GEOLOGY OF BEACHROCK ALONG THE PALISADOES SPIT, JAMAICA: A SPATIAL, TEMPORAL AND PETROGRAPHIC INVESTIGATION

In contrast to a combined beachrock outcrop length of 3 km recorded by Steers in 1940, 8.1 km total combined length of beachrock is now exposed along the windward coast of the Palisadoes spit in south eastern Jamaica. From spatial change modelling of the coastline using photographic digitizations of 1941 and 1991 aerial photographs of the Palisadoes spit windward coast using Arc-GIS: newer beachrock exposures are associated with retreating shorelines dominated by erosional processes. Beachrock crops out along the Palisadoes as massive, tabular or multi-layered units. These vary in length from less than a metre up to expanses more than a km long. In terms of lithofacies the units are variable and are classified using the Udden-Wenthworth grain size (Boggs, 2006) chart as coarse sandstone, granule and pebble conglomerates, cobble conglomerate and boulder conglomerate facies. Petrographic and modal analysis defined these sedimentary rocks as volcarenites using ternary diagrams as defined by folk (1968), they also plot on ternary diagrams presented in Dickinson (1983) in fields for Un-dissected Arc and the Lithic Recycled zones. This reflects the source area for the beach sediments in the Wagwater rift and Blue Mountains arc rocks inland. Beachrock cements consist of various polymorphs of calcium carbonate and show multiple generations with varying fabrics such as: primary rim cements, vadose ‘silts’ with surrounding cements and opaque amorphous microcrystalline cements, indicative of cementation in the coastal vadose zone with strictly marine influence as defined by stable isotope signatures.