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