SEISMICALLY-INDUCED LANDSLIDES, DEBRIS FLOWS, AND TURBIDITES IN LAKE TAHOE

Holocene sedimentation in the Lake Tahoe basin is dramatically affected by large, episodic landslides which line the basin walls and redistribute lacustrine sediments throughout the basin via debris flows and turbidity currents. Debris flows and turbidites are identified, correlated, and dated in a suite of ~3-m piston cores and ~1-m gravity cores. Debris flows consist of reworked lacustrine sediment including tilted blocks of previously flat-lying sediment, clay and silt balls, and rip-up clasts. A large turbidite immediately overlies the upper contact of the debris flow deposits suggesting close association of landslides and turbidites. Turbidites grade upward from sand to clayey silt, are capped by a thin diatom layer, and account for greater than 50% of the Holocene sedimentation. Turbidites are correlated based on lithologies, visual characteristics, magnetic susceptibility signatures, and stratigraphic position. Several lines of evidence suggest a seismic origin for the turbidites. The turbidites have greatest thicknesses and the coarsest grain size in proximity to landslides rather than stream inputs, and have multiple source areas, suggesting landsliding at multiple locations at similar times. Turbidites contain highly fractured diatoms, indicative of transport as opposed to a diatom bloom. Lacustrine sediment is reworked in the debris flows, and radiocarbon dates within turbidites are anomalously old as a result of the reworking, while non-turbidite radiocarbon dates provide a consistent age-depth curve. The dating of one core, when combined with the turbidite correlations, suggests that landslides, lacustrine debris flows, and turbidity currents have redistributed sediment in the lake on the order of every ~1000 years. Strong earthquake shaking on Tahoe basin faults and the Genoa Fault to the east likely caused the massive landsliding and major lake disruption throughout the Holocene.