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High-Energy Facies off San Salvador Patch Reefs: Mineralogy, Microfacies, and Stable-Isotope Record


The skeletal-peloid grainstone facies surrounding patch reefs is highly abundant in modern intraoceanic carbonate platform settings. Unlike the barrier and fringing reef facies, it has not received much research attention despite its potential as an analogue for highly porous reservoir units that patchily occur in many carbonate reservoirs. This study focuses on shallow-water (<5 m deep) patch reefs from a high-energy lagoon along the swell-exposed northeastern coast of San Salvador Island, Bahamas. 13 patch reefs, each up to 10 m in diameter, were analyzed in terms of biotic composition, and off-reef sediment samples were collected while scuba-diving at depths of up to 5 meters. The frequency analysis of microfacies data was done on grain mounts. Bulk sediment samples were powdered and analyzed for their mineralogy using the standard X-Ray Diffraction (XRD) method. Carbon and oxygen isotopes were obtained from powdered carbonate sediment samples; the isotopes were also obtained from the separated and powdered skeletal grains, including green algae (Halimeda incrasata, H. lacrimosa), gastropods, echinoids, corals, bivalves, foraminifera and red algae. The patch reef cores are dominated by stony corals, predominantly Montastrea, but its surface is mainly covered by brown algae (Padina, Dictyota), green and red algae, abundant octocorals (Plexaura, Briareum, Muricea) and fire corals (Millepora), and patches of stony corals Diploria and Porites. The major off-reef sediment producer is Halimeda (32% of all grains), followed by red algae (14%), bivalves (7%), foraminifera (6%), gastropods (3%), and corals and echinoids (<1%). Non-skeletal grains comprise 37% of all grains, and are predominantly peloids. Aragonite is the dominant mineralogy in 80% of the samples; in those samples aragonite content ranges from 54–73%. Where dominant, the calcite content ranges from 63–67%. Overall, the average aragonite to calcite ratio is 1.28. The studied calcite is high in magnesium, with values ranging from 14.3–16.4 molE% (mean 15.5 molE%). There appears to be a correlation between an increase in calcite content and abundance of bivalve fragments. Oxygen isotope values range from −0.28 to −2.29 ‰VPDB, with mean value of −0.99 ‰VPDB; carbon isotopes range from −0.73 to 3.18 ‰VPDB, with mean value of 2.27 ‰VPDB. The isotope values of the bulk sediment are clustered, while the individual skeletal grains exhibit much greater ranges of values.