The Sisquoc Formation is a

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

geologic unit widespread in

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...

, both on the coast and in mountains near the coast. Overlying the

Monterey Formation The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islan ...

, it is of upper

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

and lower

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Isaacs, Caroline M. and Rullkötter, Jürgen. ''The Monterey Formation: From Rocks to Molecules.'' Columbia University Press, 2001. p. 211. The formation consists of claystone, mudstone, siltstone, shale,

diatomite Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to le ...

, 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 Lompoc ( ; Chumash: ''Lum Poc'') is a city in Santa Barbara County, California. Located on the Central Coast, Lompoc has a population of 43,834 as of July 2021. Lompoc has been inhabited for thousands of years by the Chumash people, who calle ...

, 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 The Purisima Hills are a northwest-to-southeast trending low mountain range of the Outer Southern California Coast Ranges, located in Santa Barbara County, California. They divide the Santa Ynez Valley on the south from the Los Alamos Valley on t ...

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. 238Dibblee (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 A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

of the Lompoc Oil Field. The Sisquoc Formation is not as resistant to erosion as other formations in the

stratigraphic Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...

sequence in coastal California, and south of the

Santa Ynez Mountains The Santa Ynez Mountains are a portion of the Transverse Ranges, part of the Pacific Coast Ranges of the west coast of North America. It is the westernmost range in the Transverse Ranges. The range is a large fault block of Cenozoic age create ...

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 The Gaviota Coast in Santa Barbara County, California is a rural coastline along the Santa Barbara Channel roughly bounded by the city of Goleta, California, Goleta on the south and the north boundary of the county on the north. This last undeve ...

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. p. 339-340

Deposition environment

The Sisquoc Formation was deposited in a middle

bathyal The bathypelagic zone or bathyal zone (from Greek βαθύς (bathýs), deep) is the part of the open ocean that extends from a depth of below the ocean surface. It lies between the mesopelagic above, and the abyssopelagic below. The bathypelagi ...

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.Dibblee (1966) 52 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

clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

s 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 The Santa Ynez River is one of the largest rivers on the Central Coast of California. It is long, ArcExplorer GIS data viewer. flowing from east to west through the Santa Ynez Valley, reaching the Pacific Ocean at Surf, near Vandenberg Space Fo ...

was deposited as a fine mud, rich with diatoms.

Tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

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.

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.County of Santa Barbara. Paleontological Resources section from th
Lompoc Wind Project Final Environmental Impact Report
. 3.12-3 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

radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...

arenaceous Arenite ( Latin: ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00246 in) and 2 mm (0.08 in) and contain less than 15% matrix. The related adjective is ''arenaceous''. The equivalen ...

foraminifer Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an ...

s, and remains of sponges.

Economic importance

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

; in other cases, such as in the Casmalia Oil Field and the offshore portion of the

Ellwood Oil Field Ellwood Oil Field (also spelled "Elwood") and South Ellwood Offshore Oil Field are a pair of adjacent, partially active oil fields adjoining the city of Goleta, California, about west of Santa Barbara, largely in the Santa Barbara Channel. A r ...

, 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. While most of the oil found in the Sisquoc Formation has gotten there by migrating upward from the underlying Monterey Formation, the primary source rock for petroleum in southwestern California, sometimes the Sisquoc is itself a source rock. In places the formation contains enough organic carbon – up to six percent – to generate oil under the right conditions, such as burial in deep synclines with high enough temperature and sufficient time (several million years) to cause the

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

from the decomposing organic matter to be matured, via hydrocarbon cracking, into petroleum. 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

; the

USGS The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...

estimates that the Lompoc mine alone with its large reserves could meet the needs of the entire world for centuries.Dolley, Thomas P. ''Diatomite''. U.S. Geological Survey Minerals Yearbook, 2002. US Government Printing Office. Availabl
here

References and notes

{{reflist, colwidth=30em Geology of Santa Barbara County, California Pliocene California Miocene California