Abstract: A Tale of Six Consecutive Tertiary Depositional Basins in Northern Los Angeles County, California
FRITSCHE, A., California State University, Northridge, Northridge, CA
As the East Pacific Rise approached North America during the late Oligocene, the relatively young, thin portion of the subducting Farallon plate broke into several microplates. One of these, the Monterey microplate, became wedged under the North American plate in the vicinity of the future San Gabriel Mountains. As subduction tectonics was replaced by transtension, the crust northwest of the future San Gabriel Mountains area was stretched and a northeast-trending graben was created that became occupied by five, small, interior-drainage sub-basins: the Diligencia, Vasquez Rocks, Texas Canyon, Charlie Canyon, and Lockwood. By 26 Ma these sub-basins began to fill with volcanic rocks, fluvial sediment, and evaporites. By ~21 Ma, these five sub-basins became connected by a southwestward, through-going river that flowed into the Cuyama basin, thus placing this northern Los Angeles County area in the headwater reaches of the Cuyama depositional basin. By ~16 Ma, the area began to rotate clockwise in response to capture by the Pacific plate and, as rotational stress increased, brittle fracture of the crust created the San Gabriel fault ~12 Ma. Right slip on the San Gabriel fault, which cut through the west end of the San Gabriel Mountains plutonic rocks, moved the western end of the mountains across the path of the Cuyama basin river, thus beheading this headwaters region of the Cuyama basin and creating the lake-filled Soledad depositional basin. This lake eventually spilled southward out of the Soledad basin to establish a through-flowing river that connected with the Ventura basin, placing the northern Los Angeles County area in the headwaters of the Ventura depositional basin. By ~9 Ma, the ocean transgressed up the valley and flooded this upper portion of the Ventura depositional basin. As time passed, the ocean regressed and pull-apart movement along the San Gabriel fault created a new lake-filled basin, the Ridge depositional basin, at ~8 Ma. Continued deformation of the area, cessation of movement on the San Gabriel fault ~5 Ma, and uplift of the Santa Susana Mountains beginning ~700 Ka established the modern drainage of the area via the Santa Clara River, which is part of the headwaters of the Santa Barbara depositional basin.
AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California