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.
Description:
Includes bibliographical references (pages 125-129)
California State University, Northridge. Department of Geology.