Corals to rhodolites to microbialites; a community replacement sequence indicative of regressive conditions

Late Pleistocene strata on Grand Cayman and San Salvador Island contain fossil corals which became the nuclei for rhodolites that in turn became the nuclei for microbialites. The corals grew in shallow lagoons (<5 m deep) where there was constant water movement due to the breaking of waves over the nearby reef. Dead branches of Acropora cervicornis that lay on the seafloor became encrusted by red algae, foraminifera, and serpulid worms to form rhodolites. Photophilic Porolithon was the initial colonizer. Subsequent colonization by sciaphilic Neogoniolithon, Tenarea, Lithothamnium, Carpenteria utricularis, Homotrema rubrum, and serpulid worms occurred in cryptic habitats on the undersides of the branches. Periodic high energy conditions overturned the coral branches and promoted the development of the encrusting organisms on all sides. For Pocillopora, which remained in life position, the sciaphilic organisms only encrusted those parts of branches that were shaded by other branches. The final stage in the community replacement sequence involved filamentous cyanobacteria trapping micrite to form microbialites. This probably occurred in a shallow, quiet-water setting where micrite was accumulating. During this stage there was no movement of the substrates. The complex community replacement sequence from corals to rhodolites to microbialites developed in response to the drop in sea level which followed the Sangamon highstand approximately 125,000 years ago. As the water became progressively shallower and quieter the fauna and flora changed accordingly.