Masters Thesis

Stratigraphy and depositional environments of the Vaqueros Formation, Central Santa Monica Mountains, California

The Vaqueros Formation along the southern flank of the central Santa Monica Mountains is a sequence of lower upper Oligocene through lower middle Miocene sandstone and siltstone that records a rapid transgression due to rising sea level and a subsequent progradation of a deltaic system. Study of these rocks reveals nine lithosome units that are process controlled. The lithosomes, from lower to upper, include: a parallel-laminated siltstone (A); parallel-laminated, fine-grained sandstone (B): structureless, fine-grained sandstone (C); cross-bedded sandstone (D); parallel-laminated, coarse-grained sandstone (E); structureless, coarse-grained sandstone (F); bioturbated, fine-grained sandstone (G); parallel-laminated, medium-grained sandstone (H); and interbedded red-and-green siltstone (I). Provenance and paleocurrent data are scarce, but source terrains that contributed high-grade metamorphic and reworked sedimentary clastic material to the study area are indicated. The river that supplied sediment probably flowed from a northern or eastern direction. There are two possible orientations of the shoreline: one that trended north-south and another that trended east-west. A riverdominated deltaic depositional system existed in the eastern portion of the study area, whereas wave-dominated deposition along an erosional coastline took place in the western region. The western stratigraphic sections - San Nicholas Canyon, Encinal Canyon, and Latigo Canyon contain strictly marine environments from offshore to backshore. A large amount of sediment from increasing tectonics caused the shoreline to prograde and the rocks generally reflect a shallowing upward from the bottom to the top of the stratigraphic sections. The eastern sections Corral Canyon, Puerco Canyon, Malibu Canyon, and Piuma Road - contain rocks which also reflect a general shallowing upward, however, this is due to the progradation of a Riverdominated deltaic system. The inferred environments determined from these rocks are prodelta, delta front, and lower and upper delta plain. Although it is difficult to distinguish between the two, the deltaic and marine strandline deposits probably interfinger with each other between Corral and Latigo Canyons. During the early late Oligocene there was a rapid transgression of the sea. In the west, offshore and shoreface sediment of the Vaqueros was deposited unconformably on the nonmarine Sespe Formation. At the same time, deltaic deposits of the Vaqueros and fluvial deposits of the Sespe were interfingering in the east. The greatest extension of the transgression was just east of the Puerco Canyon section. Continued subsidence and deposition allowed a thick section of sand to accumulate. Although sea levels continued to rise, the delta prograded due to the high sedimentation rate. After the early medial Miocene, uplift was followed by erosion and the Topanga Canyon Formation was deposited unconformably onto much of the Vaqueros.

Items in ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.