Controls on Sedimentology and Geomorphology of Holocene Isolated, Shallow, Tropical Carbonate Platforms: Bahamas and Beyond

Gene Rankey
University of Kansas, Lawrence, Kansas ([email protected])

Abstract

Isolated carbonate platforms are the archive of much of geologic history. Similarly, in many cases, they host large hydrocarbon reservoirs (e.g., Miocene of Southeast Asia, Devonian-Carboniferous of Caspian Basin). Although we understand the general factors influencing the stratigraphic evolution of isolated platforms, factors controlling platform-scale geomorphic and sedimentologic details within individual time-slices are less well constrained. The purpose of this talk is to systematically explore controls on facies patterns of Holocene platforms from a ‘process-response’ spectrum, based on field, remote sensing, and lab observations of several Holocene isolated platforms in the Caribbean and Pacific oceans. The results provide testable conceptual models that might be used to predict stratal architecture and potential reservoir quality in subsurface analogs.

To begin to develop these predictive models, this study compares geomorphic and sedimentologic patterns on platforms across a ‘process-response’ spectrum. Sedimentologic and geomorphic comparison of Caribbean (Great Bahama, Crooked-Acklins, Caicos) and Pacific (Aitutaki, Maupiti, Nonouti) shallow-water (<25 m) platforms from settings with a range of island and reef-rim configuration, tidal amplitude and significant wave height reveals several themes: 1) neither an open windward margin nor currents driven by strong winds are fundamental controls on the occurrence of grainy platform interiors; 2) tide-dominated platforms commonly have broader, more gradational lateral facies changes normal to platform margins; wave-dominated platforms have more abrupt facies boundaries related to dissipation of wave energy at the margins; 3) well-developed reefs and reef aprons are likely on margins facing open-ocean swell, which may or may not be coincident with windward margins; 4) on individual platforms, and between platforms, grain size, sorting, and type are not strongly correlated with water depth, but are related instead to hydrodynamics (wave and tide energy) and setting (e.g., proximity to shelf margin); 5) although an open windward margin or currents driven by strong winds may indeed facilitate removal of muds, their absence does not dictate that a muddy platform interior is present.

These results from Holocene analogs illustrate and quantify the importance of waves, tides and currents on platform-scale facies patterns. As such, they provide conceptual facies models for understanding field and inter-well facies variability, and can aid in developing more robust geologic models of reservoirs in isolated carbonate platforms.

AAPG Search and Discovery Article #90086 © 2008 AAPG Foundation Distinguished Lecturer Series 2008-2009