SUBMERGED BOULDER RIDGES, TAHOE CITY SHELF, LAKE TAHOE, CA

The Tahoe City shelf in the NW part of Lake Tahoe is a broad, 3 x 4 km, wave-cut platform that is shallower than 20 meters. It is underlain by a gently tilted section of Pliocene (?) thin-bedded, fine-grained, diatomite-bearing, moderately-lithified lakebeds, and locally by a thin capping of basaltic lapilli tuffs. The shelf is truncated on the south by part of the headwall re-entrant of the McKinney Bay landslide, from which ~8 km3 of lake beds and overlying glacial deposits failed and slid eastward to the 500-m-deep floor of the lake basin. The Dollar Point fault escarpment truncates the shelf on the east.

Multibeam sonar and Lidar maps reveal a remarkable array of east-trending ridges on the shelf. About 10 ridges are 1-4 m high, 30 to 150 m in width, and may be traced up to 1 km in length. Crests are narrow and sharp. The crest-to-crest distance between the ridges varies from 100 to 200 m, and shallow closed depressions up to 2m deep separate the ridges. Ridges on the south half of the shelf are aligned sub-parallel to the headwall scar of the McKinney Bay landslide. The maps also reveal an array of N to NNE-striking normal faults in a 1-km-wide zone south of Tahoe City, and NW-trending ridges in the eastern part of the shelf that appear to be a series of SW-dipping cuestas.

In May, 2005, the ROV Triton , deployed from the UC Davis research vessel, John Le Conte, recovered continuous video images of the shelf along 4 km of track line. The images show that the east-trending ridges are composed of dark, subrounded to subangular boulders resembling volcanic rocks, averaging about 20 cm in size and attaining 1-2 m in maximum length. The boulders are heaped into the ridges and little matrix occurs between them. In fact, Recent sand and mud are not common on the shelf. Lowlying areas between ridges expose outcrops of scoured lakebeds with sparse boulders.

The origin of the boulder ridges is in doubt; two possible hypotheses include glacial origin and tsunami origin. The boulders may have been deposited on the bench by piedmont glaciers or rafted by ice. Alternatively, the assembly of the boulders to produce the ridges may have been the result of vigorous waves generated from landslides and repeatedly washing across the shelf. Further work is in progress.

We thank the engineering students of SCU, who designed, constructed, deployed, and piloted Triton, and Brant Allen, captain of the LeConte. IEEE oceanic engineering society provided funding.