Abstract

Coral pinnacles growing in Alacran Reef lagoon can be characterized by three different depositional environments. These are: (1), Pinnacle crest, an area of living colonial stony corals and unconsolidated sediment which is poorly sorted 2.5 phi coral sand, having relatively high permeability, 20% calcite, 1.6% magnesium, and 0.6% strontium; (2), Pinnacle slope, a transitional environment, which accumulates very poorly sorted mixtures of gravelly coral-- Halimeda sand and silt, having relatively low permeability, 10% calcite, 1.2% magnesium, and 0.7% strontium; and (3), Lagoon floor, a topographic basin in which accumulates poorly sorted 5 phi Halimeda and Halimeda --fecal pellet silt, having relatively low permeability, 10% calcite, 0.8% magnesium, and 0.7% strontium. Observed diagenetic processes are pelleting of lagoon floor mud, formation of galleries in the rigid coral framework by lithophagid pelecypods and in coral and mollusk grains by boring algae, growth of euhedral aragonite crystals in grain cavities, and recrystallization of coral grains. Expected diagenetic processes are cementation and dolomitization of pinnacle crest sand, compaction of lagoon floor mud, and mold porosity formation in the coral framework and in sediments of all three environments.

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