Discrimination of Seismic vs. Hydrologic Triggers for Holocene Turbidites from Lake Tahoe, California and Nevada

Osleger, David A.¹, Alan Heyvaert¹, Joseph S. Stoner², Kenneth L. Verosub¹ (1) UC Davis, Davis, CA (2) Oregon State Univ, Corvallis, OR

Turbidite paleoseismology is increasingly being recognized as a useful tool for estimating recurrence intervals of earthquakes in tectonically active regions. However, discriminating between turbidites deposited by seismic-induced failure versus those deposited by hyperpycnal flows driven by high-volume runoff from rivers is a difficult and somewhat arbitrary process. To assess the Holocene record of climate and seismicity in the Lake Tahoe basin, a variety of parameters were measured at high-resolution sampling intervals from two cores from the deep floor of Lake Tahoe, including particle-size analysis, magnetic susceptibility, total organic carbon, C/N ratio, and δ13C of the organic carbon component. AMS radiocarbon dates and a fingerprinted ash provide age control for calculating accumulation rates and timing of events. Magnetic and geochemical proxies indicate a significant terrigenous source for the majority of Holocene turbidites in Tahoe, suggesting that they were generated by extreme hydrologic events in the watershed.

Large-magnitude storms appear to have affected the Tahoe watershed between ~5600 and 3000 cal yr BP and correlate with regional data indicative of wetter climates in the Sierra Nevada and western Great Basin. Conversely, a turbidite-free interval of fine-grained sediment deposited between ~2000 to 800 cal yr BP appears to be related to a prolonged episode of drier climate, a corresponding lack of major storm events, and reduced sediment input into the lake. The intensity of storms and associated runoff events over the recent past appears to be much less severe than extreme storms that occurred earlier in the mid-Holocene.