DANIEL I. AXELROD Dept. Geology, University of California, Los Angeles, Calif. Post-Pliocene Uplift of the Sierra Nevada, California Abstract: Reconstructed stream profiles and paleo- The postulate of a high Tertiary barrier, and of botanical evidence suggest that in the Yosemite only 4000 (Yosemite sector) to 2000 feet (Lake region altitude increased approximately 4000 feet Tahoe sector) uplift in the Pleistocene raises prob- following the Broad Valley stage. Since this was lems: the Tertiary section has defied erosion; fine- preceded by a 2500- to 3000-foot uplift of the Late grained sediments were deposited well up in the Pliocene Boreal ("Eocene") surface during the range and at its summit; formations were deposited Plio-Pleistocene transition, total post-Pliocene up- across the crest; Tertiary rivers and basalt flowed lift is about 6500-7000 feet. across the summit; faulting and warping of dated With similar methods, divergent results have sections and erosion surfaces show post-Pliocene been reported for the Lake Tahoe sector. The uplift of 4000-9000 feet; and the scarps have re- presence in the summit region of Miocene de- sisted erosion. These "problems" disappear if the ciduous hardwood forests typical of mild-tem- range originated as a major post-Pliocene topo- perate climate, the absence there of montane graphic barrier. conifers, the spatial relations of the Miocene forest Two basic weaknesses to the method of de- zones, and the regional pattern of climate they sug- termining Tertiary altitude from ancient stream gest all imply altitudes near 2000 feet. Recon- profiles are: (1) The assumption that certain well- structed stream profiles indicate an altitude of graded stretches of the ancient streams had pretilt 5000-7000 feet. These results are tested for plausi- slopes equal to those of well-graded parts of the bility by utilizing (1) sea-surface temperatures to modern rivers does not agree with the nature of calculate temperature and, hence, climate, at any the Tertiary sediments in the range. (2) Recurrent altitude postulated for the Sierran summit and (2) tectonism during the Tertiary tilted the basement sequences of later Tertiary floras on opposite sides and Tertiary river channels, giving them gradients of the barrier to determine the magnitude of the more nearly like those of present-day streams, and, rain shadow. Both lines of evidence suggest only hence, the illusion that the range has not been up- low altitude lifted appreciably since Eocene. CONTENTS Introduction 184 Formations deposited across summit area ... 192 Acknowledgments 184 Origin of scarps 192 Yosemite region 184 Displacement on scarps 193 Introductory remarks 184 Uplift from stream profiles 193 Erosion surfaces for determining uplift . .184 Introductory remarks 193 Ages of erosion surfaces 185 Graded rivers 193 Total uplift 186 Tertiary tectonism 195 Amount 186 Conclusion 195 Evidence in Mount Whitney region 186 References cited 196 Matthes' Sierran upwarp 187 Lake Tahoe region 187 Figure Introductory remarks 187 1. Topographic relations of erosion surfaces in the Paleobotanical evidence for low relief 188 Sierra Nevada 185 Fossil and related modern forests 188 2. Comparison of rainfall trend on opposite sides Altitudinal belts of vegetation 188 of the Sierran axis during later Tertiary . 191 Regional climatic pattern 189 Tests for plausibility of results 189 Table Marine paleotempcratures 189 1. Major Sierran erosion surfaces, their correlation No rain shadow, no high relief 191 and age 185 High Tertiary relief makes geologic problems . 192 2. Late Miocene sea-surface temperature and in- Introductory remarks 192 ferred climatic change with increasing alti- Preservation of Tertiary section 192 tude 190 Geological Society of America Bulletin, v. 73, p. 183-198, 2 figs., February 1962 183 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/183/3416884/i0016-7606-73-2-183.pdf by guest on 29 September 2021 184 D. I. AXELROD—POST-PLIOCENE UPLIFT OF THE SIERRA NEVADA paleotemperatures for the marine Miocene of INTRODUCTION central California. The amount of post-Pliocene uplift of the Sierra Nevada has been estimated recently by YOSEMITE REGION two different methods. Following Matthes' (1930) approach, Hudson (1960) recalculated Introductory Remarks the gradients of the Merced and American Hudson's estimate of 3930-feet uplift in the rivers during the Broad Valley stage. Com- Yosemite region following the Broad Valley parison with the gradients of the modern stage is only 70 feet less than that indicated by streams led him to conclude that after the fossil floras in the eastern part of the region Broad Valley stage the summit of the range (Axelrod and Ting, 1960a). His doubt as to the increased its altitude approximately 4000 feet value of paleobotanic and paleoclimatic evi- in the Yosemite region and about 2000 feet in dence as an indicator of ancient topography the Lake Tahoe area 135 miles north. Taken results from the fact that his conclusion does in conjunction with his earlier work dealing not agree with that reached in an early study with the gradients of Eocene streams (Hudson, (Axelrod, 1957a) which suggested that the 1955), he concluded that the Sierra Nevada Sierra Nevada probably was uplifted on the had an altitude of approximately 9000 feet in order of 6000-8000 feet since the Tertiary. The the Yosemite area and about 5000 feet in the discrepancy exists because Hudson used a dif- Tahoe sector, from the Eocene to the Pleisto- ferent erosion surface from that I used as a cene. He notes (Hudson, 1960, p. 1571) that datum plane from which to measure the dis- these conclusions differ substantially from those placement. based on paleobotanical evidence provided by Tertiary floras in the Sierra Nevada and ad- Erosion Surfaces for Determining Uplift jacent areas to the east and west (Axelrod, The amount of post-Pliocene uplift in the 1957a; Axelrod, 1957B). His estimates also dif- range can be determined from either of the two fer from those derived from more recent studies major erosion surfaces (Table 1). These surfaces of the late Pliocene (Axelrod and Ting, 1960a) are best preserved on the Kern Plateau south and early Pleistocene (Axelrod and Ting, 1961) of Mount Whitney (Fig. 1A), an area that was floras from the Sierra Nevada. largely nonglaciated. Lawson (1904) first de- The writer believes it appropriate to review scribed and named them the Subsummit the problem of the post-Pliocene, and also the plateau and High Valley surface. The Sub- Tertiary, altitude of the range, because, con- summit plateau is separated from the High trary to Hudson's statement (p. 1571), the Valley by 2500 feet of relief that represents conclusions reached from paleobotanical evi- approximately the amount of initial uplift in dence do not differ wholly from those he in- that region. The High Valley is trenched 3000 ferred. Since we are in agreement as to the feet by the canyon of the Kern River, which amount of post-Broad Valley uplift following reflects the second major period of uplift1. the Early Pleistocene in the Yosemite region Knopf (1918) added materially to our under- but do differ substantially in our estimates of standing of their geomorphic and structural re- uplift in the Lake Tahoe sector (Granite Chief, lations, and Matthes (1950b) renamed these Donner Summit), these areas are discussed ancient landscapes the Boreal (Subsummit) and separately. Chagoopa (High Valley). The Boreal and Chagoopa surfaces have been ACKNOWLEDGMENTS traced into the Yosemite region. Knopf (1918) Many of the new data summarized here followed them from the Kern Plateau north- come from current studies of the late Cenozoic microfossil floras of California and the Tertiary 1 This includes both the Mountain Valley and Canyon floras of Nevada, research projects that have stages of the Yosemite and San Joaquin areas. The been generously supported by the National Mountain Valley stage, which is also represented in the Kern River Canyon area, appears to be no more than an Science Foundation. initial cycle in the development of the Canyon stage. For their careful and thoughtful appraisal Its age is not precisely known, but it is approximately of the manuscript, I am indebted to Cordell mid-Pleistocene, not Pliocene as Matthes supposed. Durrell and William C. Putnam. Thanks are Work currently under way in the San Joaquin basin also due Clarence A. Hall, Jr., for the revised should provide a more precise date. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/2/183/3416884/i0016-7606-73-2-183.pdf by guest on 29 September 2021 YOSEMITE REGION 185 ward for 90 miles into the High Sierra near Bishop where they are also separated by 2500 feet. In the region on the windward side of the range, 20-30 miles west, Birrnan (1956, unpub. Ph.D. thesis, Univ. California, Los Angeles) recognized three similar geomorphic (unnamed) features: remnants of a high upland surface, a rolling middle plateau region 2500-3000 feet lower in altitude, and the canyon of the San Joaquin River that deeply trenches the latter. He notes that a few remnants of the high up- land surface are covered with basalt and that the intermediate plateau region is locally k 6 = 9 blanketed by a younger basalt. Closely similar -5 -d 8 8 relations are displayed in the area 30 miles u $3 8 .z 0 3 s 0 w north, in the upper San Joaquin River basin at =:. - S -2 0, zz ffi 28-5 2 Boreal Figure 1. Topographic relations of erosion sur- faces in the Sierra Nevada. (A) Kern Plateau region (after Matthes 1950b). (B) Yosemite-San Joaquin River region (adapted from Matthes, 1960) the southeast corner of Yosemite National Park (Erwin, 1934). In a posthumous paper, Matthes (1960) not only identifies two major upland surfaces in the San Joaquin River basin, he confidently correlates them with the two oldest cycles in the Yosemite region (Matthes, 1930)-the "Eocene" and Broad Valley (Fig.