The Rincon Formation (or Rincon Shale) 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.[3]
The type locality of the unit is in Ventura County, on the east side of Rincon Mountain, where the formation is exposed in the south-flowing Los Sauces Canyon, north of U.S. Highway 101 and in the vicinity of the Rincon Oil Field.[4] The unit is mapped from Ventura County westward along the south slope of the Santa Ynez Mountains, as well as on portions of the north slope. It also occurs in a few places north of the Santa Ynez Fault, but outcrops nowhere north of the Santa Ynez River.[4] In the subsurface it frequently is found in well cores, underlying the Monterey Formation and overlying the Vaqueros Formation. At the ground surface, the contact with the Vaqueros is obvious along the south slope of the mountains, for it is almost always defined by the line dividing the rounded, grassy foothills from the more rugged, chaparral-covered sandstones upslope.[4]
Thickness of the unit at the surface ranges from about 1400 to 1700 feet south of the mountain crest, averaging 1600 feet. It is less thick north of the mountains, attaining a maximum thickness of around 1000 feet at the western end of its range, and thinning to the east.[4] In the subsurface under the Santa Barbara Channel the thickness is variable but generally less than onshore. In well cores in the Hondo Offshore Oil Field the thickness is around 600 feet, while in the abandoned Coal Oil Point field it is 900 feet, and in the Cuarta Offshore Oil Field it is over 1200 feet thick.[5]
Lithology and deposition environment
Close-up of the Rincon Shale in a road cut, showing weathered, highly fractured surface.
The Rincon Formation is massive to poorly bedded, and consists of predominantly argillaceous to silty shale and mudstone, with occasional dolomite. Two layers of siliceous shale are found in the middle of the unit, and they outcrop noticeably in the region west of Goleta.[6][7][8] The base of the unit, which rests conformably on the shallower-water Vaqueros Formation, contains a glauconitic layer. Several thin layers of bentonite occur within the unit, and the unit is capped by one as well, which forms the base of the conformably overlying Monterey Formation. North of the Santa Ynez River, the Rincon is typically capped not by the Monterey but by the Temblor Formation, a sandstone similar to the underlying Vaqueros; additionally, north of the river the Rincon Formation grades into this sandstone to the east.[9] Northeast of Blue Canyon and the Little Pine Fault, in the backcountry of easternmost Santa Barbara County and Ventura County, the Rincon Formation disappears, and the Temblor Formation rests unconformably on the much older Cozy Dell Shale.[10]
The Rincon Shale was deposited on the deep sea floor during the time at which the Miocene sea reached its greatest depth. The faunal assemblage found as fossils and fragmentary fossils indicates that the sea was tropical to subtropical at this time. During this period the sea deepened due to tectonic forces, as the area of deposition was at the collision boundary of the North American and Pacific Plates, shortly after the Farallon Plate had been completely buried beneath the American plate.[11]
At that time only clay- and mud-sized grains could reach the deposition location, since it was far from the shore, and sediment accumulated slowly. The unit represents a period of around 7.5 million years – from 24 Ma (million years before present) to 17.5 – covering, with the Vaqueros Formation, the entirety of the lower Miocene.[12]
The Rincon Formation weathers to a distinctive loamy, black soil with a high expansion-contraction potential, presenting a risk to structures built on soils derived from the formation. Landslides and slumps are frequent on soils produced from the Rincon, although they usually are not large, limited to about 15 feet in depth.[3]
As is common with shale units, the formation has a low permeability, and therefore where oil and gas reservoirs occur in the area it serves as the impermeable cap keeping the hydrocarbons in place. Some of these reservoirs include the Ellwood Oil Field, Gaviota Offshore Gas Field, Cuarta Offshore Oil Field, Molino Gas Field, and others.[13]
Paleontology
Microfossils are common in the Rincon, and have been helpful in dating the unit.[14]Formaniferal remains in particular are abundant, and the various types have allowed staging of the layers into Upper Zemorrian and Lower and Upper Saucesian.[9] Within Santa Barbara County, two significant fossil finds (aside from microfossils) have been made in the Rincon Formation.[15] In Ventura County, the exposure of the formation along Los Sauces Creek has yielded ten different species of ostracoda; a detailed study suggested that the sea bottom they inhabited was around 2,000 meters deep, very near to the edge of the continental shelf.[16]
Radon
Naturally high in uranium, the Rincon Formation is a notorious source of radon gas emissions in Santa Barbara and Ventura Counties, making them the counties with the greatest radon hazard in California.[17] Radon is a byproduct of radioactive decay of uranium, which is present in the Rincon at around 20 to 30 parts per million (ppm). A 1993 study found that approximately 74% of homes built on the Rincon Formation, or on alluvium or soils derived from the Rincon Formation, showed interior radon concentrations in excess of 4 picocuries per liter (pCi/l), the U.S. EPA action level, and 26% had measured levels of radon over 20 pCi/l.[18] According to the U.S. EPA, non-smokers exposed to radon at the 20 pCi/l level over a lifetime had a risk of dying from lung cancer at 36 in 1000, as opposed to 7 in 1000 at the 4 pCi/l level, and 2 per 1000 at the average indoor radon level in the United States of 1.3 pCi/l. Smokers were at a much greater risk, with fully 260 out of 1,000 expected to die of lung cancer with a lifetime exposure of 20 pCi/l of radon.[19]
Radon migrates upwards from its source, entering buildings wherever the bottom is imperfectly sealed, for example through cracks in a concrete slab. If a structure is poorly ventilated – such as when the windows remain shut during the cool season – radon can accumulate to a hazardous level.[20] Some of the mitigation measures employed in houses on the Rincon Formation since the discovery of the problem in the 1990s include building ventilation systems for the air space beneath the structure, performing "soil suction" in which the gas is pulled directly from the soil under the building, and active house pressurization, which keeps the gas from entering.[21]
References and notes
↑ Dibblee, Thomas. Geology of the central Santa Ynez Mountains, Santa Barbara County, California. Bulletin 186, California Division of Mines and Geology. San Francisco, 1966. 42
1 2 City of Goleta, California. Background Report #16. Geology/Geologic Hazards. March 19, 2004. p. 2, 4, 11. Accessed at www.cityofgoleta.org/Modules/ShowDocument.aspx?documentid=649
↑ 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 www.consrv.ca.gov. p. 616, 646, 652
↑ 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.
↑ Deborah R. Harden. California Geology. Prentice Hall, 1997. ISBN0-02-350042-5 p. 433
↑ Gregory A. Miles and Catherine A. Rigsby. "Lithostratigraphy and Depositional Environments of the Vaqueros and Upper Sespe/Alegria Formations, Hondo Field, Santa Barbara Channel, California." SEPM Core Workshop No. 14. San Francisco, June 3, 1990. ISBN0-918985-84-6 p. 47
↑ Richard G. Stanley, et al. "Stratigraphic Revelations Regarding the Rincon Shale (Lower Miocene) in the Santa Barbara Coastal Area, California." American Association of Petroleum Geologists Pacific Section Meeting, Ventura, California, April 27–29, 1994. Abstract available here
↑ Kenneth L. Finger, "Ostracoda from the Lower Rincon Formation (Oligo-Miocene) of Southern California". Micropaleontology, Vol. 29, No. 1 (1983), pp. 78-109. Stable URL: https://www.jstor.org/stable/1485653
↑ Ron Churchill: "Radon Mapping: Santa Barbara and Ventura Counties." California Department of Conservation, Division of Mines and Geology, 1997. Available here
↑ Donald Carlisle, Haydar Azzouz: "Discovery Of Radon Potential In The Rincon Shale, California – A Case History Of Deliberate Exploration." International Journal of Indoor Environment and Health. Vol 3, Issue 2, pp. 131-142, June 1993. Abstract available here.
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 South Cuyama Oil Field is a large oil and gas field in the Cuyama Valley and the adjacent northern foothills of the Sierra Madre Mountains in northeastern Santa Barbara County, California. Discovered in 1949, and with a cumulative production of around 225 million barrels (35,800,000 m3) of oil, it ranks 27th in size in the state, but is believed to retain only approximately two percent of its original oil, according to the official estimates of the California Department of Oil, Gas, and Geothermal Resources (DOGGR). Of the top forty onshore oil fields in California, it is the most recent to be discovered, but by the end of 2008 only 87 wells remained in production.
Simi Valley is a synclinal valley in Southern California in the United States. It is an enclosed or hidden valley surrounded by mountains and hills. It is connected to the San Fernando Valley to the east by the Santa Susana Pass and the 118 freeway, and in the west the narrows of the Arroyo Simi and the Reagan Freeway connect to Moorpark and Ventura, California. The relatively flat bottom of the valley contains soils formed from shales, sandstones, and conglomerates eroded from the surrounding hills of the Santa Susana Mountains to the north, which separate Simi Valley from the Santa Clara River Valley, and the Simi Hills.
Ellwood Oil Field and South Ellwood Offshore Oil Field are a pair of adjacent, partially active oil fields adjoining the city of Goleta, California, about twelve miles (19 km) west of Santa Barbara, largely in the Santa Barbara Channel. A richly productive field in the 1930s, the Ellwood Oil Field was important to the economic development of the Santa Barbara area. The Japanese submarine I-17 shelled the area during World War II, the first direct naval bombardment of the U.S. continent of the war, causing an invasion scare on the West Coast.
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 Summerland Oil Field is an inactive oil field in Santa Barbara County, California, about four miles (6 km) east of the city of Santa Barbara, within and next to the unincorporated community of Summerland. First developed in the 1890s, and richly productive in the early 20th century, the Summerland Oil Field was the location of the world's first offshore oil wells, drilled from piers in 1896. This field, which was the first significant field to be developed in Santa Barbara County, produced 3.18 million of barrels of oil during its 50-year lifespan, finally being abandoned in 1939-40. Another nearby oil field entirely offshore, discovered in 1957 and named the Summerland Offshore Oil Field, produced from two drilling platforms in the Santa Barbara Channel before being abandoned in 1996.
The Rincon Oil Field is a large oil field on the coast of southern California, about 10 miles (16 km) northwest of the city of Ventura, and about 20 miles (32 km) east-southeast of the city of Santa Barbara. It is the westernmost onshore field in a series of three fields which follow the Ventura Anticline, an east-west trending feature paralleling the Transverse Ranges. Discovered in 1927, the oil field is ranked 36th in California by size of recoverable oil reserves, and while mostly depleted – now having, by California Department of Conservation estimates, only about 2.5% of its original oil – it remains productive, with 77 wells active at the beginning of 2008. Oil produced in the field flows through the M-143 pipeline, which parallels U.S. Highway 101 southeast to the Ventura Pump Station, at which point it joins a Tosco pipeline which carries it to Los Angeles area refineries. As of 2009, the primary operators of the field were Occidental Petroleum for the onshore portion, and Greka Energy for the offshore portion. The offshore part of the field is operated mainly from Rincon Island.
The Dos Cuadras Offshore Oil Field is a large oil and gas field underneath the Santa Barbara Channel about eight miles southeast of Santa Barbara, California. Discovered in 1968, and with a cumulative production of over 260 million barrels of oil, it is the 24th-largest oil field within California and the adjacent waters. As it is in the Pacific Ocean outside of the 3-mile tidelands limit, it is a federally leased field, regulated by the U.S. Department of the Interior rather than the California Department of Conservation. It is entirely produced from four drilling and production platforms in the channel, which as of 2009 were operated by Dos Cuadras Offshore Resources (DCOR), LLC, a private firm based in Ventura. A blowout near one of these platforms – Unocal's Platform A – was responsible for the 1969 Santa Barbara oil spill that was formative for the modern environmental movement, and spurred the passage of the National Environmental Policy Act.
The Russell Ranch Oil Field is an oil and gas field in the Cuyama Valley of northern Santa Barbara and southern San Luis Obispo Counties, California, in the United States. Discovered in 1948, and reaching peak production in 1950, it has produced over 68 million barrels (10,800,000 m3) of oil in its lifetime; with only an estimated 216,000 barrels (34,300 m3) of recoverable oil remaining, and having produced around 66,000 in 2008, it is considered to be close to exhaustion. The primary operator on the field as of 2010 is E&B Natural Resources, which also runs the nearby South Cuyama Oil Field.
The Mesa Oil Field is an abandoned oil field entirely within the city limits of Santa Barbara, California, in the United States. Discovered in 1929, it was quickly developed and quickly declined, as it proved to be but a relatively small accumulation of oil in a single geologic formation. While the field was active in the 1930s, residential development in most of the Mesa neighborhood of Santa Barbara came to a halt. The field included two major productive areas with a total surface extent of only 210 acres (0.85 km2), and produced 3,700,000 barrels (590,000 m3) of oil during its brief lifetime.
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 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. 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 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 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.
The geology of California is highly complex, with numerous mountain ranges, substantial faulting and tectonic activity, rich natural resources and a history of both ancient and comparatively recent intense geological activity. The area formed as a series of small island arcs, deep-ocean sediments and mafic oceanic crust accreted to the western edge of North America, producing a series of deep basins and high mountain ranges.
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