REFINEMENT OF STRATIGRAPHIC FRAMEWORK AND VARIATIONS IN DEPOSITIONAL FACIES FROM THE LAST INTERGLACIAL HIGHSTAND, LITTLE CAYMAN ISLAND

Documenting and interpreting regional variations of the deposits of the last interglacial highstand (LIG), also referred to as marine isotope stage 5e (MIS 5e) are critical for understanding global sea level variations as well as changes in patterns of ocean chemistry and physical conditions. Studies of LIG stratigraphy have pointed out the complex facies evolution during MIS 5e at both Grand Cayman and Cayman Brac. This study uses exposures on Little Cayman to examine whether island size and average elevation influence facies development for reefal platform and grainstone shoreline/shoal facies. This study represents the first island-wide investigation of the LIG deposits of Little Cayman and recognizes the presence of the intra-5e erosion surface referred to as the Devil’s Point event on nearby Great Inagua. This study also demonstrates the utility of integrating airborne LiDAR data in mapping the geomorphology of the LIG. The LiDAR dataset of the 16 km by 3 km island was instrumental in refining the geologic map of Little Cayman Island. LiDAR mapping of karst density provided a first-order distribution of MIS 5e deposits and of the older Tertiary Bluff Formation. With the LiDAR map, 40 road cuts and coastal exposures were described and sampled for petrographic analysis. LIG facies, known regionally as the Ironshore Fm., include an early MIS 5e Acropora palmata-dominated framestone with a sharp erosional upper surface, a late MIS 5e beachrock/reefal rudstone conglomerate, and foreshore mixed skeletal/oolitic grainstone couplet that is replaced along strike by a late MIS 5e coralline-algal/Montastrea-dominated reef. Grainstones of the upper MIS 5e are distinctly thin and span the northern shore of the island, but show few beach-ridge accretion sets and consist mainly of mixed skeletal-peloid grainstones rather than extensive oolitic strandplains of other islands in the region. Similar to Grand Cayman, an interior lagoon with molluscan-dominated facies is found and south-facing foreshore deposits are notably thin and dominantly skeletal, not oolitic. Island size and average elevation is critical in facies development during the LIG. Although a closely comparable sea-level history, including the double-peaked MIS 5e signal, is observed, the unique ooid explosion of the upper 5e does not develop on Little Cayman.