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Microfacies and Mineralogy of Modern Mixed Siliciclastic-Carbonate Lagoonal Sediments of the Little Laughlands Bay, Jamaica


Microfacies and mineralogical characteristics of modern mixed carbonate-evaporite sediments yield essential data for the interpretation of depositional settings. Although many early diagenetic processes can affect mostly carbonate grain preservation potential, the overall sediment characteristics should contain enough evidence of both siliciclastic sediment input and the carbonate grain production to accurately represent the initial sediment production. This study examines the major carbonate producers in the Little Laughlands Bay, Jamaica, a tropical carbonate lagoon with riverine input of siliciclastic and Cretaceous-Tertiary carbonate lithoclast derived from the island interior. The lagoon is located on the northern coast of the island and is protected from the open-ocean waves by a barrier reef to the north. The lagoon was split into 14 shoreline-to-reef transects spaced 100 m apart. Every 10m depth was recorded and later used to create a bathymetric map. For each transect, a 1×1 m grid was laid down every 50m to collect samples and quantify biota. Major grain types were quantified for 63 thin-section grain-mounts by counting 250 points per thin-section. Bulk sediment samples were powdered and then analyzed for their mineralogy using the standard X-Ray Diffraction method. Major lithofacies types include coarse siliciclastic sand and gravel (backshore to foreshore), fine organic rich sands and muds (nearshore and lagoon center), medium to coarse skeletal sand (high-energy subtidal), and coral rubble (reef flat). Carbonate grains (excluding pre-Holocene carbonate extraclasts) account for 58% of all grain types. The major skeletal components include fragments of Halimeda (5%), red algae (2%), gastropods (3%) and bivalves/foraminifera (<1%). Carbonate producer abundance increases from west (16%) to east (24%), which is evidence by increased numbers of Halimeda (from 7% to 26%) and red algae (from 3% to 8%). There is a strong positive correlation between the abundance of red algae, gastropods and Halimeda, as well as a strong negative relationship between Halimeda and extraclasts. Mineralogy of backshore and upper foreshore sediment is predominated by quartz, feldspar, and low-Mg calcite (mean 1.5 mol% MgCO3). Elsewhere in the lagoon, away from the foreshore and the river mouth, aragonite and high-Mg calcite (mean 15 mol% MgCO3) predominate, and the abundance of non-carbonate minerals (including low-Mg calcite) becomes negligent towards the ref flat.