Rincon Formation
Encyclopedia
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
.
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
. 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
. 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.
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. 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.
. Two layers of siliceous shale are found in the middle of the unit, and they outcrop noticeably in the region west of Goleta. 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. 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
.
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 Plate
s, shortly after the Farallon Plate
had been completely buried beneath the American plate.
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.
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.
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.
a; 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
.
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. 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.
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. 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.
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
age, abundant in the coastal portions of southern Santa Barbara County, California eastward into Ventura County
Ventura County, California
Ventura County is a county in the southern part of the U.S. state of California. It is located on California's Pacific coast. It is often referred to as the Gold Coast, and has a reputation of being one of the safest populated places and one of the most affluent places in the country...
. Consisting of massive to poorly-bedded shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
, mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
, and siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...
, 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
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, and are one of the northernmost mountain ranges in Southern California.-Geography:...
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
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
content, released by radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
Type locality and extent
The type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
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
Rincon Oil Field
The Rincon Oil Field is a large oil field on the coast of southern California, about ten miles northwest of the city of Ventura, and about 20 miles 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...
. 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
Santa Ynez River
The Santa Ynez River is one of the largest rivers on the Central Coast of California. It is long, flowing from east to west through the Santa Ynez Valley, reaching the Pacific Ocean at Surf, near Vandenberg Air Force Base and the city of Lompoc....
. In the subsurface it frequently is found in well cores, underlying the Monterey Formation
Monterey Formation
The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with massive outcroppings of the formation in areas of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands...
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.
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. In the subsurface under the Santa Barbara Channel
Santa Barbara Channel
The Santa Barbara Channel is a portion of the Pacific Ocean which separates the mainland of California from the northern Channel Islands. It is generally south of the city of Santa Barbara, and west of the city of Ventura....
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.
Lithology and deposition environment
The Rincon Formation is massive to poorly-bedded, and consists of predominantly argillaceous to silty shale and mudstone, with occasional dolomiteDolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
. Two layers of siliceous shale are found in the middle of the unit, and they outcrop noticeably in the region west of Goleta. The base of the unit, which rests conformably on the shallower-water Vaqueros Formation, contains a glauconitic layer. Several thin layers of bentonite
Bentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...
occur within the unit, and the unit is capped by one as well, which forms the base of the conformably overlying Monterey Formation
Monterey Formation
The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with massive outcroppings of the formation in areas of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands...
. 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. 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
Cozy Dell Shale
The Cozy Dell Shale is a sedimentary geologic unit of late Eocene age found principally in the Santa Ynez Mountain range north of Santa Barbara, California, from near Point Arguello in the west to central Ventura County in the east...
.
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
North American Plate
The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...
and Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
s, shortly after the Farallon Plate
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...
had been completely buried beneath the American plate.
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.
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.
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
Ellwood Oil Field
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 west of Santa Barbara, largely in the Santa Barbara Channel...
, Gaviota Offshore Gas Field, Cuarta Offshore Oil Field, Molino Gas Field, and others.
Paleontology
Microfossils are common in the Rincon, and have been helpful in dating the unit. Formaniferal remains in particular are abundant, and the various types have allowed staging of the layers into Upper Zemorrian and Lower and Upper Saucesian. Within Santa Barbara County, two significant fossil finds (aside from microfossils) have been made in the Rincon Formation. In Ventura County, the exposure of the formation along Los Sauces Creek has yielded ten different species of ostracodOstracod
Ostracoda is a class of the Crustacea, sometimes known as the seed shrimp because of their appearance. Some 65,000 species have been identified, grouped into several orders....
a; 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
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
.
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. Radon is a byproduct of radioactive decayRadioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
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. 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.
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. Some of the mitigation measures
Radon mitigation
Radon mitigation is any process used to reduce radon concentrations in the breathing zones of occupied buildings.-Testing:ASTM E-2121 is a standard for reducing radon in homes as far as practicable below 4 picocuries per liter in indoor air. Radon test kits are commercially available...
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.