ABSTRACT

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

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

Based on 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 are initially cemented by grain welding and aragonite precipitation between grains. This early type of cementation is most common in the supratidal and intertidal zones. Subsequent calcite cementation is inferred to record dissolution-precipitation replacement of aragonite. After calcite precipitation has occurred, no evidence can be found of pre-existing aragonite cement.

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 responses 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.

Both primary and secondary porosity are represented in the San Andres area. Development and occulusion depend on exposure of carbonate material to a subaerial environment before mineral stabilization occurs.