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Home , Animas Formation, Arikareean, Atlantic (period), Barstow Formation, Cenozoic, Chuckanut Formation

The Rocks; a Discussion to Accompany the Geologic Map of the

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

Page Page Abstract ------­ 1 -Continued Introduction ------1 -Continued Tertiary System ------··------­ 2 Continental deposits ------­ 33 and Series ------­ 2 Sheet deposits ------­ 33 Marine stratified rocks ------3 ------­ 33 Gulf Coastal ------­ 3 --···------34 Midway (Tx) ------­ 3 Basin deposits ------­ 35 (Tel) ------­ 3 Santa Fe Formation ------­ 35 Claiborne Group (Te2) ------­ 6 Gila Formation ------­ 36 Jackson Group (Te3) ------7 Muddy Creek Formation ------­ 37 Coastal Plain ------· 7 Humboldt Formation ------­ 37 Pacific Coastal Area ------·-····· 7 Salt Lake Formation ------38 ------·-----­ 8 Deposits associated with volcanic rocks ...... 38 Oregon and Washington ------·------­ 9 Group ------­ 38 Continental deposits ------·------10 Other formations ------­ 38 Great and ...... 10 System ------· ····------­ 39 Paleocene deposits (Txc) ------·------­ 11 Gulf Coastal Plain ------­ 39 Eocene deposits (Tee) ------12 Mississippj Embayment ------39 Eocene lacustrine deposits (Tel) ______13 Other areas ---·------42 Outlying areas ------·------­ 14 Atlantic Coastal Plain ------·------42 New and west ------14 ------·------········ 43 Arizona ----·------14 ...... 44 Nevada and California ------­ 14 Undifferentiated Quaternary ...... 44 Western ------­ 15 California ------45 Washington ------15 Basin and Range province ...... 46 Eugeosynclinal deposits ------­ 15 Cenozoic volcanic rocks ...... 47 Series ------­ 16 Lower Tertiary volcanic rocks (lTv) ...... 49 Marine stratified rocks ------­ 16 Eocene marine"pillow (Teb) ...... 49 Gulf Coastal Plain ------­ 16 Lower Tertiary (ITa) ...... 49 Pacific Coastal Area ------­ 17 Clarno Formation (lTv) of northeastern Oregon .. 51 California ···------­ 17 Challis Volcanics (lTv) of south-central Idaho ... . 52 Oregon and Washington ------·------­ 17 Western Montana ...... 52 Continental deposits ------17 Absaroka volcanic field, and Montana 52 Great Plains and Rocky Mountains ------·------­ 17 Eastern Great Basin ------··········· 54 Outlying areas ------20 , southwestern ... 54 Oregon ------·------20 Other areas of lower Tertiary volcanics in ------­ 20 Rocky Mountains ...... 57 Series ------21 Southwestern Arizona ------·-················------·········· 58 Marine stratified rocks ------·------­ 21 Mogollon Plateau, southwestern ______58 Gulf Coastal Plain ------­ 21 Trans-Pecos Texas ------··········------59 Ailantic Coastal Plain ·········------····------­ 24 Upper Tertiary volcanic rocks (Tmv, Tpv) ...... 61 Pacific Coastal Area ------24 Upper Tertiary of Washington (uTa) ... . 64 California ------­ 24 Upper Tertiary andesites of Oregon and California Oregon and Washington ------­ 25 (uTa) ------64 Continental deposits ------­ 26 Columbia River Group of Washington, Oregon, and Northern Great Plains and Rocky Mountains 26 Idaho (Tmv) ...... 65 Outlying areas ------·------­ 27 Southeastern Oregon and northeastern California. 66 New Mexico and Arizona ------27 River Plain of southern Idaho ...... 67 Washington and Oregon ------27 Great Basin of Nevada and adjacent States ------­ 67 California ------·------­ 28 and Coast Ranges of California __ 68 Pliocene Series ------28 ------69 Marine stratified rocks ····------: ...... 28 Volcanic fields of ...... 70 Atlantic and Gulf Coastal Plains ------28 Upper Tertiary volcanic rocks of southern Basin Pacific Coastal Area in California ______32 and Range province in Arizona and California 71 III IV CONTENTS

Page Page Cenozoic volcanic rocks-Continued Quaternary volcanic rocks (Qv, Qf) ...... 71 Tertiary intrusive rocks (Ti) ...... 75 Andesitic volcanics of Cascade Range in Cascade Range ...... 75 Washington, Oregon, and California ...... 73 Oregon Coast Range ...... 75 of Snake River Plain (Qv) ...... 73 Central Idaho ...... 77 Felsic volcanic rocks of Yellowstone National Park Central Montana ...... 77 (Qf) ···················································································· 74 California and Great Basin ...... 77 Eastern border of Sierra Nevada, California ...... 74 Colorado Plateau ...... 77 Clear Lake area, California Coast Ranges ...... 74 Southern Rocky Mountains ...... 77 , New Mexico ...... 74 Southeastern Basin and Range province ...... 78 Basaltic fields of New Mexico and Arizona ...... 75 References cited ...... 78

ILLUSTRATIONS

Page 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 ...... 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 , the Atlantic and Gulf Coastal Plains, and the intermountain basins in the west. The Cenozoic rocks formed during the last 65 million of Volcanic rocks of Cenozoic occur throughout the western geologic time and, being the last to form, still cover nearly half the Cordillera, but they form an especially thick, continuous blanket country. They are a varied and complex assemblage, including in the northwestern States of Washington, Oregon, and Idaho. All marine stratified rocks near the present Atlantic, Gulf, and Pacific ages are represented, from Eocene to Quaternary. coasts, continental deposits in the interior region, and volcanic The lower Tertiary (Eocene and Oligocene) volcanic rocks are rocks through much of the western Cordillera. less widely exposed than the younger ones. They are dominantly The marine stratified rocks in the east form most of the Atlantic of andesitic or intermediate composition. They include the marine and Gulf Coastal Plains and range in age from Paleocene to pillow basalts of the Coast Ranges of Oregon and Washington Pliocene. They are the thin updip edges of deposits that thicken in that are interbedded in the Eocene strata and the lower Tertiary subsurface toward the coasts and are largely shall~w-water andesites of the Cascade Range not far to the east. Farther east are marine deposits, with marginal brackish-water and continental some notably thick and extensive volcanic fields, as in the phases, that change downdip in subsurface into deeper-water Absaroka Mountains of northwestern Wyoming, the San Juan marine deposits. Mountains of southwestern Colorado, and the Mogollon Plateau The marine stratified rocks in the California segment of the of southwestern New Mexico. Pacific coast likewise range in age from Paleocene to Pliocene and The upper Tertiary (Miocene and Pliocene) volcanic rocks are Pleistocene, but they are thicker, more varied, and more complex more extensively exposed and of more diverse compositions. They and have been more heavily involved in orogenic deformation. As include the upper Tertiary andesites of the Cascade Range and the a result, they do not lie in regionally consistent belts of outcrop but great mass of tholeiitic plateau basalts of the Columbia River form numerous sedimentary basins of different ages. They include Group in the Columbia Plateau. Farther south, in the Great Basin, not only shallow-water marginal deposits and fringing conti­ is an extensive body of felsic volcanic rocks, mainly ash-flow tuffs nental deposits, but nearer the coast deep-water deposits that were or that spread as broad sheets across the featureless laid down beyond the edge of the of the time. terrane prior to its disruption by Basin and Range block-faulting. The marine stratified rocks of the Pacific coastal segment to the Smaller areas of upper Tertiary volcanic rocks occur throughout north, in Oregon and Washington, are mainly of Eocene, the remainder of the Cordillera, but notably along the south­ Oligocene, and Miocene age. Like their counterparts farther south, western and southern edges of the Colorado Plateau. they range from near-shore and continental deposits on the east, Quaternary volcanic rocks have a much smaller distribution to deep-water deposits farther west, but they differ in their much and are restricted to areas that remained tectonically active in greater volcanic component, including submarine pillow , recent times. They include the chain of andesitic volcanoes on the terrestrial lavas, and a large volcanic-derived content in all the crest of the Cascade Range from Washington, through Oregon, sediments. into northern California; and a transverse belt of basaltic Continental deposits of Cenozoic age are especially prominent volcanics that extends along the Snake River Plain of southern in the Rocky Mountains and adjacent Great Plains and range in Idaho and into southeastern Oregon. Basaltic flows occur in many age from Paleocene to Pliocene. The Paleocene and Eocene other areas; in addition there are a few notable centers of felsic deposits fill to great thickness basins between the newly raised volcanism-in Yellowstone National Park, along the eastern side mountain uplifts and east of the mountains. They are mostly of the Sierra Nevada, and in the Jemez Mountains of New Mexico. flood-plain and swamp deposits but include some extensive Tertiary intrusive rocks form much smaller areas in the western deposits laid down in lakes, represented by the Paleocene Cordillera than those of ages and are of diverse ages and Flagstaff and the Eocene . The habits. They include many small hypabyssal stocks and lacco­ Oligocene, Miocene, and Pliocene deposits are thinner, but they liths, but there are a few larger masses of deep-seated origin and are even more extensive east of the Rocky Mountains, where they batholithic dimensions. The latter are especially prominent in the spread as great sheets across the Great Plains that were laid down northern Cascade Range of Washington, the region of the Idaho by streams carrying detritus derived from erosion of the batholith in Idaho, and the Southern Rocky Mountains of mountains to the west. Colorado. The Quaternary deposits are only partly represented on the Geologic Map. Not shown, for example, are the glacial drift and INTRODUCTION associated deposits that are extensive in the northern states. Quaternary deposits are shown only where they are an essential This is the last of a series of four reports dealing feature of the bedrock pattern and attain sizeable thickness, as in with features shown on the Geologic Map of the

1 2 THE CENOZOIC ROCKS United States (1974). Professional Paper 901 deals before. The great epicontinental seas of previous with general principles and problems relating to the were permanently expelled from most of the conti­ map; Professional Paper 902 with the nent, so that during the Cenozoic marine deposits rocks; Professional Paper 903 with the and were laid down only near the present coasts. Mesozoic rocks; and Professional Paper 904 (the Continental deposits continued to be laid down in present report) with the Cenozoic rocks. some of the interior areas, but other areas, such as The Cenozoic rocks formed only during the last 65 much of the eastern interior, became permanent land million years of geologic time, but, being the surfaces. The emergent regime of the Cenozoic youngest, they spread over nearly half the country. resulted in quickening of erosional processes, so that They are a varied and complex assemblage, includ­ great quantities of clastic detritus were eroded from ing marine stratified deposits along the Atlantic, the land and delivered to the areas of sedimentation Gulf, and Pacific coasts, continental deposits in the that remained. Another result of the Mesozoic interior region, and volcanic rocks in the western in the western Cordillera was the sub­ Cordillera. The Cenozoic rocks in the eastern half of sequent increase in volcanic activity, resulting in the country are flat-lying or gently tilted, but those in extensive fields of lavas, pyroclastic rocks, and the western half have been mildly to strongly volcaniclastic sediments being spread over much of deformed by various orogenies during Cenozoic time. the Western States during the Cenozoic. The treatment of the Cenozoic rocks in the present account, as in the account of the Paleozoic and TERTIARY SYSTEM Mesozoic rocks, closely follows the outline of the map legend. The rocks of successive ages are treated in PALEOCENE AND EOCENE SERIES turn, from Paleocene to . Under each The Paleocene and Eocene Series formed during a category, the first rocks described are the marine span of about 28 million years, or the first third of strata, in geographic order from east to wes~, and Tertiary time. In terms of the standard European next the continental deposits; a small body of eugeo­ sequence, the Paleocene corresponds to the , synclinal deposits of early Tertiary age is treated Montian, and Stages; and the Eocene to separately. Following this, the Cenozoic volcanic the , Cuisian, , Auversian, Bar­ rocks are dealt with, again by successive ages, from , and Ludian Stages. As explained below, one area to another. In a final section, the Tertiary several schemes of stages have been used for local intrusive rocks are treated briefly. purposes in the marine sequence in California, and The report is illustrated by a series of maps another set of stages is used for the North American showing the surface extent of the rocks of dif­ continental Tertiary. ferent ages (figs. 1-7, 12, 13, 15, 16), as they are Although the Paleocene and Eocene Epochs represented on the Geologic Map of the United occupied a sizeable fraction of Tertiary time, the States. In addition, a few figures are included that rocks of these epochs are considered together in this show special features of the volcanic rocks that account. In some places, as in the Rocky Mountain are not brought out clearly on the Geologic Map. region, the two sets of deposits are considerably The Cenozoic is conventionally divided into the different, because they formed in different deposi­ Tertiary and Quaternary Systems, the first being tional basins. In other places, however, the distinc­ much the greater, the second only representing the tions are less marked, and there is controversy as to last 2 million years of geologic time. The Tertiary is the boundary between the two series. in tum divided into the Paleocene, Eocene, Oligo­ The Paleocene and Eocene are represented on the cene, Miocene, and Pljocene Series, and the Quater­ Geologic Map of the United States by stratified nary into the Pleistocene and Holocene Series. The sedimentary rocks, mostly marine (Tx, Te); by Quaternary is distinguished from the Tertiary continental deposits (Txc, Tee); and by eugeosyn­ mainly by its glacial periods-although this climatic clinal deposits (Tee). In the Gulf Coastal Plain, the regime, in fact, actually began during the later Eocene stratified rocks are separated into three Tertiary. divisions (Tel, Te2, Te3), which correspond to the The Cenozoic succeeded the great orogenies of Wilcox, C!aibome, and Jackson Groups. In parts of later Mesozoic time-although orogenic deformation the Rocky Mountain region, Eocene lacustrine continued into the Cenozoic and continues even deposits (Tel) are differentiated amidst the conti­ today in some areas. 'I~1e Mesozoic orogenies nental deposits. Eocene volcanic rocks in the produced a major reorganization of the continent, so Western States are mostly grouped in the lower that it became more widely emergent than ever Tertiary volcanic rocks (lTv), which are discussed TERTIARY SYSTEM 3 separately later. However, in the Coast Ranges of water deposit, whose fauna also differs from Oregon and Washington, Eocene pillow basalts apparently correlative strata elsewhere, as in west­ (Teb) are separately mapped. ern and on the Pacific Coast. It is commonly Paleocene and Eocene rocks cover sizeable areas­ considered to be the sole representative of the perhaps as much as 10 percent of the area of the Paleocene in the Gulf Coastal Plain, but accumu­ Forty-eight States shown on the Geologic Map lating paleontological evidence suggests that the top (fig. 1). They form a broad outcrop belt along the of the Paleocene extends somewhat higher in terms inner part of the Gulf Coastal Plain, and smaller, of the European standard sections (see below). more discontinuous areas in the same position in the In Texas the Midway is divided into the Kinkaid Atlantic Coastal Plain. The most ex4!nsive outcrop and Wills Point Formations, the first of glauconitic areas of Paleocene and Eocene rocks are, however, in sands and clays with several lenses of coquinid the Rocky Mountain region and northern Great limestone, the second largely clay and silty clay of a Plains, where they form the centers of basins deeper water facies. In Mississippi and Alabama it is between the mountain uplifts. Along the Pacific divided into the Clayton Formation, Porters Creek Coast, Paleocene and Eocene rocks are recognized Clay, and Naheola Formation. The Clayton some­ from southern California to northwestern Wash­ what resembles the Kinkaid, being glauconitic, ington, but because of the considerable deformation sandy, and limy, and the Porters Creek the Wills to which they have been subjected, individual out­ Point, being dark laminated clay. The Porters Creek crops are small and discontinuous. extends northward beyond the remainder of the group into southern Illinois. The Naheola differs MARINE STRATIFIED ROCKS from the others in being sandy and noncalcareous. GULF COASTAL PLAIN I On the crest of the Sabine uplift, south of the main outcrop belt, in northwestern Louisiana and eastern The Paleocene and Eocene Series are exposed in Texas, a large circular area of outcrop was formerly belts around the inner half of the Gulf Coastal Plain. mapped as Midway but is now interpreted as a The outcropping rocks are the updip edges that marine downdip argillaceous facies of th,e Wilcox extend in subsurface toward the Gulf, where they Group and is so represented on the Geologic Map. form a thick, relatively unbroken, dominantly Only a small outcrop in the north part of the uplift, argillaceous marine sequence. The exposed rocks, to northwest of Shreveport, is now considered to be of which main attention is given here are thinner Midway age. more interrupted, more coarsely clasti~ deposits, that WILCOX GROUP (Tel) were laid down in marine, littoral, and continental The Wilcox Group, or initial Eocene unit of the Gulf environments along the edge of the continent. Coastal Plain, was named in 1906 for Wilcox County,

MIDWAY GROUP (Tx) Alabama, to replace the previously used lithologic The Midway Group at the base of the Tertiary term "lignitic." The term Sabine was proposed at coastal plain sequence was named in 1887 for about the same time for the same unit west of the Midway Landing on the Alabama River in Alabama. Mississippi and is still used by some geologists in The group crops out in a nearly continuous band Louisiana. The Wilcox forms an interrupted band of from the Rio Grande in Texas to Georgia, with a outcrop in the inner part of the Gulf Coastal Plain in notable deflection northward to southern Illinois in Texas eastward to Georgia, where it is cut off by the Mississippi Embayment. Exposed sections of the overstep of younger Eocene units. As indicated Midway are generally less than 500 ft (150 m) thick. above, the Wilcox is also exposed in a large area on In most places the Midway can be divided into two or the crest of the Sabine uplift in northwestern more formations. Eastward from Georgia, the Louisiana and eastern Texas. On the outcrop the Midway is overstepped by succeeding Eocene Wilcox is as much as 1,000 feet (300m) thick. In large formations. parts of its outcrop (for example, in Texas and the In contrast to parts of the succeeding Eocene Mississippi Embayment) the Wilcox is of nonmarine sequence, the Midway is marine throughout; in facies and is indivisible in many areas. In other places it much resembles the uppermost places, howe rer, intertonguing marine strata permit deposits on which it lies, although with a notably subdivision of the group into formations, which vary different fauna. The Midway is a subtropical, warm- from one region to another. In central Texas the Wilcox Group was divided by 1 A general review of the Paleocene and Eocene formations of the Gulf Coastal Plain io Plummer (1932, p. 574) into the thick medial non­ preoented by Murray (1961, p. 367-394). For an earlier more detailed summary of the Texas oegment, see Plummer (1932, p. 531-699). marine Rockdale Formation of sands, clays, and 4 THE CENOZOIC ROCKS

I I \ \ I I 1. __ , \ / ------:; I / I -- ~ I - -'~~ - I

r------· - "~------~ ------__) , __ _ I ~' ' -~ - ~'P i \ I I ____{?_ ( I I ' -- - "------~ ------~ \ \ \ "I \ .., r------'. \ I '' ; I \ ! ... ~ .Nl("" \ I~- 'WI' .---­ \ .J ------'\1, \ ~ -,: ------..----r.=::~:~:-; ------\ : ,... \J .. ' ,.t'# ..... L .-' .. '~-..r\ ( - --1 ·. I ~"\..,. , :.r "•'\, • ,"" --; ~· ,...... ' . : r -- --. _;;_:--·- .- - -- -·-J --- ·---:

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 / ,f _...,f"\ . .,..- ~ I t -----\ ) \ I I '"\

·------\ ------·"'·~.~ -- ., \ \::.------­ <-, -\ I ./ ' I ' ' (-:-·------{' __ \ ") '--, ( I ' '-, .r-' ' '

EXPLANATION

Eocene rocks

Paleocene rocks

G U L F 0 F MEXICO

.... ' '-·

FIGURE I.-Continued.

lignites, underlain and overlain by thin fossiliferous 1974) the same sequence of nonmarine beds is marine units, the Seguin and Sabinetown Forma­ divided into the Hooper Formation below, the tions. In later work (as shown on the Waco and medial-sandy Simsboro Formation (classed as a Austin sheets of the Texas Geologic Atlas, 1970, member by Plummer), and the Calvert Bluff 6 THE CENOZOIC ROCKS

Formation above; the marine units are not men­ any of the other Paleocene or Eocene units, tioned. Farther southwest in Texas, the undivided especially in the Mississippi Embayment and west of Wilcox has traditionally been termed the Indio the Mississippi River. In south Texas it forms an Formation. outcrop band 60 miles (95 km) or more broad in the In Alabama, Mississippi, and Georgia the non­ Rio Grande embayment, and in northeast Texas it marine beds of the Wilcox are principally in its upper occupies most of the East Texas embayment and part, and the group is divided into the Nanfalia, extends around the north and south sides of the Tuscahoma, and Hatchetigbee Formations (with the Sabine uplift into southern Arkansas and northern Bashi Marl Member at the base of the latter). Louisiana, where its outcrop is 120 miles (190 km) Planktonic foraminifers indicate that the Paleocene­ broad. It also underlies all the northern two-thirds of Eocene boundary, in terms of European standard the Mississippi Embayment as far north as Illinois, sections, is above the boundary between the Midway although its outcrops are interrupted in part by and Wilcox Groups (Berggren, 1965, p. 280-282). On downfolds of Jackson Group (Te3) and by Quater­ the basis of these foraminifers the Bashi Marl of nary deposits. East of the Mississippi River it forms Alabama is basal Eocene (Ypresian). The Nanfalia a narrower band of outcrop that extends eastward Formation (and the local Salt Mountain Limestone) across Georgia and South Carolina, beyond the are upper Paleocene (Sparnacian). The overlying truncated edges of the Midway and Wilcox Groups. Tuscahoma Formation and the Seguin Formation of The Claiborne is notable for its highly fossiliferous Texas do not contain planktonic foraminifers marine strata, whose have been extensively suitable for long-range correlations but are pre­ collected and investigated for years, since the days of sumed to be upper Paleocene. the early geological explorers. Its deposits character­ This creates problems of representation on the istically form a series of cyclical sequences, each Geologic Map, where the units adopted are the beginning with transgressive glauconitic, calcar­ Midway and Wilcox Groups, which are assigned to eous, fossiliferous strata; passing up into dark Paleocene and Eocene ages, respectively. It seems argillaceous deeper-water deposits; and terminating fruitless to attempt to subdivide the Wilcox on the with regressive carbonaceous to lignitic sandy and basis of these new judgements, as the Paleocene­ argillaceous deposits. Eocene boundary in the Wilcox is certainly known In Alabama the Claiborne is divided into the only in the relatively complete marine sequence in Tallahatta Formation, the Lisbon Formation, and Alabama. Elsewhere, much of the Wilcox is non­ the Gosport Sand. The Tallahatta is one of the more marine and is generally undivided on the source resistant units of the Gulf Coastal Plain and forms a maps. We consider it best to leave the boundaries prominent cuesta, as its sands and clays are where they are, pending the receipt of other data, indurated on the outcrop to "buhrstone," which was with the understanding that part of the Wilcox is used in the early days for millstones. The Gosport is actually Paleocene. the most fossiliferous of the three units; its fossils At the eastern end of the Eocene outcrop belt, in were first described by Conrad as early as 1833. South Carolina, equivalents of the Wilcox reappear In Texas the Claiborne Group is divided into the in a small area from beneath the overlap of the Carrizo Sand, the Mount Selman Formation (with younger formations and are known as the Black the Reklaw, Queen City, and Weches Members), the Mingo Formation. The Black Mingo contains some Cook Mountain Formation (with the Sparta and of the marine mollusks which occur in the Nanfalia Crockett Members), and the Yegua Formation. The and Tuscahoma Formations of Alabama, as well as units are all widely recognizable, and they or their some others that are more characteristic of the equivalents have been mapped across Texas and Midway Group in that area, so that the Black Mingo into Louisiana. The Carrizo at the base is conti­ evidently contains undifferentiated strata of both nental and is a sheet of sand that was spread widely early Eocene and Paleocene age (Cooke and Mac­ along the shore. The Yegua at the top is again of Neil, 1952, p. 20-21). continental or brackish water facies. The inter­ vening units are mainly marine or littoral. The CLAIBORNE GROUP (Te2) Weches Member is the most prominently exposed The Claiborne Group was named in 1847 for unit and forms escarpments and mesas. It is highly Claiborne Bluff on the Alabama River in Alabama. glauconitic and its weathered surfaces are deeply It is perhaps 1,000 feet (300 m) or more thick, and encrusted with brown iron ore that has been because of its thickness and various structural exploited commercially from time to time (Eckel and complications it has a much larger outcrop area than Purcell, 1934, p. 482-503). PALEOCENE AND EOCENE SERIES 7

In eastern Georgia and in South Carolina the (400 km) north of South Carolina. The next Claiborne Group is represented by the McBean Paleocene and Eocene outcrops are in the segment in Formation, 100 feet (30 m) or so thick, and northern , whence they continue to the approximately equivalent to the Lisbon and Mount termin us of the Coastal Plain in northeastern New Selman Formations of the Claiborne Group farther Jersey. The rocks form a narrow band of outcrop next west. to the Cretaceous, which is overstepped in places by Miocene deposits. jACKSON GROUP (Te3) The Paleocene and Eocene deposits in this region are not more than a few hundred feet thick on the The Jackson Group, or highest Eocene unit in the outcrop and are all shallow-water marine deposits. Gulf Coastal Plain, was named in 1856 for Jackson, The rocks are highly fossiliferous in places and are Mississippi. The group is 500 feet (150 m) or more composed mostly of glauconitic sands and clays; thick, and forms a nearly continuous band of outcrop however, some more calcareous units occur. The in the middle of the coastal plain from the Rio Paleocene is represented in Virginia and Grande in Texas to South Carolina. Several large by the , and in New Jersey by the outliers of the Jackson occur in down warps in the Hornerstown and Vincentown Formations. The Mississippi Embayment as far north as Kentucky, overlying Eocene deposits, largely of Wilcox age, are and a large inlier emerges on the crest of the Ocala more discontinuous in outcrop because of Miocene uplift in northwestern Florida. cover. In Virginia and Maryland the Eocene Strata of the Jackson Group were deposited during comprises the , and in New a separate and widespread marine transgression in Jersey the Manasquan Formation.2 the Gulf region. In the eastern part of the region the deposits are mainly calcareous, in the central part PACIFIC COASTAL AREA argillaceous, and in the western part sandy. Marine stratified Paleocene and Eocene rocks are In Texas the Jackson was formerly grouped as the exposed along the en tire length of the Paclfic coastal Fayette Fortnation, but its and clay units belt from southwestern California to northwestern are widely traceable, and it is now divided into the Washington, and in places attain thicknesses of Caddell, Wellborn, McElroy, and Whitsett Forma­ more than 10,000 feet (3,000 m). Because the rocks tions, with various members (Eargle, 1959). The have been subjected to greater deformation than in sediments are rich in volcanic detritus and contain the Gulf and Atlantic coastal areas, individual many deposits of commercially exploitable bentonite outcrops are smaller and more discontinuous, and fuller's earth (King, 1940, p. 170-178). In south especially in California where folding and faulting Texas the upper, or Whitsett Formation, also have been greatest. The largestandmostcontinuous contains large uranium deposits that have been outcrop area is in the Coast Ranges of western mined since 1960 (Eargle and others, 1975). Oregon and Washington, where folding is relatively In the central, or type area, near Jackson, open. Mississippi, the Jackson consists of the Moodys Throughout the length of the Pacific coastal belt Branch Marl, a glauconitic, sandy and shaly deposit; the Paleocene and Eocene deposits exhibit a and the Yazoo Clay, a dominantly argillaceous westward transition across a zone 80 to 120 miles deposit. Northward in the Mississippi Embayment (125-190 km) wide. They grade from nonmarine marine components fade out and the group becomes continental deposits on the east, through shallow­ largely clastic and continental. Calcareous com­ water nearshore deposits, to deep-water deposits, ponents increase southeastward, and in Florida the mainl'y formed on deep-sea fans. However, this group is represented by the solid body of the Ocala pattern varies considerably from place to place, and Limestone, poorly to well indurated and crowded in California the original geographic pattern has with foraminifers, bryozoans, mollusks, and echi­ been much disordered by large strike-slip move­ noids. In north western Florida the Ocala is an ments along the San Andreas and other faults. In the important aquifer and is the source of many large Olympic Mountains of northwestern Washington, springs. some deposits are differentiated as "eugeogeosyn­ clinal" and separately mapped (Tee). ATLANTIC COASTAL PLAIN Except for the Castle Hayne Formation of eastern 2Many details of these units are given by Spangler and Pete.rson (1960, p. 52-61, 72- North Carolina (of Claiborne and Jackson age) no 75). Their planktonic are discuBSed by Loebhch and Tappan (1957, Paleocene or Eocene rocks are exposed for 250 miles p. 1127-1132). 8 THE CENOZOIC ROCKS

Distinctions between deposits of Paleocene and the bedrock in a tropical . The lone is more or Eocene ages are unclear in many areas, partly less confluent with the ancient river channel because of the scantiness of fossils, the equivocal deposits on the slopes of the Sierra Nevada to the nature of the faunas, and uncertainty of correlation east (the "Auriferous Gravels"). A somewhat similar with type Paleocene and Eocene. Paleocene marine continental deposit, the Montgomery Creek Forma­ deposits are differentiated on the map in California tion, crops out along the western edge of the Cascade where they have been identified with certainty; Range farther north, lying unconformably on the elsewhere, both here and farther north, the Paleo­ basement rocks of the Klamath Mountains to the cene, if present, is not separated from the more west, and passing eastward beneath the Pliocene extensive Eocene deposits. lavas of the Cascade Range. The Eocene deposits change rapidly into marine CALIFORNIA~ facies when traced in subsurface beneath the Great The of the Paleocene and Eocene Valley. On its western side, in the Diablo Range and marine rocks of California has been intensively north of Bay, the Paleocene and studied for many years and has produced several Eocene still consist of deposits that were mostly laid sets of stages that are in common use. Using the down in waters of moderate depth. Nevertheless, larger invertebrate fossils, the Paleocene has been drilling in the Great Valley has revealed several deep divided into the Martinez and Meganos Stages, and gorges or submarine canyons which slope westward the Eocene into the Capay, Domengine, transition, from the Sierra Nevada and are filled with deeper­ and Tejon Stages. On the basis of benthonic water deposits. The most impressive is the Markley Foraminifera, the Paleocene has been divided into gorge in the latitude of Sacramento, formed during the Ynezian and Bulitian Stages, and the Eocene the late Eocene and Oligocene. into the Penutian, Ulatisian, and Narizian Stages On the east slope of the Diablo Range near Mount (Mallory, 1959). Many of the first set of names were Diablo, the exposed sequence is about 9,000 feet derived from formations in local areas, and in older (2,750 m) thick above the Upper Cretaceous and is reports they were used as formation names in wide divided into the Martinez Formation, the Meganos areas in California. This usage is now outmoded, and Formation, Capay Formation, Domengine Sand­ the names are used as formations only in places in or stone, and the Nortonville Member of the near where they were first defined, other names Kreyenhagen Formation. Except for the Nortonville, being substituted in other areas where the rocks are all the units are prevailingly sandy and contain of different facies although of approximately similar conglomerate; the sediments were mainly derived age. Correlation of the California Paleocene and from the Sierra Nevada to the east. Eocene units with units of these ages on the Gulf Accumulating evidence suggests that Late Creta­ coast is rather vague, although some similarity of ceous and early Tertiary movements had occurred foraminiferal species in the two areas has been along the (or its ancestor), so that noted. the Salinian block of basement rocks already projected some distance northward along the west Northeastern California, the region northeast of side of the , producing a shallow the San Andreas fault and north of the Tehachapi embayment. As a result, some of the of Mountains, is less disrupted tectonically than in the the southern area were derived from western sources rest of the State, hence best preserves the original and contain Franciscan and Salinian detritus. pattern of the Paleocene and Eocene deposits. In the northern Coast Ranges, occasional outliers In the western foothills of the Sierra Nevada, the of Paleocene and Eocene rocks of Great Valley-type lower Eocene lone Formation (Tee) overlaps the occur within the Franciscan terrane, but they are Mesozoic basement. Except for a few marine parts of klippen that have been carried westward intercalations, the lone is a continental deposit as along the Coast Range thrust. A quite different much as 1,000 feet (300 m) thick; it is composed of facies forms a part of the "coastal belt" of the sandstone members, some beds of lignite, and Franciscan assemblage (Ke of map) and is shown by members of remarkably pure kaolinitic clay that fossils to be of Paleocene and Eocene ages (Evitt and formed during a time of deep chemical of Pierce, 1975). It is less disordered than the main body 3For a general summary, with emphasis on the sedimentology, see Nilsen and Clarke of the Franciscan assemblage and probably formed (1975). Data on southern California are given by Durham (1954), on the Great Valley by during the waning stages of the Franciscan deep-sea 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 trench. presented by Reed (1933, p. 120-146), and by Reed and Hollister (1936, p. 17-27). In the Eureka district along the coast farther PALEOCENE AND EOCENE SERIES 9 north, the Franciscan is overlain by the Yeager To the south in the Peninsular Range, the sequence Formation (Tx), a fossiliferous shallow-water de­ thins to 1,600 feet (490 m). Here, the upper part, or posit, also classed as of Paleocene age, which is Poway Group, contains the distinctive "Poway­ evidently a near-shore facies of the upper part of the type" clasts, which are well-rounded gravels of hard, "coastal belt" Franciscan. resistant rocks, especially porphyritic , The rocks southwest of the San Andreas fault were whose source area has been much debated but was in shifted by strike-slip movements at least 400 miles some undetermined terrane to the east. (650 km) northwestward between early Tertiary and Near the San Andreas fault in the segment from Quaternary time, so that they are not closely related the San Gabriel Mountains southeastward, slivers of to rocks of the same age across the fault to the marine Paleocene and Eocene rocks occur which northeast. In the segment in the central and were originally parts of extensive formations, now southern Coast Ranges, Paleocene and Eocene broken and displaced by fault movements. They deposits overlap against the crystalline basement of include the Paleocene San Francisquito Formation the Salinian block, and include more continental of the Valyermo area and Cajon Pass, and the slope and ocean floor deposits than to the northeast. Eocene Maniobra Formations of the Orocopia They lie in places on Upper Cretaceous rocks, but in Mountains northeast of the Salton Sea. many other places they overstep directly onto the crystalline basement. The deposits include se­ OREGON AND WASHINGTON I quences 10,000 to as much as 20,000 feet (3,000- Marine lower Tertiary rocks reappear north of the 6,000 m) thick of sandy flysch or turbidite that Klamath Mountain barrier of Mesozoic rocks in the formed on deep-sea fans at the foot of the continental coastal belt of Oregon and Washington. In north­ slope. western Oregon, Eocene rocks form a nearly con­ The flysch sequences include the Eocene Butano tinuous belt of outcrop 40 or 50 miles (100-130 km) Sandstone of the Santa Cruz Mountains and even wide in the Oregon Coast Ranges west of the thicker flysch in the southern Santa Lucia Moun­ Willamette Valley, with only minor cover of younger tains, the La Panza Mountains, and the Sierra Tertiary. North of a downwarp along the Columbia Madre, which extend from Paleocene to upper River, they reappear in smaller outcrops in the Eocene. The latter form the largest outcrop area of Coast Ranges of southwestern Washington. Farther lower Tertiary rocks in the coastal belt of California north, Eocene rocks form a wide band of outcrop (Tx, Te). They are contiguous to, but in fault contact around the eastern and northern edges of the with, a mo'"e varied Paleocene and Eocene sequence Olympic Mountains. Except in the Olympic area, in the Santa Ynez Range to the south. In the eastern the rocks are only broadly folded. Santa Ynez Range, the main part of the sequence is The lower Tertiary rocks of Oregon and Wash­ the thick Paleocene and Eocene Juncal Formation of ington, like those of California, are continental arkosic sandstone, shale, and mudstone. Farther margin deposits, and marine fossils at various levels west the sequence forms the Anita Shale, Matilija indicate correlations with the California sequence. Sandstone, Cozy Dell Shale, and Coldwater Sand­ Fossils in the lower beds in Oregon indicate stone. The thin lenticular orbitoidal Sierra Blanca correlation with the Capay and Domengine Stages of Limestone is at the base in many places, and is the the lower Eocene of California; the Paleocene is only conspicuous carbonate in the Tertiary of the seemingly unrepresented. Nevertheless, the Oregon California coastal belt. The Eocene marine strata in and Washington Eocene is of markedly different this area are succeeded by the red continental Sespe facies, containing large volcanic components that Formation (Toe), mainly Oligocene but including are lacking in California. late Eocene at the base, which expresses a major The Eocene overlaps the Mesozoic of the Klamath regression of the sea. Mountains on the south and that of Vancouver South of the , Paleocene and Island on the north, but otherwise its base is not Eocene deposits fringe the western sides of the Santa exposed. Except at the margins, much of it probably Ana Mountains and Peninsular Ranges past the accumulated on oceanic crust off the edge of the Mexican border into Baja California; the strata are continent. The total thickness of the Eocene here is tilted westward but are not greatly deformed. In the 10,000 to 20,000 feet (3,000-6,000 m) or more. Because Santa Ana Mountains, a 4,000-foot (1,200-m) se­ of the associated sedimentation and volcanism, the quence consists of the Paleocene Silverado Forma­ oceanic basement, and other features, much of the tion and the Eocene Santiago Formation, both shallow-water deposits with nonmarine interbeds. 4 For general summaries see Snavely and Wagner (1963, 1964). 10 THE CENOZOIC ROCKS Eocene of the region can be considered eugeosyn­ Following the Eocene, there was a widespread clinal; on the Geologic Map, however, only the part of withdrawal of the sea so that the succeeding the sequence in which these features occur in Oligocene occupies much smaller areas within the exaggerated degree is so indicated. Coast Range belt. Along the margins of the depositional trough are shallow-water deposits, passing inland into CONTINENTAL DEPOSITS measures, especially east of the Puget Trough on the The Paleocene and Eocene continental deposits flanks of the Cascade Range in Washington, and occur in many areas of the interior of the western more locally at the north edge of the Klamath United States; none are preserved farther east and Mountains in Oregon. These pass westward into the were probably never laid down there. The largest deep-water marine deposits in the main area of the areas of these deposits are those in the northern trough. Great Plains and Rocky Mountains, which accumu­ Beginning in the earliest Eocene and continuing lated in basins between and east of mountains that locally to the end of the Eocene, basaltic volcanics had been newly raised during the Laramide oro­ (Teb) were erupted from a series of centers along the genies of late Mesozoic and early Tertiary time. axis of the trough. The volcanic material is estimated Smaller, more widely separated patches of deposits to have a volume of 60,000 cubic miles (250,000 km3), are preserved in various outlying areas west of the the largest of any volcanic unit in the Pacific Rocky Mountains from the Canadian to the Mexican Northwest. The volcanic bodies are known from borders and were probably never much more exten­ south to north as the Umpqua, Siletz River, and sive than they now are. Tillamook Volcanics in Oregon, and the Crescent The continental deposits have yielded many and Metchosin Volcanics in Washington. Thick remains of and , especially in the masses of pillow lavas alternate or interfinger Great Plains and Rocky Mountains, from which a marginally with water-laid fragmental basaltic system of stages has been constructed that is inde­ debris, and at times the volcanic buildup was great pendent of but parallel to the marine stages of the enough that parts of the centers emerged as islands. east and west coasts (Wood and others, 1941). The The earlier volcanics are succeeded in the south by Paleocene has been divided into the , the middle Eocene Tyee Formation, a thick-bedded, Dragonian, , , and Clark­ graded turbidite of arkosic sandstone and siltstone. forkian Stages, and the Eocene into theWasatchian, Paleocurrent data indicate that it was derived from Bridgerian, , and Duchesnian Stages. All the pre-Tertiary crystalline basement rocks of the the stages are based on formations at localities Klamath Mountains to the south. Farther north, the within the Rocky Mountain region, but they have equivalent of the Tyee is gray, tuffaceous siltstone been extended far beyond the original formations and water-laid pyroclastics. into rocks of different facies and settings. Upper Eocene rocks are somewhat more restricted than the earlier Eocene, and in many places lie GREAT PLAINS AND ROCKY MOUNTAINS 5 unconformably on them. Alkalic basalt was erupted Paleocene and Eocene continental deposits are locally and passes laterally into gray, thin-bedded to extensive in the northern Great Plains and the inter­ massive tuffaceous siltstone and mudstone, called montane basins of the Rocky Mountains. They are by such names as Nestucca and Yamhill Formations closely related to the Laramide of Late in Oregon. At this time, nonmarine coal measures Cretaceous and early Tertiary time, which created and other deposits spread farther into the trough. the basins, and the intervening uplifted ranges A tectonic situation different from the rest of the which supplied much of the sediment source. The Oregon and Washington Eocene prevailed in the orogeny also permanently expelled the seas from Olympic Peninsula of northwestern Washington. this part of the continent (with the minor exception Here, the Eocene encircles the peninsula on the north of the Cannonball Formation, noted below), so that and east, facing outward toward younger Tertiary the deposits were laid down on piedmont slopes or formations, and consists of the flood plains, in swamps, and lakes. In some of the of basaltic volcanics 20,000 feet (6,000 m) thick and deeper basins, the deposits attain thicknesses of associated sedimentary units, all of which stand 10,000 feet (3,000 m) or more. at vertically and are in thrust contact with the sheared, various levels in the sequence, especially along the partly metamorphosed eugeosynclinal rocks of the edges of the basins, attest to the progress of the core of the Olympic Mountains (Tee, Toee, Tmoe) (Tabor, 1972, p. 1805-1807). 5 For summaries, see Robinson (1972) and McDonald (1972). PALEOCENE AND EOCENE SERIES 11 orogeny, but the rocks of the basins themselves were mouth of the Yellowstone River near the present little disturbed, and lie nearly horizontal over wide town of Buford, North Dakota. In the areas. of North Dakota and in many other places, it is as The unconformities are local, and the sequence in much as 6,000 feet (1,800 m) thick. Because of its other places is conformable-from the latest Cre­ uniform character it has not been divided in many taceous into the Paleocene and from the Paleocene places, but elsewhere various components have been into the Eocene-so that there has been much contro­ recognized and named, such as the Tullock Lake, versy as to age assignments, notably as to the Lebo, and Tongue River Members (or Formations). Cretaceous-Tertiary boundary (the so-called "Lara­ Volcanic detritus is rare in the Fort Union; westward mie question"). Many of the deposits are similar from in the Crazy Mountains basin of south-central Mon­ one age to the next, but there was a gradual change tana, however, the Fort Union passes into the Liv­ in the deposits with time, reflecting different sedi­ ingston Formation, a mass of andesitic tuff that ment sources, climatic conditions, and other factors; includes highest Cretaceous below. there were also changes in the faunas and floras, Along the east edge of the outcrop area, in North reflecting both changes in environment and evolu­ Dakota and northern , the Cannonball tionary development. Despite the orogeny, the Formation (Tx) interfingers westward into the lower region remained at low altitude through the Paleo­ part of the . It consists of cene and Eocene; even the mountain areas were not shallow-water marine -the last marine de­ lofty but were worn down by erosion nearly as posits of the continental interior, probably formed rapidly as they were raised. A notable change in during a brief incursion from the Arctic. It is named depositional patterns, probably related to tectonic for the Cannonball River in North Dakota, along movements, occurs between the Paleocene and which its characteristic "cannonball" Eocene. The Paleocene forms deposits in nearly all are prominent. The formation contains oyster banks the basins, where as the Eocene is thick and extensive and about 150 species of mollusks and foraminifers, only in the intermontane basins. In the Great Plains the latter resembling those of the Midway Group of east of the. Rocky Mountains the Paleocene spreads the Gulf Coast. out over great areas; the Eocene, if present, is con­ The Paleocene deposits are preserved in two fined to small remnant patches. smaller areas along the front of the Rocky Mountains farther south, in the and Raton basins of PALEOCENE DEPOSITS (Txc) Colorado and New Mexico. In both areas they overlie Both the latest Cretaceous (Laramie, Hell Creek, beds of Laramie age (uK4) and are more or less and Lance Formations) and the Paleocene deposits gradational with them. The in the are somber-gray, lignitic sands and clays that accu­ is composed of granitic detritus from mulated in swamps in a humid, semitropical envi­ the adjacent and intergrades laterally ronment. Near the mountains, granitic detritus with the finer-grained . In the appears in the sequence. Much futile effort was once the fine-grained coal-bearing Raton expended in a search for unconformities in the basin Formation similarly intergrades northward with the deposits that presumably would mark the Cretaceous­ coars~r which contains Tertiary boundary, but the deposits are actually con­ volcanic detritus. At Spanish Peaks at the north end formable, or nearly so. Nevertheless, the highest of the basin, it is followed by the small remnant of the Cretaceous deposits contain Triceratops and others Huerfano Formation of Eocene age. of the last of the , whereas the succeeding A more complex set of Paleocene deposits occurs Paleocene contains only archaic . How­ much farther west, in north-central southwest ever, part of the Laramie flora is of Tertiary aspect, of the whose original basinal form is so much adjustment has had to be made to reconcile now much disrupted by the block-faulting in the the and animal evidence; this dilemma has High Plateaus of Utah (Spieker, 1946). Below is the now apparently been resolved. variegated shale, sandstone, and conglomerate of The term Fort Union Formation (or Group) has the North Horn Formation which bridges the virtually become the generic term for the Paleocene Cretaceous-Paleocene boundary and contains Cre­ deposits of North and South Dakota, Montana, and taceous dinosaurs in its lower part and Paleocene Wyoming; farther south other names are used. The mammals in its upper part. It is followed by the Fort Union was named by Meek and Hayden in 1862, lacustrine Flagstaff Limestone, whose white ledges during the early days of geological exploration of the form a prominent and persistent capping of the High northwest, for the now-defunct Fort Union at the Plateaus. The Flagstaff contains fresh-water mol- 12 THE CENOZOIC ROCKS lusks. Variegated clays and marls at the south end conditions from those of the Paleocene-from of the plateau that have traditionally been called somber-gray lignitic Paleocene deposits laid down in are probably of Flagstaff age a moist swampy environment, to red-banded Eocene and are so represented on the Geologic Map (Txc). clastics formed in a drier climate on piedmont slopes The lake in which the Flagstaff accumulated varied and flood plains. gave place to open in shape and size with time, but late in the and , and the mammalian probably covered most of central Utah although only community changed from -dwelling animals to fragments are now preserved. The Flagstaff is browsing herbivores. At the same time, evolutionary succeeded by the Colton Formation which is changes resulted in the extinction of the archaic considered to be the local representative of the Paleocene mammals and the substitution of mam­ Wasatch Formation, of Eocene age, on the basis of mals of more modern aspect. Primitive horses scanty molluscan remains. appeared, and higher up in the sequence (and It would have been interesting to have differen­ continuing into the Oligocene) the titanotheres, tiated the Flagstaff lacustrine deposits in the same whose gigantic bones aroused the interestofthefirst manner as the Eocene (Green River) lacustrine geological explorers, and later became the field of deposits on the Geologic Map, but the areal pattern is collectors and paleontologists such as Marsh and too complex for such representation, and they are Cope. therefore grouped with the North Horn Formation as In southwestern Wyoming, northwestern Colo­ Paleocene continental deposits (Txc). rado, and northeastern Utah, terrestrial piedmont In the to the southeast, in north­ and flood-plain sedimentation was interrupted dur­ western New Mexico and southwestern Colorado, ing the middle Eocene by lacustrine conditions Paleocene continental deposits about 2,000 feet whose deposits (the Green River Formation) are (600 m) thick lie between the highest Cretaceous differentiated from the rest on the Geologic Map (Tel) (uK4) and the Eocene (Tee) in the center of the basins. and are discussed separately below. In the southern part they comprise the thin, sandy Although the Eocene deposits formed in a tectonic and conglomeratic and the environment, the region remained at an altitude as thicker, finer-grained . These low as that of the Paleocene; the successive uplifts units are replaced along the north side of the basin never attained the height of lofty mountains. The by the much coarser (merged similarity of mammalian faunas from one basin to with the Upper Cretaceous on the Geologic Map), the next indicates that there were no topographic which contains large quantities of andesitic debris hindrances to free migration, and in places the derived from the early eruptions in the San Juan original deposits seem to have been confluent Mountains to the north. The Nacimiento and between basins, the connections having been re­ Animas include the Puerco and Torrejon Formations moved by subsequent erosion. and "Tiffany beds" of former usage, each of which The Wasatch Formation (Tee) at the base of the contains a significant Paleocene mammalian fauna Eocene was named by Hayden in 1868 for outcrops in that has served as the basis for the Puercan, Echo Canyon on the line of the Union Pacific Rail­ Torrejonian, and Tiffanian Stages of the continental road on the east side of the Wasatch Mountains in Tertiary time scale. northeastern Utah, where its red conglomeratic sandstones are prominent. The type Wasatch was EOCENE DEPOSITS (Tee) subsequently divided into the Knight, Fowkes, and Eocene continental deposits succeed the Paleocene Almy Formations, but this subdivision now appears in all the intermontane basins of the Rocky to be invalid. The Fowkes is a younger Tertiary Mountains, and attain sizeable volumes. They are as tuffaceous deposit accidentally infaulted in the much as14,000feet(4,250m)thickin the Uinta basin sequence. The lower red sandstones in Echo Canyon, on the south flank of the Uinta Mountains, and which are unconformable below the Knight, are exceed 5,000 feet (1,500 m) in many of the other actually Cretaceous (Echo Canyon Formation); else­ basins. By contrast, Eocene deposits are lacking or where, beds mapped as Almy are indistinguishable inconsequential east of the mountains, where only from the Knight, and the latter is the only true small remnants of the are representative of the original Wasatch. recorded above the Fort Union Formation in North The name Wasatch has been widely extended from Dakota; the Huerfano Formation occupies a small the type area to other parts of Utah, Wyoming, area at the north end of the Raton basin in Colorado. Colorado, and New Mexico, but the lithologic Eocene deposits record a marked change in Wasatch varies considerably in scope and age from PALEOCENE AND EOCENE SERIES 13 place to place, so that stratigraphic precisionists northeastern Utah, and have a thickness of 3,000- prefer to substitute local names in the different 5,000 feet (900-1,500 m). An outlying lacustrine areas. Nevertheless, it has become virtually a deposit, the , occurs in the generic term for the early Eocene continental to the north, where it overlies the local deposits of the region, and it is retained here in this representative of the Wasatch Formation (= Will­ sense. Its faunas are the basis for the wood Formation). of the continental Tertiary time scale. The only The Green River Formation has been extensively exception to the general application of the name investigated because of its economic potential-its Wasatch is in the , where the name· deposits of oil shale and of trona (a form of sodium Wind River Formation has traditionally been used. carbonate)-but much of its broader interpretation The name is of about the same vintage as the name and philosophical rationalization is the work of Wasatch, and applies to about the same strata, Wilmot H. Bradley (1964 and earlier reports), who although it apparently includes some younger beds. has devoted a large part of his scientific career to the Marginally, the Wasatch is a red, coarse, bouldery subject. piedmont deposit (as at the type locality) that in The Green River Formation is preserved in the many places oversteps the Paleocene and Cretaceous Green River and Washakie basins in Wyoming, the to lie on older rocks in the cores of the ranges. Farther in Colorado, and the Uinta basin in out in the basins it is a red-banded sandy clay and Utah. It accumulated in Lake Gosiute and Lake sandstone that was probably deposited by streams Uinta that lay north and south of the Uinta on flood-plains. Mountains axis, and which were partly confluent In northeastern Utah, southwestern Wyoming, around its eastern end. From time to time, the Rock and northeastern Colorado, the Green River lake Springs uplift in southwestern Wyoming formed an beds are overlain by higher Eocene continental island between the Green River and Washakie deposits (shown as Tee on the Geologic Map, like the basins. Wasatch). They include the Bridger of the Green The Green River Formation characteristically River basin and the Uinta of the Uinta basin, the forms light-colored, thin-bedded outcrops that con­ latter apparently somewhat younger than the for­ trast unmistakably with the adjacent Eocene terres­ mer. The Bridger and Uinta much resemble the trial deposits. Much of it is varved, and on the basis Wasatch Formation below but red colors are rarer, of its annual layers it is estimated that the lakes and some beds are chocolate brown. Minor lenses of endured for about 6,500,000 years. In detail, its lacustrine deposits occur, and the Bridger contains deposits exhibit a great variety of facies, depending much fine-grained tuff and other volcanic detritus. on their distance from shore, fluctuations in depth The Duchesne River Formation overlies the Uinta of water and salinity, and of the extent of the lakes Formation along the southern flank of the Uinta throughout time. During much of their life, the lakes Mountains and has been variously classed as of had exterior outlets to the north and south, but in the Eocene or of Oligocene age. On the Geologic Map it is middle of the epoch, in Wyoming at least, the lake shown as Oligocene (Toe), and it is df'scribed below shrank, the outlet disappeared, and the waters under that series. became highly saline, causing deposition of the beds In the San Juan basin of nC'rth western New of trona and other evaporites. Mexico, the Eocene of the central part was formerly The oil shale (an organic-rich marlstone), one of termed Wasatch Formation, but it has been renamed the characteristic components of the Green River, is San Juan Formation. It is about 1,500 feet (450 m) especially concentrated in the Parachute Creek thick and is divisible into various intertonguing Member in the Piceance basin of northwestern members of different lithologies. In the central part Colorado. Other facies include light gray, brown, or are units of shale and fine-grained sandstone, some buff dolomitic marlstone and shale; dolomitic marl­ red, some gray or varicolored, and around the edges stone thickly studded with salt-crystal molds; oolitic are coarse conglomeratic units that were spread as limestone and bedded algal deposits; limy sandstone fans into the basin. and sandy marls tone; anti beds of volcanic ash that become increasingly abundant upward. Marginally, EOCENE LACUSTRINE DEPOSITS (Tel) sandy deposits interfinger with the finer-grained Middle Eocene lacustrine deposits, known collec­ deposits. The formation contains , tively as the Green River Formation, cover an area of plants, pelecypods, gastropods, and larvae. , about 30,000 square miles (78,000 km2) in south­ , and bones occur in the shore facies. western Wyoming, , and Because of fluctuations of the lake, the Green River 14 THE CENOZOIC ROCKS deposits intertongue extensively with the terrestrial Cretaceous continental deposits are overlain by Wasatch deposits, especially in Wyoming, where a continental deposits of Paleocene and Eocene ages succession of tongues of Green River and Wasatch (Maxwell and others, 1967, p. 98-107). The Black have been mapped and named. Peaks Formation, an alternation of sand and clay with "cannonball" concretions, is about 800 feet OUTLYING AREAS (250m) thick and contains Paleocene mammals of NEW MEXICO AND WEST TEXAS Torrejonian and Tiffanian ages. It is followed by the South of the Raton and San Juan basins of very similar Hannold Hill Formation about 500 feet northern New Mexico, small, widely separated (150m) thick that contains mammals of Wasatchian patches of Paleocene and Eocene continental de­ age. As both formations crop out in a very small area, posits are preserved which are probably remnants of they are shown together as Paleocene (Txc) on the units that were originally more extensive but are Geologic Map. now so dismembered that the outlines of their basins ARIZONA .are no longer discernible. The basins were probably To the west in Arizona a dozen or more still smaller never as extensive, nor the deposits as thick as those areas classed as Eocene continental deposits (Tee) in the Rocky Mountains to the north. are widely despersed in the Basin and Range At the north end of the Sandia Mountains province. For the most part they are the units northeast of Albuquerque is the , designated as TKs on the State Map (1969). They are about 3,000 feet (900 m) thick, which lies unconform­ tilted and faulted, reddish-brown to gray conglomer­ ably on the Cretaceous and consists of variegated ate, sandstone, shale, and thin limestone with thick­ sand and clay, in part tuffaceous, with some beds of nesses of several thousand feet, which lie uncon­ fresh-water limestone. bones at various formably on the Meso.zoic and older rocks and lie levels indicate Wasatchian and younger Eocene ages. beneath the main body of Tertiary volcanics and West of the Rio Grande farther south, the Baca associated sediments. Aside from these structural Formation fringes the north edge of the Datil relations, there is little specific evidence as to their volcanic field for about 100 miles (160 km); lying ages, although the deposits in the Artillery Moun­ unconformably on the Cretaceous to the north and tains in the western part of the State contain palm passing southward beneath the younger Tertiary roots of possible Eocene age. volcanics. It is about 700 feet (210 m) thick and One of the more interesting of these units is the consists of a coarse basal conglomerate followed by Pantano Formation of the Tucson area, 6,500 feet red and white sandstone and clay. Indications of age (2,000 m) thick, of sandstone, mudstone, and granite­ are scanty, but the formation is presumed to be boulder fanglomerate (Finnell, 1970). Unlike the Eocene. younger Tertiary continental deposits of the sur­ At the north end of the just rounding area, the Pantano has been steeply east of the Rio Grande the McRae Formation, about upended, strongly deformed, and thrust over the 3,000 feet (900 m) thick, overlies the Upper Creta­ gneisses of the Rincon and Catalina Mountains. ceous Mesaverde Formation (Kelley and Silver, 1952, Like the other continental deposits of southern p. 115-120). It consists of a basal conglomerate Arizona, there is little evidence for its precise age; containing many volcanic clasts that is overlain by on the Geologic Map it is classed as Eocene (Tee), but maroon shales with some interbedded sandstone radiometric determinations on interbedded layers. bones have been collected in the and andesites yield ages of 25 to 36 m.y ., which lower part, and if not reworked, indicate a Creta­ indicates a range in age between early Oligocene and ceous 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 NEVADA AND CALIFORNIA Map it is assigned to the Paleocene (Txc). In southern Nevada east of Las Vegas, the Horse The Cub Mountain Formation in the Sierra Blanca Spring Formation consists of more than 1,000 feet area 80 miles (130 km) to the east is similar and (300m) of yellowish siltstone, whitish limestone, and probably correlative. It lies unconformably on the an upper unit of coarser clastic deposits (Longwell surrounding Cretaceous and is followed unconform­ and others, 1965, p. 45-48). The formation is con­ ably by the early Tertiary volcanics that form the formable on the Cretaceous and older rocks of the heights of the Sierra Blanca peaks. district, and is unconformable beneath the late Much farther south, in the Big Bend region of west Tertiary Muddy Creek Formation. Fossils are scanty Texas and east of the Chisos Mountains, Upper and the age of the Horse Spring is uncertain. It PALEOCENE AND EOCENE SERIES 15

contai~s a few plants, pelecypods, and gastropods WASHINGTON of possible Eocene age, but radiometric determina­ tions at one locality suggest a Miocene age. On the In central Washington, on the east side of the Geologic Map it is indicated as Eocene (Tee). Cascade Range in the latitude of Seattle, is a complex array of lower Tertiary continental deposits and The lower Tertiary continental deposits west of the associated volcanics that were elucidated early in Sierra Nevada in California have already been noted the century by the work of Smith (1904), and Smith (p. 7). East of the Sierra Nevada the only one and Calkins (1906); they were later reviewed by recorded is the Goler Formation of the El 'Paso Foster (1960). Mountains, a short distance north of the Barlock At the base is the Swauk Formation (Txc) more fault (Nilsen and Clarke, 1975, p. 24). It is about than 4,000 feet (1,200 m) thick, a nonvolcanic flu­ ?,500 feet (2,000 m) thick, lies unconformably on an viatile deposit composed largely of arkose, in part Igneous and metamorphic basement and is overlain conglomeratic, with some interbedded shale that by Pliocene continental deposits. it consists of a lies unconformably on pre-Tertiary metam~rphic lower. conglomerate and breccia unit, overlain by and plutonic rocks. It contains fossil plants of ~rkosic sandstone and red mudstone. Fossil plants Paleocene age. In the mountains to the west are In the Goler have been ascribed an Eocene age, but its smaller areas of the Guye Formation, once thought vertebrates are probably Paleocene. It is indicated as to be younger, but now known from paleobotanical Paleocene (Txc) on the Geologic Map. evidence to be correlative.

WESTERN MONTANA The Swauk is overlain unconformably southward by the Teneaway Basalt and the local Silver Pass Within the mountains of western Montana are a Volcanics 1,000-5,000 feet (300-1,500 m) thick (lTv), few early Tertiary continental deposits. which are followed conformably by another conti­ In the Flathead Valley west of the Lewis and nental deposit, the Roslyn Arkose, whose leaves and Livingston Ranges, or frontal ridges of the Belt rare vertebrate remains are middle or upper Eocene. terrane at the Canadian border is the Kishenehn The Guye Formation to the west is also foMowed by Formation, an early Tertiary deposit much obscured Eocene continental deposits and volcanics, such as by Pleistocene glacial deposits (Ross, 1959, p. 67-72; the Naches Formation which contains fossil leaves. described as "older alluvium"). It consists of well­ The lower Tertiary rocks of central Washington bedded, well-indurated, fine-grained sand silt and are overlain unconformably by the lower part of the clay, ~th so~e interbedded lignite. Fro~ pl~ce to great volcanic sequences of the Cascade Range and place It contains plant remains, mollusks, and . Columbia Plateau-on the west the Miocene Keeche­ The bulk of the material seems to be late Eocene but lus Andesite (uTa) and on the east theYakima Basalt some collections may be of Miocene or other Tertiary at the base of the Miocene Columbia River Group ages. It is represented as Eocene (Tee) on the (Tmv). Geologic Map. Farther north, on the west side of the Cascade Much farther south, in the Lima region of Range, is the Chuckanut Formation (Txc). It is southwestern Montana, is the Beaverhead Con­ lithically much like the Swauk, of similar origin glo~erate (~cholten and others, 1955, p. 368-369), similarly unconformably on the pre-Tertiary rocks' which occupies the low ground in .front of many of and with fossil plants of about the same age, but it the ranges, and is overthrust in places by the attains thicknesses of 15,000 feet ( 4,500 m) or more. Mesozoic and Paleozoic rocks; in other places it lies Various small to large outliers of Paleocene conti­ unconformably on them. It is as much as 9 000 feet nental deposits occur in the Cascade Range between (2,700 m) thick and consists of coarse red c~nglom­ the Swauk and Chuckanut, which suggest that the erate, with interbedded sandstone and some fresh­ two formations were once connected. water. limestone.. It is evidently a synorogenic deposit, formed d unng the deformation of this part of EUGEOSYNCLINAL DEPOSITS6 the Rocky Mountains, and is probably Paleocene (Txc), although fossil control is sparse. At one The only Tertiary rocks shown as eugeosynclinal locality north of Lima it is overlain unconformably on the Geologic Map are those in the core of the by deposits that contain late Eocene mammals (Sage Olympic Mountains of northwestern Washington, Creek Formation). About 40 miles (65 km) to the east whose sparse fossils indicate an early Eocene to of the Lima region, the Sphinx Conglomerate forms Miocene age. However, a considerable part of the a sm~ll outlier on the summit of the Madison Range and Is probably correlative with the Beaverhead. 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 consistent, outward-facing sequences, but no general Washington also have many eugeosynclinal quali­ stratigraphic sequence is determinable; most of the fications, being marine turbidites and interbedded rock is "broken-formation," and passes in places into submarine basaltic volcanics that accumulated on melange. an oceanic crust. OLIGOCENE SERIES The peripheral rocks of the Olympic Mountains surround the core on the north, east, and southeast The Oligocene Series, which formed during about and are a thick, steeply-dipping, outward-facing 11 million years of mid-Tertiary time, corresponds to sequence that ranges in age from Eocene to Miocene. the Tongrian, , and Stages of the The Eocene part, mainly the Crescent Formation, European marine sequence and is divided into the resembles that farther south in the coastal belt, , Orwellian, and Stages of already described (p. 8). the American continental sequence. The Oligocene is The core rocks are thrust-faulted against the only meagerly represented on the Geologic Map of peripheral rocks. These faults, including the Hurri­ the United States (fig. 2)-among the marine cane Ridge fault on the north, now dip steeply, but deposits of the Gulf Coastal Plain and the Pacific probably originally dipped eastward at a much lower northwest (To), among the continental deposits of angle; they and other concentric faults within the the northern Great Plains and Rocky Mountains core form the boundaries of crustal slabs that were (Toe), and in smaller areas elsewhere. Some other subducted eastward from the ocean floor beneath the deposits may be of Oligocene age but are mapped continent and its marginal deposits. Westward into with the series above or below on the Geologic Map. the core, each slab consists of rocks somewhat Oligocene volcanic rocks in the western United younger than those in the slab to the east; hence, the States are grouped with others of the lower Tertiary Geologic Map distinguishes units from east to west (lTv). as of Eocene age (Tee), Oligocene and Eocene age MARINE STRATIFIED ROCKS (Toee), and Miocene and Oligocene age (Tmoe). GULF COASTAL PLAIN The core rocks are marine shales, turbidites, and Strata of Oligocene age are shown on the Geologic minor basalts. To the east they are penetratively Map of the United States as a narrow band of outcrop sheared, and metamorphosed to phyllite and semi­ in the eastern Gulf Coastal Plain extending from schist of prehnite-pump~llyite and low-rank green­ Mississippi into Georgia, and less continuously schist facies (Tabor, 1972) (on the Geologic Map they eastward into South Carolina and westward into are shown with a metamorphic overprint, although Louisiana. They emerge again in northern and elsewhere on the map this overprint is reserved for central Florida on the flanks of the Ocala uplift. No rocks of amphibolite grade or higher). Beds and folds Oligocene is known in the Atlantic Coastal Plain are commonly.sheared out and the sandstones form north of South Carolina, at least at the surface. No boudins. To the west, the rocks are still highly folded Oligocene is mapped in the western Gulf Coastal and faulted but are less penetratively sheared and Plain in Texas, although the non-marine Frio Clay of the only metamorphic minerals are zeolites. the southern part of the State may be of this age; it Sparse microfossils and megafossils indicate that was grouped with the Miocene on the Geologic Map the core rocks to the east are Eocene (a few collections on the advice of D. H. Eargle (oral commun., 1971). may be Paleocene~ but nothing older is known). The Oligocene of the outcrops in the Gulf Coastal Westward in the core more numerous fossils indicate Plain is only a few hundred feet thick, but it thickens late Eocene and Oligocene ages, and near the coast to many thousands of feet downdip in subsurface early to . where a complex stratigraphy has been worked out. Rocks in the core include pillow basalt, basaltic The outcrops in the eastern half of the Gulf Coastal breccia and tuff, tuffaceous argillite, and associated Plain are dominantly calcareous, with many lime­ limestone. The basaltic rocks are especially abun­ stone beds and formations, which are interbedded dant in the eastern, or Eocene block, where they form with marl and lesser sand and clay, giving rise to pods and lenses in sheared rock but were probably many thin stratigraphic units. originally autochthonous. The rarer lenses of ba­ The most prominent part of the Oligocene is the saltic rock in the blocks farther west might be alloch­ middle, or Vicksburg Group, named in 1848 for thonous, as the youngest foraminifers so far exposures near Vicksburg, Mississippi, at the identified in the associated are all west end of the main outcrop belt. The Vicksburg is Eocene. divided in Mississippi and Alabama into the Mint The sediments in individual outcrops indicate Spring Marl, , and Byram OLIGOCENE SERIES 17

Limestone, as well as various minor units or Geologic Map. Much of the anticlinorium of the members. The Marianna and Byram Limestones Oregon Coast Ranges was emergent, and the form the main part of the Oligocene in Florida. Oligocene deposits occur along its west and east In the type region the Vicksburg is underlain by edges and around its north end. the Forest Hill Sand and Red Bluff Clay, which lie on Vigorous pyroclastic volcanism in the Cascade the Jackson Group; and it is overlain by the Range to the east caused the Oligocene marine Chickasawhay Marl and its equivalent, the Flint deposits to be dominantly tuffaceous; they pass River Formation, which in Florida passes into the eastward into volcanic-rich continental deposits Suwannee Limestone. along the edge of the Cascade Range, which lack the To the west, across the Mississippi River in coal measures of the preceding Eocene. The Oligo­ Louisiana, only equivalents of the Vicksburg persist cene deposits include the Yaquina Formation of the in the outcrops, which are here dark shale with some Oregon coast, composed of cross-bedded arkosic and sandy and lignitic layers. They are overlapped near tuffaceous sandstone, and the Lincoln Formation of the west edge of the State by the Miocene Catahoula the Washington coast, consisting of massive tuffa­ Formation and do not come to the surface in Texas­ ceous siltstone and fine-grained sandstone. East of unless the Frio Clay in the far southwest is of the Coast Range uplift in Oregon is the richly Oligocene age. fossiliferous Eugene Formation. North of the PACIFIC COASTAL AREA Olympic Mountains along the Strait of Juan de

CALIFORNIA Fuca, more than 10,000 feet (3,000 m) of Oligocene The Oligocene of California is represented by the silts and clays were deposited. As this area was Lincoln and Blakeley Stages of marine megafossil farther removed from the volcanic activity to the zonation, and the greater part of the Refugian Stage east, these deposits contain much less tuffaceous of microfossils. However, there is much uncertainty material than the other Oligocene units. as to the correlation of beds assigned to the CONTINENTAL DEPOSITS Oligocene in California with the type Oligocene in Europe, so that there is controversy as to the exact Continental deposits of Oligocene age are widely limits of the true Oligocene. In addition, Oligocene distributed in the northern part of the interior region was a time of widespread emergence in the coastal of the Western United States, but individual outcrop area of California, so that the supposed Oligocene areas are mostly small and widely dispersed, the formations are very locally distributed and rather largest being in the Great Plains and eastern part of thin. On the Geologic Map, the Oligocene is thus not the Rocky Mountains. In many places, especially shown for the most part, its strata being merged with toward the west, the Oligocene continental deposits the underlying Eocene or overlying Miocene. are intermingled with contemporaneous volcanic The largest area of Oligocene shown on the map is rocks, which are grouped with the other lower the Sespe Formation, a nonmarine unit about Tertiary volcanics (lTv) and are described separately 5,000 feet (1,500 m) thick that is intercalated in the later. marine sequence in the western part of the Trans­ GREAT PLAINS AND ROCKY MOUNTAINS verse Ranges; from place to place the Sespe extends The (or Group) has become from the upper Eocene into the lower Miocene. It is a the generic name for most of the Oligocene deposits red clastic deposit largely of granitic debris that in the Great Plains of South Dakota, , and contains vertebrate bones at several levels. At the Colorado, and westward into the Rocky Mountains western end of the Santa Ynez Mountains it grades of Wyoming. The formation was named by Meek and into marine beds (Alegria and Gaviota Formations)~ Hayden in 1858 for the White River in southwestern Farther north, marine formations of Oligocene age South Dakota southeast of the , along (not represented on the Geologic Map) include parts which it is spectacularly exposed in the Big of the San Lorenzo Formation of the Santa Cruz . To the south, the White River reappears Mountains and the Kreyenhagen Shale of the from beneath the Miocene tnd Pliocene deposits in western part of the San Joaquin Valley. the valleys of the North and South Platte Rivers, where it extends from the front of the Rocky OREGON AND WASHINGTON Mountains in Wyoming and Colorado, eastward into In the northern coastal belt of Oregon and Nebraska. Outliers of the White River to the west in Washington, as in California, the Oligocene is Wyoming are mostly in intermontane basins, but thinner and more restricted than the Eocene, but its some occur high up in the mountains, indicating the outcrops are large enough to be represented on the great original extent of this broad sheet of deposits. 18 THE CENOZOIC ROCKS

·----­------( ··-r-·· ------..---- I ~ : I •, I I \ \ ~, __ ...... ______( 'L, I I, I - . : ... ;.. \ ~· ~) .. ~------; .. ) ·-, • r-·----- "I .' ' . ,_---~-!"',j ·--- : l / I f~ I I' I f ------1___ .,. i I ~- ,; ·------~---- , _____ "', _, / ---- ., \ I I ' \' I I I I I

!. __ \._ I ------\ ,-\.: \: \} ' ·"" L ' -' I '•-" t, ·--_...., ·- "'"'·:r '·'\.. __ _ ----,.._' I ·-...... I

I r·----..i:-----. ______J ------:... _;

'> \ z \ \ \ \ '-,

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

,·--.. ./, ...... , : \ I ., : \ ( ',-, -~ ,J \ · ...... _r·-,_.,"\.,_,._ .• ,f r\ ---r----~ \ .. ( \ I : . '. I \ "\ ' ' \ ' \:- -~------I -·. -"'. t-- \. \ ------\ \,_\: ______.I , ..r \ \ I t.------,;' -·->~ \,, ~. \..: '.r-, ' I I / ,.._( 5;.r·f'·' I ., -' 1 \r-5 , ---·--·- \------2-1;:-:1----' J ;._ .. -----~------\ r' I \ ) \ ; \ ( ' -"""""""" .. -., < \ :______.. -i, ' \. ;,~ ., ,' ' \/,. ! \ ~-----: ~ ! \

G U L F OF MEXICO ,.,.. . -·

FIGURE 2.-Continued. In its type area in the Great Plains, the White River cene deposit forms Slim Buttes northwest of the is 600 to 700 feet (180-210 m) thick, but elsewhere it is main area in South Dakota and is locally present mostly thinner. In the Great Plains it is divided into beneath the Chadron elsewhere. Both the Chadron the of clay, sandy clay, and and Brule abound in vertebrate bones, notably the river-channel sandstone, and the overlying thicker, gigantic titanotheres (Osborn, 1929, p. 103-112). finer-grained Brule Clay. A somewhat older Oligo- The White River Formation is a terrestrial deposit 20 THE CENOZOIC ROCKS that was spread across a broad plain sloping gently deposit that represents a new cycle of sedimentation, eastward from the Rocky Mountains and has been and it overlaps northward on the pre-Tertiary rocks. little deformed since. (An early idea that it was a Opinions differ as to whether the formation is of subaqueous deposit laid down in a vast lake has long Eocene or Oligocene age; the Duchesnian Stage of since been discredited.) It includes massive, len­ the continental Tertiary is conventionally placed in ticular river-channel sandstones, surrounded by the Eocene, but at least the uppermost members (La more extensive finer-grained flood-plain or overflow Point and Star Flat) are certainly Oligocene (Clark, deposits, and occasional deposits that were laid 1975, p. 110). It has yielded a K/ Ar age of 39.3 m.y. down in local ponds and marshes. Volcanic ash OUTLYING AREAS forms occasional layers. The river channels can be traced for considerable distances, and extend east­ OREGON ward and southeastward from the mountains. Their In north-central Oregon the John Day Formation mineral content can be matched with their respective flanks the Blue Mountains uplift on the northwest, sources-the Black Hills, the Laramie Range, and where it lies unconformably on the Eocene Clarno the Front Range (Clark, 1975, p. 99-104). Probably Formation (lTv), and is overlain unconformably by the rivers coalesced eastward into streams draining the basalts of the Columbia River Group (Tmv). Both south into the . The excess of the Clarno and John Day are of volcanic origin, but deposition in the region can be attributed to a general the John Day is more dominantly sedimentary, change from the moist climate of Eocene time to one hence is shown as continental deposits (Toe) on the of increasing aridity. Geologic Map. The John Day is about 2,000 feet (600 The vertebrate faunas of the White River reflect m) thick and is composed largely of tuffaceous these varied environments. The fine-grained flood­ sediments-water-laid tuffaceous siltstone and rhy­ plain deposits contain chiefly plains and olitic air-fall tuff and ash-flow tuff. It has been animals, whereas the river-channel deposits contain famous since the turn of the century for its chiefly forest and fluviatile animals. The lagoon and mammalian remains, which are assigned to the swamp deposits contain fish, , and aquatic Whitneyan and Stages of the conti­ plants. nental Tertiary (upper Oligocene and lower Miocene). South of the White River area, in the Great Plains Farther west in Oregon, in the westel'Il part of the of Colorado, the Castle Rock Conglomerate forms a Cascade Range, are the lower Tertiary andesites of small outlier in the center of the Denver basin south the Little Butte Volcanic Series (ITa) (Peck and of Denver. It is essentially an extension of the White others, 1964, p. 11-26). In some segments of its River deposits and contains many of the same outcrops, nearly three-quarters of the series consists vertebrate bones. It is underlain by a rhyolite ash­ of poorly bedded andesitic tuff and breccia; andesitic flow tuff lying unconformably on the Paleocene lapilli tuff, welded tuff, and breccia; stratified tuff deposits that has been dated radiometrically at 34.8 and tuffaceous sandstone and conglomerate. These m.y. and is probably an extension of the Thirtymile volcaniclastic components of the Little Butte Series volcanic field in the Front Range to the southwest are shown separately on the Geologic Map as (Steven, 1975, p. 83). continental deposits (Toe). Fossil plants indicate that they are of middle Oligocene to lower Miocene Within the southern Front Range itself are the age. Florissant Lake Beds, famed for their plant and remains, but with very few vertebrates In the Basin and Range part of southern Oregon, (MacGinitie, 1953). Underlying tuff beds have a the lowest unit exposed in Steens Mountain and radiometric age of 35 to 36 m.y., and the lake beds other block-faulted ranges is the Cedarville Forma­ cannot be greatly younger. The lake beds accumu­ tion (Toe), typified by exposures in the Warner Range lated in a shallow basin on a surface carved in late of northeastern California, where as much as Eocene time and dammed by the eruptions of the 7,500 feet (1,200 m) is exposed (Russell, 1928, p. 402- Thirtymile volcanic field to the west. Their floras 416). It consists of andesitic tuffs, agglomerates, and indicate that the lake beds formed in a warm, mudflow deposits, with minor . The lower beds subhumid climate at an altitude of about 3,000 feet contain a middle Oligocene flora, but the upper part (1,000 m), as compared with their present altitude of extends into the Miocene. The Cedarville is uncon­ 9,000 feet (2, 750 m). formable beneath the main Miocene and Pliocene On the south flank of the Uinta Mountains is the volcanic sequences in the ranges. Duchesne River formation (Toe). It lies conformably CALIFORNIA on the Eocene , but it is a coarser In the southwestern Klamath Mountains of MIOCENE SERIES 21 northern California are several small areas of the Coastal Plain that extends continuously from the Weaverville Formation (Toe) (Irwin, 1966, p. 29-30), Rio Grande in Texas to South Carolina, except for a which is a deposit of sandstone, shale, conglomerate, 20-mile (32-km) alluvium-covered gap along the tuff, and lignite as much as 2,000 feet (600 m) thick, Mississippi River. Miocene rocks are also extensive that contains fossil plants of Oligocene age. The throughout the Florida peninsula. On the outcrop the Weaverville lies unconformably on the pre-Tertiary Miocene is about 2,000 feet (600 m) thick, and most of basement rocks of the Klamath Mountains and was it from Alabama westward is brackish-water or probably deposited in local, partly down-faulted continental deposits. It changes to marine in depressions. subsurface downdip where it contains a sequence of In eastern California, in the Grapevine Mountains widely traceable foraminiferal zones. It thickens east of Death Valley, the Paleozoic rocks are overlain dramatically, especially across growth faults down­ by the Titus Canyon Formation (Toe), about 3,000 thrown toward the coast, so that near the Texas and feet (900 m) thick, composed of sandstone, in part Louisiana coast it attains 48,000 feet (14,500 m) tuffaceous, conglomerate, mudstone, and algal (Crouch, 1959, p. 1285-1286). limestone, that contains lower Oligocene (Chad­ Terminology of the Miocene deposits has been ronian) mammals (Stock and Bode, 1935). Farther much confused by a host of stratigraphic names that south in the Death Valley region the Oligocene is have been used in different senses by successive volcanic (Artists Drive Formation, lTv). authors (Murray, 1961, p. 407-412). Actually, in In the Orocopia Mountains of southeastern Calif­ surface outcrops at least, the gross subdivisions are ornia, east of the Salton Sea and the San Andreas fairly clear. fault, marine Eocene rocks (Maniobra Formation, At the base in the western half of the Coastal Plain Te) are overlain by an unnamed unit of nonmarine is the Catahoula Sandstone, named for Catahoula conglomerate, sandstone, and mudstone (Toe) of Parish, Louisiana, and recognized from Alabama to probable Oligocene and possible lower Miocene age. Texas. In Texas it is generally 200-300 feet (60-90 m) thick and is formed of clay and sandstone. The clay MIOCENE SERIES is bentonitic and noncalcareous, and the sandstone The Miocene Series, whose rocks formed during fine- to medium-grained, cross-bedded and tuffa­ about 14 million years of mid-Tertiary time, corres­ ceous. In south Texas the formation thickens to ponds to the , , Helvetian, 1,000 feet (300m) and is more coarsely volcanic; this , Samartian, and Pentian Stages of the part was called the Gueydan Formation by Bailey European marine sequence. In , as in (1926) and is now distinguished as the Catahoula the Eocene and Oligocene, the marine sequence in Tuff. Its volcanic clasts include boulders, pebbles, California, and the continental sequence in the and chunks of lava, porphyry, and . Many of interior region, are divided into stages of local these can be specifically matched with contempo­ application. Next to the Eocene and Paleocene, the raneous volcanic formations in the trans-Pecos Miocene is the most extensively exposed Tertiary mountains and upstream along the Rio Grande, but series in the United States (fig. 3). Marine deposits some show so little water-rounding that they may (Tm) occur in the Atlantic and Gulf Coastal Plains have come from now-concealed volcanic centers in and the Pacific coastal areas of California and the Coastal Plain itself. The Catahoula in Texas farther north. Continental deposits (Tmc) are exten­ contains a few brackish-water mollusks and more sive in the northern Great Plain1::~ and Rocky abundant plants, including palms that must have Mountains and in smaller areas in the interior region grown in tropical coastal swamps. farther west. Miocene volcanic rocks (Tmv, Tmf) are The Catahoula Sandstone contains workable widely distributed in the western interilor, where they deposits of fuller's earth in east Texas (King, 1940, interfinger in many places with the continental p. 179-182) and important deposits of uranium in deposits; they are treated under a later heading. south Texas (Eargle and others, 1975, p. 773-777). In south Texas the Catahoula is underlain by the MARINE STRATIFIED ROCKS Frio Clay 150 feet (45 m) thick, of lacustrine origin. GULF COASTAL PLAIN 7 Its age is uncertain and it might be Oligocene, The Miocene formations are exposed in a narrow to although it is included with the Miocene on the wide band of outcrop in the shoreward part of the Geologic Map. Northward, the Catahoula oversteps the Frio and lies unconformably on the Eocene.

'Compiled from many sources; few modem summaries are available, but earlier In Texas, from the Brazos River southwestward, summaries by Plummer (1932, p. 710-749) and Cooke (1945, p. 190-195) are still useful. the Catahoula is followed unconformably by the 22 THE CENOZOIC ROCKS

·----~--.r-r------..---- ~ a.~. : I \ I : ·-...... ---...... -...... ------~( .. .._., <~ I; ': ,.,. ·. ' I ( ) ~' y ; ', )' (-~f/t !.____ ~------~ (., ' - ". ------l l ---- ~-· 'j ------~ : I - , ..: ------I ------:I I ,' -~-- : ~ 1- ---r------! --- ~ ------,_----~\ I I 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 23

J'\../''~ ... , : \ I ._ : \ ( ·., -~ / ·-...... _r--t..•• ""'-·-··--. ,f f"\ ~- ...... ~ '. ---r._ I ) '\ I : . \ I , "'\. '. , \_J------

1'r __ ...... - -"\"\ \. -, ------""'\ \ ·------\ _..,..... '-,...... \ ~ \:,----) ---r-- .,_ /. I \\ -- ' \ \ \ :'· ----:-..P~ I • I \.--- \ / ,....._~'!__ (.------.;' \ ,.,: ,, ;' ' ( \ : i' __\·-.~ t \ . ..-v r·· . ·,, C \ .. ..,.._.\.. I I ', ,.- ·.- I '-: .r-: I / \ J I ( ; :"-, _'-/.. - -·· /-. ' t' t;.r·f'··} ;' - - __ .. _...... ---·- . -' ,.·· ---)---- 1 ~~~-S ------,,/'! ----·'------?__r-· ~~ --' -___ .1 \ ~ ,/ / ___ .~.----( ,.(------\\ --, __

) ~ / \ _...... _..,_.,I \~ .\ :_.. ------L, I I .\ .. ,~· ,.~.

GULF OF MEXICO

FIGURE 3.-Continued. Oakville Sandstone 200-500 feet (60-150 m) thick, a near and beyond the Balcones Escarpment, and the coarser deposit formed during a new cycle of gravels contain notable quantities of reworked sedimentation. Drainage during its deposition was Cretaceous marine shells (as much as 5 percent of the now from the northwest, from Cretaceous outcrops volume). The Oakville Sandstone also contains 24 THE CENOZOIC ROCKS

abundant bones of horses, , , and and sandy formations, all marine and very fossilif­ that are of Arikareean and Heming­ erous, all separated by unconformities. In Maryland, fordian (middle Miocene) age. No marine fossils are Delaware, and northern Virginia, it consists of known on the outcrop, but occur in subsurface a short the middle Miocene Calvert, Choptank, and St. distance downdip. Mary's Formation, overlai)l toward the south by the The Oakville is followed conformably by the Yorktown Formation, traditionally classed as upper Fleming Formation (= Lagarto Clay) and merges Miocene, which is the most extensive unit in eastward with its lower part. The Fleming is 1,200 southern Virginia and North Carolina. In New feet (360m) thick and is similar to the Oakville but Jersey, the middle Miocene is represented by the finer grained. It contains brackish-water mollusks Kirkwood and Cohansey Formations, and no equiv­ and mammalian bones of (latest Mio­ alents of the Yorktown are present. cene) age. The is prominently exposed in The nonmarine facies of the Miocene continues in the Calvert Bluffs on the west shore of Chesapeake surface outcrops eastward across Louisiana and Bay in Maryland, where it consists of diatomaceous Mississippi into Alabama. The lower sandy beds are clay with a shell bed in the middle that is a mass of everywhere termed Catahoula, but the upper beds pelecypods and gastropods; the shells weather free are divided into the Hattiesburg and Pascagoula and strew the modern beaches of the bay. The Clays, each about 400 feet (120 m) thick. The overlying Choptank is more sandy but is also very Pascagoula contains a few layers with marine fossiliferous. The St. Mary's is clay, sandy clay, and fossils. sand. In the eastern part of the Gulf Coastal Plain, the The Yorktown at the top is typically exposed in exposed Miocene passes into a marine, calcareous, the bluffs of the York River at Yorktown, Virginia, highly fossiliferous facies of about the same thick­ and consists of coquina, marly sand and clay, and ness as the nonmarine deposits farther west. At the glauconitic sand. Recent paleontological work indi­ base in Florida and Georgia is the Tampa Limestone, cates that it and the Duplin Marl (its extension into more variable and less pure than the Oligocene and South Carolina) are lower Pliocene in age, rather Eocene limestones beneath it; it is approximately than upper Miocene in terms of European standard equivalent to the Catahoula Sandstone farther west. sections (J. E. , written commun. April, 1975). It is followed by the Alum Bluff Group, comprising Moreover, in the Yorktown has yielded a the Chipola and Shoal River Formations and the Pliocene K/ Ar age of 4.4 m.y. The Yorktown is the local Grove Sand. The Chipola includes coarse only upper Tertiary unit exposed south of the James siliceous sand and limestone, and the Shoal River is River in Virginia and beyond through North Caro­ chiefly sand and clay. The Alum Bluff passes lina; hence, all the Miocene shown on the Geologic northeastward into the Hawthorne Formation, which Map in this segment of the Atlantic Coastal Plain is extensive in Georgia and extends as far as South should be transferred to the Pliocene. Carolina. The Hawthorne consists of gray phos­ phatic sand with lenses of fuller's earth that are PACIFIC COASTAL AREA mined commercially in many places. Besides its CALIFORNIA 9 invertebrate shells, it contains mammalian bones of The Miocene of the coastal area in California has age. the most complex display of rocks and formations,

ATLANTIC COASTAL PLAINS and the thickest sequences in surface exposures (9,000 feet, 2, 700 m, or more) of any Miocene rocks in North of a gap inNorth Carolina where Cretaceous the United States. It occurs in numerous small formations extend nearly to the coast on the Cape outcrops in the Coast Ranges south of San Francisco arch, Miocene deposits are extensive in the bay, eastward across the San Joaquin Valley, Atlantic Coastal Plain as far as New Jersey. The south ward across the western part of the Trans­ Miocene units are, in fact, the most extensive of the verse Ranges, around the Los Angeles Basin, and Tertiary deposits of this part of the Coastal Plain, along the western flank of the Peninsular Ranges and in places overstep the older Tertiary and Creta­ nearly to the Mexican border. Narrow fault-bounded ceous so that they extend onto the pre-Mesozoic slices of what were originally extensive Miocene crystalline rocks of the Piedmont province. formations extend eastward along the north side of The Miocene consists of a number of thin shaly

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 8For summary, see Spangler and Peter10n (1950, p. 61, 75-79). Bramlette (1946). MIOCENE SERIES 25 the . Continental deposits of stone, which in some areas replace them entirely. Miocene age are widespread in southeastern Calif­ The upper Miocene in the Salinas Valley of the ornia, especially in the Mojave region. · central Coast Ranges is again a coarse clastic A large body of knowledge regarding the Cali­ deposit, the Santa Margarita Formation. fornia Miocene has accumulated, partly as a result Volcanic rocks (Tmv), mainly andesite and basalt, of academic research, partly from petroleum explo­ are a locally significant component of the California ration, and can only be summarized briefly here. Miocene. They occur, for example, at The Pinnacles The California Miocene has long been divided on in the Gabilan Range of central California, and in the basis of marine megafossils into the Vaqueros, the western farther south, Temblor, Briones, Cierbo, and Neroly Stages, but where they are more than 5,000 feet (1,500 m) thick. these are rather vague and not entirely satisfactory, Tuffaceous deposits and bentonite are widely dis­ as they are · based on rock-stratigraphic units tributed in the middle and upper Miocene. There is a mainly in the Central Coast Ranges. Later, it was possible, although unproved relation between the divided on the basis of foraminifers into the volcanic episode and the widespread distribution of Zemorrian, Saucesian, Relizian, Luisian, Mohnian, siliceous sediments. and DelmontianStages (Kleinpell, 1938, p.103-157). The marine Miocene deposits grade eastward and These stages are believed to be more nearly time­ southeastward into continental deposits. The latter stratigraphic units, which cut through the complex occur at the western foot of the Sierra Nevada along array of formations and rock facies, and they have the eastern side of the San Joaquin Valley (Mehrten come in to wide use. and Zilch Formations), which to the west toward the North of the Transverse Ranges, the San Andreas San Andreas fault in the Bakersfield area change to fault zone extends through the Miocene formations. marine deposits. West of the San Andreas fault in This segment of the fault has undergone more than the latitude of Bakersfield, the continental deposits 300 miles (480 km) of right-lateral displacement are offset northward, so that the transition into during and since Miocene time, which has resulted marine deposits occurs near Caliente Mountain. To in marked offsets of Miocene facies across it, such the south in the Transverse Ranges are the Tick as the boundary between marine and continental Canyon and Tecuya Formations with Arikareean deposits, and the boundary between warm-water and Hemingfordian (lower Miocene) mammals and and cold-water marine faunas (Addicott, 1968). the with Barstovian (upper Clearly, the geography during Miocene time was Miocene) mammals. There are unsolved discrepan­ quite different from that at present, which greatly cies in age assignments between these fossiliferous complicates paleogeographic reconstructions. continental deposits and the fossiliferous marine deposits with which they intergrade (Durham and Lower Miocene deposits (Zemorrian and Sauce­ others, 1954). sian) include much fairly coarse clastic material, Structural unconformities occur at many places and in various places have been termed the below, above, and within the California Miocene, Vaqueros and Temblor Sandstones. The most but individual unconformities are of local extent characteristic rocks of the younger Miocene are and are seemingly represented elsewhere by con­ diatomite and associated siliceous sediments, whose formable sequences. There were thus no universal white, thin-bedded ledges are a characteristic fea­ times of orogeny and def~rmation during the epoch. ture of the landscape of the Coast Ranges and other Nevertheless, there seems to have been a general parts of California (Bramlette, 1946). They are reorganization of depositional regimes in middle known collectively as the Monterey Formation, but Miocene time as a result of tectonic movements; they are a facies that varies greatly in scope and age middle and upper Miocene deposits appear to have from place to place-in places only middle Miocene, been more extensive and varied than those of the in others both middle and upper Miocene and even lower Miocene. basal Pliocene. The siliceous rocks include por­ celanite or siliceous mudstone, laminated siliceous shale, and thin-bedded chert and cherty shale. OREGON AND WASHINGTON Diatomite, or -rich siliceous rock, occurs In the north western coastal area of Oregon and mainly in the upper parts of individual sequences; Washington, Miocene marine deposits are much however, the lower siliceous rocks were also prob­ less extensive than in California; most of the ably originally diatomaceous and have been altered Tertiary rocks of the Coast Ranges are Eocene and by diagenesis. The siliceous rocks intertongue Oligocene. The Miocene is mostly confined to marginally with mudstone, siltstone, and sand- narrow strips along the coast, although it extends 26 THE CENOZOIC ROCKS

somewhat farther inland in the Grays Harbor area and Colorado, the Miocene deposits are preserved in of southwestern Washington. separate basins, in some of which they attain In the Newport embayment of western Oregon, thicknesses much greater than those in the Great the lower Miocene is represented by 3,500 feet Plains. The basins approximate the original deposi­ (1,000 m) of the organic-rich Nye Mudstone, but the tional sites, but during the later Miocene, to judge most extensive formation along the coast is the from the occurrence of scattered outliers, sediments middle Miocene Astoria Formation, which consists seem to have been spread widely between the basins of about 1,000 feet (300 m) of sandstone and and across some of the intervening ranges. siltstone. One of the most extensive Miocene deposits is the The Astoria is interbedded with basalt flows Split Rock Formation, typically developed in the erupted from local centers and is succeeded along Sweetwater area in the center of Wyoming, where it the downwarp near the Columbia River by basalt of surrounds and partly buries the Precambrian rocks the Columbia River Group (Tmv) that flowed from of the Granite Mountains (Wg) (Love, 1961, p. 5-25). the main eruptive area of flood basalts east of the It is about 2, 700 feet (830 m) thick, and vertebrate Cascade Range. Near the coast, the normal terres­ evidence indicates that it includes most of the trial basalts of the Columbia River pass into Arikareean and Hemingford Stages. It consists accumulations of pillow lava and breccia. mainly of fine-grained silty sandstone with some units of coarser porous sandstone. Much of the CONTINENTAL DEPOSITS sediment was derived from the northwest, and some NORTHERN GREAT PLAINS AND ROCKY MOUNTAINS of it is of aeolian origin; the Rocky Mountains were In the north-central Great Plains, in the segment not actively shedding detritus during Split Rock centering in western Nebraska and southeastern time. Wyoming, Miocene continental deposits are part of In the vicinity of the eastern Uinta Mountains, the great sheet of outwash that was spread on the 130 miles (210 km) southwest of the Sweetwater slope east of the Rocky Mountains from Oligocene area, is the . It is typically through Pliocene time. Much of the Miocene has developed in a graben that splits the crest of the traditionally been assigned to the Arikaree Group, Uinta Mountains at their eastern end, but it extends but an overlying Hemingford Group has been farther east along the valley of the Yampa River; recognized later (Lugn, 1939, p. 1251-1258). These remnants also occur along the south flank of the groups are the types of the Arikareean and Uinta Mountains and along the west flank of the Hemingfordian Stages of the continental Tertiary, Park Range, indicating that the deposit was although the Hemingford apparently contains beds originally much more extensive. The Browns Park of the Barstovian Stage at the top. is about 1,800 feet (550 m) thick, and most of it is The Miocene deposits are 500-1,000 feet (150- white or gray, fine-grained, tuffaceous sandstone of 200 m) thick in western Nebraska, but wedge out aeolian and fluviatile origin, but at the base is a eastward and southward between the underlying coarse conglomerate, composed of quartzite of the and overlying Tertiary. Channeling is character­ Uinta Mountain Group (Y). Mammalian remains istic, and each successive formation lies on a indicate that the formation is mainly upper Miocene channeled surface of the one beneath. For example, (Hemingfordian and Barstovian); radiometric ages the Gering Formation at the base of the Arikaree range from 26 m.y. to 9 m.y. (Izett, 1975, p. 186-189). lies on a deeply channeled surface cut on the On the north flank of the Uinta Mountains is the Oligocene Brule Clay, and the Marsland Formation Bishop Conglomerate, which caps a series of high at the base of the Hemingford lies with marked terrace remnants. Its conglomerates resemble those erosional on the Harrison Formation at the base of the Browns Park Formation, with of the Arikaree. Some of the channels, such as those which it is not in contact, which has led to the of the Gering Formation, contain coarse sand and suggestion that it is of the same age, but present gravel, but for the most part the Miocene sediments indications are that it is earlier Miocene. To the are fine-grained sands and silts, indicating that east, between the Park and Front Ranges, are during the epoch the Rocky Mountains were not smaller areas of Miocene continental deposits, the actively shedding material. The upper part of the North Park and Troublesome Formations, largely Sheep Creek Formation at the top of the Heming­ uppermost Miocene. ford is less channeled than the rest and consists of In the high plains of northwestern Montana, close fine-grained flood-plain material and aeolian silt. to the Canadian border and far from the formations Westward in the Rocky Mountains of Wyoming just discussed, the Flaxville Gravel forms the cap of MIOCENE SERIES 27 remnant plateaus on top of the Paleocene and Grande Valley. Aside from being obviously middle high Upper Cretaceous rocks. It is a few hundred Tertiary, its exact age is undetermined. feet thick and consists of gravel derived from the Farther south, at the south end of the Caballo Rocky Mountains far to the west intermingled with Mountains, are the Palm Park and Thurman some sand and clay. Rather abundant vertebrates Formations (Kelley and Silver, 1952, p. 120-123). suggest a or early Pliocene age; it is The first is a red bouldery deposit with many shown on the Geologic Map as Miocene (Tmc). volcanic clasts, the second a light buff tuffaceous A different set of deposits occurs in the basins clay. They overlap the Paleozoic rocks of the within the mountains of southwestern Montana. Caballo Mountains and are overlain by the Pliocene They were originally termed the "Bozeman lake Santa Fe Formation of the Rio Grande Valley. The beds," but most of them are fluviatile rather than formations seem to be younger than the Paleocene lacustrine in origin, and modern work indicates that McRae Formation (Tx) at the north end of the they are heterogeneous, including deposits of many mountains, although not in contact with it, and are Tertiary ages, now collectively referred to as the probably of middle Tertiary age. Bozeman Group (Robinson, 1961, p. 1003-1004). West of the Rio Grande in southwestern New Included in the Bozeman Group in some of the Mexico is the extensive Mogollon-Datil volcanic basins are Eocene and Oligocene strata that are province, which continues into southeastern Ari­ unconformable beneath the younger Tertiary. The zona. The volcanic rocks have been called collec­ younger Tertiary forms the greater part of the tively the Datil Formation, but recent work indi­ surface areas of the basins, and its vertebrates cates that they include at least three major sub­ indicate late Miocene and Pliocene ages; it is shown divisions of Oligocene and Miocene ages (Elston on the Geologic Map as Miocene (Tmc). The deposits and others, 1973, p. 2261-2263). Most of the Datil include coarse conglomerate and fanglomerate rocks are lavas, but the New Mexico Geologic Map along the edges of the ranges, and finer-grained (1965) represents the northwestern part as clastic sediments farther out. They were laid down in rocks of volcanic fragments (Tds) and boulder beds intermontane depressions produced by downwarp­ and coarse clastic rocks (Tdc). Their westward ing and downfaulting during various stages of extension is shown on the Arizona Geologic Map Tertiary time. The Miocene and Pliocene deposits (1969) as undifferentiated Tertiary sediments (Ts). much resemble those that were laid down in They are shown on the Geologic Map of the United intermontane basins farther south in the western States as Miocene continental deposits (Tmc), but interior that are shown on the Geologic Map as they may well be Oligocene. Pliocene and Quaternary (Tpc, Q), but the average Farther southwest, in the Basin and Range age of the deposits in Montana seems to be province of southern Arizona, widely separated somewhat greater. small areas in the ranges are shown on the Arizona Geologic Map (1969) as Miocene and Miocene(?) OUTLYING AREAS sedimentary rocks (Tms). They are shown on the NEW MEXICO AND ARIZONA Geologic Map of the United States as Miocene Continental deposits classed as Miocene are continental deposits (Tmc). They consist of con­ shown in many small areas in western New Mexico glomerate, sandstone, siltstone, limestone, tuff, and and southern Arizona. A few have been dated by minor lavas. Some are known to be of Miocene age fossils or by radiometric means, but most are from fossils or radiometric dating. inferred to be middle Tertiary from th~ir relation to underlying and overlying Tertiary sediments and WASHINGTON AND OREGON volcanics. Most of the deposits are within or on the In the Spokane area of northeastern Washington borders of mountain areas, hence are older than the is the Latah Formation, a lacustrine deposit that Basin and Range faulting or contemporaneous with accumulated in valleys dammed during the eruption its early phases. of the basalts of the Columbia River Group. The In northern New Mexico, west of the Rio Grande Latah consists of clay and sand, in part tuffaceous, Valley, is the Carson Conglomerate, a coarse and beds of diatomite. Rather abundant fossil bouldery deposit composed of clasts of volcanic and plants indicate middle and upper Miocene ages. Precambrian rocks. It surrounds and half buries the Although the formation is extensive in small areas Precambrian rocks of the Petaca area (Xm), and along the northeast edge of the basalt plateau, only overlaps Mesozoic rocks to the west. It is overlapped a few of the larger areas can be indicated on the in tum on the east by the Pliocene rocks of the Rio Geologic Map where they are shown as Miocene 28 THE CENOZOIC ROCKS continental deposits (Tmc). San Andreas fault, and the Cajon Formation 30 In the volcanic area of southeastern Oregon, miles (90 km) to the east lies north of it. The two Miocene lavas cover wide areas, and sediments are formations may be parts of the same deposit, now interbedded with them in many places, some separated by right-lateral displacement on the fault, forming areas large enough to be represented on the but it has been objected that the Punchbowl Geologic Map as Miocene continental deposits contains vertebrates and the Cajon (Tmc). They include the of the Barstovian vertebrates. This argument loses force western part of the Blue Mountains uplift, which when it is realized that both units are sparsely contains Barstovian vertebrates and plants, and fossiliferous and very thick-the Punchbowl 6,000 the Payette Formation of eastern Oregon and feet (1,800 m) and the Cajon 8,000 feet (2,400 m). The southwestern Idaho, which contains Miocene plants. discrepancy in ages of the known faunas may well be due to the vagaries of collecting.

CALIFORNIA Some of the Miocene continental deposits that PLIOCENE SERIES interfinger with nearby marine deposits in the The Pliocene Series is commonly considered to Coast Ranges and Transverse Ranges have already have formed during the final10 million years of the been noted (p. 25). Miocene continental deposits also Tertiary Period (2-10 m.y. ago) and to correspond to extend much farther east in southeastern California, the Plaisancian and Astian Stages of the European into the Mojave Desert and thence eastward to the marine sequence and to the Clarendonian, Hemphil­ . As in adjoining southern Arizona, lian, and Stages of the American conti­ the deposits are preserved in small patches in the nental sequence. However, a considerable body of ranges where they overlie pre-Tertiary sedimentary opinion now favors reducing the length of the and plutonic rocks, and they are intermingled with Pliocene to 3 million years (2-5 m.y. ago) (Berggren volcanic rocks of Miocene age (Tmv). Many small and Van Couvering, 1974) and placing the Claren­ areas are shown on the atlas sheets of the Geologic donian and part of the continental Map of California, where they are indicated as Me. stages in the upper Miocene. Also, the marine stages Not all of them can be represented on the Geologic that have been proposed in Europe for the upper Map of the United States, and some of them are Miocene and lower Pliocene are suspect as time merged with the associated volcanic rocks. units; some may be lateral facies of others. There is The Miocene continental deposits of the Mojave difference of opinion, besides, as to the Pliocene­ Desert have long been referred to as the "Rosamund Pleistocene boundary. The Pleistocene is commonly Series," but this term has been loosely applied to assumed to begin with the first glaciation (Ne­ beds with a wide range of ages, and the Miocene braskan in North America), but in terms of the part has now been renamed the Tropico Group marine sequences farther south in Europe, the base (Dibblee, 1958, p. 136). In the western part of the is defined as the boundary between the Astian and desert, it is about 2,800 feet (850 m) thick and is (=Villafranchian) Stages, which appears divided into an assortment of sedimentary and to be older than the first glaciation, lending support volcanic formations, none of which are of more than for the concept of a "preglacial Pleistocene" epoch. local extent. The Tropico (= Rosamond) is not very Pliocene marine deposits (Tp) are very scantily fossiliferous in its type area, but the probably developed within the United States (fig. 4), due equivalent Barstow Formation farther east con­ probably to a general emergence of the continent; on tains abundant mammalian remains of upper the Geologic Map they are shown in the Gulf Miocene age, which are the basis of the Barstovian Coastal Plain and Atlantic Coastal Plain and in the Stage of the continental Tertiary. The Ricardo Pacific coastal belt of California. Pliocene conti­ Formation, northwest of Barstow across the Garlock nental deposits (Tpc) are much more extensive, fault, has sometimes been compared with the especially in the Great Plains and the Cordilleran Barstow Formation, but its mammalian fauna is region of the western United States. In the Cordil­ Clarendonian, or early Pliocene. leran region there are also many occurrences of Along the northern border of the San Gabriel Pliocene volcanic rocks (Tpv, Tpf) which are dis­ Mountains, at the south edge of the Mojave Desert, cussed under a later heading. are narrow fault wedges of formerly more extensive middle Tertiary continental deposits; pink, massive­ MARINE STRATIFIED ROCKS ly bedded coarse sandstone and interbedded silt­ ATLANTIC AND GULF COASTAL PLAINS stone. The lies south of the The only truly marine Pliocene rocks shown on PLIOCENE SERIES 29 the Geologic Map are in the southeastern part of the in discontinuous outcrops from central Texas to Atlantic Coastal Plain (although, as noted above, Georgia, at the outer edge of the Tertiary deposits, the Yorktown Formation and Duplin Marl farther and next to the Quaternary deposits along the coast. north may also be early Pliocene). Farther west, in The Citronelle was named for a locality in the Gulf Coastal Plain, the Pliocene deposits are southwestern Alabama (Matson, 1916) to replace brackish water or continental, but they are dis­ the discredited term "Lafayette," which had been cussed here for convenience. used indiscriminately for a wide variety of terres­ In Florida the principal Pliocene formation is the trial deposits fringing the coastal part of the Caloosahatchee Marl (Tp) exposed near Lake Okee­ Coastal Plain Tertiary. In Texas equivalent deposits chobee in the southern part of the peninsula. It are termed the Willis Sand. In Louisiana termi­ consists of sand and beds of molluscan shells, many nology has been confused by the Pleistocene terrace of which are indistinguishable from modern species sequence proposed by Fisk (1938, ff), who believed (Cooke, 1945, p. 214-227). As the terrain in this part that his oldest and highest terrace surface and of Florida is low and flat, only a few feet of the deposits, the Williana, were equivalent to the formation is exposed at any place, but the total Citronelle, but if the Williana exists in the form thickness is probably not great. The Caloosahat­ envisioned by Fisk, it is probably younger (Doering, chee grades laterally into the Buckingham Marl 1956, .p. 1832-1837). and the which, although The Citronelle forms a thin sheet on the uplands separately named, are merely facies of the same along the coastward part of the Tertiary outcrop deposit. and extends long distances inland as erosion Farther north in Florida, remnants of Pliocene remnants along the interfluves, truncating the deposits are scattered over the older Tertiary rocks, earlier (mainly Miocene) deposits. Its basal surface but are not shown on the Geologic Map. They and its deposits slope coastward at a lower angle include the Alachua and Bone Valley Formations, than the dip of the underlying strata, but steeper which are estuarine or terrestrial deposits, some­ than the nearly undisturbed Pleistocene deposits times filling sinkholes, with a fauna mainly of that border it toward the coast. The Citronelle was vertebrates. They are notable for their phosphate, laid down after the older Tertiary had been tilted concentrated by weathering from the underlying and peneplaned and was tilted in turn prior to the formations. Pleistocene; these tiltings express a regional epei­ Near the coast in eastern South Carolina is the rogenic upwarp of the continental interior late in Waccamaw Formation (Tp), which is approximately Tertiary time. equivalent to the Caloosahatchee, and like it is a The regional extent of the Citronelle has been thin fossiliferous layer of sand and shells. reviewed by Doering (1956, 1960), who extends its Far to the west, in south Texas, is the Goliad equivalents northeastward in the the Atlantic Formation (Tp), which forms a band of outcrop on Coastal Plain beyond the last outcrops shown on the downdip side of the Miocene Formations the Geologic Map, in Georgia, across South Caro­ extending from the Rio Grande as far north as the lina, North Carolina, into Virginia. In these States, Colorado River (Plummer, 1932, p. 750-760). Al­ as farther south, thin deposits overlie the older though shown as marine on the Geologic Map, it is Tertiary and Cretaceous on the interfluves, but primarily a brackish water and fluviatile deposit. since they are not clearly differentiated on our The Goliad consists of 100-200 feet (30-60 m) of source maps, they are omitted from the Geologic clays and fine to coarse sands, with much gravel in Map. Across the Potomac, in the Coastal Plain of the lower part, cemented in places by caliche. The southern Maryland is the Brandywine Formation, gravels are mainly chert and pebbles but likewise an upland remnant of fluviatile deposits of include bone fragments from the underlying Mio­ possible late Pliocene or age. The cene. It contains a few indigenous mollusks and Brandywine may be equivalent of the Citronelle, vertebrate bones, the latter being of Pliocene age although the great distance and the lack of fossil and comparable to those of the Ogallala Formation control renders this correlation speculative at best. of northwestern Texas. Northeast of the Colorado The Brandywine is not shown on the Geologic Map River the Goliad is overlapped by the younger Willis (but see King and Beikman, fig. 15). Sand (= Citronelle). The Citronelle deposits consist of a basal gravelly The (Tpc) and its equiva­ sand overlain by sandy clay and clayey sand, laid lents are the most extensive Pliocene deposits in the down by streams that were vigorously eroding the Coastal Plain and are shown on the Geologic Map continental interior as a result of the late Tertiary 30 THE CENOZOIC ROCKS

·------. ------i------..---- ~ ... , I \ _, : 1 I \ ·\--~-;-----( I I I "'I ~. / (., \ ~ ~~./ ) ~------; .,\ ..f :~!0.·. - - '\.-.~~-:'.;·-----·-- :i '. l I -- ~ IIJ ' ' ' : ' ------·--- ~ • • .. / ( ! ' -i -----~----- ! . ~.. ! ' ,,., i ::~--~-l~-~tij I ./'A '' . I " • : i_ ~~~: , a. , ------r--·----·---- • ~-- , I : I \ f ,. I \ : \ ·. • ~ : I '• \~l I • \'. ~ . ' \ -'~;" I ~ .> \ 9t ------L ______·: .. ') ."' -~ ~. " ·r .."- \. ' / \", ~.. L •' I '~"""" ~ '•--" ' • ~ I ,- ...... -, "'' (, •· -A;' '·"'-·:r·-'\.._,- ----'--;::: ______' ~~~'

"-- I "-._.. 1 : r----- 0 -·~-·oao·-:

'> tl 'Z.

\ \ '-, 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). MIOCENE SERIES 31 j\.. /'' ...... , ,' \ I ._ : \ { ·-,, ,f \ ,-,,f __ .... _.....~ \ ---r ) ' ', ( \ I : ,

'."'\ II '" 'I\------( ~

1 --- .-"\- 'r-- \. \ ------'"'\ I ·------\ __. ... - ' \,_\:: ______-I

I ./ I \ I (,..------.;-' t I '-, ( ' '..r-, J ' ; A_( ( ~;.r-·f'·' ., -' \~-5, ------1 ;!.}___ __, .. ------c_ -1 ~ _,../ ) ___ _,_ .. ( .. ~------1"------\ --\_ ) / \

·: ~I \ ·------t, > I 1

I \

' ,-~-)

G U L F OF MEXICO

. ..,.,. .

FiGURE 4.-Continued.

epeirogenic uplifts. Toward the coast, some of the top is commonly eroded. In the type area, the basal deposits accumulated in lagoons, estuaries, and deposits contain fossil plants at one locality, and marshes. In the type area in southwestern Alabama, abundant vertebrate bones at another, both of late the formation is as much as 300 feet (90 m) thick; Pliocene age (Isphording and Lamb, 1971, p. 776- elsewhere it is 100-150 feet (30-45 m) thick, but the 777). The upper part of the formation may extend 32 THE CENOZOIC ROCKS

into the Pleistocene, and arguments have been occurrences and greatest thicknesses are in embay­ presented that it is mainly a "pre-glacial Pleisto­ ments where the ocean extended inland from the cene" deposit (a stage not recognized by the U.S. present coast-the large Los Angeles basin in the Geological Survey, hence the assignment of the south, and the smaller Ventura and Santa Maria formation to the Pliocene on the Geologic Map). basins farther northwest. In addition, a thick sequence of marine deposits occurs in the southwest PACIFIC COASTAL AREA IN CALIFORNIA part of the San Joaquin Valley farther inland, In the Pacific coastal area, Pliocene rocks occur which was formed in a tongue of the sea somehow only in California, where they are extensive. As connected with the ocean across the site of the shown on the Geologic Map, they are exposed from southern Coast Ranges (the connection probably the north to the south ends of the State, but they are now offset from its original position by displace­ most abundant in the central and southern Coast ment on the San Andreas fault). Another marine Ranges and Transverse Ranges, and eastward to embayment, represented by small outcrops on the the border of the San Joaquin Valley. Many of the west side of the Imperial Valley to the southeast, is deposits are marine (Tp), but continental deposits unrelated to the rest, and it formed in a former (Tpc) are more abundant eastward, and upward in northward extension of the Gulf of California. the sequence. Pliocene continental deposits also In the Los Angeles and Ventura basins, the fringe the western foot of the Sierra Nevada along Pliocene marine strata are divided into the Repetto the east edge of the San Joaquin and Sacramento and Pico Formations. The deposits are mostly Valleys. sandy silts and sandy clays, some diatomaceous, a The Pliocene deposits of California are notable for little coarser in the Repetto than the Pico. Sand­ their great thickness, especially in rather local stone and conglomerate are interbedded in the lower areas. Surface sections of marine Pliocene strata in part, and interfinger marginally toward the east, the Ventura basin exceed 15,000 feet (4,500 m), and where they pass eventually into continental de­ similar thicknesses may occur in subsurface in posits. Foraminiferal studies indicate that in the some of the other basins. Continental deposits Ventura basin, and probably also in the Los nearly as thick occur in some of the enclosed basins Angeles basin, the initial Pliocene deposits were in the Coast Ranges and elsewhere. laid down in water 4,000-5,000 ft (1,200-1,500 m) As with the older Tertiary, the Pliocene rocks deep, and that the water gradually shoaled during have been deformed, in places steeply. In some of the epoch, as a result of filling of the basin with the best known areas, especially in the Los Angeles sediments (Natland and Kuenen, 1951, p. 80-85). and Ventura basins and the south part of the San The basin was filled by deposits brought in by Joaquin Valley, the sequences are conformable turbidity currents, which show most of the classic through the Pliocene into the lower Pleistocene, and turbidite structures. The coarse sandstones were the two are strongly unconformable with the over­ brought in by submarine slumps off adjacent lying upper Pleistocene. This has given rise to a gravelly shores and beaches. Many shallow-water widely held belief that the major orogeny in the fossils and fragments of terrestrial plants were coastal belt was mid-Pleistocene ("the greatest since swept in by the turbidity currents and were re­ the Cretaceous"), but in other areas major uncon­ deposited in deep water. formities occur within the Pliocene or at its base, so In the southwestern part of the San Joaquin that the supposed mid-Pleistocene orogeny was Valley, in the Coalinga district and Kettleman merely the terminal phase of a long sequence of Hills, the marine Pliocene is 5,000-7,000 feet (1,500- tectonic instability. During this interval, the ranges 2,100 m) thick and comprises the Jacalitos and between the basins were in process of uplift with Etchegoin Formations. It is followed by the brackish­ little internal deformation. water or fresh-water that bridges The marine Pliocene of California is divided into the upper Pliocene and lower Pleistocene. As in the the Jacalitos, Etchegoin, and San Joaquin mega­ Los Angeles and Ventura basins, the deposits are fossil stages, and into the Repettian, Venturian, and rather fine-grained clastics-silts below and above Wheelerian foraminiferal stages. In addition, the and sands in the middle. Clarendonian, Hemphillian, and Blancan Stages of In the San Francisco region farther north is a the continental Pliocene sequence are frequently correspondingly thick but more complex array of referred to. Pliocene formations. In the Santa Cruz Mountains Pliocene marine deposits are exposed in many west of the San Andreas fault is the marine areas near the coast of California, but the principal Purisima Formation about 6,000 feet (1,800 m) thick; PLIOCENE SERIES 33 it is composed of volcanic sandstone, mudstone, and deposits of wide extent but small thickness (mainly siliceous mudstone, which passes into shallow­ in the Great Plains; (b) basin(= bolson) deposits of water or littoral deposits toward the top. In the San narrow dimensions but great thickness (in basins Francisco Peninsula east of the San Andreas fault between the ranges in the Basin and Range is the Merced Formation of similar origin, compo­ province); and (c) deposits associated with and sition, and thicknesses, somewhat younger than the grading into contemporaneous volcanic rocks (main­ Purisima and including lower Pleistocene in its ly in the northwestern United States). These upper part. Across San Francisco Bay to the east, as distinctions can serve as a basis for the following in the Berkeley Hills, all the Pliocene is nonmarine. treatment of the continental deposits. At the base is the Orinda Formation, 6,000 feet SHEET DEPOSITS (1,800 m) or more thick, of fluvial, alluvial, and lacustrine sandstone and mudstone, in part con­ OGALLALA fORMATION 10 glomeratic. It is followed by the dominantly vol­ By far the most extensive and continuous Plio­ canic Berkeley Group, with several persistent lava cene continental deposit in the United States is that units separated by clastic units. North of the bay of the Ogallala Formation, which covers the Great the similar (Tpv) are extensive. Plains east of the Rocky Mountains (with only These Pliocene deposits are succeeded in several minor interruptions from later dissection) from parts of the central Coast Ranges by coarse conti­ southern South Dakota to western Texas, a distance nental deposits which bridge the Pliocene-Pleistocene of 800 miles (1,300 km), with a preserved width of as boundary. They include the Livermore Gravels, much as 400 miles (650 km). The Ogallala Forma­ 5,000 feet (1,500 m) thick in the intermontane tion has traditionally been classed as Pliocene and Livermore Valley east of the Berkeley Hills, and the is so represented on the Geologic Map, but if the equally thick but more extensive Paso Robles recently proposed upper age limit of the Miocene at Formation farther south at the head of the Salinas 5 m.y. is accepted, all the Ogallala except for the Valley. Blancan Stage at the top would be uppet Miocene. Most of the northern Coast Ranges north of San Throughout this expanse, the Ogallala lies at or Francisco Bay are a terrane of Franciscan rocks, near the surface of the plains, and as shown as but Pliocene marine deposits encroach on them in continuous in outcrop of the Geologic Map. In actual places in embayments near the coast. Not far north fact, much of the flat upper surface of the plains is of the bay are extensions of the Merced Formation, masked by small thicknesses of Quaternary mate­ and the Ohlson Ranch Formation. Much farther rials, which are shown in considerable detail on north, beyond Cape Mendocino, the Wildcat Can­ some of the StP.te Maps-drifted sand and sand yon Group occupies the trough of a along dunes, in the north, and occasional basins the Eel River, where its rather fine-grained clastics filled with various Pleistocene formations. The true attain a thickness of 12,000 feet (3,600 m). outcrops of the Ogallala are mostly in the erosion East of the Coast Ranges in the Sacramento scarps or "breaks" around the edges of the plains or Valley and northern San Joaquin Valley, the in the walls of valleys and canyons that trench the Pliocene deposits are all continental. They are deposit. extensively exposed at the north end, south of The Ogallala was originally more extensive east Redding, the dominant sediments on the west side and west. Westward. it extended up to the foot of the being the Tehama Formation and the dominant Rocky Mountains, from which most of its sediment andesitic tuff on the east side of the Tuscan was derived (a narrow remnant of the deposit Formation. Farther south on the east side of the extending up to the mountains, known as "The valley, near the latitude of Sacramento, is the Gangplank," is still preserved in southeastern similarly andesitic Mehrten Formation, which in­ Wyoming). Eastwiird the Ogallala thinned to a cludes rocks of both Miocene and Pliocene ages. The feather edge across the Midcontinent States. Ero­ andesitic debris was derived from eruptive centers sion after the Pliocene has cut back the original in the southern part of the Cascade Range to the edges into frayed-out escarpments or "breaks"­ northeast and in the Sierra Nevada to the east. prominent on the east, more subdued on the west. In the north, the Ogallala lies nearly conformably CONTINENTAL DEPOSITS on similar earlier Tertiary continental deposits Continental deposits of Pliocene age, which are 10'fhe many publications on the Ogallala Formation include Plummer (1932, p. 763- extensive in the western interior of the United 776; called the "Panhandle Formation"), Lugn' 1939, p. l:z55-1263), Frye and othera States, have several different habits: (a) sheet (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 occurrence. Terrestrial gastropods are preserved in from Kansas southward it lies on the deeply eroded, some of the more limy beds. Far more abundant are channeled surfaces of Cretaceous, , and plant remains-grass seeds and nuts of small strata. The formation is from 200-500 feet shrubs, which are present in nearly every exposure (60-150 m) thick and consists of poorly consolidated and every lithology. The plant remains can be used silt, sand, and gravel in varying proportions, with for zonation, and the zones can be recognized from rare layers of volcanic ash, bentonite, diatomite, one end of the formation to the other. and fresh-water limestone. Some parts are cemented The origin of the Ogallala deposits was brilliantly by silica, and others by lime. The most firmly lime­ expounded by Willard Johnson in a classic report of cemented parts are "mortar beds" or "caprock" the turn of the century (Johnson, 1901, p. 612-656). which stand in ledges, the most prominent of which He showed that they were formed on a long slope is everywhere at the top of the formation. The lime extending eastward from the Rocky Mountains, cement of these layers is caliche, a product of with about the same gradient (10-15 feet per mile; formation in a semiarid climate, and it was formed 1.2-1.4 m per km) and in the same semi-arid climate during or shortly after deposition of the sediments, as at present. Streams flowed eastward across the as clasts of the rna terial are reworked in the plains, where they dropped their loads of sediments overlying Quaternary deposits. derived from erosion of the mountains. Johnson Although the Ogallala deposits are heterogeneous, supposed that the streams spread out in distribu­ there is monotony in the heterogeneity, and there taries on a smooth surface, but modern data on the are no truly valid rock-stratigraphic subdivisions­ deposit (in part from drilling) demonstrates that the merely gross contrasts between the lower and upper streams actually flowed in valleys, which they filled parts. Nevertheless, in Nebraska subdivisions have with coarse material, spreading finer material been recognized, named, and dignified as "forma­ across the interfluves from time to time. tions" (although they are little more than incon­ With the end of the Pliocene, the aggradational stant members)-the basal Valentine, followed by epoch was terminated by climatic change and the Ash Hollow, Sidney (local), and Kimball. These possibly by tectonic movements, so that the sheet of have paleontological basis, however, and their sediments was dissected and partly destroyed by equivalents can be recognized as "zones" as far erosion during the Pleistocene-a process which south as Texas. In Texas, two well-marked verte­ continues to the present. brate faunas have served as the basis for the Clarendonian and Hemphillian Stages of the BIDAHOCHI FoRMATION American continental Cenozoic sequence. In addi­ On the opposite side of the Rocky Mountains, in tion, at the top of the Ogallala at a few places in northeastern Arizona and northwestern New Mex­ Texas are deposits separated as the Blanco Forma­ ico, is the Bidahochi Formation, another sheet of tion, which is the basis for the Blancan continental deposits of about the same age as the Ogallala, stage. The age of the Blancan has been variously although less extensive (Repenning and Irwin, assigned; weight of present opinion is that it is 1954). (In early reports this was correlated with the uppermost Pliocene (C. A. Repenning, oral com­ Eocene Wasatch Formation and is so represented on munication, 1975), although in the past it has the Geologic Map of 1932.) The Bidahochi lies on the sometimes been equated with the Nebraskan or Mesozoic formations of the Colorado Plateau in the even the Kansan Stages of the Pleistocene. Hopi country southwest of the Defiance Plateau and Age determinations by potassium-argon and has a thickness of about 450 feet (140m). It consists fission-track methods have been made on volcanic of a lower mudstone unit of lacustrine origin and an ash from various layers in the Ogallala Formation upper cross-bedded sandstone of fluviatile origin, at localities from Nebraska to Texas. They range between which are basalt flows (Tpv) in many from 16 or 17 m.y. for layers near the base to 5 m.y. places. The formation contains the remains of fossil for a layer immediately above the type Hemphillian of middle Pliocene (Hemphillian) age. (Izett, 1975, p. 201-202). The fluviatile parts of the Bidahochi Formation Mammalian bones are locally abundant in the were laid down by southwestward-flowing streams Ogallala Formation and have been collected and and contain much detritus derived from volcanic studied for many years. They are useful for indi­ areas farther northeast, as well as from the Rocky eating regional correlations with other parts of Mountains. The original southwestern extent of the North America or abroad, but they are of less use for deposit is poorly known, as much of it has been detailed stratigraphic work because of their erratic eroded, but a few outliers occur in this direction, PLIOCENE SERIES 35 including some on the rim of the Mogollon Plateau. placed them in structural contact with the bordering Northeast of the area of Bidahochi deposits, on ranges and has faulted and tilted them internally, the Arizona-New Mexico border, is the unfossilifer­ the deformation being greatest close to the moun­ ous Ch uska Sandstone, also shown as a Pliocene tains and passing into less deformed or flat-lying continental deposit (Tpc) on the Geologic Map. The contemporaneous deposits in the centers of the Chuska has sometimes been correlated with the basins. Mter the earlier deposits were deformed, Bidahochi but is of another character, being mainly their upper surfaces were eroded into pediments a coarsely crossbedded dune deposit. Probably it is before the Quaternary deposits were laid over them. somewhat older, although still middle or upper Tertiary (Wright, 1956, p. 427-431). SANTA FE FORMATION The Pliocene continental deposits of New Mexico, BASIN DEPOSITS especially near the Rio Grande in the center of the Within the Basin and Range province of the State, are known as the Santa Fe Formation, which southwestern United States the basins, or bolsons, was named by Hayden in 1869 for exposures near were thickly filled during later Cenozoic time with Santa Fe, New Mexico. Much of the modern erosional waste derived from the intervening ranges. knowledge of the formation is derived from the work Filling of the basins began with the initiation of the of Kirk Bryan and his students at Harvard Univer­ Basin and Range block-faulting in late Miocene or sity, whose many publications include those of early Pliocene time and has continued with some Bryan and McCann (1937), Smith (1938), Denny interruptions to the present. (1940), and Wright (1946). The earlier basin fill is largely of Pliocene age, as The Santa Fe Formation characterizes especially shown by fossils, although the lower parts of some the series of structural depressions drained by the of the deposits extend into the upper Miocene and Rio Grande that extend from the in the upper parts may be lower Pleistocene; they are southern Colorado, southward the length of New shown as Pliocene continental deposits (Tpc) on the Mexico into western Texas. As shown on the Geologic Map. These deposits probably underlie Geologic Map, exposures of the Santa Fe are nearly most of the basins, but in many areas they are continuous· through this distance, broken only by masked at the surface by similar Quaternary the Quaternary alluvial cover and occasional cap­ deposits (Q). They have been uncovered mainly in pings of Quaternary basalt flows (Qv). In the areas of through-flowing drainage, such as those of southwestern part of the State, discontinuous out­ the Rio Grande, Gila, Colorado, and Humboldt crops of similar deposits occur westward along the Rivers. On the Geologic Map, the basin areas thus south edge of the Mogollon Plateau, to connect appear partly in colors of gray (Q) and partly of vaguely with the Gila Formation of southeastern yellow (Tpc). Arizona. The earlier basin fill is known variously as the The Santa Fe Formation is a heterogeneous body Santa Fe Formation in New Mexico, the Gila of continental deposits that was laid down in Formation in southeastern Arizona, the Muddy structural basins like those at present and in a Creek Formation in southern Nevada, the Hum­ similar climate. The basins were outlined by warp­ boldt Formation in northeastern Nevada, and the ing and faulting late in Miocene time and had Salt Lake Formation in northern Utah. Neverthe­ somewhat different dimensions from the present less, despite their wide dispersal and diverse ter­ ones, in some places larger. Along the margins are minology, they share many common characters. fanglomerates with clasts largely derived from the Close to the bordering mountains the deposits are rocks of the adjacent mountains-Precambrian coarse fanglomerates, but they become finer-grained crystallines, Paleozoic and Mesozoic sediments, and farther out, and near the centers of the basins they older Tertiary volcanics. Farther out are well­ are fine-grained sands, silts, and clays that are bedded sands, and in a few places thinly stratified partly of lacustrine origin. The deposits accumu­ clays and evaporites that accumulated in playas. lated after the great eruptions of mid-Tertiary time The Santa Fe is unique among the basin forma­ and are mainly non-volcanic, although minor vol­ tions in containing traces of a through-flowing canic units occur in places, mainly in the lower river-channels containing well-rounded gravels parts. The deposits were laid down after the first distantly derived from the north. The gravels have Basin and Range faulting had blocked out the been traced from the latitude of Santa Fe at least as basins and ranges, but before renewed faulting far south as the latitude of Socorro. Although the during Quaternary time. Renewed faulting has river was ancestral to the present Rio Grande and 36 THE CENOZOIC ROCKS lies near its course, it diverges widely in places, and southeastern Arizona. It was named by Gilbert in apparently extended across modern ranges that 1875 for exposures near the Gila River in the eastern were raised after Santa Fe time. Where the river part of the State and in adjacent New Mexico. In went farther south is conjectural; it may have subsequent mapping the name has been loosely drained in to a closed depression in north western applied to almost any poorly consolidated late Chihuahua, for through-flowing drainage to the Cenozoic continental deposit of diverse lithology Gulf of Mexico across the mountain barriers in the and age, including (improperly) Quaternary alluvial Big Bend area of Texas does not seem to have been terrace deposits at the top. As a result, some established until after the Santa Fe epoch. geologists class the Gila as a group, and others wish The Santa Fe contains vertebrate bones at a good to abandon the term entirely. Nevertheless. the many localities. The original collections made near name is, with proper restrictions, still useful as a Santa Fe have been determined to be of upper regional designation. Miocene to middle Pliocene age (Barstovian, Claren­ Like the deposits of the Santa Fe Formation, donian, and Hemphillian). Farther south only those of the Gila Formation include coarse fan­ Pliocene bones have been collected, and at a few glomerates along the margins of the basins, with localities fossils of the Blancan Stage-a stage clasts derived from the diverse rocks of the adjoin­ sometimes considered to be lower Pleistocene. The ing mountains; fine-grained, better bedded sandy bulk of the deposit is certainly Pliocene (Tpc) and it deposits farther out; and fine-grained, thinly bedded is so represented on the Geologic Map. playa deposits in the centers of the basins (Knechtel, Subdivisions have been recognized in the Santa 1936). Unlike the Santa Fe deposits, those of the Fe Formation in places, and the unit has been Gila contain no trace of a through-flowing river; classed as a "group" by some authors. These are, through-flowing drainage by the Gila and other however, merely local members, which from the rivers was not established in the region until after very nature of the deposit could not extend far in the depositional epoch during Quaternary time. any direction-considering the complex interfinger­ Where the Gila Formation has been studied in ing of different sorts of deposits laid down in detail, it seems to be capable of more meaningful contrasting environments in different parts of the subdivision than the Santa Fe, and the epoch was basin. In the Santa Fe area, however, a lower unit, broken into several depositional cycles with slightly the Abiquiu, is widespread and is decidedly more different deposits and separated by unconformities. tuffaceous than the overlying beds; it is usually The Gila in the valley of the San Pedro River south­ considered to be a basal member of the Santa Fe. ward from its junction with the Gila River at Observed surface sections of the Santa Fe range Winkelman is now known in much detail (Heindl, from 1,000 to 4,500 feet (300-1,300 m) thick but are 1963; Krieger and others, 1974). The oldest unit of uncertain because of the considerable minor fault­ the group, the San Manuel Formation, was de­ ing within the formation. Well data suggest that in posited in a longitudinal basin west of the present the centers of the deeper basins the thickness of the axis of the San Pedro Valley. It consists of formation may exceed 5,000 feet (1,500 m). fanglomerate, and alluvial and possible playa The Santa Fe Formation has been considerably deposits, and attains a thickness of 5,000 feet deformed by subsequent faulting and tilting, and (1,500 m). It is notable for its lenses of megabreccia the later movements have much modified the composed of large masses of the hard pre-Tertiary Pliocene form of the Rio Grande depression. Younger rocks, which were probably carried into the basin in block-faulting has raised new mountains along its gigantic landslips. An overlying tuff unit has been borders which have reduced the size of the original dated radiometrically as middle Miocene, so that depression, and in a few places, as in the Valles the San Manuel deposits are evidently lower Mio­ Mountains west of Santa Fe, volcanic edifices have cene. Unconformably on the San Manuel at the been built up in the original depression. The upper north end of the valley is the Big Dome Formation, surface of the Santa Fe is not the original deposi­ a coarse alluvial deposit about 600 feet (180 m) tional surface but is a widespread planation surface thick, which was laid·down in the present structural (the Ortiz pediment) that was cut in early Pleisto­ depression. It is younger than the radiometrically cene time. dated middle Miocene tuff, and radiometric dates on GILA FORMATION interbedded tuffs suggest an upper Miocene age. The term Gila Formation (originally called "Con­ Following the Big Dome epoch, the structural glomerate") is commonly used for late Tertiary depression was further restricted by faulting, and continental deposits in the intermontane basins of the alluvial and lake-bed deposits of the Quiburis PLIOCENE SERIES 37

Formation were deposited along the axis of the environment not unlike that of the present inter­ valley. They are about 1,500 feet (450 m) thick and montane basins in a region of interior drainage contain middle Pliocene (Hemphillian) mammalian which wa~ ponded in places to produce lacustrine bones. deposits such as the evaporites and fresh-water Other areas of the Gila have not been studied in limestones. All the sediments are locally derived, as much detail as the one just described, so that the and there is no indication of any through-flowing regional history and extent of subdivisions of the river, such as the Colorado River that now traverses Gila are still imperfectly known. Deposits with the area (Longwell, 1946, p. 821-826). Deposits of the Pliocene bones occur southward in the San Pedro Colorado River, including well-rounded foreign Valley nearly to the Mexican border and indicate gravels, lie on the eroded surface of the Muddy the wide extent of the Quiburis in this direction. In Creek and were deposited during several stages, the Safford Valley farther east, the main body of the none of which are older than Pleistocene. Gila, or "Solomonsville beds," about 2,000 feet (600 Deposits like those of the Muddy Creek occur in m) thick, also contains Pliocene fossils (Wood, 1959, the intermontane basins northward from the p. 60) but is underlain unconformably by the earlier Colorado River into eastern Nevada and adjacent "Bonita beds" of massive volcanic conglomerate. southwestern Utah. Part of these deposits is the Above are the "Frye Mesa beds" of bouldery Panaca Formation, from which Pliocene vertebrate conglomerate that fringe the edges of the bordering bones have been collected. mountains and are clearly Pleistocene. All the Gila of southeastern Arizona is indicated as Pliocene Hl'MBOLDT FORMATION continental deposits (Tpc) on the Geologic Map. The late Tertiary Basin deposits of northeastern Drilling in the basins of south western Arizona Nevada are the Humboldt Formation (Sharp, 1939), indicates the presence of evaporites in the Pliocene which was named by Clarence King in 1878 for deposits, suggesting a tenuous connection with the exposures in the upper drainage of the Humboldt sea in the Gulf of California. River. It is well exposed near the river, as in the vicinity of Elko and the Ruby Mountains, and Mt:onv CREEK FoRMATION occurs in most of the intermontane basins north­ The late Tertiary basin deposits of southern ward to the edge of the State. South of the Humboldt Nevada are the Muddy Creek Formation (Longwell River the formation is mostly concealed by Quater­ and others, 1965, p. 48-49), typically developed in nary basin deposits. In some publications the term Muddy Valley northeast of Las Vegas. The Muddy Humboldt has been loosely applied to almost any of Creek occurs in all the intermontane depressions the later Tertiary deposits of the region, but some near the Colorado River from Las Vegas eastward earlier Tertiary deposits are not properly a part of it to the Grand Wash Cliffs at the edge of the Colorado and should be excluded. Except where these occur, Plateau and has been partly submerged by the the formation lies unconformably on the pre­ waters of Lake Mead. Tertiary rocks of the bordering ranges. Like the basin deposits previously described, the The Humboldt Formation is about 3,000 feet Muddy Creek consists of fanglomerate and other (900 m) thick and consists of fanglomerate and coarse deposits near the mountain borders, passing conglomerate near the mountains, which pass basinward into well-stratified deposits of sand­ upward and outward into sandstone, mudstone, and stone, siltstone, and clay. In the centers of some of siltstone, much of which is tuffaceous. It is of the basins are evaporite deposits-chiefly gypsum, fluviatile and lacustrine origin. The fanglomerates but with thick beds of rock salt in the Virgin Valley. near the Ruby Mountains consist of clasts of Bedded manganese oxides occur at one place, and Paleozoic limestone below, passing up into clasts of fresh-water limestone in others. One or two basalt crystalline rocks, indicating progressive unroofing flows are interbedded. Except near some of the of the range, probably ·by uplift along block faults bordering faults, the formation is virtually uncle­ during deposition of the formation. Other ranges in formed and flat-lying. The age of the Muddy Creek the region were probably also undergoing block is uncertain, as fossil remains are scarce; it is uplift during Humboldt time. At the margins of the probably mainly Pliocene, although a Miocene age deposit, as at Palisades Canyon on the Humboldt has been ascribed to some of the vertebrate material. River west of Elko, rhyolite flows intertongue with The Muddy Creek Formation is 2,000 feet (600 m) or the upper part of the deposits. more thick. The Humboldt Formation contains rather abun­ The Muddy Creek was deposited in an arid dant vertebrates, as well as fresh-water inverte- 38 THE CENOZOIC ROCKS brates and plants. The vertebrates are generally flows of the Snake River Group (Qv) of Pleistocene ascribed a late Miocene age, and there is no and Holocene age. indication of any of Pliocene age. Nevertheless, it is The Idaho Group itself is a body of fluviatile and shown along with the other basin deposits as a lacustrine deposits and interbedded basalt flows Pliocene continental deposit (Tpc) on the Geologic about 3,000 feet (500 m) thick and of middle and Map. upper Pliocene and lower Pleistocene age, which is

SALT LAKE FORMATION divisible into a set of named formations. All the In northeastern Utah and southeastern Idaho the formations are lenticular and are mostly uncon­ late Tertiary basin deposits are the Salt Lake formable with each other because of progressive Formation, named by Hayden in 1869. It is exposed down warping and downfaulting of the plain during on the flanks of the Wasatch Range north of Salt accumulation. All the sedimentary units are rather Lake City and between other ranges farther north, abundantly fossiliferous and contain vertebrate as far as the Snake River Plain. The formation bones, plants, and fresh-water mollusks which consists of tuff, tuffaceous sandstone, shale, and provide many data on the different ages. The marl, partly of fluviatile and partly of lacustrine subdivisions are the Poison Creek Formation, Ban­ origin. Exposed sections 1,500 feet ( 450 m) thick are bury Basalt, and Chalk Hills Formation, all of common, and the total may exceed 3,000 feet (900 Pliocene age; the Glens Ferry Formation which m). Subdivisions have been recognized in some bridges the Pliocene-Pleistocene boundary; and the areas, and units of several different ages may be T9ana Gravel, Bruneau Formation, and present, although the bulk of the formation is Gravel of Pleistocene age. believed to be Pliocene. The Salt Lake lies uncon­ formably on the Wasatch and other Eocene conti­ OTHER FORMATIOi\'S nental deposits, and in some of the valleys it is On the eastern flank of the Cascade Range in overlain by sheets of basalt of Quaternary age. In central Washington is the Ellensburg Formation, a some of the ranges the formation has been much fine- to coarse-grained clastic deposit, largely de­ disturbed and offset by later block-faulting, but in rived from erosion of the middle Tertiary andesitic other areas it is little deformed. volcanics of the Cascades (Waters, 1955, p. 670-675).

DEPOSITS ASSOCIATED WITH VOLCANIC ROCKS The Ellensburg overlies the Yakima Basalt of the Columbia River Group and contains in its lower Pliocene continental deposits (Tpc) are shown on part thin interbedded basalt flows like those be­ the Geologic Map in many parts of the volcanic neath. Eastward it interfingers with and is appar­ province of the northwestern United States, where ently largely replaced by the upper basalts of the they are associated with and grade into Pliocene Columbia River Group. The Ellensburg contains volcanic rocks and are not ordinarily basin-fill plants, fresh-water gastropods, and vertebrates of deposits like the group just described. In many early Pliocene age. areas they are merely end-members of a sequence Along the valley of the John Day River, in the ranging from lava flows, through ash-flow tuffs and Blue Mountains uplift of northeastern Oregon, is tuffs, to strata more clearly sedimentary, and thus the Rattlesnake Formation, composed of fanglomer­ they have no special designations. In some places, ate, bouldery gravel, and welded tuff. It lies uncon­ however, they have more stratigraphic entity and formably on the Miocene and earlier sedimentary have been given formation names. and volcanic rocks of the region, and contains IDAHO GROlTP middle Miocene (Hemphillian) vertebrates, but its In the western third of the Snake River Plain of upper part may extend into the Pliocene. south western Idaho is a complex sequence of In western Nevada are the Truckee Formation of Pliocene and early Pleistocene continental deposits the Reno area (Thompson and White, 1964, p. 16-17) and interbedded volcanics, the Idaho Group, which and the Esmeralda Formation of Esmeralda County accumulated in this northwest-trending segment farther southeast (Robinson and others, 1968). Both during its downwarping and downfaulting (Malde are late Tertiary fluviatile and lacustrine deposits, and Powers, 1962, p. 1201-1212). laid down in disconnected structural basins that Beneath the Idaho Group, especially on its south­ correspond closely to the modern blockfaulted de­ ern border, are the largely felsic ldavada Volcanics pressions. Both formations contain large volumes of (Tpf) whose sedimentary members contain plants volcanic debris-basaltic and andesitic in the and other fossils of lower Pliocene age. Above the Truckee, rhyolitic in the Esmeralda. This debris is group, especially toward the east, are the basalt intermingled with non-volcanic sedimentary com- QUATERNARY SYSTEM 39

ponents, including siltstone, diatomite, beds of coal West, where they are an essential feature of the (in the Esmeralda), and pebble conglomerates bedrock pattern (fig. 5). In such areas the Quater­ formed of clasts of the pre-Tertiary rocks. The nary deposits attain sizeable thicknesses, and in Truckee Formation is at least 2,000 feet (600 m) many areas the bedrock pattern beneath them is thick, and the Esmeralda in places attains 9,000 feet poorly known or unknown. In the Coastal Plains of (2,700 m). The Truckee Formation interfingers with the , the Quaternary deposits the upper part of the andesitic Kate Peak Formation are divided into the Pleistocene (Qp) and Holocene and is overlain by younger basalts. The Esmeralda (Qh) Series. The Quaternary deposits of the Western interfingers with and is overlain by the volcanic United States are not separated on the Geologic rocks of the Silver Peak center. The Truckee Map and are represented as an undifferentiated unit contains fossil plants of middle Pliocene age. Fossil (Q). In many of these areas, the respective outcrops remains in the Esmeralda are more diverse, and of the Pleistocene and Holocene Series would be too include plants, mammals, fish, ostracodes, and small to show on the scale of the map, and the mollusks of Hemingfordian, Barstovian, and Clar­ differentiation between them has not been made on endonian ages; radiometric dates by the K/ Ar many of the source maps. method range from 13 to 4 m.y., or from late Miocene to late Pliocene. GULF COASTAL PLAIN In eastern California, east of the south end of the MISSISSIPPI EMBAYMENT Sierra Nevada, is the Ricardo Formation of conti­ By far the largest area of Quaternary deposits in nental and lacustrine sediments and interbedded the Coastal Plains is that in the Mississippi lavas and tuffs, which attains a thickness of Embayment, which extends northward from the 7,000 feet (2,100 m) and contains lower Pliocene Louisiana coast for 600 miles (950 km) to southern (Clarendonian) vertebrates. Illinois. Extending centrally through the embay­ ment is the alluvial valley of the Mississippi River, QUATERNARY SYSTEM filled largely by Holocene deposits, panked on one or both sides by Pleistocene terrace and other Quaternary deposits are only partly represented upland deposits. on the Geologic Map of the United States, because Much of our knowledge of the alluvial valley we they are primarily a surficial mantle over the pre­ owe to the classic memoir of Fisk (1944), in which Quaternary bedrock-representation of which is the the modern landforms and the processes at work on principal objective of the map. The appropriate them are ably set forth, and an attempt made to place for a complete representation of the Quater­ reconstruct its earlier Quaternary history. Further nary deposits is on a surficial geology map, several investigations, and the accumulation of more data of which have been published of different parts of since publication of this memoir, have resulted in the country. The U.S. Geological Survey is now considerable modifications of Fisk's proposed his­ planning a surficial geology map of the whole tory (Durham and others, 1967; Saucier, 1974). A United States, as a companion to the present major change that affects representation on the geologic map. Geologic Map is the reassignment of the lower The Geologic Map of the United States does not alluvial terraces in the valley from after the end of represent the glacial and other deposits of Pleisto­ the last , 10,000 to 12,000 years ago, to cene age that blanket large parts of the Northern earlier periods, some as early as the Sangamon · Interior States, nor the loess and drifted sand that Stage about 100,000 years ago; all the are extensive in some other places. In such areas the terraces in the valley are mapped as Pleistocene bedrock may be partly or wholly concealed, and its rather than Holocene. representation must be by subcrop or subdrift The oldest continental deposits near the Missis­ mapping, based more on the results of drilling and sippi River valley are the Citronelle Formation of geophysical data than on outcrops. In the Northern probable late Plio~ene age, already described, which Interior States we have, however, marked the limits forms a coastward-dipping body of sediments in the of the earlier and later Pleistocene glaciations, to uplands to the east and west. In addition, Fisk suggest areas in which the bedrock is likely to be identified two high-level terrace deposits and sur­ extensively concealed. faces on the uplands df supposed early Pleistocene The Geologic Map does, however, represent the age, the Williana and Bentley. While it is true that Quaternary deposits along the Atlantic and Gulf vague, deeply dissected surfaces can be recognized Coastal Plains and in intermontane areas in the in the uplands, the surfaces were probably cut from 40 THE CENOZOIC ROCKS

------r------..---- : I I : \ I

------/ ,. \ -~ .. ~" I ____ --~. "·. __ ,vi ·r'' 1 I ~~ ·­

' lf \ .. 4: ' \ \ \ l. ____ / ______1 ______I I \ \ I ------'lor----' I '\ ~. I ' I ~ ------:---I ------~--- I !------!"~ I I 1 ~ : ~ I I L ',...p. ~-- .... ·- "'"'-:.r·- .... _.-

'> z

\ ' '-,

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). QUATERNARY SYSTEM 41

,-,_,, ...... , : \ I '. \ \ \ ', '· ,! ' ,f (\ ----r----·') \ ', ( .. t I \ ', i "\ ' .. ( _: \------~--~ \ I

~. ---- ~' --~--::'_;~·" I 1.--- \ / ~"--'-!..... ~: ,, / , (' .. -v ;-· _,? I

\ '-._ _) ,1'-.._.•-./ ;' ------~·:-~-~_-:T __ _;-----·--- J ·.' I ,; _,/ \ ___ .!.---' ( ------\ ·-\ \ ',"\ \ \

G U L F OF MEXICO

FIGURE 5.-Continued. 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 tween them are clear, but there is disagreement on bedrock floor. Their existence, which was first details (compare Doering, 1956, p. 1,831-1,840; and suspected as early as 1881 by drilling in the Bernard and LeBlanc, 1965, p. 176-179). Tradition­ alluvium, have been abundantly documented since ally, the Pleistocene has been divided into the lissie by a closer net of drill holes. During each inter­ and Beaumont Formations. On the Houston and glacial stage, sea-level to nearly its original Beaumont sheets of the Geologic Atlas of Texas level, and the valley was aggraded; the streams, at (1968), the lissie is divided into the Bentley and first heavily loaded with glacial outwash, had a Montgomery Formations, supposedly correlative braided pattern, and passed later into a meander­ with the Louisiana units, and the Beaumont is belt pattern. During the braided stream stage, silt correlated with the Prairie Formation of Louisiana, was carried eastward by the wind and deposited as with which it is physically continuous; in addition, loess on the uplands on this side of the valley. the Deweyville Terrace Formation is recognized During last post-glacial, or Holocene cycle, the river along the upstream parts of the major rivers, as it is has now attained the meander-belt stage through­ in Louisiana. out the alluvial valley (Qh). The earlier aggrada­ This segment of the Coastal Plain differs from tional stages are represented by the terrace deposits that in the Mississippi Embayment in that none of that border the valley (Qp). its rivers headed in a glaciated area farther north, The oldest identifiable Pleistocene terrace forma­ so that direct glacial control was lacking; otherwise, tion (Qp) is the Montgomery, of inferred Yarmouth the deposits and conditions of deposition were much age, represented by deeply weathered, dissected the same. Most of the plains were deltaic, and the remnants. Its deposits attain thicknesses of 300 or surface of the Beaumont, especially, is marked by 400 feet (90-120 m), and slope seaward at a rate of branching distributary channels whose natural 3-4 feet per mile (0.6-0. 7 m per km), probably levees are sandy deposits separated by areas of clay passing beneath the succeeding Prairie Terrace deposits. Toward the coast, the Beaumont passes Formation at steadily increasing depths. The Prairie into brackish-water and- near-shore deposits which Terrace Formation, of presumed Sangamon age, is contain many shell banks. far more extensive, especially in the coastal belt Holocene deposits are confined to offshore bars of southern Louisiana, and its surface includes and to broad belts of stream alluvium that extend meander-9elt deposits inland, and deltaic plains, far up the major river valleys. On the Geologic Map, lagoons, and barrier islands near the coast. At these alluvial belts are arbitrarily cut off about levels below the Prairie Terrace are smaller terraces 100 miles (160 km) from the coast so as not to formed during the Wisconsin glacial and inter­ obscure the bedrock formations, but they actually glacial substages, including the fluvial Deweyville extend nearly to the inner edge of the Coastal Plain. Terrace in upstream areas and various braided­ In south Texas, not far north of the Rio Grande, an stream terraces within the alluvial valley. extensive body of drifted sand spreads inland over The Holocene deposits (Qp) of the alluvial valley the Pleistocene deposits and is mapped with the include meander belts formed at different times by Holocene. the Mississippi River and its tributaries, as they East of the Mississippi Embayment, the Citronelle shifted this way and that across the valley. Between and older Tertiary formations reach close to the the meander belts are back-swamp deposits. Down­ coast so that the Pleistocene deposits are mostly stream, the deposits form deltaic plains with branch­ missing and the Holocene is represented by only a ing distributaries and along the coast chenier plains narrow coastal strip. The only exception is in north­ with elongate beach ridges ("cheniers"). western Florida where the Appalachicola River has built a delta which includes several Pleistocene OTHER AREAS terraces that are seemingly correJative with those West of the- Mississippi Embayment, in south­ farther west (Doering, 1956, p. 1,854-1,858). western Louisiana and in Texas, a belt of Quater­ ATLANTIC COASTAL PLAIN nary deposits 40-80 miles (65-130 km) wide lies Representation of the Quaternary of the Atlantic between the Tertiary outcrops and the coast. Their Coastal Plain on the Geologic Map is based, for the surfaces are a succession of broad plains or benches most part, on the available State Geologic Maps. stepped down coastward, which express a set of These maps show as Quaternary a belt of varying formations. Stratigraphic terminology in Texas width along the coast where the Tertiary rocks antedates that in the Mississippi alluvial valley and farther inland are no longer exposed, and they elsewhere in Louisiana; broader correlations be- ignore areas of Quaternary deposits farther inland QUATERNARY SYSTEM 43 where they are thinner and more discontinuous. on surfaces representing different stands of the sea This procedure is desirable on the Geologic Map, during the Pleistocene), their relation to glaciation, whose purpose is to emphasize the pre-Quaternary and their possible relation to crustal warping. The bedrock. Nevertheless, as examination of the litera­ publications up to 1940 were summarized by Flint ture of the Quaternary of the Atlantic Coastal Plain (1940), and since then still other views have been will show, the results are empirical rather than expressed (for example, MacNeil, 1950; Doering, morphological, as explained below. 1960; and Alt and Brooks, 1965), some of them widely The only modifications of representation from the divergent. source maps were made in Maryland, Delaware, The terrace concept was championed especially and New Jersey, and in northeastern Florida. In the by C. W. Cooke, who recognized as many as seven first three States, the inland extent of the Quater­ surfaces forming a descending from as high nary deposits is shown in much greater detail on the as 270 feet (90 m) to sea-level. All the surfaces were State Maps than farther south, interspersed with supposed to be unwarped and horizontal from one outcrops of the Tertiary rocks. Here, the same end of the Coastal Plain to the other and to have been procedure was adopted as farther south, and the formed by marine invasions during the Pleistocene Quaternary was mapped only beyond the limits of glacial period, each marine submergence being less Tertiary outcrops. For the actual extent of the than the one preceding, for reasons not entirely Quaternary in these States, see King and Beikman clear. These interpretations have been accepted (1974, fig. 15, p. 37). only in part by other geologists. In northeastern Florida, the State Map (1964) In Virginia and North Carolina two scarps indicates the Quaternary as "Recent and Pleisto­ created by marine erosion are widely traceable-the cene; several lower marine and estuarine terrace Surry at 90 feet (27 m) and the Suffolk at 25 feet deposits." In this area, further subdhjsion was (7.5 m). Above the Surry there is no convincing made on the basis of MacNeil's map (1950, pl. 19), evidence of any marine deposits (Flint, 1940, p. 777). which represents the Quaternary terrace formations All the Quaternary shown on the Geologic Map in and their shorelines. Shown as Pleistocene were the these States is below the Suffolk scarp. In Florida higher terrace formations-the Okefenokee at about and Georgia, evidence of marine submergence to 150 feet (45 m), the Wicomico at about 100 feet higher levels is better documented; MacNeil (1950, (30 m), and "the Pamlico at about 30 feet (10 m). p. 99) reports evidence of marine scarps, bars, and Shown as Holocene were the Silver Bluff at about deposits at .altitudes as high as 150 feet (45 m). 10 feet (3 m), which MacNeil classes as post­ These higher features are shown as Pleistocene on Wisconsin, and surfaces below it. The results are the Geologic Map. For this area a strongly dis­ somewhat questionable, but no better map represen­ senting view is taken by Alt and Brooks (1965, tation was available to us. p. 407-410) who recognize shorelines as high as On the Geologic Map, the Holocene of the Atlantic 250 feet (75 m), but interpret the higher ones as of Coastal Plain is represented as a strip along the Miocene and Pliocene age and concede as Pleisto­ coast from Florida through Georgia into South cene only those below 30 feet (10 m). Carolina; farther north all the Quaternary is mapped The only marine surface and deposit which can as Pleistocene. It is true that Holocene deposits form definitely be correlated with the glacial events to the the offshore bars here, as well as narrow strips north is the Cape May Formation of coastal New along the inlets, but in areas too small to separate. Jersey, standing at 30 feet (10 m), which can be Moreover, the Pleistocene and Holocene are not traced into Sangamon interglacial deposits (be­ separated on many of the source maps. Beyond New tween the Illinoisan and Wisconsin glaciations). Jersey, on Long Island, Martha's Vineyard, Nan­ GREAT PLAINS tucket, and Cape Cod, the Quaternary deposits are For the most part, Quaternary deposits in the clearly a product of the Pleistocene glacial period­ continental interior east of the Rocky Mountains the massive Wisconsin terminal moraines and are not shown on the Geologic Map, but some are associated deposits. represented in the southern part of the Great Plains, An extensive literature on the Quaternary deposits part of which are classed as Pleistocene, part as of the Atlantic Coastal Plain exists, and geologists undifferentiated Quaternary, as explained below. have much debated their meaning, with confusing Omitted from the map are the sand hills of north­ results-as to what parts are marine and what parts western Nebraska, of Quaternary age, which lie on continental, as to whether they are classifiable the Pliocene Ogallala Formation (King and Beik­ according to altitude (that is, whether they formed man, 1974, fig. 14, p. 35), and the glacial deposits 44 THE CENOZOIC ROCKS farther north which form a nearly complete blanket (Frye and Leonard, 1964, p. 16-19). The basin was over the bedrock east of the (King probably created by subsidence, resulting from and Beikman, 1974, fig. 11, p. 31). The extent of the solution of the underlying bedrock. Similar but glacial deposits is suggested on the Geologic Map by smaller basins containing Pleistocene lake deposits, the lines representing the limits of glaciation. termed the Tahoka Formation, are scattered here and there over the surface of the Great Plains (= ) in Texas, but they are not PLEISTOCENE represented on the Geologic Map. The largest area shown as Pleistocene is an South of Lake Lomax a large area of Pleistocene irregularly shaped body in southeastern Colorado, is shown on the Geologic Map surrounded by Lower whose deposits have commonly been shown as Cretaceous limestones of the Edwards Plateau. This Pliocene Ogallala Formation on published geologic area is usually shown as Ogallala Formation on maps (although they are shown as Miocene on the published geologic maps, but the deposits which Geologic Map of North America of 1965, for reasons cover its surface are only a few feet thick, are locally not apparent). The unit lies on an eastward-sloping derived, and are probably younger. surface on the divide north of the Arkansas River A final area is indicated as Pleistocene on the that projects in low scarps above the surrounding Geologic Map in southeastern New Mexico, between lowlands. This surface is distinctly lower than that the Pecos River and the Ogallala Formation of the of the Ogallala Formation to the east-more than Great Plains at the east edge of the State. It is 500 feet (150 m) as projected-and must be a shown on the New Mexico Map (1965) as "Qab­ younger feature. Alluvium and bolson deposits and other surficial The deposits which underlie the western part of deposits." The area is an irregular surface of inter­ the surface are the Nussbaum Formation, named by mediate height between the Roswell basin along Gilbert in 1897, which crops out around its edges. the Pecos and the Great Plains to the east. Most of it The surface itself is mantled by windblown sand, is covered with heterogeneous surface deposits, which extends to the east end of the area; the east­ including windblown sand and caliche, probably of ward extent of the Nussbaum beneath the surface is variable thickness due to solution-collapse of the uncertain. The Nussbaum is an alluvial deposit, underlying bedrock. To distinguish these deposits much less consolidated than the Ogallala deposits from the alluvium along the Pecos River they are and more like the Quaternary alluvium on lower tentatively shown as Pleistocene (Qp) on the Geo­ surfaces in the area (Scott, 1963). No diagnostic logic Map. 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 UNDIFFERENTIATED QUATERNARY accordingly marked as Pleistocene (Qp) on the The Roswell and Toyah Basins along the Pecos Geologic Map. River in southeastern New Mexico and western A smaller area of Pleistocene is indicated farther Texas much resemble the intermontane basins east, in the Great Plains of Meade County at the farther west, except that they are not defined by south edge of Kansas. The deposits of the area are faults and other structures but have been produced the Meade Formation, a body of sand and gravel by erosional excavation that has been accentuated that fills a basinlike area to a thickness of more by collapse resulting from solution of salt and other than 200 feet (60 m) (Frye, 1942, p. 103-109). It evaporites in the underlying Permian bedrock. contains vertebrates and mollusks of early Pleisto­ The Roswell Basin in New Mexico, the smaller cene (Nebraskan) age, and it is overlain by one of and better defined of the two, is bounded on the west the layers of the widespread Pearlette Ash of by the limestone foothills of the Sacramento Moun­ Kansan age. Pleistocene deposits occur on the tains and on the east by lower scarps of Permian Tertiary and older 'rocks in many other places in redbeds and Triassic sandstones. Southward, near this part of the Great Plains, but they do not attain Carlsbad, New Mexico, the Pecos River flows from the thickness or have the basinlike form of those in the basin onto Permian and Triassic bedrock, before the Meade County area. passing into the Toyah Basin in Texas. The Toyah Another area of Pleistocene is indicated at the Basin is much larger, more irregular, and less well south edge of the Great Plains west of Big Spring, defined; it is bounded by the trans-Pecos mountains Texas. This was the site of Lake Lomax, that on the west and by Lower Cretaceous limestones of occupied a basin on the surface of the Ogallala the Edwards Plateau on the south and east. At its Formation during (Wisconsin) time lower end, the Pecos again flows on bedrock in a QUATERNARY SYSTEM 45 canyon in the Edwards Plateau to its junction with results would be too minute and complex for the the Rio Grande. scale of the map. Also, the marine lower Pleistocene The surface cover of the Roswell Basin is com­ of southern California, which is conformable on the plete, but that in the western part of the Toyah Pliocene and forms a narrow band of outcrop Basin is interrupted by many outcrops of bedrock. adjacent to it, is for convenience mapped with the The surface materials in the two basins form a Pliocene (Tp ). succession of descending terraces whose formation The marine Pleistocene is best developed in the has been ascribed to events during the glacial Ventura and Los Angeles basins of southern Cali­ period (Fiedler and Nye, 1933, p. 106-113) The fornia. In this region, a major structural uncon­ Quaternary deposits beneath the surface have a formity has long been known between deformed variable thickness, commonly more than 100 feet strata and nearly undisturbed overlying deposits, (30 m), but are much thicker in places. Drill holes which Lawson (1893) first supposed to mark the indicate depths of more than 300 feet (90 m) in parts Tertiary-Quaternary boundary. Later, however, pa­ of the Roswell Basin, and well over 500 feet (150 m) leontologists determined that the upper part of the in parts of the Toyah Basin. The deepest fill in the deformed sequence was also Pleistocene, so that the Toyah Basin is along its northeast side, where there period of orogeny indicated by the unconformity are few rock outcrops and where the underlying actually lay within the Pleistocene Epoch. Paleon­ Permian salt beds were originally thickest (Maley tological differentiation of Pliocene and Pleistocene and Huffington, 195"3). The thick alluvial fill was in the region was based on Lyellian principles of produced in areas of deep circulation of ground relative percentages of modern genera and species, water and consequent salt solution and collapse. principally of the marine megafossils (Woodring, Collapse due to solution is evident near the Pecos 1952, p. 402), but it has since been largely confirmed River at the lower end of the Toyah Basin, where the by planktonic Foraminifera. otherwise flat-lying Lower Cretaceous limestones The classic section in the region is that in the dip steeply and erratically toward the valley. Palos Verdes Hills on the coast south west of the Los East of the Great Plains in Kansas and Okla­ Angeles basin, where marine Pleistocene fossils homa, the source maps indicate broad, lengthy were first reported by Ralph Arnold in 1903 and strips of Quaternary along the Arkansas, Cim­ more fully documented in later reports (for example, maron, Canadian, and other rivers. These appear to Woodring and others, 1946). Here, the lower Pleisto­ be chiefly windblown sand and silt derived from the cene is about 600 feet (180 m) thick and comprises river valleys. We have been reluctant to mark these the Lomita Marl, Tims Point Silt, and San Pedro surficial deposits on the Geologic Map but have Sand. These beds have been tilted and are overlain shown the larger strips. The strip along the Arkan­ unconformably by flat-lying upper Pleistocene de­ sas River, especially, is too extensive to ignore, as it posits, including the Palos Verdes Sand, on uplifted interrupts the continuity of the bedrock formations marine terraces. Middle Pleistocene is missing. whose patterns beneath are conjectural. Northeastward in the Los Angeles basin, the CALIFORNIA 11 Pleistocene thickens in subsurface to 3,000 feet The nonglacial Quaternary of California exceeds (900 m) and the mid-Pleistocene hiatus may in variety and complexity any other in the United disappear. Most of the fossils in the lower Pleistocene of the States-marine deposits in coastal embayments Palos Verdes Hills are northern cool-water forms, a and uplifted marine terraces along the coasts, fact which was once thought to indicate the onset of terrestrial deposits in the inland basins, and indi­ glacial conditions, but which might well reflect cations of major orogeny within the period. More­ depth of water and coastal upwellings. A radio­ over, the sequence of glacial deposits in the Sierra metric date of three million years on glauconite Nevada can be related at least tenuously with non­ from the Lomita Marl, obtained by J. S. Obradovich, glacial sequences in the nearby lowlands. Never­ is disconcertingly old for Pleistocene and is suspect theless, the system is shown as an undifferentiated unit (Q) on the Geologic Map, and a subdivision into for a number of reasons. In the Ventura basin to the northwest, surface Pleistocene and Holocene Series is not attempted as sections of the Pleistocene are considerably thicker it was in the Mississippi Embayment. Data for such than in the Palos Verdes Hills-as much as 2,500 a subdivision are available in most areas, but the feet (760 m), a sizeable figure but modest compared with the 15,000 feet (4,500 m) or so of the underlying liThe Quaternary of California is summarized by Wahrhaftig and Birman (1965). An earlier summary by Reed (1933, p. 255-273) is of historical interest. Pliocene. The lower Pleistocene follows conform- 46 THE CENOZOIC ROCKS ably on the Pliocene Pico Formation and comprises with them along the along the edge of the the Santa Barbara and San Pedro Formations. valley during mid-Pleistocene time. The deforma­ Here, the mid-Pleistocene unconformity is even tion fades out northeastward across the valley, more pronounced than in the Los Angeles basin. although an unconformity persists. Near the middle The Pliocene and lower Pleistocene are steeply of the Tulare in subsurface is a thin, widespread, upended and even overturned and are overlain with diatomaceous silty clay layer, the Corcoran Clay right-angled unconformity by upper Pleistocene Member. On the east side of the valley this is terrace deposits. As explained earlier (p. 32) the overlain locally by pumiceous ash which has been orogeny implied by this unconformity, although dated by the K/ Ar method at 600,000 years. impressive here and at some other places, is merely On the northeast side of the San Joaquin Valley a local and terminal event in a long sequence of are a set of alluvial formations derived mainly from tectonic instability. erosion of the Sierra Nevada. Below is the Turlock Farther north, in the San Francisco Peninsula, is Lake Formation, which is traceable in subsurface the marine Merced Formation, whose upper part, into the Tulare Formation, followed by the middle conformable on the lower, is of early Pleistocene and late Pleistocene Riverband and Modesto For­ age. Both the lower and upper Merced are moder­ mations. The latter is essentially undeformed, but it ately deformed. stands in low benches above the Holocene alluvium The marine upper Pleistocene of the coastal area of the present valley bottoms. mostly forms the capping of of uplifted wave­ The Quaternary deposits of the Sacramento Valley cut terraces. In the Palos Verdes Hills there are 13 to the north are much thinner and contain some marine terraces between sea-level and 1,300 feet volcanic detritus. They lie unconformably on the (400 m). The number and maximum height of the eroded surfaces of the rertiary and older rocks. terraces varies from one coastal segment to another, BASIN AND RANGE PROVINCE'~ and their altitudes vary as a result of warping; the cover of marine fossiliferous deposits is best pre­ Undifferentiated Quaternary (Q) is shown on the served on the lower surfaces. Some of the sea-level Geologic Map in most of the intermontane basins of changes implied by the terraces are probably the .Basin and Range province, from northern attributable to subtraction and addition of sea Nevada and Utah to southern Arizona and New water during the glacial and interglacial stages, but Mexico. With the exception of some minor marine the uplift of the higher terraces is more likely layers in the Salton Trough of southern California, tectonic. The lowest terrace, standing at about all are continental deposits. Both Pleistocene and 100 feet (30m) has commonly been correlated with Holocene deposits are present but are not separated the Sangamon Interglacial Stage, and near Santa on the Geologic Map. Cruz has yielded radiometric ages of about 110,000 The Quaternary deposits generally overlie similar years. An unsolved problem is that the terraces are intermontane deposits of Pliocene and Miocene age restricted to the present coastal slopes; none of the (Tpc), but these are only partly exposed, and in nearby interior valleys show any indications of the many basins the Quaternary deposits are the only deep submergence or successive marine planations surface formations. The thickness of the Quater­ implied by the coastal terraces. nary is variable and in many areas uncertain. In Most of the Pleistocene deposits just described are basins with through-flowing drainage, such as only indifferently represented on the Geologic Map. those of the Rio Grande, Gila, and Colorado Rivers, On the map, by far the largest area of Quaternary is the Quaternary is generally a rather thin set of that in the San Joaquin and Sacramento Valleys terrace deposits on top of the much thicker Tertiary, and consists of varied terrestrial deposits of dif­ but in other basins it attains considerable thick­ ferent ages. The total thickness of Quaternary nesses in its own right. Surface sections of as much deposits attains 15,000 feet (4,500 m) in the southern as 2,000 feet (600 m) occur in the Salton Trough, and part of the San Joaquin Valley but is much less the deposits may attain 3,000 feet (900 m) in farther north. subsurface. Thicknesses as great or greater prob­ On the southwest side of the San Joaquin Valley ably occur in the basins with interior drainage in (as near the Kettleman Hills) is the lower Pleisto­ the northern part of the province. Thicknesses in cene Tulare Formation, a lacustrine or marsh the centers of the basins are uncertain because of deposit containing fresh-water mollusks. It con­ formably overlies the marine Pliocene Etchegoin and San Joaquin Formations and was deformed t2Jncludes data from Morrison (1965) and Kottlowski, Cooley, and Rube (1965). CENOZOIC VOLCANIC ROCKS 47 the difficulty of distinguishing in well records produced by these lakes and their predecessors is between the Quaternary deposits and the similar uncertain, but definitely exceeds 500 feet (150 m). underlying Tertiary deposits. Much of the present topography of the Basin and CENOZOIC VOLCANIC ROCKS Range province is a product of Quaternary time, and very little of the late Tertiary topography has Volcanic rocks of Cenozoic age cover at least a persisted. An important part of Basin and Range third of the surface of the Western United States, structure is a product of early Pleistocene block­ with the greatest areas in the Pacific Northwest faulting, which accentuated earlier fault blocks and (Washington, Oregon, and Idaho), and smaller raised or lowered new fault blocks. Subsequent areas elsewhere (fig. 6). The rocks range in age from erosion and deposition has profoundly modified the Paleocene and Eocene to Quaternary, the most ranges and basins. Erosional products of the ranges extensive being of Miocene and Pliocene ages. have been carried into the adjacent basins; in Classification of the Cenozoic volcanic rocks for basins with exterior drainage, large parts of these purposes of the Geologic Map of the United States erosional products have been carried out of the presents various problems, some still partly un­ region, but in basins without outlet they have solved. Enough is now known, however, to apply a remained as Quaternary deposits. general classification throughout the map area, and A significant result of Quaternary tectonic move­ to avoid undifferentiated categories such as "Ter­ ments has been the epeirogenic arching of the tiary volcanics (Tv)" and "Quaternary and Tertiary central part of the Great Basin with respect to its volcanics (QTv)" which have appeared on earlier edges, so that even the basin floors in the central maps. part stand 2,000-3,000 feet (600-900 m) higher than The volcanics are divided on the Geologic Map those of its east and west sides. As a result, all the into those of early Tertiary (lTv), Miocene (Tmv), present lakes of the region, and to an even greater Pliocene (Tpv) and Quaternary (Qv) ages. The early extent the large Pleistocene lakes, lie close to the Tertiary· volcanics- include those, of Paleocene, Eo- east and west sides rather than in the center. cene, and Oligocene ages, and they are generally The Quaternary deposits in the closed basins more deformed and hydrothermally altered than the resemble those of the late Tertiary-sloping alluvial younger volcanics. Their exact ages are undeter­ fans and bajadas along the edges of the mountains, mined in many areas, and they crop out in such passing into nearly flat-lying finer-grained allu­ relatively small areas that it appeared fruitless to vium in the centers of the basins, with deposits of attempt a more detailed subdivision. Although the ephemeral lakes or playas in the lowest parts. volcanics are mostly not divided according to Climatic variations during the Quaternary from composition, several exceptions are made. The felsic arid to sub-humid are reflected in the nature of the or rhyolitic volcanics are important in many areas, successive deposits and the formed from them. have somewhat different habits from the rest, and During the rainy periods of Pleistocene time, the express a significant history. They are mapped basins were filled with more permanent water separately wherever data are available (lTf, Tmf, bodies than now exist, the most important of which Tpf, Qf). The remaining volcanics, indicated by "v," are those formed during the last, or Wisconsin have compositions varying from andesitic to ba­ Glaciation; traces of lacustrine epochs during the saltic but include a few undifferentiated felsic earlier glaciations also survive. volcanics. The largest and best-known Wisconsin lakes were In the Coast Ranges of Oregon and Washington, Lake Bonneville and Lake Lahontan on the east large masses of submarine pillow basalts (Teb) are and west sides of the Great Basin, although smaller interbedded in the Eocene stratified sequence. In the lakes existed in most of the other closed depressions Cascade Range to the east, the dominant andesitic throughout the Basin and Range province. Lake eruptives are divisible into lower Tertiary andesites Bonneville at its maximum covered most of the (ITa) of Eocene and Oligocene ages, unconformably western half of Utah, had a depth of 1,100 feet overlain by upper Tertiary andesites (uTa) of (335 m) and an area of 19,940 square miles Miocene and Pliocene ages. The basalts of the 2 (51,700 km ). Lake Lahontan was smaller and more Columbia River Group east of the Cascade Range irregular, with many intervening mountain ridges are mapped with the remaining non-felsic Miocene rising above it. It had a maximum depth of 700 feet volcanics (Tmv). (213 m) and an area of 8,665 square miles (22,442 Ages of the volcanic rocks can be inferred to some 2 km ). The thickness of the lacustrine deposits extent by their stratigraphic and structural rela- 48 THE CENOZOIC ROCKS

------r-r----- .,: 1: ------.. ___ _ 1'------._---. ~- • \ \ 'L, I .,. \. ~------~ i '. ______o l

0 : I l ~- ,------'-----"'~- ---- \

\ '· : : \ ""-.. ~~~~...... ' ... w, ' .:· . L------.------I 0 ' .. t/ \- ,,~ ~~t 'I '" \. ! --·:·-----'------~ •• ' ; " ' • i ·~' ' , 0 ~)·; ! '\ • • i . ••• ~ ' ! ------

• 'II·-··~~~~:- [~•~ I! -~-Itt :

• ,.. .---- 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 upper part of the lower Tertiary sequence in the San vertebrate remains in interbedded tuffaceous layers. Juan Mountains of southwestern Colorado are To an increasing extent, however, ages are being indicated separately (ITt). obtained by radiometric dating. Radiometric data The general composition of the lower Tertiary are relatively complete in a few areas, but in many volcanic rocks has been summarized as follows others the data are incomplete or lacking. Further (Lipman and others, 1972, p. 220): radiometric dating will no doubt bring about impor­ Lower and middle Cenozoic volcanic fields and associated tant revisions of the ages of the volcanics in many intrusives *** constitute a broad petrologic association of areas. predominantly intermediate compositions. Lavas are typically andesite, dacite, and quartz latite; the common hypabyssal The known ages of the volcanic rocks indicate intrusives are granodiorite, monzonite, and quartz monzonite. that the groupings of the volcanics do not corres­ Somewhat more silicic rocks, especially ash-flow tuffs of quartz pond neatly to the different Tertiary epochs, so that latite and low-silica rhyolite that are abundant in some areas, there are frequently Oligo-Miocene groups, Mio­ can plausibly be interpreted as high-level differentiates of Pliocene groups, and the like. The age designations intermediate-composition magmas. Very mafic and very silicic rocks are much less abundant; in many fields they are sparse or by the conventional epochs as shown on the map absent. Deviations from this broad outline are most conspicuous are thus to some extent unrealistic, and the assign­ at the Pacific and continental interior margins of the Cenozoic ments of the volcanic groups to one epoch or volcanic province, where mafic rocks are more abundant. another have had to be arbitrary. Some of these EOCENE MARINE PILLOW BASALT (Teb} problems are discussed below. The Eocene pillow basalts of the Coast Ranges of LOWER TERTIARY VOLCANIC ROCKS (lTv) Oregon and Washington have already been treated The lower Tertiary volcanic rocks are largely of in connection with the Eocene sedimentary rocks of Eocene and Oligocene age. Paleocene volcanic rocks that area (p. 10), so they will be mentioned only are of minor extent and belong to a different cycle of briefly here. They occur throughout the length of the eruption more closely related to the immediately Coast Ranges, from southwestern Oregon to north­ preceding . Where present, they western Washington, and are interbedded with, or are generally included with the Cretaceous vol­ form thick lenses in, the lower and middle Eocene canics (Kv). The rocks mapped as lower Tertiary geosynclinal marine sediments; they are less im­ volcanics range in age from about 55-25 m.y. portant in the upper Eocene. The basalts are Despite the rather lengthy period represented, estimated to have a volume of 60,000 cubic miles separation of the Eocene from the Oligocene vol­ (250,000 km3) and were built up in a series of canic rocks is generally not feasible at the present volcanic centers along the axis of the geosynclinal state of knowledge, and many of the sequences in trough, interfingering marginally with the sedi­ different areas include rocks of both ages. ments and in places built up to such a thickness that Lower Tertiary volcanic rocks occur throughout they emerged as islands. the western United States, but their present areal extent is much less than that of the younger LOWER TERTIARY ANDESITE (ITa) volcanic rocks (fig. 7), due partly to subsequent The Cascade Range, in its 550-mile (890-km) removal by erosion and to burial beneath younger length from northern California, through Oregon, rocks. Although both Eocene and Oligocene vol­ to northern Washington, is a complex accumulation canic rocks occur throughout the region, the lower of andesitic volcanics varying in age from early Tertiary volcanic rocks average somewhat older in Tertiary through Quaternary. The range is asym­ the north than in the south; north of the 41st metrical, with a broad, deeply dissected western parallel Eocene components dominate, farther south part that slopes westward to the Puget Trough, the Oligocene. Willamette Valley, and other lowlands, and with a Most of the lower Tertiary volcanic rocks are narrower higher part to the east, crowned by a shown on the Geologic Map as a single unit (lTv). In chain of young volcanoes. The western, lower the Pacific Northwest, however, two categories are Cascades are formed of a mass of slightly alterP.d shown separately-the Eocene submarine basalts of and somewhat folded· andesitic eruptives, ranging the Coast Ranges (Teb) and the lower Tertiary in age from late Eocene, through the Oligocene, into andesites of the western part of the Cascade Range the early Miocene. The High Cascades to the east (ITa). In addition, the felsic volcanic rocks of the are built of little dissected andesitic eruptives of 50 THE CENOZOIC ROCKS

------,~--- ~ ------\ \ \ \

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

j______.-J-----I --.\ ------• . ' t. .I " : ______----- t ·a.. f.· ! ' ,. : \• I • , , . "' I : · .. 1 • J. ".. ' : I •• 0 , I . '··,_ ,:f.1f·•· ! : t I J• 't: ~0, I ~.. 'o '... , ~ I "II- ,,• :I \. ------'·. ~.~0 •,fi : ~ J------'·., • 1------\------1..;------~=------.. ·• 1· I I \ '" : 0 ~;-,_ ~: ! I \ • • 0 "···) •. I: I l '--"·---4,,---.,'J'-_.,__,' .... ~ I / : • #, •• • I J ...... I ~.··r.'"'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. Ohanapecosh Formation, named for typical expo­ In Oregon, the great bulk of the western Cascades sures in Mount Rainier National Park (Fiske and is formed of the Little Butte Volcanic Series (Peck others, 1963, p. 4-20). Exposed sections of the for­ and others, 1964, p.ll-26), between 5,000 and 10,000 mation are as much as 10,000 feet (3,000 m) thick, feet (1,500-3,000 m) thick in most places, but in some and the total may exceed 15,000 feet (4,500 m). The as much as 15,000 feet (4,500 m). On the west it lies Ohanapecosh consists of volcaniclastic rocks and in places on the upper Eocene Colestin Formation, a basaltic andesite lava. Lava flows and coarse continental deposit with its own volcanic compo­ volcaniclastic rocks form thick local accumulations nents (Tee), and elsewhere on the pre-Tertiary that grade laterally into finer grained clastic rocks. basement. Northwestward, the Little Butte Series Ash-flow tuffs and rhyolites are a very minor intertongues with marine tuff and sandstone of constituent. Large parts of the formation were laid Oligocene and early Miocene age. Toward the north down under water, probably in lakes and coastal it is overlain by the middle Miocene Columbia River lagoons. The clastic rocks contain fossil plants of basalt (Tmv) and toward the east by the upper middle and upper Eocene age at some localities; the Tertiary andesites of the High Cascades (uTa). highest unfossiliferous part of the unit may extend The Little Butte Series (ITa) is formed of massive into the Oligocene. beds of andesitic and dacitic tuff, deposited by In Washington, the boundary between the lower ashflows, and interbedded flows and breccias of and upper Tertiary andesites of the Cascade Range olivine basalt, and of bedded tuff, welded tuff, and is placed on the Geologic Map at the unconformable flows of dacite and rhyodacite. In some segments, contact between the Ohanapecosh and Stevens pyroclastic and volcaniclastic rocks make up as Ridge Formations on the advice of C. A. Hopson much as three-quarters of the sequence and are (written commun. 1972). The Stevens Ridge is of shown separately on the Geologic Map as conti­ upper Oligocene and Miocene age, so that the nental deposits (Toe). The rocks were erupted from mapped bouudary is at a lower level than that in chains of volcanic vents, of which at least 30 have Oregon. been identified. Most of the volcanic rocks are partly West ward in the Washington Cascades, the ande­ or completely. altered by low-grade (zeolite facies) sitic components fade out and intertongue with metamorphism, and they have been altered to continental deposits, such as the coal-bearing upper hornfels near intrusives. Eocene Cowlitz Formation of the Centralia-Chehalis Fossil plants of middle to late Oligocene and district. The underlying N orthcroft Formation is early Miocene age occur at many levels and largely andesitic, but its lower part is sedimentary localities in the Little Butte Volcanic Series. The and contains basaltic detritus derived from the upper Oligocene floras are like those in the John west. The upper Eocene formations of this area lie Day Formation farther east in Oregon. The older on the east edge of the lower and middle Eocene floras indicate a tropical or subtropical climate, and pillow basalts of the Coast Ranges to the west. the youngest a somewhat cooler climate. The lower Tertiary andesites extend about 45 CLARNO FORMATION (lTv) OF NORTHEASTERN OREGON miles (72 km) southward into California to the East of the Cascade Range, the next extensive latitude of Mount Shasta, beyond which they are body of lower Tertiary volcanic rocks is in the overstepped by the younger Cenozoic volcanics northern and western parts of the Blue Mountains (Macdonald, 1966, p. 69-70). The California se­ uplift, the Clarno and John Day Formations. The quence has about the same thickness as in Oregon; John Day is more dominantly volcaniclastic than the rocks are generally less altered because of lack the Clarno, is shown as Oligocene continental of intrusives. There are some units as much as 1,000 deposits (Toe) on the Geologic Map, and is described feet (300 m) thick of coarse andesitic tuff-breccia, under an earlier heading (p. 20). containing angular blocks which reach 4 feet (1.2 m) The Clarno, lying unconformably beneath the across. John Day and unconformably on the pre-Tertiary In northern Oregon the lower Tertiary andesites basement rocks of the Blue Mountains uplift, is are covered by younger volcanics for about 50 miles more than 5,000 feet (1,500 m) thick. It is composed (80 km) toward the Columbia River, but they of basaltic, andesitic, and rhyolitic flows, breccia, reappear in southern Washington, west of Mount St. tuff, and volcanic conglomerate, with thick lenses of Helens and Mount Rainier, where they form the of fine-grained water-laid ash; thick flows of 52 THE CENOZOIC ROCKS hornblende-bearing porphyritic andesite and dacite between lower country to the east and west (Axelrod, and associated breccia occur locally. Silicified wood 1968). As a result of later deformation, the early i~ common in the volcanic breccia, and leaf impres­ Tertiary volcanics north of the Snake River Plain sions are abundant in some of the silty beds. have been raised 5,000 feet (1,500 m) higher, to Vertebrate remains are rare or absent. The floras altitudes of 10,000 feet (3,000 m) or more. are lower to upper Eocene in age, and the highest WESTERN MONTANA are perhaps Oligocene. Radiometric ages of 41 to 34 m.y. (Eocene and Oligocene) are reported from Many small areas of lower Tertiary volcanics the Clarno. occur in western Montana, both within the Rocky Mountains and in outlying mountain groups in the

CII.\I.LIS \'OI.C.\:\'ICS (ITY) OF SOUTH-CENTRAL IDAHO Great Plains to the east. They are probably rem­ nants of formerly more extensive volcanic fields In south-central Idaho east of the Idaho batholith now much reduced by erosion, whose extent the early Tertiary Challis Volcanics overlie i~ Paleo~ suggested by the even wider scatter of early Tertiary zoic rocks (Ross, 1962, p. 73-102). They cover an 2 hypabyssal intrusions, especially in the region east ar~~ of 3,000 square miles (8,000 km ); they were of the Rocky Mountains. onginally even more extensive but have now been One of the larger bodies of the lower Tertiary fragmented by deformation and erosion. For the volcanics of western Montana is the Lowland Creek mos~ part they were spread out on a nearly level Volcanics on the western flank of the Boulder erosion surface of the Paleozoic rocks but some batholith near Butte (Smedes, 1962), which covers higher peaks projected, adjoined by bouidery basal 2 an area of 800 square miles (2,000 km ) and has a conglomerate. The Challis Volcanics were origin­ thickness of nearly 6,000 feet (1,800 m). The vol­ ally more extensive to the south, across the site of canics lie on an eroded surface with 1,000-2,000 feet the Snake River Plain, and the remnants of lower (300-600 m) of relief cut on the Boulder batholith Tertiary volcanics in the ranges of northernmost and the associated Elkhorn Mountain Volcanics. Nevada are probably their former extensions· the The Lowland Creek is a sequence of quartz latitic intervening downwarp and rift of the Snake River volcanic rocks, divisible into as many as six units, Plain, thickly filled by younger volcanics did not each separated by unconformities, some of which begin to develop until the Miocene. ' involved block-faulting and other deformation. The The Challis Volcanics are 2,500 to more than units include ash-flow sheets, volcanic breccias, 5,000 feet (750-1,500 m) thick. Subdivisions can be quartz latite lava, and quartz latite vitrophyre, and recognized in most areas. The thickest and most th~y are cut by many small contemporaneous persistent unit is the latite-andesite member at the intrusives. The lower part contains a few non­ base. Above this member are flows of basalt and diagnostic plants (once thought to be of Oligocene calcic andesite, commonly interbedded with tuff the age) and has yielded early Eocene K/ Ar ages of most conspicuous of which are in the Ge;mer 48-50 m.y. Tuffaceous Member in the upper part. East of the Rocky Mountains intrusives occur in In the north west part of the volcanic area, a more the Crazy Mountains, Castle Mountains, Little Belt al~red phase of the lavas was formerly distin­ Mountains, Highwood Mountains, Bearpaw Moun­ guished as the Casto volcanics, thought to be of tains, Little Rocky Mountains, and Sweetgrass Permo-Triassic age, but it is now interpreted to be Hills. These are mostly near-surface stocks and ?n. altered phase of the Challis, produced in prox­ laccoliths, and in some of the areas, notably the ~mity to the large early Tertiary granodiorite pluton Highwood and Bearpaw Mountains, effusive equiv­ In that area; only one volcanic unit is thus present. alents are preserved. The igneous rocks range in The Challis Volcanics contain fossil plants at composition from ordinary rhyolites, andesites,and many localities and levels, assigned to the Eocene syenites, to strongly alkalic rocks rich in potash and and to the Oligocene by various paleobotanists. A soda (Larsen, 1940). The igneous rocks east of the few vertebrate remains, reported to be of Miocene Rocky Mountains are generally credited to be of age, occur. Lead/alpha determinations on intru­ Eocene age. sives in the Challis Volcanics yield Eocene ages of 40 to 50 m.y. The floras are a product of a sub-alpine ABSAROKA VOLCANIC FIELD, WYOMING AND MONTANA forest that lived at an altitude of about 4,000 feet One of the major lower Tertiary volcanic areas in (1 ,200 m), which indicates the existence of a the western United States is the Absaroka volcanic highland along the central a~ds of the Cordillera field in northwestern Wyoming and southern Mon- ' LOWER TERTIARY \'OI.CANIC ROCKS (IT\') 53 tana, which forms the high, deeply dissected vol­ mudflows. The early basalt sheets, which average canic plateau of the Absaroka Mountains with a 1,000 feet (300 m) thick, occur throughout the northwestward extension into the Gallatin Range. eruptive field. The late acid breccia of the Thorofare The volcanic rocks, called the Absaroka Volcanic Creek Group is as much as 2,500 feet (760 m) thick, Supergroup (Smedes and Proskta, 1972), have an and the late basic breccia as much as 2,500 feet 2 area of 10,000 square miles (2,500 km ), and a (825 m). The late basalt sheets are preserved only as volume of 7,000 cubic miles (29,000 km3 ). The remnants on the high divides. volcanic area extends southeastward for 160 miles In the Gallatin Range to the northwest, where the (250 km), with a width of as much as 60 miles rocks are mostly Washburn Group, flows dominate (95 km). Toward the southwest, the deeply eroded over breccias (Chadwick, 1970), but at the southeast surface of the Absaroka rocks is overlapped by the end of the field in Wyoming, the sequence in the young~r volcanics of Yellowstone National Park, Thorofare Creek Group is dominantly sedimentary and toward the northeast they border the Pre­ and volcaniclastic (Love, 1939, p. 73-85). Here, the massif of the Beartooth Mountains, which group is represented by the Tepee Trail Formation probably formed a barrier to the eruptions. East­ and Wiggins Formation. ward and southeastward the volcanics rise in The Absaroka volcanics were erupted from a imposing erosional escarpments that overlook the series of centers extending northwestward through Bighorn and Wind River basins, and the volcanics the field. The centers lie in two belts which are 15 were originally more extensive in these directions. miles (25 km) apart in the Gallatin Range and Thicknesses of the volcanic rocks in continuous diverge southeastward to 35 miles (55 km) in the sections are as much as 6,500 feet (2,000 m), and the Absaroka Mountains. The vent areas were compo­ total composite thickness probably exceeds 12,000 site stratovolcanoes and shield volcanoes composed feet (3,650 m). of flow breccias, lava flows, mudflows, and ava­ The Absaroka volcanic rocks are principally calc­ lanche debris, with steep primary radial dips; the alkalic andesite, with some dacite flows and brec­ vents were intruded by stocks, plugs, laccoliths, cias and minor potassic lavas of the shonkinite and radial swarms of dikes. Away from the vents, suite. In the area east of Yellowstone Park, the the rocks pass into low-dipping volcanic conglom­ volcanics fall into two groups, each with a lower erate and breccia, volcanic sandstone, and siltstone, acid breccia, a middle basic breccia, and an upper and air-fall tuff. set of basalt sheets (Rouse, 1937). These have The Absaroka volcanic rocks lie stratigraphically traditionally been called the early acid breccia, over the non-volcanic early Eocene Wasatchian early basic breccia, early basalt sheets, late acid beds in the nearby Bighorn and Wind River basins, breccia, late basic breccia, and late basalt sheets. but they are themselves early Tertiary. They con­ Later, the units of the sequence were formally tain abundant plant remains and some vertebrate named (Smedes and Proskta, 1972, p. 10-11); the fossils and have been dated radiometrically. This early acid breccia is the Washburn Group, the early evidence indicates that the Washburn Group is late basic breccia and basalt sheets are the Sunlight Wasatchian and early Bridgerian in age, and that Group, and the late acid breccia, late basic breccia, the Sunlight and Thorofare Creek Groups are of and the late basalt sheets are the Thorofare Creek Bridgerian and Uintan age. Earlier interpretations Group. The acid breccias are light colored and that the upper part of the Absaroka sequence consist of hornblende andesite and hornblende­ extends into the Oligocene have now been dis­ andesite. The basic breccias are dark-colored, proved. owing to their higher content of ferro-magnesian Genetically related to the volcanic activity is the minerals. Heart Mountain detachment thrust in the Paleozoic The Washburn Group constitutes much of the rocks beneath the volcanics along the northeast sequence in the Gallatin Range, but farther south­ side of the Absaroka Range (Pierce, 1957). Here, east it has a patchy distribution owing to the to carbonates have be­ irregular surface on which it was laid and to erosion come detached from the underlying Cambriun of its top, so that it is no more than 1,000 feet strata and have moved southeastward, probably by (300 m) thick. The early basic breccia of the gravity sliding; on the ea~t edge of the mountains Sunlight Group is much more prominent, more they ride up and over the Wasatch Formation, with widely distributed, and thicker, reaching 5,000 feet remnants of the carbonate rocks overlying the (1,700 m) near the eruptive centers. It is a complex of Wasatch in Heart Mountain and McCulloch Peaks agglomerates, volcanic conglomerates, tuffs, and well out in the Bighorn Basin. The upper carbonates 54 THE CENOZOIC ROCKS have moved independently as detached blocks, but it probably includes Paleocene and Eocene. The leaving voids that were filled by the early basic Needles Range Formation, which 'lies on it conform­ breccia at the base of the Sunlight Group. Move­ ably and interfingers with it laterally, has a radio­ ment of the blocks may have been nearly contem­ metric age of 29-28 m.y. (Oligocene). It consists of poraneous with the eruptions of the Washburn two to five flows that are recognizable over an area 2 Group, which is scantily preserved in the area. The of about 13,000 square miles (33,000 km ) in Utah gravity sliding was somehow triggered by the and Nevada. To the west, the Needles Range is volcanism in the Absaroka volcanic field. underlain by two other ash-flow units, the Stone Cabin and Windous Butte Formations, probably EASTERN GREAT BASIN equivalent to the upper part of the Claron and 32 Lower Tertiary volcanic rocks (lTv), mostly Oligo­ m.y. or more old (Cook, 1965, p. 12-16). Above the cene, form the lower part of the Tertiary sequence in Needles Range are other ash-flow units, one of many of the ranges in the eastern part of the Great which, the Isom Formation, has an age of about Basin, in southwestern Utah and east-central 25 m.y., or late Oligocene. The higher ash-flow units Nevada (figs. 8, 9). The volcanics are of several of the Great Basin, of Miocene and later ages, are contrasting compositions as indicated below, but discussed under a later heading. they are not differentiated on the map. In many places the lower Tertiary volcanics lie on an erosion SAN JUAN VOLCANIC FIELD, SOUTHWESTERN COLORADO surface cut on the Mesozoic and older rocks, but in The San Juan Mountains of southwestern Colo­ others thin units of older Tertiary continental rado are a lofty, deeply dissected volcanic plateau deposits intervene. The lower Tertiary volcanics with an area of about 9, 700 square miles (25,000 wedge out westward and southward, so that younger km2). Mter discovery of precious metals in the Tertiary volcanics lie directly on the pre-Tertiary mountains in the 1870's, the region was investi­ rocks. Eastward, however, they extend beyond the gated by many geologists, especially by the staff of edge of the Colorado Plateau. the U.S. Geological Survey who produced a set of In the western part of the region, the lower geologic folios. The results were summarized in a Tertiary volcanics are lavas of intermediate compo­ classic report by Larsen and Cross (1956). Much sition, mostly andesite and dacite, with a thickness new work has been done in the last few decades, of about 1,000 feet (300 m). Their extrusion began resulting in many revisions and reinterpretations; abruptly about 39 m.y. ago and ceased at about 34 for a summary, see especially Lipman, Steven and m.y. ago· (McKee and Silberman, 1970, p. 2326). Mehnert (1970). Eastward, these lavas give place to and are overlain The San Juan volcanic field developed over the by felsic volcanics. Uncompahgre-San Luis geanticline, an intermit­ The felsic volcanics are mainly rhyolitic ash-flow tently positive axis that was raised during late tuffs or ignimbrites, which were spread as broad Paleozoic time, and again in Late Cretaceou~ and sheets over a nearly level surface for thousands of Paleocene time as a part of the Laramide orogeny. square miles. Successive flows thus form a recog­ During the Laramide orogeny, andesitic volcanism nizable stratigraphic sequence that can be traced occurred along the axis. Although most of the and correlated from one mountain range to another. andesitic rocks were eroded, their detritus occurs in Eruption of the felsic volcanics began about 34 m.y. the Paleocene and Eocene deposits of the San Juan ago and continued from the Oligocene into the basin. During the late Eocene, the eroded surface of Miocene. the Precambrian along the axis was covered by the In the Iron Springs district of southwestern Utah thin, non-volcanic Telluride and Blanco Basin the basal ash-flow tuff sequence, or Needles Range Formations; the main period of volcanism was later. Formation, lies on the non-volcanic Claron Forma­ Initial eruptions from numerous volcanoes pro­ tion (Txc of Geologic Map) (Mackin, 1960, p. 99-100; duced voluminous intermediate lavas and breccias Anderson and Rowley, 1975, p. 10-13). The Claron, (lTv), mostly of alkali andesite, rhyodacite, and about 1,500 feet (450 m) thick, is a continental mafic quartz latite that make up the San Juan, Lake deposit of clastics and interbedded limestone. It is Fork, and Conejos Formation. The early volcanics lithically similar to and probably equivalent to the blanketed most of the field and have a volume of so-called "Wasatch Formation" at the south end of more than 10,000 cubic miles (40,000 km3 ) (fig. 10). the High Plateaus of Utah to the east, which is of These eruptions occurred between 35 and 40 m.y. Paleocene and Eocene (Flagstaff and Green River) ago, or during the early Oligocene. age. The age of the Claron itself is undetermined, 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 , ,"' . "· r---I --

" 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 ash­ ash-flow eruptions all occurred within 5 m.y., flow tuffs (lTf). More than 15 separate sheets between 30 and 25 m.y. ago. Although areally derived from in the western and central extensive, their volume is only about half that of the San Juan Mountains have been recognized. The older intermediate lavas. Four of the largest cal- 56 THE CENOZOIC ROCKS

~ ~------_l -·r--·- _j j NEVADA - ----.---~UTAH __ _ . ------r------

I : I ' I I I I I , I I I . I I .... ;...... I .· I I I .. ' ···~, i : . :. ·.·:~·~··. I\ 1 I . ·.. : .. i . . .:. . !J I ;: .·· .: 1 .·::- ·.. · .. ~ ...· "' . '""' ·_ .. { '""' .. . :· . . .. -1 ""' , ·=···. .·. •'I c.~ I

·! 1--·;-----·-· __l ' ___ ------r--~------L ______r-______. ! . . . i j f j

I I I I I .I I . I _._.------.. ·. .j I I i ).-:·· ··· ... _, I I i ,' 0 I • h ·l I. I\L .j. i) " 1: ,""': A '"'·. _/·r ... ""-., . i

" I '""". . ! ""' I .. '"' , !I :"' ' ' .. '·.I " .. ~------""" '""".. r------' '"'. ,. c '"' D '""" i 10-20m.y. 0- 10 m.y. 0 100 200 Ml LES I I

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 Silverton and Lake City calderas. Those in the many others (fig. 10). Those in the western part of central part formed a cauldron complex 50 miles the field formed the San Juan volcanic depression (80 km) long and 15-25 miles (25-40 km) wide, about 25 miles (40 km) long, in which are nested the containing six or more partly coalesced calderas. LOWER TERTIARY \'OI.CA:\:IC ROCKS (II\)

390r1T09_a ______-= 0 G~'r_a_nd__ Ju_n_c_ti_on--~ 10~8-o------~1~0~7 o------~--~106°

I .I I I I EXPLANATION I { Late basalt I• and rhyolite I I Lavas erupted con­ currently with ash-flow- tuffs I C1) 38°r-- c C1) u 0 l .!:!l I 0 Ash-flow tuffs I Early intermediate lavas and breccias I I 0 I I 0 10 20 MILES I I I I 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 are afforded by congeneric intrusions in the Meso­ overlain by intermediate to silicic lava flows and zoic and older rocks. Eastward, the field is covered breccias, the youngest of which is the Fisher Quartz by later Tertiary deposits (Tpc) in the downwarp of Latite, erupted about 25 m.y. ago (near the end of the San Luis Valley, but its edge probably coalesced the Oligocene). with the Oligocene volcanic fields of the southern Minor eruptions, which continued into the Mio­ Front Range and Wet Mountains (see below). cene are represented by erosional remnants on the crests of the mountains. These, and the Pliocene OTHER AREAS OF LOWER TERTIARY VOLCANICS IN volcanics of nearby areas, are discussed under later SOUTHERN ROCKY MOUNTAINS headings. The San Juan volcanic field is the largest of many The San Juan volcanic field was originally much areas of lower Tertiary volcanics in the Southern more extensive than its present limits. An outlier Rocky Mountains of Colorado. At the close of the with a nearly identical volcanic sequence occurs in early Tertiary eruptions, lava flows and volcani­ the West Mountains north of the canyon of the clastic sediments probably blanketed most of the Gunnison River. The spread of the volcanics was present mountainous areas south of the central limited on the northeast by the uplift of the Sawatch Front Range. There were outlying volcanic areas to Mountains, and partially on the southeast side by the north, in the Never Summer Mountains of the the Precambrian massif of the Needle Mountains. northern Front Range and the Rabbit Ears Range To the northwest, west, and southwest, the vol­ between North and South Parks (Steven, 1975). canics spread widely. Hints as to their former extent The largest of the outlying areas is the Thirty- 58 THE CENOZOIC ROCKS

nine Mile field in the southern Front Range (Epis lent Miocene and Pliocene volcanics (Tmv, Tpv). The and Chapin, 1968) with an area of 1,500 square older volcanics of the region were classed on the miles (3,900 km2). The field was spread over an Geologic Map of Arizona (1969) as Cretaceous (Kv). erosion surface of low relief cut on Precambrian Cretaceous volcanics occur farther east in the State, rocks in Eocene time following the Laramide where they intertongue with fossiliferous strata; orogeny. Initial eruptions from a cauldron in the those farther west were assigned a Cretaceous age on northern part of the field were andesite flows and the tenuous assumption that they were involved in breccias with a late Eocene age of 40 m.y ., or the Laramide orogeny, but most of them are somewhat earlier than the first eruptions in the San certainly Tertiary, as shown by radiometric dating Juan field to the west. Later eruptions from centers at various localities. farther south first produced andesite flows and The rocks shown as lower Tertiary volcanics are pyroclastics, followed by silicic ash-flow tuffs with generally more deformed and altered and less fresh­ an early Oligocene age of about 34 m.y. They were appearing than those of younger ages, and in many spread northeastward into a lake dammed by the places they are separated by angular unconform­ eruptions in which the Florissant Lake Beds accu­ ities, but they are probably of diverse ages. mulated. Minor andesitic eruptions continued into In northwestern Arizona south of Boulder Dam, a the Miocene, from which 19 m.y. dates have been 17,000-foot (5,200 m) sequence of Tertiary volcanics obtained. overlying the Precambrian basement is divided into A smaller area of lower Tertiary volcanics occurs the Patsy Mine, Golden Door, and Mount Davis in the Silver Cliff and Rosita Hills districts on the Volcanics (Longwell, 1963, p. 18-29); the lower part west slope of the Wet Mountains to the south. of these is represented as lower Tertiary volcanics on Eruptions were from local cauldrons, and as in the the Geologic Map. The Patsy Mine Volcanics are a other fields, include both andesite flows and rhyo­ plausible candidate for lower Tertiary, because they litic ash-flow tuffs. The oldest are 40 m.y. old (late are more deformed and altered than the higher units. Eocene), but the bulk of the volcanics is Oligocene, However, radiometric determinations give little with ages of 38-33 m.y. comfort to the notion. Radiometric ages determined Extensive andesitic eruptions in the Mineral Belt on the Patsy Mine are between 14 and 18 m.y. of the central Front Range during the Laramide (Miocene), except for a doubtful 40 m.y. date in the orogeny contributed large quantities of andesitic lower part (Anderson and others, 1972, p. 275). To the debris to the and Paleocene forma­ west in the Mojave Desert region of California, no tions in the Denver and North Park basins to the volcanics older than Miocene are known. east and west, but few outcrops of the andesite Much farther southeast, in the Tucson Mountains remain in the source areas. Volcanism was renewed west of Tucson, the lowermost volcanic unit (Cat during the Oligocene, but except for a few remnants Mountain Rhyolite), shown as lower Tertiary on the of flows, most indications of the event are in the Geologic Map, has yielded Cretaceous ages of 65-70 associated intrusives, which were formerly confused m.y. (Bikerman and Damon, 1966, p. 1228). with the Laramide intrusives (Kg3) that occur in the In the intervening area, the designation of lower same area. The large Mount Princeton pluton in the Tertiary for the older volcanics is generally more Sawatch Range to the west has been dated radio­ appropriate, as Oligocene dates of 26-29 m.y. have metrically as late Eocene or early Oligocene and been reported from a number of localities, as in the probably was the root of a volcanic center of that Sierrita Mountains and the Yuma area. Composi­ time whose surface rocks have been eroded. tions range from andesite to rhyolite. The lower Tertiary volcanic fields of the Southern Rocky Mountains may have formed over a large MOGOLLON PLATEAU, SOUTHWESTERN NEW MEXICO subjacent batholith or chain of batholiths (Steven, The Mogollon Plateau is a volcanic field covering 1975, p. 88-91). A series of pronounced gravity lows more than 20,000 square miles (50,000 km2) at the extending northeastward from the San Juan Moun­ southeast corner of the Colorado Plateau in south­ tains to the Mineral Belt of the central Front Range western New Mexico and adjacent Arizona. On the is probably a geophysical expression of the batho­ Geologic Map its rocks are shown as Miocene and lith. Pliocene volcanics (Tmv, Tpv), with minor areas of SOUTHWESTERN ARIZONA lower Tertiary volcanics (lTv) along the southeast Lower Tertiary volcanic rocks (lTv) are shown on edge. This representation was based on an interpre­ the Geologic Map in many of the desert ranges of tation of units shown on the Geologic Maps of New southwestern Arizona, along with the more preva- Mexico (1965) and Arizona (1968). On the New LO\\'FR TERTIARY \'OLCANIC ROCKS (lTv) 59

Mexico map, the older volcanics are classed as Datil deposited in the block-faulted depressions. Very little Formation (Td), divided into various lithologic of these later basalts was erupted within the plateau facies, and on the Arizona map as "intermediate itself. volcanic rocks" (Tvi); these were assigned a Miocene TRANS-PE<:OS TEXAS age, following prevailing opinion. The younger In the southwestern part of the trans-Pecos region basaltic rocks are shown on both maps as QTb; these of west Texas, 240 miles (380 km) southeast of the were assigned a Pliocene age. Mogollon field, a broad lower Tertiary volcanic field Further studies of the field were in progress at the extends south westward across the Rio Grande into time of map compilation and suggested important Mexico. The volcanic field includes the Davis revisions of age assignments, but specific details did Mountains on the northeast, extends westward into not become available until the Geologic Map was in the Sierra Vieja or Rim Rock area, and southward press. The new data, supported by radiometric into the Chisos Mountains of the Big Bend area. determinations, indicate that in general the rocks Large parts of the field have been studied in recent designated as Miocene on the Geologic Map are decades, mostly by the staff of the Bureau of actually Oligocene (lTv), and those designated as Economic Geology and by students of the University Pliocene are actually Miocene (Tmv) (fig. 11) (Elston of Texas, the results of which have been summarized and others, 1973; Elston and others, 1976). and correlated by Maxwell and Dietrich (1970). A The volcanic rocks of the Mogollon field comprise notable feature of the volcanic rocks is their high three overlapping magma suites: (a) an early calc­ alkalic content, which is especially evident in the alkalic andesite to rhyolite suite with ages of about associated intrusions. 29-43 m.y.; a high-silica alkali rhyolite suite with The volcanic rocks were erupted onto an irregular ages of about 21-31 m.y.; and (c) a basaltic suite, erosion surface of Cretaceous and Paleozoic rocks, contemporaneous with the other two suites, but from several centers, each providing a succession of continuing from about 37 m.y. to the present, but volcanic and volcaniclastic units, having a wide reaching its peak during and just after the high­ variety of local names. These sequences have been silica alkali rhyolite eruptions. The volcanic rocks of correlated by widespread layers of ash-flow tuff, of the Mogollori field evolved in a single direction, and which the Mitchell Mesa or Brite Tuffs are most not in cycles. Calc-alkalic rocks appeared early and persistent, and have been used as a base-line for then faded out, becoming scarcer just as the high­ correlation by Maxwell and Dietrich (1970, pl. 1). On silica alkali rhyolite became abundant; basaltic the Geologic Map this base-line has been adopted for andesite and basalt occur at many stratigraphic separating lower Tertiary volcanics (lTv) from levels. Miocene volcanics (Tmv). Although this separation The rocks of the rhyolite suite are believed to have is convenient for map purposes, it is misleading as to originated in a large subjacent batholith. They were age, for the rocks below the base-line are mainly erupted from a series of cauldrons and volcanic­ Eocene, and those above it mainly Oligocene. tectonic depressions, mostly confined to the Mogol­ The oldest Tertiary rocks in the trans-Pecos region lon Plateau, some more than 35 miles (55 km) in are in the Big Bend area surrounding the Chisos diameter. They are bordered by broad ring-fracture Mountains. Here, most of the former extent of the zones. Within the cauldrons ash-flow tuffs exceed volcanics has been eroded, leaving only remnants in 3,000 feet (1 ,000 m) in thickness, but they thin in the the higher mountains. Congeneric intrusive stocks outflow sheets. Ring-fracture zones are character­ abound in the Cretaceous of the surrounding lower ized by domes and flows of flow-banded rhyolite country. East of the Chisos Mountains the Creta­ associated with moat and flank deposits of bedded ceous rocks are succeeded by the nonvolcanic Black pumice. Lower temperature pumice leaked out along Peaks, Hannold Hill, and Canoe Formations, which ring fractures and filled moats around resurgent contain Paleocene, lower Eocene, and middle Eocene domes. Lake beds, landslide blocks, and conglomer­ vertebrates, respectively. Thin basalt flows occur in ate beds are common in the moats. the Canoe Formation, but the main volcanic During the main period of eruption, the region was sequence consists of the conformably overlying tectonically quiet. Following the eruptions, Basin Chisos and South Rim Formations, totaling 3,600 and Range structure, produced by regional tension, feet (1,100 m) thick. The Chisos is mainly volcani­ began to develop about 21 m.y. ago, but the block­ clastic, but it includes widely traceable basalt faulting thereby created was confined to the members; the South Rim is massive rhyolite flows periphery of the plateau. Basalts are interbedded in and associated flow-breccia. A few late Eocene the lower part of the Gila Formation that was mammals occur in the lower part of the Chisos 60 THE CENOZOIC ROCKS

Q Q ~ ~Tof ~@ ~

~ 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 by thinner, widely traceable flows and ignimbrites project in prominent escarpments and cuestas. The volcanic components thicken and merge northward Alluvium toward eruptive centers in the core of the Davis Mountains. The upper part of the sequence, approx­ imately equivalent to the South Rim Formation, Basaltic lava includes the Mitchell Mesa , and at the top the Rawls Basalt, both of which are extensive. ~ The succession in the Rim Rock area farther west Santa Fe and is notable for its abundant vertebrate remains, J{ Gila Formations which have been correlated with radiometric dates j{ from associated volcanic layers (Wilson and others, Basaltic andesite 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 Felsic volcanic rocks (Rhyolite lava and ash­ four higher vertebrate faunas represent different flow tuff) parts of the Duchesnian Stage, near the Eocene­ [E] Oligocene boundary, and associated volcanic layers Volcaniclastic have yielded radiometric ages of 38-34 m.y. Above rocks 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 Basaltic andesite the area have Miocene ages of23-18m.y. (Dasch and others, 1969). c:Ill Ill u One of the youngest units in the volcanic sequence 0 Felsic volcanic rocks .!!!> (Rhyolite lava and ash­ is the Perdiz Conglomerate, which forms extensive 5 flow tuff) gravel hills and plains east of the Rim Rock and W!Yi~)J south of Marfa. It is composed of clasts of a wide Volcaniclastic variety of volcanic rocks eroded from the older units, rocks as well as clasts of Cretaceous rocks. It contains a few Tertiary fossils, but its precise age is un­

Andesite determined.

UPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv)

>- Upper Tertiary volcanic rocks form nearly two­ thirds of the surface area of the Cenozoic volcanic fields in the western United States (figs. 12, 13) and Precambrian,B Paleozoic, }~~ and Mesozoic rocks E-;' cover a span extending from about 26 m.y. to about 2 ~ .---.. 0:: m.y. ago, or about 23 m.y. On the Geologic Map the / " c.. volcanics are differentiated by age as Miocene and Outlines of cauldrons and calderas Pliocene volcanics (Tmv, Tpv), in places on rather tenuous evidence, but in this account it is convenient Contact to consider them together, area by area, rather than by age. Fault Dashed where approximate The upper Tertiary volcanic rocks form a large and or concealed complex assemblage. On the Geologic Map, a compositional distinction is made between upper FIGURE 11.-Continued. Tertiary andesites (uTa) in the Cascade Range of the Formation, and it has yielded radiometric dates of Northwestern States and felsic volcanics (Tmf, Tpf), 42-31 m.y., or late Eocene to early Oligocene. The mainly rhyolites and ash-flow tuffs, in the region to South Rim Formation has been dated as 30-29 m.y. the southeast. The remaining undifferentiated A different but partly equivalent volcanic se­ volcanics (Tmv, Tpv) are dominantly basaltic but quence occurs in the sou them Davis Mountains to include mixed associations and some small areas the north, where thick volcaniclastic units separated of felsic volcanics. 62 THE CENOZOIC ROCKS -- -r-;------: ' ------...... I t ------r-·------!, \ ' I 'L, I \ I I \ ) r·------; '\.-.-~.:'.:·------I ._ 1 ---·. ~: I I I I I I I ~- ----... I .------,_----,~ --,------: ,, I \ ' ' L------.--- I \ I I ·-----.... ______-L. ______• I 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). UPPER TERTIARY VOLCANIC ROCKS (fmv, Tpv) 63

-,---· ------~r------\

1 \, I I \ I I '\ ·-...... ·------~-t ... ~' ~ L------:; I C·\ : .'1 _;' ) ! : ·- ,------, \ t.. \-.-~-r-.: I : ----·, U./ ·r. ! I ,\_ ..... v :l .___.. .. ! : ~

I ~ .,.- ,· I:------, - ·-·--,·', -!.L ..- I • • l I \ -~f' -~------: \. ,. I J ·; " . I ~ ' : I ~.,.~ .. i•l~ .• • , t 1 ~ 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). 64 THE CENOZOIC ROCKS

The passage from early Tertiary to late Tertiary 30). The lower subdivision, the Stevens Ridge time is characterized by a change from a domi­ Formation, lies unconformably on the Ohanape­ nantly calc-alkalic intermediate or andesitic regime cosh Formation, described previously (p. 51), and is to one of more varied volcanism (Christiansen and about 3,000 feet (900 m) thick. In consists of flows Lipman, 1972, p. 279-284). Basaltic fields are and volcaniclastic rocks, generally light-colored extensive, notably in the Columbia River Plateau to and quartz-bearing. Ash-flows are abundant in the the northwest, but there are also bimodal basalt­ lower part and give place upward to coarse volcani­ rhyolite fields, as in the Great Basin, as well as clastic rocks. The Stevens Ridge contains a middle differentiated basaltic and alkalic fields, and com­ Oligocene to lower Miocene oreodont and other plex fields. The change from the earlier to the later fossils which indicate about the same age. The volcanic regimes was not simultaneous across the overlying Fife Peak Formation, about 2,400 feet region, but appears to have begun earlier in the (730 m) thick, consists of olivine basaltic and south, toward the end of the Oligocene, and to have andesitic lavas, darker colored than the Stevens progressed northward in Miocene time and later. Ridge, with minor interbedded rhyolite flows and Onset of the later volcanic regime was a manifes­ volcaniclastic rocks. No fossils have been found in tation of the dominant tensional tectonic state that the Fife Peak, but it lies stratigraphically between began in the western part of the United States the late Oligocene to early Miocene Stevens Ridge during the later Cenozoic. It was accompanied at Formation, and the late Miocene Yakima Basalt least in its earlier phases, by extensional Basin and that overlaps it on the east. Range block-faulting. The earlier mixed volcanics The Stevens Ridge and Fife Peak are intruded by spread indiscriminately across an unfaulted terrain, the Tatoosh granodiorite pluton which forms the but later volcanics were closely related to the foundation of the Mount Rainier ; this development of block-faulted basins and ranges. pluton is equivalent to the Snoqualmie Grano­ Ultimately, this tectonism and volcanism is related diorite that intrudes the Keechelus rocks of the to plate motions on the Pacific Ocean floor to the Snoqualmie quadrangle. Both the Tatoosh and west (Christiansen and Lipman, 1972, p. 279-284). Snoqualmie plutons were explosively unroofed late in their history and gave rise to associated ande­ UPPER TERTIARY ANDESITES OF WASHINGTON (uTa) sitic volcanics. These products have largely been eroded in the Mount Rainier area but are exten­ The andesitic volcanism of the Cascade Range in sively preserved in the Snoqualmie area, where they Washington, Oregon, and California, which began are the upper unit of the Keechelus Andesite. Unlike during the early Tertiary, continued during the later the lower Keechelus, which underlies the Yakima Tertiary and the Quaternary. The later Tertiary Basalt, the upper Keechelus overlies it, and inter­ andesitic eruptions built the High Cascades east of fingers eastward with the clastics of the Ellensburg the lower western Cascades that had been formed Formation of Pliocene age. by the older andesites. In Washington, the upper Tertiary andesites form lTPPER TERTIARY ANDESITES OF OREGON Ar\D smaller, more disconnected areas than those farther CALIFORNIA (uTa) south. Early in the century they were designated the The upper Tertiary andesites of Oregon and Keechelus Andesite by Smith and Calkins (1906) California form a broader, more continuous outcrop from exposures in the Snoqualmie quadrangle on band than in Washington, and they are mainly the east side of the range. These geologists recog­ younger or largely of Pliocene age (Williams, 1942, nized, however, that their unit was apparently p. 17-19; Macdonald, 1966, p. 71-74). composite, with a body of older altered andesitic The High Cascades of Oregon form a lofty plateau volcanics, separated from younger, much fresher surmounted by steep-sided shield volcanoes of volcanics by an obscure unconformity. Subse­ olivine basalt and olivine-bearing andesite, largely quently, the name Keechelus has been extended to of Pliocene age, above which rise the still higher other parts of the Cascade Range in Washington, Quaternary composite andesitic cones that form a and misapplied to a considerable variety of ande­ chain along the crest. The Qua ternary volcanics sitic units, ranging in age from Eocene to Pliocene form only a small part of the volume of the High (Waters, 1961a, p. 43-47). Cascades; the preceding Pliocene volcanics were far Much later, volcanics equivalent to the lower more voluminous. The earlier eruptions were largely Keechelus were subdivided and named in Mount quiet effusions of mafic lava of striking uniformity. Rainier National Park, southwest of the Sno­ Part of the lava escaped from fissures along the qualmie quadrangle (Fiske and others, 1963, p. 20- western border of the belt and flowed westward

66 THE CENOZOIC ROCKS

Snake River area farther east. The last two units are are difficult to trace across the intervening forested probably nearly contemporaneous but differ some­ uplands, where they are much weathered. They are what in composition. They are separated from the therefore incompletely mapped and are only inade­ overlying Yakima by unconformities with slight quately represented on the Geologic Map, where a angular discordances. The Yakima Basalt, in turn, few of the many dikes of the Chief Joseph and is divisible into three unnamed members (Wright Monument swarms are shown. and others, 1973, p. 371-374), the lower two of which As a result of the northward flow of the basalt, correspond to the original Yakima Basalt. The marginal lakes and sedimentary basins were ponded upper member, preserved only in the center of the against the adjacent highlands, producing the field, is more or less equivalent to the sedimentary Latah Formation on the north and Payette Forma­ rocks of the Ellensburg Formation on the west side. tion on the east, both of Miocene age, and the Radiometric ages by the K/ Ar method of the somewhat younger Ellensburg Formation on the basalts of the Columbia River Group indicate that west. The basalts also flowed westward into western they were extruded during an interval of about 9 Oregon and Washington through a sag in the older m.y. The Picture Gorge Basalt has been dated volcanics of the Cascade Range near the site of the between 21 and 15 m.y., and the Yakima Basalt present Columbia River, where they assumed a between 20 and 15 m.y. (Gray and Kittleman, 1967), pillowed subaqueous phase along the Pacific shore­ or from early to late Miocene. The dates do not seem line of the time and mingled with other Miocene to confirm the considerable hiatus between the older basalts derived from local centers (Snaveley and and younger basalt formations that is suggested by others, 1973, p. 388-391). the stratigraphic evidence. The highest member of SOUTHEASTERN OREGON AND NORTHEASTERN the Yakima, which is equivalent to the Ellensburg CALIFORNIA Formation, probably extends into the early Pliocene. The basalts of the Columbia River Group are South of the basalt field of the Columbia River monotonously uniform tholeiites, composed of Group, beyond the Blue Mountains uplift, Cenozoic pigeonite, labradorite, and magnetite-rich glass, volcanic rocks underlie all the surface in south­ and virtually without olivine. They differ little in eastern Oregon and in the Modoc Plateau of composition through 3,000-foot (900-m) sections. northeastern California as far as northern Nevada. There are, nevertheless, subtle compositional dif­ Unlike the Columbia River basalts, these volcanics ferences between the successive stratigraphic units are much broken by Basin and Range block-faulting (Waters, 1961b, p. 591-604; Wright and others, 1973, and are much more varied, both in composition and p. 377 -383). The Picture Gorge and Imnaha flows age. Miocene volcanic rocks emerge in the higher contain about 5 percent olivine, 47-50 percent silica, uplifts, followed by Pliocene volcanics and volcani­ and notably higher percentages of alumina, mag­ clastic sediments, and in the lower areas by nesia, and lime than the younger flows. The Yakima Quaternary volcanics (Walker, 1970; Macdonald, flows contain 53-54 percent silica, much potash and 1966, p. 89-95). titania, and very little olivine. The flows of the The Miocene includes several thick bodies of highest member are again olivine-bearing. basalt (Tmv), such as the Steens Basalt of Steens The Columbia River flows were erupted from Mountain in southern Oregon, and the Warner fissures rather than from volcanoes; they were Basalt of the Warner Range in northeastern Cali­ produced in a tensional regime. Several dike swarms fornia. These units are also identifiable in adjacent have been identified, mostly on the southeast and ranges, although it is doubtful that they formed a south sides of the plateau, from which the basalt continuous sheet. In California, moreover, the term flowed north ward. The largest of these is the Chief Warner Basalt has been used for basalts at different Joseph (= Grand Ronde) swarm in northeastern stratigraphic levels, some of which are Pliocene. Oregon, where there may be as many as 21,000 The Steens Basalt is as much as 5,000 feet (1,500 m) north-northwest-trending dikes that extend for 165 thick in its type area, but the Warner Basalt is miles (265 km) or more (Taubeneck, 1970; Waters, thinner. The Steens Basalt has been dated by the 1961b, p. 586). Another group, the Monument Kl Ar method as 15-16 m.y. old, or about the same Swarm, occurs farther west along the John Day age as the main part of the Columbia River Group, River on the north side of the Blue Mountains uplift. but it and the other basalts of the region are There are smaller swarms along the edge of the field dominantly alkali olivine basalts, rather than to the west and northwest. The dikes are promi­ tholeiitic like the Columbia River lavas; some of the nently exposed on canyon walls and in roadcuts but flows are nonporphyritic high-alumina basalts. lJPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv) 67 Individual flows are mostly thinner than those of are partly silicic latite and partly rhyolite. A the Columbia River, being mostly 20 feet (6 m) thick prominent component is the Cougar Point Tuff that or less, and columnar jointing is rare. The Straw­ forms the rim of the south-facing cuestas along the berry Volcanics on the south flank of the Blue Nevada border, overlooking the older volcanics Mountains uplift are mainly basaltic like the rest, (Coats, 1964, p. 13-15). The sedimentary parts of the but they pass upward into more felsic dacitic and formation in the western part of the plain contain rhyolitic lavas (Tmf). fossil plants, land snails, and of lower The overlying rocks, mainly Pliocene, are a more Pliocene age, but fossils of apparently somewhat complex assemblage, formed while block-faulting younger age occur farther east. was in progress. They include sheet basalts and Overlying the ldavada Volcanics in the uplands basalts erupted from shield volcanoes (Tpv), some south of the western part of the plain is the Banbury dated at 9 m.y., and also large volumes of felsic Basalt of the Idaho Group (the various subdivisions volcanic rocks (Tpf), which form tuffs, flows, and of the group have already been described in the domes. In places these pass into volcaniclastic section on "Idaho Group"). The Banbury is as much sedimentary rocks (Tpc). There is, however, a as 900 feet (290m) thick, but it thins southward to a notable absence of andesites and other intermediate feather edge along the Nevada border. Mammals volcanics. For the most part, the subdivisions of the and other fossils in sedimentary intercalations are Pliocene rocks have not been named, but the of middle Pliocene age, and the basalt has yielded a Danforth and Harney Formations have been dis­ radiometric age of 9 m.y. Overlying volcanic rocks tinguished in southern Oregon, where they are of the Snake River Plain are of Quaternary age. applied to mafic and felsic tuffs and volcaniclastic sediments that overlie the Steens Basalt. In north­ GREAT BASIN OF NEVADA AND ADJACENT STATES eastern California is the Alturas Formation, largely Miocene and Pliocene volcanic rocks (Tmv, Tpv) of lacustrine origin, composed of diatomite, tuffa­ form an extensive and complex assemblage in the ceous silty and sandy shale, and sandstone, which Great Basin of Nevada, and extend into Utah and contains Pliocene plants and mammals. Westward, California. Important parts of the assemblage are the Pliocene volcanics mingle with the east edges of felsic (Tmf, Tpf); they are shown separately on the the ur 'ler Tertiary andesites of the Cascade Range. Geologic Map. Felsic eruptions began in eastern Nevada and southwestern Utah in the late Oligo­ SNAKE RIVER PLAIN OF SOUTHERN IDAHO cene, as described in the section on "Eastern Great East of the region just discussed is the Snake Basin," and progressed westward and southwest­ River Plain, which extends across southern Idaho. ward during Miocene and Pliocene time (Armstrong The plain is a downwarp transverse to the earlier and other, 1969; McKee and Silberman, 1970; McKee Cordilleran structures, extending northwestward in and others, 1971; Silberman and McKee, 1972). its western part and northeastward farther east, The Tertiary volcanic sequence in central and with a length of nearly 350 miles (550 km). It is western Nevada falls into three groups: andesitic entirely a late Cenozoic feature, probably caused by and dacitic flows with ages of 36 to 34 m.y. (included crustal rifting, and has been filled thickly by on the Geologic Map in the lower Tertiary volcanics Pliocene and Quaternary volcanic rocks and asso­ (lTv); rhyolite ash-flows with ages from 34 to 20 m.y. ciated sediments, beginning with felsic volcanics (divided on the map between lower Tertiary and (Tpf) and changing to mafic later (Tpv, Qv) (Malde Miocene volcanics, lTv, Tmf); and alkali basaltic and Powers, 1962). and andesitic flows with ages less than 17 m.y. The initial part of the sequence is the Idavada (mainly shown as Tpv). The first two groups were Volcanics, which are exposed in a wide band in spread widely over a featureless terrain; the third southwestern Idaho, sloping northward toward the group accompanied and followed the beginning of axis of the plain. Small outliers project from beneath block-fault~ng and the development of Basin and the younger lavas on the north side of the plain, and Range structure. in smaller areas along the edges farther east, The sequence of volcanic rocks in Lander and suggesting that the ldavada extends beneath the Churchill Counties is typical of central Nevada axis of the plain. In southwestern Idaho the (McKee and Silberman, 1970; McKee and Stewart, Idavada is as much as 3,000 feet (900 m) thick and 1971). The lower Tertiary andesite and dacite flows lies unconformably on older mineralized volcanics, (lTv) are followed by Miocene ash-flow tuffs (Tmf) largely of Miocene age. The ldavada Volcanics more than 1,500 feet (450 m) thick, which consist of consist of welded ash-flow tuffs and rarer lavas that a succession of ash-flows or cooling units. These are 68 THE CENOZOIC ROCKS widely traceable from range to range but were A different sequence of upper Tertiary volcanic derived from different centers and wedge out later­ rocks occurs in southern Nevada, where they form ally in various directions, so that not all the units an extensive and nearly unbroken field centering in occur in single sections. The oldest formation is the the Nevada Test Site of southern Nye County and Edwards Creek Tuff, composed of five cooling units, attain a thickness of as much as 40,000 feet with an age of 27 m.y. The Bates Mountain Tuff, (12,000 m) (Ekren, 1968). Miocene ash-flow tuffs commonly composed of three cooling units, has an crop out around the edges, but the most extensive age of 24 m.y.; and the New Pass Tuff, a single surface rocks are Pliocene ash-flow tuffs (Tpf) that cooling unit, has an age of 22 m.y. In northern were erupted from local calderas, two of which, the Lander County the oldest tuff formation is the Timber Mountain and Black Mountain calderas, are Caetano Tuff, which is preserved in an east-west still expressed at the surface and are shown on the belt that extends across three or four of the ranges. Geologic Map. Older calderas have been covered by In northern Lander County, the ash-flow tuff the younger volcanics and are known only from sequence is followed unconformably by the upper drilling and geophysical surveys. Radiometric ages volcanic sequence (Tpv); here, basalt and basaltic by the K/ Ar method range from 27 m.y. (early andesite flows as much as 1,000 feet (300m) thick, Miocene) for the oldest ash-flow tuffs to 6 m.y. with ages of 15-10 m.y., stand in mesas and cuestas (Pliocene) for the youngest (Marvin and others, that overlook the older volcanics. These basalts and 1970). Minor basalt flows (Tpv) overlie the youngest andesites are closely related to the volcanics adjoin­ ash-flows and are related to the development of the ing the Snake River Plain to the north, already Basin and Range structure. described. In western Nevada the Miocene ash-flow deposits SIERRA NEVADA AND COAST RANGES OF CALIFORNIA are scantily developed, and most of the volcanic On the long western slope of the Sierra Nevada, rocks belong to the younger sequence (Tpv). They between its crest and the Great Valley, are many were erupted from local centers, where they may erosional remnants of Tertiary volcanic rocks­ attain thicknesses of several thousand feet, but abundant in the north, sparse in the south-which their total volume is less than that of the earlier are parts of the "Superjacent Series" of the early volcanics. Their composition is varied, with domi­ reports which lies unconformably on the "Bedrock nant calc-alkaline andesite and dacite, but includ­ Series" of the Mesozoic and earlier rocks. The ing some flows of rhyolite and alkali-olivine basalt. Superjacent Series includes accumulations that They include the volcanic sequence in the Virginia formed during a prolonged period, from Eocene to and Carson Ranges (Thompson and White, 1964, p. Pliocene, and in the northern Sierra, at least, was 12-18), and farther south those in the Bodie Hills, originally a much more extensive cover. The super­ the Goldfield Hills, and the Silver Peak Range facent rocks were laid down on a surface that sloped (Robinson and others, 1968). The best known westward like the present range, but much more sequence, in the Virginia and Carson Ranges, has gently, as much of the great tilting and uplift of the at the base the Alta Formation, of soda trachyte and Sierra occurred during the late Pliocene and early hornblende andesite, with some fossil plants of Pleistocene, before which the surface was more or Oligocene or Miocene age, followed by the thicker less confluent with that of the Great Basin to the and more varied mass of the Kate Peak Formation, east. Most of the earlier Tertiary deposits, although composed largely of hornblende andesite flows and significant, are of small extent so that on the flow-breccias, with some flows of basalt and rhyo­ Geologic Map the superjacent rocks are largely lite, and much tuff-breccia. Hydrothermally altered indicated as Pliocene volcanics (Tpv), themselves parts of the Alta and Kate Peak Formations were generalized and reduced in area at the expense of the hosts of the fabulous Comstock Lode of the the bedrock formations. Virginia City district. The Kate Peak is apparently The oldest Tertiary deposits, of Eocene age, occur intergradational with the Alta Formation, and its only in the northern Sierra Nevada. The lone upper part interfingers with the sedimentary Formation (Tee) of the Sierra foothills, a continental Truckee Formation of the adjoining basins. Fossil and littoral deposit, extends upslope into the Auri­ plants indicate that the Kate Peak is largely of ferous Gravels, which were laid down in the Pliocene age (Tpv). The Kate Peak and Truckee are channels of vanished rivers. They are followed by overlain unconformably by the basalts of the various basaltic and andesitic units of Eocene and Lousetown Formation that is largely Pleistocene Oligocene age (Durrell, 1966, p. 187-189), a few of (Qv). which are shown on the Geologic Map as lTv. UPPER TERTIARY VOLCANIC ROCKS (Tmv, Tpv) 69 Much more extensive, both in the northern and Farther north in the Coast Ranges, east of the central Sierra Nevada, are the Miocene and Plio­ San Andreas fault, the volcanics are younger. In the cene volcanic formations (Durrell, 1966, p. 189-192; Berkeley Hills east of San Francisco Bay, the Slemmons, 1966). In the central Sierra they include Berkeley Group at the top of the thick nonmarine the rhyolitic Valley Springs Formation, the ande­ Pliocene sequence consists of several persistent lava sitic Relief Peak Formation, the latitic Stanislaus units, separated by clastic units; it has been dated Formation, and the andesitic Disaster Peak Forma­ by the K/ Ar method as between 12 m.y. and 5 m.y. tion, the first two Miocene, the last two Pliocene, as old. Closely related to the Berkeley Group, although indicated by radiometric dating and fossil plants. somewhat younger, are the more extensive Sonoma An interesting feature of the younger lavas are the Volcanics north of the bay, of andesite, basalt, and "Table Mountains" which follow the sinuous courses rhyolite flows, tuffs, breccias, and agglomerates, of ancient river beds but now rise above the with minor pumice and , and water-laid surrounding terrain. In the foothills the equivalent volcaniclastic sediments. The Sonoma Volcanics of the younger volcanics is the Mehrten Formation, have yielded an age of about 3 m.y. composed largely of andesitic debris. In the southern Sierra Nevada, Tertiary volcanic SOUTHERN ROCKY MOUNTAINS rocks form only widely spaced outliers (Bateman Upper Tertiary volcanics are less extensive in the and Wahrhaftig, 1966, p. 139-145), a few of which Southern Rocky Mountains than the more volumin­ are shown on the Geologic Map (Tpv). Probably ous lower Tertiary volcanics already described, but they were never much more extensive, as this part of they occur in widely separated areas. the Sierra had much more erosional relief during the West of the Rocky Mountains in northwestern Tertiary than farther north, perhaps 4,000-6,000 Colorado are many erosional outliers of basaltic . feet (1,200-1,800 m); the flows accumulated in volcanics. They were labeled QTv on the Geologic valleys below the higher summits. The flows are Map of Colorado (1935), but obviously could not be mainly basaltic and andesitic, and different outliers Quaternary, as they form mesas on the "top of the have been dated radiometrically as between ·9 m.y. country," such as Grand Mesa east of Grand and 2 m.y. Junction and the in the northern part of In the Coast Ranges west of the Sierra Nevada the White River Plateau. On the Geologic Map they upper Tertiary volcanic rocks were erupted from a are classed as Pliocene (Tpv), but later geologic number of widely spaced centers and interfinger work indicates that the outliers have a greater laterally with the contemporaneous Tertiary sedi­ range of later Tertiary ages (Larson and others, ments, some of which themselves contain much 1975). volcanic debris. The rocks shown as Tpv on the map are mainly of In the southern Coast Ranges and farther south, two age groups. The older volcanics are of Miocene the volcanics are mainly of lower Miocene (pre­ age, with radiometric ages between 24 and 20 m.y., Monterey) age. In the western Santa Monica Moun­ and interfinger laterally with the Browns Park tains of the Transverse Ranges is a Miocene Formation. A younger group is of late Miocene and volcanic pile 5,000 feet (1,500 m) thick of submarine early Pliocene age, with radiometric ages between andesites and basalts, with extensive associated 15 and 9 m.y. Both groups are represented in the shallow intrusives. The volcanic field extends west­ Flat Tops outlier in the northern White River ward into the Channel Islands and is well repre­ Plateau, but only the second occurs in Grand Mesa sented on Santa Cruz Island. Much farther south, farther west. A third group about 8 m.y. old was San Clemente Island is largely formed of sub­ erupted after considerable uplift and faulting had horizontal Miocene lava flows. Farther north, in the occurred in the region. Minor Pleistocene basalts southern Coast Ranges west of the San Andreas (not shown on the map) occur in some of the fault, are the Miocene volcanics of the Pinnacles canyons of the area. area in the Gabilan Range, composed of rhyolite The Miocene volcanics at the top of the sequence flows, tuff, and breccia; and the lower Miocene in the San Juan Mountains have already been noted Mindego Formation of the Santa Cruz Mountains of (p. 57). These, and the earlier Oligocene volcanics, basalt, pillow lava, and tuff, with an age of about preceded the downwarping and downfaulting of the 23 m.y. East of the San Andreas fault in the southern Rio Grande depression east of the mountains, and Diablo Range (not far from the Pinnacles area) are its local representative, the San Luis Valley. Forma­ the middle Miocene andesite and basalt flows and tion of the depression was accompanied by a new agglomerates of the Quien Sabe area. cycle of basaltic volcanism, beginning in Pliocene 70 THE CENOZOIC ROCKS time in the north, and continuing into Quaternary thickness, the lower part (Bullion Canyon Forma­ time farther south (Lipman and Mehnert, 1975). The tion in the north, Mount Dutton Formation farther most extensive flows, the Servilleta Formation south) consists of rhyodacite and basalt flows and (Tpv) are flat-lying olivine tholeiites which spread associated shallow intrusives, which pass outward over the Santa Fe Formation in the depression to a into volcanic breccias, ash-flow tuffs, and inter­ thickness of 650 feet (200m) to form a broad plateau bedded sediments. In the upper part of the pile are west of Taos, New Mexico, and have been dated as several more persistent named units of basalt, between 4.5 and 3.6 m.y. rhyolite, and tuff, the youngest of which has been In the Great Plains east of the Sangre de Cristo dated at 19 m.y. (late Miocene). The outer edges of Mountains in northeastern New Mexico and south­ the accumulation cap the Fish Lake and Aquarius eastern Colorado a unique volcanic field developed Plateaus on the east, and the in late Cenozoic time in a region that had had little and other mountains on the southwest and west. prior igneous history (Stormer, 1972). Along the Although the edges of the volcanic field have been New Mexico-Colorado boundary, the lofty Raton worn back by erosion, its present extent probably Mesa and Mesa de Maya are capped by olivine approximates the original outlines. basalt flows with a Pliocene age of about 8 m.y. In Pliocene time, the Miocene volcanic rocks were (Tpv); the eastern tip of the volcanic outliers extends broken into separate blocks by the great faults into the west corner of Oklahoma. During the along the west side of the Colorado Plateau (the Quaternary basalt flows covered the area to the Paunsagunt, Sevier, and others), and the conti­ south, each flow lying on successively lower erosion nental beds of the Sevier River Formation were surfaces. Eruptions in this field were probably deposited on the downfaulted blocks. genetically related to those that accompanied the On the Geologic Map of Arizona (1969), the upper formation of the Rio Grande depression to the west. Cenozoic volcanics along the edge of the Colorado Plateau are represented as Qv, TQv, and Tv. In VOLCANIC FIELDS OF COLORADO PLATEAll compiling the Geologic Map, it was assumed that The west and south edges of the Colorado Plateau, the first group was Quaternary (Qv), the second southwest of the Rocky Mountains, in Utah, Ari­ Pliocene (Tpv), and the third Miocene (Tmv). In zona, and New Mexico, are marked by a chain of general, this classification has been confirmed by volcanic fields erupted from different centers. later more detailed data. For the northernmost field, that in the High The plateaus in the western part of the Grand Plateaus of southern Utah, little information was Canyon district of northwestern Arizona exhibit a available at the time of compilation of the Geologic sequence of basalts of Pliocene and Quaternary Map, except for Major Dutton's report for the ages (shown as Tpv on the map), whose eruptions Territorial Surveys (1880), and local reports on the were closely related to the block-faulting and Marysville mining district. Only recently have more canyon cutting (Hamblin, 1970). They are divisible comprehensive data become available (Anderson into four groups, of which only the first is Pliocene, and Rowley, 1975), which indicate that some re­ with ages of 6 or 7 m.y. This lava flowed onto a vision of representation on the Geologic Map will be plain unrelated to the modern topography and is required. interbedded with stream gravels derived from the On the Geologic Map the volcanics of the High south; all the block-faulting and canyon-cutting was Plateaus are shown as of Miocene and Pliocene age later. The three later groups are more closely related (Tmv, Tpv). More recent data indicate that although to modern drainage, and formed during progressive the base is Oligocene with a radiometric date of erosional lowering. The youngest basalt, of late 30-25 m.y., the bulk is of Miocene age with radio­ Quaternary age, poured into the present canyon at metric dates between 25 and 19 m.y. (Bassett and Vulcan's Throne, temporarily damming the Colo­ others, 1963; Anderson and Rowley, 1975). The rado River. overlying sedimentary Sevier River Formation (Tpc) Farther southeast in Arizona is an extensive and associated basalt flows are Pliocene. complex of volcanic fields extending southward The broad Oligocene ash-flow tuffs of the Great from San Fancisco Peaks near Flagstaff to the Basin, such as the Needles Range and Isom Prescott-Jerome area (McKee and Anderson, 1971). F(}rmations, extend into the western part of the The San Francisco Peaks are a much dissected High Plateaus, where they are succeeded by varied volcanic center, whose summit rises to an altitude of local volcanic accumulations. In the Marysville 13,370 feet (4,090 m), or about 6,000 feet (1,800 m) area, where the latter exceed 5,000 feet (1,500 m) in above the surrounding plateau. Its eruptions began QllATERNARY VOLCANIC ROCKS (Qv. Ql) 71 during the middle Pliocene, about 7 m.y. ago, Tertiary and Cretaceous Lower Tertiary volcanics beginning with basalt, followed by intermediate volcanics (lTv) lavas, and ending with Quaternary basalt. In the (TKv, TKa, TKr) Prescott-Jerome area to the south, an older series, Cretaceous volcanics Do. the Sullivan Buttes Latite, with radiometric dates of (Kv, Kr, Ka) 26-23 m.y. (late Oligocene and early Miocene) is In general, the age assignments made on the followed by the more extensive Hickey and Milk Geologic Map of the United States have been Creek Formations, consisting of basalt flows and confirmed by more detailed surveys and by radio­ interbedded continental deposits, with ages of 14-10 metric dating. The age relations of the lower m.y. (late Miocene and early Pliocene), confirmed by Tertiary volcanics have already been discussed (see mammalian bones of Barstovian and Clarendonian section on "Southwestern Arizona"); most are prob­ ages. The edges of the somewhat younger basalts of ably Oligocene, but volcanics of Cretaceous and the San Francisco field overlap the formations of Miocene ages have been included in places. The the Prescott-Jerome field on the north. Block­ Miocene (Tmv, Tmf) and Pliocene (Tpv, Tpf) vol­ faulting succeeded the volcanic epoch, and the continental Verde Formation was laid down in the canics have yielded ages of 25 m.y. and younger at many localities. In the Mojave Desert region to the ensuing depressions. It contains late Hemphillian mammals and its interbedded basalts have an age west, all the volcanics yield ages of 22 m.y. and younger, and lower Tertiary volcanics are absent. of about 5 m.y. The Mogollon field at the southeast corner of the The different categories of volcanic rocks show Colorado Plateau in New Mexico has already been progressively less deformation with age, and some discussed (see section on "Mogollon Plateau, south­ of the groups are separated by unconformities. The lower Tertiary volcanics are much deformed and western New Mexico"). It is largely of Oligocene mineralized, whereas the upper Tertiary volcanics age, but the upper part of the sequence is Miocene. are merely tilted and almost unaltered; the Quater­ nary volcanics are flat-lying. The Basin and Range UPPER TERTIARY VOLCANIC ROCKS OF SOUTHERN HASIN block-faulting generally began immediately after AND RANCE I;ROVINCE IN ARIZONA AND CALIFORNIA the Miocene. For example, the Galiuro Mountains The desert ranges of southern Arizona and east of Tucson are composed of a thick sequence of southeastern California present an array of Ceno­ eastward-tilted Miocene volcanics, which with their zoic volcanic rocks lying on an eroded surface of the basement are upfaulted against the Pliocene de­ plutonic and metamorphic basement. Some of the posits in the San Pedro Valley to the west. volcanic areas have been studied in detail, but most of them are known only in a general way, and no QUATERNARY VOLCANIC ROCKS (Qv, Qf) comprehensive summary is available. Many of the Quaternary volcanic rocks occupy notably smaller volcanic rocks in Arizona have been dated radio­ areas than the lower and upper Tertiary volcanic metrically by Paul Damon and his associates at the rocks, but their distribution is significant in that it University of Arizona (Damon and Bikerman, reflects the areas of latest crustal activity in the 1964), and Armstrong and Higgins (1973) have Cordillera (fig. 15). One belt extends southward reported on radiometric ages in the Mojave Desert of along the Cascade Range of the Northwestern California. States in a series of volcanic cones that form its On the Geolo~ric Map of Arizona (1969), the crest. To the east is a transverse belt centering in volcanic rocks of the southern part of the State were the downwarp or rift of the Snake River Plain and placed in five categories. They are listed below, extending eastward into Yellowstone National Park along with designations adopted for them on the and westward into central Oregon. Another more Geologic Map of the United States: disconnected belt of Quaternary volcanics extends from northeastern New Mexico, across the Rio Geologic Map of Arizona (1969) Geologic Map of United States Grande depression, and into the southern Colorado Quaternary volcanics Quaternary volcanics Plateau of northern Arizona. Smaller areas occur (Qb, Qr) (Qv) elsewhere, notably along the edge of the Sierra Quaternary and Tertiary Pliocene volcanics Nevada in eastern California. volcanics (Tpv, Tpf) On the Geologic Map all the Quaternary volcanics (QTv, QTb, QTa) are grouped together. It would have been instructive Tertiary volcanics Miocene volcanics to separate the lower Pleistocene, upper Pleistocene, (Tv, Tvs, Tvi, Tvm) (Tmv, Tmf) and Holocene volcanics, but we believe that this is 72 THE CENOZOIC ROCKS

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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 (Macdonald, 1966, p. 83-94). Newberry Caldera is at areas. Most of the Quaternary volcanics are ba­ the summit of a broad shield volcano, much more saltic, but those of the Cascade Range are andesitic, extensive than the steep-sided cones of the main and there are significant areas of felsic volcanics Cascades. (differentiated as Qf), notably in Yellowstone Park, Most of the Quaternary lavas of the Cascade along the edge of the Sierra Nevada and around the Range are pyroxene andesites, but in some of the Valles Caldera in the Jemez Mountains of northern cones there is also subordinate basalt and dacite. New Mexico. The Quaternary volcanics east of the main range are less andesitic and more varied. The Newberry Ai\:DFSITIC \'OI.CANICS OF CASCADE RAN(;E 1:\: center includes basalt and rhyolite, and several WASIIJN(;TON, ORFGON, AND CALIFORNIA prominant late flows of obsidian. Similar rocks are The Pliocene and older andesites of the High also prevalent in the Medicine Lake Highlands and Cascades in the Northwestern States (uTa) are Modoc Plateau in California. crowned by a chain of lofty volcanic cones that were built by eruptions during Quaternary time (Qv)­ BASALTS OF SNAKE RIVER PLAIN (Qv) Glacier Peak, Mount Baker, Mount Rainier, Mount In the downwarp or rift of the Snake River Plain St. Helens, and Mount Adams in Washington; is the Idaho Group, Pliocene and early Pleistocene, Mount Hood, Mount Jefferson, Mount Washington, consisting of sediments and interbedded basalt "Mount Mazama" (Crater Lake), and Mount Mc­ flows, which is succeeded by the Pleistocene and Loughlin in Oregon; Mount Shasta and Lassen Peak Holocene Snake River Group of basalt with minor in California. Forty miles (64 km) east of the main sedimentary layers (Malde and Powers, 1962, p. chain in central Oregon is the shield volcano of 1212-1217). The Snake River Group forms most of Newberry Caldera. The volcanoes project to impos­ the surface in the eastern half of the plain and less ing heights; Mount Rainier rises to 14,408 feet (4,392 extensive areas farther west. The individual units m), and Mount Shasta to 14,162 feet (4,317 m); some are discontinuous, partly because the flows were of the others are nearly as lofty. derived from different centers and partly because The geology of the High Cascades volcanoes is they spread on an irregular topography formed dealt with in many reports. Fiske, Hopson, and during the progressive downcutting of the Snake Waters (1963) have described Mount Rainier. Most River and its tributaries. Drill holes indicate that of the fundamental work on the southern volcanoes the rocks of the group attain a thickness of 900 feet is by Howel Williams (1932a, b, 1942, and many (275 m) or more near the central axis of the plain. other papers). Higgins and Waters (1967) have The Snake River Group is divided into the reported on Newberry Caldera. Madison Basalt, Sugarbowl Gravel, Thousand Growth of the Cascade volcanoes began well back Springs Basalt, Crowsnest Gravel, Sand Springs in the Pleistocene, and some of them have a complex Basalt, Bancroft Springs Basalt, Melon Gravel, and glacial as well as a volcanic history. Important Holocene lava flows. The gravels were derived from eruptions continued well into the Holocene. Mount streams flowing onto the plain from surrounding Mazama "blew its top" in a great eruption 6,600 highlands, probably during times of excessive years ago, creating a gaping hole that was filled by runoff during the glacial periods; the Melon Gravel the waters of Crater Lake, and deposited a layer of at the top was derived from catastrophic floods ash widely toward the northeast. Mount St. Helens resulting from rapid draining of Lake Bonneville to was built only a few thousand years ago, and the southeast. The basalts are all olivine-bearing, Lassen Peak erupted vigorously in 1914 and 1915- and different units each comprise several flows, the only active volcano in the Forty-eight States. most of which have conspicuous columnar jointing. Many of the other volcanoes are merely dormant, as In the areas between the present streams, the older shown by modern fumarolic activity. basalts underlie level surfaces, which are covered by The volcanoes are surrounded by small to large windblown silt and are heavily farmed. The Holo­ areas of Quaternary volcanics (Qv). In many of the cene flows are fresh and unaltered and lack sJil northern ones the lavas extend little farther than cover. At the Craters of the Moon, about midlength the bases of the cones, but in California the in the Snake River Plain, a cluster of very recent Quaternary lavas spread well east of the Cascades craters follows a north-south fracture in the older in the Medicine Lake Highlands and Modoc Plateau lavas. 74 THE CENOZOIC ROCKS

FELSIC VOLCANIC ROCKS OF Y~LLOWSTONE NATIONAL overlies the Sherwin Till of Illinoisan or Kansan PARK (Qf) age. After eruption the tuff was faulted and warped, East of the termin us of the Snake River Plain producing a fairly steep slope from Long Valley is a very different body of Quaternary volcanic toward Owens Valley. rocks, largely of felsic composition (Qf) in the Events following the eruption of the Bishop Tuff high plateau of the Yellowstone National Park include the growth of Mono Craters and other area (Christiansen and Blank, 1972). It covers an rhyolite and dacite domes on the west and south area of 2,500 square miles (6,500 km2) and radio­ sides of Mono Lake during late Pleistocene and metric dating indicates that all the rocks are less Holocene time. They were extruded as stiff magma than 2 m.y. old, the youngest being about 70,000 which formed steep-sided domes and craters. years. The Mono Lake basin and Long Valley are The first eruptions were great sheets of ash-flow structural depressions, as indicated by their strong tuff that spread widely over the terrain and cul­ negative gravity anomalies. The depth of their minated explosively about 600,000 years ago with volcanic and sedimentary fill has been variously the collapse of an enormous central caldera 25 by 45 interpreted as more than 10,000 feet (3,000 m) and miles (40 by 70 km) across in the area northwest of only a few thousand feet, depending on what the present Yellowstone Lake. Because of later assumptions are made as to the densities of the eruptive cover, the caldera is little expressed in filling rnaterial. modern topography; its original configuration is CLEAR LAKE AREA, CALIFORNIA COAST RANGES marked by a dashed and dotted line on the Geologic Map. The ash-flow tuffs erupted from it are exposed The Clear Lake area of the northern Coast Ranges, mostly in the uplands surrounding the caldera about 60 miles (95 km) north of San Francisco itself. By contrast, the smaller, nearly contempo­ Bay, records an epoch of Quaternary volcanism raneous Island Park caldera west of Yellowstone unique in this part of California (Anderson, 1936). Park is a prominent topographic feature. The lake itself was created by volcanic damming Mter the early ash-flow eruptions and the collapse in late Pleistocene or Holocene time. The volcanics of the caldera, its site was buried by a younger series were erupted onto the eroded surface of older rocks­ of rhyolitic lava flows whose upper surface forms to the east the thick tilted continental deposits of the present Yellowstone Plateau. Late in the erup­ the Cache Formation of late Pliocene and early tive cycle, minor sheets of basalt spread out in areas Pleistocene age (Tpc), and farther west the Fran­ beyond the caldera. ciscan and other Mesozoic rocks. Following the end of eruption, the volcanic The area is dominated by Mount Konocti, a plateau was heavily covered by ice during the volcanic cone that rises 2,500 feet (760 m) above the Wisconsin Glacial Stage (minor earlier glacial lake on its south side, which is formed of rhyodacite deposits are interbedded in places with the vol­ lava and associated tuff and breccia. At the east end canics). The region remains a thermally active area, of the lake are younger flows and cinder cones of as attested by its numerous hot springs and geysers. andesite, dacite, and obsidian. Solfataric activity and mineralization of sulfur and cinnabar in the EASTERN BORDER OF SIERRA NEVADA, CALIFORNIA lavas and bedrock still continues. Probably related A smaller, but significant area of Quaternary to the Clear Lake volcanism is the Geysers geo­ felsic volcanism lies along the eastern base of the thermal area southwest of the lake, although it is Sierra Nevada between Mono Lake and the head of not accompanied by eruptive activity. Owens Valley to the south, where eruptions occurred from about 3 m.y. ago to less than 13,000 years ago JEMEZ MOUNTAINS, NEW MEXICO (Gilbert and others, 1968, p. 288-299). The vol­ Another area of dominantly felsic Quaternary canism was genetically related to the growth of volcanism occurs far to the east, in the Jemez tectonic-volcanic depressions in the Mono Lake Mountains of northern New Mexico (Smith and basin and Long Valley farther south. Bailey, 1968). The Jemez Mountains are a late The felsic eruptions and development of present Tertiary and Quaternary volcanic edifice, built structures began about 3 m.y. ago. About 700,000 mostly over the Tertiary deposits of the western part years ago the Bishop Tuff spread out as an ash-flow of the Rio Grande depression, but overlapping sheet from a center near Long Valley, flowing both westward onto Mesozoic, Paleozoic, and Precam­ southward into the head of Owens Valley near brian rocks of the Nacimiento uplift. The volcanics Bishop and northward into the Mono Lake basin. It 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). less than that of the Mesozoic plutonic rocks of the Initial eruptions during the Pliocene were domi­ same region. Although diverse in both character nantly basaltic, but during later Pliocene time and age, they are shown as a single unit (Ti) on the eruptions progressed from basalt to andesite to Geologic Map. They include a few coarse-grained rhyodacite, quartz latite, and rhyolite. During the granitic bodies of batholithic dimensions, notably early Pleistocene they culminated in the great in the northern Cascade Range of Washington, in ash-flow sheets of the Bandelier Tuff, which were the region of the Idaho batholith of central Idaho, erupted from central calderas and spread widely and in the Southern Rocky Mountains of Colorado, over the surrounding terrain, where they are pre­ but most are smaller hypabyssal intrusions of served as flat-topped mesas (Qf). Two main phases shallow origin-mainly stocks and laccoliths. Most of ash-flow eruptions are dated at 1.4 and 1.0 m.y., of the intrusive rocks are of early to middle Tertiary respectively. The second phase was accompanied by age, the earlier ones a continuation of the plutonic the subsidence of the great Valles Caldera, 12-14 activity of Cretaceous time, the later intrusions miles (19-22 km) across, still a prominent topo­ representing one of more cycles of subsequent graphic feature. Small rhyolite domes were built igneous activity. Some of the Tertiary intrusive inside the caldera, the youngest about 400,000 years rocks have already been mentioned in the dis­ ago. The region remains a high thermal area, with cussions of the closely related volcanic rocks. many solfataras and hot springs inside the caldera. CASCADE RAN

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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 1973). The large Little Cottonwood stock rises at the In central Idaho to the east, the great Idaho mountain front; the smaller Alta and Clayton Peak batholith was the scene of long-continued plutonic stocks lie within the mountains to the east. Fission­ activity that culminated during the Cretaceous, but track and K/ Ar dating indicate that the stocks are lesser intrusive activity persisted into the early progressively younger from east to west-the Clay­ Tertiary, giving rise to several sizeable plutons ton Peak being 37-41 m.y. old (late Eocene), the Alta embedded in the older complex. One of the larger 32-33 m.y. old (early Oligocene), and the little bodies is in the Casto area of the central part of the Cottonwood 24-31 m.y. old (early Miocene). batholith, where a mass of granite and· quartz monzonite intrudes the Eocene Challis Volcanics COLORADO PLATEAU (Ross, 1934, p. 54-67). A radiometric age of 59:m.y. is The Colorado Plateau east of the Great Basin is not reported from it. Another somewhat smaller body of notably an igneous area, except for the volcanic quartz monzonite, nearly surrounded by Cretaceous fields along its southwest and south edges. Within plutonic rocks, forms the Sawtooth Range farther the plateau itself, a wide region is without igneous southwest (Reid, 1963, p. 14). rocks, except for laccoliths in a few clusters-in the Henry Mountains, the La Sal Mountains, the Abajo CENTRAL MONTANA Mountains, and the Carrizo Mountains. The Henry To the east in-Montana, east of the front of the Mountains laccoliths, made famous by the pioneer Northern Rocky Mountains, numerous near-surface work of G. K. Gilbert, have yielded Eocene K/Ar stocks, laccoliths, and satellitic dikes occur in the ages of 44-46 m.y.; the La Sal Mountains laccoliths Crazy Mountains, Castle Mountains, Little Belt are late Oligocene, with ages of 23-26 m.y. (Arm­ Mountains, Highwood Mountains, Bearpaw Moun­ strong, 1969, p. 2085). tains, Little Rocky Mountains, and Sweetgrass Hills (Larsen, 1940). In a few places, effusive SOUTHERN ROCKY MOUNTAINS equivalents are preserved. The igneous rocks range The Sa watch Range and the Elk Mountains to the in composition from ordinary rhyolites, andesites, west, in the Southern Rocky Mountains of central and syenites, to strongly alkalic rocks rich in potash Colorado, contain the only other area in the western and soda-shonkinite, nepheline syenite, and phono­ United States besides those of the Northwestern lite, almost devoid of plagioclase-with consider­ States of truly deep-seated plutonic rocks of Tertiary able variation from one area to another. The age. These culminate in the large Mount Princeton igneous rocks are generally considered to be Eocene. pluton of the southern Sawatch Range, which has been dated radiometrically as late Eocene or early CALIFORNIA AND GREAT BASIN Oligocene. The Snowmass and Whiterock grano­ In California and western Nevada, plutonic ac­ diorite plutons and others in the Elk Mountains are tivity virtually ended during Mesozoic time, except apparently somewhat younger, with Oligocene ages for a few widely spaced small Tertiary stocks. From of about 34 m.y., and the Treasure Mountain pluton central Nevada eastward in the Great Basin, has a Miocene age of 12 m.y. (Obradovich and however, Tertiary intrusive bodies are mingled with others, 1969). those of Mesozoic age and finally dominate at its These plutons all form part of the Colorado east edge. They include the small intrusions of the Mineral Belt, which contains numerous intrusions northern Shoshone Range, with a lead-alpha age of from the Front Range on the northeast, southwest­ 50 m.y. (Gilluly and Gates, 1965, p. 60), the larger ward into the San Juan Mountains. Those in the Harrison Pass pluton of the Ruby Range, Nevada Front Range are mainly Late Cretaceous, or "Lara­ (Coats and others, 1965), and the Almo pluton of the mide" (Kg4), those farther west are Tertiary. They Albion Range, southern Idaho (Armstrong, 1968, lie within a series of pronounced gravity lows which p. 1305), with Rb/Sr ages of 35 and 30 m.y., suggest the presence of a large subjacent batholith, respectively. or batholithic complexes, of which the visible At the east edge of the Great Basin, in the intrusive bodies are cupolas (Steven, 1975, p. 88-9.~.). Wasatch Mountains southeast of Salt Lake City, a The plutonic rocks of the Southern Rocky Moun­ cluster of large stocks of quartz diorite and quartz tains are of interest because, unlike those farther monzonite lie on the western prolongation of the west, they invade a rather thin sedimentary cover Uinta Mountains axis (Crittenden and others, over a Precambrian basement; they must have risen 78 THE CENOZOIC ROCKS through the Precambrian from a source in the Bateman, P. C., and Wahrhaftig, Clyde, 1966, Geology of the underlying mantle. Sierra Nevada, in Bailey, E. H., ed., Geology of northern California: California Div. Mines and Geology Bull. 190, SOUTHEASTERN BASIN AND RANGE PROVINCE p. 107-172. Bassett, W. A., Kerr, P. F., Schaeffer, 0. A., and Stoener, R. W., In southern Arizona the ages of the plutonic rocks 1963, Potassium-argon dating of the late Tertiary volcanic become progressively younger eastward from mid­ rocks and mineralization of Marysville, Utah: Geol. Soc. Mesozoic to dominantly Late Cretaceous ("Lara­ America Bull., v. 74, no. 2, p. 213-220. mide") (Kg4), into southern New Mexico where all Berggren, W. A., 1965, Some problems of Paleocene-lower Eocene the intrusive rocks are early Tertiary. Most of these planktonic foraminiferal correlations: Micropaleontology, v. 11, no. 3, p. 278-300. form relatively small stocks, but a larger pluton Berggren, W. A., and Van Couvering, J. A., 1974, The late Neo­ occurs in the Organ Mountains east of Las Cruces, gene: , , and paleclimatology and there are others in the Sierra Blanca and in of the last 15 million years in marine and continental se­ Capitan Mountain farther northeast. In the trans­ quences: Developments in paleontology and stratigraphy: 2, Pecos area of Texas to the south are numerous Elsevier Sci. Publ. Co., Amsterdam, 216 p. Bernard, H. A., and LeBlanc, R. J., 1965, Resume of the Quater­ stocks, sills, and laccoliths, at least partly related to nary geology of the northwestern Gulf of Mexico province, the early Tertiary volcanic activity; they are notable in Wright, H. E., Jr., and Frey, D. G., eds., The Quaternary of for their highly alkalic composition (Maxwell and the United States: Princeton Univ. Press, p. 137-185. others, 1967, p. 172-175). Similar alkalic intrusions Bikerman, Michael, and Damon, P. E., 1966, K/ Ar chronology of extend northward from the trans-Pecos volcanic the Tucson Mountains, Pima County, Arizona: Geol. Soc. America Bull., v. 77, no. 11, p. 1225-1234. area as far as the Cornudas Mountains on the Bradley, W. H., 1964, Geology of the Green River Formation and Texas-New Mexico boundary. associated Eocene rocks of southwestern Wyoming and adja­ cent parts of Colorado and Utah: U.S. Geol. Survey Prof. REFERENCES CITED Paper 496-A, 86 p. Bramlette, M. N., 1946, The Monterey Formation of California, Addicott, W. 0., 1968, Mid-Tertiary zoogeographic and paleo­ and the origin of its siliceous rocks: U.S. Geol. Survey Prof. geographic discontinuities across the San Andreas fault Paper 212, 57 p. California, in Dicktmson, W. R., and Grantz, Arthur, eds.: Bryan, Kirk, and McCann, F. T., 1937, The Ceja del Rio Puerco; Proceedings of conference on geologic problems -of San a border feature of the Basin and Range province in New Andreas fault system: Stanford Univ. Publ. Geol. Sci., v. 11, Mexico. 1. Stratigraphy and structure: Jour. Geology, v. 48, p. 144-165. no. 8, p. 801-828. Alt, D., and Brooks, H. K., 1965, Age of the Florida marine ter­ Chadwick, R. A., 1970, Belts of eruptive centers in the Absaroka­ races: Jour. Geology, v. 73, p. 406-411. Gallatin volcanic province, Wyoming and Montana: Geol. Anderson, C. A., 1936, Volcanic history of the Clear Lake area, Soc. America Bull., v. 81, no. 1, p. 267-274. California: Geol. Soc. 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