MESOSCALE BIOTURBATION PATTERNS IN CARBONATE AND SILICICLASTIC COASTAL SEQUENCES: INSIGHTS FROM GEORADAR IMAGING

Bioturbation by vertebrates and large invertebrates in terrestrial coastal settings can result in substantial reworking of both siliciclastic and carbonate substrates. In order to assess burrow characteristics (dimensions, geometry, connectivity, fill type), ground-penetrating radar (GPR) imaging is being successfully implemented at a number of sites along the U.S. Mid-Atlantic coast and the Bahama Archipelago. To date, surveys have focused on active and abandoned burrows of ghost crabs (Ocypode quadrata, the only species found in both sites), and the Bahamian blackback land crabs (Gecarcinus lateralis), blue land crabs (Cardisoma guanhumi), and San Salvador rock iguanas (Cyclura rileyi, burrows surveyed in a captive habitat). Whereas the majority of recent efforts address comparative neoichnology of the ghost crab (Psilonichnus isp.) and blackback crab, little is known about the two latter tracemakers. Due to their large burrow size (width >10 cm; length >60 cm) and massive spoil mounds, blue land crabs may play an important role as zoogeomorphic agents in low supratidal settings throughout the archipelago and beyond. Despite signal attenuation by saltwater, GPR is emerging as a viable tool for describing C. guanhumi burrow complexes. Subsurface characterization of these and other bioturbation structures along shore-normal geomorphic and ecological gradients will form the basis for assessing the tracemaker-habitat feedbacks. By utilizing diagnostic GPR signatures to differentiate the varying ichnotaxa, the nature of overlapping or compound bioturbation types (ichnoassemblages) can be attributed to either ecological factors (new ichnocoenosis) or habitat shift (changes in sea level, water table, vegetation type, etc.). In addition, high-frequency (800-2,300 MHz) imaging can help quantify the role of bioturbation structures as mesoscale fluid conduits through otherwise impermeable horizons. For example, burrow fill that differs from the matrix may influence fluid flow, both within a stratigraphic layer and between layers, particularly in areas of intense bioturbation. Therefore, our findings have implications not only for the ichnology of key tracemakers, but also for assessment of micro- and megaporosity patterns and connectivity in carbonate and siliciclastic reservoirs.