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A discontinuous layer of lithified carbonate sandstone underlies a small part of the Lac, a large lagoon on the southeastern coast of Bonaire. The layer lies 35 cm below the sediment surface, varies from 5 to 20 cm in thickness, and is restricted to an area beneath a broad intertidal and subtidal flat. Beachrock crops out in the high intertidal zone.
The lithified layer consists of grainstone cemented by acicular aragonite. Numerous lines of evidence indicate the cementation occurred in the marine environment. The lithified layer is at present continuously saturated with normal seawater. The submarine-cemented rocks lack the gray algal coating which is characteristic of beachrock and subaerially exposed coral rubble. Carbon-14-dating of the rock indicates cementation occurred less than 900 years ago. Study of the constituent particles of the lithified layer and the sediment above and below indicates continuous marine sedimentation.
Several distinct types of micrite are present in the beachrock and submarine-cemented layer. The most common is a high- and low-Mg calcite which coats single and multiple skeletal fragments with a sharp contact between the grain and micrite. The coating results from micritization of the high-Mg calcite of the encrusting coralline algae. Electron microprobe and X-ray diffraction analysis of these coatings demonstrated that as micritization proceeds and the microstructure of the algal coating is destroyed, the mineralogy changes from high- to low-Mg calcite.
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