Differentiation of environments of dolomite formation, Lower Cretaceous of Central Tunisia



    1. Laboratoire de Pétrologie Sédimentaire et Paléontologie (ERA 765), Université Paris Sud, Centre d'Orsay, Bât. 504, 91405 Orsay Cedex, France
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Combined field, sedimentological, mineralogical, isotopic and geochemical study of the Lower Cretaceous dolomites of Central Tunisia has demonstrated considerable diversity in origin. Environments of dolomite formation include deep phreatic, karst, lacustrine and evaporitic sabkha. All four groups of dolomite are composed of non-stoichiometric and/or disordered crystals which are more or less rich in calcium and in iron. Petrographic fabrics are of three types: replacement, recrystallization and cementation. These three fabrics are proposed among the various criteria for the different environments of dolomitization. Average isotope (δ18 O and δ13 C) contents for these four dolomite groups range from −10·5 to +0·4%o (PDB) and −3·9 to + 3·7%o respectively. The distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity. Ferrous iron, also very common, is regarded as an indicator of relatively deep reducing conditions, mainly in meteoric groundwaters. Sodium distribution is related to inclusions within the dolomite, its distribution being relatively constant in all four groups; it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids. This study confirms that dolomitization may occur under widely different palaeoenvironments, either at the surface or during burial. With the exception of the sabkha environment, dolomitizing fluids seem to have been essentially meteoric.