The late Oligocene San Sebastian Formation of northern Puerto Rico was deposited on an erosional surface of Eocene and Cretaceous rocks. It constitutes a transgressive sequence representing deltaic, coastal lagoon, and shallow-water marine paleoenvironments.

The northward extension of the formation has been traced as far as the Kewanee Interamerican Oil Co. 4CPR well, 8 km east of Arecibo and about 12 km north of the closest outcrop. In the well, the formation occurs at depths between 1,352 and 1,652 m. In previous descriptions of the well cuttings, "silicified tests of planktonic foraminifers" were reported; review of the assemblages has revealed that the "siliceous tests" are actually light-brown spherulites of siderite, some of which have shapes similar to Orbulina, Biorbulina, Globigerinoides, and Globigerina. They probably formed in a freshwater or brackish-water paleoenvironment.