DISTINCTIVE FEATURES OF SUBAERIAL SEEPS IN HOLOCENE CARBONATE EOLIAN STRATA FROM CAT ISLAND, BAHAMAS

Numerous small seeps are present along the southern slope of the Trial Hill ridge in the Pigeon Cay area to the east of Alligator Point, Cat Island, Bahamas. This ridge is elongated west to east, paralleling the Exuma Sound shoreline to the south and a tidal inlet to the north. Although small (about 1 km long, less than 100 m wide, and almost 19 m tall), the ridge is the area’s most prominent topographic feature. A thin soil cover and dense vegetation cap this ridge made of carbonate eolianite from the Hanna Bay Mb. of the Holocene Rice Bay Fm. Underlying eolianite of the North Point Mb. of the same formation is well exposed along the ridge’s southern flanks as prominent foreset beds of uniform 10-15° dip to the south.

The seeps form small triangular aprons below discharge points aligned about 18-20 m upslope from the shore and defining a field about 400 m long immediately south of the highest parts of Trial Hill. Aprons are shallow (up to 10 cm deep) erosional incisions into North Point eolianite. Within the aprons there are two types of carbonate precipitates: 1) dense, brown laminated crusts, up to 1.5 cm thick and made of elongated calcite crystals; and 2) porous microbial bushes, stained dark brown to black, and containing micritic to finely crystalline equant calcite. These laminated crusts are an example of travertine that forms in areas of active water flow, surrounded and underlined by microbial bushes that represent tufa-like material forming in moist areas between flow events. Seeps were inactive during our field observations in January 2009, but likely have relatively low discharge related to immediate antecedent rainfall conditions.

Such seeps are rare in Holocene deposits of the Bahamas. Their formation and distribution requires a unique combination of stratigraphy and topography. Trial Hill provides a localized area where water recharges through the porous Hanna Bay eolianite and surfaces when perched and deflected down-dip along bedding planes of underlying, better indurated North Point eolianite. Water interaction with soil and vegetation of the recharge area is likely responsible for both the corrosive nature of seepage and dark color of associated carbonate precipitates. Communities of microbes and other organisms also provide sites of localized carbonate precipitation and organic staining within these seeps.