Abstract: Tertiary Microfaunas of the Gualala and Pt. Arena Basins and their Relation to Local and Global Events

McDOUGALL, KRISTIN, U. S. Geological Survey, Menlo Park, California

Tertiary microfaunas in the Gualala and Point Arena basins have intermittently been the focus of studies which attempt to interpret the complex tectonic history of the California margin during a period of transition from a convergent to translational margin. New microfaunal data as well as re-examined published studies and oil company data from the Tertiary sections in the Gualala and Pt. Arena basins provide critical ages and paleobathymetry estimates. Biostratigraphic analysis of these microfaunas indicates the presence of two depositional cycles; the first occurred during the late Paleocene through early Eocene (Anchor Bay and German Rancho strata) and the second during the latest Oligocene through middle Miocene (Iversen Basalt, Skooner Gulch, Gallaway and Pt. Arena Formations). The early Paleogene, middle Eocene through early Oligocene, and late Miocene are represented by hiatuses.

The Paleocene faunas are sparse; primarily containing nondiagnostic lower bathyal to abyssal species with an occasional age diagnostic species. Near the Paleocene/Eocene boundary the foraminiferal assemblages are dominated by arenaceous species characteristic of a deep water mass which moved northward along the California margin. Early Eocene benthic foraminiferal assemblages are largely arenaceous and indicative of lower bathyal to abyssal depths. Coeval faunas are recognized in the San Francisco Bay region and southwest side of the Salinian Block. The late Oligocene to middle Miocene benthic foraminiferal assemblages contain age diagnostic calcareous species which suggest assignment to the late Zemorrian through Luisian Stages and are indicative of middle to lower bathyal deposition. Although not well developed, the middle Miocene foraminiferal assemblages are the northernmost Monterey-like faunas in California.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California