Abstract
A series of four continuously-cored boreholes (76 to 95 meters deep) across Little Bahama Bank serves as the basis for a study of the facies evolution, biostratigraphy, and diagenesis of post-Late Miocene limestones and dolomites. A general four-stage sequence of facies evolution is documented: (1) a late Miocene flat-topped bank upon which shallow-water reefs and reefal sands were deposited during a period of rapidly fluctuating sea level; (2) an open-circulation bank flooded to ca. 10 meters, having more sloping margins upon which open fore-reef rhodolith/skeletal packstone prevailed (latest Miocene-early Pliocene); (3) a possible atoll-like bank characterized by raised reef rims and open circulation skeletal deposition in the bank interior; and (4) a later Pliocene to Recent section of predominantly non-skeletal sediments (Lucayan Fm.) deposited on a flat-topped, "awash" bank during a period of rapidly fluctuating sea-level.A biostratigraphic framework based on the occurrence of a distinctive and datable molluscan assemblage has been established. The top of this Bowden-equivalent-assemblage (at ca. 18-20 meters depth bankwide) is interpreted as marking the late Pliocene onset of northern hemisphere glaciation. Regional application of this biostratigraphy suggest that LBB is subsiding at approximately half the rate of the remainder of the Bahamas.Cement types and distributions were established. Lithification generally increases with depth with local anomalously well-cemented shallower intervals associated with subaerial exposure horizons, unusually well-developed syndepositional submarine cements, and the marginal reef facies. Porosity is high throughout, with primary yielding to secondary vuggy porosity with depth. Submarine cementation is common, but meteoric leaching and cementation are predominant.A thick sequence of dolomite consisting of four major macroscopic and five microscopic types occurs bankwide at depths as shallow as 16 meters. Much of this dolomite is texturally supermature, resembling certain Paleozoic dolomites in density. The texturally supermature dolomite is late Miocene in age, while the remainder results from post-late Pliocene dolomitization. The high degree of textural maturity of these young dolomites is significant because it clearly results from near-surface, early diagenetic processes acting over relatively short periods of time.