Hurricane Trajectory and Irregular Bedrock Topography as Drivers of Washover Fan Geomorphology on an Isolated Carbonate Platform

Mattheus, C.R. and Yovichin, R.D., III, 2018. Hurricane trajectory and irregular bedrock topography as drivers of washover fan geomorphology on an isolated carbonate platform. Although washover fan geomorphology is widely studied along sandy passive continental margins, little is known about rocky carbonate platform analogs. This paper addresses the multidecadal evolution of a compound washover fan complex on San Salvador Island, an isolated and hurricane impact-prone Bahamian carbonate platform. Its geomorphic development is assessed from pre- and post-Hurricane Joaquin topographic measurements, historic aerial photographs, ground-penetrating radar imagery, and sediment cores. Survey information provides insight into event-based surficial processes, while subsurface mapping and aerial photography offers a window into past events. Hurricane Frances left the fan largely unmodified in 2004, whereas the comparably strong Hurricane Joaquin was affiliated with heavy scouring of the fan apex and extensive accretion across the distal fan in 2015. Mimicking the surficial geomorphic changes affiliated with Joaquin, a superposition of five distinct event-based depositional units is elucidated from subsurface architectures. Cores from the distal, subaqueous fan sample interbedded lagoon muds and overwash sands, while truncated subsurface radar units near the fan apex map erosional events. The susceptibility of this particular island locale to repeated modification relates to the irregularity of the carbonate bedrock terrain, which funnels surge waters inland through low topography under favorable hydrodynamic conditions. These were met during the NE-trending Hurricane Joaquin, but not the NW-trending Frances. Storm characteristics, particularly trajectory, appear to play a significant role in determining overwash potentials along highly irregular bedrock-dominated coastlines. These implications for overwash dynamics must be considered and reconciled when attempting to reconstruct regional storm histories from coastal sedimentary archives.