Paleontology of the Santa Margarita formation on the Coalinga Antieline, Fresno County, California

The upper middle to lower upper Miocene "Santa Margarita" Formation of the Coalinga anticline is on the west side of the San Joaquin valley. This formation, which is exposed in hillsides that extend about 32 km in a north-to- south direction, varies in thickness from 46 to 60 m. Three informal members are recognized. The lower member lies unconformably on top of the middle Miocene Big Blue Formation. The upper member is unconformably overlain by the upper Miocene and lower Pliocene Etchegoin Formation. To the south, this unconformity cuts into the middle member. The lower member consists of a slightly fossiliferous fine to medium sandstone interpreted to be an upper shoreface deposit. The middle member consists of slightly fossiliferous, silty fine sandstone and highly fossiliferous, pebbly medium to coarse sandstone. The latter contains in situ, large-sized Crassostrea titan biostromes. Both types of sandstone are interpreted as lower shoreface deposits. The Crassostrea biostromes, furthermore, indicate warm shallow-waters with low to variable salinities. The upper member is similar to the middle member except that it contains thinner-bedded in situ Crassostrea titan biostromes and is interpreted to be a lower shoreface deposit slightly deeper than the middle member. All the members are interpreted to have been deposited in a large embayment along a low-energy coastline. "Santa Margarita" fossils are predominantly macro-invertebrates with some vertebrates and microfossils. Macro-invertebrates consist of 37 species in 35 genera collected from 66 localities. The more common species include the bivalves Crassostrea titan, Dendostrea vespertina, Pacipecten discus, Lyropecten crassicardo, Lyropecten estrellanus, the gastropod Forreria careasensis and barnacle Balanus gregarius. Vertebrates consist of 6 genera of sharks, 5 genera of rays, teleost fish, dolphins, whales, and 1 species of penniped. The microfossils consist of 6 species and 13 genera of forams, 2 genera of ostracods, spumellarian and nassellarian radiolarians, sponge spicules, and 3 genera of diatoms. Three paleocommunities are present in the "Santa Margarita" Formation. These include the biostromal Crassostrea-Dendostrea?-Lyropecten-Forreria paleocommunity in the middle member and the Crassostrea-Lucinoma-"Spisula"- Macoma-Nassarius-naticid and Nuculana-"Spisula"-TurritellaNassarius- Discanisca paleocommunities in the upper member. All of the macro-invertebrate genera in the "Santa Margarita" Formation were present on the west coast by the middle Miocene. These genera include Tegula, Turritella, Crepidula, Crassostrea, Lyropecten, and Lucinoma to name a few.