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CENOZOIC VOLCANISM OF THE CENTRAL ,

By David B. Slemmons Mackay School of Mines, UNivtRsiTY of Nevada, Reno, Nevada

The purpose of this article is to review the Cenozoic active tectonism in the Basin and Range province volcanic activity of the central Sierra Nevada and, under NSF Grant GP-5034. more briefly, the volcanism in the southern part of I w ish also to express my gratitude to Gary Ballew, the range. In a companion article in this bulletin. Dr. Peter Chapman, Jim Sjoberg, David Sterling, and Wil- Cordell Durrell revie\\ s the Cenozoic volcanic activity liam Tafuri for assistance in compiling isopach maps of the northern Sierra Nevada. included with this article. The editorial and technical Although nearly all of the central Sierra Nevada assistance of Harold Bonham, Ira Lutse\', Dick Paul, has been mapped by reconnaissance methods, the exact and Ron Gunderson, of the Nevada Bureau of .Mines, age of the volcanic units has been a matter of broad is also gratefully acknowledged. The qualit\' of the generalization, due to sparse preservation of fossil detail supporting the distribution of the various vol- faunas and floras and general lack of published de- canic units was greatly improved by the preliminary tailed mapping. Because sophisticated methods for dat- release of data bv John Burnett, Robert .Matthews, and ing volcanic rocks by potassium-argon isotope analysis Rudolph Strand of the California Division of .Mines have recently been developed, it is now possible to and Geolog>'. subdivide more accurately the volcanic sequences and RESUM6 OF THE CENOZOIC HISTORY OF THE SIERRA NEVADA to correlate between widely separated, nonfossiliferous units. The present paper, therefore, \vi\\ utilize the The Cenozoic volcanic history can be divided into recent age dates of Dalrymple (1963, 1964a, 1964b), four major episodes: (1) an Oligocene to .Miocene Evernden and James (1964), and Evernden, Savage, period of eruption and deposition of the \'alley Curtis, and James (1964) as summarized in figure 1. Springs rhyolite tuffs, (2) a late .Miocene or early The problem of describing the volcanic activity of Pliocene period of andesite eruptions resulting in the this region is further confused b\' the general lack of accumulation of the mudflows and volcanic sediments formational assignment of many of the volcanic units of the Relief Peak Formation (new), (3) an early and b\' the absence of data regarding detailed field Pliocene period of eruption of latite and quartz latite relations of those formations now recognized. Al- flows and tuffs of the Stanislaus Formation (new), and though most of the major rock units were recognized (4) later eruptions of Pliocene andesites of the Dis- by Ransome and Turner in the late 1800's, no formal aster Peak Formation (new) and late Pliocene to Qua- formational names were proposed by them. The de- ternar\' andesites, basalts, and rh>olites. scription of map units in the foothill region was as- The earliest activity in the central Sierra Nevada is sisted by Piper and others (19.^9) who proposed the much \ounger than that of the northern Sierra Nevada formational names, "\"alle\' Springs Formation" for where andesites were erupted as earl\- as 53.5 m. y. the early rhyolites, and "Alehrten Formation" for ago (Dalrymple, 1964a), that is, in the late Paleocene younger andesites of the foothill area. Near the sum- or earlv Eocene. The initial activity of the central mit areas of the range, however, the volcanic section Sierra Nevada consisted mainh- of deposition of rhyo- is much more diverse and additional formational as- litic tuffs, which are widely distributed in the range signments were found to be necessary during thesis north of the Madera-iMariposa County line. These studies by Curtis (1951), Halsey (1953), and Slem- rhyolitic eruptions were of nuee ardente type, and mons (1953). Although the results of these studies the sporadic eruptions, which commenced about 33 are available, none of the ne\\' formational names in- m. v. ago, spread a succession of welded tuffs that cluded have been formally proposed. This summary blanketed most of the northern and central parts of article will utilize their data in subdividing the vol- the range. The thickness of rhyolite that has escaped canic stratigraphy into formations that are consistent erosion decreases from a maximum of more than 4,000 with this more recent research. feet near P\ramid Lake and in the \'irginia Range of Some of the research \\hich contributed to this Nevada, to about 420 feet near \'alle\- Springs in the paper was developed during a study of the Sonora western foothills of the range. The eruptive centers Pass Remote Sensing Test Site, under NASA Research were both along and east of the Sierran divide. .Many Grant NGR-29-001-015, and also during studies of sources lay to the east, for the northeast-trending belts

[ 199 1 200 Gkolocv ok Northirn Caliiornia Bull. 190

lESTEDN FOOTHILLS mODLE tLTITUDES H ICH S lORt Stinltlaus anil Slams laus and Slams laus and

Holia luana df a inaga t Tuolumne diainagei Tuo lu»ne d r a tnaga

tnatlllic Harnblanda-i ich DISISTCI bttecia andat ilal act! OOO' ) (1000') FoaiHiioii

Eutaki Valli) IMi 9.0. 9.2 a.T. ilOt ill-iUlili lajile TiDIl ountlir UOO' )

Lai I It Hiallir (200-) Tabia launlai Tabli •ounlain Latila laiba Lama laabai OOO') (1500')

RELIEF RELIEF PEtl FOUHTIOM FOaUtlON

18.9. 21.9 >.l

V«ILET »»LUY SFRtmS *h«olilt tufli saainos FODUTIOII Foawiioii

20.5 H.T.-

Figure 1. Correlation and summary diagram of Tertiary cks of the central Sierra Nevada (after Dalrymple, 1964): A, Composite section from Valley Springs Formation type section and Knights Ferry a; B, Composite section from Ponderosa Way, Rattlesnake Hill, McKay, and Jawbone Ridge; C, Composite section from Sonoro Poss ond RoncI of rh\()litc increase in tiiickness to the northeast, and applied by Piper and others (1939) to a sequence of some rii\oIite intrusions have been recognized near predominantly andesitic clays, sandstones, and breccias the summit of the range. In the central Sierra Nevada, exposed in a belt in the Sierra Foothills extending from the rhyolites are generally called the "Valley Springs Bellota on the to Cosumnes on the

Formation," but ranges in actual age dates (16.1 to . The type locality- is near the Alehrten

33.4 m. >•.) indicate that activity was sporadic during damsite on the Alokelumne River. .Mthough it was an extended period. Slemmons (1953) and Dalrymple recognized that other andesitic rocks elsewhere in the

( 1 964a) have recognized more than one member both Sierra Nevada were correlative. Piper and others in the summit region and at the type locality in the (1939, p. 69) clearly did not include the rocks east of foothills (fig. 1). This further confirms the intermit- the Foothill belt in their Alehrten P'ormation. Detailed tent character of the eruptions. In the foothill region mapping by Slemmons (1953) and recent age dates by the basal unconformit>- generall\- has low relief, w ith Dalrymple (1964a), by Evernden and James (1964), broad floored valleys and narrow inner gorges, sug- and b\' Evernden and others ( 1964), however, indicate gesting that some uplift and rejuvenation preceded the that in the Sonora Pa.ss area (fig. 1), there are three eruptions. The basal unconformity commonly is above main postrhyoiite petrographic sequences, two of rocks that are w eathered to a hematite-red color and, which are older than the Alehrten Formation at its iocall>-, overlies lateritic soils. This ancient weathering type locality- probably developed during a subtropical climatic The lowermost of the three postrhNolitc units con- phase f)f the I'.ocene. sists of the andesites of the Relief Peak Formation that The rhyolitic extrusives were sufficiently extensive arc probal)l\- mainly lower Pliocene. These overlie the to force the establishment of new drainage systems N'allcy Springs rhyolites unconformably on a surface that no longer followed the former trellis pattern of exhibiting a xouthful stage of erosion. This surface longitudinal valleys and ridges, but generally drained has a relief of about 1,500 feet near the summit of the directly southwcstward across the general structural range, where great thicknesses of andesitic mudflows grain of the basement rocks. (lahars) predominate. In going westward from the The andesitic deposits forming the main postrh\-o- cre.st, the mudflows gradually thin, and near the west- lite volcanic accumulations of the range have been ern edge of the range the\' grade into andesitic sands grouped together and loosely referred to as the "Mehr- and gravels. The age of the andesites is more than the ten Formation." The Alehrten Formation was the name 9-m.\-.-old latites of the area. 1966 Slkmmons: Ci'mral Sikrra Nb-.vada 201

Unconformably on the Relief Peak Formation arc foothill town of Valley Springs where two welded tiie iatites of the Stanislaus Formation (new), which tuffs were described. The section there includes the occur in a belt up to 20 miles in width. These Iatites two rhyolitic tuffs, and also cla\s, sandstones, and con- and quartz Iatites were erupted from sources near glomerates, lying unconformably on the lone Forma- , Lighting Ridge near Sonora Pass, tion (Eocene), on auriferous gravels, or locall\- on Sonora Pass, and from important but as \'ct undiscov- pre-Tertiary basement rocks. ered sources east of Sonora Pass. At least one flow The V^alley Springs rhyolites are widely distributed extended westward 60 miles or more to Knights Ferry from the northern end of the range to about 37° lat. on the , and for several tens of miles near the western foot of the range. On the western eastward into Nevada. Although there is some evi- slope the rhyolite occupies narrow belts, 10 miles or dence that uplift of the range had already begun so in width, extending from the summit w estward to (Slemmons, 1953), and that Basin and Range faulting the foothills, where the rhvolite disappears underneath had possibly begun, the uplift could not have amounted the younger sedimentary and volcanic rocks that form to more than a third of the total Late Cenoxoic uplift the uppermost parr of the filling in the Great \'allcy. and tilt of the range, since there is no abrupt change Few outcrops of rhyolite are present in the main in the thickness of the Iatites as the westernmost Basin Sierran slopes south of the 38° lat., near the north end and Range faults are crossed. Pre-latite faulting is of , although a narrow belt of believed to be relatively minor, although near Sonora rhyolites extends along the edge of the Great \'alley Pass the unconformity between the Iatites and the at least as far southward as Friant (.Macdonald, 1941). underlying andesites had up to 1,500 feet of relief. The thickness of the rhyolites is highly variable, Andesitic and basaltic lavas were deposited on the owing not only to vagaries of initial distribution from Stanislaus latite, and as only a weakly defined uncon- various vents east of the Sierra Nevada causing the formit\' separates the two units the erosional interval pyroclastic materials to be channeled down different between the eruption of the Iatites and the extrusion westward-trending valleys, but also to extensive ero- of andesites and basalts was brief. Relatively few dates sion that occurred during different intervals after are available for the andesitic and basaltic section, but their deposition. The thicknesses generally- decrease the Oakdale flora in the youngest andesitic sediments southwestward from maximum values of 1,200 feet in the Sierran foothills has been dated middle Pliocene near Sonora Pass and more than 1,000 feet near River- (Hemphillian) by Axelrod (1958)-—supported by a ton, to 450 feet at the type locality near V^alley recent potassium-argon date of 5.7 m.\'. (Dalrymple. Springs and 270 feet at La Grange near the Merced 1964a). Dalrymple (1964a; 1964b) has also suggested River. The rhyolite of the Hartford Hill Formation that the most recent vigorous uplift and tilting of the in the \'irginia Range is of comparable geologic age Sierra Nevada was initiated during the middle to late (Schilling, 1965), and similar rhyolite that is widely Pliocene (between 3.5 and 9 m.y. ago). Typical Basin distributed in western Nevada has thicknesses of over faulting younger; it and Range may be somewhat 3,000 feet (figs. 1 and 2). probably began in the area, less than 2.6 The rhyolites unconformably overlie either older and 3.2 m.y. ago, and in the Truckee area near the Tertiary gravels and sands or the lone Formation. The end of the interval between 7.4 m.y. and 2.2 to 2.3 rhyolites were sufficiently thick to block the valleys m.y. ago. Thus there seems to be a time lag between in w hich they flowed, requiring the establishment of the tilting of the Sierra Nevada and the beginning of new drainage patterns during the subsequent erosion the present Basin and Range faulting. c\cle. The surface developed on the rh\olite was one Following the deposition of these young andesites in of moderate relief, with up to 1,000 feet of local relief the Sierra Nevada, uplift and tilting of the Sierran toward the source areas. The extensive erosion which block has caused erosion to cut deeply into the range. followed both the rh\olitic and andesitic eruptions Downcutting by extensive glaciers and streams has in- stripped much of the rh\olite from valley walls, but cised the canyons far below the gently inclined surface small bands of protected portions of rh\olites still at the top of the volcanic section, and the erosion cycle remained sporadically below the very irregular base of is still in a youthful stage with a consequent stream overlying andesites or Iatites. pattern. The \^alle\' Springs Formation was assigned a late EARLY RHYOimC ACTIVITY Miocene age on the basis of a small flora (Axelrod, Extensive remnants of rhyolite welded tuff, water- 1944, p. 217). Recently, more than 25 potassium-argon sorted tuffs, and rh\olite-bearing gravels commonly dates have become available for early Sierran rhyolites, form the basal unit of the Cenozoic section in the (Evernden and others, 1962; Evernden and James, central Sierra Nevada (fig. 1). The deposits generally 1964; and Dalrymple, 1964a). These dates show a grade from tuffs near the eastern part of the range broad range in time, from as much as 33.4 m.y. (late into types that show more extensive sedimentar\- re- Oligocene) to as little as 16.1 m.y. (middle Miocene). working toward the western edge of the range. The According to Dalrymple (1964a, p. 21), part of this basal rhyolites ^\'ere named "Valley Springs Forma- disparit)' of ages is due to discordance in dates from tion" by Piper and others (1939, p. 72-73), for the various minerals in the same rock, probably as a result 202 Gkologv ok Northern Calikorma Bull. 190

solid contouri denoting their Figure 2. Mop jhowing diifribution of Oligocene to Miocene rhyoliles of the central Sierra Nevodo, with rhyolites. The deep opproximote moximum original thickness. The dashed contours indicole the opproximate elevation of the inferred top of the Although there may hove and repeated dissection of the rhyolites mokes the Indicoted isopochs minimol volues for the thickness in deeper valleys. contours of ele- been several oreas of nondeposition between some of the southwest-trending channels of maximal thickness, the isopochs and vation ore projected between the main channels. 1966 Slkm.mons: Ckntrai. Surra Nivada 203 of contamination by older bedrock minerals. Most of with a relief as great as that on the prerhyolite ter- the variation, however, is believed to indicate a pro- rain. In the old vailevs at the base of the andesites in longed, but intermittent, period of rh\olitic activity, places are included sands and gravels, generall\' con- with the main eruptions occurring in the early Mio- taminated both with basement rocks and easil\' eroded cene. Cenozoic rhyolites. fMost, if not all, the rh\olites had a northeastcrl\- From andesites underlying the Table Mountain La- source. This is indicated by the progressive increase tite Member of the Stanislaus Formation near James- in thickness of the rhyolites in a northeast, upslope town that are herein assigned to the Relief Peak For- direction, the presence of some Tertiary rhyolite in- mation, Condit (1944) has described a Mio-Pliocene trusions toward the eastern edge of the range, and the flora, the Table Mountain flora. Stirton and Goeriz progressive change from rhyolitic sediments in the (1942) assign an early Clarendonian age to the same southwest, to massive tuffs and welded tuffs toward rocks on the basis of a small fauna. The potassium- the northeast. argon dates for the overlying Stanislaus Formation \^ EARLIER ANDESITIC ACTIVITY ranges between 8.8 and 9.3 m.v., and the underlying Postrhyolite flows, mudflows, gravels, and sands, rhyolites are probably older tlian 19.9 m.y., which is derived mainly from centers of basic andesite erup- the age of the youngest of the underlying V'alley tions in the central Sierra Nevada, extend in a continu- Springs rhyolites. ous zone from at least as far east as and On the basis of a flora found on Niagara Creek, the Sonora Pass to the western foothills at Knights Ferrw basal sediments of the andesites of the Relief Peak This sequence of andesites and associated rocks is Formation higher in the range appear to have a late herein named the Relief Peak Formation. It uncon- Miocene age (D. I. Axelrod, oral communication, formably overlies the \"alley Springs Formation, and 1965). In this area the underlying rhyolite yields a is unconformably overlain by the Stanislaus Formation. potassium-argon date of 23.3 m.y. on sanidine and The thickness of the Relief Peak Formation decreases 25.7 and 26.1 m.y. on biotite samples; of the three from the crest of the range toward the \\ estern foot- dates, Dalrymple (1964a, p. 22) considers the sanidine hills, where it contains a Mio-Pliocene flora and under- date as the most reliable. In either case, the lower lies latites with ages of about 9 m.y. Miocene age of the rhyolite is well established. The The Relief Peak Formation crops out on the slopes Stanislaus Formation, which unconformably overlies of Relief Peak, designated herein as the type area, in the Relief Peak, is Hemphjllian, with potassium-argon a sequence that is not only of the unusually great ages of about 9 m.\-. The weaki\- eroded nature of thickness of about 3,000 feet, but also is of varied the unconformity at the base of the latites in this area lithology. The sequence includes on the north flank suggests that the youngest of the Relief Peak ande- of the mountain, in sees. 18, 19, and 20, T. 5 N., R. 21 sites may be lower Pliocene, and these early andesites E., about 20 feet of gravel with abundant granitic and may have been erupted at various stages during late metamorphic rocks of the Emigrant Basin type, andes- Miocene to lower Pliocene. itic sands, and, near the base, much reworked rhyolitic Dikes and plugs of andesite are abundant in the area material, including cobbles and sands from the under- from Sonora Pass and Relief Peak to Mount Emma, lying Valley Springs Formation. Above these volcanic and this must have been at least one of the major sediments, the section is composed mainly of mudflow source areas for the Relief Peak mudflow breccias, breccias and autobrecciated andesitic flows. tuffs, and flows. Extensive andesitic volcanic activit\- The thickness of the Relief Peak Formation varies in nearby parts of Nevada could have provided addi- from 3,000 feet near the source at Relief Peak, and at tional sources for some of the mudflows, sands, and Mount Emma near Sonora Pass, to about 300 feet near gravels of the Relief Peak Formation. the Jamestown-Knights Ferry area shown on figure STANISLAUS FORMATION la, where the formation disappears under younger deposits of the Great Valley of California. Latites erupted after the Relief Peak andesitic ac- This unconformity, between the early andesites of tivity, and herein named the Stanislaus Formation, the Relief Peak Formation and underlying basement may prove to be of singular importance in decipher- rocks or Cenozoic rhyolites and sediments, is generally ing the structural and geomorphologica! development one of moderate relief in the foothills, but to the ea.st of the central Sierra Nevada, for they seem to form the relief increases to about 2,000 feet and provides a the onl>- well-defined stratigraphic and time marker buried youthful erosion surface with slopes of over that extends from under the Great X'alley of Cali- 25°, which is as steep as the present mountain slopes. fornia, across the range, and into the eastern portion The unconformity preserves dendritic channels in the of the Basin and Range province. Because the Stanis- underlying rhyolites that followed paths which led laus Formation predates b\- a short interval most of more or less directly downslope to the southwest. The the Sierra Nevada uplift and the development of the erosional interlude between the deposition of the 20- present Basin and Range topography by faulting, it to 33-m.y. old rhyolites and the Relief Peak andesites not only provides a measure of fault displacements but was sufficiently long to create new, youthful valleys also a means of correlating between the r\\o provinces. 204 Gkol

^i^f^

Figure 3. Map showing inferred al distribution of the Stanislaus Formation, and the original distribution of the Table Mountain Lo ember and the welded biotite-quartz lotite tuffs of the Eureka Valley Member, The volcanic centers le Table Mountain Member are in the area indicoted by the diagonal pattern near Sonoro Pass, 3 possible intrusive source for the Eureka Valley Member is similarly indicated about 15 miles eo Pass.

The latites now assigned to the Stanislaus Formation Mountain Latite Member has been traced from its type \\ere divided into three units by Ransonie (1898) and localit\- at Table Mountain almost continuously (Ran- Slemmons (1953). The three units, now considered some, 1898; Slemmons, 19.v3) to its source area near to he members, are herein defined from oldest to Sonora Pass, where at and as flows of this same t\pe of \()ungest as: ( 1 ) Table A4ountain Latite Member, there are man\' as 40 mostl\' olivine-augite latite flows including the type latite lava comprising a thickness of 1,.500 feet. locality for latites, (2) Eureka \'alley Member, con- The middle member, the "biotite-augite-latite" of sisting of several latite flows with interbedded welded Raasome (1898, p. 37-46), is here named the Eureka biotite-augite quartz-latite tuff, probably the type X'alley Member for Eureka \'alley in the Dardanelles "quartz latites," and (3) Dardanelles Member, con- quadrangle (1:62,500). The type locality is the ridge taining flows ranging from latitic to basaltic in com- between Bald Peak and Red Mountain where the sec- position. tion shows great diversity of lithology and a thickness

The Stanislaus Formation is named for its conspicu- of 200 feet (Slemmons, 1953, p. 67-71). This mem- ous development along the various tributaries of the ber conformably overlies the Table Mountain Latite Stanislaus River of California, and the type locality Member.

is designated as the Bald Peak-Red Peak ridge which The uppermost member of the Stanislaus Formation separates the Clarks Fork and Deadman Creek branches is here named the Dardanelles .Member (as currently of the Middle Fork of the Stanislaus River. The three spelled) after the "DardancUc flow" of Ransome Dardanelle of the Dar- formally named members arc defined on the basis of ( 1898, p. 46-52) for the West the three most characteristic localities for each of the danelles Cone quadrangle. At the West Dardanelle, flows nicmbcrs, and w ith an attempt to fit the terminolog\- u hich is designated as the type locality, the are as closcl\- as possible to the nomenclature proposed similar to the augite latites of the Table Mountain informally in the classic description of type latites and, Member, but plagioclase phenocrysts in them are pr()babl\-, quartz latites, by Ransome (1898). smaller and more sparse. Eastward from West Dar-

The lowermost unit is here designated the Table danelle, the Dardanelles Member includes a number Mountain Latite .Member, in conformity with Ran- of basalt and olivine basalt flows. This member con- some's (1898, p. 14-27) references to the "Table formably overlies the Eureka X'alley Member. Mountain facics" of the latites, and to the "Table The Stanislaus Formation attains a maximum thick-

Mountain flow." This member is, accordingly, named ness of more than 1,500 feet near Sonora Pass, but it for "Tuolumne Tabic .Mountain," half a mile west of thins southwestward to where, in the Table Mountain

Shaws Flat in the Columbia (1:24,000) quadrangle, area near Sonora, it consists of a single augite latite

California, where it is almost 200 feet thick. The Table flow only 200 feet thick. It is unfossiliferous, but Dal- 1966 Sl.F.M.MONS: Cf.NTRAI, SlKRRA NfVADA 205 ryniple has dated six samples b\- the potassium-argon quartz latite welded tuffs of the Eureka Valley Mem- method and obtained dates of from 8.8 to 9.3 m.y., ber are 9.2 m.y. at Big Trees, 8.8 and 8.9 m.y. for indicating a lower Pliocene (Hemphillian) age. Some tiie summit region, and 9.0 m.y. for the Jawbone of the latite lava erupted from dikes and fissures be- Ridge area. The Dardanelles Member, the uppermost tween Dardanelles Cone and Sonora Pass, and other unit, is represented by a 9.3 m.y. date (Dalrympie, flows mav have erupted east of the present divide 1964a). The onlv other potassium-argon age date sim-

(Halsey, 1953). ilar to those of the latites is from the Table Mountain The great area! extent of the Stanislaus Formation is basalt (Sugar Loaf Hill) of the Friant area (Dal- shown by figure 3. The lower unit, the Table Moun- rympie, 1963; Wahrhaftig, 1965) and Coyote Flat. tain Latite .Member, extends eastward from near This unit is probabl\' an olivine basalt (Macdonald,

Knights Ferry, where it emerges from beneath 1941; Dalrympie, 1963; and Wahrhaftig, 1965), al- \ounger volcanics and sediments that mantle the Great though Dalr\-mple's whole-rock anal>"ses indicate a \'alle>'. It attains a maximum width near Sonora Pass potassium content of 2.01 percent at Sugar Loaf Hill in the source area and fans out east of the summit and 1.82 percent at Co\ote Flat, suggestive of alkaline- area. Still farther east it narrows and becomes sporadi- potassium affinities. Their potassium-argon ages are cally distributed as it approaches its eastern limit near 9.5 m.y. and 9.6 m.\-., respectively. the Nevada-California boundary. The overlying The source of at least part of the Stanislaus Forma- welded augite-biotite quartz latite tuffs do not ex- tion is indicated by the occurrence of more than 20 tend as far w estward as the flows of Table /Mountain augite latite dikes recognized between Dardanelles augite latite, but their total area is greater. The quartz Cone and Sonora Pass. Halse\- (1953) also has noted latite outcrop width increases greatly as the summit the possibility of quartz latitic centers of eruption near of the range is approached, its limits extending farther Fales Hot Springs. The thickest sections are between north, east, and south than the flows of the Table Sonora Pass and Pinecrest, which suggests that there Mountain Latite .Member. The basalts and latites of mav be other sources to the west of the present the uppermost unit, the Dardanelles .Member, are be- divide. lieved to be more limited in areal extent, having a POSTLATITE VOLCANIC ROCKS known distribution from Big Trees on the west to the Postlatite lavas, mostly andesites and basalts, form a Nevada-California boundary on the east (Ransome, thick succession near Sonora Pass, and extend across 1898; Slemmons, 1953; and Halsey, 1953). Future de- the entire width of the range to the Great X'alley of tailed mapping, however, will probably extend the California, where they form the type locality for the limits as indicated in figure 3. .Mehrten Formation (Piper and others, 1938). These The thickness of the Stanislaus Formation varies lavas probably extend northward to, or be\ond, Don- greatly because of the irregular unconformit)' at the ner Pass, but the lack of adequate potassium-argon base of the formation, the irregular initial deposition, dates or paleontological control, and the absence of and because of subsequent deep dissection. The Table latitic lavas, have made it difficult to distinguish them Mountain Latite .Member at Knights Ferry has a from the older andesites of the central Sierra Nevada. thickness of 200 feet, and near the source at Sonora In addition to this thick sequence of lavas, there are in Pass has its maximum thickness of 1,500 feet. It thins the Sierra a number of isolated outcrops of flows of out to the east at Fales Hot Springs, but a small aux- basalt, andesite, and rh\olite that are mainl>- \ounger iliary field of similar augite latites is developed near than this thick sequence. .\s many of these postlatite Bridgeport, California. The Eureka Valley Member rocks occupy especially significant positions on ero- varies in thickness from a few tens of feet near Big sion surfaces of var_\ing geomorphological age, they Trees to 400 feet in the Sonora Pass area and Bell have received special attention (Axelrod and Ting, Meadows, 3 miles southeast of Pinecrest Reservoir. 1960, 1961; Dalrympie, 1963a, 1963b, 1964; Wahr- The overlying latites and basalts of the Dardanelles haftig, 1965). Member are developed mainl\' between the Darda- The thickest and most w idespread of the post-Stan- nelles, where they are 200 feet thick, and the eastern islaus Formation lavas of the Sonora Pass area are edge of the Fales Hot Springs quadrangle. hornblende-rich andesites herein designated as the Dis- The unconformit>' separates the latites of the which aster Peak Formation (fig. Ic). They arc ver\- well Stanislaus Formation from the underlying Relief Peak exposed in a section preserved on the slopes of Disaster andesites is generally one of low relief, and may indi- Peak, Sonora Pass quadrangle, which is designated as cate only a short hiatus in deposition. No lateral the type locality. This formation can be recognized equivalents to the latites are known. in the source area between Ebbetts Pass and Relief The age of the Stanislaus Formation can be placed Peak by the presence in it of large cobbles and breccia rather closely in spite of the absence of fossil localities clasts of hornblende andesites, generalK" with abundant in it because the consistenc\' of the several available to 1 in length. The potassium-argon dates restricts the latitic activities to hornblende phenocrysts up inch a brief period. The date for the Table Mountain Alem- hornblendes are sufficiently large to "twinkle" in the ber is 9 m.y., the four values for the biotite-augite sunlight as the outcrops are crossed, even when one is 206 Gkology of Northfrn CaI.IF()RM\ Bull. 190 1966 Slkm.mons: Cf.ntrai, Sikrra Nfvada 207 crossing rapidly in a car. The deposits arc mainl\- mudflow breccias, autobrecciated flows, and subordi- nate volcanic sediments. The section is approximately 1,000 feet thick, both near Disaster Peak and also farther south near Castle Rock and East Flange Rock where the Disaster Peak andesites lie on latite. The Disaster Peak Formation may prove to be the equiva- lent of the Mehrten Formation in the type area, but owing to large gaps in the mapping it seems best to apply a new name rather than guess at the correlation. The composition of the younger volcanic rocks varies from dominantly hornblendic andesites, which contrast with the p>'roxene-bearing t>pes that predom- inate in the older andesitic volcanics of the central Sierra Nevada (Curtis, 1951), to rhyolite and basalt. South of the Sonora Pass area, the volcanic rocks are limited in extent and are mainly basalts, rhyolite in domes, and pyroclastic beds. The deposits attain maxi- mum thicknesses in excess of 1,500 feet between the Sonora Pass area (fig. Ic) and the south end of . East of the Sierra Nevada, volcanics of similar age are even thicker, suggesting that many of the vol- canics present in the Sierra Nevada may have had an diagram for Cenozoic volcanic rocks of eastern source. That the latest volcanic activity is more Figure 5. Silica variation the alkolic Stanislaus Formation (solid lines), the calc-olkoline volconic in earlier parts of the Cenozoic is diversified than rocks of the Mehrten Formation (dashed lines), and calc-alkaline shown in figure 1. Mesozoic plutonic rocks of the central Sierra Nevada (dotted lines). The principal sources of analytical data ore from Curtis (1951), The thickness of the young volcanic rocks is highl\- Halsey (1953), Slemmons (1953), and Washington (1917). variable, for much of the southern part of the range

is either unafi^ected by volcanic activity, or contains of the three main periods of the Cenozoic (fig. 1), only sporadic flows, mainly of basalt. There is a gen- although the small number of analyses of Mesozoic eral increase in thickness of the Disaster Peak Forma- intrusive rocks prevents a good comparison. tion to the east in the central part of the range, with The Oligocene and Miocene magmas of the Valley a variation of from several hundred feet near the t\pe Spring Formation were dominantl\- rhyolitic. The locality of the Mehrten Formation at the edge of the overlying Relief Peak andesites, though not repre- Great Valley to over 3,000 feet near Sonora Pass and sented by many chemical analyses, were mainly ba- Lake Tahoe. saltic, and basic to intermediate, andesites. The latites The Disaster Peak Formation unconformably over- of the Stanislaus Formation are much different from lies either the Stanislaus Formation, usualh' with an both younger and older volcanic rocks because they erosion surface of low relief, or the early andesites and are alkalic, and potassium predominates over sodium Valley Springs rhyolites with a deeply dissected sur- in most of them (fig. 5). Their alkaline character is face of moderate relief. indicated by the Peacock calc-alkali index being lower The postlatite lavas vary widely in age, and include for these rocks than for most Sierra Nevada volcanic andesites only slightly younger than the 9-m.y.-old rocks. Postlatitic activity has, except for the major latites of the Stanislaus Formation, as well as andesitic period of eruption of the Disaster Peak hornblendic material from the Verdi area in adjoining Nevada that andesites, been marked by diverse activity in which has been dated at 5.7 m.y. (Evernden and James, 1964), extreme chemical types seem to be dominant. Rhyo- and the Oakdale flora and faunas of the foothill region lites and basalts were erupted intemiittenth" during the (Axelrod, 1944; Stirton and Goeriz, 1942). The last part of the Pliocene and the Quaternary. This

younger basalts include a wide range of dates from activitv is well marked east of the range both at .Mono scattered sources in the summit region. Lake and in Owens \'alle\-, as well as within the main The source of these young volcanic rocks of the Sierran block. Sierra Nevada lies mainly in many separate vents, most frhe eruptive record provides sharp contrasts in of which are near the crest of the range or east of chemical character, from the mainly rhyolitic charac- the range. ter of the early activity- in the Oligocene and Miocene, PETROCHEMISTRY to the andesitic character of the Mio-Pliocene or early The petrochemical character of the Sierra Nevada Pliocene, to latites and quartz latites with highly alka- aflJinities, to the youngest volcanic activity is different for each of the separate line and strongly potassic and independent periods of eruption. Activity during periods of andesitic, rhyolitic, and basaltic activity Mesozoic appears to be distinct in character from that with a calc-alkaline characten 208 Gi'oi,(h;y ok Noriukrn Cai,ikirni\ Bull. 190

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volconic rocks of the Sierro Nevada, Colifor Geol. So . America Stirton, R. A., ond Goeriz, H. F., 1942, Fossil vertebrates from Super- Bull., V. 74, no. 4, p. 379-390. jacent deposits near Knights Ferry, Colifornio: California Univ., 447-472. 1964o, Cenozoic chronology of the Sierra Nevodo Colifornio: Dept. Geol. Sci. Bull., v. 26, no. 5, p. California Univ. Pub. Geol. Sci., v. 47, 41 p. P. Strand, R. G., 196 , Geologic mop of California, Olof Jenkins 1964b, Potossium-orgon dates of three Pleistocene interglociol edition, Mariposa sheet: Colifornio Div. Mines and Geology, scale basalt flows from the Sierra Nevada, California: Geol. So 1:250,000. (In press 1965) Bull., V. 753-758. 75, no. 8, p. Turner, H. W., 1894, Description of the Jackson quodrongle, California:

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