ROLE OF ROOTS AND MYCORRHIZAL FUNGI IN EARLY TOUCHING VUG FORMATION AND POROSITY-PERMEABILITY RELATIONSHIPS ON A FORESTED CARBONATE ISLAND, SAN SALVADOR ISLAND, BAHAMAS

Mycorrhizal fungi form a symbiotic relationship with tree roots, in which the tree provides the fungi glucose, and the fungi assists the tree with mineral uptake from the soil. Recent studies have shown that mycorrhizal fungi are one of the main weathering agents of silicate rocks, with the ability to bore directly into minerals and along crystal planes, and secrete organic acids that dissolve substrate. Mycorrhizal fungi also play an important role in carbonate rock weathering, but none have yet demonstrated the effect on evolution of the early pore system.

San Salvador Island is a heavily forested carbonate island located at the far eastern edge of the Bahamian archipelago. Because of the dense tree cover, San Salvador Island provides an opportunity to study the effect of rootlets and mycorrhizal fungi on carbonate sediments and pore systems. 190 thin sections made from core plugs were described for microfacies, cements, and pore types and compared to accompanying petrophysical data. A subset of 100 plugs from a horizontal transect near the southern end of the island were further studied to determine the effect of roots and mycorrhizal fungi on early pore system evolution in otherwise homogeneous grainstones. While six different microfacies were identified from the subset of samples, the vast majority were peloid and ooid-peloid grainstones with nearly all interparticle space was occluded by blocky calcite cement. Root and fungal borings were identified in ~60% of the samples. When compared to petrophysical data, bored samples showed a marked increase in permeability values compared to non-bored samples of similar facies. Petrographic observations indicate that rootlets and mycorrhizal fungi bore through young carbonate rocks, optimizing pathways between contacts of microporous grains. The result is the creation of relatively high permeability pathways through otherwise low permeability rocks. Because of the ability to widen pore throats across grains and cement, these borings act to connect otherwise isolated pores and may contribute to touching vug and cave formation in the vadose zone.