Late Quaternary deposits near Point Sal, south-central California; a time frame for coastal-dune emplacement

Extensive coastal dunes rise southward from the Santa Maria Valley to the Point Sal Ridge, south-central California. In Mussel Rock ravine and elsewhere, eolian sands occur in association with fluvial deposits. Radiocarbon ages and correlative deposits in nearby areas indicate a sequence of events postdating marine-terrace deposits of oxygen isotope stage 5. Dissected paleodunes (Qe1), probably related to stage 4, are overlain by mostly sandy fluvial deposits (Qf) whose higher units yield l4C ages in the 30- to 23-ka range. Transverse paleodunes (Qe2) began to form after 26 ka, as sea level fell during the transition from stage 3 to stage 2, and probably continued to accumulate westward during stage 2. Parabolic dunes (Qe3) formed from new and reactivated sand masses during the Flandrian transgression and were stabilized before 3 ka. Lobate dunes (Qe4) subsequently formed and the presently active transverse dunes (Qe5) have developed within the past 200 years. The chronology revealed near Point Sal may provide a valuable time frame for coastal-dune development elsewhere in California. The investigation indicates, for example, that extensive transverse dunes form during periods of sediment abundance, ideally when sea level falls and large quantities of sand are exposed on emergent continental shelves, but also when stabilized dunes are reactivated by environmental changes, including human land use. Parabolic dunes develop during periods of sediment deficiency-for example, as sea level stabilizes following a marine transgression.