Anatomy of a late Quaternary carbonate island: Constraints on timing and magnitude of sea-level fluctuations, West Caicos, Turks and Caicos Islands, BWI

The data set of the island of West Caicos consists of a combination of high-resolution lidar and digital imagery, radiometric data, and amino acid racemization (AAR) D/L values as well as extraordinary preservation of sedimentary and bio-constructed deposits on the island. Together with a well-established regional stratigraphic framework from the neighboring Bahamian island chain and detailed field mapping, the West Caicos data set provides an opportunity to advance our understanding of the link between carbonate stratigraphy and the controlling parameters of sea level, sediment supply, tectonics, and climate. The exceptionally well-preserved stratigraphic record of the mid-late Pleistocene from West Caicos allows recognition of marine isotope stages (MIS) 9/11, 5e, and Holocene. The west coast of the island provides a continuous 8.4 km exposure of the MIS 5e record that enhances our understanding of the intra-MIS 5e bipartite succession in this region and supports a sea-level lowering that occurred during the last interglacial (LIG). West Caicos accreted as a series of eolian, foreshore-shoreface, barrier-reef, and fringing-reef fades. Shoreline positions for all but the MIS 7 and MIS 5a/c deposits are now captured in the sub-meter geodetic-quality topobathy lidar survey. The Star Town unit of MIS 9/11 age forms the southern margin of the island as well as the central karst-modified spine, with uranium-thorium(U-Th) coral ages of between 368 and 365 thousand years ago (ka) and a D/L Glu average of >0.60. Early MIS 5e strata include the Railroad Ridge unit in the center of the island, which was previously identified as one of the older units. The Railroad Ridge unit is a north-south linear dune ridge that has a D/L Glu average of 0.52 and indicates the onset of sediment sources from the east associated with a flooded central Caicos Platform. Approximately synchronously, the early MIS 5e South Reef unit (+3.75-4 m sea-level elevation) nucleated on the western coast of West Caicos, attached to the middle Pleistocene Star Town unit. The South Reef corals yield an average coral U-Th age of 126.5 ka, with an average D/L Glu from skeletal matrix of 0.46. This transitional barrier/fringing reef is unconformably overlain by the late MIS 5e Boat Cove unit. Corals in the basal transgressive phase of this upper MIS 5e Boat Cove unit yield an average 120.6 ka U-Th and average D/L Glu of 0.45. Shallow-marine, foreshore, and eolian facies of the late MIS 5e rest on a wave-cut terrace that cuts the early MIS 5e South Reef unit by as much as 3 m down to a +0.5 m above present sea-level elevation. The upper MIS 5e Boat Cove unit displays a maximum of +4.5 m elevation above present-day sea level, measured from the middle of the foreshore. This estimate may be a minimum value as the exposures on the west coast are part of a forced regressive foreshore sequence. Last stages of the upper MIS 5e deposition are seen in a series of regressive dune beach ridge deposits referred to as the Northeast Ridges unit, with sea-level positions ranging from +4.5 m early in the central part of West Caicos, to +0.5 m on the far northeast coast, eventually building down below present-day sea level. This sea-level history fits well with previously documented records in the Bahamas and globally, and the extensive erosion supports a short-lived (between 126 and 120 ka) intra-MIS 5e relative-sea-level fall. Holocene deposition on West Caicos is restricted to northern and eastern margins of the island as a combination of strandplain and dune ridge mixed skeletal-oolitic units. Using the high-resolution digital elevation model (DEM) and linked digital geologic map makes it possible to compare areas of the different Pleistocene and Holocene units quantitatively and to constrain volumetric assessments and accumulation rates on the island for different units. The peak sediment volume production/accumulation rate-between 6.5 and 8 x 10(6) m(3) or between 2.2 and 2.7 x 10(6) m(3) per ka-is observed in the upper MIS 5e unit. This rate is 5 times greater than the 0.5 x 10(6) m(3) per ka estimated for the Holocene ooid dune-ridge/strandplain complex, suggesting that the combination of higher sea level, greater wave and wind energy, and warmer ocean temperatures may have fostered the higher rate of chemically precipitated sediment supply. (C) 2018 Elsevier Ltd. All rights reserved.