The Role of Islands in Influencing Carbonate Platform-Top Deposition

Abstract

Great Bahama Bank (GBB) is a useful Modern analog to aid subsurface interpretation, particularly in the correlation of depositional facies within high-frequency sequences (cycles) across a platform top. Islands atop GBB primarily consist of exposed Pleistocene limestones (eolian, beach and locally subtidal deposits from previous highstands MIS 5, 9 and 11). Although numerous (n = 1,430), islands occupy only 8% (or 8,700 km²) of the platform-top. Nevertheless, islands play a critical role in focusing and shaping the filling of accommodation space during the Holocene by significantly influencing the local environment (tidal currents, wave energy, wind transport, etc.) to localize high-energy grainy factories and shape their deposits around them. Islands also provide protection in their lee for mud accumulation. Remote sensing, facies and water depth mapping reveal that the likelihood of encountering filled accommodation decreases exponentially with increasing distance from islands, and the probability of encountering under-filled accommodation is 70% for the first 50 km offset from the islands. Islands that have built back from the platform-margin into the more protected platform-interior support particularly broad leeward expanses of filled accommodation, whereas those situated on the platform's windward promontories have conspicuously poorly developed windward accumulations of filled accommodation. Seventy percent of sediment in the areas of filled accommodation is rudstone, high-energy grainstone, grainstone and mud-poor packstone; thus there is a linear increase in the probability of encountering grainy facies (potential reservoir) and a corresponding decrease in the likelihood of encountering potential baffle (= muddy facies) with increasing distance from islands. The most abrupt lateral changes of depositional facies across the platform top are observed leeward of islands, areas that also hold the highest diversity in facies type. Most islands on the platform are preferentially distributed along the eastern (windward) margin of GBB and align with the NW-SE strike of the margin; these islands, in turn, exert control on the shape and orientation of facies belts that develop in proximity to them. For this reason, regions of the platform that contain prevalent islands host facies belts that align with the principal axis of the platform, whereas for regions lacking islands, the facies belts adopt an E-W trend consistent with prevailing winds and currents.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016