This study investigates the amount and variability of porosity in the approximately 130,000-year-old politic grainstones of the Miami Limestone in the lower Florida Keys. Porosity is derived from porosimeter measurements of 326 plugs and core pieces representing 11 locations spanning about 65 km. Because larger voids in our samples result primarily from scouring by circulating water used in coring and plugging, measurements of matrix porosity, rather than of bulk porosity, represent the formation better in situ. X-ray diffraction analyses show that the rocks have undergone substantial inversion of aragonite to calcite (mean aragonite content of the matrix is 40 percent), but are not yet approaching mineralogical stabilization. Porosity of these partially stabilized rocks averages 36 percent, a decrease of 5-10 percent from that of the initial sediments. A qualitative interpretation suggests that the reduction in pore volume correlates with the degree of inversion of aragonite to calcite, and that the porosity decrease associated with complete mineralogical stabilization could be on the order of 15-20 percent. Porosity is narrowly distributed. The standard deviation of porosity for the entire data set is only 4 percent, and much of this variability is on the relatively small scale of primary sedimentary fabric. Although the pre-burial, near-surface meteoric environment may set the stage for subsequent porosity evolution, results imply that significant porosity loss and porosity diversity in similar but much older rocks develop during burial diagenesis.

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