Longshore Transport-Driven Accretionary Growth of Carbonate Islands: Holocene and Pleistocene Examples from the Lucayan Archipelago

Abstract:

A newly available 12-meter horizontal resolution Digital Elevation Model with complete coverage over The Bahamas and the Turks and Caicos Islands allows for detailed characterization of coastal morphology and interpretation of geometries shaped by longshore currents. Previously a comprehensive assessment of these features was impractical due to subtle relief and logistical challenges of visiting numerous islands. A total of 396 examples of landforms constructed by longshore transport of sediment have been identified spanning latitudes ranging from N 20^o 40’ to N 27^o. Holocene alongshore sand waves, reconnecting spits, and flying spits characteristic of high-angle wave approach rim portions of many of the Bahamian islands. Wavelengths and amplitudes of these features range from approximately 500m to 13km and 100m to 3km, respectively. The features form predominantly on coasts oriented parallel-to-oblique to the eastward trade winds, but also show some evidence of development on protected lees of islands. Larger, more asymmetric Holocene examples show elongation of land-attached spits and along shore sand waves often form enclosed lakes or sheltered lagoons in the protected lee of the features, ranging from small isolated ponds to larger lakes up to 2 km². Additional Holocene examples illustrates linked chains of spits or sand waves extending away from the islands, enclosing inland lakes ranging from 0.1 km² or 29 km². Similar off-lapping asymmetric patterns occur in many Pleistocene dune ridges, suggesting longshore sand bodies framed many of these carbonate islands in the Middle – Late Pleistocene. Observations of longshore-driven morphologies in the pattern of aeolianite distribution may further help evaluate middle Pleistocene island development. We propose longshore transport patterns driven by high-angle waves and delivery of sediment for eolian reworking may represent a significant but underappreciated contributor to the construction and evolution of other modern and ancient carbonate islands.