Polyphased dolomitization of a shoal-rimmed carbonate platform: example from the middle Turonian Bireno dolomites of central Tunisia

The middle Turonian Bireno carbonate platform of central Tunisia consists of a shoal-rimmed platform including at least three carbonate units deposited in inner to outer shelf depositional settings. The Bireno middle unit is dominantly composed of dolomites capped by a subaerial exposure surface correlative with the global SB 90.5 Ma. Dolomitization occurred during a major sea-level fall which resulted in a progressive modification of the marine groundwater by meteoric water or by evaporation The multidisciplinary study, including petrographic and geochemical analyses, shows that the dolomites include three dolomite phases which consecutively occurred in quite different platform environments, according to different mechanisms: (1) the first dolomite phase resulted from a replacive dolomitization that affected the fine-grained limestone intercalations associated with the layered anhydrites in the evaporitic supratidal zone; (2) the second dolomite phase resulted from a replacive dolomitization that simultaneously affected the rudist lithosomes on the platform border as well as the alternation of the peritidal laminated limestone and desiccation breccias of the intertidal zone, the resulting dolomicrite was more or less recrystallized during shallow burial into turbid dolomicrosparite and dolosparite; (3) the third dolomite phase resulted from the precipitation of limpid dolomite cement in the tectonic fractures affecting especially the rudist shoal carbonates on the border of the platform.Through this study it was established that dolomitization occured in close relation with the major lowring of the sea level in different sub-environments belonging to the same sedimentary system, but following different mechanisms that can result in different dolomite phases displaying quite different petrographic and geochemical signatures. In addition to the role of the sea-level fall, the local tectonic framework played a great role in controlling dolomitizing fluids circulation and distribution of the dolomitized bodies within the platform.