The Mariana Limestone is an emerged reef. It forms 75% of the exposed limestone of Guam and dominates the surface of northern Guam and eastern flank of southern Guam. The ariana Limestone was mapped in the field as five facies (Tracey et al., 1964). The most prominent are the reef facies (a coral-algal incrustate limestone), detrital facies and molluskan facies (packestones and wackestones of lagoonal origin), and argillaceous facies (clay-rich rocks adjacent to volcanic terrain).

The Mariana Limestone is undergoing recrystallization, with original carbonate still locally present as aragonite (Schlanger, 1964). Porosity is high and is estimated to average about 10 percent. That includes both primary pore spaces, which can be as high as 30 percent in some specimens (but largely filled with calcite in others), as well as secondary (karst) voids.

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