DIVERSE SEDIMENTARY STRUCTURES OF A HOLOCENE CARBONATE EOLIANITE COMPLEX ON CAT ISLAND, BAHAMAS

Alligator Point is a prominent, cape-like feature on the leeward, Exuma Sound coast of northern Cat Island, Bahamas. Extending seaward for about 3.5 km, the point consists of Holocene eolianite composed dominantly of very fine to fine ooid grainstone, in contrast to the skeletal-peloidal grainstones of Holocene eolianite common elsewhere in the Bahamas. The high southern ridge of the point has excellent eolianite exposures along the coast that are assigned to the Rice Bay Formation and reveal diverse physical and biogenic sedimentary structures. Deposits of the older North Point Member form the spine of the ridge. Well-exposed contacts with overlapping Hanna Bay Member beds are present in several places. Thin, weakly lithified beds of Hanna Bay eolian ooid grainstone are characterized by distinctive patterns of large 4- to 6-sided polygonal fractures extending through beds to maximum depths of ~80 cm. These beds are topped by well-developed “fossil” dunelets and surfaces with unusual rounded to tear-shaped structures developed by wind scouring of beds during their rapid lithification. Along the coast, dip of Hanna Bay beds decreases to near horizontal, reflecting backshore deposition. Insect-generated trace fossils are present in the dunal beds, along with crab burrows (Psilonichnus upsilon) in backshore strata. Wind-ripple sets are well developed on some surfaces of North Point beds, and a typical suite of mesoscale eolian-bedding structures is displayed. In addition, thin caliche layers, springs with shallow corrosional channels and microbially influenced travertine precipitates, and rhizomorphs of a variety of forms are present. In one area, palm frond fossils, likely formed by the molding of silver thatch (Coccothrinax argentata) fronds in the wind-deposited sediment, are abundant. The diverse suite of physical and biogenic structures so well preserved in the Holocene beds of Alligator Point characterizes a variety of sub-environments within this carbonate eolian setting and enables detailed reconstruction of depositional conditions and processes. Recognition of these and other similar features in ancient eolianites will permit more comprehensive environmental reconstructions of these unique and important carbonate deposits in the future.