Dolomitization and evolution of the Puerto Rico North Coast confined aquifer system

The Lares Limestone and Montebello Member of the Cibao Formation form the lower confined aquifer system of the Puerto Rico North Coast. These extensively cored Neogene limestone units dip from 4° to 6° toward the north, are exposed over a broad area, and have a relatively well-known geologic setting and history.; Dolomite is absent in the exposed sections of the units but is abundant in the subsurface. Dolomitization was apparently limited to the subsurface and seaward portions of the units.; Petrography indicates the following paragenetic sequence: (1) shallow submarine diagenesis, (2) extensive dissolution and/or neomorphism of calcitic and aragonitic allochems, (3) burial compaction and pressure solution, (4) dolomitization, (5) dolomite dissolution and generation of coarse calcite cements. Dolomites are not affected by pressure-solution features, indicating dolomitization after at least 300 m burial. Modern aquifer water is undersaturated with respect to dolomite and high Mg2+ content in the water indicates dissolution of dolomite is occurring, therefore dolomitization occurred after some burial (middle Miocene) and before modern aquifer conditions developed (late Miocene). The dolomites are calcian with CaCO₃ mean mole percentage of 56. Average δ18O and δ13C values relative to PDB are +1.0‰ (±0.4‰) and +0.2‰ (±0.5‰) respectively. Mean dolomitic Sr concentration is 360 ppm. Fe and Mn mean concentrations in the dolomites are 266 ppm and 25 ppm respectively. 87Sr/86Sr ranges from 0.708276 to 0.708509 ± 0.000008 in the dolomites and from 0.708199 to 0.708295 ± 0.000008 in the aquifer waters.; These geochemical data are consistent with dolomitization by both marine waters or a mixture of marine and aquifer waters. High concentrations of Fe and Mn indicate non-marine waters. Meteoric fabrics before and after dolomitization suggest meteoric water influences. Sr isotopic ratios are consistent with mixing of marine and aquifer waters.; Although there are other viable explanations, mixing-zone dolomitization in a deep confined aquifer provides the best way to explain all the data obtained. Miocene eustatic sea-level fluctuations probably maintained flow and movement of the mixing zone producing thick dolomitic intervals.