Late Quaternary pipe- or well-like paleokarst features are being exhumed and modified by modern coastal processes along the north-western and northern coasts of Puerto Rico. These features are cigar-shaped tubes dissolved into host rock, with depths up to 4 m, and widths of similar to 0.5 m. They can be so densely packed that much of the original deposit has been removed. Most contain evidence of a few millimeters thick calcrete lining, consisting of micrite laminae, and a zone of indurated rock up to several centimeters thick of micrite and microspar. Many pipes are filled with insoluble material similar in appearance to the insolubles of the host rock but more concentrated, and augmented by material which resembles terra-rossa. At one site the pipes have retained this primary fill material, now somewhat cemented. At the other site the primary fill material, probably sand rather than terra-rossa, was completely removed, the pipes re-filled with marine debris and the whole complex cemented. Some pipes show more than one cycle of filling, emptying and re-filling, and some areas show more than one phase of pipe formation. The pipes formed in the vadose zone, in poorly lithified, coarse-grained Late Quaternary sandy limestones, by dissolution and re-precipitation along focused flow paths in a climatic regime with rain and strong evaporation. They may have formed within a few thousand years of host rock emplacement.
