Modern submergent reefs and Pleistocene emergent reefs fringe the coasts of the Red Sea. In the southern Sinai Peninsula the emerged reefs have been dated (1) at about 110,000 years; (2) 200,000 to 250,000 years; and (3) older than 250,000 years. Progressive diagenetic changes can be traced from living reefs, through modern dead but still submerged reefs, to emerged reefs of progressively greater ages. We recognize four stages in the diagenetic sequence: stage I, initial fabric of living scleractinian corals; stage II, introduction of marine cements and decomposition of organic tissue in the submerged reefs; stage III, leaching of sclerodermites (the elementary structural units of corals) under subaerial conditions, and stage IV, precipitation of low-magnesian calcite thr ugh meteoric fresh waters.

Between stages I and II, as organic tissue decomposes, the voids in the centers of sclerodermites, which previously were filled with organic tissue and were unconnected, become joined. Aragonite and high-magnesian calcite precipitate as cement in such voids and reduce porosity and permeability. As reefs emerge, leading to stage III, leaching of the aragonite in the coral skeletons begins in the centers of sclerodermites. Ultimately during stage III the aragonitic skeleton is entirely removed, preserving only the original micritic envelope that lined the coral. The original high-magnesian calcite of the micritic envelope becomes low-magnesian calcite without a change in fabric. Any aragonitic void filling that may have formed at stage II is ultimately removed during stage III. At this tage, removal of aragonite may lead to effective porosity of up to 60% and permeability in excess of 10,000 md. During stage IV, low-magnesian calcite nucleates on the thin films of the micritic envelope and grows as drusy calcite into the newly formed pores; however, most of the void space of stage III is retained.

In the arid environment of the Red Sea, the fresh waters of the desert that leach aragonite at stage III remain mostly undersaturated with respect to calcium carbonate; thus progressive leaching increases the porosity of emergent reefs. At stage IV precipitated calcite only coats the micritic envelopes; limited saturation with respect to calcium carbonate causes little precipitation of calcium carbonate and does not eliminate existing void space. Porosity and permeability remain high.