A REMARKABLE FOSSIL FAUNULE FROM THE BASAL WILSON GROVE FORMATION (LATE MIOCENE), SONOMA COUNTY, CALIFORNIA

A small quarry north of the town of Bloomfield in Sonoma County, California has yielded a remarkable fauna of invertebrate and vertebrate fossils from the basal Wilson Grove Formation of late Miocene age (“Jacalitos” California provincial molluscan stage/Hemphillian North American land mammal age/upper Tortonian to lower Messinian of the International Chronostratigraphic Chart). Seventy-seven taxa have been recovered including 4 brachiopods, 42 mollusks (28 bivalves, 14 gastropods), 6 arthropods (1 crab, 1 shrimp, 4 barnacles), and 25 vertebrates (3 sharks, 1 ray, 8 bony fishes, 9 marine mammals, and 4 birds). Unusual in the fauna are the abundant and diverse brachiopod fauna, the diverse barnacle assemblage, and the wide variety of vertebrates, including four taxa of birds and the most diverse assemblage of walrus (4) from a single locality in the world.

Paleoenvironmental interpretation of the fauna indicates that it was deposited in a shallow subtidal to rocky intertidal environment at temperatures similar to the intertidal zone off the adjacent coast today. These interpretations are based on 1) the taxa present and their paleoecologic interpretations, and 2) the outcrops sitting on a slightly irregular contact with detached rocks from underlying Franciscan Complex basement. In addition, we determined the fauna has undergone little transport because of the large number of brachiopod valves that would quickly degrade during transport.

This site is well dated using: 1) argon-40/argon-39 age determination from an underlying stratigraphic horizon, 2) a single strontium analysis from a pectinid (Mollusca: Bivalvia) from the deposit, 3) volcanic tuff correlations of the overlying Roblar tuff of Sarna-Wojcicki (1992), 4) revised dating of the Roblar tuff, and 5) biostratigraphy of mollusks and vertebrates. These data indicate an age range less than 9.27±0.06 Ma and more than 6.203±0.011 Ma.