DIAGENESIS OF AN OOLITE-CARBONATE MOUND-TIDAL FLAT SEDIMENTARY SUCCESSION, MISSISSIPPIAN BANGOR LIMESTONE, NORTHERN ALABAMA

The Bangor Limestone is a Mississippian shallow marine carbonate formation exposed over a large portion of the Interior Low Plateaus province of northern Alabama. Although the unit is dominated by oolitic and skeletal grainstones, recent work has shown the Bangor to contain isolated mound complexes and peritidal facies. At our study site near Moulton, Alabama, the Bangor exposure consists of an oolite - rugose mound complex - tidal flat succession which we interpret as comprising a single 4th order sea level cycle and part of another. The diagenetic history of the Bangor Limestone at this location supports this conclusion.

The oolite contains a first generation, isopachous, synsedimentary cement as well as early diagenetic meniscus\pendant calcite cement and infiltrated lime mud ("vadose silt"). Much of the oolite may have been indurated prior to the transgression that initiated mound development. The mound complex contains limited synsedimentary cement in void spaces within rugose corals. Diagenesis of the mound and overlying peritidal facies was dominated by early meteoric processes driven by subaerial exposure that occurred near the top of the sedimentary succession at this location. Microspar formation, dissolution of aragonitic allochems and drusy calcite infilling of voids all occurred during exposure, but the most significant exposure-related diagenetic event was dolomitization. The dolomite underlies the exposure surface, overprints some early diagenetic phases (e.g., microspar), but predates others (e.g., infilling of voids), suggesting that dolomite also formed comparatively early. Petrographic and geochemical data suggest a hypersaline origin for the dolomite.

The section of Bangor Limestone that we have examined is relatively minor given the areal extent of the formation in northern Alabama, but it is important for demonstrating the importance of diagenesis and sea level change for initiating and terminating mound development in shallow marine carbonate environments.