PLEISTOCENE LACUSTRINE SEDIMENTS IN THE LAKE TAHOE BASIN, CALIFORNIA

Geotechnical drilling onshore at the outlet of Lake Tahoe has revealed the existence of a lacustrine/fluviatile sedimentary sequence consisting of an upper clay layer (0-7 meters), a sand layer (7 to 12 meters), a middle clay layer (12 to 17 meters), a gravel layer (17 to 24 meters) and a lower clay layer (24 to at least 64 meters). At least eight tephra layers are present in the sequence. In the upper 10-30 meters are three tephras which can be correlated to the either the Bishop tephra or the closely-related Glass Mountain ash beds (0.75-1.0 Ma). Two additional tephras below this group correlate with slightly older Glass Mountain ash beds (0.8-1.2 Ma). At 37 meters is a tephra that correlates with one dated at 1.35 Ma in the Tule Lake core. Near the base of the sampled sequence is another tephra which correlates with one dated at 2.01 Ma in the Tule Lake core as well as a mixed population of shards including some that match with the Huckleberry Ridge ash bed from the Yellowstone area, dated at 2.09 Ma. Paleomagnetic samples collected from the clay portions of the sequence are generally well-behaved during alternating field demagnetization. For the most part, the samples are of reversed polarity, except for an interval of normal polarity between 95 and 105 meters. The simplest interpretation of the tephra and paleomagnetic data is that the sequence spans the interval from about 750,000 years to 2,000,000 million years with the interval between 95 and 105 meters representing the Jaramillo subchron These results provide strong evidence for the presence of early and middle Pleistocene sedimentary sequences around the shores of Lake Tahoe. The sequences were probably deposited in an enlarged proto-Lake Tahoe, created when the outlet of the lake was dammed by either ice, landslides or lava flows.