GEOLOGIC MAPPING, CHARACTERISTICS, AND MAP ANALYSIS OF EARLY- TO MID-CENOZOIC PALEOCHANNEL DEPOSITS IN THE SIERRA NEVADA WEST OF LAKE TAHOE

Paleogene sedimentary and volcanic deposits are present within the central and northern Sierra Nevada as paleochannel remnants which record and constrain Paleogene topography and Neogene to Quaternary deformation.

Within a study area west and northwest of Lake Tahoe, broadly defined by the path of the Ancestral South Yuba River as interpreted by Lindgren (1911), large (>1:24,000) scale geologic mapping of Paleogene deposits was conducted. This involved compilation of extant geologic mapping in GIS, and significant revision of compiled mapping based on fieldwork and high-resolution imagery and topographic data. ⁴⁰Ar/³⁹Ar tuff geochronology, tuff compositional data determined by XRF, and sparse subsurface data were also utilized to inform geologic mapping and characterization of paleochannel stratigraphy and paleovalley geometry. Field observations and lidar geomorphology were also used to map landslides, in order to reduce potential error in paleochannel deposit base elevations introduced by unrecognized landsliding.

The resulting geologic map and other data were used in conjunction with high-resolution elevation data to evaluate and constrain paleotopography and paleochannel paths. Several tilt scenarios were modelled to evaluate changes in paleotopography and paleochannel path viability under these tilt scenarios. The results were compared with previous paleochannel path interpretations.

Additionally, the revised geologic mapping of paleochannel deposits was used in conjunction with geomorphic profiles constructed from a lidar DEM to evaluate Neogene to Quaternary faulting within the study area and west of the Sierran Frontal Fault System. These results were compared with previous mapping of late-Cenozoic faults within the study area and were found to alternately support previously mapped faults, identify previously unmapped faults, and question or constrain other previous fault interpretations.