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Characterizing the Stratigraphy of Wave-Dominated Deltas at the Oxnard Plain, California and the Elwha Delta, Washington Using Insights From Ground-Penetrating Radar


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. One geomorphic distinction between the two depositional environments is offshore bar formation from flooding events on deltas. The formation of offshore bars and subsequent welding onto the shoreline is a well-documented process, yet little work has examined the stratigraphic signature of the process. Flooding along the Santa Clara River in the Oxnard Plain in 2004 provided the opportunity to observe the formation and subsequent welding of an offshore bar. Ground-penetrating radar (GPR) profiles proximal to the river's delta reveal a distinct geometric pattern of landward-dipping reflections truncated by an erosional surface. This pattern is interpreted to be the stratigraphic signature of an offshore bar welding to the shoreline through overwash deposits, and the subsequent straightening of the shoreline. To further test this hypothesis, a GPR survey was conducted at the Elwha Delta, WA in July 2016. Beginning in September 2011, two dams were removed along the river and over two years released ~8.2 million tons of sediment, prograding the delta ~200 m northward. The GPR survey is supplemented by biannual topographic and bathymetric surveys, as well as aerial photography, from the USGS, which are used to constrain the timing of surfaces identified in the GPR profiles. Similar signatures of bar welding have been identified in the GPR data from the Elwha Delta, where bar welding has been imaged aerially since the dam removal. Vibracores taken from both locations help to ground-truth GPR interpretations as well as constrain timing of events in the older Oxnard Plain. Overall, the GPR profiles from both study sites reveal a distinct stratigraphic signature for bar welding and wave-dominated deltas that can be used to identify wave-dominated deltas in outcrops with limited exposure. This insight into wave-dominated delta sediment architecture will improve the scientific understanding of facies heterogeneities and may be used to improve delta models.