The Wilson Grove Formation is exposed from Petaluma north to northern Santa Rosa, and from Bennett Valley west to Bodega Bay. A fauna of at least 107 invertebrate taxa consisting of two brachiopods, 95 mollusks (48 bivalves and 46 gastropods), at least eight arthropods, and at least two echinoids have been collected, ranging in age from late Miocene to late Pliocene. Rocks and fossils from the southwest part of the outcrop area, along the Estero de San Antonio, were deposited in a deep-water marine environment. At Meacham Hill, near the Stony Point Rock Quarry, and along the northern margin of the outcrop area at River Road and Wilson Grove, the Wilson Grove Formation was deposited in shallow marine to continental environments. At Meacham Hill, these shallow water deposits represent a brackish bay to continental environment, whereas at River Road and Wilson Grove, fossils suggest normal, euhaline (normal marine salinity) conditions.
A few taxa from the River Road area suggest water temperatures slightly warmer than along the adjacent coast today because their modern ranges do not extend as far north in latitude as River Road. In addition, fossil collections from along River Road contain the bivalve mollusks Macoma addicotti (Nikas) and Nuttallia jamesii Roth and Naidu, both of which are restricted to the late Pliocene. The late Miocene Roblar tuff of Sarna-Wojcicki (1992) also crops out northeast of the River Road area and underlies the late Pliocene section at Wilson Grove by almost 300 m. Outcrops in the central part of the region are older than those to the northeast, and presumably younger than deposits to the southwest. The Roblar tuff of Sarna-Wojcicki (1992) occurs at Steinbeck Ranch in the central portion of the outcrop area. At Spring Hill, also in the central part of the outcrop area, the sanddollar Scutellaster sp., cf. S. oregonensis (Clark) has been recently collected. This species, questionably identified here, is restricted to the late Miocene from central California through Oregon. Outcrops at Salmon Creek, northeast of Steinbeck Ranch and also in the central part of the outcrop area, contain Aulacofusus? recurva (Gabb) and Turcica brevis Stewart, which are both restricted to the Pliocene, as well as Lirabuccinum portolaensis (Arnold) known from the early Pliocene of central and northern California and into the late Pliocene in southern California. These data suggest an overall pattern of older rocks and deeper water to the south and west, and younger rocks and shallower water to the east and north. Outcrops to the southwest, south of the Bloomfield fault, are not well dated but presumably are older than the late Miocene Roblar tuff of Sarna-Wojcicki (1992). Fossils in this part of the section are rare and are not useful in determining a precise age or environment of deposition for the lower part of the Wilson Grove Formation. However, sedimentary sequences and structures in the rocks here are useful and suggest probable outer shelf and slope water depths. Lituyapecten turneri (Arnold) which occurs in this part of the section has previously been restricted to the Pliocene, but its occurrence below the Roblar tuff of Sarna-Wojcicki (1992) indicates a revised late Miocene age for this taxon.
Three possibly new gastropods (Mollusca) are reported here: Calyptraea (Trochita) n. sp. and Nucella sp., aff. N. lamellosa (Gmelin), both from the Bloomfield Quarry area, and Acanthinucella? n. sp. from the River Road area. These species are not described here because this venue is deemed insufficient for the description of new taxa.
Additional publication details
USGS Numbered Series
Invertebrate Paleontology of the Wilson Grove Formation (Late Miocene to Late Pliocene), Sonoma and Marin Counties, California, with some Observations on Its Stratigraphy, Thickness, and Structure