DISSOLUTION PIPES IN NORTHERN PUERTO-RICO - AN EXHUMED PALEOKARST

Late Quaternary pipe- or well-like paleokarst features are being exhum ed and modified by modern coastal processes along the north-western an d northern coasts of Puerto Rico. These features are cigar-shaped tube s dissolved into host rock, with depths up to 4 m, and widths of simil ar to 0.5 m. They can be so densely packed that much of the original d eposit has been removed. Most contain evidence of a few millimeters th ick calcrete lining, consisting of micrite laminae, and a zone of indu rated rock up to several centimeters thick of micrite and microspar. M any pipes are filled with insoluble material similar in appearance to the insolubles of the host rock but more concentrated, and augmented b y material which resembles terra-rossa. At one site the pipes have ret ained 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 th e whole complex cemented. Some pipes show more than one cycle of filli ng, emptying and re-filling, and some areas show more than one phase o f pipe formation. The pipes formed in the vadose zone, in poorly lithi fied, coarse-grained Late Quaternary sandy limestones, by dissolution and re-precipitation along focused flow paths in a climatic regime wit h rain and strong evaporation. They may have formed within a few thous and years of host rock emplacement.