The Vaqueros Formation is a

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

geologic unit primarily of Upper

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

and Lower

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

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.

Type locality and deposition environment

The type locality of the Vaqueros is from Vaqueros Canyon in the

Santa Lucia Mountains The Santa Lucia Mountains (sæntə luˈsiːə) or Santa Lucia Range is a rugged mountain range in coastal central California, running from Carmel southeast for to the Cuyama River in San Luis Obispo County. The range is never more than from t ...

, about eight miles southwest of Greenfield. The formation was first described by Homer Hamlin in 1904, as part of a report on the water resources of the

Salinas Valley The Salinas Valley is one of the major valleys and most productive Agriculture, agricultural regions in California. It is located west of the San Joaquin Valley and south of San Francisco Bay and the Santa Clara Valley. The Salinas River (Califo ...

. The sandstone unit consists of well-sorted grains, averaging medium-size, typically

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

and

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

with some black flecks, and in form it ranges from cross-bedded to massive and thick-bedded. Occasionally it contains pebbles, especially near its base where it sits on the red non-marine Sespe Formation. Some fossils – including mollusks and barnacles – can be found in the Vaqueros, also near the base of the unit where the depositional environment was nearest shore.Dibblee (1966) 42Minor et al., map legend The unit was deposited by runoff from highlands to the east into a shallow, warm marine environment, as the ocean transgressed on the subsiding

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

containing the Sespe in the late

age, between 26 and 28 Ma (million years before present) to 24 to 25 Ma.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. p. 46 As the land continued to subside, the ocean depth increased with a corresponding drop in grain size in higher strata. The topmost part of the Vaqueros contains interbedded mudstones, silstones, and fine-grained sandstones, representing this shift. The unit above the Vaqueros, the

Rincon Formation 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, ...

, consists of deepwater shales. The Vaqueros weathers to a clayey soil which supports

chaparral Chaparral ( ) is a shrubland plant community and geographical feature found primarily in the U.S. state of California, in southern Oregon, and in the northern portion of the Baja California Peninsula in Mexico. It is shaped by a Mediterranean c ...

, and on the southern slopes of the

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. It is the westernmost range in the Transverse Ranges. The range is a large fault block of Cenozoic age created ...

in southern Santa Barbara County, its contact with the Rincon Formation is easily visible for it correlates closely to the line where the grassland or

coastal sage scrub Coastal sage scrub, also known as coastal scrub, CSS, or soft chaparral, is a low scrubland plant community of the California coastal sage and chaparral subecoregion, found in coastal California and northwestern coastal Baja California. It is w ...

, nearer the coast, abruptly changes to dense chaparral on the mountainside.

Paleontology

Fossils found in the Vaqueros are mostly near-shore marine organisms, such as mollusks, scallops, and oysters (''Turritella'' sp., ''Pecten'' sp., ''Ostrea'' sp.) While the molluscan stage is hard to date and ranges from the

epoch,

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

from

Simi Valley Simi Valley (; Chumash: ''Shimiyi'') is a city in the valley of the same name in the southeast region of Ventura County, California, United States. Simi Valley is from Downtown Los Angeles, making it part of the Greater Los Angeles Area. The ...

have sampled in the upper Oligocene period.

Mammals

As a petroleum-bearing unit

In some places, the Vaqueros has been deformed into

anticlinal Anticlinal may refer to: *Anticline, in structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. *Anticlinal, in stereochemistry, a torsion angle between 90° to 150°, and –90° to –150°; see Alkane_st ...

structures, or pinched out into structural traps, allowing petroleum to become trapped in economically recoverable quantities. Some locations where this has occurred include the Ellwood and

Mesa 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 accumulat ...

s in Santa Barbara County, and the Kettleman North Dome and

Coalinga Oil Field The Coalinga Oil Field is a large oil field in western Fresno County, California, in the United States. It surrounds the town of Coalinga, about halfway between Los Angeles and San Francisco, to the west of Interstate 5, at the foot of the Diablo ...

s in the Central Valley. When grouped with the underlying

Sespe Formation 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 ...

, because of its high porosity and the presence of an impermeable cap in the overlying

, it is the second-most important producing horizon in Southern California.James M. Galloway. "Santa Barbara-Ventura Basin Province."
100.

Notes

References

{{commons category, Vaqueros Formation * C. Michael Hogan, Leda Patmore, David Crimp et al., ''San Lorenzo Basin Groundwater Recharge and Water Quality Study'', Earth Metrics Incorporated,

Association of Monterey Bay Area Governments The Association of Monterey Bay Area Governments (AMBAG) is a regional governmental organization which consists of representation of a large number of public agencies within Monterey County, Santa Cruz County and San Benito County, California. AMB ...

, July 7, 1978 * United States Geological Survey. 1921. ''Bulletin, Volume 721'', US Government Printing Office, Washington DC Geologic formations of California Miocene California Paleogene California Geology of Fresno County, California Geology of Monterey County, California Geology of San Luis Obispo County, California Geology of Santa Barbara County, California Geology of Ventura County, California Geography of the San Joaquin Valley Santa Lucia Range Santa Ynez Mountains Transverse Ranges