Salina sedimentation and diagenesis: West Caicos Island, British West Indies
East Salina on West Caicos Island, BWI, is the site of active gypsum, calcite and dolomite formation. Up to 2.4 m of Holocene sediment have accumulated in an elongate topographic low (3 km long by 0.5 km wide) bounded on the west by Pleistocene aeolianite and isolated from shallow platform waters on the east by a series of oolitic beach and beach/dune sequences of Holocene age. The salina is fed by marine groundwaters that primarily seep through the underlying Pleistocene bedrock. Hydraulic head created by the elevation difference between the salina surface and mean sea level drives the marine influx. The stratigraphic succession of the salina indicates a trend towards increasing marine restriction, grading from marine wackestones, packstones and grainstones at the base, upward through microbially laminated mudstones into gypsum mush in the uppermost part of the section. A thin microbial mat encrusted with gypsum and ephemeral halite covers the salina surface. Faunal diversity analysis suggests that two separate cycles of salina sedimentation exist within the overall succession. This cyclicity indicates that the depression occupied by the present-day salina was opened and closed at least twice during the Holocene. Sedimentary porewaters approach normal marine salinities near the base of the succession, but become more saline upwards. Evaporation near the salina surface elevates porewater salinities to as much as seven times normal marine water. Chlorinity profiles suggest that reflux of dense brines is not presently occurring within the sediments examined. More subtle changes in interstitial water chemistry indicate that the following reactions are presently taking place at various places in the sediment column: precipitation and dissolution of gypsum, formation of dolomite, precipitation of calcite and Mg-calcite, and microbial sulphate reduction. Carbonate phases present in small quantities in the gypsum layer include aragonite, calcite, Mg-calcitc and dolomite. Below the gypsum zone, aragonite and Mg-calcite dominate the mineralogy. Porewater chemistry suggests that dolomite and Mg-calcite are actively forming in this interval. Mg-calcite occurs in the form of sheaves of prismatic crystals; dolomite occurs as rhombohedral and subhexagonal crystals. Dolomite has also been observed to replace precursor carbonate mud in organic-rich zones presently undergoing microbial sulphate reduction. All authigenic carbonates examined have 813C values ranging from-1 to-9%o (PDB), reflecting a contribution from organic carbon. 618O values range from-2.6 to +4.2%O (PDB) and are consistent with formation from hypersaline marine-derived fluids.
