Partially dolomitized Lower Jurassic limestone in the Kálvária Hill, Tata

Abstract

A few meter sized dolomite body was found in the Lower Jurassic limestone strata of the Kálvária Hill, Gerecse Mts. The dolomite is unique, both in the outcrop and in the Jurassic beds of the Gerecse Mts. It is laminated, yellow-red and follows a fracture, further invading the host Jurassic limestone along a bedding plane. Dolomite, replacing the host rock is fine to medium crystalline, and locally saddle dolomite crystals were observed. A breccia zone cuts across the dolomite body, however brecciation was not observed within the dolomite, only calcite veins are present. The breccia zone is cemented by white-grey-yellow calcite that incorporates dolomite crystal fragments. Calcitized dolomite crystals were found in the host Jurassic limestone next to the breccia zone. The three different dolomite types are most probably associated with the same dolomitization event. Dolomitizing fluids could have migrated through fractures and occasionally along bedding planes. Dolomite crystal fragments – found within zones of the breccia cementing calcites – could have been derived from the dolomitized host rock. The stable isotope data of the dolomite suggest that dolomitization took place most probably in the burial realm, either from the existing pore water or hydrothermal fluids, migrating through fractures in the lithified Lower Jurassic limestone. Post-sedimentary character of the fractures suggests post Late Jurassic age for the dolomitization. Meteoric origin of the calcite found in fractures and as cement in the breccia zone, points to subaerial exposure. This would mean pre-Late Cretaceous–Paleogene dolomitization event.