EOCENE-OLIGOCENE PALEOVALLEY CHARACTERIZATION AND FLUVIAL SYSTEM EVOLUTION IN THE NORTHERN SIERRA NEVADA, CALIFORNIA

In the northern Sierra Nevada, Eocene-Oligocene fluvial sediments (the ‘auriferous gravels’) rest unconformably above well-defined basement-incised paleovalleys, providing a record of paleo-landscape geomorphology and drainage system evolution. Paleovalley surface mapping shows that bedrock structure and lithology controlled the location of broad, relatively lower-gradient paleovalley segments, which formed in metamorphic bedrock between the eastern and western plutonic belts. High-gradient, high-energy deposits are present near the edges of this area, suggesting that tributaries and outlets may have occupied steeper, narrow paleovalleys. Multiple strath terraces are common in the broad paleovalleys, indicating that incision events occurred both before and after paleovalley widening, followed by periods of aggradation. Sedimentology and stratigraphy of broad paleovalley deposits show predominantly braided stream facies deposited in an energetically variable system. Four upward-fining cycles, bounded by erosional surfaces, comprise an overall upward-fining succession, suggesting that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled an intermontane basin.

In the uppermost part of the fluvial succession, Oligocene volcanic ignimbrites overlie fine-grained deposits, and tuffaceous fluvial deposits are present, showing that aggradation continued in the drainage system from Eocene to Oligocene. These ignimbrites contain hydrated volcanic glass, which is used as a proxy for the stable isotope composition of meteoric water. Paleoaltimetry data from Sierra Nevada ignimbrites reflect the effect of a steep western slope on precipitation, indicating that the area had elevations similar to the present day range. This relatively steep topographic gradient is consistent with the pattern of paleovalley morphology and the localized, energetically variable braided fluvial deposits. Age and geochemical correlations to volcanic units in central Nevada indicate that source calderas were located >200 km to the east, likely in a region of high topography. The Sierra Nevada may have been the western edge of a high elevation plateau – the “Nevadaplano” that covered much of what is now Nevada and western Utah – similar to the Andean Altiplano.