HR: 1340h
AN: T33A-1141    [Abstracts]
TI: The Eocene to Oligocene Landscape of the Northern Sierra Nevada
AU: * Cassel, E J
EM: ecassel@stanford.edu
AF: Stanford University, GES Dept., Stanford, CA 94305, United States
AU: Graham, S A
EM: sagraham@stanford.edu
AF: Stanford University, GES Dept., Stanford, CA 94305, United States
AB: To gain a better understanding of the Cenozoic tectonic and landscape evolution of the northern Sierra Nevada, well-preserved Eocene to Oligocene sedimentary and volcanic units form the focus of a detailed stratigraphic study which incorporates geochemical and stable isotopic analyses. Widespread silicic ash-flow tuffs (31-28 Ma) crop out across the northern Sierra from near paleo-sea level at the eastern edge of the Great Valley across the modern crest of the range into Nevada. On the western flank of the northern Sierra, they cap Eocene prevolcanic fluvial sediments of the ancestral Yuba and Feather Rivers. The Eocene fluvial system was dominantly controlled by bedrock structure, consisting of two types of coeval valley morphologies: steep, narrow high-energy valley segments and broader, lower-gradient braided stream valley segments. The braided fluvial sequence contains four upward-fining cycles: coarse lower intervals consist of gravel-sand dunes and lateral accretion elements deposited within higher energy channels; upper fine intervals consist of 1-5 m thick lignite-bearing clay and silt marsh deposits. Full-valley width exposures of clay eliminate the possibility of autocyclic controls, indicating that the multiple upward-fining cycles reflect base-level change. Two distinctive overlying ash-flow tuffs were identified and correlated by trace and rare earth element composition of volcanic glass and lithologic criteria. Hydrated glass was used as a proxy for hydrogen isotopic composition of precipitation to determine the paleoelevation gradient in the Oligocene. The δD of ancient meteoric waters, which scales at a predictable rate with change in elevation, decreases steadily across a range-perpendicular transect, from -125‰ ± 1‰ in the west to -160‰ ± 4‰ in the east. This 35‰ decrease in the δD of precipitation is similar to the compositional gradient of the range today, and reflects an increase in ancient mean elevation along the transect. These results suggest that the northern Sierra Nevada stood as an area of high topography in the early Oligocene, similar to the modern range. This relatively steep topographic gradient is consistent with the landscape reflected in the stratigraphic sequence: steep high-energy fluvial channels emptying into structurally-controlled broad braided stream valleys.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 8169 Sedimentary basin processes
DE: 8175 Tectonics and landscape evolution
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2007 Fall Meeting