Facies and diagenesis of late Pleistocene carbonates from Big Pine Key, Florida

Nine boreholes in Big Pine Key, Florida penetrated three Upper Pleistocene stratigraphic units equivalent to the Q3, Q4 and Q5 (in ascending order) subdivisions of Perkins (1977). Three major facies occur within the Q5 unit: an ooid grainstone facies grades into an ooid grainstone-packstone facies which in turn grades into a peloid-bioclast packstone-grainstone facies. The ooid grainstone and ooid grainstone-packstone facies are regarded as Miami Limestone (Hoffmeister et al., 1967), whereas the peloid-bioclast packstone-grainstone facies is considered to be Key Largo Limestone (Hoffmeister and Multer, 1968). This last facies is not only a lateral facies equivalent of the Q5 Miami Limestone, but also underlies it as the Q4 and Q3 units. Grains of the Q5 unit are generally less altered than their Q3 and Q4 counterparts, and the Q5 unit is also typically less well cemented. Both differences simply reflect the more prolonged diagenetic history of the older sediments. Cementation inhomogeneities, meniscus cements, and prominent vug and channel porosity indicate that the vadose zone has been the principal influence on the diagenesis of all three Q units. Present-day freshwater lenses within the Q5 unit have had little diagenetic effect. Although total porosity in the three units examined is comparable, secondary porosity is dominant in packstones within the Q5 unit, and is the main porosity type in the older units.