Vaqueros sandstone
Encyclopedia
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.
of the Vaqueros is from Vaqueros Canyon in the Santa Lucia Mountains
, 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 sandstone unit consists of well-sorted grains, averaging medium-size, typically quartz
and feldspar
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.
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
containing the Sespe in the late Oligocene
age, between 26 to 28 Ma (million years before present) to 24 to 25 Ma. 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
, consists of deepwater shales.
The Vaqueros weathers to a clayey soil which supports chaparral
, and on the southern slopes of the Santa Ynez Mountains
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, nearer the coast, abruptly changes to dense chaparral on the mountainside.
epoch, strata
from Simi Valley
have sampled in the upper Oligocene period.
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
s in Santa Barbara County, and the Kettleman North Dome
and Coalinga Oil Field
s in the Central Valley. When grouped with the underlying Sespe Formation
, because of its high porosity and the presence of an impermeable cap in the overlying Rincon Formation
, it is the second-most important producing horizon in Southern California.
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...
and Lower 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...
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 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 Vaqueros is from Vaqueros Canyon in the Santa Lucia Mountains
Santa Lucia Mountains
The Santa Lucia Mountains or Santa Lucia Range is a mountain range in coastal California, running from Monterey southeast for 105 miles to San Luis Obispo. The highest summit is Junipero Serra Peak, in Monterey County...
, about eight miles southwest of Greenfield
Greenfield, California
Greenfield is a city in Monterey County, California, United States. Greenfield is located southeast of Salinas, at an elevation of 289 feet . The city was the fastest growing in the county during the 2000s, the population was 12,583 in 2000, increasing to 16,330 in the 2010 census. Its most...
. The formation was first described by Homer Hamlin in 1904, as part of a report on the water resources of the Salinas Valley
Salinas Valley
The Salinas Valley lies south of San Francisco, California.The word "salina" is spanish for salt marsh, salt lake or salt pan.-Geography:The Salinas Valley runs approximately south-east from Salinas towards King City. The valley lends its name to the geologic province in which it's located, the...
.
The sandstone unit consists of well-sorted grains, averaging medium-size, typically quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
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.
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
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...
containing the Sespe in the late 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...
age, between 26 to 28 Ma (million years before present) to 24 to 25 Ma. 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
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...
, consists of deepwater shales.
The Vaqueros weathers to a clayey soil which supports chaparral
Chaparral
Chaparral is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico...
, and on the southern 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:...
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, 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 MioceneMiocene
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...
epoch, strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
from 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...
have sampled in the upper Oligocene period.
As a petroleum-bearing unit
In some places, the Vaqueros has 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, or pinched out into structural traps, allowing petroleum to become trapped in economically recoverable quantities. Some locations where this has occurred include the Ellwood
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...
and Mesa Oil Field
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 accumulation of oil in a single geologic formation...
s in Santa Barbara County, and the Kettleman North Dome
Kettleman North Dome Oil Field
The Kettleman North Dome Oil Field is a large oil and gas field in Kings and Fresno Counties, California. Discovered in 1928, it is fifteenth largest field in the state by total ultimate oil recovery, and of the top twenty oil fields it is the closest to exhaustion, with less than one-half of one...
and Coalinga Oil Field
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 half-way between Los Angeles and San Francisco, to the west of Interstate 5, at the foot of the Diablo Range...
s in the Central Valley. When grouped with the underlying Sespe Formation
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, shoreline, and floodplain environment between the...
, because of its high porosity and the presence of an impermeable cap in 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...
, it is the second-most important producing horizon in Southern California.