DOLOMITIZATION IN THE AYMAMÓN LIMESTONE (MIOCENE) OF NORTHERN PUERTO RICO

The Aymamón Limestone forms an unconfined aquifer system that is part of the Puerto Rico North Coast Hydrologic Province. This Miocene unit has been extensively cored, is exposed over a broad area, and has a relatively well-known geologic setting and history. Dolomite is abundant in the unit but decreases gradually with stratigraphic depth. Skeletal components, micrite cements, and the matrix are dolomitic. Red Algae and Rhodolite facies are pervasively dolomitic. The hydrologic regime and/or geochemical character of the fluids was somewhat influenced by these facies causing localized dolomitization. Dolomitization was preceded by early marine cementation, dissolution and/or neomorphism of calcitic and aragonitic components, and infilling of skeletal and intraskeletal porosity by equant/blocky calcite spar. After dolomitization infilling of skeletal and intraskeletal porosity by equant/blocky calcite spar continued. The dolomites are calcian with CaCO₃ mean mole percentage of 53. Average d ¹⁸O and d ¹³C values relative to PDB are +3.7 ‰ (± 0.9 ‰) and +2.0 ‰ (± 0.8 ‰) respectively. Mean dolomitic Sr concentration is 243 ppm. Fe and Mn mean concentrations in the dolomites are 4,183 ppm and 100 ppm respectively. The petrographical and geochemical data obtained suggest dolomitization by a mixture of marine and aquifer waters. Mixing-zone dolomitization under unconfined aquifer conditions provide the best way to explain all the data obtained. Dolomitization by marine water is not consistent with most of the geochemical data obtained. High concentrations of Fe and Mn suggest non-marine water influences. The presence of meteoric fabrics before and after dolomitization also suggests non-marine water involvement. ⁸⁷Sr/⁸⁶Sr isotopic ratios are bimodal in distribution. Surface samples ratios are higher (0.709018 ± 0.000008) than subsurface ratios (0.708965 ± 0.000008). If effected by marine waters these Sr isotopic ratios would suggest two different times of dolomitization, middle and late Miocene. However, these variations can be explained also by Sr contamination from non-marine waters. If this is the case dolomites with higher ⁸⁷Sr/⁸⁶Sr ratios provide a better estimate of the time of dolomitization since petrography suggests late dolomitization. Dolomitization probably occurred during the late Miocene.