--> ABSTRACT: Coral Distribution Patterns in Miocene Reefs of Anguilla, Leeward Islands, West Indies, by Ann Budd Foster and Kenneth G. Johnson; #91030 (2010)

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Coral Distribution Patterns in Miocene Reefs of Anguilla, Leeward Islands, West Indies

Ann Budd Foster, Kenneth G. Johnson

Anguilla, a 27 by 5 km island at 18°13^primeN, 63°05^primeW, parallels the northwest edge of the Anguilla bank (St. Martin plateau) in the outer Lesser Antilles volcanic arc, which was active from the Eocene to Oligocene. Except for scattered exposures of tuff or basalt, the island is composed predominantly of reefal limestones and marls of the 70-m thick, middle Miocene Anguilla Formation, deposited on a shallow inner shelf platform extending from volcanoes near St. Martin. The reef framework consists of branched and platy corals interspersed with calcareous sand lenses. Although the limestones have been uplifted and subjected to minor faulting, little evidence supports extensive transport across a slope.

Coral distribution patterns have been quantified across the reefal units by point-counting species occurrences at 0.16-m intervals within 1-m2 quadrants placed haphazardly across vertical exposures. Eight coral species (of possibly 18 total) were recorded. Cluster analysis delineated four facies: (1) a low-diversity facies dominated by branched Porites, (2) an intermediate diversity facies dominated by branched Porites, (3) a high-diversity facies dominated by massive Montastraea, Siderastrea, and Porites, and (4) an intermediate diversity facies dominated by platy Porites. These facies consist of lenses, no more than 100 m long and 2 m high, arranged in no apparent regular sequence. Thus, they do not represent zones across a depth gradient.

Comparisons with living Caribbean reefs suggests that the Anguilla Miocene reefs were similar to small, modern, backreef fringing and patch reefs near the San Blas Islands of Panama, reefs whose variable composition and patchy distribution depend largely on sedimentation and current patterns.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.