DOLOMITE MATURATION BY A REGIONALLY EXTENSIVE MIXING ZONE IN THE PROTO FLORIDAN AQUIFER

Mixing-zone dolomite has been widely documented as cement on existing dolomite crystals and as a recrystallization product. More hotly debated is the scale and areal extent at which a mixing zone can drive these maturation processes. This study addresses the extent of the mixing-zone dolomite in the Early Oligocene Suwannee and Late Eocene Ocala dolostone bodies, elucidates the timing of initial dolomitization and addition of mixing-zone dolomite, and answers the question of whether the mixing-zone maturation of the dolomite was the result of multiple mixing-zone events or a single episode.

In west-central Florida, mixing-zone dolomite is identified as a secondary luminescent dolomite developed over an initial non-luminscent replacive dolomite . Volumetrically, the luminescent dolomite forms 3-40% of the dolomite in any thin section and represents 1.8 km3 of material in the Ocala. When present as a cement, it can reduce porosity by 10-20% and permeability by many hundreds of millidarcies in sucrosic dolomite and up to 10% and a few millidarcies in indurated dolomite. Stable isotopes and trace-element geochemistry confirm a marine origin for the initial Ocala and Suwannee dolomites and a dilute freshwater-seawater origin for the secondary luminescent phases. 87Sr/86Sr values for matrix dolomite of the Suwannee and Ocala indicate initial dolomitization occurred during the late Early Oligocene, coincident with a major mid-Oligocene sea-level fall, suggesting significant circulation of seawater induced by an overlying mixing zone. Modeling of the luminescent dolomite's 87Sr/86Sr values indicates the mixing-zone phase precipitated during a sea-level rise and highstand in the early Late Miocene. This age is compatible with the age proposed for mixing-zone overprinting in the underlying Middle Eocene Avon Park Formation, indicating a single, regionally widespread and volumetrically significant mixing zone maturation of existing dolomites in the proto Floridan Aquifer.