TRANSITIONAL MARINE, OLIGOHALINE, LIMNIC OSTRACODE ASSEMBLAGES OF THE LOWER PETALUMA FORMATION, SONOMA COUNTY, CALIFORNIA

The Upper Miocene (~9.3-8 Ma) Lower Petaluma Formation of Sonoma County, California yields a diverse ostracode fauna, including the genera: Cypria, Eucypris, Candona, Heterocypris, Ilyocypris, Darwinula, Limnocythere, Cytheromorpha, and Cyprideis. These taxa are divided into three assemblages and environments: transitional marine, oligohaline and limnic reflecting multiple hydrochemical settings in the Lower Petaluma basin.

Samples from the Mossi Well #1 (1790-958 ft. bgs) contain ostracode species Heterocypris glaucus, Candona sp., Limnocythere cf. L. herricki, Ilyocypris gibba, and Darwinula stevensoni are indicative of high PH, lacustrine environments.

Samples from the Murphy Well #1 from depths (2208-1856 ft. bgs) contain a single ostracode, Eucypris sp., well known from low TDS (Total Dissolved Solids), moderate PH vernal pools and freshwater ponds.

Samples from (199.5 - 228.4 ft. bgs) in a boring from the Stony Point Rock Quarry contain ostracode species: Heterocypris glaucus, Candona patzcuaro, Candona sp., Darwinula cf., D. stevensoni, and Limnocythere cf., L. itasca, suggesting lacustrine environments with elevated TDS and PH may be inferred from the overall ostracode and charophyte assemblage.

Along the NW side of Stage Gulch Road immediately NE of Tolay Fault, samples yield sparse valves of the oligohaline lacustrine species Heterocypris glaucus and Candona patzcuaro, Candona sp., Heterocypris cf. H. glaucus, Limnocythere cf. herricki, Darwinula cf. D. stevensoni, and Cytheromorpha sp.

Samples from the Tolay Valley immediately north of the Roche-Cardoza fault contains Eucypris sp., a parthenogenetic species most typical of vernal pools and ponds, and transitional marine diatoms Surirella striatella, suggesting a freshwater ecosystem periodically flushed by brackish water. An ostracode assemblage recovered from another sample at this locality contains the transitional marine species, Cyprideis beaconensis, as well as Cytheromorpha sp., Heterocypris cf. H. glaucus and Candona sp. Extensive pyritization of most ostracode specimens indicates anoxic, high H2S tidal marshland environment. A tergal opercular plate from the brackish water barnacle species, Balanus crenatus is associated with this assemblage.