Stratigraphy of the pliocene pleistocene strata in the twelvemile creek area san francisco peninsula california usa

Yancey, T.E.

Proceedings of the California Academy of Sciences 41(15): 357-370


ISSN/ISBN: 0068-547X
Accession: 006486658

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Rocks in the Twelvemile Creek area [California, USA] are mostly sedimentary, belonging to the Franciscan, Merced and Colma formations. The Merced Formation consists of about 1000 m of richly fossiliferous marine strata, which correlate with the upper marine part of the type section of the Merced and are entirely of upper Pliocene age. The nonmarine Colma Formation is subdivided into a lower massive sand unit a few hundred meters thick, and a thin clay-rich upper unit and these units are suggested to have been deposited respectively, during a high stand and a low stand of sea level during the Pleistocene. The Merced and Colma formations in this area are separated by a high angle fault, which is a northward continuation of the Serra Fault. The Serra Fault in this area nearly parallels the San Andreas Fault, gradually diverging from it towards the north. The Merced Formation in the Twelvemile Creek area is abundantly fossiliferous and contains a known fauna of over 50 spp. The echinoids (Scutellaster), nassariid gastropods (Nassarius) and turrid gastropods (Ophiodermella) are the most useful fossils for correlations of Merced Formation strata. Species of the gastropod Ophiodermella have restricted stratigraphic ranges in the Merced Formation and probably can be used for regional correlations of Plio-Pleistocene strata. The species O. graciosana (Arnold, 1907), O. mercedensis (Martin, 1914), and O. incisa (Carpenter, 1864) occur in stratigraphic succession, and differ in having progressively weaker ornament and increasing spire height from O. graciosana to O. incisa.