Holocene bioherms (algal ridges and bank-barrier reefs) of the eastern Caribbean

Abstract

Based on aerial and surface reconnaissance of shallow-water reefs and algal ridges throughout the Lesser Antilles and coring of reef-ridge structures on St. Croix, Martinique, Antigua and Guadeloupe, major emergent and shallow-water Holocene bioherms are characterized by maps and sections. On St. Croix an extensive bank-barrier reef about 13-18 m thick is developed on the inner part of the carbonate shelf. Stratigraphically, this reef has a lower Diploria-Montastrea-rubble-sand (or bank) facies, formed during middle to late Holocene time, overlain by a shallow-water Acropora palmata facies, formed during the past 1000 yr. In exposed areas, at depths of less than 2 m, crustose coralline pavements are presently forming incipient algal ridges on the reef crest. Shoreward of the barrier reef, on preexisting benches at depths of 3-10 m, algal ridges had developed from 5000-2000 B.P. The developing barrier reef has since blocked wave action from the bench algal ridges, and many are now degenerating. This same pattern occurs on the windward sides of many Lesser Antillean islands with the following variations: where wave energy is generally greater than on St. Croix, an extensive coralline-Millepore crust in the form of mounds or spurs, incipient algal ridges, or even well-developed algal ridges cap bank-barrier systems; where bank barriers have not blocked inshore bench algal ridges (especially on limestone islands), they remain well-developed and active; and under some conditions, apparently associated with high energy and turbidity, fleshy algal pavements cap some bank-barriers structures. In the Lesser Antilles where present shelf or bench depths are less than 10-12 m, the uppermost surfaces of Holocene bioherms have had sufficient time to build to maturity and now have reef flats and (or) algal ridges. Where shelf depths are between 10-20 m, modern A. palmata reef or algal ridge systems are forming on late Holocene deeper water bank bioherms. Where shelf depths are greater than 20-25 m, sufficient Holocene time has not elapsed to allow the extension of bioherms into shallow water. Shelf-edge reefs, especially without the possibility of a bank base, fall into this latter category and are generally immature and submerged.