CORAL REEFS AND PATCH REEFS AT DEAD MAN'S BONES: RAPID BURIAL OR SLOW SEDIMENT STRESS

The Pleistocene mixed clastic-carbonate Port Morant Formation, exposed at Dead Man's Bones, St. Thomas, Jamaica, shows a well-defined coral biofacies zonation similar to that of modern Caribbean reefs. The reef crest zone is characterized by A. palmata and D. strigosa corals and consists of horizontally bedded limestones. Behind the reef crest zone, packstones contain fragmented corals and large coral patch reefs. Landward of the reef zone the sediments contain extensive rhizocretions indicating growth of extensive trees on the beach as the lagoon behind the reef was filled. Also exposed in the lagoon fill are large (metre-size) coral heads of Solenastrea bournoni which shows extensive growth lines and borings that correspond with the surrounding sediment. These patch reefs provide an excellent opportunity to look at corals under stress.

Transects through the coral patches were used to assess coral community composition, coral growth fabric and interstitial sediments. Throughout the largest patch reef (~80 m wide and ~4 m in height) the sediment is consistently carbonate rich with in situ superstratal growth fabric. There is variation in coral composition throughout the patch but no unequivocal evidence that this is due to sedimentation stress. Transects through smaller patches show primarily columnar, massive forms at base of section with gradual loss up section of continuous growth fabric and in situ branched colonies.

The transects indicate a variety of responses to progradation of the Port Morant delta with respect to the relative positions of the patch reefs and delta mouth. Observations suggest that the closest reef is killed relatively quickly whereas patches further from the delta persisted longer. Did these corals die of stress from high sediment fluxes as the delta built out or did they die from a major event?