Abstract
This study evaluates the characteristics, deformation, and tilt-dependent flow paths of the Ancestral South Yuba River in the Sierra Nevada west of Lake Tahoe. Previous studies have produced several differing interpretations of paleochannel paths, and some interpreted paths assume and require late-Cenozoic tilting of the range. Paleochannel deposits of the Tertiary gravels and Valley Springs Formation represent a probably greater than 30 m.y. timespan, and their relative characteristics, distribution, and implications for possible drainage network reconfiguration are not fully understood. Paleochannel deposits represent an important strain marker and can be used to assess late-Cenozoic deformation within the range (west of the Sierra Nevada frontal fault system).
For this study, geologic mapping was compiled and substantially revised based on fieldwork and high-resolution imagery and topographic data. Geologic mapping and field investigations were used to improve characterization of the paleochannel, including cross sectional geometry, thickness, stratigraphy, and paleovalley morphology. Oligocene deposit base elevation data was generated from revised geologic mapping and used in conjunction with constraining bedrock topography, tuff stratigraphy, and other information to revise mapping of the Oligocene paleochannel path. Tilt corrections were applied to elevation data using parameters from eight different tilt scenarios to assess changes in viability of flow paths in each scenario. Geologic mapping and geomorphic anomaly mapping were used to assess known Cenozoic faults and to identify possible unmapped Cenozoic faults west of the frontal fault system.
The paleochannel segment between Soda Springs and the west end of Ralston Ridge is found to host an especially thick (>300 m) section of Oligocene tuffs and minor interbedded conglomerates which record 6.8 m.y. of alternating cycles of pyroclastic flow deposition, fluvial transport, and incision. Paleochannel geometry across this segment demonstrates paleovalleys much wider than deep, with generally broad flat valley floors and some inset terraces. Paleovalley cross-sectional geometries and base elevations are found to differ significantly from those of adjacent modern canyons.
Oligocene paleochannel paths interpreted for this study generally agree with Tertiary channel path interpretations from Lindgren (1911) in many areas, with some divergence representing different or additional paths and/or presence of unrecognized faulting. However, northward flow from Foresthill towards Scotts Flat Reservoir, in the Oligocene, is not supported by modern or tilt-corrected deposit base elevation data and constraining bedrock topography.
Several previously mapped Cenozoic faults are supported by new data, while some are questioned or constrained, and additional possible and probable unmapped Cenozoic faults and geomorphic anomalies of ambiguous origin were identified.