TOPOGRAPHY OF THE EOCENE SIERRA NEVADA: EVIDENCE FROM STABLE ISOTOPES OF KAOLINITE IN PALEO-STREAM CHANNELS

The links and feedbacks between topography, tectonics, and climate remains a poorly understood yet important problem in the field of earth sciences. Large mountains and high-elevation plateaus exert a strong control on global climate and it is, therefore, critical to understand their topographic history. One region that is of critical importance to global climate is the topography of western North America. Despite its importance to global climate change relatively little is know of the Cenozoic topographic development of this mountainous region. 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 taking place in the last 10 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. Our results show that the δD of kaolinite decreases by approximately 15 per mil from a high of -91 per mil in sediments that were deposited at Eocene sea level at the current base of the Sierra to -106 per mil 80 kilometers eastward on the flank of the Sierra Nevada. This isotopic difference suggests that our highest altitude samples, collected at ca. 1300 m current elevation, were deposited at Eocene elevations of 700 m to 1000 m. Thus, Eocene topographic gradients were half to three-quarters of today's, but still reflecting relatively mountainous topography, consistent with pebble- to cobble-sized clasts that dominate the Eocene fluvial deposits. Viewed in context of other isotopic studies (Poage and Chamberlain, 2002, Tectonics), we therefore suggest that that mountainous topography characterized the Sierra Nevada from the Eocene to the Recent. Nevertheless, our data also indicate uplift of the mid-elevations of the western Sierran slope of by a minimum 300 m since the Eocene.