Selected geochemical factors influencing diagenesis of eocene carbonate rocks, peninsular Florida, U.S.A.

Abstract

The geochemical significance of three selected ions (Mg 2+, Na +, and Sr 2+) supports a model of dolomitization by brackish groundwater. This groundwater zone contains sufficient quantities of Mg 2+ to facilitate dolomitization ( Mg Ca ratios 1 ). Rising and falling of sea level and fluctuations of the phreatic zone related to climatic variations account for the thickness of the dolomite layers and the chemical distributions within these layers. Sodium concentrations in the calcite are 70–185 ppm, indicating formation in brackish water. Dolomite has sodium concentrations between 50–1400 ppm, suggesting formation in waters of similar salinity. Strontium in calcite ranges from 320–600 ppm, suggesting diagenesis in slightly saline waters in an open system. Dolomite contains 241 ppm Sr 2+ on the average and calcite has 418 ppm Sr 2+. The Sr 2+ concentrations of the dolomite are characteristic of diagenesis in water less saline than sea water. Average strontium concentrations in the dolomite occur in two distinct groups, 260 ppm for dolomite with 39–43 mole-% MgCo 3 and 195 ppm for the dolomite with 44–50 mole-% MgCO 3. The difference in the Sr 2+ concentrations of the two dolomite groups indicates the higher mole-% MgCO 3 dolomite recrystallized in a less saline environment than the lower mole-% MgCO 3 dolomite. These different environments are attributed to a relatively more saline coastal environment and a less saline inland environment. The more nearly stoichiometric dolomite (44–50 mole-% MgCO 3) has less scatter when mole-% MgCO 3 is plotted against Sr 2+ and Na +. This suggests a greater approach to equilibrium with the dolomitizing fluid than the lower mole-% MgCO 3 (39–43) dolomite. The more saline environment has higher Mg/Ca ratios and promotes more calcium-rich dolomite during diagenesis because of the inhibition from competing foreign ions and because it is thermodynamically a more favorable environment which causes more rapid crystallization. The less saline waters allow recrystallization to proceed more slowly, producing better ordering in the dolomites, textural preservation and development of subhedral to euhedral rhombic crystals.