ENCRUSTERS ON CORALS FROM PLEISTOCENE REEFS IN THE BAHAMAS: EXAMPLES FROM SAN SALVADOR AND GREAT INAGUA

Outcrops of Pleistocene coral reef deposits of the Cockburn Town Member (Grotto Beach Formation; Eemian; MIS 5e) on San Salvador and Great Inagua Islands in the Bahamas provide an important source of information about paleoenvironmental conditions and sea-level fluctuations during the last interglacial highstand. This research builds upon our previous field and petrographic work from two sites on San Salvador, aimed at analyzing in detail the abundance, distribution and succession of various types of encrusters on corals, with the addition of two locations on Great Inagua. Stable isotope analysis was also conducted to provide insights into the depositional and diagenetic history of these deposits.

Cockburn Town Fossil Reef on the west coast of San Salvador has in situ reef exposures ~3 m above modern sea-level, that are separated by an erosional discontinuity into Reef I and II deposits (RI & RII). RI contains branching corals encrusted by red crustose coralline (RCC) algae, foraminifera, serpulids, stromatolites, and clotted microbialites, while RII corals lack thick encrustations. The same unconformity separates RI and RII deposits at Devil’s Point on the west coast of Great Inagua. However, corals in RI exhibit only typical taphonomic modification with very thin encrustations, but an in situ RII exposure of Acropora palmata corals has a crust up to 2 cm thick, forming an encrusted branching-coral framework with irregular, bumpy exterior. Our Matthew Town Marina site, also on the west coast of Great Inagua, is a spoil pile of boulders, likely from RI, containing Orbicella annularis corals with encrustations similar to those on San Salvador.

Transition from coral to encrusting RCC algae to microbialites in RI on San Salvador has been interpreted as a change in reef development from bank barrier to restricted backreef and lagoonal environments. The absence of thick encrusters in RI at Devil’s Point on Great Inagua and the encrusted branching framework in the lower part of RII illustrate important differences between Pleistocene coral reefs on San Salvador and Great Inagua. The presence of microbialites on displaced Marina boulders on Great Inagua, however, reveals some similarities with microbial encrusters from San Salvador. Stable isotope results further illustrate these important similarities and differences.