Small mollusk faunas characteristic of the uppermost part of the Wilson Grove Formation at Wilson Grove and along River Road at Trenton (Pliocene) were encountered in Sonoma County Water Agency pilot wells at Occidental Road well field between 320-500 ft (98-152 m), depth, and in the Sebastopol Road pilot well field between 560-570 ft (171-174 m), depth. These mollusks support correlations between the two wells made on lithologic grounds.
A benthic foraminifer was recovered from between 380-390 ft (116-119 m), depth, in the Sonoma County Water Agency Occidental Road pilot well. Though an isolated specimen, the presence of this well-preserved foraminifer supports the environmental interpretation of less than 100 m on the continental shelf indicated by the molluscan assemblages at this site.
For this marine stratigraphic interval of the Wilson Grove Formation, we suggest a relatively narrow age range of 5.3 (Miocene-Pliocene boundary) to ~ 4.5 Ma based on the stratigraphic relations of correlative marine strata around the upland margin of Santa Rosa plain and correlative strata in the Santa Cruz area, although an age between 5.3 and ~ 2.8 Ma cannot be discounted.
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Biostratigraphic and lithologic correlations of two Sonoma County Water Agency pilot wells with the type Wilson Grove Formation, Sonoma County, central California