LITHOLOGY AND VERTEBRATE FOSSILS OF THE PETALUMA FORMATION, SONOMA COUNTY, CALIFORNIA

The late Miocene – Pliocene Petaluma Formation (PF) crops out in a large area of Sonoma County, California. PF vertebrate fossil collections at the University of California Museum of Paleontology along with our findings from the formation were examined, localities were plotted on maps and field checked with respect to lithology.

The easternmost portion of the formation crops out approximately along a northwesterly-trending line from the south at Sears Point, along western Sonoma Valley, through NW Glen Ellen (fault contact with the Glen Ellen Fm. (GEF)), from Bennett Valley to the northern terminus at NW Santa Rosa (Weaver, 1949). The later three areas erroneously mapped as Huichica Fm. (HF) by Fox (1983). The westernmost portion of the formation interfingers with the Wilson Grove Fm. and crops out approximately along a northwesterly-trending line from the south along Lakeville Hwy., through the towns of Stony Point, Sebastopol and to the north at Trenton.

The PF differs from the GEF and HF by unique lithologies, source rocks, age to some extent and is separated from the two by wide valleys and/or faults. The PF consists of three distinct lithologic members that are, from oldest to youngest, lower, middle and upper PF. The lower PF overlies approximately 9.28 Ma volcanic rocks (Youngman, 1989), contains an ~8.5 Ma flow (Fox et al., 1985a) and is composed dominantly of shale (Morse and Bailey, 1935; Allen, 2003). The middle PF is composed of siltstone to coarse sandstone and conglomerate derived from the Franciscan Complex. The upper PF consists of siltstone to coarse sandstone and conglomerate derived dominantly from the Monterey Group (Allen, 2003).

Areas mapped as lower and middle PF in the greater Petaluma Oil Field and Lakeville areas contain these Hemphillian fossils: Neohipparion gidleyi, Equidae (Stirton, 1939; 1952), Leporidae, Camelidae, Rhinocerotidae and Testudines. Neohipparion gidleyi and Camelidae were recovered from the lower PF at the Stony Point Rock Quarry (Allen, 2003). Upper PF in the Sears Point and Bennett Valley areas contain these Blancan fossils: Equus. These fossils indicate ages for the lower, middle and upper PF lithologies described above. Correlations between these unique fossiliferous lithologic horizons to similar lithologic horizons devoid of fossils in the PF can be made.