INTERPRETING PLEISTOCENE MARINE TERRACE DEPOSITS OVERLYING THE 82 KA WAVE-CUT PLATFORM, POINT REYES PENINSULA, MARIN COUNTY, CALIFORNIA

The southwestern coast of the Point Reyes Peninsula is characterized by marine terraces consisting of uplifted wave-cut platforms with overlying sedimentary sequences (Qt). The youngest uplifted wave-cut platform was formed ~82 ka, during the sea-level high stand associated with oxygen isotope stage (OIS) 5a. It consists of beach and other coastal deposits overlain by a thick wedge of alluvial sediments. The entire peninsula has been deformed into a large-scale syncline, and the Qt terrace deposits are deformed as part of the syncline, showing that deformation has continued for the last 82 k.y. The age of the Qt deposits is bracketed between ~12 ka and ~82 ka. The ~12 ka minimum age was derived from a previous study of younger inset alluvial deposits, and the ~82 ka maximum age was determined by applying luminescence techniques to beach sand at the base of the Qt sequence. The Qt unit, which is a regressive sequence, contains two distinct deposit types that record variations in depositional environment as controlled by sea-level, climate, and tectonics. The lower section of Qt is beach and coastal dune sand, whereas the upper section is alluvial gravel. Field observations were used to create a cross section and stratigraphic columns to illustrate vertical and lateral variations along the coast. Based on previous studies, including paleoclimate reconstructions and sedimentary responses to climate changes, we correlated the beach sand to OIS 5a, the dune sand to a time of decreasing sea level leading to OIS 4, and the alluvial unit to aggradational events associated with climate changes during OIS 4-3. Differential uplift rates along the coast affected depositional energy, as well as the thickness and distribution of the alluvium, causing an increase in alluvial thickness from southeast to northwest, between Bolinas and Limantour Estero.