Unit is named the Wilson Grove Formation for rocks previously assigned to the Merced(?) Formation in the Sebastopol block in Sonoma Co., CA. Consists of unconsolidated, fine-grained, massive sand and minor amounts of gravel and tuff deposited under beach and shallow-marine conditions. A conspicuous but discontinuous interbed of pumice lapilli tuff 15 to 75 m above the base of the Wilson Grove was erupted from the Sonoma volcanic center and probably correlates with a tuff in the Petaluma Formation. Local deposits of sand and gravel of alluvial origin overlying the marine beds and previously included with them in the Merced(?) Formation are here treated as a separate unit, the informal sand and gravel of Cotati. Overlies a surface of low to moderate relief beveled across rocks of the Franciscan Complex and Great Valley sequence; contact with the Petaluma Formation to the east is concealed by younger alluvial deposits. Correlates with the lower Merced of Glen (1959) at the Merced Formation type section. Thickness attains 150 m. Age is late Miocene and Pliocene and spans the revised boundary between the Miocene and Pliocene, based on radiometric (K-Ar) ages of 6.1 and 5.7 Ma on tuff in the Wilson Grove and Pliocene fossils [mollusks]. (B1565)

Source: GNU records (USGS DDS-6; Menlo GNULEX).