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Sedimentology and Stratigraphy of the Cenozoic Sequence in the Northern Sierra Nevada: Implications for Channel Morphology and Topographic Development

Elizabeth Cassel

Stanford University, Geological and Environmental Sciences Department

Stanford, CA, USA

[email protected]

Along the western slopes of the northern Sierra Nevada, Eocene auriferous fluvial deposits and Oligocene-Pleistocene volcanics form a complex record of river incision and deposition reflecting the evolving paleo-landscape. A detailed analysis of sedimentary structures, facies, channel patterns, and system evolution is in progress. The Eocene fluvial sequence consists of two units, marking a major change in the depositional system. Lower unit conglomerates fill a narrow bedrock channel and clasts display a bi-modal size distribution: larger sub-rounded clasts (averaging 95 centimeters diameter) were likely landslide blocks which only moved in traction during peak flows, surrounded by small well-rounded clasts (averaging 16 centimeters). Upper unit conglomerates are composed of clasts less than 22 centimeters diameter, interbedded with sand and clay. The upper unit defines a broader flood plain with lower flow capacity and beds exhibit characteristics of braided streams. The overlying volcanic sequence consists of rhyolite ash fall tuffs and reworked ash and sand beds, overlain by andesite pyroclastic density current deposits and mudflow breccias.

Future work for this project consists of improving the resolution and correlation of this sequence across drainages, including Ar/Ar dating of volcanic units and stable isotope paleoaltimetry transects up paleo-drainages, to provide an independent measure of mean surface elevations at the time of deposition. This record of paleo-elevation and landscape development will improve understanding of the uplift history of the northern Sierra Nevada, the tectonic and climatic dynamics behind surface movement, and the responses of the fluvial depositional system to tectonism and episodic volcanism.

AAPG Search and Discovery Article #90060©2006 AAPG Foundation Grants-in-Aid