CANDER, HARRIS, Amoco Production Company, Houston, TX

ABSTRACT: Origin of Mixing-Zone Dolomite and Associated Pore Fluids, Floridan Aquifer System: Geochemical and Isotopic Evidence

The middle Eocene Avon Park Formation is a 400 m thick pervasively dolomitized peritidal platform carbonate in the Floridan aquifer system. Typical Avon Park dolomite is inclusion-rich, fine-grained (<40 (sym, mu)m), and highly porous (avg. 20%), and formed during the Eocene by normal to hypersaline seawater (Dolomite: (sym, delta){18}O = +3.7, PDB; (sym, delta){13}C = +2.0; {87}Sr/{86}Sr = 0.70778). In a 20 m interval in a core from coastal southwest Florida, cathodoluminescent dolomite cement overgrows early-formed nonluminescent marine dolomite. The cement overgrowths transform early-formed marine Avon Park dolomite from porous, poorly-crystalline, "Cenozoic-like" dolomite to hard, highly crystalline, "Paleozoic-like" dolomite. Based on analyses of samples containing both the ea ly marine dolomite and late dolomite cement, quantitative estimates of the composition of pure cathodoluminescent dolomite cement are: (sym, delta){18}O = +1.7, PDB; (sym, delta){13}C = +0.5, {87}Sr/{86}Sr = 0.7085. These data indicate that the dolomite cement precipitated from a post-Middle Miocene mixed marine-meteoric fluid.

Present day Avon Park pore fluids in this same core have 3400 ppm TDS; Br, Cl, and Na concentrations indicate that the pore fluids are a mixture of 94% Floridan aquifer meteoric water and 6% normal seawater. These pore fluids are calcite and dolomite supersaturated, have excess Sr (24 ppm), Ca (340 ppm) and SO<4> (755.5 ppm) and {87}Sr/{86}Sr compositions (0.7078) in equilibrium with Avon Park host rock. Calculated mixing of the dilute mixing-zone fluids with modern seawater (8 ppm Sr, {87}Sr/{86}Sr = 0.7092) provides an estimation that the late-stage dolomite cement precipitated from a fluid with salinity 75% that of normal seawater. This same fluid would be both dolomite and calcite supersaturated and would have 12 ppm Sr. Such a fluid could precipitate dolomite cement with 22 ppm Sr using a reasonable K<d> = 0.05; this accounts for the higher Sr concentrations in the mixing-zone dolomite cement (225 ppm) relative to Avon Park marine dolomite (167 ppm). Since the modern pore fluids have coeval Eocene {87}Sr/{86}Sr compositions and a positive correlation between Sr, Ca, and SO<4>, these constituents must be derived from extensive dissolution of Avon Park gypsum, not carbonate rock-water interaction.

This study demonstrates that: (1) quantitative modelling can be used to relate modern pore fluid chemistry to the geochemistry and origin of diagenetic phases in the host rock; and (2) dolomite cement precipitated from a mixed marine-meteoric fluid that was both calcite supersaturated and contained high SO<4> concentrations.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.