The presence in the Florida Keys of a Paleocene-Upper Cretaceous barrier-reef complex of regional extent is indicated by data derived from drill cuttings, electric logs, and drillers' reports. The reef section is characterized by lost circulation and reports of cavities, caverns, and "boulders." Drill cuttings show the section to be lithologically anomalous. In the south Florida back-reef basin, the normal section consists of Upper Cretaceous Pine Key chalk overlain by interbedded microcrystalline euhedral dolomite and anhydrite of the Cedar Keys Formation. The equivalent barrier-reef section is composed of very fine to fine crystalline anhedral dolomite, characterized by vugs or cavernous porosity. In the south Florida back-reef basin north of its greatest development, t e Rebecca Shoal dolomite-reef facies breaks up into the Tavernier and Plantation Tongues which eventually interfinger with the "basin" facies. In the south Florida back-reef basin, the chalk of the Pine Key Formation persisted until the barrier reef had encircled the Florida peninsula, producing the Cedar Keys cyclic dolomite and evaporite more typical of a back-reef facies.