Push cores from the branching-coral framework of lagoonal reefs in Belize and Panama indicate that the micritic Mg-calcite in the unlithified mud matrix is similar in composition to the well documented submarine lithified micritic crusts and infillings found in open reef framework. The sediment matrix of the lagoonal framework contains significant concentrations of 12 to 13 mole % MgCO3 calcite that shows almost no signs of skeletal origins, as indicated by a fairly consistent mineralogy and independent distribution patterns in sand-, silt-, and clay-size fractions. Its sedimentological and mineralogical similarity to cemented micritic crusts and infillings suggests that the precipitation of micritic Mg-calcite is widespread in tropical-reef waters and becomes lithified when precipitated in open frameworks but remains unlithified when trapped in impermeable lagoonal muddy sediments.

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