Paleohydrological modeling of penesaline reflux dolomitization: Avon Park Formation (Middle Eocene), East Central Florida

Abstract

Previous geochemical, petrological, and sedimentological investigations of the Middle Eocene Avon Park Formation (Middle Eocene) of Florida suggest that dolomitization occurred in the Eocene from normal to hypersaline seawater. Reflux of penesaline water (i.e., seawater evaporatively concentrated below evaporite mineral precipitation) is the most likely mechanism to supply the required magnesium. Density-dependent solute-transport modeling was performed to evaluate whether penesaline reflux under known or inferred Middle Eocene paleohydrological conditions could have provided enough magnesium to account for the mass of dolomite present in three cores of the upper part of the Avon Park Formation from the City of Daytona Beach. Rather than a simple down and out flow pattern, the modeling results indicate that an initial rapid vertical convection-dominated flow regime first occurs in which native groundwater is displaced by refluxed brine, and followed by a longer-duration regime dominated by steady-state lateral flow. Dolomitization from a brine with a 89.6 ppt TDS concentration would require an average minimum reflux (at 100% dolomitization efficiency) on the Florida Platform of about 8130 m3 per m2 to provide the required magnesium. Modeling results indicate that such reflux could occur over 0.7–3 million years, depending upon the aquifer hydraulic parameter values. Longer time periods would be required for the reflux of lower salinity brines and lower dolomitization efficiencies. Penesaline reflux is thus hydrologically plausible for the dolomites of the upper Avon Park Formation.