PETROGRAPHIC AND STABLE ISOTOPE ANALYSIS OF BIOTURBATED AND SUBAERIALLY ALTERED SUBTIDAL PLEISTOCENE LIMESTONE IN THE BAHAMAS

Pleistocene subtidal ooid-skeletal-peloidal grainstone of the Cockburn Town Mb., Grotto Beach Fm. at Harry Cay on Little Exuma Island contains well-developed Ophiomorpha, Conichnus, Planolites and Skolithos. Samples of these trace fossils were analyzed petrographically and for stable isotopes to determine extent of modification of matrix sediment, which was initially lithified in the marine realm, but later experienced meteoric diagenesis and formation of a caliche cap.

Ophiomorpha (=O.) from Harry Cay was compared with Pleistocene samples from San Salvador, Rum Cay, Great Inagua, Great Exuma and Grand Cayman Islands, as well as Holocene O. from San Salvador and Lee Stocking Islands. Pleistocene O. wall pellets have more micrite than host rock, indicating that callianassid shrimp concentrate mud during burrow construction. Stable isotope values of O. range -1.5 to -4 ‰ VPDB δ¹⁸O and 0 to 3.5 ‰ VPDB δ¹³C, with complete overlap among multiple sites and substantial overlap with calcarenite host rock and burrow fills. Holocene O. has higher values, typical of unaltered marine carbonate, clustering ~0 ‰ δ¹⁸O and 4.5 ‰ δ¹³C.

Some O. tubes are filled with caliche similar to the cap at Harry Cay. δ¹⁸O values of these caliche fills overlap with O. walls, but their low δ¹³C values (-8 to -9 ‰) reflect soil-derived ¹²C. δ¹³C values of some calcarenite fills are between those of burrow walls and caliche, forming an inverted J trend indicative of meteoric diagenesis, with burrows providing fluid pathways. This resulted in preferential meteoric diagenesis of micrite in burrow walls as supported by their δ¹⁸O values lower than associated matrix, and their calcitic composition compared to the mixed aragonite-calcite host rock.

Planolites traces appear finer grained than the host rock, suggesting that ballanoglossid worms can sort sediment while ingesting it. Conichnus contains sand grains that appear more loosely packed than host sediment, indicating that burrowing activity, possibly by sea anemones, created a porous fabric. Skolithos tubes are too thin to examine petrographically, but isotopic composition of calcarenite within these three trace fossils is not systematically different from the host rock, reflecting similarity in their composition and diagenesis despite textural and fabric modifications by burrowing organisms.