COMPARATIVE SEDIMENTOLOGY OF CARBONATE PLATFORMS–LESSONS LEARNED FROM THE BAHAMAS

Recent studies of modern carbonate environments in the Bahamas continue to refine depositional and diagenetic models. New insights into the stratigraphic framework of carbonate platforms include an understanding of how isolated platforms may coalesce through progradation along leeward margins by highstand shedding of bank-top derived sediment. The resulting “highstand wedge” mimics in many ways the “lowstand wedge” of classical sequence stratigraphy, suggesting that further refinement of these models is needed. Additionally, seismic reflectors in pure carbonate systems have been shown to be the result of lithologic and diagenetic change, many regionally correlatable seismic sequence boundaries are indeed chronostratigraphic horizons, and failure of platform margins and slopes and subsequent deposition of megabreccias may occur during both lowstands and highstands of sea level.

For depositional models, key lessons include that lithofacies types and distribution are relatively consistent across platforms but are dependent upon paleogeography and paleoceanography. The role of antecedent topography in initiating development of both reefal and sand bodies is strongly coupled to the windward margin positioning of these facies, while the sedimentary make-up (grain vs. mud dominated) of proximal slope facies is also dependent on the windward/leeward orientation of the margin. In addition, details of the genesis of shallowing upward cycles in different environments, coupled with the realization that unfilled accommodation space is common, adds to our understanding of ancient platform equivalents and suggest limitations inherent to cyclostratigraphic correlation.

Important advances from a diagenetic perspective include a realization that syndepositional marine cementation takes place not only in shallow subtidal and intertidal environments, but also at depths of at least 60-75 m. These findings suggest that paradigms associated with slope stabilization and the formation of submarine hardgrounds and seismic reflector horizons need to be revisited. Other recent work has focused on the role of micro-organisms in the cementation process while also documenting the presence of “meteoric-like” moldic porosity fabrics occurring in a marine phreatic environment.