Stratigraphy of Paleocene to Neogene Transition Rocks of Western Transverse Range Province: Reevaluation

Catherine A. Rigsby

Reexamination of stratigraphic sequences along the Santa Barbara coast suggests that strata mapped as the Vaqueros Formation in coastal areas east of Gaviota are more complex than previously reported. By convention, the base of the Vaqueros in this area is mapped at the top of the last Sespe Formation red bed. At Tajiguas Canyon, the formation consists of 200 m of fine to medium-grained, yellow, quartzofeldspathic deltaic to inner-shelf sandstone sharply overlain by 200 m of coarse-grained to granular, bluish-white, lithic-rich sandstone and conglomerate, also of marine origin. These two units are distinct and mappable.

The upper unit correlates with strata mapped as Vaqueros at Gaviota Gorge. It pinches out eastward in the area of Lauro Canyon (approximately 15 mi to the east). The lower unit correlates lithologically with strata mapped as Alegria to the west and with strata mapped as Vaqueros to the east and north. The Alegria and Sespe Formations are Oligocene whereas the Vaqueros Formation has been considered lower Miocene. The contract between the formations has been considered a sharp one (even unconformable) in the western Transverse Range area. Recent work, however, suggests that the Vaqueros Formation may be, in part, Oligocene.

Efforts to solve these stratigraphic problems include strontium isotope dating of marine fauna and detailed petrographic analysis of the Vaqueros Formation sandstones. Preliminary results of these studies confirm an abrupt change in sandstone petrology within these strata and provide new insight into the understanding of the sedimentologic and tectonic history of this region.

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