Insights into the formation of wave-dominated deltas from ground-penetrating radar profiles of the Oxnard Plain, California and the Elwha River Delta, Washington

Distinguishing wave-dominated deltas from prograding clastic shorelines in ancient rock settings is a difficult task due to the lithologic similarities of the two depositional environments. Nevertheless, it is an important distinction in reconstructing the paleogeography essential in interpreting the rock record. Ground-penetrating radar (GPR) surveys were collected at the shoreline of the Oxnard Plain, Southern California and at the Elwha River Delta, Washington to determine if a stratigraphic signature for wave-dominated deltas could be identified. GPR profiles from the shoreline of the Oxnard Plain, CA show landward-dipping reflections near the mouth of the Holocene wave-dominated delta of the Santa Clara River. The landward-dipping reflections do not appear in GPR profiles away from the river mouth and may be one way to distinguish wave-dominated deltas from prograding clastic shorelines in outcrops with limited exposure. The landward-dipping reflections are likely formed by the amalgamation of offshore bars onto the shoreline. Inland storms often cause flooding along rivers, increasing the sediment supplied to the delta mouth. This leads to the formation of offshore bars and back-bar lagoons, which slowly fill with overwash deposits as the bars weld to the shoreline. We propose that the overwash deposits form landward-dipping sediment packages, imaged in GPR profiles as landward-dipping reflections and are unique to wave-dominated deltas. The Elwha River Delta provides a test ground for our model of offshore bar-amalgamation and its geometries within the wave-dominated delta deposits. The geomorphic response of the delta to increased sediment supply due to two dam removals on the Elwha River, initiated in September 2011, has been well documented with aerial photography, topographic surveys, and bathymetric surveys by the USGS. The dam removal greatly increased the sediment supply to the delta and led to delta growth and the formation and subsequent welding of numerous offshore bars to the new shoreline. A GPR survey conducted in July 2016 imaged the sediment geometries in the delta five years after dam removal and confirms the origin of the landward-dipping reflections.