SUBMARINE GEOLOGY WITHIN THE WESTERN PART OF LAKE TAHOE, CALIFORNIA

Along the west shore of Lake Tahoe from Meeks Bay to McKinney Bay, subhorizontal strata are exposed on steep topographic features to depths of 90 m below lake surface. Observations are from submarine video images taken with the remotely operated submersible Triton, designed and built by students in the Robotic Systems Laboratory at Santa Clara University.

At Meeks Bay, bouldery, poorly sorted deposits at depths of 55-85 m form an elongate mound approximately perpendicular to the shore and are interpreted as the submarine continuation of a right-lateral Tioga moraine. These unstratified deposits overlie well-stratified sands and muds observed between 58 and 82 m depth. We estimate the bedding thicknesses range from 5-50 cm. The deposits appear orangish-brown (algae encrustation?) to gray under Triton's lighting. Similar stratified deposits were also observed offshore of Ehrman Mansion at depths of ~90 m.

On the north and south walls of a submarine canyon near General Creek, thin bluish layers (of mudstone?) are rhythmically interbedded with thicker, more resistant orangish layers (of resistant sandstone?) at depths of 46-76 m; small folds therein are interpreted as soft sediment deformation. At 66 m depth, a matrix-supported pebble-cobble deposit ~ 2 m thick occurs within the sequence.

An escarpment at the south end of McKinney Bay exposes a continuous, 40-m-thick sequence of stratified deposits. These are characterized by consistent thin bedding in the finer sediment (apparent mudstone) and medium to thin bedding in coarser sediment (apparent sandstone). A reflective white layer may be an interstratified tuff or diatomite.

Our observations suggest that in the Meeks-Sugar Pine-McKinney vicinity of west Lake Tahoe, at least 90 m of lacustrine deposits are preserved at shallow depths. This is consistent with published core data from the northwest Tahoe basin showing substantial thicknesses of clay, mud, and fine sand, with varying degrees of lithification. Collectively, these suggest that an extensive lacustrine system existed in the west Lake Tahoe area prior to the formation of what is now the modern depositional center. Age constraints are sparse, but we tentatively suggest a Late Pliocene to Late Pleistocene age for these west Lake Tahoe deposits based on correlation and stratigraphic relations in the Tahoe City area.