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

William A. Morgan
ConocoPhillips, Houston, TX

A combination of sediment-probing, push coring, and vibracoring has resulted in better documentation of the sedimentary facies, vertical successions, and Holocene evolution of northern Caicos Platform and its importance as an analogue for ancient carbonate platforms. Approximately 32 m of core from 22 locations on Caicos Platform were collected from a suite of depositional environments, including salina, tidal flat, ooid and skeletal beaches, muddy and grainy platform interior, incipient algal bank, and back-reef sands. Carbonate sands, situated some 750 m behind the barrier reef crest north of Providenciales Island, were the thickest sediments penetrated (5.75 m). In contrast, a thin veneer or no Holocene sediment was locally encountered between low-relief ooid bars off the southern shore of Providenciales Island and in tidal channels of North Caicos Island.

Sedimentation appears to be dominated by aggradation to retrogradation of a complex mosaic of depositional environments, determined in large part by paleotopography, including the location of exposed Pleistocene aeolian dunes, relative to dominant easterly winds and currents. Bank-top flooding began at least 4,800 YBP, based on radiocarbon dating of tidal-flat sediments of North Caicos Island. Salinas formed locally as marine waters flooded lows between Pleistocene dunes, commencing at least 3,220 YBP on West Caicos Island. Sediments from marine environments are dominantly aragonite (> 80%) with lesser amounts of high- and low-Mg calcite, based on x-ray diffraction analysis of 122 samples, 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, but is volumetrically insignificant.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas