The Quebradillas Limestone (latest Miocene—Pliocene) consists mostly of limestone and lesser amounts of dolomite and marl. It overlies the “Aymamon limestone” whose uppermost part represents a shallow fore-reef environment. The Quebradillas is divided into three units which represent three distinct environments. The lower unit (latest Miocene or earliest Pliocene or both) consists mostly of planktonic foraminifers and is equivalent to a globigerine ooze. It represents the only known deep-water condition in northern Puerto Rico since the beginning of the Oligocene. The middle unit (early Pliocene) consists mainly of Nummulites cojimarensis and planktonic foraminifers. It represents an outer-shelf environment and marks a transition from deep to shallow-water conditions. The upper unit (Pliocene) is made mostly of encrusting coralline algae and Amphistegina, and in places ooids fill fissures and solution cavities. The three units of the Quebradillas constitute a regressive sequence.

The transition from shallow-water conditions that prevailed during the Oligocene and most of the Miocene to the deep-water conditions that characterized the latest Miocene and earliest Pliocene represents a sudden change in relative sea level of more than 200 m.

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