Recent geologic mapping in the Clear Lake basin shows that the nonmarine Cache Formation can be subdivided into several members. New names are herein informally used for three distinct deposits called the B, L, and K members. The three members are geographically separate, except where the L member locally unconformably overlies the B member. This study of the Cache Formation provides a framework for understanding the late Cenozoic tectonic, geologic, and climatic histories of the Clear Lake, California, area.
The B member is characterized by cross-stratified sandstone with interbedded reddish-brown conglomerate around the margins of the basin, and by gray sandstone and persistent, light-olive-gray conglomerate lower in the section, as it is exposed in the center of the basin. Minor amounts of basaltic tuff are present near the top of the B member in the Clear Lake basin. The B member has a maximum thickness of at least 1,500 m and represents dominantly fluvial and minor lacustrine deposition. This Blancan (Pliocene and early Pleistocene(?)) unit was deposited in a fault-controlled, subsiding basin. The L member is a heterogeneous deposit of pebble conglomerate, sandstone, siltstone, calcareous siltstone, limestone, tuff, and diatomite. This lacustrine unit has a maximum thickness of at least 130 m, and is mid-Pleistocene in age. The K member is composed of lacustrine sandstone and siltstone, and locally interbedded fluvial pebble conglomerate and sandstone. Several tuffs, including the tuff of Adobe Creek, an informally named marker bed, occur in the K member. The K member has a maximum thickness of at least 140 m and is mid-Pleistocene in age. The presence of fossil leaves and cones that represent growth during warm and cold climates indicate that the K member was deposited during both interglacial and glacial climatic conditions. The tuff of Adobe Creek of the K member has been recognized in water wells west and north of surface exposures of the K member.
The B member is probably correlative with the Tehama Formation and is apparently unrelated to the sediment of Clear Lake. The L and K members intertongue with and are underlain by flows of the Clear Lake Volcanics. The L and K members are dominated by partly coeval lacustrine deposits that were possibly deposited in different subbasins of the same lake, and are possibly correlative with sediment concealed beneath Clear Lake.
