Holocene Sediments of Northern and Western Caicos Platform, British West Indies

Abstract A combination of sediment-probing for thickness, push coring, and vibracoring has resulted in better documentation of the sedimentary facies, vertical successions, and Holocene evolution of northern and western Caicos Platform - adding to its value as an excellent modern analogue for ancient carbonate platforms and associated hydrocarbon reservoirs, especially those with a grain-rich interior. The Holocene history of sedimentation on northern and western Caicos Platform appears to have been dominated by aggradation of a complex arrangement of depositional environments that were greatly influenced by Pleistocene paleotopography (especially the location and topography of Pleistocene aeolian dunes that form the back-bone of the present-day islands) relative to dominant wind directions. Bank-top flooding began at least approximately 4,800 ybp, based on radiocarbon dates of peat and skeletal grainstone immediately overlying Pleistocene bedrock in cores from tidal flats of North Caicos Island. Tidal flat sediments of North, Middle, and East Caicos Islands accumulated in the lee of Pleistocene islands, whereas skeletal sands accumulated in the lee of aggrading reefs on the windward margins of the bank. Ooid sands formed locally within the platform interior and on beaches exposed to the easterly prevailing winds. Platform interior sediments are mostly a mix of pelletal, skeletal, and ooid grain-rich sediments reflecting extensive bioturbation and exposure of the platform to the winnowing effects of prevailing easterly winds. Mud-dominated platform-interior sediments are rare. Salinas locally formed between Pleistocene dunes on the western margin of the platform as marine waters flooded inter-dune lows of West Caicos at least 3,220 ybp, based on radiocarbon dating. Sediments in the East (Great) Salina consist of interbedded skeletal wackestone to grainstone, massive gypsum, and Scytonema mats reflecting alternating periods of marine influence and restriction. Sediments from marine environments are dominantly aragonite (> 80%) with lesser amounts of high- and low-Mg calcite, based on x-ray diffraction analysis, although subtle differences between marine environments are discernable. Gypsum is dominant in salina sediments, with lesser amounts of aragonite, low- and high-Mg calcite, halite, and dolomite. Nonstoichiometric dolomite was found in a few samples from tidal-channel levees on North Caicos and at the base of the salina sediments on West Caicos, but is volumetrically insignificant. Sediment accumulation rates range from 22 cm/1,000 yrs in gypsum-dominated salina sediments to a high of 106 cm/1,000 yrs for skeletal sands deposited behind the Providenciales Island barrier reef.