Geologic map of lower surface of sedimentary mantle of the East European Platform

An unusual type of metasomatic dolomitization occurs in the region. Here numerous zones of dolomitization are developed in Ordovician and Silurian sediments, confined to tectonic disturbances and zones of jointing with northeasterly strike. The zones of dolomitization have been traced along fractures and are up to 10-12 km long and up to 400 wide. The dolomitization has a regional character and is apparently associated with the effects of rising magnesian waters on the Ordovician and Silurian carbonate sequence. Possible formation of the zones of dolomitization embraces the interval from the Late Silurian to the Middle Devonian, that is, these zones are of Caledonian age.—Authors.