Whole-island approach to relative age models and stratigraphic frameworks of the Bahamian-Caicos Archipelago

Abstract:

The Quaternary stratigraphic record of the Bahamas-Caicos archipelago (B-C) is exceptional in its three-dimensional representation of ancient shallow-water carbonate seascapes. During deposition of the last interglacial (LIG or MIS 5e) when sea level was +2 to +6m (+9?), strand plains, reefs, tidal bars, polygonal bars, lagoons, tidal flats/channels, and aeolian dune complexes portray a system both closely analogous to present-day and yet with several distinct differences. Advances in the availability of high-resolution digital terrain models from airborne lidar (optimum) and lower resolution but more broadly available SAR data reveal this paleogeography with exquisite detail. Bare earth digital terrain models from airborne lidar permit detailed 3D mapping of shorelines and their elevations, directions of longshore drift, windward-leeward asymmetry, relative age of units (determined by karst density), and critically mapping of shoreline trajectory, an approach used in many sequence stratigraphic analyses of ancient strata. Through this approach, it is possible to build a robust and complete depositional system model of islands where previous attempts typically left 50-80% of the surface unmapped.

Many reconstructions of sea level from the B-C archipelago focus on a limited number of well-exposed reef or foreshore features and associated radiometric age dates to document SL position at a fixed time or limited time window. While these data points are critical, it is now possible to integrate these into a complete three-dimensional dynamic shoreline trajectory. This continuous record and the stratigraphic elements tied to it provides a robust constraint on the LIG sea level record. Using the whole-island mapping approach, the LIG SL record for the BC archipelago is seen to include early MIS 5e fringing reef systems to +3m (Cockburn Town and similar, 132-126 ka), followed by a brief fall to approximately 0 m and attendant wave-cut erosion. A second rise upper MIS 5e or MIS5e_2), including minor reef development (125-120 ka), peak SL oolitic shorelines that cluster at +6m (peak typically around 119-118 ka), and, finally, forced-regressive strandplains that must date between 118-116 ka.