Sespe Formation
Encyclopedia
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 sandstone
s and conglomerate
s laid down in a riverine, shoreline, and floodplain
environment between the upper Eocene
Epoch (around 40 million years ago) to the end of the Oligocene
Epoch (around 24 million years ago). 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.
of the Sespe is along Sespe Creek
, about four miles (6 km) north of Fillmore
, where it was first named in 1897 and redefined in 1924. The Sespe Formation has been divided into three clearly differentiable subunits, designed Lower, Middle, and Upper. The Lower unit has a large proportion of conglomerate, as well as sandstone, and a smaller amount of mudstone and shale. Distinguishing this unit are the clasts within the conglomerate, which originated from a source terrane
with granitic and volcanic rocks, probably in the vicinity of the modern-day Mojave Desert
. The Middle Unit also contains conglomerates and sandstones, but the clasts within the conglomerates include chert
and sandstones of the Franciscan complex
. The Upper member of the Sespe Formation consists of interbedded sandstone, mudstone, and siltstones, without the conglomerates; the depositional environment at this time no longer included the pebbles and boulders which result in conglomerate beds.
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. The sedimentary rocks which resulted from the millions of years of deposition included mudstone
s, siltstone
s, and abundant sandstones and conglomerates. During this time the seashore gradually regressed to the west, as the large mountains eroded and filled the floodplain with alluvium. 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. 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
; in the vicinity of Simi Valley
, the gap corresponded to an erosional event lasting about 8 million years.
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 Dibble – to demarcate the beginning of the Sespe from the end of the Coldwater Sandstone
in the stratigraphic column.
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.
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.
structures, and contain considerable amounts of petroleum
and has produced 400 Moilbbl of oil in Ventura County, California
. For example, both the South Mountain
and West Montalvo Oil Field
s 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.
Sandstone from the Sespe also is used as a building material, and colorful boulders of its sandstones are sometimes used for landscaping.
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s and conglomerate
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...
s laid down in a riverine, shoreline, and floodplain
Floodplain
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...
environment between the upper Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
Epoch (around 40 million years ago) to the end of the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
Epoch (around 24 million years ago). 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.
Type locality and subunits
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 Sespe is along Sespe Creek
Sespe Creek
Sespe Creek is a stream, some long, in Ventura County, California, in the United States. The creek starts at Potrero Seco in the Sierra Madre Mountains, and is formed by more than thirty tributary streams before it empties into the Santa Clara River in Fillmore. Thirty-one miles of Sespe Creek are...
, about four miles (6 km) north of Fillmore
Fillmore, California
Fillmore is a city in Ventura County, California, United States. The population was 15,002 at the 2010 census, up from 13,643 at the 2000 census.- History :...
, where it was first named in 1897 and redefined in 1924. The Sespe Formation has been divided into three clearly differentiable subunits, designed Lower, Middle, and Upper. The Lower unit has a large proportion of conglomerate, as well as sandstone, and a smaller amount of mudstone and shale. Distinguishing this unit are the clasts within the conglomerate, which originated from a source terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
with granitic and volcanic rocks, probably in the vicinity of the modern-day Mojave Desert
Mojave Desert
The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...
. The Middle Unit also contains conglomerates and sandstones, but the clasts within the conglomerates include chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
and sandstones of the Franciscan complex
Franciscan Assemblage
The Franciscan Assemblage is a geological term for an accreted terrane of heterogeneous rocks found on and near the San Francisco Peninsula. It was named by geologist Andrew Lawson who also named the San Andreas Fault which bounds the Franciscan Assemblage....
. The Upper member of the Sespe Formation consists of interbedded sandstone, mudstone, and siltstones, without the conglomerates; the depositional environment at this time no longer included the pebbles and boulders which result in conglomerate beds.
Depositional environment and paleontology
During the Oligocene Epoch, the graniticGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
and volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
mountain ranges in the present-day vicinity of San Diego County eroded over many millions of years, with their alluvium
Alluvium
Alluvium is loose, unconsolidated soil or sediments, eroded, deposited, and reshaped by water in some form in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel...
, ranging in size from microscopic particles to pebbles and even larger rocks, deposited in an extensive coastal floodplain intersected with rivers. The sedimentary rocks which resulted from the millions of years of deposition included 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...
s, 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...
s, and abundant sandstones and conglomerates. During this time the seashore gradually regressed to the west, as the large mountains eroded and filled the floodplain with alluvium. 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. A peculiarity of the Sespe Formation is the presence of an unconformity
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
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
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
; in the vicinity of Simi Valley
Simi Valley
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 & 118 freeway, and in the west the narrows of the Arroyo Simi and 118...
, the gap corresponded to an erosional event lasting about 8 million years.
The redbeds found in the Sespe are similar to those characteristic of the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
period, and the sudden occurrence of redbeds have been used – for example by Thomas Dibble – to demarcate the beginning of the Sespe from the end of the Coldwater Sandstone
Coldwater Sandstone
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...
in the stratigraphic column.
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.
Numerous vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
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.
Economic importance
In some places, certain high-porosity sandstones within the Sespe unit have been deformed into anticlinalAnticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
structures, and contain considerable amounts of petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
and has produced 400 Moilbbl of oil in Ventura County, California
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...
. For example, both the South Mountain
South Mountain Oil Field
The South Mountain Oil Field is a large and productive oil field in Ventura County, California, in the United States, in and adjacent to the city of Santa Paula. Discovered in 1916, and having a cumulative production of over of oil, it is the 37th largest oil field in California and the second...
and West Montalvo Oil Field
West Montalvo Oil Field
The West Montalvo Oil Field is a large and productive oil field on the coast of Ventura County, California, in the United States, in and adjacent to the city of Oxnard. Discovered in 1947, it has produced approximately of oil, and retains about 2.2 million recoverable with current technology, in...
s 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
Rincon Formation
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...
, a shale deposited in a deep-sea environment, serves as an impermeable cap.
Sandstone from the Sespe also is used as a building material, and colorful boulders of its sandstones are sometimes used for landscaping.
See also
- Vaqueros Formation
- Monterey FormationMonterey FormationThe 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...
- Coldwater Formation
- Rincon FormationRincon FormationThe 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...