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The clay minerals of Late Neoproterozoic–Early Cambrian rock salt of Salt Range Formation of Pakistan have been studied by means of X-ray diffraction, scanning electron microscopy and dispersive X-ray spectrometry, complex thermal and chemical analyses. The clay minerals association of pelitic fraction of water-insoluble residue of these deposits consists of corrensite, chlorite and illite with the admixture of unordered mixed-layered chlorite–corrensite and chlorite–smectite; in some samples, the admixture of smectite occurs. The expandable layers in corrensite are determined as smectite. In studied samples the chlorite, corrensite and mixed-layered species are presented by trioctahedral Mg-rich type and illite is dioctahedral and enriched by Fe; this association of clay minerals is typical for evaporite deposits. Transformation of clay minerals proceeded under the impact of several factors of different direction and intensity. In evaporite basin, the elevated salinity of brines reinforces the processes of clay minerals structure ordering causing disappearance of mixed-layered minerals and thus decreasing the number of clay mineral species; in the brines originated from SO4-rich seawater, the clay mineral associations are richer comparing to Ca-rich brines. Local factors—volcanic ash input, elevated content of organic matter slow down the transformation processes thus also increasing the number of clay mineral species. We explain the unexpectedly rich clay mineral association (as for the halite stage of evaporation) in studied rocks by the strong effect of local factors.

Information on the associations of clay minerals in Upper Proterozoic and Phanerozoic marine evaporite formations suggests that cyclic changes in the (SO4-rich and Ca-rich) chemical type of seawater during the Phanerozoic could affect the composition of associations of authigenic clay minerals in marine evaporite deposits. The vast majority of evaporite clay minerals are authigenic. The most common are illite, chlorite, smectite and disordered mixed-layer illite-smectite and chlorite-smectite, all the clay minerals are included regardless of their quantity. Corrensite, sepiolite, palygorskite and talc are very unevenly distributed in the Phanerozoic. Other clay minerals (perhaps with the exception of kaolinite) are very rare. Evaporites precipitated during periods of SO4-rich seawater type are characterized by both a greater number and a greater variety of clay minerals&mdash, smectite and mixed-layer minerals, as well as Mg-corrensite, palygorskite, sepiolite, and talc, are more common in associations. The composition of clay mineral association in marine evaporites clearly depends on the chemical type of seawater and upon the brine concentration in the evaporite basin. Along with increasing salinity, aggradational transformations of clay minerals lead to the ordering of their structure and, ideally, to a decrease in the number of minerals. In fact, evaporite deposits of higher stages of brine concentration often still contain unstable clay minerals. This is due to the intense simultaneous volcanic activity that brought a significant amount of pyroclastic material into the evaporite basin, intermediate products of its transformation (in the form of swelling minerals) often remained in the deposits of the potassium salt precipitation stage.

Polyhalite is a common constituent of many ancient evaporite sequences, especially Permian and Neogene ones, that is related to the Na−K−Mg−Cl−SO4 type of marine brines in those time intervals. There are four polyhalite deposits in the Zechstein of northern Poland, and more than ten polyhalite-bearing areas in the adjacent part of Russia, and they are commonly accompanied by K−Mg chlorides. Most polyhalite occurrences are related to the upper part of the Lower Werra Anhydrite and in most cases, polyhalite deposits are concentrated at the sulfate platform close to its boundary with platform slope, where they can pass horizontally into polyhalite beds occurring in the Oldest Halite. The bromine content in samples of the Oldest Halite range from 40–120 ppm and the composition of fluid inclusions in halite are characteristic of halite precipitated from seawater concentrated to the early and middle stages of halite precipitation. The δ18O and δ34S values for sulfates are 10.03‰–13.50‰ and 10.03‰–12.14‰, respectively, and the δ37Cl values for halites from −0.1‰ to +0.4‰ support their marine origin. Bromine distribution in the Oldest Halite and the occurrence of anhydrite intercalations indicate fluctuations of the brine density during the Oldest Halite deposition. The formation of polyhalite was preceded by the syndepositional dehydration of the original gypsum deposit and it appears that the anhydrite was then transformed to polyhalite by reaction with marine brines more evolved than those from which precipitated precursor calcium sulfate minerals. These concentrated brines could have been derived from the evaporation of marine brines and/or inflow of K- and Mg-rich brines that were formed in nearby shallow salt pans occurring in sulfate platform areas and thus sulfate platform areas and adjacent slopes of those platforms were predestined for polyhalite formation.

Paleokarst developed in sulfate deposits is common, and it is usually formed along the contact with the overlying permeable rocks or it is due to near-surface dissolution of bedded evaporites. In the Lower Werra Anhydrite (Zechstein) of northern Poland the paleokarst cavities are usually filled by bluish semitransparent anhydrite and more rarely by celestite, polyhalite, halite, and carbonate. In small cavities (a few centimeters across), a rim of rod-like anhydrite crystals arranged in narrow bundles occurs, and the inner part of the cavity is filled with a mosaic aggregate of short prismatic crystals of anhydrite and celestite as well as coarse irregular anhydrite. Celestite crystals and fan-shaped aggregates as well as spherulites of anhydrite are rare. In bigger cavities (some ten centimeters across), multiple zones of fibrous anhydrite are arranged in different directions in the middle part of the cavity fill. The innermost parts of large karst cavities remain hollow in some cases, with the cavity walls encrusted by coarse, well-developed crystals of anhydrite and celestite. The karst cavities in the Lower Werra Anhydrite developed in the subsurface by dissolution of CaSO4 strata in halite-rich intervals due to gypsum dehydration water. During gypsum dehydration, dissolution of that halite would have increased the sodium chloride content of the solution and thus the solubility of calcium sulfate. Dissolved calcium sulfate was removed from a leaching zone by diffusion and/or downward flow in interstitial space, and the minerals in karst cavities precipitated from the same solutions as those solutions became oversaturated because of decreases in NaCl concentration over time. This study suggests that karst in sulfate deposits can develop in the subsurface and without uplift and/or near-surface conditions.