Miocene gastropods and biostratigraphy of the Kern River area, California

* Description
* Abstract
* Described species

**Marine formations of early and middle Miocene age are exposed in the Kern River area. Faunal assemblages from the subsurface Vedder Sand of this area and from exposures of the overlying Jewett Sand are referred to the "Vaqueros Stage," the early Miocene part of the Pacific coast megainvertebrate chronology of Weaver and others (HJ44). A few long-ranging molluslcs occur in the overlying Freeman Silt. Foraminifera from this formation are indicative of an early Miocene age. Mollusks from the Olcese Sand and the overlying Round Mountain Silt form the faunal l.>asis for recognition of the "Temblor Stage,"a unit long regarded as the middle Miocene provincial megafaunal standard. i'llolluslcs of late Mio~ene age are not known to occur in surface exposures in the Kern River area, although several assemblages have l.>een reported from well cores of the Santa Margarita Formation in oil-field areas a few miles west of the lower and middle Miocene exposures.**
**Formational nomenclature for the Miocene sequence was originated during early exploratory and development drilling for petroleum in the Round Mountain-Mount Poso area, principally from subsurface data. Most of these units, though locally applicable in surface mapping, have not been adequately defined.**
**Accordingly, formal description and review of the Miocene formations is undertaken to facilitate biostratigraphic characterization of the Miocene sequence. The fauna of the lower part of the .Jewett Sand, the lowest exposed· marine formation in the Kern River area, includes 46 gastropod taxa, many of which are newly described herein.**
**Almost half of these taxa do not occur higher in the local section. In most of the upper part of the formation, mollusks are widely scattered, but tl.1ere are a few oyster biostromes near the top which contain Ostrea eldridgei yneziana and scattered Ohlam118 h01·tleini., a pectinid restricted to the "Vaqueros Stage."**
**Important "Vaqueros" index species in the Jewett Sand include the gastropods T1tn·itella inezana, Ocenebra milicentana, and l1Jpitoni1tm clallamense and the pelecypods Lyropecten magnolia and Ostrea vaq1terosensis. Cores of the overlying Freeman Silt contain an assemblage characterized by the small pelecypods Acila conrad,i, 01}clocard,la 81tbtenta, and Nuculana ochsneri.**
**Also occurring with this assemblage is the gastropod Priscotusus rnedi.alis, which seems to be restricted to the Jewett Sand and**
Freeman Silt. The small subsurface molluscan assemblages from the Freeman Silt are not diagnostic of position in the provincial megaitwertebrate sequence, but indirect evidence from foraminiferal assemblages is suggestive of an early Miocene age.
Molluscan assemiJlages from the lower part of the overlying Olcese Sand, although relatively small, are similar to those from higher in the Olcese Sand and from the overlying Round Mountain Silt. Gastropods restricted to the "Temblor Stage" that make their initial appearance in the lower part of the Olcese Sand include 0'tC1Jill1tla rost1·a.Zis, Mcuasurcula howei, and Ophiodcnnclla tcmblorcnsis.
The gastropod fauna of the Olcese Sand and Round Mountain Silt consists of 157 taxa, about one-third of which are described herein as new. A significant number of the gastropods, about 15 percent, are stratigraphically restricted and of sufficiently widespread occurrence in the California Coast Ranges to be considered as useful index species. Moreover, there are many widespread gastropod taxa with stratigraphic ranges that overlap with the limits of the "Temblor Stage."
Scattered records of megainvertebrates from subsurface cores of the Santa Margarita Formation basinward from the lower and middle Miocene exposures of the Kern River area are indicative of an assemblage that includes several middle Miocene relicts.
There is also an element with recorded ranges of Pliocene to Holocene, and this joint occurrence, together with records of the stratigraphically restricted gastropod Nassaritts 1Jablocns1s and the pelecypod Aeqwi,pecten (liscus, provides evidence for a late Miocene age in the provincial chronology.

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