IMPACT OF BIOTURBATION ON PLEISTOCENE CARBONATE SUBTIDAL SEDIMENTS, HARRY CAY SITE, LITTLE EXUMA, BAHAMAS: INSIGHTS FROM PETROGRAPHIC ANALYSIS

Pleistocene subtidal carbonate deposits of the Cockburn Town Member, Grotto Beach Formation at Harry Cay on Little Exuma Island contain a diverse assemblage of trace fossils, which were analyzed petrographically to determine how they differ from and have modified the host rocks. The stratigraphic section exposed in a large boulder at Harry Cay Marina reveals 3 units as follows: Unit 1) 30-50 cm of ooid-peloidal grainstone, with some horizontal laminations and an ichnofabric index of 3 with individual well-developed examples of Ophiomorpha, Conichnus, Planolites, and Skolithos (the latter not sampled due to its small diameter); Unit 2) 25-30 cm of ooid-peloidal grainstone, with an ichnofabric index of 5 dominated by Ophiomorpha; and Unit 3) 45 cm of ooid-skeletal grainstone with well-developed cross-bedding and an ichnofabric index of 1. All units were initially lithified in the marine realm, but experienced meteoric diagenesis, including the formation of a cap caliche crust during subsequent subaerial exposure.

Petrographic study indicated that Ophiomorpha burrows, formed by callianassid shrimp, have much more micrite in their burrow wall pellets relative to the host rock. This suggests that the shrimp concentrate micrite during burrow-construction activities. Interiors of Planolites, formed by ballanoglossid worms, are finer grained than the host rock, suggesting that the worms sorted sediment while ingesting it. Conichnus, likely formed by burrowing sea anemones, contains sand grains that appear more loosely packed and better lithified relative to the host sediment. This suggests that burrowing activity created a more porous fabric that allowed for greater concentration of diagenetic fluids and more cement precipitation.

These results demonstrate that burrowing organisms modify the composition, texture, and fabric of subtidal carbonate sand-rich sediment through processes of micrite concentration and compaction (Ophiomorpha), sand sorting (Planolites), and repacking (Conichnus), respectively. In conjunction with formation of ichnogenic megaporosity associated with open-burrow networks and differential lithification of sediment, these processes can increase heterogeneity of porosity and permeability distribution in Quaternary carbonate rocks, impacting their aquifer properties.