CRITERIA FOR RECOGNIZING CLOTTED MICROBIALITES IN PLEISTOCENE CORAL-RICH, SHALLOW-MARINE CARBONATES FROM THE BAHAMAS

In field examination of cm-thick encrusters on corals from Pleistocene shallow-marine carbonate deposits in the Bahamas, laminated fabrics can be relatively easily recognized as microbial in origin (i.e., stromatolites), especially in well exposed outcrops. However, in 50 mm diameter core samples, clotted or thrombolitic microbialites are more difficult to identify in the absence of associated stromatolites and coral substrates.

Up to ~15 cm thick microbial encrusters are present on Eemian (MIS 5e or Last Interglacial) Acropora cervicornis coral branches in outcrop at Cockburn Town on the west coast of San Salvador Island, and in cores from the same locality and The Gulf on the island’s south coast. Dead coral fragments were first coated with mm-scale skeletal encrusters (coralline red algae, serpulids, foraminifera), and then overlain by microbialites consisting of: 1) stromatolites on the upper sides of corals; and 2) clotted microbialites on the lower surfaces and eventually engulfing the stromatolites.

Petrographic analyses of coral encrusters reveal that stromatolites incorporated fine-grained skeletal-peloidal-ooid sand within micritic laminae. Clotted microbialites contained abundant micrite, organized in patches and incorporating fine- to medium-grained carbonate sand. Some examples of clotted microbialites also displayed crudely laminated fabric. All clotted microbialites have irregular fenestral pores and are associated with encrusting foraminifera, with Homotrema rubra and Carpenteria utricularis dominant, and Gypsina plana and Planogypsina acervalis less common.

These observations allowed us to establish the following criteria for recognition of clotted microbialites: 1) the presence of micritic patches that incorporate carbonate sand; 2) abundant fenestrae; 3) common encrusting foraminifera; and 4) a crudely laminated fabric. When these criteria were applied during re-examination of cores from San Salvador, we recognized that microbialites are more abundant in some sections of these cores than previously thought. Having a clear set of criteria for recognition of microbialites provides more accurate information on their abundance and for interpreting the paleo-environmental conditions that led to their formation.