Origin and Paleogeographic Development of Tertiary Cuyama Depositional Basin, Southern California

A. Eugene Fritsche

The Cuyama depositional basin of southern California, as opposed to the more recent Cuyama structural basin, originated during the latest Oligocene and early Miocene as a chain of small interior-drainage basins that filled rapidly with the nonmarine alluvial-fan, flood plain, and lacustrine deposits of the Simmler, Plush Ranch, and Vasquez Formations and their associated volcanics. During the early Miocene, the ocean transgressed rapidly eastward across this chain of small basins, and as they filled they became united into one large, narrow basin that was confined to the northern part of the area. The basin deepened to bathyal depths and the mountains to the south subsided, allowing the ocean to transgress southward and greatly enlarge the area of the basin.

Following expansion and deepening, a large delta prograded westward into the northern part of the basin and began to fill it. Upper delta-plain and flood plain deposits were preserved as the Caliente Formation, and lower delta-plain and shallow marine deposits as the Painted Rock Sandstone Member. In the southern part of the basin, an unnamed, deep-water, submarine-fan sandstone unit was deposited. The topographic low point of the basin was on the south side, and the depositional center, or locus of thickest deposition, was on the north.

During the latest early Miocene, subsidence occurred and the ocean began a second transgression, and most of the basin became occupied by basin-plain deposits. Submarine-fan deposition occurred in the central part of the basin, and shallow-marine and lower delta-plain deposition occurred in the northeast. Upper delta-plain and flood plain deposition extended eastward across the future location of the San Gabriel fault. The Cuyama basin at this time extended northward to the present-day vicinity of the San Juan Valley and Indian Creek areas and southward to the region just north of Figueroa Mountain. The western margin of the basin is not known.

Near the beginning of the middle Miocene, westward progradation of the delta was renewed. Progradation continued until about 13 Ma, when the San Gabriel fault began to move and clockwise rotation of some parts of the basin began. Associated uplift created a local unconformity in the remaining eastern part of the basin. Delta progradation across the northern part of the basin and syndepositional tectonic rotation continued through the rest of the middle Miocene and into the late Miocene. The northern half of the basin was covered with nonmarine deposits by medial late Miocene. In the southern half of the basin, the rock record has been eroded, so the total extent of nonmarine filling of the basin is not known.

Tectonic deformation of the region during the Pliocene and early Pleistocene created the Soledad, Cuyama, and San Rafael structural basins out of the Miocene Cuyama depositional basin. The depositional and paleogeographic history of many California basins has been misunderstood because of the confusion of the depositional basin with the structural basins that were created postdepositionally, and understanding will come only through careful stratigraphic and paleogeographic analysis.

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