Sisquoc Formation

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. ==Deposition environment==

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. ==Paleontology==

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. ==Economic importance==

(2009), are a major source of diatomaceous earth. The Sisquoc Formation is important as a source of diatomaceous earth and as a petroleum reservoir. As a stratigraphic member of petroleum reservoirs, it can be both a reservoir and a sealing unit. Sometimes, as in the Lompoc Oil Field, it serves as an impermeable cap rock to an oil-bearing unit, in that case in the Monterey Formation; in other cases, such as in the Casmalia Oil Field and the offshore portion of the Ellwood Oil Field, it is an oil-bearing unit in its own right. In the Casmalia field, west of Santa Maria, pebbly and high-porosity units contain a considerable amount of oil, while clay-rich and diatomaceous mudstones contain almost none. The diatomites of the Sisquoc, like those of the Monterey, can function both as oil-bearing units – where they have been capped with impermeable beds – or they may be oil-free, in which case, if they are near the surface and of sufficient purity, they can be mined as diatomaceous earth. The world's largest such deposit, operated by Imerys, is in the Sisquoc and Monterey formations in the hills south of Lompoc, California; the USGS estimates that the Lompoc mine alone with its large reserves could meet the needs of the entire world for centuries. ==References and notes==