Miocene geology of the Southwestern portion of the San Rafael Wilderness, Santa Barbara County, California

Abstract

The Hurricane Deck area is in the San Rafael Mountains of northern Santa Barbara County, California. Upper Cretaceous and middle Tertiary strata crop out locally and have been folded into a broad northwest-trending syncline. The area is geographically situated between three well-studied Tertiary-age depositional basins, each of which has a separate and distinctive stratigraphic nomenclature. Tertiary strata within the study area were mapped in detail in order to determine their relationship and correlation to known formational units in the adjacent basins. It was determined that lower to middle Miocene strata of the Hurricane Deck were deposited in the Cuyama depositional basin, which is located northeast of the study area. Portions of the Simmler, Vaqueros, Hurricane Deck, and Monterey Formations were identified and mapped. Mapped exposures of the Simmler Formation consist of interbedded sandstone, conglomerate, and minor amounts of mudstone deposited in a nonmarine environment. Two members of the Vaqueros Formation are present in the study area: a shallow-marine, well-bedded sandstone member and an offshore-marine, predominantly structureless, grey shale. Lower and upper members of the Hurricane Deck formation are present in the study area. The lower member consists of thick-bedded, generally structureless sandstone beds containing thin mudstone interbeds. The upper member consists of a more heterogenous mixture of interbedded sandstone, mudstone, and limestone. These units were deposited during early and middle Miocene time in a local depositional low as lobes of a submarine fan. The youngest Tertiary unit mapped is the Monterey Shale which consists of brown mudstone, limestone, and minor amounts of sandstone deposited in a bathyal marine environment. Strata have been folded into the northwest-trending Hurricane Deck syncline. Some minor faulting has occurred and has offset both Tertiary and Cretaceous strata in portions of the area. Local drainage is structurally controlled and generally parallels the structural grain of the area.