Growth and Depositional Facies of a Windward Reef Complex

Macintyre, I.; Multer, H.; Zankl, H.; Hubbard, D.; Weiss, M.; Stuckenrath, R.

The history of a Caribbean windward reef complex (bank barrier, patch, and fringing reef) is revealed in eight core holes across Nonsuch Bay, Antigua. This complex (up to 16 m thick) is established on Oligocene limestone and contains five reef facies: fore-reef talus, fore-reef coral heads fore-reef A. palmata, back-reef rubble, and back-reef sediments. Ten radiocarbon dates indicate that 1 about 5,000 years B.P. the bank barrier reef consisted of an A. palmata community with a seaward accumulation of fore-reef talus debris. In the subsequent 2,000 years, a well-developed coral-head community (accumulation rates of up to 4.6 m/1000 years) thrived seaward of the A. palmata community and the back-reef sediments (maximum accumulation rate of 5.6 m/1000 years). In the last 3000 years the A. palmata facies has prograded seaward over the coral-head facies. Today, the limited accumulation of this shallow A. palmata community (0'.9 m/1000 years) indicates that much of this coral is being transported shoreward and is providing rubble to the reef-crest and shallow back-reef. Because of space limitations, this material has been accumulating at a rate of only 0.7 m/1000 years. The cored patch reef rests on an Oligocene erosional high and consists of an A. palmata community 6745 years old. It gives way to !· cervicornis and coral head communities that formed a loose back-reef sediment facies trapping large quantities of fine sediment (overall accumulation rate of 2. 2 m/1000 years). The fringing reef consists of a similar facies (accumulation rate of 1.6 m/1000 years) capped by a thin A. palmata section. Submarine lithification appears primarily in the fore-reef coral head and A. palmata facies, where it forms microcrystalline crust/infill rims on corals. Traces of multtcyclic board and cemented pavement linestone were found in the back-reef rubble facies directly shoreward of the reef crest. As in many other reef areas, this submarine cement is predominantly magnesium calcite (18.52 15.1 mole percent Mg co3), with minor traces of acicular aragonite, generally in coral skeletal cavities.