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The Los Angeles Basin

LA Geologic Setting

The modern-day Los Angeles Basin is one of many Neogene-aged basins located along the San Andreas transform system between the Pacific and North American plates on the western margins of California. It is a north-west trending alluviated lowland plain that lies along the intersection of the Peninsular Range, Transverse Ranges, and the continental borderlands of southern California. The basin is a member of the general class of strike-slip basins that form in transform margin settings. The basin was initiated in the mid-Miocene due to extension associated with significant strike-slip and rotation of the Transverse Ranges. There was another extensional even between the Miocene and Pliocene that caused the Gulf of California to open. Shortening began in the early Pliocene and continues today and can be seen in the deformational history of the basin. Although it is considered one of the world's smallest petroleum systems, with an area of approximately 50 miles long by 20 miles wide, it contains extremely thick sedimentary fill that makes it one of the richest basins in the world in terms of hydrocarbons per volume of sedimentary fill.

Block Structure

The Los Angeles basin has been divided into four subdivisions based on the distribution, thickness, and structure of the rocks in different areas of the basin. These differences have been noted through regional subsurface studies, exposed sections, and drilling.

Southwestern Block

This rectangular-shaped block is about 5 miles long and 12 miles wide and lies mostly beneath the Pacific Ocean. It is a low plain that extends from Santa Monica in the northwest down to Long Beach in the southeast. The most prominent feature of the block are the Palos Verdes Hills, which are about 1,300 feet tall. These hills are underlain by the Newport-Inglewood zone of deformation. The basement rocks of this block are exposed in an area of the Palos Verdes hills. The superjacent rocks are 20,500 feet thick and are mostly marine sedimentary strata of middle Miocene to recent age with some igneous rocks from the middle Miocene. Many anticline structures underly these hills and many oil wells have been successfully drilled here.

Northwestern Block

SoCal Topography

This block includes parts of the Santa Monica Mountains, the Verdugo Mountains, and the San Fernando Valley. The basement rocks of this block are exposed in the eastern Santa Monica and Verdugo Mountains.The rocks above the basement rock are 14,500 feet thick and are comprised of marine clastic sedimentary strata of Late Cretaceous to Pleistocene age and volcanic rocks from the middle Miocene.

Central Block

This block is wedge-shaped and is about 55 miles long from the Santa Monica Mountains to the San Joaquin Hills and ranges between 10-15 miles in width. The block includes aggraded central lowland plans, the Elysian Hills, parts of the Repetto Hills, the Coyote Hills, the synclinal La Habra Valley, the Santa Ana Mountains, and the San Joaquin Hills. Basement rocks are exposed in the core of the Santa Ana Mountains and the superjacent rocks have a maximum thickness of 32,000 feet and consist of marine and nonmarine clastic sedimentary rocks that are Late Cretaceous to Pleistocene aged and are interbedded with middle Miocene-aged volcanic rocks. The dominant structural feature of this block is a north-west trending, double plunging synclinal trough.

Northeastern Block

This block is a triangular wedge that is about 35 miles long and 18 miles wide. The low Repetto Hills and Puente Hills form an arc along the south part of the block and are separated by the Whittier Narrows. The basement rock of this block are exposed at the north of the Puente and San Jose Hills. The superjacent rocks are up to 24,000 feet thick and are composed of Cenozoic-aged fine to course grained marine clastic sedimentary rocks. The eastern part of the block has 4,000 feet of middle Miocene volcanic rocks of late Eocene to early Miocene age.

Tectonic History of Southern California

Pacific plate subducting underneath North American plate

85 Mya:

This was a time when a series of oceanic plates were subducting underneath North America. The western edge of the North American plate was considered a subduction zone and most of the basement rocks in Southern California were created in this environment. The subduction during this time resulted in the north-south orientation of some of California's mountains such as the Sierras and Coastal Ranges.

40 Mya:

Tectonic Setting

The Farallon plate is the last of a series of plates that subducted underneath North America. The Pacific rise hits the North American plate and splits the Farallon plate into two smaller pieces. The contact between those two plates is the San Andreas Fault. The upper plate is now called the Juan de Fuca plate and the lower plate is called the Rivera plate which is part of the larger Cocos plate.

30 Mya:

The subduction eventually ceased and the plate margin was converted to a transform boundary. The North American-Pacific transform boundary began moving north and created crustal extension. This rifting was accompanied with the rotation of the western Transverse Ranges, which is responsible for the placement and northwest-southeast orientation of the Los Angeles Basin.

18 Mya:

The San Andreas fault has grown as the Mendocino triple junction and the Rivera triple junction have grown apart from each other. The San Andreas fault now extends all the way from Mexico to Mendicino and a segment of the continental crust has been torn off of the North America and carried along with the Pacific plate. The western U.S. was stretched and left Nevada and California much wider.

Basin Evolution

The evolution of the Los Angeles Basin is still debated upon, but the five step model presented below by R.F. Yerkes is currently the most accepted. There are two prebasinal phases that describe the basement and Upper Cretaceous to lower Miocene sedimentary rocks. Then comes the middle Miocene basin-inception phase, followed by a late Miocene to early Pleistocene subsidence and deposition phase, and finally a post-mid-Pleistocene basin-disruption phase.

Pre-Depositional Phase-Basement Rock:

The floor of the basin is a heterogeneous assembly of metamorphosed sedimentary and volcanic rocks that were intruded by plutonic rocks. The basement rocks are pre-Late Cretaceous in age and have been divided into two distinct groups by the Newport-Inglewood fault. The western basement complex is made up of southwestern block and contains the Catalina Schist, which represents the ancient oceanic plate. The eastern basement complex is made up by the northwestern, central, and northeastern blocks and contains slates and granitic rocks that belong to the North American plate. A strength discontinuity lies between these two different basement rocks and became a zone of weakness during Middle Miocene rifting and during the transform faulting and distributed right-lateral shear occurring presently.

Cross Section of the LA Basin

Pre-Basin Phase of Deposition:

The Upper Cretaceous to lower Miocene rocks contain about 5,900 feet of Late Cretaceous-aged marine clastic sedimentary deposits and 11,000 feet of Paleocene to early Miocene-aged shallow-water marine and non-marine clastic sedimentary deposits. These rocks are found above the eastern basement and were deposited over a large marine or non-marine environment during three cycles of marine transgression and regression.

Basin Inception-Middle Miocene Rocks

After the deposition of thick Upper Cretaceous and lower Tertiary rocks, a significant episode of emergence and erosion occurred due to sea level change and a basin-wide unconformity can be seen at the base of the middle Miocene. The basin's middle Miocene rock, which has a thickness of 10,000 feet, is a varied and widespread succession of marine clastic sedimentary rocks, fine-grained siliceous organic sedimentary rocks, and interbedded volcanic rocks. This succession of rocks can be seen in the Santa Monica Mountains and San Joaquin Hills.

Principle Phase of Subsidence and Deposition-Upper Miocene to Lower Pleistocene Rocks:

There was a phase of accelerated subsidence and deposition that began in the late Miocene and continued uninterrupted into the early Pleistocene. The middle Miocene embayment became covered by the rapid accumulation of clastic sedimentary rocks that were derived from the highland areas and down submarine slopes in a water depth of 3,000 feet by the end of the Miocene. The large amounts of organic matter contained in the finer grained sediments formed the source for large volumes of hydrocarbons. Subsidence and deposition occurred simultaneously and uninterrupted until the late Pliocene when the rate of deposition gradually overtook the rate of subsidence and the basin's margins began to rise above sea level.

Basin Disruption-Upper Pleistocene to Recent Strata:

Throughout the late Pleistocene, the shoreline continued to retreat as the basin continued to subside and receive coarse clastic sediments from the San Gabriel Mountains and rapidly rising Puente Hills, Santa Ana Mountains, and Santa Monica Mountains. This ongoing phase is constituted by compressional deformation which has resulted in folding that traps hydrocarbons.

Basin Features

The walls of the Los Angeles basin are formed by the San Gabriel, Santa Monica, and Santa Ana Mountains, and the Palos Verdes Peninsula.

Newport-Inglewood Fault:

Structural Features of LA Basin

This is the most notable feature of the basin. It is defined as a „right-lateral strike-slip fault that separates the southwestern & central blocks. Low hills can be observed and at least 10 anticlinal folds are present. This feature receives an abundance of attention due to the numerous oil fields that run parallel to it.

Whittier Fault:

„This fault is an active part of San Andreas transform system and is defined as a reverse right-oblique fault. It runs parallel to the Newport-Inglewood fault and also contains anticlines, which are evidence for the compressional deformation that occurred during the late Miocene to early Pliocene.

Stratigraphy

Generalized Stratigraphy of the LA Basin

The strata of the Los Angeles basin are separated into two large groups by a mid-Cretaceous aged unconformity. Below the unconformity lies basement rocks, metamorphic, and igneous rocks of Precambrian to early Late Cretaceous age. The rocks above the unconformity are a thick succession of marine and nonmarine sedimentary and volcanic rocks that are Late Cretaceous to recent age.

The Moneterey, Modelo, and Puente Formations are middle to late Miocene aged source rocks that have a total organic content (TOC) as high as 10-16%. Miocene to Pliocene aged submarine fan sands form important reservoir units that include the Fernando, Pico, and Repetto Formation sands.

Hydrocarbon Potential

Oil Fields LA Basin

The LA basin provides a nearly optimum combination of conditions favorable for petroleum generation and accumulation. The combination of „rich and abundant organic source rocks, „adequate maturation temperatures,„ widespread layers of porous reservoir sands, and „the early development of structural traps around most of the basin’s margins are responsible for the high productivity of this basin. Ultimate oil recovery from known fields in this basin exceeds 9 billion bbl or a volumetric oil recovery of 4.5 billion bbl per cubic miles of basinal sediment. This makes the basin the richest basin in the world in terms of hydrocarbons per volume of sedimentary fill.