LITHOLOGICAL AND HYDROGEOCHEMICAL IMPLICATIONS OF ION EXCHANGE IN SEDIMENTARY BASINS: EVIDENCE FROM THE VOLGA-URAL BASIN

Abstract

Implications of ion exchange processes for the formation of lithological and hydrogeochemical systems under various thermobaric conditions of the subsurface hydrosphere are discussed on the basis of on-location, experimental, and thermodynamic studies. It is shown for the case of the Volga–Ural sedimentary basin that ion exchange interaction in the water–rock system bears a zonal character and is differentiated with depth. Exchange and adsorption processes are most efficient in the upper 500-m-thick supergene zone composed of terrigenous clayey rocks, where the HCO3–Na- and SO4–Na-waters with a mineralization reaching 20 g/dm3 are formed. In cata- and metagenetic zones at a depth of more than 1000–1500 m, these processes do not strongly affect the brine composition. Metasomatic dolomitization of limestones as an exchange adsorption process plays the crucial role in the formation of Cl–Ca-brines at that depth.