INSIGHTS INTO THE DOLOMITIZATION PROCESS AND ASSOCIATED POROSITY MODIFICATION FROM THE UPPER AVON PARK FORMATION (MIDDLE EOCENE), DAYTONA BEACH, FLORIDA

The Paleogene carbonates of peninsular Florida have had a relatively simple history of largely progressive burial with episodic changes in paleohydrology in response to sea level fluctuations. The carbonates thus provide an excellent window into an intermediate stage of carbonate diagenesis, after early diagenesis but before the onset of intense late burial diagenesis. Three 305-meter long cores were taken from land surface through the upper Avon Park Formation (Middle Eocene) in Daytona Beach, Florida. The upper Avon Park Formation contains sucrosic dolomites in varying stages of development, ranging from scattered crystals in limestones to very pure dolomites in which all of the intercrystalline calcite has dissolved. The upper Avon Park carbonates consist of stacked shoaling upwards cycles deposited in a peritidal environment. Oxygen stable isotope data suggest that dolomite precipitation occurred in waters that were likely moderately more saline than coeval seawater. The cores contain only minor isolated evidence for the presence of former evaporite minerals, which suggests a penesaline diagenetic environment.

The removal of residual calcite in sucrosic dolomites has often been attributed to post-dolomitization leaching by undersaturated waters. However, the Daytona Beach cores have a very close proximity of beds with varying degrees of dissolution of calcitic allochems, particularly large benthic foraminifera, which cannot be attributed to leaching by the inflow of undersaturated groundwaters. The dolomite distribution and textures instead indicate that the precipitation of dolomite itself was responsible for calcite dissolution. Dolomite crystal growth caused local calcite undersaturation at the solution film between the growing dolomite calcite crystal and host carbonate, as well as more diffuse undersaturation in the beds undergoing dissolution by the reduction in calcium and carbonate activities. Bed-scale porosity modification during dolomitization reflects the net influx of calcium and carbonate ions during dolomitization and the smaller molar volume of dolomite compared to calcite. Dolomitization may thus result in either a local increase or decrease in porosity.