Diagenesis of scleractinian corals in the subaerial vadose environment

Abstract

Exoskeletons of scleractinian corals in the elevated late Pleistocene reefs on northern Barbados illustrate a series of diagenetic textures that document aragonite coral alteration under subaerial vadose conditions. Two major solution-precipitation processes are recognized: (1) concomitant solution-precipitation on a fine scale leading to preservation of coral microstructure, and (2) total leaching and destruction of the microstructure followed by later precipitation of void-filling spar. The pathway of aragonite solution is similar in both processes, and is controlled by the original coral microstructure. Solution begins in the fine equant crystallites forming the axis (center of calcification) of each trabecula and moves outward into the surrounding zone of closely packed aragonite needles by preferential solution along the linear intercrystalline contacts between needles. This results in the friable aragonite chalk commonly observed in Pleistocene corals. Aragonite chalk is also observed as a zone between aragonite and calcite in corals undergoing solution-precipitation on a fine scale. The coral microstructure is preserved by concomitant precipitation of calcite between separated aragonite needles adjacent to the calcite alteration front, incorporating minor impurities and irregularities of the original structure into the calcite crystals. This results in a new calcite texture that mimics the original aragonite one. Corals completely altered to calcite by concomitant solution-precipitation on a fine scale exhibit a coarse mosaic of blocky calcite with a relict fibrous texture. The once dark trabecular axes are preserved as a clear central canal. A similar calcite trabecular texture is observed in certain Devonian tabulate corals and other fossil calcareous organisms, suggesting that their skeletons may have been aragonite, altered to calcite in a manner similar to that described above.