NEW PALEONTOLOGICAL EVIDENCE SUPPORTING THE NEOGENE TRANSITION FROM MARINE TO NON-MARINE CONDITIONS IN MARIN AND SONOMA COUNTIES, CALIFORNIA, USA

Micro- and macro-fossils from ongoing mapping, coring, and museum collections are used here to help refine the stratigraphic, paleoenvironmental, and paleogeographic framework of late Neogene rocks in Sonoma and Marin counties. During the late Miocene to late Pliocene, marine conditions existed in the western part of the study area (Wilson Grove Formation), changing to a continental environment characterized by streams, ponds, and small estuaries (Petaluma Formation). To the east across the Rogers Creek Fault, the late Pliocene is represented by continental deposits of the Glen Ellen Formation. Age control is provided by tephrochronology and paleontology. Diatom floras from the east side of the Cotati Valley indicate shallow, slightly alkaline mesotrophic to eutrophic, freshwater ponds associated with the continental deposits of the Petaluma Formation. Where biochronologic markers (Stephanodiscus spp.) are present, the diatomaceous sediments are early Pliocene. Diatomaceous rocks from the western side of the Cotati Valley and the Santa Rosa plain contain taxa representative of brackish environments indicating a gradual change from lacustrine to estuarine conditions from east to west.

Ostracodes and foraminifers from the Shell Oil Company Mossi #1 well in the Petaluma Oil Field suggest a gradual change from slightly brackish, estuarine conditions in the lower part of the core (1751-1510 ft) to fresher water conditions in the upper part (958-968 ft) of the core. Additional borehole and surface samples support a northeast-southwest transition from slightly alkaline ponds to brackish, estuarine conditions.

Invertebrate macrofossils from the Wilson Grove Formation on the western side of the study area, from Spring Hill to Wilson Grove, document the transition from late Miocene deep marine outer continental shelf/slope facies to the southwest to late Pliocene shallow subtidal marine facies to the northeast, and continental facies further to the east. The marine fauna is dominated by molluscs (bivalves and gastropods), but also contains brachiopods, arthropods, and echinoderms. Macrofossils (molluscs and arthropods) from Edom Hill and east of Santa Rosa associated with the Petaluma Formation indicate marginal marine to brackish conditions, and are likely late Miocene.