Formation and evolution of Pleistocene (MIS 5e) strandplain grainstones along the leeward margin of West Caicos island, BWI
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The island of West Caicos is a typical Caribbean Pleistocene-Holocene carbonate strandplain system that developed by accretion of successive dune-foreshore-upper shoreface intraclastic-peloidal-oolitic limestone grainstones. Although strandplains are the most volumetrically significant element of low-latitude carbonate island interglacial highstand deposits, documentation of the 3D facies architecture and accumulation rates for these basic building blocks is sparse. This detailed mapping project documents for the first time on the Caicos platform the timing and origin of a foreshore-upper shoreface ooid grainstone body along the west coast of West Caicos. Timing has been constrained for this grainstone body to the latter portion of the composite Marine Isotope Stage (MIS) 5e signal (5e_2), providing very tight constraints on duration (2-4 ky) and sea level history; there was a rapid rise from near present sea level to +6 m, which lasted from 121-118 kya. Subsequently, deposition ended with a rapid fall to -60m by 5d time (118-110 kya). Using these age constraints and high-resolution airborne LIDAR mapping, this study analyzes vertical and 3D rates of accumulation, discrete facies architecture, provenance, and transport for this MIS 5e_2 strandplain. Dip-oriented 2D cross sectional mapping across the 9 km north to south length of the west coast outcrops was carried out at 3 locations, and several 3D facies models were made. These maps fit into the larger regional map of the west coast MIS 5e and are enhanced with paleocurrent data from longshore-directed trough-cross-strata and foreshore strata. Across the profile, 53 thin sections document grain types, size, and sorting. The profiles consist of 3-6 degree dipping foreshore strata with dip widths of 10-20 m and plunge zone to upper shoreface facies with trough-cross-strata showing a distinct southward transport direction. Along the northern section of the profile, massive bioturbated lower upper shoreface grain-dominated packstones are present down dip from the upper shoreface. These massive grain-dominated packstones accumulated to 1 m thick, occur in greater than 2 meters of water depth, and rest locally on top of corals of the 5e_1 reef system. The rise to a sea level six meters higher than present during MIS 5e_2 time increased ooid production on the Caicos platform and throughout the Caribbean (Kindler and Hearty 1996). Although a large amount of ooids were likely winnowed off the platform top due to prevailing easterlies, many of the ooids were transported from north to south along the leeward coast of West Caicos via longshore drift. This interpretation is supported by a unidirectional southward paleoflow direction, decreasing lateral (along-strike) continuity in 5e_2 grainstone units from north-south, and the poorly sorted nature of grain type in the petrography from north-south. The heterogeneity in grain type suggests a fair amount of grain mixing, indicative of transport over several kilometers; active ooid factories on West Caicos such as Ambergris and Long Bay Beach have >90% ooid compositions, much greater than ooid compositions along the leeward margin of West Caicos. Assuming a 3 ky period of deposition, vertical accumulation rates for this MIS 5e_2 strandplain are as large as 2 m/ky (6 m total), and 3D accumulation is 115,000 m3/ky (345,000 m3 total). Understanding the depositional and accumulation history during sub-Milankovitch sea level fluctuations underscores the stratigraphic complexity of Pleistocene icehouse systems, and shows short-lived rises in sea level can have significant effects on sediment production and carbonate island building.