Exposed, Pleistocene age Key Largo Limestone forms the upper Florida Keys. About 30 percent is organic framework of coral colonies (Montastrea annularis predominates), and 70 percent is a two-facies, interstitial calcarenite. The predominant facies has high calcilutite content, is poorly sorted, and surrounds coral heads; the well sorted facies forms channel deposits between coral buildups. Original aragonite and high-magnesium calcite are currently being dissolved, and magnesium removed by leaching, in the meteoric zone. CaCO 3 is redeposited as a lithifying, sparry cement and aragonite fossils are now molds of sparry calcite. Exposure accelerates these processes. Similar diagenetic fabrics may characterize ancient limestone units that were subjected to subaerial exposure after deposition.