GEOLOGY OF LAKE TAHOE, CA-NV, FROM SUBMERSIBLE ROV STUDIES

Lake Tahoe lies at the tectonically active boundary between the Sierra Nevada and Basin and Range provinces, coincides with numerous active faults, and records a dramatic history that may be typical of lakes in other active regions. Guided by 1998 USGS multibeam maps and sonar images, we have deployed the submersible remotely operated vehicle (ROV) Triton at Tahoe from 2004 to 2014. Over 70 hours of video images from over 100 dives (depths from 3-200 meters) cover most of the periphery of the lake. Our observations include lithology, sedimentology, structure, evidence of mass wasting, geomorphology, submerged moraines and terraces, boulder ridges, and active faults.

Findings from the dives have obvious hazard implications, and include: 1) shallow shelves and steep sidewalls around ~80% of the lake are made up of diatomaceous siltstone, sandstone, and clay of proto-Tahoe, a shallow precursor lake that existed from > 2 Ma to ~mid Pleistocene; 2) these nearly flat-lying, thin bedded lacustrine deposits are weakly jointed and crop out on shelves and along steep cliffs 10's-100's of m in height; 3) in places proto-Tahoe sediments are unconformably overlain by bouldery till and moraines of Tahoe and Tioga age; (4) the West Tahoe-Dollar Point fault zone (WTDPFZ), a zone of large, down-to-the-east, high-angle normal faults, displaces the proto-Tahoe section from the steep western sidewalls to the deepest parts of the lake; 5) repeated failure of the lacustrine section in the footwall of the WTDPFZ carved out large amphitheatres and produced major debris avalanches; 6) numerous, deep submarine canyons along the steep sidewalls have incised the proto-Tahoe sediments; 7) shallow shelves in places are capped by sets of elongate boulder ridges or megaripples; 8) at least two submerged terrace sets ring the lake; 9) published work indicates that 5 (above) led to megatsunami that produced features such as 6, 7, and 8; 10) numerous active faults and scarps of the Tahoe-Sierra frontal fault zone cut across the floor of the Emerald Bay fiord.

Although more data on the age and character of the proto-Tahoe deposits is needed to unravel lake history and to assess hazards, ROV work has revealed important tectonic processes and events at Lake Tahoe and underscores the importance of such studies in other lakes in tectonically active regions.