LAKE TAHOE SUBMARINE GEOLOGY: IMPLICATIONS FOR ANCESTRAL LAKE, LANDSLIDES, AND FAULTS

Submarine observations in 2004 and 2005 using the submersible ROV Triton deployed by the UC Davis research vessel John LeConte have led to a number of important findings. A Pliocene and older lacustrine section, locally over 200m thick, and part of an ancestral lake, underlies a ~20 x 8 km area including most of the nearshore parts of Lake Tahoe west of the West Tahoe-Dollar Point fault. Lithologic units include quiet-water muds and clays, diatomaceous sands, and sparse turbidites. These strata crop out in cliffs along the headwall scar of the McKinney Bay area and appear to have failed in the McKinney Bay landslides because they are a close lithologic match with dredge hauls from the two largest blocks of the megalandslide, taken at lat. 39⁰06.5'N and long. 119⁰06.5'W, and at lat. 39⁰05.3'N and long. 120⁰01.3'W. Around the headwall scar of McKinney Bay, ongoing mass wasting of the lacustrine strata is indicated by arcuate cliffs with detached, tilted blocks and rubble piles at bases of cliffs.

Cliff exposures of lacustrine sediments near Homewood and at Blackwood Canyon verify the importance of displacements on onshore east-side-down normal faults in the Homewood area. Observations in these areas also show a complete absence of glacial debris near the mouths of glacial canyons, confirming the removal of glacial moraines during major collapse and tsunami events. Several meters of glacial till resting on lacustrine deposits has been imaged on shallow submerged shelves near and north of Sugar Pine Point, where tsunamis beveled the land surface.

Exposures of the lacustrine section in cliffs along the escarpment of the West Tahoe-Dollar Point fault zone indicate that most displacement along this fault zone is post-Pliocene and that the pre-Tahoe lacustrine section must underlie some of the deeper parts of Lake Tahoe east of these faults.

We gratefully acknowledge the IEEE Oceanic Engineering Society for partial funding, and the students of SCU for designing, building, deploying, maintaining, and piloting the Triton. We also thank Brant Allen, captain of the LeConte, for his support.