We have identified ash beds in sediment cores of Clear Lake, California, by the chemistry of their volcanic glasses and petrography. These identifications enable us to correlate between cores, and to correlate three ash beds to several localities outside the Clear Lake basin where they have been isotopically dated or their ages estimated by stratigraphically bracketing dates. The three dated ash beds are ash bed 1 (Olema ash bed), estimated to be between 55 and 75 ka, in two deep cores CL-80-1 and CL-73-4, and two ash beds in core CL-80-1, ash bed 6 (Loleta ash bed), estimated to be between 0.30 and 0.39 Ma, and ash bed 7, estimated to be about 0.4 Ma.

Available age control from extrapolation of radiocarbon ages downward in the two cores, age constraints from correlations of ash beds, and etching of mafic minerals in ash beds at depths below about 118 m in core CL-80-1 suggest the following depositional histories for the two cores: in core CL-73-4, sedimentation appears to have been rapid (about 1 mm/yr) and continuous from about 120 ka to the present, corresponding to a depth interval from about 115 m to the present lake bottom. In the deeper core CL-80-1, sedimentation took place at a relatively moderate rate (0.4 mm/yr) from about 460 ka until sometime between about 300 and 140 ka, corresponding to a depth interval from about 168 to 118 m. Slow deposition or erosion took place sometime during the interval from about 300 to 140 ka, corresponding to an inferred hiatus at a depth of about 118 m. From 140 ka to the present, rapid sedimentation took place at about the same rate (about 0.8 mm/yr) as in core CL-73-4, corresponding to a depth interval from about 118 m to the present lake bottom.

The age of sediments in Clear Lake is not well constrained within the depth interval of about 70 to 130 m in the two deep cores, and the duration of the putative hiatus at about 118 m in the core CL-80-1 may be shorter than we propose. The presence of a hiatus at about 118 m depth in this core, however, is suggested by etching of mafic minerals in tephra layers below this level but not above, indicating that a period of subaerial exposure, or exposure above the groundwater table, had occurred for sediments below this level.