PETROGRAPHY OF MONA ISLAND DOLOMITES, PUERTO RICO

Although detailed studies on dolomite and dolomitization are available for Caribbean and Bahamian rocks, few data are available on the dolomites of Puerto Rico. The abundant dolomites present in the limestones located along the northern cliff front of Mona Island, Puerto Rico, have never been sampled before due to logistic and safety concerns. Five stratigraphic sections along the north coast cliff front of the island were measured (Cabo Barrionuevo, Cabo Noroeste, Cabo Norte, Cueva Gato, and Cueva del Frio). Thin sections (n=50) were made and the petrography of the rocks described from them. Seven depositional facies were identified; Red Algal Wackestone, Skeletal Packstone, Foram-Red Algal Packstone, Grainstone, Red Algal Packstone and Skeletal Wackestone. The most abundant bioclasts identified were red algae and benthic foraminiferans. Selective dissolution of foraminifera is common and has produced abundant moldic porosity. In some sections the forams have not been leached and are preserved in some detail. Lateral as well as stratigraphic variations are present in the degree of dolomitization. Dolomite occurs in a wide range of crystals forms, fabrics and mosaics and ranges from fabric destructive to retentive and from fabric selective to pervasive. The dolomite crystals in the replacement mosaics vary from anhedral to euhedral rhombs (xenotopic to idiotopic). The most common crystals forms found were planar crystals in an idiotopic mosaic composed of euhedral rhombs. Microcrystalline dolomite is common and widely distributed. Euhedral, limpid dolomite commonly fills the intergranular porosity. Lower in the stratigraphic sections (10 to 20 meters below the island surface) the units show extensive dolomitization. Stratigraphically higher in the sections (upper 10 meters just below the island surface) dolomitization mostly affect matrix material. Due to its isolation and its 100% carbonate rock composition, Mona Island is an ideal place to study the diagenesis of limestones. Future work will study the geochemistry of the dolomites and will integrate the new data on the northern part of the island to the already available data on the southern section of the island.