Sisquoc Formation
| Sisquoc Formation | |
|---|---|
| Stratigraphic range: Late Miocene to early Pliocene, | |
Sisquoc Formation at the stairs to More Mesa Beach, Santa Barbara, California | |
| Type | Sedimentary |
| Unit of | Los Angeles Basin |
| Underlies | Pico Formation, Santa Barbara Formation, Foxen Mudstone, 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.