San Andres is a small coralline island 136 km east of the Caribbean coast of Nicaragua. Surface mapping and paleontologic data indicate reefs have been growing actively in the area at least since early Miocene time. Because of the offlap relations of the ancient reef complexes, accessibility of progressively younger rocks is simplified.

Field and laboratory examination of both modern and ancient reef complexes indicates five major bottom facies: (1) forereef flank, (2) reef, (3) backreef platform, (4) backreef lagoon with patch reefs, and (5) shore area. Principal builders of the reefs were Millepora and lime-secreting algae with abundant Acropora, Diploria, Montastrea, and Porites.

From X-ray and petrographic studies of the limestones, a sequence of carbonate diagenesis has been determined for the San Andres area. During early stages of diagenesis, sediments initially are cemented by grain welding and aragonite precipitation between grains. This early type of cementation is most common in the supertidal and intertidal zones. Subsequent calcite cementation occurs by dissolution-precipitation after aragonite. After calcite precipitation, no evidence can be found of the preexisting aragonite cement. Although inversion of aragonite to calcite commonly occurs in grain material, dissolution-precipitation seems to be the major process of cementation.

During the diagenetic sequence, four responses may be expected: (1) magnesium is removed and high-magnesium calcite stabilizes to low-magnesium calcite, (2) aragonite can stabilize to calcite, (3) aragonite may be dissolved, and (4) sediments may become dolomitized. The time required for any of these reactions is variable and depends on the chemical environment. X-ray analyses indicate that all samples older than middle Pleistocene have stabilized to low-magnesium calcite or dolomite. The result of the diagenetic sequence is the development of a low-magnesium calcite limestone, a dolomite, or a combination of the two.

Porosity in the San Andres area is primary, secondary, or occluded and development depends on exposure of carbonate material to a subaerial environment before mineral stabilization occurs.

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