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 of
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
in the
Paleogene Period
The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
, found in Santa Barbara and Ventura Counties in Southern California.
It consists of thick layers of
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
, made up of grains of
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 ...
and
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 ...
from a granitic source rock, interbedded with thin layers of
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...
and
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
. Hard, massive, and exceptionally resistant to weathering, it forms the high rocky summits of the Santa Ynez Mountain range north of Santa Barbara, eastward into Ventura County.
Type locality, description, and distribution
The type locality of the unit is at Matilija Hot Springs, along the
Ventura River
The Ventura River, in western Ventura County in southern California, United States, flows from its headwaters to the Pacific Ocean. The smallest of the three major rivers in Ventura County, it flows through the steeply sloped, narrow Ventura V ...
about northwest of Ojai, near to California State Route 33. At its type locality it is around thick, and consists of both marine and non-marine sandstones with occasional thin beds of micaceous shale separating massive sandstone layers.Donald R. Prothero and Justin R. Britt. "Magnetic stratigraphy and tectonic rotation of the Middle Eocene Matilija Sandstone and Cozy Dell Shale, Ventura County, California: implications for sequence stratigraphic correlations." ''Earth and Planetary Science Letters'', Volume 163, Issues 1-4, November 1998, Pages 261-273. The sandstone layers are made up of well-sorted grains of quartz and feldspar. The unit can be found along the crest of the Santa Ynez range all the way from the western extremity of the mountains near Point Arguello, to its type locality north of Ojai, and east and northeast into the Ventura County backcountry, where Piru and Sespe creeks cross through the formation. It accounts for the highest peaks in the Santa Ynez range, dipping underneath the younger Coldwater Sandstone,
Cozy Dell Shale
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 ...
, and Sespe Formation near San Marcos Pass in the center of the range.
The thickness of the unit is widely variable, generally decreasing to the west. In its type locality it is around thick; in the vicinity of La Cumbre Peak it is thick; and while it thins to only thick underneath San Marcos Pass, it thickens again to around at the high summits of Santa Ynez and Broadcast Peaks. It thins westward from there, being only thick at Refugio Pass, and less than 300 in the Santa Rosa Hills and west to Point Conception.Dibblee, Thomas. ''Geology of Southwestern Santa Barbara County, California''. Bulletin 150, California Division of Mines and Geology. San Francisco, 1950. 95 p.
Along with the younger Coldwater Formation, the Matilija is the source of the enormous sandstone boulders which are found abundantly along the creeks and shoreline of Santa Barbara and Goleta. Additionally, boulders are strewn on hillsides in the upper Riviera, and elsewhere on hillsides and floodplains. These boulders, none of which can be moved even in the most intense modern-day flood events, tumbled down the mountains during Pleistocene-age storms and mudflows of unimaginable intensity. Boulders of the Matilija were included in the mud and debris flows in the January 2018 Montecito mudflows.
The Matilija Sandstone produces little soil when it weathers, and generally supports only hard
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 ...
on slopes that are not bare rock. North-facing slopes have some stands of pine and fir in the higher elevations.
The formation appears in boreholes drilled into oil fields offshore, indicating the formation dips steeply underneath the Santa Barbara Channel. At the Point Conception Oil Field, the Matilija is at least thick, and first appears approximately below ground surface.California Department of Conservation, Division of Oil, Gas, and Geothermal Resources (DOGGR). ''California Oil and Gas Fields, Volumes I, II and III''. Vol. I (1998), Vol. II (1992), Vol. III (1982). PDF file available on CD from www.consrv.ca.gov. p. 674-675 In the Molino Offshore Gas Field, southeast of Gaviota about offshore, the formation is at an average of below the seafloor, and a well drilled to had not reached the bottom of the formation.
Deposition environment and tectonic history
Sometime in the early Eocene, around 50 million years ago, the landmass containing present-day Santa Barbara County became submerged, allowing sediment deposition offshore. During this time the sea alternately deepened and became shallow again, with the deposition environment supporting either sandstones or shales corresponding to shallow or deep conditions. The Matilija Sandstone dates from the earliest sustained shallow-water episode during this time, the period from 48.5 to 46 million years ago. Additionally, the piece of the crustal block on which the sandstone was deposited has rotated approximately 90 degrees clockwise since the time of deposition, moving from a location approximately adjacent to present-day San Diego to the region it occupies in the present day.
After reaching its current position along the coast, the entire Santa Ynez Range was uplifted, mostly during the
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
period and late
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Pacific and
North American Plate
The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...
s. The Matilija Sandstone and other units, formerly submerged, rose over from their original positions. In some places the forces causing this uplift were so abrupt that sedimentary layers deposited as recently as the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
are now angled 60 degrees from horizontal.
Paleontology
While the formation is rich in microfossils, it is considered to be of low paleontologic sensitivity, i.e. larger fossils are unlikely to be encountered. In the Santa Rosa Hills, the unit has yielded numerous shells of
mollusc
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
s, including
bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
s and
gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
s.
Economic importance
As a potential reservoir for oil and gas deposits, the Matilija Sandstone is grouped with the above-lying
Cozy Dell Shale
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 ...
and Coldwater Sandstone as the ''Gaviota-Sacate-Matilija Sandstone Play''. The likely source rocks for any petroleum accumulation in the Gaviota-Sacate-Matilija Play are the lower-lying organic-rich shale units, such as the
Juncal
Juncal is a civil parish in the municipality of Porto de Mós
Porto de Mós () is a town and a municipality of Estremadura province in Leiria District. It is in the Centro Region and the Pinhal Litoral subregion. The population in 2011 was 24, ...
-
Anita Formation
Anita or ANITA may refer to:
Arts
* ''Anita'' (1967 film), an Indian film
* ''Anita'' (2009 film), an Argentine film
* ''Anita'' (2021 film), a Hong Kong film
*'' Anita: Swedish Nymphet'', a 1973 erotic film
People
*Anita (given name), people w ...
s.James M. Galloway. "Santa Barbara-Ventura Basin Province." (BOEMRE) Structure and Petroleum Geology, Santa Barbara Channel, California, 1998 Pages 63-72, Overview and Issues Section, Pacific Section of AAPG, 97, 102, 112-114.
Since this deep-lying play is incompletely explored, with many boreholes not even reaching the Matilija, the estimates for hydrocarbon resources are speculative, with the United States Bureau of Ocean Energy Management, Regulation and Enforcement giving a range of of oil recoverable using current technology for the entire Santa Barbara-Ventura Basin Province. In general, in the eastern and central parts of the basin, the sandstone would be too deeply buried to have reservoir potential. However, the area potentially containing oil and gas is large, including approximately within the Federal Outer Continental Shelf. The largest single hydrocarbon accumulation yet identified in the Matilija Sandstone is in the Molino Offshore Gas Field, discovered in 1983, and contains approximately of oil equivalent.