The Toa Baja Well drilled in northern Puerto Rico to a depth of 2705 m (8872 ft.) penetrated over 550 m (1800 ft.) of Tertiary carbonates. The limestone-dominated portion of the well consists mostly of shallow-water backreef carbonates. Metastable carbonates have been either calcitized, dolomitized or dissolved. The petrographic character of the Tertiary carbonates in the Toa Baja Well, and those reported by Monroe [1980>, the cathodoluminescence petrography, and the stable isotopic compositons indicate that these carbonates were rapidly cemented inthe marine environment (limiting compaction), that replacement of metastable carbonates by calcite and precipitation of sparry calcite took place mostly in meteoric diagenetic environments and dolomitization probably occurred in a meteoric-marine mixing zone. Given the history of numerous drainage systems that dissected the Tertiary carbonates throughout their depositional history [Monroe, 1980> and the repeated Cenozoic sea level oscillations [Monroe, 1980; Seiglie and Moussa, 1984>, it is likely that alteration of metastable carbonates took place during lowstands. Interstratifed fluvial deposits suggest the continued influence of meteoric fluids in local highlands [Monroe, 1980>. ¿American Geophysical Union 1991