MODERN MIXED CARBONATE-SILICICLASTIC DEPOSITIONAL ENVIRONMENT OF THE SALEM BAY, JAMAICA

This study focuses on on the identification of the major sediment producers in eastern Salem Bay, Jamaica, a muddy carbonate embayment with riverine siliciclastic input. Salem Bay lagoon stretches E-W along the northern coast of Jamaica and is protected by a barrier reef that completely encloses the lagoon at its eastern end, thus creating a low energy embayment with predominantly muddy sediment. To the west, the barrier reef breaks up into a series of smaller patch reefs, which results in less protection from the incoming waves and consequently in much lower fraction of silt-size particles.

Sixty-two samples of the lagoon-floor sediment were collected along six submarine transects in the eastern part of the lagoon. All transects were measured from the shoreline to the barrier reef (average 500m in length, 200 meters between each transect), and the water depth was measured at 10m intervals to map the bathymetry. The present living biota was identified and quantified in situ by laying down a 1 x 1 meter grid every 50 meters along each transect. Grainmounts of the collected samples were used for frequency analysis of microfacies data, with a minimum of 250 grains counted and analyzed per grain mount.

The mud content of sediment decreases both reefwards (northwards) and westwards. The eastern, most protected part of the lagoon shows the greatest mud content (44%); in the western part of the study area the mud content decreases to only 4%. The abundance of siliciclastics whose provenance is the interior of the island is greatest close to the shoreline; the siliciclastic grains gradually completely disappear towards the reef. The major carbonate producers are green algae (Halimeda) and red algae, whose fragments comprise 21% of all grains, and with abundance increasing from eastern (3%) to western (26%) part of the lagoon. Less common are bivalves and gastropods that account for 7% of carbonate grains. Both the diversity and abundance of individual skeletal grains surge with energy levels in the lagoon, as suggested by their increase northwards towards the reef and westwards where the barrier reef is incontinuous. The relationship between mineralogy of the powdered bulk sediment samples and frequency of microfacies data is evaluated.