The Guayanilla superhighway outcrop section of southwestern Puerto Rico offers an unusual chance to examine the development and diagenesis of an Oligocene fringing reef and carbonate platform. The stratigraphic section embodies a complex sequence of three cycles of reef development. Reef cycle 2, some 48 m (150 ft) thick, comprises the development of a major fringing or barrier reef at this locality. Reef growth in cycle 2 was inhibited by intermittent ecological stress, perhaps by upwelling colder waters, so that rhodolite-paved ramps replaced shelf margin framework. Tectonic downwarp drowned the reef tract, replacing it by deep open shelf conditions,...