Episodic Coastal Progradation of the Oxnard Plain, Southern California, USA

As droughts are projected to intensify over the next century, determining the coastal response is pivotal for hazard mitigation in the future. Ground-penetrating radar (GPR), in combination with radiocarbon and luminescence (optically stimulated luminescence [OSL]) dating methods, provides insight into the coastal response of the Oxnard Plain, Southern California, to past droughts and storms through the imaging of sedimentary stratigraphy. GPR profiles contain three radar facies and two surfaces. Two of the radar facies and the two surfaces reflect cut and fill as the beach accreted and eroded. The third GPR facies is interpreted as aeolian deposits. For the first time, beach cusps are recognized in shore-parallel profiles as concave-up surfaces. Progradation rates at the 150- to 200-year timescale along the coastal Oxnard Plain have remained relatively constant, prograding at rates between 0.3 to 1.4 m a(-1). However, on decadal timescales, progradation has been episodic through time, with episodes of progradation after large floods along the nearby Santa Clara River, whereas wave action during periods of nonprogradation associated with prolonged droughts have resulted in the erosion of up to 90 m of the shoreline, equivalent to 5 to 120 years of the sediment record. Erosional surfaces often used to reconstruct past storms and tsunamis are more likely to be preserved in locations marked by higher rates of progradation, such as closer to the Santa Clara River delta.