Diagenesis of Spathian-Anisian dolomites from the Western Balkanides, Bulgaria

The calcian crypto- to microcrystalline dolomites (Mogila Formation) origin ated in a mild arid peritidal depositional environment through complete ear ly diagenetic replacement of lime sediments, at low Mg/Ca ratio of the dolo mitizing fluids and salinity closer to normal marine water. The preservatio n of their non-stoichiometry was mainly controlled by a closed diagenetic s ystem with a low water/rock ratio. The fine crystalline dolomites from dolo mitic limestones (Babino Formation) are also calcian but the process of par tial dolomitization took place in the late diagenesis, within a closed diag enetic system, at short reaction time and probably through autochthonous su pply of magnesium cations. The nearly stoichiometric crypto- to microcrysta lline dolomites (Svidol Formation) were also formed as an early diagenetic replacement phase in a peritidal setting but at elevated Mg/Ca ratio of the dolomitizing solutions and a hypersalinity of the water milieu which both have been controlled by highly arid climatic conditions. The nearly stoichi ometric medium to very coarse crystalline late diagenetic dolomites from pu re dolostones (Mogila and Babino Formation) show a higher degree of order c ompared to the early diagenetic dolomites. Complete replacement of limeston es at slow crystallization rates by diluted fluids at elevated temperatures is suggested, but neomorphism of earlier metastable dolomite phases may ha ve proceeded as well.