Datapages, Inc.Print this page

Spongiform Texture and Pipe Structures in Holocene Grainstones from the Bahamas: Implications for Porosity and Permeability Development and Evolution

Glumac, Bosiljka *1; Curran, H. Allen 1; Savarese, Michael 2; Hoeflein, Fritz 2
(1) Department of Geosciences, Smith College, Northampton, MA.
(2) Department of Marine and Ecological Sciences, Florida Gulf Coast University, Fort Myers, FL.

Highly porous carbonate layers characterized by an intricate network of openings with variable morphology, referred to as spongiform textural fabric, are commonly found within Bahamian Holocene eolian and beach grainstone deposits. The texture of sand with spongiform features ranges from fine grained and well sorted to very poorly sorted with fine to very coarse grains. Sand composition also varies from predominantly skeletal (e.g., Eleuthera Island) to oolitic (e.g., Cat Island). The presence of menisci and thin rims of clear equant meteoric calcite cement results in clumps and clusters of generally poorly lithified sand. Pores in between are commonly 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 obvious sedimentary structures. In some field examples, spongiform texture is found in eolian deposits with large vertical cylindrical pipes, commonly up to 10-15 cm in diameter and 1.5-2 m in height. Pipes can be closely spaced (20-30 cm apart) and individual examples have relatively uniform diameter without any obvious evidence for branching or tapering. In most cases the pipe walls are well defined, some with better-lithified rims, and with sediment-free or partially infilled interiors.

Modern analogs suggest that accumulation 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 with the texture of sand. Associated vertical pipes represent either molds of tree trunks (e.g., palms), or possibly clusters of larger vertical roots. Weathering and erosion can enhance the expression of spongiform texture and vertical pipes because sand cemented around moisture retaining organic material is more firmly lithified than sediment infilling pores created by organic matter decay. Understanding the origin, distribution and diagenesis of grainstone with spongiform texture and pipe structures is important for characterizing reservoir quality of these strata because of their potential for abundant large-scale porosity and permeability relative to interbedded laminated grainstone.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California