HR: 09:15h
AN: T31E-06    [Abstracts]
TI: Eocene Topography of the Northern Sierra Nevada: Direct Paleoelevation Evidence from Hydrogen Isotopes in Kaolinite of Paleostream Channels
AU: * Mulch, A
EM: mulch@pangea.stanford.edu
AF: Stanford University, Earth and Environmental Sciences 450 Serra Mall, Bldg.320, Braun Hall, Stanford, CA 94035 United States
AU: Graham, S A
EM: graham@pangea.Stanford.EDU
AF: Stanford University, Earth and Environmental Sciences 450 Serra Mall, Bldg.320, Braun Hall, Stanford, CA 94035 United States
AU: Chamberlain, C P
EM: chamb@pangea.stanford.edu
AF: Stanford University, Earth and Environmental Sciences 450 Serra Mall, Bldg.320, Braun Hall, Stanford, CA 94035 United States
AB: The links and feedbacks among topography, tectonics, and climate remain a poorly understood yet important problem in Earth Sciences. Large mountains and high-elevation plateaux exert a strong control on global climate and it is, therefore, critical to understand their topographic history. Despite its importance to global climate change relatively little is known of the Cenozoic topographic development of the western North America. For example, there is considerable debate as to when the Sierra Nevada developed as a mountain range, with one view that the bulk of elevation gain took place in the last 3-5 Ma and the other that it already existed as a major topographic feature throughout much of the Cenozoic. To address this debate we examined the hydrogen isotope composition of kaolinite from weathered Eocene fluvial sediments. These sediments, well known because of past gold mining, occur within Eocene river channels cut into the western flank of the northern Sierra Nevada and are found from paleo-sea level upstream into the modern range. Our results show that the deltaD of kaolinite along paleoslopes decreases systematically by up to 25 per mil within different paleodrainage systems from a high of -80 per mil in sediments deposited at the current base of the Sierra to -106 per mil about 60 km eastward on the flank of the Sierra Nevada. The observed isotopic difference between downstream and upstream samples suggests that the highest altitude samples, collected at ca. 1600 m current elevation, were deposited at Eocene elevations of 1100 m to 1300 m. Thus, Eocene topographic gradients may have been lower than todays, but still reflect mountainous topography, consistent with pebble- to cobble-sized clasts that dominate the Eocene fluvial deposits. Viewed in context of other isotopic and geomorphic studies, we therefore suggest that mountainous topography characterized the Eocene northern Sierra Nevada whose western flank was occupied by high discharge river systems draining a range with elevations up to 2000 m above sea level. Nevertheless, our data also indicate uplift of the mid- and high elevations of the western Sierran slope of by a minimum 300-500 m since the Eocene.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 8175 Tectonics and landscape evolution
DE: 8177 Tectonics and climatic interactions
SC: Tectonophysics [T]
MN: Fall Meeting 2005