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Stratigraphy and Depositional Environments of Miocene Branch Canyon Sandstone in Sierra Madre, Caliente Range, and Sespe Creek Areas, California

Michael L. Perri, A. Eugene Fritsche

The lower to middle Miocene Branch Canyon formation is a light-gray, medium to thick-bedded, medium to coarse-grained, fossiliferous sandstone that crops out in the Caliente Range, the Sierra Madre, and the Sespe Creek areas of the Coast and Transverse Ranges, California. The type section, west of Branch Canyon in the Sierra Madre, contains three conformable formational units: (1) a lower, light-brown, silty shale that is ± 90 m (± 300 ft) thick and may correlate with the Soda Lake Shale Member of the Vaqueros Formation, (2) a middle, light-gray, structureless, unnamed sandstone unit that is ± 75 m (± 250 ft) thick, and (3) the "true" Branch Canyon Sandstone, as recognized in this report, that is ± 750 m (± 2,450 ft) thick. Eastward from the ype section, a thick lens of Monterey Shale intervenes conformably between the "true" Branch Canyon and the underlying unnamed sandstone. Conformably overlying the Branch Canyon at its type section is the Santa Margarita Formation. The Branch Canyon achieves its maximum thickness at the type section; to the east it is thinner because of the presence of an intraformational parallel unconformity.

North of the Sierra Madre in the eastern Caliente Range, the Branch Canyon Sandstone contains three basalt flows that date the top of the unit at around 17-14 Ma. The depositional environments of the Branch Canyon in the Caliente Range are shoreface to foreshore on the front of a prograding, wave-dominated delta. Unconformably overlying the Branch Canyon Sandstone are the nonmarine delta deposits of the Caliente Formation.

South of the Sierra Madre at Sespe Creek, the Santa Margarita Formation lies in slightly angular unconformity on the Monterey Shale. The basal member of the Santa Margarita Formation here is correlative with the upper part of the Branch Canyon.

The unconformities at the three localities are a reflection of the same tectonic event. The varying positions of the unconformities in the three stratigraphic sequences show that the Branch Canyon becomes younger toward the south and west, which is due to delta progradation across the Cuyama basin.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.