MODEL FOR DOLOMITE FORMATION IN NORTHWEST FLORIDA

Petrographic methods are used to examine the nature of the Floridan po st-Oligocene unconformity and diagenetic responses in the associated c arbonate units. The study addresses the depositional environment and t he character of sediments associated with the unconformity, types and timing of diagenetic changes, and mode of dolomitization for the Suwan nee Limestone dolomites and basal Hawthorn Group dolosilts. The purpos e of the study is to determine if the original texture of geologic mat erials is preserved more frequently in rocks deposited in a supratidal environment in the uppermost Floridan carbonates, to examine possible sources of magnesium for the dolomitization that has occurred in the Floridan aquifer, and to determine the chemical environment of dolomit e formation. On the basis of lithology, the tri-county study area can be divided into two regions; one in which the uppermost Floridan carbo nate is calcareous or locally silicified, and one in which the aquifer system is capped with dolomite. Lateral interfingering between the tw o regions is apparent, and it is inferred that the boundary between th e limestone and dolomite represents a geochemical boundary between a m ixed marine and freshwater environment and a subaerially exposed fresh water environment. Where dolomite has replaced the uppermost Suwannee Limestone, the dolomite is overlain by a thin unit of dolosilt. The pr edominant source of magnesium for the dolomitization that occurred in the study area is probably seawater.