Sisquoc Formation
Encyclopedia
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). 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 –. 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.
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.
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 along the beaches 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.
; 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.
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.
s, arenaceous foraminifers, and remains of sponges.
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.
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 syncline
s 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.
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.
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
geologic unit widespread in Southern California
Southern California
Southern California is a megaregion, or megapolitan area, in the southern area of the U.S. state of California. Large urban areas include Greater Los Angeles and Greater San Diego. The urban area stretches along the coast from Ventura through the Southland and Inland Empire to San Diego...
, both on the coast and in mountains near the coast. Overlying 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...
, it is of upper Miocene
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...
and lower Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
age (from about 4 to 6 million years old). The formation consists of claystone
Claystone
Claystone is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles ....
, 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...
, 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...
, 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...
, diatomite, and conglomerates
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
, with considerable regional variation, and was deposited in a moderately deep marine environment at a depth of approximately 500 –. 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
Diatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...
. France-based Imerys
Imerys
Imerys is a French multinational company. It is a constituent of the CAC Mid 60 index.-History:The Company was founded in 1880 and for many years was known as Imetal....
operates a mine in the Sisquoc and Monterey Formations in the hills south of Lompoc, California
Lompoc, California
Lompoc is a city in Santa Barbara County, California, United States. The city was incorporated in 1888. The population was 42,434 at the 2010 census, up from 41,103 at the 2000 census....
, the largest such operation in the world.
Type locality, description, and occurrence
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....
for the Sisquoc Formation is along the Sisquoc River
Sisquoc River
The Sisquoc River is a river in northeastern Santa Barbara County, California. It is a tributary of the Santa Maria River, which is formed when the Sisquoc River meets the Cuyama River. The river is long and originates on the north slopes of Big Pine Mountain, at approximately...
, 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
Lompoc Oil Field
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...
, 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.
The Sisquoc Formation is not as resistant to erosion as other formations in the stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
sequence in coastal California, and south 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:...
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 along the beaches from Santa Barbara
Santa Barbara, California
Santa Barbara is the county seat of Santa Barbara County, California, United States. Situated on an east-west trending section of coastline, the longest such section on the West Coast of the United States, the city lies between the steeply-rising Santa Ynez Mountains and the Pacific Ocean...
west to Gaviota. In the Santa Maria Basin – the area surrounding the city of Santa Maria
Santa Maria, California
Santa Maria is a city in Santa Barbara County, on the Central Coast of California. The 2010 census population was 100,062, putting it ahead of Santa Barbara for the first time and making it the largest city in the county...
and the hills to the south and southwest – well-weathered outcrops of the formation are often bleached white.
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 subsideSubsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
; 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
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....
was deposited as a fine mud, rich with diatoms. Tests
Test (biology)
A test is a term used to refer to the shell of sea urchins, and also the shell of certain microorganisms, such as testate foraminifera and testate amoebae....
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. 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 radiolarianRadiolarian
Radiolarians are amoeboid protozoa that produce intricate mineral skeletons, typically with a central capsule dividing the cell into inner and outer portions, called endoplasm and ectoplasm. They are found as zooplankton throughout the ocean, and their skeletal remains cover large portions of the...
s, arenaceous foraminifers, and remains of sponges.
Economic importance
The Sisquoc Formation is important as a source of diatomaceous earthDiatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...
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
Lompoc Oil Field
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...
, it serves as an impermeable cap rock to an oil-bearing unit, in that case in 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...
; in other cases, such as in the Casmalia Oil Field and the offshore portion of 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...
, it is an oil-bearing unit in its own right. In the Casmalia field, west of Santa Maria
Santa Maria, California
Santa Maria is a city in Santa Barbara County, on the Central Coast of California. The 2010 census population was 100,062, putting it ahead of Santa Barbara for the first time and making it the largest city in the county...
, 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 syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
s with high enough temperature and sufficient time (several million years) to cause the kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
from the decomposing organic matter to be matured, via hydrocarbon cracking
Cracking (chemistry)
In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products...
, 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
Diatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...
. The world's largest such deposit, operated by Imerys
Imerys
Imerys is a French multinational company. It is a constituent of the CAC Mid 60 index.-History:The Company was founded in 1880 and for many years was known as Imetal....
, is in the Sisquoc and Monterey formations in the hills south of Lompoc, California
Lompoc, California
Lompoc is a city in Santa Barbara County, California, United States. The city was incorporated in 1888. The population was 42,434 at the 2010 census, up from 41,103 at the 2000 census....
; the USGS
United States Geological Survey
The United States 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, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
estimates that the Lompoc mine alone with its large reserves could meet the needs of the entire world for centuries.