Sespe Formation | |
---|---|
Stratigraphic range: Late Eocene-Early Miocene | |
Type | sedimentary |
Underlies | Vaqueros Formation, Monterey Formation |
Overlies | Coldwater Formation ("Coldwater Sandstone", "Coldwater Shale") |
Thickness | 0–7,500 ft (0–2,286 m) [1] |
Lithology | |
Primary | Sandstone, conglomerate |
Other | Mudstone, occasional shale |
Location | |
Region | Southern and south central California |
Country | United States |
Type section | |
Named for | Sespe Creek |
Named by | Watts (1897); redefined by Kew (1924) [2] |
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 (around 40 million years ago) through the lower Miocene. [3] [4] 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. [5] [6]
The type locality of the Sespe is along Sespe Creek in the Topatopa Mountains, about four miles (6 km) north of Fillmore, where it was first named in 1897 and redefined in 1924. [7]
The Sespe Formation has been divided into three clearly differentiable subunits, designed Lower, Middle, and Upper.
During the Oligocene Epoch, the granitic and volcanic mountain ranges in the present-day vicinity of San Diego County eroded over many millions of years, with their alluvium, ranging in size from microscopic particles to pebbles and even larger rocks, deposited in an extensive coastal floodplain intersected with rivers. [9] The sedimentary rocks which resulted from the millions of years of deposition included mudstones, siltstones, and abundant sandstones and conglomerates. [10] During this time the seashore gradually regressed to the west, as the large mountains eroded and filled the floodplain with alluvium. [10] [11] Over time the depositional environment changed from continental to marine, due to changes in surface elevation and sea level; the marine sedimentary formation equivalent to the Sespe is known as the Alegria Formation, and is more often found to the west, for example in southwestern Santa Barbara County. [12] A peculiarity of the Sespe Formation is the presence of an unconformity throughout much of the geographic distribution of the unit, representing a gap of millions of years and including most or all of the early Oligocene; [13] in the vicinity of Simi Valley, the gap corresponded to an erosional event lasting about 8 million years. [14]
The redbeds found in the Sespe are similar to those characteristic of the Permian period, and the sudden occurrence of redbeds have been used – for example by Thomas Dibblee – to demarcate the beginning of the Sespe from the end of the Coldwater Sandstone in the stratigraphic column. [15] [16]
In the Oligocene the land subsided, creating a shallow, warm marine environment. Sediments continued to accrete, but now they were mixed with the shells of near-shore creatures, similar to those in modern shallow tropical seas. The geologic unit which resulted from deposition in this environment, and which follows the Sespe in lithologic sequence, is named the Vaqueros Formation. [13] [17]
Numerous vertebrate fossils have been found in the Sespe, with the principal locations of the finds north of Simi Valley in Ventura County. A few of the many species associated with the Sespe include Amynodontopsis (an Eocene rhinoceros), Simimys, a rodent, and Sespedectes singularis. [18]
In some places, certain high-porosity sandstones within the Sespe unit have been deformed into anticlinal structures, and contain considerable amounts of petroleum and has produced 400 million barrels (64,000,000 m3) of oil in Ventura County, California. [19] For example, both the South Mountain and West Montalvo Oil Fields have producing horizons within the Sespe, and as such the unit is of economic importance. Within Southern California, the combined Sespe-Vaqueros coarse-grained clastic rocks form the second-most important petroleum-bearing unit, second only to the Pliocene-age Pico and Repetto Formations, which are usually much nearer the surface. The overlying Rincon Formation, a shale deposited in a deep-sea environment, serves as an impermeable cap. [20] [21] [22]
The Sespe Formation contains Uranium deposits of potential economic value. [23]
Sandstone from the Sespe also is used as a building material, and colorful boulders of its sandstones are sometimes used for landscaping. [24]
The Florissant Formation is a sedimentary geologic formation outcropping around Florissant, Teller County, Colorado. The formation is noted for the abundant and exceptionally preserved insect and plant fossils that are found in the mudstones and shales. Based on argon radiometric dating, the formation is Eocene in age and has been interpreted as a lake environment. The fossils have been preserved because of the interaction of the volcanic ash from the nearby Thirtynine Mile volcanic field with diatoms in the lake, causing a diatom bloom. As the diatoms fell to the bottom of the lake, any plants or animals that had recently died were preserved by the diatom falls. Fine layers of clays and muds interspersed with layers of ash form "paper shales" holding beautifully-preserved fossils. The Florissant Fossil Beds National Monument is a national monument established to preserve and study the geology and history of the area.
The Los Angeles Basin is a sedimentary basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an anomalous group of east-west trending chains of mountains collectively known as the Transverse Ranges. The present basin is a coastal lowland area, whose floor is marked by elongate low ridges and groups of hills that is located on the edge of the Pacific Plate. The Los Angeles Basin, along with the Santa Barbara Channel, the Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, lies within the greater Southern California region. The majority of the jurisdictional land area of the city of Los Angeles physically lies within this basin.
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.
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 connection to Moorpark. 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.
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 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 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 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 Himalayan foreland basin is an active collisional foreland basin system in South Asia. Uplift and loading of the Eurasian Plate on to the Indian Plate resulted in the flexure (bending) of the Indian Plate, and the creation of a depression adjacent to the Himalayan mountain belt. This depression was filled with sediment eroded from the Himalaya, that lithified and produced a sedimentary basin ~3 to >7 km deep. The foreland basin spans approximately 2,000 kilometres (1,200 mi) in length and 450 kilometres (280 mi) in width. From west to east the foreland basin stretches across five countries: Pakistan, India, Nepal, Bangladesh, and Bhutan.
The geology of Morocco formed beginning up to two billion years ago, in the Paleoproterozoic and potentially even earlier. It was affected by the Pan-African orogeny, although the later Hercynian orogeny produced fewer changes and left the Maseta Domain, a large area of remnant Paleozoic massifs. During the Paleozoic, extensive sedimentary deposits preserved marine fossils. Throughout the Mesozoic, the rifting apart of Pangaea to form the Atlantic Ocean created basins and fault blocks, which were blanketed in terrestrial and marine sediments—particularly as a major marine transgression flooded much of the region. In the Cenozoic, a microcontinent covered in sedimentary rocks from the Triassic and Cretaceous collided with northern Morocco, forming the Rif region. Morocco has extensive phosphate and salt reserves, as well as resources such as lead, zinc, copper and silver.
The geology of Utah, in the western United States, includes rocks formed at the edge of the proto-North American continent during the Precambrian. A shallow marine sedimentary environment covered the region for much of the Paleozoic and Mesozoic, followed by dryland conditions, volcanism, and the formation of the basin and range terrain in the Cenozoic.
The geology of Montana includes thick sequences of Paleozoic, Mesozoic and Cenozoic sedimentary rocks overlying ancient Archean and Proterozoic crystalline basement rock. Eastern Montana has considerable oil and gas resources, while the uplifted Rocky Mountains in the west, which resulted from the Laramide orogeny and other tectonic events have locations with metal ore.
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.
The Vasquez Formation (Tvz) is a geologic formation cropping out at the eponymous Vasquez Rocks in southern California. The formation dates to the Late Oligocene to Early Miocene.
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