2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 15
Presentation Time: 5:00 PM

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


GASWIRTH, Stephanie B., ExxonMobil Upstream Rsch Co, P.O. Box 2189, Houston, TX 77252, BUDD, David A., Dept. of Geological Sciences, Univ of Colorado, 399 UCB, Boulder, CO 80309-0399 and FARMER, G. Lang, Dept. of Geol. Sci, Univ. of CO, Boulder, CO 80309, stephanie.b.gaswirth@exxonmobil.com

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.