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Sisquoc Formation
The Sisquoc Formation is a sedimentary geologic unit widespread in Southern California, both on the coast and in mountains near the coast. Overlying the Monterey Formation, it is of upper Miocene and lower Pliocene age (from about 4 to 6 million years old).〔Isaacs, Caroline M. and Rullkötter, Jürgen. ''The Monterey Formation: From Rocks to Molecules.'' Columbia University Press, 2001. ISBN 0-231-10585-1 p. 211.〕 The formation consists of claystone, mudstone, siltstone, shale, diatomite, and conglomerates, with considerable regional variation, and was deposited in a moderately deep marine environment at a depth of approximately . Since some of its diatomites, along with those of the underlying Monterey Formation, are of unusual purity and extent, they can be mined as diatomaceous earth. France-based Imerys operates a mine in the Sisquoc and Monterey Formations in the hills south of Lompoc, California, the largest such operation in the world.
==Type locality, description, and occurrence==
The type locality for the Sisquoc Formation is along the Sisquoc River, in northern Santa Barbara County, about a mile east of its confluence with Foxen Canyon.〔Dibblee (1966) 51〕 In this location the formation consists mainly of sandstones, but also some siltstone and diatomite, and is about 1,100 feet thick. In other places, such as in the Purisima Hills north of Lompoc, in and adjacent to the Lompoc Oil Field, its total thickness reaches 5,000 feet.〔〔California Department of Conservation, Division of Oil, Gas, and Geothermal Resources (DOGGR). ''California Oil and Gas Fields, Volumes I, II and III''. Vol. I (1998), Vol. II (1992), Vol. III (1982). PDF file available on CD from consrv.ca.gov. p. 238〕〔Dibblee (1950), 43-44〕 Diatomite is a major component of the formation in the hills north and south of Lompoc, where it is interbedded with diatomaceous clay shale. The lowest portion of the formation in the Purisima Hills contains tar, for it is here that the unit forms an impermeable cap on the underlying Monterey reservoir of the Lompoc Oil Field.〔〔
The Sisquoc Formation is not as resistant to erosion as other formations in the stratigraphic sequence in coastal California, and south of the Santa Ynez Mountains it weathers to hilly terrain with gray soil that supports grasses. It outcrops rarely, being best exposed in road cuts, along rivers, and especially along the cliffs on the coast, where it is easily visible from the beach. Many prominent exposures of the Sisquoc occur along the beaches from Santa Barbara west to Gaviota.〔 In the Santa Maria Basin – the area surrounding the city of Santa Maria and the hills to the south and southwest – well-weathered outcrops of the formation are often bleached white.〔Nuel C. Henderson, Jr. and Pedro C. Ramirez. "Control exerted by lithologic variations and pebbly units on petroleum occurrences in the Pliocene Upper Sisquoc Formation, Casmalia Hills, Santa Maria Basin, California." SEPM Core Workshop No. 14. San Francisco, June 3, 1990. ISBN 0-918985-84-6 p. 339-340〕
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