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

Clays deposited in marine evaporite sequences are strongly altered, and the most important factor determining their transformation is brine concentration. An 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 assemblages 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 assemblages resulted from the 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 the sorption of organic components on the mineral structure, which slows the transformation processes. The assemblage of clay minerals in the weathering zone of the evaporite deposits, besides inherited illite and chlorite, also contains 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.