Isotopic composition of dolomite associated with Middle Miocene Badenian anhydrites in the Carpathian Foredeep Basin of SE Poland

Abstract

Dolomite is a quite common, although usually minor, constituent of the Badenian sulphate deposits of the Carpathian Foredeep Basin. In the autochthonous member of the Wola Ró aniecka 7 borehole (SE Poland) which contains well preserved, large anhydritic pseudomorphs after selenitic gypsum, dolomite constitutes up to 75% of the rock volume, the rest being anhydrite. Dolomites mostly show peloidal clotted microfabrics typical of microbialites and are interpreted as products of microbially induced precipitation and/or dolomitisation at temperatures around 40°C (as interpreted from the d18 O values that range from 0.4 to 2.6 VPDB) and in water not very distant from equilibrium with atmospheric carbon dioxide (d13 C values usually within the range of +1.0 to +1.6 VPDB). Dolomites associated with laminated anhydrite show similar d18 O values (from +0.9 to +1.5 VPDB) and different d13 C values (from -9.2 to -12.6 VPDB) than the dolomites from the peloidal clotted microfabrics, and are linked to dolomitisation of carbonate mud by evaporitic hypersaline fluids in the early diagenetic/shallow burial environment. The deposits of the laminated anhydrite originated mostly in a deeper, probably stratified, basin, and carbon from the oxidation of organic matter dominated the pool of carbonate ions. 87 Sr/86 Sr values of anhydrite (0.71058-0.71947) and carbonates (0.71024-0.71148 in both members of the Wola Ró aniecka 7 borehole are much higher than the values of contemporaneous Badenian seawater. They indicate strong modification of seawater-derived brines by highly radiogenic continental waters.