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Abstract

Abstract Neogene and Holocene volcanism in the Lesser Antillean arc has been explosive in character, and bioherms and carbonate platforms are only slowly established on young volcanic islands owing to the general instability of the pyroclastic shorelines. However, islands inactive for more than several million years begin to develop massive carbonate platforms, and the size and maturity of the eventual platforms depend in large part on the length of the quiescent period. The resulting platforms have been variously subjected to sea-level changes, minor tectonic uplift or subsidence, and, in a few cases, renewed volcanism.

Windward Holocene coral reefs up to at least 14 m thick and algal ridges up to 10 m thick are abundant on older island cores or carbonate caps in the eastern Caribbean. The Holocene bioherms are structurally of two types: bench and bank barrier. Bench bioherms are predominantly coral or coralline frameworks throughout, and occur at present depths of less than 6 to 10 m on wave-cut Pleistocene benches or other mound or edge features. They are most abundant on raised or emergent carbonate islands. Bank-barrier reefs formed initially as elongate horseshoe or spit-shaped arcs of carbonate sand and rubble trapped by head corals. They are well developed where extensive carbonate shelfs are found at present depths of 10 to 20 m. The cap facies of bank barriers are coral or coralline frameworks similar to those found in bench bioherms.

The deeper layers of these bioherms (greater than 6 to 10 m) are dominated by Montastrea annularis. Acropora cervicornis commonly is present in mid-reef zones but, perhaps because of the common heavy swell, it is not as important a framework builder here as it is in the central and western Caribbean. Above 6 to 10 m, the reef frameworks consist mostly of Acropora palmata, Millepora, or coralline algae, depending on wave energy. High island reefs with turbid coastal waters are characterized by reef crests coated with fleshy algae.

Although the crests of some shelf-edge reefs in the Lesser Antilles are emergent and therefore barrier reefs, most shelf edges lie at depths of 15 to 30 m and have little Holocene framework. Extensive shelf-edge barrier or atoll reefs, such as are characteristic of the central Pacific and Australian plates and which occur in the westernmost Caribbean, are not present. In this respect, the Lesser Antillean arc is similar to the seismically active island arcs of plate margins in the Indo-Pacific.

The glaciation of Antarctica in Miocene to Pliocene time produced a marked drop of sea level which had the effect of raising all Miocene and older reefs. However, the generally subsiding shelf-edge reefs on islands removed from plate margins resumed Pliocene-Pleistocene building on the older reef structures after a hiatus of several million years. Nonsubsiding shelf-edge reefs were “permanently” raised and new shelf-edge reefs formed against the seaward walls of the original platforms.

During the Pleistocene, with interglacial “high” sea levels at intervals of 100,000 to 200,000 years, the reefs of subsiding platforms would have had to build only a few meters at each high stand to remain at their present emergent levels. The stable platforms, developing new shelf-edge reefs, would have their shelf edges built to the average high sea stand of the more numerous inter-stadials (about 25 m below present sea level).

We conclude that the reefs of the Lesser Antilles are not “inferior” to Indo-Pacific reefs because of climatic or biotic factors, but rather are typical of reef development in seismic belts where general subsidence is absent.

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