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.

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 at the beaches along the Gaviota Coast 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.

The Sisquoc Formation was deposited in a middle bathyal environment, at a depth of between 150 and 1500 meters, between approximately 4 and 6 million years ago – the upper Miocene and lower Pliocene eras. During this time, the region was continuing to subside; as a result, sediments deposited later in the period tend to be finer-grained, since the deeper the water, the finer the sediment deposited there. Even though the deposition environment was far from shore, the unit contains occasional conglomerates. One such unit near More Mesa Beach in Santa Barbara, containing clasts from the underlying Monterey Formation, was probably the result of a submarine landslide, bringing down pebbles and boulders from that older formation, which had already been uplifted onshore.

The bulk of the Sisquoc north of the Santa Ynez River was deposited as a fine mud, rich with diatoms. Tests of these tiny marine creatures form diatomite, and some of their organic remains persist as the high organic carbon content of parts of the formation (when conditions are right, these organic remains form petroleum reservoirs). South of the Santa Ynez Mountains, the ocean was deeper and the formation consists of finer muds and clays.

Going east along the Santa Ynez River, the Sisquoc Formation grades into the Tequepis Sandstone, in which diatomite is gradually replaced by sandstones of granitic origin.

Numerous fossils have been found in the Sisquoc Formation. Within Santa Barbara County, the Sisquoc alone has produced 127 separate finds, which are catalogued by the University of California, Berkeley Museum of Paleontology. In addition to the abundant diatoms which make up the diatomite, fossils include vertebrates such as sea lions and walruses, bony and cartilaginous fishes, and birds. Additional fossils include radiolarians, arenaceous foraminifers, and remains of sponges.

The Sisquoc Formation is important as a source of diatomaceous earth and as a petroleum reservoir.