![Los Angeles basin](https://upload.wikimedia.org/wikipedia/commons/d/d2/Los_Angeles_Basin.jpg)
The Los Angeles Basin is a sedimentary
basin located in
Southern California
Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...
, 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 Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa ...
. 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 Santa Barbara Channel is a portion of the Southern California Bight and separates the mainland of California from the northern Channel Islands. It is generally south of the city of Santa Barbara, and west of the Oxnard Plain in Ventura Cou ...
, the
Ventura Basin, the
San Fernando Valley, and the
San Gabriel Basin, lies within the greater southern California region.
On the north, northeast, and east, the lowland
basin is bound by the
Santa Monica Mountains
The Santa Monica Mountains is a coastal mountain range in Southern California, next to the Pacific Ocean. It is part of the Transverse Ranges. Because of its proximity to densely populated regions, it is one of the most visited natural areas in ...
and Puente, Elysian, and Repetto hills.
To the southeast, the basin is bordered by the
Santa Ana Mountains
The Santa Ana Mountains are a short peninsular mountain range along the coast of Southern California in the United States. They extend for approximately southeast of the Los Angeles Basin largely along the border between Orange and Riverside c ...
and the
San Joaquin Hills
The San Joaquin Hills are a low mountain range of the Peninsular Ranges System, located in coastal Orange County, California.
They extend in a northwest–southeast direction, starting in the northwest in Newport Beach at the southern edge of t ...
.
The western boundary of the basin is marked by the Continental Borderland and is part of the onshore portion. The California borderland is characterized by northwest trending offshore ridges and basins.
The Los Angeles Basin is notable for its great structural relief and complexity in relation to its geologic youth and small size for its prolific oil production.
Yerkes et al. identify five major stages of the basin's evolution, which began in the
Upper Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', th ...
and ended in the
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological 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 finally confirmed in ...
. This basin can be classified as an irregular pull-apart basin accompanied by rotational tectonics during the post-
early Miocene
The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages.
The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...
.
Basin development
Before the formation of the basin, the area that encompasses the Los Angeles basin began above ground. A rapid transgression and regression of the shoreline moved the area to a shallow marine environment. Tectonic instability coupled with volcanic activity in rapidly subsiding areas during the Middle Miocene set the stage for the modern basin.
The basin formed in a submarine environment and was later brought back above sea level when the rate of
subsidence slowed. There is much discussion in the literature about the geologic time boundaries when each basin forming event took place. While exact ages may not be clear, Yerkes et al. (1965) provided a general timeline to categorize the sequence of depositional events in the LA Basin's evolution and they are as follows:
Phase 1: Pre-extension
During pre-
Turonian
The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded b ...
, metamorphosed sedimentary and volcanic rocks are present that serve as the two major basement rock units for the LA Basin. Large-scale movement along the Newport–Inglewood zone juxtaposed the two bedrock units along the east and west margins.
During this phase, the basin was above sea level.
Phase 2: Pre-basin phase of deposition
The hallmarks of this phase were successive shoreline transgression and regression cycles. Deposition of older marine and non-marine sediments began to fill the basin. Towards the end of this phase, the shoreline began to retreat and deposition continued.
Phase 3: Basin inception
After the deposition of the pre-Turonian units, there was a large emergence and erosion that can be observed as a major unconformity at the base of the middle Miocene units.
Emergence did not occur at the same rate or in all sections of the basin. During this time, the basin was covered by a marine embayment. Rivers sourced in the highlands brought large amounts of detritus to the northeastern edge of the basin.
During this period, the Topanga formation was also being deposited.
Phase 4: Principal phase of subsidence and deposition
The present form and structural relief of the basin was largely established during this phase of accelerated subsidence and deposition which occurred during the late Miocene and continued through the early Pleistocene.
Clastic sedimentary rocks from the highland areas (to the north and east) moved down the submarine slopes and infilled the basin floor. Subsidence and sedimentation most likely began in the southern portion basin.
Subsidence and Deposition occurred simultaneously, without interruption, until the late Pliocene. Until the rate of deposition gradually overtook the rate of subsidence, and the sea level began to fall. Towards the end of this phase, the margins of the basin began to rise above sea level. During the early Pleistocene, deposition began to outpace subsidence in the depressed parts of the basin and the shoreline began to move southward.
This phase also had movement along the Newport–Inglewood fault zone that resulted in the initiation of the modern basin. This movement caused the southwestern block to be uplifted relative to the central basin block.
Phase 5: Basin disruption
The central part of the basin continued to experience sediment deposition through the Pleistocene from flooding and erosional debris from the surrounding mountains and Puente Hills. This infill was responsible for the final retreat of the shoreline from the basin. Deposition in the Holocene is characterized by non marine gravel, sand and silt.
This phase also includes the late stage compressional deformation responsible for the formation of the hydrocarbon traps.
Basin blocks
Four major faults are present in the region and divide the basin in the central, northwest, southwest, and northeast structural blocks.
These blocks not only denote their geographic location, but they indicate the strata present and major structural features. The southwestern block was uplifted prior to the middle Miocene and is composed mostly of marine strata and contains two major anticlines.
This block also contains the steeply-dipping Palos Verdes Hills fault zone. The middle Miocene volcanics can be seen locally within the southwest block.
The northwestern block consists of clastic marine sediments of Late Cretaceous to Pleistocene age. Middle Miocene volcanics are also present. This block has a broad anticline that is truncated by the Santa Monica fault zone. The central block contains both marine and non-marine clastic rock units interbedded with volcanic rocks that are late Cretaceous to Pliocene in age. Pliocene and Quaternary strata are most visible within the central block. Structurally, there is a synclinal trough.
The northeastern block contains fine to coarse grained clastic marine rocks of Cenozoic age.
Locally, middle Miocene volcanics can be seen as well as Eocene to Miocene aged non-marine sedimentary rocks. There is also an anticline in the northeastern block.
Basin stratigraphy
Homogeneous evolution of this basin did not occur due to dynamic tectonic activity. Despite the active setting, there are over 9,100 m of strata within the basin.
The dynamic setting was also responsible for the heterogeneous deposition of each formation. It is common for rock units of the same depositional event to have different names in different locations within the basin. This may be a result of large variation in clast size as with the upper
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Pico Formation
The Pico Formation is a Pliocene epoch stratigraphic unit and geologic formation in the greater Los Angeles Basin, the Santa Monica Mountains, and the Santa Susana Mountains, in Los Angeles County of Southern California.
Geology
It was formed du ...](_blank)
in the northwestern part of the basin and the Upper
Fernando Formation
The Fernando Formation is a Plio-Pleistocene marine mudstone, siltstone and sandstone formation in the greater Los Angeles Basin, Ventura Basin, and Santa Monica Mountains, in Los Angeles County of Southern California.
Geology
Outcrops of the ...
in the southwest part of the basin.
The Los Angeles Basin contains what is known as the "
Great Unconformity
Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland,Rance, H (1999''Historical Geology: The ...
" which has been interpreted as a large scale erosional event in the basement rock unit. This unconformity is used to correlate strata throughout the basin. The record of the
Cenozoic activity begins above this unconformity.
The stratigraphic record for this basin indicates that it began as a non-marine environment and then transgressed to a deep ocean system. The oldest basement units of this basin are of both
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
ary and
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
origin. The sedimentary unit was
metamorphosed
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
as a result of slippage of the
Newport–Inglewood fault
The Newport–Inglewood Fault is a right-lateral strike-slip fault in Southern California. The fault extends for from Culver City southeast through Inglewood and other coastal communities to Newport Beach at which point the fault extends east- ...
and is known as the
Catalina Schist
The Catalina Schist is a metamorphic rock complex primarily exposed on Santa Catalina Island of the Channel Islands of California.
It formed during the Cretaceous Period of the Mesozoic Era.
Geology
The Catalina Schist is broadly correlated wit ...
. The Catalina Schist can be found on the southwestern edge of the basin and is predominantly a
chlorite-quartz schist. Closer to the Newport–Inglewood fault zone,
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
-bearing schists and
metagabbros occur.
The
Santa Monica Slate can be observed in the northwestern block of the basin. The eastern complex is characterized by
Santiago Peak Volcanics. This rock unit contains
andesitic
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...
s, flow,
agglomerates and
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...
s.
The
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 ...
is the first to appear above the "Great Unconformity" and is marked by interbedded mudstones, sandstones and pebbly sandstones. This bed sequence indicates an alluvial fan, meandering stream or braided stream origin.
Upward from the Sespe Formation toward the
Vaqueros
The ''vaquero'' (; pt, vaqueiro, , ) is a horse-mounted livestock herder of a tradition that has its roots in the Iberian Peninsula and extensively developed in Mexico from a methodology brought to Latin America from Spain. The vaquero became t ...
, the grains become finer and the beds become thinner; indicating a transition to a shallow marine environment. The Vaqueros Formation is marked by two sandstone, siltstone and shale units. There are also characteristic mollusk fossils that indicate the area was dominately shallow marine.
The
Topanga Group is the next major formation in the stratigraphic sequence and infills the topography on older rocks.
It is a mixed sedimentary and volcanic unit whose base is an erosional unconformity.
The unit consists of 3 parts: First is a basal marine conglomeratic sandstone, followed by a dominantly
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
ic middle layer of multiple
submarine lava flows and tuffs. The youngest part of this unit is a sedimentary breccia, conglomerate, sandstone, and a
siltstone. The earliest deposits of the Topanga Group appear to reflect the continuation of a shift in shoreline that can be seen in both the Sespe and Vaqueros formations.
Eruptions from one or more of volcanic centers locally and temporarily interrupted sedimentation.
The
Puente Formation
The Puente Formation is a geologic formation in California. It preserves fossils dating back to the Neogene period.
See also
* List of fossiliferous stratigraphic units in California
* Paleontology in California
Paleontology in California r ...
is a deep-marine formation that is characterized by pro-delta sediments and an overlapping fan system.
This unit lies above the Topanga Group giving it a Late Miocene depositional age and is divided into four members. The
La Vida Member is a micaceous, platy siltstone with subordinate amounts of thin-bedded feldspathic sandstone. The next member is the
Soquel
Soquel (; Ohlone: ''Sokel'') is an unincorporated town and census-designated place (CDP) in Santa Cruz County, California, located on the northern coast of Monterey Bay. The population was 9,980 at the 2020 census.
Geography
Soquel is located a ...
, which is a thick bedded to massive
micaceous sandstone. Locally abundant siltstone, conglomerate, and intraformational breccia can also be seen in this member.
Above the Soquel lies the
Yorba Member Yorba may refer to:
People:
*Bernardo Yorba (1800–1858), native of Nueva California and the son of Spanish soldier, José Antonio Yorba
*José Antonio Yorba (1743–1825), Spanish soldier and early settler of Spanish California
Music:
*"Hotel Yo ...
. This member is a sandy siltstone that is interbedded with a fine-grained sandstone. The
Sycamore Canyon Member contains lenses of conglomerate, conglomeratic sandstone, and sandstone. Sandy siltstone and fine-grained sandstones are interbedded with the aforementioned rock types.
The
Monterey Formation
The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islan ...
is characterized by abnormally high
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
content compared to most clastic rocks. There are also silica-cemented rocks known as
porcelanite and porcelanite
shale.
While this formation has distinguishable beds, there are many shale, sandstone, and
mudstone beds that have normal amounts of silica.
This sequence of this formation indicates an
off-shore marine environment.
The
Fernando Formation
The Fernando Formation is a Plio-Pleistocene marine mudstone, siltstone and sandstone formation in the greater Los Angeles Basin, Ventura Basin, and Santa Monica Mountains, in Los Angeles County of Southern California.
Geology
Outcrops of the ...
is split into two sub-facies known as the
Pico
Pico may refer to:
Places The Moon
* Mons Pico, a lunar mountain in the northern part of the Mare Imbrium basin
Portugal
* Pico, a civil parish in the municipality of Vila Verde
* Pico da Pedra, a civil parish in the municipality of Ribeir ...
and
Repetto Members. These members represent a distinct change in the depositional environment and are of
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological 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 finally confirmed in ...
age.
The
Repetto
Repetto is a French ballet shoe company. It was founded in 1947 by Rose Repetto, after her son, choreographer Roland Petit, would come home from classes complaining of sore feet. Repetto provides ballet shoes for
the Opéra National de Paris and ...
is the older of the two members and is composed of interbedded fine to coarse grained siltstone, mudstone, and sandstone. The
Pico Member is mostly made of massive siltstones and sandstones interbedded with minor silty-sandstones.
Holocene
The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...
alluvium
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...
and
Quaternary sediments is a largely unconsolidated unit and is composed mostly of gravel and floodplain sediments. The sediments that mark the top of the basin can be found in modern streams/rivers and at the base of the foothills.
Tectonic setting
The history of this basin begins with the subduction of Pacific plate underneath the North American plate in the beginning of the Mesozoic.
During this subduction event, two smaller plates, the Monterey and Juan de Fuca plates, also began to subduct underneath the North American plate. Around 20Ma, the Monterey plate attached to and followed the motion of the Pacific plate. Later, subduction of the Pacific-Monterey ceased and the plate margin was converted to a transform boundary. The North America/Pacific-Monterey transform boundary began to move north and created crustal extension. This rifting was accompanied with the rotation of the western Transverse Ranges. This rotation is responsible for the placement and northwest-southeast orientation of the LA Basin.
Early in the Miocene, before deposition of the Topanga, high heat flow and
transtension Transtension is the state in which a rock mass or area of the Earth's crust experiences both ''extensive'' and ''transtensive'' shear. As such, transtensional regions are characterised by both extensional structures ( normal faults, grabens) and ...
caused the extension of the basin.
As the crust thinned, the basin began to subside from
isostatic pressure as a result of large amounts of sediment deposition.
Because the basin lies on the boundary of the Transverse and Peninsular Ranges, this basin experiences both compressional and strike slip tectonics.
During the early Pliocene, also identified as the "Basin Disruption" phase, deformation and folding occurred as a result of fault movement and a slight rotation event. While movement along the
San Andreas Fault
The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...
is responsible for the placement of the basin, it is the Whittier and Newport–Inglewood faults that have dictated the seismic behavior within the basin.
Earthquakes
The Los Angeles basin is still active tectonically and the region continues to experience earthquakes as a result. Due to the number of faults and fault splays, seismic activity is not concentrated in one particular area.
The cities that are overlain by the Newport–Inglewood and Whittier fault zones have a higher probability of experiencing seismic activity. The region experiences earthquakes that are mostly mild (magnitude ≤2.25). However moderate earthquakes (magnitude 4.9 to 6.4) have been reported. Earthquakes of moderate magnitude are very infrequent.
Basin features
The Newport–Inglewood Fault Zone
This fault zone is the most notable feature within the basin that is a single strand with local (fault) splays.
The fault zone is also marked by low hills, scarps, and ten anticlinal folds in a right-stepping en echelon pattern.
It is located in the southwest portion of the basin and is a
strike-slip margin. There are several oil fields that run parallel to this fault.
The Whittier Fault
This fault lies on the eastern border of the basin and mergers with the Elsinore Fault in the canyon of the Santa Ana river, one of the upper branches of the fault.
This fault is a reverse right-oblique fault. It is most known for the Whittier, Brea-Olinda, Sansinena, oil fields. There is an anticline that runs parallel to the Whittier fault that is evidence for compressional deformation during the late Miocene to early Pliocene. Thinning and pinch-out of the Pliocene sandstones are evidence for uplift during this same time period.
The Anaheim Nose
The Anaheim nose is a subsurface feature that was discovered by geophysical surveys and exploratory drilling in 1930.
It is a mid-Miocene fault block that revealed a northwest trending ridge of Paleocene age rocks.
This structural feature is important because it revealed many oil traps and orientation of the beds indicate the age of subsidence in this portion of the basin.
The Wilmington Anticline
This particular
anticline
In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
is the most notable subsurface feature within the basin.
Deformation events such as erosion of the uplifted crustal blocks, initiation of various faults, and the development of the submarine channel led to the anticline's formation.
Fold initiation began in the late-Miocene to early Pliocene period of deformation. There are many other anticlines within the basin and
isopach
An isopach map () illustrates thickness variations within a tabular unit, layer or stratum. Isopachs are contour lines of equal thickness over an area. Isopach maps are utilized in hydrographic survey, stratigraphy, sedimentology, structural geolo ...
data suggests that the formation of these folds occurred mostly during the Pliocene.
The La Brea Tar Pits
The La Brea Tar Pits are pools of stagnant asphaltum that have been found on the basin's surface. These "pools" are important because hundreds of thousands of late Pleistocene bones and plants have been found.
These pits allowed scientists to better understand the ecosystem at that particular point in the geologic past.
Petroleum
![LosAngelesBasinOilGasFields](https://upload.wikimedia.org/wikipedia/commons/7/73/LosAngelesBasinOilGasFields.png)
Accumulations of oil and gas occur almost wholly within strata of the younger sequence and in areas that are within or adjacent to the coastal belt.
The Puente formation has proved to be the most notable reservoir for petroleum in the basin.
The primary reason for the high abundance of oil is because the
oil sands
Oil sands, tar sands, crude bitumen, or bituminous sands, are a type of unconventional petroleum deposit. Oil sands are either loose sands or partially consolidated sandstone containing a naturally occurring mixture of sand, clay, and wate ...
are well saturated within the basin. The thickness of these oil sands range from hundreds to thousands of feet.
Anticlines and faulted anticlines are the structural features that are also responsible for trapping oil.
The first reported oil-producing well was discovered in 1892 on the land that is presently beneath
Dodger Stadium
Dodger Stadium is a baseball stadium in the Elysian Park neighborhood of Los Angeles, California. It is the home stadium of Major League Baseball's Los Angeles Dodgers. Opened in 1962, it was constructed in less than three years at a cost of ...
.
This basin was responsible for half of the states oil production until the (90's?). This is remarkable due to the relatively small size and youth of the basin.
The basin currently has about 40 active oil fields that collectively have 4,000 operating wells.
In 1904, there were over 1,150 wells in the city of Los Angeles alone. Tight spacing and continued pumping of the wells resulted in most of the wells to dry up. Most recent data indicates that 255 million barrels of oil were produced in 2013. This is a large decline from the almost 1 billion barrels per year produced in the late 1970s.
See also
References
External links
Significant LA earthquakes
{{Coord missing, Los Angeles County, California
Valleys of Los Angeles County, California
Valleys of Orange County, California
Watersheds of California
Landforms of Southern California
Los Angeles metropolitan area
Regions of California
Geology of Los Angeles County, California
Geology of Orange County, California
Pull-apart basins
Greater Los Angeles
Los Angeles River
Peninsular Ranges
San Gabriel River (California)
Santa Ana River
Transverse Ranges
Sedimentary basins of North America
Structural basins of the United States
Valleys of California
Watersheds of the West Coast (U.S.)