Depositional environments of the Vaqueros Formation along upper Sespe Creek, Ventura County, California

Abstract

The Vaqueros Formation (Oligocene-lower Miocene) along upper Sespe Creek in Ventura County, California, is a transgressive marine sequence overlying the Sespe Formation and underlying the Rincon and Monterey Formations. The Vaqueros is exposed in a narrow graben between the Pine Mountain and Santa Ynez faults on the northern edge of the Ventura basin. Three. members are distinguished: a lower limestone, sandstone, and mudstone, which averages 65 m thick; a middle mudstone, which averages 110 m thick; and an upper sandstone, which averages 75 m thick. Rocks of the lower and middle members represent deposits in bay and inner shelf environments. The lowest mudstone and sandstone beds are muddy beach and bay margin deposits. Fossiliferous mudstone represents deposition in an open bay environment characterized by water depth of 1 to 10 m. Fossiliferous limestone containing abundant shell debris interbedded with mudstone is storm-lag or swell-lag deposits. Potamides-bearing limestone indicates a grassy bay environment with water depth of 1 to 3 m. Limestone beds near the top of the lower member which contain Anadara, Anomia, Chione, Macoma, and Ostrea represent an inlet influenced bay environment and near normal oceanic circulation. Mudstone of the middle member represents deposition in a shallow inner shelf environment with few mollusks and water depth less than 10 m. Rocks of the upper member represent environments of a shelf-depth sand sheet. Cross-bedded sandstone represents dune field deposits which accumulated from the southward migration of megaripples in water 15 to 30 m deep. Plane-bedded and massive sandstone are interdune deposits which collected sand at lower current velocities than that in the dune fields. Muddy conglomerate beds near the top of the Vaqueros represent debris flows and indicate a deepening of the shelf to 100 m or more. Glauconitic sandstone at the top of the formation indicates slackened deposition and outer shelf water depths. Pebbles from the upper member indicate a sedimentary source terrane near the Pine Mountain area north of Sespe Creek and a granitic source terrane near the Alamo Mountain area northeast of Sespe Creek. These source terranes define the San Rafael uplift, which was an eastwest trending highland separating Vaqueros exposures in the Sespe Creek and Cuyama Valley areas. Paleogeographic features which controlled Vaqueros deposition were established before the end of Sespe Formation deposition. Alluvial fans on the edges of a large floodplain sloped upward to a highland to the west and upward to the San Rafael uplift to the north. With the beginning of the Vaqueros transgression, the flat floodplain was rapidly covered by the water of Sespe Bay, a sheltered area between the western highland and the San Rafael uplift. Low energy beaches formed north and west of the Sespe Creek area. At the beginning of upper member deposition, the rising water of the transgression separated the western highland from the San Rafael uplift, forming Ynez Island and San Rafael Strait. Oceanic circulation moved through the strait and created the dune fields in the Vaqueros Formation.