GPR IMAGING AND SEDIMENTOLOGY OF HOLOCENE STRAND PLAIN DEPOSITS, CROOKED ISLAND, BAHAMAS

Direct observations of modern carbonate systems can be used to constrain models of their ancient counterparts. Carbonate strand plains make up much of the terrestrial Bahamas platform and are thus important architectural components. Carbonate strand plains can be difficult to identify in the ancient rock record due to their limited composite vertical extent and very fine-scale internal structure. Ground-penetrating radar (GPR) provides high-resolution (decimeter or better, vertically) imaging of strand plain deposits that aid in determining if they are created by catastrophic storm events or by more gradual relative sea-level fluctuations. Much of the Acklins and Crooked Island archipelago, Bahamas, displays clear geomorphic expressions of lateral sedimentary accretion expressed as arcuate ridges and swales visible from satellite imagery, interpreted as a Holocene(?) strand plains. In order to obtain subsurface images of the strand plain deposits, we conducted GPR profiling on Crooked Island. We used a 400-MHz antenna in continuous mode with a 100-ns record length at ~0.05 ns/sample, and horizontal sampling of ~59 traces per meter. Our survey program provided geological profiles as much as 600 m long, approximately 2 m depth, in six different strand plain settings. The profiles furnish detailed, high-resolution images of stacked and/or imbricated clinoforms, dipping northward, toward the Atlantic Ocean. In places, the clinoforms appear within a discrete sequence, limited vertically and horizontally; elsewhere, clinoforms occur in a long, monotonous and continuous package with unvarying dip magnitude for over 100 m length. To date, we cannot observe any firm correlation in reflectivity patterns between ridges and swales (i.e., the subsurface beneath each are apparently equally reflective). Additionally, sediment samples were collected along selected GPR traverses down to a depth of 1 m that will be compared with the radar reflectivity and with sediment samples collected across the adjacent modern foreshore and shoreface immediately offshore of Crooked Island, in line with the GPR profiles. The integration of subsurface and offshore sediment samples and the radar reflectivity patterns constrains the interpretation of the vertical facies stacking patterns in the radar profiles.