PALEOGEOMORPHIC CONTROLS ON ICHNOLOGY AND SEDIMENTOLOGY OF CARBONATE SHOREFACES: PLEISTOCENE, CROOKED-ACKLINS PLATFORM, SOUTHERN BAHAMAS

Trace fossils are indicators of depositional environment, useful for determining the physicochemical conditions at the time of deposition. Although ichnology of marine siliciclastic systems is well understood, the nature, controls, and impacts effects of bioturbation in carbonate systems are not well constrained. To address this issue, this study documents the distribution and diversity of trace fossils in Pleistocene shallow marine carbonate successions on Crooked-Acklins Platform, southern Bahamas. Pleistocene strata of Crooked Island and Long Cay include coralline boundstone and rudstone deposits <0.5 km wide along strike, interpreted as patch reefs, and cross-stratified ooid-peloid grainstone, interpreted as shoreface and backshore deposits. Pleistocene strata form elongate, margin-parallel topographic ridges characterized by an upward decrease in grain size and ichnofabric index, and an increase in rhizolith content. Paleotopographic lows between ridges exhibit traces that are predominantly vertical to oblique to bedding planes (e.g., Rosselia, Cylindrichnus, escape burrows), and generally have ichnofabric indices of 4-5. These highly bioturbated areas represent stabilized, subtidal sediments deposited in an upper shoreface environment. Conversely, trace fossils within the topographic highs of ridge successions include vertical burrows (e.g., Skolithos, Conichnus) and boxwork burrows (e.g., Ophiomorpha) that have lined or reinforced walls, indicating a higher energy, shifting seafloor. These areas exhibit ichnofabric indices of 2-3, and generally have less diverse trace fossil assemblages than the low-lying areas. This study illustrates that the degree of bioturbation, diversity, and depth of penetration of invertebrate traces is closely associated with paleotopography and depositional energy. These results indicate that both trace fossil associations and ichnofabric indices are related to position within geomorphic bodies, insight that can aid in predicting environments of deposition and depositional geometries in the rock record. These ichnological and sedimentological insights can be used to reconstruct facies distributions within heterogeneous ancient carbonate systems.