SPONGIFORM TEXTURE IN BAHAMIAN HOLOCENE GRAINSTONE: ORIGIN BY LITHIFICATION OF BEACH AND DUNE CARBONATE SAND AROUND VEGETATION

Commonly found within typical Bahamian Holocene eolian and beach grainstone are highly porous layers with what we refer to as spongiform textural fabric. The texture of sand with spongiform features varies from fine grained and well sorted to very poorly sorted with fine to very coarse sand grains. Sand is predominantly skeletal (e.g., Eleuthera Island), and/or oolitic (e.g., Cat Island), and poorly lithified with menisci and thin rims of clear equant meteoric calcite cement.

Deposits with spongiform texture have an intricate network of pores with variable morphology. Most pores are irregular in shape and <1 cm wide, some are circular or elliptical, and others are tubular and up to several cm long. In some cases the pores are visible only in weathered exposures. Besides some faint lamination, there are no other visible sedimentary structures. In some field examples, spongiform texture is found in deposits with large open vertical pipes.

Examination of potential modern analogs suggests that accumulations of marine algae (e.g., Sargassum) and growth of grass and shrub vegetation in coastal zones can produce spongiform texture by the trapping and lithification of sand around small but dense roots, stems and organic litter with various microbial, fungal and insect communities. This is consistent with the observed morphology and distribution of pores and can account for the coarse-grained and poorly sorted texture of trapped and stabilized sand. Associated open pipes represent either molds of tree trunks (e.g., palms), or clusters of larger diameter vertical roots. The sediment infilling pore space produced by decay of organic material is less firmly lithified than the one cemented around moisture retaining organics, and therefore differential weathering and erosion enhance the expression of spongiform texture.

Spongiform texture may indicate moist periods with vegetated landscape whereas the interbedded laminated eolianites likely represent active dune migration over dry, barren terrain and/or rapid deposition during major storm events. Thus, understanding the origin and distribution of grainstone with spongiform texture can offer insights into the dynamics of climate, sedimentation and vegetation during the Holocene in the Bahamas, and is important for characterizing the reservoir potential of these strata.