Formation and Evolution of Strandplain Grainstones and Facies Variability Along the Leeward Margin of West Caicos, BWI

Abstract

Pleistocene outcrops of Marine Isotope Stage (MIS) 5e age along the leeward margin of West Caicos show exceptional facies variability along strike (north-south). Diagnostic facies assemblages transition from predominantly skeletal-oolitic strandplain grainstones in the north to reef-dominated facies assemblages in the south. Strandplain grainstones are essential building blocks for many shallow platform carbonate islands. These grainstone assemblages are laterally continuous with good potential reservoir quality at deposition. Understanding the provenance, formation, and evolution of modern strandplain grainstones and their contribution to island formation could prove useful for ancient isolated carbonate platform analogs—particularly those deposited during similar icehouse conditions, such as the Tengiz and Kashagan fields in Kazakhstan. This study attempts to explain the formation and evolution of Pleistocene MIS 5e strandplain grainstones through analysis of facies variability in several representative cross sections, paleocurrent data, and mapped petrographic transects. Cross sections emphasize the overall thinning of the grainstone assemblages from north to south. Thin sections sampled over 16 petrographic transects were taken along the coast at 0.5 km intervals in order to systematically describe the facies petrographically and look for trends in grain size and type. Finally, paleocurrent measurements were taken to demonstrate the persistently southward-directed paleoflow. Mapped cross sections quantify the along strike complexity of these strandplain grainstones. On the northwest coast, the grainstone assemblages are upwards of five meters thick with dip widths on the order of tens of meters. On the southwest coast, the grainstone assemblages are 0-2 meters thick with less continuity (dip widths between 5-10 meters). The along-strike continuity of grainstone units decreases from north to south as reefal facies become predominant. Paleocurrent measurements from upper shoreface troughs support the assertion that these strandplain grainstones form from northern-sourced ooids wrapping around the northwest coast of West Caicos and deposit along the leeward margin. Understanding the nature of modern carbonate platform strandplain grainstones provides insight into the depositional nature and geometry of ancient subsurface isolated carbonate platform reservoirs, which allows for more accurate constraints on exploration plays and prospect assessment.

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