The uppermost formation of the limestone belt of northern Puerto Rico is herein redesignated the “Quebradillas Limestone.” A type section is established within the original type locality north of the town of Quebradillas. As here redefined, the Quebradillas Limestone consists of a limestone sequence overlying the uppermost Amphistegina-algal limestone of the “Aymamón limestone” along Highway 2 north of the town of Quebradillas. The formation consists of three types of limestone biomicrite which form the basis for subdividing the formation into three members. The lower member consists of globigerine biomicrite and is 16 m thick. In many areas, the lower member contains thin beds of oyster shells and shell fragments inter-bedded within marl. Both the matrix of the oyster-bearing beds and the marl are rich in planktonic foraminifers. The middle member is essentially Operculinoides biomicrite and is slightly <12 m thick. The upper member is Amphistegina-algal biomicrite, and its thickness is estimated to be 44 ± 4 m. The threefold subdivision has been recognized throughout the outcrop belt of the formation from the west coast of the island eastward to Barceloneta; it probably applies to the easternmost outcrops as well.
The Quebradillas-“Aymamón” boundary is an erosional unconformity between the two formations. The globigerine limestone of the lower member of the Quebradillas fills solution cavities in the uppermost “Aymamón,” and in places, the upper “Aymamón” shows marked relief. Most of the Quebradillas is Pliocene in age, although part of the lower member may be latest Miocene. The Quebradillas Limestone is a regressive sequence. The lower member represents a lower neritic to upper bathyal environment; the middle member, an outer shelf environment; and the upper member, a forereef, subtidal, and intertidal beach environment. The upper member is veneered by what appears to be a paleosol which in turn, is overlain by Quaternary eolianite, marginal marine, and marine deposits.
The Quebradillas is not limited to only the onshore coastal area of the island but extends northward into the Puerto Rico Trench. Above a water depth of 5,000 m, the Quebradillas covers an area of at least 4,000 km2 on the southern slope of the trench, and the total area could be as much as 9,000 km2. Seismic and dredge-sample data suggest that the Quebradillas consists of the same facies offshore as in both outcrop and one well onshore. The presence of widespread Quebradillas in the Puerto Rico Trench has important tectonic implications for regional studies of both the Puerto Rico Trench and the northern Caribbean region.