Large-scale carbonate platform development of Cay Sal Bank, Bahamas, and implications for associated reef geomorphology

Abstract

The Bahama Archipelago consists of an arcuate chain of carbonate platforms. Average water depths on the platform-tops, such as the Great Bahama Bank (GBB), are typically 10m or less, with coral reef-rimmed margins, thick sediment accumulations, and the frequent occurrence of islands. There are, however, exceptions. For example, Cay Sal Bank (CSB), a little studied detached Bahamian carbonate platform with depths ranging from 30 to 7m, is only slightly deeper than the GBB, but devoid of islands, lacks platform-margin coral reefs and holds little sediment on the platform-top; the platform is incipiently drowned. CSB is interesting as it is conspicuously larger (6000sq.km) than other incipiently drowned platforms in the region, such as Serranilla Bank (1100sq.km) and the Cat Island platform (1500sq.km). Field and remote sensing data are assembled to provide insight into the sedimentology and geomorphology of the CSB. The influence of ocean climate, regional hydrodynamics, and Holocene flooding history are investigated to understand why platform-margin coral reef growth on CSB has been unable to keep pace with Holocene sea-level rise. A decade of regional sea-surface temperature data for the Bahamas report CSB to be situated in the same ocean climate regime as GBB. Temperature cannot explain the platform's different morphologies. The Florida Current has been evoked as a possible reason for the immature development of platform-top processes on the CSB, but numeric modeling suggests its influence to be restricted to the deep flanks of the bank. Further, sediment distribution on CSB, including infill patterns of karst depressions, suggest trade winds (easterlies) to drive platform-top hydrodynamics. By assembling a satellite-derived bathymetry map, it can be shown that CSB flooded earlier and at relatively higher rates of Holocene sea-level rise than its neighboring platforms. Flooding history is identified as the most feasible explanation for the atypical morphology of the CSB. By contrasting the present-day morphology of the CSB and the GBB, the work emphasizes how subtle differences in relative sea-level history can influence the growth of platform-margin coral reefs, features that in turn can conspire to set even closely neighboring carbonate platforms on divergent paths with regard to the development of marine landforms. This insight is relevant to interpreting the morphological diversity of carbonate platforms in the modern ocean and in the rock record.