Abstract: Holocene Freshwater Diagenesis of Modern Oolites, Joulters Cay, Bahamas

Conrad D. Gebelein, William C. Benmore

Ooid sand islands, consisting of a series of seaward-prograding, linear to arcuate beach-dune ridges, have developed over the oolite shoal in several places along the platform margin north of Andros Island. The largest of the islands, south Joulters Cay, is more than 1 km wide and consists of linear dune ridges, each 25 to 75 m apart and 3 to 5 m above sea level. The age of the oldest dune ridge is 1,500 to 1,800 years B.P.

Preliminary geochemical, petrographic, and mineralogic data indicate that all sediments on the island above +1 m MSL are undergoing vadose freshwater diagenesis. Sediments beneath the island are undergoing freshwater phreatic diagenesis to depths as much as 3 m below MSL.

Vadose cementation is characterized by: (1) 5 to 25% low-magnesium calcite; (2) 8 to 20-µ diameter, clear calcite spar cement crystals; (3) a "meniscus" fabric in the cement, where menisci are formed by multiple decimicron crystals; (4) progressive pore filling by multiple generations of menisci, with constant crystal size toward the pore interior and rare preserved curved crystal faces. Early vadose diagenesis of ooid grains involves preferential dissolution of nuclei, dissolution of more permeable lamellae within ooids and recrystallization of ooid grains to decimicron spar (often with preservation of lamellae), beginning at the cement-grain boundary and working inward.

Cementation in the upper parts of the phreatic zone is characterized by: (1) 10 to 30% low-magnesium calcite; (2) 10 to 50-µ diameter clear, equant calcite spar cement; (3) "dog-tooth" isopachous cement fabrics, formed by multiple sparry crystals; and (4) progressive pore filling with an increase in crystal size toward the pore center and abundant enfacial angles. Phreatic diagenesis of grains and preexisting cements is similar to that in the vadose zone, but with more intensive dissolution, including wholesale removal of aragonitic skeletal fragments, and recrystallization of marine isopachous aragonitic needle cements to elongate calcite scalenahedra (10 × 100 µ) which have a radial isopachous fabric around the ooids.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC