EVENT-SCALE MORPHOLOGICAL AND GEOPHYSICAL (GPR) SIGNATURES IN BAHAMIAN COASTAL LITHOSOMES

Coastal accumulation forms of the Bahamian archipelago span multiple azimuth orientations and archive geological signatures of extreme events. At eleven coastal sites on Exumas and Eleuthera, satellite images and topographic surveys were complemented with >4 km of 800 MHz georadar profiles to assess styles of event deposition and erosion. Holocene coastal lithosomes of bioclastic and oolitic sand (Hanna Bay Member) range from simple reef-top washover fans and single-ridge welded barriers to catenary prograded strandplains comprising 10-20 ridges (e.g., Freedom Bay Beach, Eleuthera; Moriah Harbour Cay strandplain, Little Exuma). Surficial erosional indicators include dune and berm scarps with steep to vertical faces (angle of erosional surface: ψ~90°). Occasional wide gaps in ridge trends may be due to either storm-surge breaching or focused deflation prior to lithification. Wave-induced scalloping and subsequent infilling of exposed aeolianites generate compound unconformities and onlap packages. In subsurface images of the upper 1.0-2.5 m of beach-ridge plains, truncations of berm (bi-directional dip of β<5°) and beachface (β=7-10°) reflections are interpreted as paleo-scarps (ψ>25°). Subsequent onlapping sequences display chaotic signal return characteristic of aeolian aggradation. At Lighthouse Point, Eleuthera, one of the headland-separated barriers exhibits a prominent foredune ridge, which is backed by a 40-65-m-wide landward-sloping platform with scattered limestone blocks (some >0.5 m in size). Geophysical images help trace shallow beachrock (North Point Member) beneath gently landward-dipping accretionary surfaces terminating at hypersaline water table. Although a cyclone genesis of this “surge barrier” is likely, with its SW exposure and 900 km distance from the Puerto Rico trench, a tsunami origin cannot be ruled out. With carbonate lithification taking place in less than 50-100 years at some sites, GPR imaging offers a viable tool for reconstructing the modes of emplacement and punctuated progradation of Bahamian coastal successions. Coupled morphostratigraphic and sequence-stratigraphic datasets provide the framework for subsequent age dating of indicative bounding surfaces and associated sedimentological, paleontological, and archaeological materials.