TROPICAL SHALLOW-MARINE CARBONATE- SILICICLASTIC DEPOSITIONAL ENVIRONMENT OF THE LITTLE LAUGHLANDS BAY, JAMAICA

This study focuses on major carbonate producers in the Little Laughlands Bay, Jamaica, a tropical carbonate lagoon with a riverine input of siliciclastics and Cretaceous-Tertiary carbonate lithoclasts. The lagoon is located on the northern coast of the island and is protected from the incoming swell by a barrier reef to the north. The water energy is higher in the western part of the lagoon where the Little Laughlands River empties and the strong current runs parallel to the shoreline.

Fourteen shoreline-to-reef transects spaced 100m apart were analyzed for surface sediment composition and the biota present. Bathymetric map was constructed based on depth measurements taken every 10m along each transect. For each transect, every 50m a 1x1 m grid was laid down to quantify biota and collect the sediment sample. Sixty-three thin-section grain-mounts of the samples collected were used to quantify major grain types by point-counting 250 points per thin-section.

Major lithofacies types are coarse siliciclastic sand and gravel, fine organic-rich sand and muds, medium to coarse skeletal sand, and coral rubble. Coarse siliciclastic sand and gravel dominates along the western shoreline and in the tidal channel, i.e. in high energy areas. The abundance of fine organic-rich sand and mud increases towards the center of the lagoon and away from the river mouth. The eastern part of the lagoon is mainly composed of medium and coarse skeletal sand, while shallow reef-flat area is predominated by coral rubble. Based on point-count analysis, carbonate grains (excluding pre-Holocene carbonate extraclasts) account for 58% of all grain types. The most abundant non-skeletal carbonate grains are carbonate lithoclasts (54%) and peloids (3%). The major skeletal components include fragments of Halimeda (5%), red algae (2%), gastropods (3%), and bivalves/foraminifera (<1%). The abundance of major carbonate producers increase from west (16%) to east (24%), as evidenced by increased numbers of Halimeda (from 7% to 26%) and red algae (from 3% to 8%). Gastropods have a constant low abundance (3-4%) throughout the lagoon. There is a strong positive correlation between the abundance of red algae, gastropods and Halimeda, and a strong negative relationship between Halimeda and extraclasts.