Magnetostratigraphic dating and magnetization of Cenozoic platform carbonates from the Bahamas

The application of magnetostratigraphy to Cenozoic shallow-water Bahamian carbonates obviates one of the most significant barriers to our understanding of temporal and spatial platform development, that of age-dating. This study investigates the process of magnetization, utilization of the technique, and the applicability towards refined stratigraphic correlation.Magnetization of subsurface Bahamian carbonates is largely a result of fine-grained single-domain magnetite or maghemite crystals, based on magnetic characteristics, coercivity spectral analysis, and examination of magnetic separates. The single-domain crystals are interpreted as biogenic in origin owing to size, morphology, and similarity with Holocene biogenic magnetite crystals. These data suggest that an in situ magnetic source exists in isolated carbonate environments that is capable of providing a measurable remanent magnetization. This early magnetization is preserved through inversion to low-magnesium calcite and even pervasive dolomitization.Magnetostratigraphic dating of cores from San Salvador, Great Bahama Bank, and Little Bahama Bank provides many new chronostratigraphic markers where only two biostratigraphic markers existed before. These new data have refined the transition from a skeletal to non-skeletal dominated platform in the late Pliocene and suggest restricted circulation as the primary cause. Magnetically dated age/depth curves for the four cores show three main phases of platform accumulation: (1) relatively high rates from the late Miocene to late Pliocene (2.4 Ma); (2) lower accumulation from 2.4 to 0.8 Ma; and (3) a return to higher rates throughout the middle and late Pleistocene. The individual platforms do show considerable differences in net accumulation which may be related to differential subsidence, as suggested earlier and now confirmed. Similar accumulation patterns are interpreted to reflect long-term sea level changes (3rd-order). On the windward margin of Great Bahama Bank, cyclic packages containing high-energy ooid-sand sheets, and separated by subaerial exposure surfaces, record the punctuated depositional history of the platform. Eleven preserved cycles, in conjunction with the new time-lines, provide a generalized Bahamas flooding (sea level) curve for the past 3.2 Ma. This curve is in general agreement with relative onlap patterns derived from seismic stratigraphy on the leeward side of the platform