PALEOALTIMETRY OF THE NORTHERN SIERRA NEVADA IN THE OLIGOCENE USING HYDROGEN ISOTOPES IN HYDRATED VOLCANIC GLASS

The paleoelevation gradient of the northern Sierra Nevada in the Oligocene was determined using hydration waters of volcanic glass sampled from widespread silicic ash-flow tuffs as a proxy for hydrogen isotopic composition of precipitation. Lower Oligocene rhyolitic ash-flow tuffs (~31-28 Ma) were sampled along an 85 km range-perpendicular transect at roughly the latitude of the Middle Fork of the Yuba River, from the western foothills of the range to the California-Nevada border (modern sample elevations: 856-1924 m). The Oligocene rhyolitic ash-flow tuffs overlie paleovalley-filling Eocene conglomerate and sandstone, referred to as “Auriferous Gravel,” but are more widespread, with well-preserved deposits cropping out up the western flank and across the modern crest of the range into Nevada, providing an isotopically-derived paleoaltimetric profile of the range within an as yet poorly understood time interval. Individual tuffs were correlated based on phenocryst assemblage, texture, pattern and degree of welding, and geochemical analysis. Pristine volcanic glass separated from tuff samples taken along the transect was used to determine the change in hydrogen isotopic composition of ancient meteoric waters, which scales at a predictable rate with change in elevation. Volcanic glass hydrates within ~5,000 years of deposition, and does not exchange further once saturated, thus recording the isotopic composition of hydration waters at the time of deposition. δD of volcanic glass decreases at a steady rate from -125‰ +/- 1‰ at the most westerly locations to -160‰ +/- 4‰ at the location furthest to the east. This 35‰ decrease in the hydrogen isotopic composition of precipitation is similar to the compositional gradient of the range today, and is interpreted to reflect an increase in mean elevation along the transect in the Oligocene. These results suggest that (1) contrary to many previous studies, the northern Sierra Nevada stood as an area of high topography in the early Oligocene, comparable to or higher than the modern range; and (2) combined with sedimentologic data from underlying Eocene conglomerate and other stable isotope paleoaltimetry studies from the Eocene and the Upper Miocene, the range was likely a significant topographic feature throughout the Cenozoic.