This work includes new interpretations of shallow offshore geologic structure between Point Arguello and the Santa Maria River within California’s 3-mi coastal limit. These interpretations are based on multi-sensor high-resolution seismic reflection data collected during January and February 1986. Water depths within the survey area range from 16 m (50 ft) nearshore to 70 m (230 ft) 3 mi west of Point Arguello. The sea floor slopes between 0.3 and 0.5° south-westward. The thickness of unconsolidated Quaternary sediment in the survey area ranges from 0 (bedrock outcrop) to almost 50 m (165 ft) off Point Arguello. The survey area crosses the boundary between the northwest-trending Coast Ranges and the east-trending Transverse Ranges. The onshore faults and folds can be traced offshore in the seismic sections. From north to south, these faults include: (1) Pezzoni-Casmalia-Orcutt frontal fault, (2) Lions Head fault, (3) Santa Ynez River fault system, (4) Lompoc-Solvang fault, (5) Cañada-Honda fault, and (6) several unnamed faults offshore Point Arguello. These faults are tentatively classified as potentially active because they do not offset a Pleistocene erosion surface and the Holocene unconsolidated sediments overlying that surface do not show offset in the seismic records. Although the faults are tentatively classified as potentially active, they may be seismically active as suggested by the limited earthquake data in the area. The seismic data show that the north-northwest-striking Hosgri fault zone decreases in both vertical and right-slip displacement toward the south. In the northern and central parts of the survey area, the fault zone consists of two subparallel branches. In the south, near Purisima Point and near the boundary between the Transverse and Coast Ranges, the north-northwest strike of the fault zone changes toward the east and the fault zone shows splays. Our interpretation of the data is that this area of splays may be the terminus of the Hosgri. If that is correct, then the amount of surface rupture due to earthquakes along this segment of the Hosgri is likely to be small, if surface ruptures occur at all.

Abstract 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 ( Qe ,), probably related to stage 4, are overlain by mostly sandy fluvial deposits ( Qf ) whose higher units yield 14 C ages in the 30- to 23-ka range. Transverse paleodunes ( Qe 2 ) 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 ( Qe 3 ) formed from new and reactivated sand masses during the Flandrian transgression and were stabilized before 3 ka. Lobate dunes ( Qe 4 ) subsequently formed and the presently active transverse dunes ( Qe 5 ) 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.