AGE AND PALEOGEOGRAPHY OF THE SANTA BARBARA FORMATION IN THE SANTA BARBARA AND GOLETA QUADRANGLES, CALIFORNIA, BASED ON MOLLUSKS

Fieldwork and examination of over 120 collections from West Coast museums has yielded a molluscan fauna of 273 taxa (91 bivalves, 173 gastropods, 6 chitons, 3 scaphopods). Included are 11 extinct taxa that have stratigraphic ranges from late Pliocene to middle Pleistocene. One taxon, Dendostrea vespertina Conrad, does not occur in the Pleistocene. It is questionably represented by a single valve that may be reworked. Our best guess of the age based on the fossils and published data (amino acids, foraminiferal paleoecology, paleomagnetic analysis, and strontium isotopes) is between 790 and 400 ka.

Most of the fossils occur at four areas: Bathhouse Beach (BB) and Packard’s Hill (PH) in Santa Barbara, and the County Dump (CD) and More Mesa beach (MMB) in Goleta. Molluscan assemblages at all of these outcrops represent offshore mid-shelf water depths. At BB and PH outcrops and fossils suggest an offshore bank or bank margin with low terrigenous input. Outcrops of fossiliferous conglomeratic strata at MMB represent a submarine channel; these strata were previously attributed to the Pico? Formation, and were thought to be slightly older. Our work shows that the MMB fauna is identical to the Santa Barbara fauna, except for the occurrence of Tivela stultorum (Mawe) at MMB. Nonfossiliferous alluvial deposits interfinger and overlie the Santa Barbara Formation north of the modern shoreline suggesting that Santa Barbara shelfal sedimentation was coeval with, and eventually replaced by, deltaic and on-shore deposition.

Our analysis of molluscan faunas and reconnaissance observations suggest the following paleogeographic model. The rising Santa Ynez Mountains were bordered on the south by a narrow coastal plain and broad shelf. Some coarse clastic sediment shed from the mountains was deposited in alluvial-fan, fan-delta, and shore face settings, but most bypassed the shelf into deeper water via submarine canyons. Much of the shelf was then characterized by low rates of siliciclastic sedimentation allowing local bioclastic banks. Further refinement of this paleogeography model will arise from (1) sedimentological studies of depositional processes and systems, (2) more precise stratigraphic correlations within the Formation, and (3) better understanding of local tectonics (small-block rotations, and strike-slip faults).