Paleobotanical Estimates of the Eocene Elevation of the Sierra Nevada

University Of Arizona, Tucson AZ

Investigators

Abstract

Much of what is now the North American Cordillera was near sea level in the Early Cretaceous. Knowing when uplift to its present elevation occurred is essential to understanding the large-scale tectonic processes that led to its formation. For the Sierra Nevada of California, it has been thought since the 1890s that geomorphic and other evidence required middle Miocene and later uplift of the range, despite cessation of major magmatic activity in the Late Cretaceous. New geophysical, geochemical, and paleoaltimetric data have cast doubt on this concept of late uplift. We propose new paleobotanical studies to determine the Eocene elevation of the Sierra Nevada, which will allow us to decide between early and late uplift models. Analyses of the physical morphology of modern leaf assemblages are able to produce reasonably accurate estimates of present-day climate and altitude (750 m). The relationship is especially well calibrated in North America. Leaf morphology studies have indicated that western Nevada was higher at 16 Ma (middle Miocene) than at the present, and this conclusion is supported by other lines of evidence. The history of elevation of the Sierra Nevada - western Nevada region prior to 16 Ma, however, is very uncertain. We propose to collect and analyze numerous Eocene leaf assemblages that occur in this region with ages from 45 - 50 Ma. This is the probable age of the lower part of the "auriferous gravels" of the Sierra Nevada, the focus of so much of the California gold rush. Paleobotanical work will focus first on fossil leaves known as the Chalk Bluffs flora found in the lower part of the auriferous gravels. These gravels are probably of early and/or middle Eocene age and were deposited by the ancestral Yuba River. Differences in inferred paleotemperature could result from global climatic change or from local climatic change due to altitudinal change. There are low-altitude leaf assemblages of the same age to the west that we will use as standards to control for regional climate change and estimate paleoaltitudes for the inland assemblages. The low altitude assemblages will also be compared to other low-altitude early to middle Eocene leaf assemblages in a range of 20 latitude along the Pacific Coast. Some of the localities we wish to sample are threatened by development; there is some urgency in making these collections and analyses before the opportunity to constrain the early Tertiary elevation of the north and central Sierra Nevada slips away.