DISTRIBUTION AND ORIGIN OF MULTIPLE THIN CALICHE CRUSTS IN HOLOCENE CARBONATE EOLIAN DEPOSITS ON SAN SALVADOR ISLAND, BAHAMAS

Numerous thin caliche crusts are present in carbonate eolianites of the North Point Member of the Rice Bay Formation exposed at North Point on San Salvador. These eolianites formed during Holocene transgression about 6 to 5 ka and consist of complexly stratified deposits produced by wind-ripple migration, grainfall and sandflow processes.

Caliche is a hard micritic crust formed by the dissolution of carbonate and subsequent reprecipitation of low-Mg calcite on the surface and/or at shallow depth. In the study area caliche coats most of the prominent bedding planes of eolian ooid-skeletal grainstone deposits in the upper part of the exposure including dune crests and both windward and steep (up to 34°) leeward surfaces. In weathered exposures caliche also forms a step-like pattern of up to 12 crusts, 1-4 mm thick, separating 3 to 32 cm thick eolian beds that generally thin upsection. Most of these crusts are parallel to wind-ripple lamination and vary in orientation from horizontal to 18°. Others encrust cross-laminated beds as well as vertical fractures forming caliche dikes. Caliche crusts have a sharp, smooth upper surface with varying degree of weathering that imparts a pitted, irregular appearance. In most cases caliche crusts are associated with dense and laterally extensive rhizoliths.

Multiple caliche horizons likely represent coeval precipitation as penetrative caliche. Plant roots penetrated through the eolianite and then spread laterally along more firmly lithified laminae in the search for water and nutrients as indicated by common rhizoliths. The presence of water and plant material facilitated formation of caliche and produced a unique stratification pattern with numerous thin crusts delineating distinct beds of mainly wind-ripple laminated strata. This distinguishes the Holocene deposits from their Pleistocene counterparts, which commonly have thicker packages of eolian strata separated by more varied and better-developed paleosol horizons. These differences reflect varying climatic and environmental conditions and differing duration and styles of eolian sedimentation and diagenesis in the Pleistocene vs. the Holocene in the Bahamas.