Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Stratigraphy and Paleogeography of the Tertiary Cuyama Depositional Basin, Southern California

Abstract

The purpose of this presentation is to briefly remember work done by and at the behest of Dr. Eugene Fritsche in and around the Cuyama area of central California. This was an area of great interest to Gene, and the understanding gained here is recalled in tribute to his work and in gratitude for the guidance he gave to his students.

The upper Oligocene to upper Miocene stratigraphy in the Cuyama depositional basin records two pair of transgressions and regressions. Alluvial fan, fluvial, delta, nearshore, and submarine fan deposits are represented in the exposures in the Cuyama Badlands, and Sierra Madre, and Caliente Ranges in Kern, Ventura, Santa Barbara and San Luis Obispo Counties.

The depositional basin initially was created as several smaller basins formed due to extensional tectonics associated with the approach of the East Pacific Rise spreading center. Restoration of slip along the San Andreas and San Gabriel faults shows these basins extended as far east as the Soledad Basin and the Orocopia Mountains. The first deposits in the Cuyama area were a series of northwest-prograding alluvial fan, stream, and fluvial dominated deltaic sediments of the Oligocene Simmler Formation that unconformably overlaid faulted and tilted Cretaceous to Eocene forearc terrane.

The Simmler is overlain by the Vaqueros Formation and marks the beginning of the first transgression. The lower Vaqueros member contains retrogradational delta and delta front environments, followed by deeper water deposits of the middle member. The lower Vaqueros in the Cuyama Badlands contains erosional basal contacts and localized boulder breccia that may record initial movement along the proto-San Gabriel fault system.

In the first major regression in the Cuyama Basin, a large delta prograded basinward. In the eastern portion of the Caliente Range the upper Painted Rock Sandstone Member of the Vaqueros Formation this progradational sequence of fluvial-dominated deltaic deposits filled that portion of the basin. In the western Caliente Range the delta progressed into relatively shallow water, where a series of distributary deposits repetitively subsided into interdistributary bays.

To the south, in the area of the Sierra Madre the middle shale member is overlain by gravity-flow deposits of the Hurricane Deck Formation. This series records a southwest-trending submarine fan complex. Correlation of the Hurricane Deck is uncertain but it may represent the deep-water continuation of the shallow marine Painted Rock Sandstone. Alternatively the Hurricane Deck may be a lowstand submarine fan that was deposited after the Painted Rock Sandstone but before the next transgressive sequence containing the Branch Canyon Sandstone.

In the second transgression the deep-water gravity flow deposits are overlain by fine-grained sediments of the Monterey Formation. At this time the coarse clastic deposition reverted to the basin margins.

Deposits of the Branch Canyon Sandstone in the Sierra Madre and western Caliente Ranges contain progradational deltaic and shallow marine deposits and mark the beginning of the second major regression. Deposition rates matched sea level change and through this regression the relative positions of the Monterey deep water, Branch Canyon delta and shallow marine, and Caliente non-marine remained stable through the middle Miocene.

In the Sierra Madre Range deposits of the Miocene Santa Margarita Formation record a last and localized transgression into the area. These areas were eventually overcome by terrestrial deposits.