DOLOMITIZATION ASSOCIATED WITH TRANSGRESSIVE SURFACES - A MIDCRETACEOUS EXAMPLE

Increased seawater circulation within a carbonate platform, generated through transgression and flooding of the platform, is proposed here a s a cause of the common selective dolomitization of calcitic silts in the upper part of limestone sequences of the Barremian-Aptian Urgonian platform of SE France, which underwent selective porosity creation as a result of karstification during an Aptian relative sea-level fall. Parts of this porosity are infilled by microcrystalline dolomite, and some of the host limestone is partially to completely dolomitized. The microcrystalline dolomite is considered to have formed by dolomitizat ion of calcitic silt internal sediment within the karstic cavities as a result of the pumping of seawater through the upper layers of the pl atform during subsequent transgression. The calcitic silt provided abu ndant nucleation sites for the dolomitization. Seawater dolomitization following subaerial exposure of limestone is likely to be a common ph enomenon if there is sufficient meteoric diagenesis and porosity produ ction and a prolonged period of submarine erosion, winnowing and conde nsed sequence formation, enhancing the amount of circulation of seawat er as seafloor sediment is lithified.