Late Quaternary deposits beneath Clear Lake, California; physical stratigraphy, age, and paleogeographic implications

Clear Lake, California, lies in a volcano-tectonic depression that received nearly continuous lacustrine deposition for the past 500,000 years and probably longer. The lake has been shallow (<30 meters) and eutrophic throughout its history. Sediments beneath the floor of the lake are fine grained (chiefly >7.0 phi) and contain fossils of a large lacustrine biota, as well as a pollen record of land plants that lived in the basin. The sediments also contain tephra units of local and regional extent. The ages of the sediments in Clear Lake are determined from radiocarbon dates on the sediments, from correlation of regionally distributed tephra units, and from inferred correlation of oak-pollen spectra with the marine oxygen-isotope record. From the chronology of events recorded in the cores from Clear Lake, the late Quaternary history of the lake can be deciphered and the sediments correlated with other basins in northern California. Comparison of cores from Clear Lake with strata of the Kelseyville Formation, exposed south of the main basin, suggests a general northward migration of lacustrine sedimentation, which in turn suggests a northward tilt of the basin. Migration of the lake was a response to volcanism and tectonism. Volcanic rocks erupted from Mt. Konocti , on the southern margin of the lake, and displaced the shoreline to the west and north. Clear Lake is bounded by faults that are part of the San Andreas fault system. These faults strongly influenced the position, depth, and longevity of Clear Lake. Movement on these boundary faults deepened the Highlands and Oaks arms of the lake about 10 ka that have been inferred from the subbottom stratigraphy of the lake. Climate change, although responsible for large variation in the composition of the terrestrial flora of the Clear Lake drainage basin, has not influenced the areal extent, depth, or position of the lake. (See also W89-10137) (Author 's abstract)