Oligocene Provenance, Drainage Morphology, and Topography of the Nevadaplano

Debate surrounds the origin, uplift, and evolution of the northern Sierra Nevada and western Basin and Range. Many consider the Late Cretaceous to early Cenozoic Sierra to have formed the western edge of a high elevation plateau that covered much of what is now Nevada. The topography, extent, and drainage of this plateau remain uncertain. We integrate local paleovalley morphology and alluvial conglomerate stratigraphy with regional assessments of sediment and volcanic provenance and paleo-elevations across the proposed ancestral "Nevadaplano" to gain a better understanding of the early Cenozoic geomorphic evolution of the region, and to assess the possible tectonic and climatic drivers for that evolution. A sequence of Oligocene rhyolitic ignimbrites is preserved from eastern Nevada to the western foothills of the modern Sierra, commonly overlying a basal alluvial/fluvial coarse sand- to cobble-size conglomerate. The stratigraphy and provenance of these units, based on clast composition, detrital zircon and 40Ar/39Ar geochronology, and TREE geochemistry, demonstrates the presence of an active westward-draining fluvial system in the late Eocene to early Oligocene. Although dominated by Mesozoic batholith sources, detrital zircon populations from fluvial deposits in the northern Sierra contain 2-20% zircon grains with U-Pb ages from 42.7 to 33.0 Ma, which were likely sourced from Eocene volcanic rocks in northern Nevada, and 5-30% Precambrian zircon grains, reflecting derivation from basement metamorphic belts of the Sierra, as well as possible contributions from Nevada. Hydrated volcanic glass from overlying ignimbrites was used as a proxy for δD of Oligocene meteoric water, which reflects the effect of ancient topography on precipitation. δD decreases gradually from west to east across what is now northern Nevada, although at a significantly lower gradient than Oligocene δD across the Sierra. δD of volcanic glass across Nevada decreases from values ranging from -139% ± 3% to -146% ± 3% at locations that are now ~180 km east of the Great Valley, to values ranging from -153% ± 3% to -167% ± 3% at locations ~470 km to the east (near Carlin). This 7-28 ± 3% decrease in Oligocene δD occurs over less distance than a similar magnitude isotopic change in modern meteoric waters or in 2-5.5 Ma old hydrated volcanic glass, but this decrease does not necessitate orographic fractionation. These data show a significant reduction in the rate of increase of paleo-elevation with distance, in comparison with the region to the west (now the Sierra Nevada), and may reflect a gradual increase in mean elevation or partly closed hydrologic system behavior. Evidence of an active west-flowing fluvial system, providing a conduit for Late Eocene sediment and Oligocene ignimbrites, extending from what is now north-central Nevada to eastern California, supports a gradual increase in elevation from west to east as shown by the paleoaltimetry data. The region that is now northern Nevada was likely a gradually sloping high-elevation (~3-4 km) plateau in the Oligocene.