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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. CENOZOIC VOLCANISM OF THE CENTRAL SIERRA NEVADA, CALIFORNIA 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 Calaveras River to Cosumnes on the Formation," but ranges in actual age dates (16.1 to Cosumnes River. 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 Sonora Pass area.