RELATION BETWEEN STRATIFICATION AND SURFICIAL VS. PENETRATIVE ORIGIN OF CALICHE CRUSTS IN CARBONATE EOLIANITES ON SAN SALVADOR ISLAND, BAHAMAS

Both surficial and penetrative processes have been proposed for the origin of caliche crusts in uppermost parts of Quaternary carbonate eolianite successions on San Salvador Island, Bahamas. We re-examined the Holocene North Point Member of the Rice Bay Formation exposed along the northeastern coast of the island, and found evidence for crusts having formed by both processes.

Observed surficial crusts are laterally continuous and uniformly coat prominent bedding planes. Weathering and erosion have stripped resistant crusts and eolian laminae to produce a terraced exposure of up to 12 crusts in sets about 1.5 m thick. Individual crusts are 1 to 4 mm thick and separate bedsets that thin upward from about 30 to 5 cm. Although in places, caliche veins transect these beds, field evidence shows that these veins are not contiguous with bedding-parallel crusts. Furthermore, there is absence of vertical penetration of eolian beds by rhizoliths, which together with impressions of plant runners and stems occur within bedding-parallel caliche crusts.

Upward-thinning Holocene eolian bedsets composed of wind-ripple strata and separated by syn-depositional caliche crusts suggest a transition from times of increased sediment production and supply, active dunes, and drier weather to times of reduced sediment production and supply, inactive dunes, and wetter weather. The wind-ripple sets from the uppermost part of eolian successions were deposited during windy, dry and/or stormy periods and were stabilized by vegetation and caliche crusts during less windy, wet periods. The presence of meteoric water and plant material on dune surfaces facilitated the formation of caliche by dissolution of allochems and precipitation of microcrystalline calcite. In contrast to these syn-depositional crusts that formed on or near dune surfaces, penetrative caliche crusts are post-depositional, generally cut across multiple eolian beds, are irregular in distribution, and variable in lateral thickness. Ongoing study will emphasize petrographic and geochemical distinction between penetrative and surficial caliche crusts.