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Stratigraphic and Geomorphologic Heterogeneity of a Windward Carbonate Platform Margin, Exumas, Great Bahama Bank

Abstract

In the Exumas portion of the platform margin of Great Bahama Bank, the combined product of high-frequency orbital sea-level changes and suborbital oscillations produces a complex lateral and vertical stratigraphic architecture that juxtaposes grainstones of different ages deposited in different environments. The 365 Exuma Cays and surrounding sand bodies span 170 km North-South and 5-10 km West-East and are composed of Holocene (<6,000 ybp), MIS 5e (∼125,000 ybp), and older Pleistocene strata. Shallow water environments are dominated by ooid, peloid, and skeletal grains within high-energy shoals, tidal channels, patch reefs, stable seagrass beds, and low-energy, bioturbated flats. Stratigraphic heterogeneity is documented through the: (1) distribution of modern carbonate sediments; (2) relationship between facies and ages exposed on islands; and (3) vertical stacking patterns deciphered through cores. Field mapping combined with satellite imagery of islands reveals that older highstand deposits create the template for subsequent dynamic deposition and accretion during the next highstand. Within a single highstand, more than one depositional cycle can be deposited and also eroded. For example, the eastern margin of the Exuma Cays features older Holocene dunes that were deposited and are now actively eroding away since the current Holocene transgression. Currently a new set of Holocene dunes is actively being deposited. Fourteen cores, drilled from +1 m to a maximum depth of −23 m, display a complex vertical stratigraphic heterogeneity featuring subtidal to beach to dune facies prograding eastward, towards the platform margin. A core from Darby Island features two MIS 5e subtidal units separated by an exposure horizon. In other cores, MIS 5e beach to eolian dune transitions occur at different depths, documenting pulsed down-stepping of sea level. The stratigraphic complexity of the Exumas is also reflected in the early distribution of porosity. Although we see the trend of porosity decreasing with depth (and age), we see early differentiation by subfacies/environments of deposition. Porosity ranges from 2 to 57% from 0-5 m in core with an average of 38%. These facies changes occur on orders of meters to tens of meters and therefore have significant implications for reservoir characterization and modeling of large grainstone bodies in the ancient.