Sisquoc Formation

Sisquoc Formation Stratigraphic range: Lower Pliocene, upper Miocene
Sisquoc Formation at the stairs to More Mesa Beach, Santa Barbara, California
Type	Sedimentary
Underlies	Pico Formation, Santa Barbara Formation, others
Overlies	Monterey Formation
Thickness	Up to 1000 feet in Santa Barbara area; 1100 feet at type locality; up to 5000 feet south of Lompoc.
Lithology
Primary	mudstone, shale, diatomite
Other	conglomerate
Location
Region	Southern California
Country	United States
Type section
Named for	Sisquoc River, about 10 miles southeast of Santa Maria, California
Named by	Porter (1932)

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). 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 500–5,000 feet (150–1,520 m). 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.

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. 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. 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.