SURFICIAL VS PENETRATIVE ORIGIN FOR MULTIPLE THIN CALICHE CRUSTS IN HOLOCENE CARBONATE EOLIAN DEPOSITS ON SAN SALVADOR ISLAND, BAHAMAS

Multiple thin caliche crusts are present in the upper part of the carbonate eolianite succession of the North Point Member, Holocene Rice Bay Formation, exposed at North Point on San Salvador. Here caliche coats prominent bedding planes including both windward and leeward surfaces as well as crests of dunes composed of ooid-skeletal grainstone deposits. Most caliche crusts are parallel to wind-ripple lamination and some also encrust cross-laminated beds and vertical fractures forming caliche dikes. Weathering of caliche-encrusted eolianites can produce a step-like pattern of up to 12 crusts in about 1.5 m of stratigraphic thickness. Individual crusts are 1-4 mm thick, and separate eolian beds that generally thin upsection from ~30 to 5 cm.

These multiple caliche crusts may represent simultaneous formation as penetrative caliche horizons. In this scenario, plant roots penetrate through the eolianite and then spread laterally along more firmly lithified laminae and bedding planes. The presence of meteoric water and plant material along these horizons would facilitate formation of caliche by dissolution of carbonate and subsequent precipitation of microcrystalline calcite. Arguments against this hypothesis include: 1) lack of field evidence for common vertical penetration of eolian beds by roots, although rhizoliths are abundant within the caliche crusts and dikes; and 2) no systematic vertical trends observed in the morphology, petrology or isotope geochemistry of caliche crusts. Alternatively, caliche crusts may have formed on or very near the surface with each caliche-topped eolian bed representing a wet and dry cycle. The thinning-upward of eolian beds and the appearance of common caliche crusts could indicate significant changes in depositional dynamics and climate from: 1) an earlier Holocene time characterized by rising sea levels, high rates of carbonate production, and active dune migration and deposition during generally cooler and drier climate; to 2) a later Holocene time of slowed sea-level rise and reduced sediment supply, which in conjunction with warmer and wetter climate resulted in small, vegetated dunes represented by thin eolian layers separated by rhizolith-rich caliche crusts. Our ongoing research efforts are focused on delineating between these two mechanisms of caliche formation.