Rotary-drill rock cores and vibrocores of sediments were used to investigate the origin and sedimentary history of patch reefs and a "rhomboid" shoal in offshore Belize (Central America). All lagoonal patch reefs and shoals examined are localized on preexisting Pleistocene topography. The buried topographic highs are composed of Pleistocene coralline limestone, suggesting a constructional patch-reef origin rather than an erosional or karst origin. The principal Holocene sediment- and framework-builder is the branching coral Acropora cervicornis, but A. palmata, various massive corals, and the lettuce coral Agaricia sp. are also contributors. These accumulations, up to 27 m thick, amplify Pleistocene topography. The accumulations are totally uncemented, and metal probes ca easily be inserted more than 5 m into the living reef slope. Steep dips, commonly greater than 45° and extending from the surface to a least 30 m in depth, provide a sedimentary paradox. Even though these reefs are uncemented and periodically subjected to hurricanes, reef debris has not been found in the surrounding lagoonal muds more than a few meters away from the reef "toe." The surrounding coral-free lagoonal sediments, consisting both of clays and carbonate materials, are more than 6 m thick.

Understanding the mechanics of patch-reef formation may provide clues important to oil exploration: (1) many of the rhomboid shoal reefs are of reservoir size even though they are less than 9,000 years old; (2) their position adjacent to a humid mountainous hinterland makes them susceptible to burial by terrigenous clays as the coastal plain progrades; (3) both the clays and carbonate muds are in a favorable stratigraphic position to serve as source beds.

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