Abstract
The Avon Park Formation (middle Eocene) in central Florida, U.S.A., contains shallow-water carbonates that have been replaced by dolomite to varying degrees, ranging from partially replaced limestones, to highly porous sucrosic dolostones, to, less commonly, low-porosity dense dolostones. The relationships between dolomitization and porosity and permeability were studied focusing on three 305-m-long cores taken in the City of Daytona Beach. Stable-isotope data from pure dolostones (mean delta O-18 = +3.91 parts per thousand V-PDB) indicate dolomite precipitation in Eocene penesaline pore waters, which would be expected to have been at or above saturation with respect to calcite. Nuclear magnetic log-derived porosity and permeability data indicate that dolomitization did not materially change total porosity values at the bed and formation scale, but did result in a general increase in pore size and an associated substantial increase in permeability compared to limestone precursors. Dolomitization differentially affects the porosity and permeability of carbonate strata on the scale of individual crystals, beds, and formations. At the crystal scale, dolomitization occurs in a volume-for-volume manner in which the space occupied by the former porous calcium carbonate is replaced by a solid dolomite crystal with an associated reduction in porosity.
Dolomite crystal precipitation was principally responsible for calcite dissolution both at the actual site of dolomite crystal growth and in the adjoining rock mass. Carbonate is passively scavenged from the formation, which results in no significant porosity change at the formation scale. Moldic pores after allochems formed mainly in beds that experienced high degrees of dolomitization, which demonstrates the intimate association of the dolomitization process with carbonate dissolution.
The model of force of crystallization-controlled replacement provides a plausible explanation for key observations concerning the dolomitization process in the Avon Park Formation and elsewhere: (1) volume-for-volume replacement at a crystal scale, (2) coupled growth of dolomite crystals and dissolution of host calcium carbonate matrix, and (3) automorphic replacement by euhedral dolomite crystals. The force-of-crystallization model also does not require an influx of externally derived water that is undersaturated with respect to calcite to dissolve calcite, a fact that could simplify diagenetic models of porosity generation in dolostones. The later addition of external carbonate can result in a substantial reduction in porosity by the precipitation of dolomite cement, which could convert a high porosity sucrosic dolostone into a dense "Paleozoic type" dolostone.