Los Angeles Basin
The Los Angeles Basin is a sedimentary basin located in Southern California, 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 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 Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, lies within the greater Southern California region. The majority of the jurisdictional land area of the city of Los Angeles physically lies within this basin.
On the north, northeast, and east, the lowland basin is bound by the Santa Monica Mountains and Puente, Elysian, and Repetto hills. To the southeast, the basin is bordered by the Santa Ana Mountains and the San Joaquin Hills. 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 and ended in the Pleistocene. This basin can be classified as an irregular pull-apart basin accompanied by rotational tectonics during the post-early Miocene.
Formation
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. 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, 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 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.Geology
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.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 Pico Formation in the northwestern part of the basin and the Upper Fernando Formation in the southwest part of the basin.The Los Angeles Basin contains what is known as the "Great Unconformity" 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 sedimentary and igneous origin. The sedimentary unit was metamorphosed as a result of slippage of the Newport–Inglewood fault and is known as the Catalina Schist. 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-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 breccias, flow, agglomerates and tuffs.
The Sespe Formation 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 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 basaltic 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 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, 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. 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 is characterized by abnormally high silica 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 is split into two sub-facies known as the Pico and Repetto Members. These members represent a distinct change in the depositional environment and are of Pleistocene age. The Repetto 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 alluvium 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.