Lithophaga borings and their influence on the diagenesis of corals in the Pleistocene Ironshore Formation of Grand Cayman Island, British West Indies

Corals in the Pleistocene Ironshore Formation of Grand Cayman Island have been infested by Lithophaga sp. that formed borings up to 15 cm long and 4 cm in diameter. Although some of the borings were probably generated while the corals were alive most were formed after the corals had died. Some of the large coral heads (up to 1.5 m diameter) of Montastrea annularis and Diploria labyrinthiformis have had a 14 to 15 cm wide outer band almost totally transformed by the Lithophaga borings. The borings, which have a distinct clavate form, are assigned to the ichnospecies Gastrochaenolites torpedo Kelly and Bromley, 1984. Many of the borings are lined with laminated, fluorescent cryptocrystalline calcite that may have been formed by the merger of peloids. Similar material also encrusts many of the Lithophaga shells. After death the borings were filled with (1) cryptocrystalline calcite, (2) porous cryptocrystalline calcite, (3) honeycombed cryptocrystalline calcite, (3) pelsparite, and (4) pelmicrite which locally contains small calcitic ostracod(?) shells. The cryptocrystalline calcite is always fluorescent and thus contrasts sharply with the nonfluorescent calcite of the shells and the spar cement. The corallites around the Lithophaga borings are partly or totally occluded by (1) first generation, non-fluorescent aragonite cement, (2) second generation, non-fluorescent spar calcite, (3) fluorescent peloids, and (4) featureless, fluorescent cryptocrystalline calcite. The fact that the cryptocrystalline calcite in the corallites has the same fluorescent character as the cryptocrystalline calcite in the Lithophaga borings suggests that the two may be genetically related. The infestation of the coral heads by Lithophaga played a major role in the evolution of the diagenetic fabric in the coral heads. It is evident that the Lithophaga borings can (1) substantially weaken the coral heads thereby making them more susceptible to erosion, (2) serve to bind the substrate together by virtue of the dense calcareous linings in the borings, (3) liberate substantial amounts of calcium carbonate from the coral heads, (4) increase the rock-area available for further bioerosion by sponges, algae and fungi, and (5) by virtue of the empty borings provide protected sites for sediment accumulation.