COMPUTED TOMOGRAPHY (CT) IMAGING OF PLEISTOCENE CORALS FROM SAN SALVADOR ISLAND, BAHAMAS: INFLUENCE OF THICK MICROBIAL ENCRUSTERS ON POROSITY AND PERMEABILITY OF SHALLOW-MARINE CARBONATES

High-resolution CT scans of core segments with fragments of Eemian (MIS 5e or Last Interglacial) Acropora cervicornis coral were analyzed using Dragonfly software. Two segments from a 54 mm-diameter core drilled in 2019 into the Cockburn Town Member of the Grotto Beach Formation at The Gulf (TG) on the south coast of San Salvador, Bahamas, compare A. cervicornis encrusted with 10 cm-thick laminated microbialite to a non-encrusted specimen surrounded by skeletal-peloidal-ooid grainstone. Two additional segments from a similar diameter core, drilled through the same succession in 1997 at Cockburn Town on the west coast of the island, include a comparison between A. cervicornis encrusted by laminated vs clotted microbialites.

Lowest porosity of 3-4 % was determined for core segments of corals encrusted with thick laminated microbialites, and the highest value of 14% was associated with the coral without encrusters. This difference in overall core segment porosity stems from contrasting properties of the deposits that surround the corals – well-lithified micritic microbialites vs friable carbonate grainstone – and illustrates the role that thick microbial encrusters can play in reducing porosity of coral-bearing subtidal carbonate rocks.

The core segment with coral surrounded by clotted microbialite has 9% porosity, which is greater than the values determined for segments with laminated microbialites. Even though both types of microbialites are made of micrite that incorporates carbonate sand, clotted microbial fabric contains common fenestral voids that contribute towards its higher porosity. Intraskeletal pores of associated encrusting foraminifera and open pore-space within macroborings in microbialites also impact porosity of these deposits.

In comparison to friable grainstones with elevated intergranular porosity as well as permeability (as demonstrated by standard core-plug measurements of overlying beach and eolian strata in TG core), the presence of micrite with isolated fenestral, intraskeletal and borehole pores also contributes to permeability reduction of the coral-rich shallow-marine carbonate deposits. Future research will attempt quantification of permeability from the CT scan data.