ORIGIN AND AGE OF PLEISTOCENE SHELLY MARINE DEPOSITS, TRINIDAD HEADLANDS, HUMBOLDT COUNTY, NORTHWESTERN CALIFORNIA, USA

The Trinidad Headlands reveal exposures of the late Mesozoic Franciscan Complex, overlain by 15-20 m of lower-to-middle Pleistocene shelly marine sand and gravel, in turn, capped by a 1-2 m thick unit of upper Pleistocene terrace sand. Fossils from the Pleistocene units were previously described by G. L. Kennedy in 1978 and B. Roth in 1979. A reexamination of the stratigraphic sequences exposed at Moonstone Beach and Megwil Point, and compilation of new fossil assemblage data allow for a new interpretation of the origins and ages for the fossiliferous assemblages. Historically, the marine deposits have been interpreted as nearshore, but our observations suggest they accumulated farther offshore, below fair-weather wave base, in channels or bars that were active during storms. These channels and bars are associated with sea stacks and bedrock topographic irregularities. The fossils consist of mixed assemblages of bivalves, gastropods, barnacles, sand dollars, and bryozoans from epilithic, sandy-bottom, and coelobitic sources. The high proportion of epilithic invertebrates combined with the overall large body size of the fossils and the geometry of the shelly beds supports our farther offshore, higher energy interpretation of depositional environment. G. L. Kennedy and K. R. Lajoie, in 1982, reported amino acid racemization age estimates for the Megwil Point and Moonstone Beach units that suggested possible correlation with oxygen isotope Stages 11 and 21, respectively. New age estimates using fossil ranges and new U-series analyses of Balanophyllia elegans for both areas indicate slightly elevated initial U-isotope ratios, but otherwise closed-system histories. Probable ages are ~ 560 ka, suggesting that deposits at both localities may correlate with isotope Stages 13 or 15. Further investigations into the ages of these deposits are being pursued.