mesa 

farther west. A third group about 8 m.y. old was 

erupted after considerable uplift and faulting had 

occurred in the region. Minor Pleistocene basalts 

(not shown on the map) occur in some of the 

canyons of the area. 

The Miocene volcanics at the top of the sequence 

in the San Juan Mountains have already been noted 

(p. 57). These, and the earlier Oligocene volcanics, 

preceded the downwarping and downfaulting of the 

Rio Grande depression east of the mountains, and 

its local representative, the San Luis Valley. Formation of the depression was accompanied by a new 

cycle of basaltic volcanism, beginning in Pliocene 
70 THE CENOZOIC ROCKS 

time in the north, and continuing into Quaternary 

time farther south (Lipman and Mehnert, 1975). The 

most extensive flows, the Servilleta Formation 

(Tpv) are flat-lying olivine tholeiites which spread 

over the Santa Fe Formation in the depression to a 

thickness of 650 feet (200m) to form a broad plateau 

west of Taos, New Mexico, and have been dated as 

between 4.5 and 3.6 m.y. 

In the Great Plains east of the Sangre de Cristo 

Mountains in northeastern New Mexico and southeastern Colorado a unique volcanic field developed 

in late Cenozoic time in a region that had had little 

prior igneous history (Stormer, 1972). Along the 

New Mexico-Colorado boundary, the lofty Raton 

Mesa and Mesa de Maya are capped by olivine 

basalt flows with a Pliocene age of about 8 m.y. 

(Tpv); the eastern tip of the volcanic outliers extends 

into the west corner of Oklahoma. During the 

Quaternary basalt flows covered the area to the 

south, each flow lying on successively lower erosion 

surfaces. Eruptions in this field were probably 

genetically related to those that accompanied the 

formation of the Rio Grande depression to the west. 

VOLCANIC FIELDS OF COLORADO PLATEAll 

The west and south edges of the Colorado Plateau, 

southwest of the Rocky Mountains, in Utah, Arizona, and New Mexico, are marked by a chain of 

volcanic fields erupted from different centers. 

For the northernmost field, that in the High 

Plateaus of southern Utah, little information was 

available at the time of compilation of the Geologic 

Map, except for Major Dutton's report for the 

Territorial Surveys (1880), and local reports on the 

Marysville mining district. Only recently have more 

comprehensive data become available (Anderson 

and Rowley, 1975), which indicate that some revision of representation on the Geologic Map will be 

required. 

On the Geologic Map the volcanics of the High 

Plateaus are shown as of Miocene and Pliocene age 

(Tmv, Tpv). More recent data indicate that although 

the base is Oligocene with a radiometric date of 

30-25 m.y., the bulk is of Miocene age with radiometric dates between 25 and 19 m.y. (Bassett and 

others, 1963; Anderson and Rowley, 1975). The 

overlying sedimentary Sevier River Formation (Tpc) 

and associated basalt flows are Pliocene. 

The broad Oligocene ash-flow tuffs of the Great 

Basin, such as the Needles Range and Isom 

F(}rmations, extend into the western part of the 

High Plateaus, where they are succeeded by varied 

local volcanic accumulations. In the Marysville 

area, where the latter exceed 5,000 feet (1,500 m) in 

thickness, the lower part (Bullion Canyon Formation in the north, Mount Dutton Formation farther 

south) consists of rhyodacite and basalt flows and 

associated shallow intrusives, which pass outward 

into volcanic breccias, ash-flow tuffs, and interbedded sediments. In the upper part of the pile are 

several more persistent named units of basalt, 

rhyolite, and tuff, the youngest of which has been 

dated at 19 m.y. (late Miocene). The outer edges of 

the accumulation cap the Fish Lake and Aquarius 

Plateaus on the east, and the Markagunt Plateau 

and other mountains on the southwest and west. 

Although the edges of the volcanic field have been 

worn back by erosion, its present extent probably 

approximates the original outlines. 

In Pliocene time, the Miocene volcanic rocks were 

broken into separate blocks by the great faults 

along the west side of the Colorado Plateau (the 

Paunsagunt, Sevier, and others), and the continental beds of the Sevier River Formation were 

deposited on the downfaulted blocks. 

On the Geologic Map of Arizona (1969), the upper 

Cenozoic volcanics along the edge of the Colorado 

Plateau are represented as Qv, TQv, and Tv. In 

compiling the Geologic Map, it was assumed that 

the first group was Quaternary (Qv), the second 

Pliocene (Tpv), and the third Miocene (Tmv). In 

general, this classification has been confirmed by 

later more detailed data. 

The plateaus in the western part of the Grand 

Canyon district of northwestern Arizona exhibit a 

sequence of basalts of Pliocene and Quaternary 

ages (shown as Tpv on the map), whose eruptions 

were closely related to the block-faulting and 

canyon cutting (Hamblin, 1970). They are divisible 

into four groups, of which only the first is Pliocene, 

with ages of 6 or 7 m.y. This lava flowed onto a 

plain unrelated to the modern topography and is 

interbedded with stream gravels derived from the 

south; all the block-faulting and canyon-cutting was 

later. The three later groups are more closely related 

to modern drainage, and formed during progressive 

erosional lowering. The youngest basalt, of late 

Quaternary age, poured into the present canyon at 

Vulcan's Throne, temporarily damming the Colorado River. 

Farther southeast in Arizona is an extensive 

complex of volcanic fields extending southward 

from San Fancisco Peaks near Flagstaff to the 

Prescott-Jerome area (McKee and Anderson, 1971). 

The San Francisco Peaks are a much dissected 

volcanic center, whose summit rises to an altitude of 

13,370 feet (4,090 m), or about 6,000 feet (1,800 m) 

above the surrounding plateau. Its eruptions began 
QllATERNARY VOLCANIC ROCKS (Qv. Ql) 71 

during the middle Pliocene, about 7 m.y. ago, 

beginning with basalt, followed by intermediate 

lavas, and ending with Quaternary basalt. In the 

Prescott-Jerome area to the south, an older series, 

the Sullivan Buttes Latite, with radiometric dates of 

26-23 m.y. (late Oligocene and early Miocene) is 

followed by the more extensive Hickey and Milk 

Creek Formations, consisting of basalt flows and 

interbedded continental deposits, with ages of 14-10 

m.y. (late Miocene and early Pliocene), confirmed by 

mammalian bones of Barstovian and Clarendonian 

ages. The edges of the somewhat younger basalts of 

the San Francisco field overlap the formations of 

the Prescott-Jerome field on the north. Blockfaulting succeeded the volcanic epoch, and the 

continental Verde Formation was laid down in the 

ensuing depressions. It contains late Hemphillian 

mammals and its interbedded basalts have an age 

of about 5 m.y. 

The Mogollon field at the southeast corner of the 

Colorado Plateau in New Mexico has already been 

discussed (see section on "Mogollon Plateau, southwestern New Mexico"). It is largely of Oligocene 

age, but the upper part of the sequence is Miocene. 

UPPER TERTIARY VOLCANIC ROCKS OF SOUTHERN HASIN 

AND RANCE I;ROVINCE IN ARIZONA AND CALIFORNIA 

The desert ranges of southern Arizona and 

southeastern California present an array of Cenozoic volcanic rocks lying on an eroded surface of the 

plutonic and metamorphic basement. Some of the 

volcanic areas have been studied in detail, but most 

of them are known only in a general way, and no 

comprehensive summary is available. Many of the 

volcanic rocks in Arizona have been dated radiometrically by Paul Damon and his associates at the 

University of Arizona (Damon and Bikerman, 

1964), and Armstrong and Higgins (1973) have 

reported on radiometric ages in the Mojave Desert of 

California. 

On the Geolo~ric Map of Arizona (1969), the 

volcanic rocks of the southern part of the State were 

placed in five categories. They are listed below, 

along with designations adopted for them on the 

Geologic Map of the United States: 

Geologic Map of Arizona (1969) 

Quaternary volcanics 

(Qb, Qr) 

Quaternary and Tertiary 

volcanics 

(QTv, QTb, QTa) 

Tertiary volcanics 

(Tv, Tvs, Tvi, Tvm) 

Geologic Map of United States 

Quaternary volcanics 

(Qv) 

Pliocene volcanics 

(Tpv, Tpf) 

Miocene volcanics 

(Tmv, Tmf) 

Tertiary and Cretaceous 

volcanics 

(TKv, TKa, TKr) 

Cretaceous volcanics 

(Kv, Kr, Ka) 

Lower Tertiary volcanics 

(lTv) 

Do. 

In general, the age assignments made on the 

Geologic Map of the United States have been 

confirmed by more detailed surveys and by radiometric dating. The age relations of the lower 

Tertiary volcanics have already been discussed (see 

section on "Southwestern Arizona"); most are probably Oligocene, but volcanics of Cretaceous and 

Miocene ages have been included in places. The 

Miocene (Tmv, Tmf) and Pliocene (Tpv, Tpf) volcanics have yielded ages of 25 m.y. and younger at 

many localities. In the Mojave Desert region to the 

west, all the volcanics yield ages of 22 m.y. and 

younger, and lower Tertiary volcanics are absent. 

The different categories of volcanic rocks show 

progressively less deformation with age, and some 

of the groups are separated by unconformities. The 

lower Tertiary volcanics are much deformed and 

mineralized, whereas the upper Tertiary volcanics 

are merely tilted and almost unaltered; the Quaternary volcanics are flat-lying. The Basin and Range 

block-faulting generally began immediately after 

the Miocene. For example, the Galiuro Mountains 

east of Tucson are composed of a thick sequence of 

eastward-tilted Miocene volcanics, which with their 

basement are upfaulted against the Pliocene deposits in the San Pedro Valley to the west. 

QUATERNARY VOLCANIC ROCKS (Qv, Qf) 

Quaternary volcanic rocks occupy notably smaller 

areas than the lower and upper Tertiary volcanic 

rocks, but their distribution is significant in that it 

reflects the areas of latest crustal activity in the 

Cordillera (fig. 15). One belt extends southward 

along the Cascade Range of the Northwestern 

States in a series of volcanic cones that form its 

crest. To the east is a transverse belt centering in 

the downwarp or rift of the Snake River Plain and 

extending eastward into Yellowstone National Park 

and westward into central Oregon. Another more 

disconnected belt of Quaternary volcanics extends 

from northeastern New Mexico, across the Rio 

Grande depression, and into the southern Colorado 

Plateau of northern Arizona. Smaller areas occur 

elsewhere, notably along the edge of the Sierra 

Nevada in eastern California. 

On the Geologic Map all the Quaternary volcanics 

are grouped together. It would have been instructive 

to separate the lower Pleistocene, upper Pleistocene, 

and Holocene volcanics, but we believe that this is 
72 THE CENOZOIC ROCKS 

'<; ~'5 ------- '~\~ . --------- "-- ;--;-------- • ! i -------------------------·-r-··-----------------;--· 

I ~- I \ 

l..a.. \ I \ ~ .«rW ! 'L, 

,_' ·---------------t, I I \ 

.: ; i L---------------------7 . . , ' --'. : -, )4~ >' --------------------~ \ 

.. .. : ...... - \· • fl.! ~ ': ·1. ~- ------- ~ . \' I -r·------.L________ l i ~------------------,_-------\ 

i ~---------: I '- . : I . ' 

: i I : j i l _______________________ _._ ______ • 

I : j .: _., I . ---------------- 1. : \-----

·,_ I 

~: : I : : 

\, I : I • ',, : "' I r ' I : , .. , --, r----------------.-----------------------_1 ____________________ _ 

,, '," ,-\.: ! -~~----------; l . ~ ,I' ' 

•. '- · I t1i.a ... ' I I 

J ·-,) -.If I ~ • : \ 

> .. ...L~ I L ___ ·---~ .. ~-:t·V -' .. . ,...,-r l~ ~~· • • l --------~,.!' ~ •• 

• 

-

- ' . --~ " ' . . . ' I ~ --,--, ________ ~ ___ r··- __ c_~-----------J 

\ 

\ 

' '-, I 

t 

\ 

FIGURE 15.-Map of western United States showing distribution of Quaternary volcanic rocks as represented on Geologic 

Map of the United States (units Qv, Qf). 
QllATERNARY VOLCANIC ROCKS((.!\, Qf) 73 

more appropriate on larger-scaled maps of special 

areas. Most of the Quaternary volcanics are basaltic, but those of the Cascade Range are andesitic, 

and there are significant areas of felsic volcanics 

(differentiated as Qf), notably in Yellowstone Park, 

along the edge of the Sierra Nevada and around the 

Valles Caldera in the Jemez Mountains of northern 

New Mexico. 

Ai\:DFSITIC \'OI.CANICS OF CASCADE RAN(;E 1:\: 

WASIIJN(;TON, ORFGON, AND CALIFORNIA 

The Pliocene and older andesites of the High 

Cascades in the Northwestern States (uTa) are 

crowned by a chain of lofty volcanic cones that were 

built by eruptions during Quaternary time (Qv)-

Glacier Peak, Mount Baker, Mount Rainier, Mount 

St. Helens, and Mount Adams in Washington; 

Mount Hood, Mount Jefferson, Mount Washington, 

"Mount Mazama" (Crater Lake), and Mount McLoughlin in Oregon; Mount Shasta and Lassen Peak 

in California. Forty miles (64 km) east of the main 

chain in central Oregon is the shield volcano of 

Newberry Caldera. The volcanoes project to imposing heights; Mount Rainier rises to 14,408 feet (4,392 

m), and Mount Shasta to 14,162 feet (4,317 m); some 

of the others are nearly as lofty. 

The geology of the High Cascades volcanoes is 

dealt with in many reports. Fiske, Hopson, and 

Waters (1963) have described Mount Rainier. Most 

of the fundamental work on the southern volcanoes 

is by Howel Williams (1932a, b, 1942, and many 

other papers). Higgins and Waters (1967) have 

reported on Newberry Caldera. 

Growth of the Cascade volcanoes began well back 

in the Pleistocene, and some of them have a complex 

glacial as well as a volcanic history. Important 

eruptions continued well into the Holocene. Mount 

Mazama "blew its top" in a great eruption 6,600 

years ago, creating a gaping hole that was filled by 

the waters of Crater Lake, and deposited a layer of 

ash widely toward the northeast. Mount St. Helens 

was built only a few thousand years ago, and 

Lassen Peak erupted vigorously in 1914 and 1915-

the only active volcano in the Forty-eight States. 

Many of the other volcanoes are merely dormant, as 

shown by modern fumarolic activity. 

The volcanoes are surrounded by small to large 

areas of Quaternary volcanics (Qv). In many of the 

northern ones the lavas extend little farther than 

the bases of the cones, but in California the 

Quaternary lavas spread well east of the Cascades 

in the Medicine Lake Highlands and Modoc Plateau 

(Macdonald, 1966, p. 83-94). Newberry Caldera is at 

the summit of a broad shield volcano, much more 

extensive than the steep-sided cones of the main 

Cascades. 

Most of the Quaternary lavas of the Cascade 

Range are pyroxene andesites, but in some of the 

cones there is also subordinate basalt and dacite. 

The Quaternary volcanics east of the main range 

are less andesitic and more varied. The Newberry 

center includes basalt and rhyolite, and several 

prominant late flows of obsidian. Similar rocks are 

also prevalent in the Medicine Lake Highlands and 

Modoc Plateau in California. 

BASALTS OF SNAKE RIVER PLAIN (Qv) 

In the downwarp or rift of the Snake River Plain 

is the Idaho Group, Pliocene and early Pleistocene, 

consisting of sediments and interbedded basalt 

flows, which is succeeded by the Pleistocene and 

Holocene Snake River Group of basalt with minor 

sedimentary layers (Malde and Powers, 1962, p. 

1212-1217). The Snake River Group forms most of 

the surface in the eastern half of the plain and less 

extensive areas farther west. The individual units 

are discontinuous, partly because the flows were 

derived from different centers and partly because 

they spread on an irregular topography formed 

during the progressive downcutting of the Snake 

River and its tributaries. Drill holes indicate that 

the rocks of the group attain a thickness of 900 feet 

(275 m) or more near the central axis of the plain. 

The Snake River Group is divided into the 

Madison Basalt, Sugarbowl Gravel, Thousand 

Springs Basalt, Crowsnest Gravel, Sand Springs 

Basalt, Bancroft Springs Basalt, Melon Gravel, and 

Holocene lava flows. The gravels were derived from 

streams flowing onto the plain from surrounding 

highlands, probably during times of excessive 

runoff during the glacial periods; the Melon Gravel 

at the top was derived from catastrophic floods 

resulting from rapid draining of Lake Bonneville to 

the southeast. The basalts are all olivine-bearing, 

and different units each comprise several flows, 

most of which have conspicuous columnar jointing. 

In the areas between the present streams, the older 

basalts underlie level surfaces, which are covered by 

windblown silt and are heavily farmed. The Holocene flows are fresh and unaltered and lack sJil 

cover. At the Craters of the Moon, about midlength 

in the Snake River Plain, a cluster of very recent 

craters follows a north-south fracture in the older 

lavas. 
74 THE CENOZOIC ROCKS 

FELSIC VOLCANIC ROCKS OF Y~LLOWSTONE NATIONAL 

PARK (Qf) 

East of the term in us of the Snake River Plain 

is a very different body of Quaternary volcanic 

rocks, largely of felsic composition (Qf) in the 

high plateau of the Yellowstone National Park 

area (Christiansen and Blank, 1972). It covers an 

area of 2,500 square miles (6,500 km2) and radiometric dating indicates that all the rocks are less 

than 2 m.y. old, the youngest being about 70,000 

years. 

The first eruptions were great sheets of ash-flow 

tuff that spread widely over the terrain and culminated explosively about 600,000 years ago with 

the collapse of an enormous central caldera 25 by 45 

miles (40 by 70 km) across in the area northwest of 

the present Yellowstone Lake. Because of later 

eruptive cover, the caldera is little expressed in 

modern topography; its original configuration is 

marked by a dashed and dotted line on the Geologic 

Map. The ash-flow tuffs erupted from it are exposed 

mostly in the uplands surrounding the caldera 

itself. By contrast, the smaller, nearly contemporaneous Island Park caldera west of Yellowstone 

Park is a prominent topographic feature. 

Mter the early ash-flow eruptions and the collapse 

of the caldera, its site was buried by a younger series 

of rhyolitic lava flows whose upper surface forms 

the present Yellowstone Plateau. Late in the eruptive cycle, minor sheets of basalt spread out in areas 

beyond the caldera. 

Following the end of eruption, the volcanic 

plateau was heavily covered by ice during the 

Wisconsin Glacial Stage (minor earlier glacial 

deposits are interbedded in places with the volcanics). The region remains a thermally active area, 

as attested by its numerous hot springs and geysers. 

EASTERN BORDER OF SIERRA NEVADA, CALIFORNIA 

A smaller, but significant area of Quaternary 

felsic volcanism lies along the eastern base of the 

Sierra Nevada between Mono Lake and the head of 

Owens Valley to the south, where eruptions occurred 

from about 3 m.y. ago to less than 13,000 years ago 

(Gilbert and others, 1968, p. 288-299). The volcanism was genetically related to the growth of 

tectonic-volcanic depressions in the Mono Lake 

basin and Long Valley farther south. 

The felsic eruptions and development of present 

structures began about 3 m.y. ago. About 700,000 

years ago the Bishop Tuff spread out as an ash-flow 

sheet from a center near Long Valley, flowing both 

southward into the head of Owens Valley near 

Bishop and northward into the Mono Lake basin. It 

overlies the Sherwin Till of Illinoisan or Kansan 

age. After eruption the tuff was faulted and warped, 

producing a fairly steep slope from Long Valley 

toward Owens Valley. 

Events following the eruption of the Bishop Tuff 

include the growth of Mono Craters and other 

rhyolite and dacite domes on the west and south 

sides of Mono Lake during late Pleistocene and 

Holocene time. They were extruded as stiff magma 

which formed steep-sided domes and craters. 

The Mono Lake basin and Long Valley are 

structural depressions, as indicated by their strong 

negative gravity anomalies. The depth of their 

volcanic and sedimentary fill has been variously 

interpreted as more than 10,000 feet (3,000 m) and 

only a few thousand feet, depending on what 

assumptions are made as to the densities of the 

filling rna terial. 

CLEAR LAKE AREA, CALIFORNIA COAST RANGES 

The Clear Lake area of the northern Coast Ranges, 

about 60 miles (95 km) north of San Francisco 

Bay, records an epoch of Quaternary volcanism 

unique in this part of California (Anderson, 1936). 

The lake itself was created by volcanic damming 

in late Pleistocene or Holocene time. The volcanics 

were erupted onto the eroded surface of older rocksto the east the thick tilted continental deposits of 

the Cache Formation of late Pliocene and early 

Pleistocene age (Tpc), and farther west the Franciscan and other Mesozoic rocks. 

The area is dominated by Mount Konocti, a 

volcanic cone that rises 2,500 feet (760 m) above the 

lake on its south side, which is formed of rhyodacite 

lava and associated tuff and breccia. At the east end 

of the lake are younger flows and cinder cones of 

andesite, dacite, and obsidian. Solfataric activity 

and mineralization of sulfur and cinnabar in the 

lavas and bedrock still continues. Probably related 

to the Clear Lake volcanism is the Geysers geothermal area southwest of the lake, although it is 

not accompanied by eruptive activity. 

JEMEZ MOUNTAINS, NEW MEXICO 

Another area of dominantly felsic Quaternary 

volcanism occurs far to the east, in the Jemez 

Mountains of northern New Mexico (Smith and 

Bailey, 1968). The Jemez Mountains are a late 

Tertiary and Quaternary volcanic edifice, built 

mostly over the Tertiary deposits of the western part 

of the Rio Grande depression, but overlapping 

westward onto Mesozoic, Paleozoic, and Precambrian rocks of the Nacimiento uplift. The volcanics 

have an area of 1,500 square miles (3,900 km2) and a 
TERTIARY INTRUSIVE ROCKS (Ti) 75 

maximum thickness of 5,000 feet (1,500 m). 

Initial eruptions during the Pliocene were dominantly basaltic, but during later Pliocene time 

eruptions progressed from basalt to andesite to 

rhyodacite, quartz latite, and rhyolite. During the 

early Pleistocene they culminated in the great 

ash-flow sheets of the Bandelier Tuff, which were 

erupted from central calderas and spread widely 

over the surrounding terrain, where they are preserved as flat-topped mesas (Qf). Two main phases 

of ash-flow eruptions are dated at 1.4 and 1.0 m.y., 

respectively. The second phase was accompanied by 

the subsidence of the great Valles Caldera, 12-14 

miles (19-22 km) across, still a prominent topographic feature. Small rhyolite domes were built 

inside the caldera, the youngest about 400,000 years 

ago. The region remains a high thermal area, with 

many solfataras and hot springs inside the caldera. 

BASALTIC FIELDS OF NEW MEXICO AND ARIZONA 

Aside from the felsic volcanics just described, all 

the Quaternary volcanism in New Mexico and 

Arizona was mafic. Basalts were erupted from 

various centers, especially across the northern parts 

of the two States, where they form a chain of 

disconnected fields extending from east of the 

Rocky Mountains in northeastern New Mexico, 

westward across the Rio Grande depression, into 

the southern Colorado Plateau of Arizona. The 

basalts flowed over the eroded surface of older 

rocks, and still remain as broad, nearly level plains, 

punctuated here and there by small craters and 

cinder cones. In northeastern New Mexico one of the 

volcanoes, Capulin Mountain, rises 2,000 feet (600 

m) above its surroundings. In some areas the lavas 

spread on successively lower erosion surfaces, 

indicating that several epochs of basaltic eruption 

occurred. Some of the fields lie upon, or are adjacent 

to, areas of late Tertiary volcanism; the westermost 

field of northern Arizona flanks the Pliocene volcanic center of San Francisco Peaks. Other fields 

have little relation to prior volcanic activity. 

In New Mexico, minor basaltic fields like those to 

the north occur at intervals down the Rio Grande 

depression to the Mexican border. Others are 

scattered over the Basin and Range province westward into southwestern Arizona, where one of the 

largest is in the Gila Bend area. 

TERTIARY INTRUSIVE ROCKS (Ti) 

Intrusive igneous rocks of Tertiary age are widely 

distributed in the Cordillearan region of western 

United States (fig. 16), but their areal extent is far 

less than that of the Mesozoic plutonic rocks of the 

same region. Although diverse in both character 

and age, they are shown as a single unit (Ti) on the 

Geologic Map. They include a few coarse-grained 

granitic bodies of batholithic dimensions, notably 

in the northern Cascade Range of Washington, in 

the region of the Idaho batholith of central Idaho, 

and in the Southern Rocky Mountains of Colorado, 

but most are smaller hypabyssal intrusions of 

shallow origin-mainly stocks and laccoliths. Most 

of the intrusive rocks are of early to middle Tertiary 

age, the earlier ones a continuation of the plutonic 

activity of Cretaceous time, the later intrusions 

representing one of more cycles of subsequent 

igneous activity. Some of the Tertiary intrusive 

rocks have already been mentioned in the discussions of the closely related volcanic rocks. 

CASCADE RAN<a~ 

Some of the larger Tertiary intrusive bodies are in 

the northern Cascade Range of Washington, where 

they are embedded in the earlier metamorphic and 

plutonic complex but also invade the overlying 

lower and middle Tertiary volcanic sequence. The 

Chilliwack batholith of quartz diorite and granodiorite straddles the International Boundary, extending north into British Columbia and south into 

Washington (Misch, 1966, p. 140-141). It was emplaced during several episodes; the oldest cuts the 

Paleocene Chuckanut Formation but not the overlying Oligocene volcanics, and the youngest cuts 

the Oligocene volcanics themselves. The younger 

phases have yielded radiometric ages of 30-18 m.y. 

Farther south in the Cascade Range are the 

batholith of Snoqualmie Granodiorite (Smith and 

Calkins, 1906), and the smaller Tatoosh batholith of 

similar composition in the Mount Rainier area 

(Fiske and others, 1963). Both lie mainly in the 

lower and upper Tertiary andesites and were emplaced during Miocene time. They rose to high levels 

in the crust, and late in their histories were 

explosively unroofed, giving rise to andesite flows, 

such as those in the upper part of the Keechelus 

Formation of the Snoqualmie area. Smaller intrusive bodies of similar character occur farther south 

in the andesites of the Cascade Range in Washington and Oregon. 

OREGON COAST RAN(;E 

In the Coast Range of Oregon, mafic intrusives 

are associated with the Eocene pillow basalts and 

the surrounding sediments. Although they occupy 

relatively large areas on the Geologic Map, they are 

merely thick sills. 
76 

... 

-

I 

. I • 4 • 

'--·-,------------ _( 

I 

,J' 

1.., 

/ . ,; 

I 

THE CENOZOIC ROCKS 

----

,' 

-.-;-------

.... 

. .... 

.. .. 

·-~ 

-------------r-------------------.--

I 

I 

I 

\ 

I 

\. 

~----------------------:) ) .. l .. , \.~--~-:'r--~------------------:, . 1 

... . ~ ...•• 

I 

I ~-

------- I I 

f --------~---------' I ,------------------...... _----..._~_ 

., ~---------; \ 

:... I 

I f, . 

I n •. ' ! • I ' • 

.•.. l.-.--- -~---- -.-· ~.:- -------_l..-----: 

: ! 

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~---------------------

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I :J . .. · 

·\, .. \, ;_ .;. : -~--

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··~r 

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,/ 

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I . .. I . : : ''1• ~} l I . . -----·-:::· ---- --~--- -- ; _________ _j---------------------

'1 ·:.·~. 4.. 1---------: 

I 

I 

l , _ __,.. 

- I 

~ (.' 

I 

I 

'---l 

---,..... ·, I • .... • • 

··: . .· , . ·,l . · ·r·"'- ---\:~-·::;--· · ------

·-· .. _; \"". ·.;: 

'-..: \. ·: .. ~:: 

·~~:;__:;:'£,./ ---,_ \ 

'· ........ :.r·-..... _.~ 

\ 

' '-, 

FIGURE 16.-Map of western United States showing distribution of Tertiary intrusive rocks as represented on Geologic Map 

of the United States (unit Ti). 
TERTIARY 11\'TRVSI\'E ROCKS (Ti) 77 

CENTRAL IDAHO 

In central Idaho to the east, the great Idaho 

batholith was the scene of long-continued plutonic 

activity that culminated during the Cretaceous, but 

lesser intrusive activity persisted into the early 

Tertiary, giving rise to several sizeable plutons 

embedded in the older complex. One of the larger 

bodies is in the Casto area of the central part of the 

batholith, where a mass of granite and· quartz 

monzonite intrudes the Eocene Challis Volcanics 

(Ross, 1934, p. 54-67). A radiometric age of 59:m.y. is 

reported from it. Another somewhat smaller body of 

quartz monzonite, nearly surrounded by Cretaceous 

plutonic rocks, forms the Sawtooth Range farther 

southwest (Reid, 1963, p. 14). 

CENTRAL MONTANA 

To the east in-Montana, east of the front of the 

Northern Rocky Mountains, numerous near-surface 

stocks, laccoliths, and satellitic dikes occur in the 

Crazy Mountains, Castle Mountains, Little Belt 

Mountains, Highwood Mountains, Bearpaw Mountains, Little Rocky Mountains, and Sweetgrass 

Hills (Larsen, 1940). In a few places, effusive 

equivalents are preserved. The igneous rocks range 

in composition from ordinary rhyolites, andesites, 

and syenites, to strongly alkalic rocks rich in potash 

and soda-shonkinite, nepheline syenite, and phonolite, almost devoid of plagioclase-with considerable variation from one area to another. The 

igneous rocks are generally considered to be Eocene. 

CALIFORNIA AND GREAT BASIN 

In California and western Nevada, plutonic activity virtually ended during Mesozoic time, except 

for a few widely spaced small Tertiary stocks. From 

central Nevada eastward in the Great Basin, 

however, Tertiary intrusive bodies are mingled with 

those of Mesozoic age and finally dominate at its 

east edge. They include the small intrusions of the 

northern Shoshone Range, with a lead-alpha age of 

50 m.y. (Gilluly and Gates, 1965, p. 60), the larger 

Harrison Pass pluton of the Ruby Range, Nevada 

(Coats and others, 1965), and the Almo pluton of the 

Albion Range, southern Idaho (Armstrong, 1968, 

p. 1305), with Rb/Sr ages of 35 and 30 m.y., 

respectively. 

At the east edge of the Great Basin, in the 

Wasatch Mountains southeast of Salt Lake City, a 

cluster of large stocks of quartz diorite and quartz 

monzonite lie on the western prolongation of the 

Uinta Mountains axis (Crittenden and others, 

1973). The large Little Cottonwood stock rises at the 

mountain front; the smaller Alta and Clayton Peak 

stocks lie within the mountains to the east. Fissiontrack and K/ Ar dating indicate that the stocks are 

progressively younger from east to west-the Clayton Peak being 37-41 m.y. old (late Eocene), the Alta 

32-33 m.y. old (early Oligocene), and the little 

Cottonwood 24-31 m.y. old (early Miocene). 

COLORADO PLATEAU 

The Colorado Plateau east of the Great Basin is not 

notably an igneous area, except for the volcanic 

fields along its southwest and south edges. Within 

the plateau itself, a wide region is without igneous 

rocks, except for laccoliths in a few clusters-in the 

Henry Mountains, the La Sal Mountains, the Abajo 

Mountains, and the Carrizo Mountains. The Henry 

Mountains laccoliths, made famous by the pioneer 

work of G. K. Gilbert, have yielded Eocene K/ Ar 

ages of 44-46 m.y.; the La Sal Mountains laccoliths 

are late Oligocene, with ages of 23-26 m.y. (Armstrong, 1969, p. 2085). 

SOUTHERN ROCKY MOUNTAINS 

The Sa watch Range and the Elk Mountains to the 

west, in the Southern Rocky Mountains of central 

Colorado, contain the only other area in the western 

United States besides those of the Northwestern 

States of truly deep-seated plutonic rocks of Tertiary 

age. These culminate in the large Mount Princeton 

pluton of the southern Sawatch Range, which has 

been dated radiometrically as late Eocene or early 

Oligocene. The Snowmass and Whiterock granodiorite plutons and others in the Elk Mountains are 

apparently somewhat younger, with Oligocene ages 

of about 34 m.y., and the Treasure Mountain pluton 

has a Miocene age of 12 m.y. (Obradovich and 

others, 1969). 

These plutons all form part of the Colorado 

Mineral Belt, which contains numerous intrusions 

from the Front Range on the northeast, south westward into the San Juan Mountains. Those in the 

Front Range are mainly Late Cretaceous, or "Laramide" (Kg4), those farther west are Tertiary. They 

lie within a series of pronounced gravity lows which 

suggest the presence of a large subjacent batholith, 

or batholithic complexes, of which the visible 

intrusive bodies are cupolas (Steven, 1975, p. 88-9.~.). 

The plutonic rocks of the Southern Rocky Mountains are of interest because, unlike those farther 

west, they invade a rather thin sedimentary cover 

over a Precambrian basement; they must have risen 
78 THE CENOZOIC ROCKS 

through the Precambrian from a source in the 

underlying mantle. 

SOUTHEASTERN BASIN AND RANGE PROVINCE 

In southern Arizona the ages of the plutonic rocks 

become progressively younger eastward from midMesozoic to dominantly Late Cretaceous ("Laramide") (Kg4), into southern New Mexico where all 

the intrusive rocks are early Tertiary. Most of these 

form relatively small stocks, but a larger pluton 

occurs in the Organ Mountains east of Las Cruces, 

and there are others in the Sierra Blanca and in 

Capitan Mountain farther northeast. In the transPecos area of Texas to the south are numerous 

stocks, sills, and laccoliths, at least partly related to 

the early Tertiary volcanic activity; they are notable 

for their highly alkalic composition (Maxwell and 

others, 1967, p. 172-175). Similar alkalic intrusions 

extend northward from the trans-Pecos volcanic 

area as far as the Cornudas Mountains on the 

Texas-New Mexico boundary. 

REFERENCES CITED 

Addicott, W. 0., 1968, Mid-Tertiary zoogeographic and paleogeographic discontinuities across the San Andreas fault 

California, in Dicktmson, W. R., and Grantz, Arthur, eds.: 

Proceedings of conference on geologic problems -of San 

Andreas fault system: Stanford Univ. Publ. Geol. Sci., v. 11, 

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