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