14.3: Cenozoic Sedimentation and Basin Development

Basin Formation Across the Transverse Ranges Province

The Cenozoic geology of this region is dominated by basin formation, deposition, and deformation. Beginning in the Late Cretaceous, the changing nature of the North American and Pacific plate boundary led to dramatic changes across the region that would become the modern Transverse Ranges Province. Initially situated in a fore-arc basin that is suggested to be related to that of the Great Valley at this time, this region then experienced significant crustal extension and rotation that led to the formation of numerous deep basins, including the Ventura Basin, the Los Angeles Basin, and those separating the offshore Channel Islands from one another (Figure \(\PageIndex{1}\)). These basins preserve a record of deep to shallow marine environments, along with periodic uplift and erosion.

Figure \(\PageIndex{1}\): Digital elevation model of the Transverse Ranges Province highlighting mountains and valleys of Southern California. The Transverse Ranges Province is encircled; Los Angeles is on the boundary between this province and the Peninsular Ranges Province to the south. "Southern California digital elevation map" by USGS is in the public domain.

As the modern San Andreas fault system organized beginning at approximately 5 Ma, transpressional deformation inverted and deformed many of these basins and led to pronounced uplift of the adjacent ranges that continues today. While much of the sedimentary history of these deep (and in some cases, still underwater) basins is known from drill cores, the sedimentary record of basin formation in some areas can be directly accessed in fault-bounded hills and ranges within and adjacent to the modern basins. Indeed, geomorphic features across the modern landscape, along with historical earthquakes demonstrate that the forces responsible for this deformation continue today.

Ventura Basin

An example of the dramatic changes experienced by this region is found in the confines of the Ventura Basin in the Western Transverse Ranges (Figure \(\PageIndex{2}\)). The Ventura Basin occupies the region to the north of the Santa Monica Mountains and generally south of the Santa Ynez Mountains. The basin is named for the City of San Buenaventura located in its central region. Extending east-west, this synclinal basin is continuous with the offshore Santa Barbara Basin to the west and the Soledad Basin to the east.

Figure \(\PageIndex{2}\): Principal late Tertiary to Quaternary depositional basins and upland areas where bedrock units are exposed within the Los Angeles 30' x 60' quadrangle. The Soledad basin is represented by Miocene strata, which are truncated to the southwest by the San Gabriel Fault and subsequently overlapped by Pleistocene deposits of the Ventura basin; the Ventura and Los Angeles basin boundaries represent the limits of associated Pliocene and Pleistocene formations. "Preliminary RGM Los Angeles" by the USGS, is in the public domain.

The central part of the Ventura Basin is unusually deep, containing approximately 17,700 m of mostly marine sediment that is as old as Late Cretaceous in age. Marine basin deposits here are like those found in the Great Valley to the north and other basins across southern California. Based on the sedimentary rocks within this basin, workers have inferred that this region occupied a position offshore of a volcanic arc early in its history; this arc was likely the southern extent of the ancient Sierra Nevada arc. Sometime during the Late Eocene, regional uplift and erosion of this region led to an influx of shallow marine and non-marine sediments that continued through the Early Miocene. Sedimentary units of this period are arkosic sandstones, conglomerates, and siltstones, some of which are fossiliferous. These units record a gradual, but widespread regional uplift as sedimentary facies seem to shift from shallow water to non-marine in character. These deposits are comparable to ancient fore-arc units found in the Great Valley Sequence at this time.

Within the basin, sedimentary units are folded into a broad syncline, with anticlinal structures along the northern and southern flanks, where important oil fields are located (Figure \(\PageIndex{3}\)). These anticlinal structures are associated with Pleistocene reverse faults which push ranges to the north and south over the intervening basin area.

Figure \(\PageIndex{3}\): Simplified cross-section through the central Ventura Basin. Units are folded into an anticline/syncline pair and then offset by north and south dipping reverse faults. The Miocene Monterey Formation and the Oligocene/Upper Eocene Sespe Formation are highlighted. Areas of oil production are near the top of the southern anticline, in the Sespe Formation, and also near the Santa Paula Ridge in the north, where the Monterey Formation comes close to the surface. "Simplified cross-section through the central Ventura Bssin", by M. A. House is licensed under CC BY-NC /a derivative of the original by the California Division of Mines and Geology (now the California Department of Conservation). Access a detailed description of this image.

The Late Eocene-Early Miocene Sespe formation is a distinctive unit deposited during this period. This collection of non-marine conglomerates, sandstones and siltstones weathers a distinctive red and green color (Figure \(\PageIndex{4}\)); the distinctive red appearance in some areas is comparable to that of Permian Red Beds (Figure \(\PageIndex{5}\)). The Sespe formation is found in other basins throughout the western Transverse Ranges Province, indicating that the uplift and erosion that led to its formation impacted much of this region. Overall, these rocks are generally very resistant to erosion. Where subsequent faulting has uplifted them, they are ridge forming. For example, the ~40 Ma shallow marine Coldwater Sandstone, which precedes the deposition of the Sespe Formation, forms much of the mountainous Santa Ynez Range on the north side of Santa Barbara.

Figure \(\PageIndex{4}\): Lower Sespe formation outcrop is a weathered, poorly-sorted conglomerate (with all sediment sizes from boulders on to silt); Santa Ynez Mountains, CA. "Sespe Conglomerate" by Antandrus is licensed under CC BY-SA 3.0.

Figure \(\PageIndex{5}\): Typical outcrop of the Sespe Formation, north of Santa Barbara, California. The red rocks in the center are Sespe; lighter-colored rocks on the mountainside in the background are the Coldwater Formation. "Sespe Formation Outcrop, SB" by Antandrus is licensed under CC BY-SA 3.0.

The lithology of clasts within the Sespe Formation have been used to study the pattern of uplift and the paleogeography at the time of its deposition. Clasts within the lower Sespe Formation are granites and volcanic rocks that appear to be derived from source rocks found in the modern Mojave region, indicating that this basin was located further to the east and south at the time of deposition of this part of the formation. Higher in the section, blocks of metamorphic Franciscan basement rocks appear, indicating that the basement rocks of the western San Gabriel Mountains were exposed and eroding at this time.

As the plate boundary continued to evolve, renewed basin subsidence led to a return of deep marine conditions in the central and western Ventura Basin at approximately 17 Ma. Along with the deposition of deep marine units, rhyolitic and basalt volcanic debris, which is quite extensive in some areas, appears in the basin. In areas to the south, these units (called the Conejo Volcanics) can be quite substantial.

The shift to deep marine basin conditions led to deposition of the deep-water Rincon Formation (shales) and the Monterey Formation (marine shales and siltstones, and abundant diatomaceous oozes; Figure \(\PageIndex{6}\)). These units are key elements of oil reserves in central and southern California: the Monterey Formation and its equivalents is the source rock for oil and gas reserves throughout the Transverse Ranges and the Peninsular Ranges Provinces, as well as the Great Valley Province. To learn more, see Great Valley.

Figure \(\PageIndex{6}\): Monterey Formation, Gaviota State Park, California, USA. Fine grained, well laminated shales tilt to the left. "Monterey Formation Outcrop, SB Area" by Antandrus is licensed under CC BY-SA 3.0.

Soledad Basin

The complexity of basin geometries and histories in this region can be appreciated by comparing the history of the central Ventura Basin to that of its eastern equivalent: the Soledad Basin. This collection of shallow marine and non-marine sediments occupies much of the western San Gabriel Mountains today (Figure \(\PageIndex{2}\)).

While deep marine conditions prevailed in the central and western Ventura Basin in the Miocene, the depositional setting of the area that would become the Soledad Basin was shallow marine to non-marine in nature. Take, for example, the Vasquez Formation, a non-marine alluvial fan deposit which is broadly contemporaneous with deposition of the Monterey Formation (Figure \(\PageIndex{7}\)). While siliceous oozes were actively deposited in the Ventura basin to the west, non-marine alluvial fan deposits of the Vasquez Formation were shed off the surrounding ranges and deposited in the Soledad basin. These very different depositional settings co-existed along a structural trend and attest to the complexity of reconstructing the history of the region.

Figure \(\PageIndex{7}\): Tilted outcrops of the Vasquez formation at Vasquez Rocks County Park. Bedded sandstones and conglomerates of fluvial and lacustrine origin were deposited in Late Oligocene-Early Miocene and tilted by motion on the developing San Andreas Fault System. These units are part of the Soledad Basin. "Vasquez Rocks County Park" by Thomas, is licensed under CC BY 2.0.

As is the case with other non-marine units of the Transverse Ranges, the ridge-forming conglomerates and sandstones of the Vasquez Formation are resistant to erosion. This unit can be observed at Vasquez Rocks Natural Area, where layers of conglomerate and sandstone have been uplifted and tilted due to interactions with the San Andreas fault system. Subsequent erosion has created interesting landforms that have been the backdrop for numerous films and TV shows over the years.

Los Angeles Basin

The Los Angeles Basin and coastal plain region lies at the junction of the Transverse Ranges and the Peninsular Ranges Provinces (Figure \(\PageIndex{8}\)). The Santa Monica Mountains and the Puente, Repetto, and Elysian Hills mark the northern limits of the modern basin; these anticlinal, fault bounded uplifts expose much of the basin stratigraphy. Smaller basins to the north of these uplifts (such as those beneath the modern San Gabriel and San Fernando Valleys) contain units like those in the central part of the basin, and so are an extension of the modern Los Angeles basin.

Figure \(\PageIndex{8}\): Approximate extent of the Los Angeles Basin (outlined in yellow). Details of the extent of the basin are described in the text. Major mountains and hills are labeled as described in the text. The major rivers that run across the basin are marked in blue. These include, from west to east, the Los Angeles River, the Rio Hondo River, the San Gabriel River, and the Santa Ana River. These units are part of the Soledad Basin. "LA Overhead" by NASA is in the public domain.

To the southwest, the basin continues to the base of the Palos Verdes hills, an anticlinal uplift in which basin units are uplifted across the north verging Palos Verdes Fault. Units then plunge southward of this anticline to the offshore termination of the basin.

The general structure within the basin is a northwest trending syncline (Figure \(\PageIndex{9}\)). This structure, along with the anticlinal uplifts, anticlines like the Wilmington Anticline, and associated reverse faults are the product of transpressional deformation beginning at approximately 5 Ma. In addition to these compressional structures, the Los Angeles basin is sliced into four distinct blocks. These blocks are separated from one another by the northwest trending right-lateral Newport-Inglewood and Whittier-Elsinore Faults and the Santa Monica Mountain-Raymond Hill fault system (Figure \(\PageIndex{9}\)). While the Newport-Inglewood and Whittier-Elsinore faults, which are truncated by the Santa Monica Mountain fault, have an older history (see Peninsular Ranges Chapter), they are also responsible for vertically shuffling basin blocks such that they have distinctive depths and depositional histories. These histories result in different sediment thicknesses, but across the region, key units such as the Sespe Formation, Miocene Volcanic units, and equivalents of the Monterey Formation (which are locally called the Puente Formation and the Modelo Formation) are preserved.

Figure \(\PageIndex{9}\): Major structural blocks of the Los Angeles Basin. Details of this image are discussed in the text. "Major Structural Blocks of the Los Angeles Basin" by USGS/R. F. Yerkes, T. H. McCulloh, J. E. Schoellhamer and J. G. Vedder is in the public domain. Access a detailed description of this image.

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