Abstract

The carbonate cements in a specimen from the living cup-shaped algal reefs of Bermuda were studied with scanning electron and light microscopes. These small cup reefs positioned on the margins of the platform are built by encrusting organisms; principally coralline algae, secondaril hydrocorals, gastropods and worm tubes; tire cavities of this framework are partly to completely filled with internal sediment. The principal cement is a mosaic of anhedral high-magnesium calcite, 15 tool % MgCO 3 that lines and fills pores of skeletons, borings, and interstices of internal sediments. The size frequency maxima of the crystals is 2-4mu , the range of micrite. The other much less common cement is fibrous aragonite whose preferred habitat is as linings and fillings of gastropod shells. The SEM micrographs show that the micrite cement crystallizes directly within voids and is not the recrystallization product of earlier carbonate cement. The micrographs also show all stages from initial linings to complete fillings of voids as well as multiple generations of micrite. The crystallization of micrite within voids offers an alternative explanation to the accepted view that fossil micrites result from recrystallization. Because the cup reefs are extremely permeable and are located on the seaward margins of an oceanic platform where there is strong and continual wave surge, the cement must form from seawater of oceanic position. The predominance of repeated generations of micrite-sized crystals indicates that the mechanism and chemistry of precipitation must be studied within voids and on the scale of microns.

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