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).[5][6] 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 (150–1,520m). 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.[7][8]
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.[9] 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.[9][10][11] 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.[10][11]
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.[9] 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.[12]
Deposition environment
Close-up photograph of the Sisquoc Formation, showing the friable, planiform nature of the rock on a weathered surface. Rock hammer for scale.
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.[13] 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.[14] 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.[15]
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.[14]
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.[16]
Paleontology
Numerous fossils have been found in the Sisquoc Formation. Within Santa Barbara County, the Sisquoc alone has produced 127separate finds, which are catalogued by the University of California, Berkeley Museum of Paleontology.[17] 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.[17] Additional fossils include radiolarians, arenaceousforaminifers, and remains of sponges.[14]
Economic importance
Diatomite quarries near Lompoc, California (2009), are a major source of diatomaceous earth.
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.[18] 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.[12]
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.[12] 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 from the decomposing organic matter to be matured, via hydrocarbon cracking, into petroleum.[19]
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.[20]
References and notes
↑ Minor, S.A., Kellogg, K.S., Stanley, R.G., Gurrola, L.D., Keller, E.A., and Brandt, T.R., 2009, Geologic Map of the Santa Barbara Coastal Plain Area, Santa Barbara County, California: U.S. Geological Survey Scientific Investigations Map 3001, scale 1:25,000, 1 sheet, pamphlet, 38 p.
↑ Dibblee, Thomas. Geology of the central Santa Ynez Mountains, Santa Barbara County, California. Bulletin 186, California Division of Mines and Geology. San Francisco, 1966. 51.
↑ Dibblee, Thomas. Geology of Southwestern Santa Barbara County, California. State of California, Department of Natural Resources, Division of Mines, Bulletin 150. 1950. 43.
↑ Isaacs, Caroline M. and Rullkötter, Jürgen. The Monterey Formation: From Rocks to Molecules. Columbia University Press, 2001. ISBN0-231-10585-1 p. 211.
1 2 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. 238
1 2 3 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. ISBN0-918985-84-6 p. 339-340
↑ Dolley, Thomas P. Diatomite. U.S. Geological Survey Minerals Yearbook, 2002. US Government Printing Office. Available here.
Related Research Articles
Santa Barbara County, officially the County of Santa Barbara, is a county located in Southern California. As of the 2020 census, the population was 448,229. The county seat is Santa Barbara, and the largest city is Santa Maria.
Diatomaceous earth, also known as diatomite, celite, or 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 mm to less than 1 μm, but typically 10 to 200 μm. Depending on the granularity, this powder can have an abrasive feel, similar to pumice powder, and has a low density as a result of its high porosity. The typical chemical composition of oven-dried diatomaceous earth is 80–90% silica, with 2–4% alumina, and 0.5–2% iron oxide.
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.
Area codes 805 and 820 are telephone area codes in the North American Numbering Plan (NANP) for the U.S. state of California. The numbering plan area (NPA) includes most or all of the counties of San Luis Obispo, Santa Barbara, Ventura, and the southernmost portions of Monterey County. 805 was split from area code 213 in 1957, and area code 820 was added to the NPA in 2018, creating an area code overlay.
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 islands. The type locality is near the city of Monterey, California. The Monterey Formation is the major source-rock for 37 to 38 billion barrels of oil in conventional traps such as sandstones. This is most of California's known oil resources. The Monterey has been extensively investigated and mapped for petroleum potential, and is of major importance for understanding the complex geological history of California. Its rocks are mostly highly siliceous strata that vary greatly in composition, stratigraphy, and tectono-stratigraphic history.
The Vaqueros Formation is a sedimentary geologic unit primarily of Upper Oligocene and Lower Miocene age, which is widespread on the California coast and coastal ranges in approximately the southern half of the state. It is predominantly a medium-grained sandstone unit, deposited in a shallow marine environment. Because of its high porosity and nearness to petroleum source rocks, in many places it is an oil-bearing unit, wherever it has been configured into structural or stratigraphic traps by folding and faulting. Being resistant to erosion, it forms dramatic outcrops in the coastal mountains. Its color ranges from grayish-green to light gray when freshly broken, and it weathers to a light brown or buff color.
The Lompoc Oil Field is a large oil field in the Purisima Hills north of Lompoc, California, in Santa Barbara County. Discovered in 1903, two years after the discovery of the Orcutt Oil Field in the Solomon Hills, it is one of the oldest oil fields in northern Santa Barbara County, and one of the closest to exhaustion, reporting only 1.7 million barrels (270,000 m3) of recoverable oil remaining out of its original 50 million barrels (7,900,000 m3) as of the end of 2008. Its sole operator is Sentinel Peak Resources, who acquired it from Freeport-McMoRan. In 2009, the proposed decommissioning and habitat restoration of the 3,700-acre (15 km2) field was part of a controversial and so-far unsuccessful deal between Plains, several environmental groups, Santa Barbara County, and the State of California, to allow Plains to carry out new offshore oil drilling on the Tranquillon Ridge, in the Pacific Ocean about twenty miles (32 km) southwest of the Lompoc field.
The Orcutt Oil Field is a large oil field in the Solomon Hills south of Orcutt, in Santa Barbara County, California, United States. Discovered in 1901 by William Warren Orcutt, it was the first giant field to be found in Santa Barbara County, and its development led to the boom town of Orcutt, now the major unincorporated southern suburb of Santa Maria. With a cumulative production in 2008 of 870,000 barrels (138,000 m3) of oil, it is the largest onshore producing field in Santa Barbara County.
The Oxnard Oil Field is a large and productive oil field in and adjacent to the city of Oxnard, in Ventura County, California in the United States. Its conventional oil reserves are close to exhaustion, with only an estimated one percent of the original oil recoverable with current technology remaining: 434,000 barrels (69,000 m3) out of an original 43.5 million. However, the reservoir includes an enormous deposit of tar sands, ultra-heavy oil classed as an unconventional petroleum reserve, and potentially containing 600 million barrels (95,000,000 m3) of oil equivalent, should it become economically feasible to extract. Present operators on the field include Tri-Valley Oil & Gas Co., Anterra Energy Services, Inc., Chase Production Co., and Occidental Petroleum through its Vintage Production subsidiary. As of the beginning of 2009, there were 34 active wells on the field.
The Sespe Formation is a widespread fossiliferous sedimentary geologic unit in southern and south central California in the United States. It is of nonmarine origin, consisting predominantly of sandstones and conglomerates laid down in a riverine, shoreline, and floodplain environment between the upper Eocene Epoch through the lower Miocene. It is often distinctive in appearance, with its sandstones weathering to reddish-brown, maroon, pinkish-gray, tan, and green. Since many of its sandstones are more resistant to erosion than many other regional sedimentary units it often forms dramatic outcrops and ridgelines in many local mountain ranges.
The Rincon Formation is a sedimentary geologic unit of Lower Miocene age, abundant in the coastal portions of southern Santa Barbara County, California eastward into Ventura County. Consisting of massive to poorly bedded shale, mudstone, and siltstone, it weathers readily to a rounded hilly topography with clayey, loamy soils in which landslides and slumps are frequent. It is recognizable on the south slopes of the Santa Ynez Mountains as the band at the base of the mountains which supports grasses rather than chaparral. Outcrops of the unit are infrequent, with the best exposures on the coastal bluffs near Naples, in the San Marcos Foothills, at the Tajiguas Landfill, and in road cuts. The geologic unit is notorious as a source of radon gas related to its high uranium content, released by radioactive decay.
The Cozy Dell Shale is a geologic formation of middle Eocene age that crops out in the Santa Ynez Mountains and Topatopa Mountains of California, extending from north of Fillmore in Ventura County westward to near Point Arguello, north of Santa Barbara. Because the Cozy Dell easily weathers to a clay-rich soil, it crops out infrequently and generally forms dense stands of chaparral in saddles between peaks and ridges of the more resistant Matilija and Coldwater formations.
The Coldwater Sandstone is a sedimentary geologic unit of Eocene age found in Southern California, primarily in and south of the Santa Ynez Mountains of Santa Barbara County, and east into Ventura County. It consists primarily of massive arkosic sandstone with some siltstone and shale. Being exceptionally resistant to erosion, outcrops of the Coldwater form some of the most dramatic terrain on the south slope of the Santa Ynez Mountains, with immense white sculpted slabs forming peaks, hogback ridges, and sheer cliff faces.
The Matilija Sandstone is a sedimentary geologic unit of Eocene epoch in the Paleogene Period, found in Santa Barbara and Ventura Counties in Southern California.
The Juncal Formation is a prominent sedimentary geologic unit of Eocene age found in and north of the Santa Ynez Mountain range in southern and central Santa Barbara County and central Ventura County, California. An enormously thick series of sediments deposited over millions of years in environments ranging from nearshore to deep water, it makes up much of the crest of the Santa Ynez range north of Montecito, as well as portions of the San Rafael Mountains in the interior of the county. Its softer shales weather to saddles and swales, supporting a dense growth of brush, and its sandstones form prominent outcrops.
The Jalama Formation is a sedimentary rock formation widespread in southern Santa Barbara County and northern Ventura County, southern California. Of the Late Cretaceous epoch, the unit consists predominantly of clay shale with some beds of sandstone.
The Espada Formation is a sedimentary rock formation widespread in Santa Barbara County, California. Of late Jurassic to Cretaceous age, the unit consists primarily of shale with some interbedded thin layers of sandstone, conglomerate, and limestone.
The Zaca Oil Field is an oil field in central Santa Barbara County, California, about 20 miles southeast of Santa Maria. One of several oil fields in the county which produce heavy oil from the Monterey Formation, the field is hidden within a region of rolling hills, north of the Santa Ynez Valley. As of 2011, the principal operator of the oil field is Greka Energy and the operator of the "Zaca Field Extension Project" is Underground Energy. The field is known to contain heavy crude oil and Underground Energy has recently discovered a lower sub-thrust block in the field, which was not previously produced by the field's historical operators. The field was discovered in 1942, reached peak production in 1954, and remains active with more than thirty oil wells and continues to grow.
The Cat Canyon Oil Field is a large oil field in the Solomon Hills of central Santa Barbara County, California, about 10 miles southeast of Santa Maria. It is the largest oil field in Santa Barbara County, and as of 2010 is the 20th-largest in California by cumulative production.
The La Goleta Gas Field is a natural gas field in unincorporated Santa Barbara County, California, adjacent to the city of Goleta. Discovered in 1929, and first put into production in 1932, it has been in continuous use ever since, producing approximately 12 billion cubic feet of gas. With production declining, the field was converted into a gas storage reservoir in 1941. As of 2016 it remains one of the four gas storage facilities maintained by Southern California Gas Company (SoCalGas), a division of Sempra Energy, with the others being Aliso Canyon, Honor Rancho and Playa del Rey. It is the oldest storage facility of the four and the third largest, with a maximum capacity of 21.5 billion cubic feet. The storage facilities are necessary to balance load for the over ten million customers of SoCalGas: during summer months, when gas usage is at a minimum, gas is pumped into the reservoirs; and in the winter when usage is high, gas is withdrawn. The La Goleta field serves the northern portion of SoCalGas's geographic range.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
