Geology of the middle Tertiary formations of Puerto Rico

Rocks of Oligocene and Miocene age crop out in both N and S Puerto Rico. They rest with angular unconformity on volcanic and related sedimentary and intrusive rocks that range in age from Early Cretaceous to M. Eocene. The older rocks crop out on the upland that extends E-W through the center of the island. The surface on which the M. Tertiary rocks were deposited is very irregular, having a local relief of at least 200m. Although the rocks of N and S Puerto Rico are approximately the same age, there was no nearby marine connection during Oligocene time; as a result, neither the lithologic characteristics nor the faunas contained in the rocks permit exact correlation from one side of Puerto Rico to the other. During Miocene time, there may have been a connection of the two seas around the W end of present-day Puerto Rico. -from Author