Controls on the Transformation of Clay Minerals in the Miocene Evaporite Deposits of the Ukrainian Carpathian Foredeep: An Overview

Clays deposited in marine evaporite sequences are strongly altered and the most important factor determining their transformation is the brine concentration. X-ray diffraction study of clay minerals associated with the Lower and Middle Miocene evaporite formations of the Ukrainian Carpathian Foredeep indicated that the clay mineral associations in the gypsum facies are composed of smectite and illite, and in some samples mixed-layer chlorite-smectite and illite-smectite as well as chlorite. In the halite facies, illite, chlorite and mixed-layer illite-smectite occur in rock salt of Eggenburgian age (Vorotyshcha Suite); in addition to those minerals, smectite, corrensite and mixed-layer chlorite-smectite occur in the Badenian rock salt (Tyras Suite); and in the potash facies illite and chlorite were recorded. Such clay mineral associations resulted from aggradational transformation of unstable and labile minerals and phases (kaolinite, smectite and mixed-layer phases) that finally pass into illite and chlorite: minerals that are stable in an evaporite environment. In addition to brine concentration control, another important factor in the transformations of clay minerals was sorption of organic components on the mineral structure, that slows the transformation processes. The association of clay minerals in the weathering zone of the evaporite deposits, besides inherited illite and chlorite, contains also mixed-layer illite-smectite and kaolinite. The appearance of those clay minerals in hypergene deposits is the consequence of two processes: degradational transformation (illite-smectite) and neoformation (kaolinite) in conditions of decreased ionic concentrations during desalination.