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The hydrography and brine flow patterns in the Middle Miocene (Badenian) evaporite basin of the northern Carpathian Foredeep (in Ukraine, Poland and the Czech Republic) are reconstructed based on studies of the peculiar, conformably oriented, bottom-grown gypsum crystals present in the selenite deposits along the basin margin. The crystal apices are turned in a similar horizontal direction that is interpreted as the product of consistent flow of the bottom brines during crystal formation. Similarly the regular millimetre-scale growth zoning in these crystals presumably reflects the annual stratification-mixing pattern in the brine column typical of monomictic basins. In the central, deeper parts of the basin deposition was dominated by Na-chloride, and the selenitic facies are lacking. These central areas are interpreted as being meromictic during the oriented selenite deposition. The permanent pycnocline separated a mixolimnion, at the surface, from an anoxic (euxinic) monimolimnion, at the bottom, where direct evaporative crystallization of gypsum was not possible. The mixolimnion, which extended far onto the shallow margin of the basin, showed only a seasonal (annual) pycnocline and monomictic hydrography. Oriented selenites grew just in this mixolimnetic marginal zone, under predominantly counterclockwise (cyclonic) flow.

An integrated group of conceptual models for evaporite deposition is presented for a shallow salina-type basin, supplied both with marine and non-marine water, and with a water level separated from and drawn down below world sea-level. Knowledge of, and terminology for, modern limnology is used in these models in order to build a new and more comprehensive link between the hydrography and hydrochemistry of brines and the depositional and stratigraphical record anticipated in such basins. In these modelled basins it is assumed that (as in saline lakes): (1) the evaporite deposition reflects the stratification-mixing cycles in the brine column; (2) the evaporative crystallization of salts culminates during the mixing periods; and (3) the evaporite facies are linked to the position of water zones separated by a horizontal pycnocline. The models are prepared especially for interpretation of subaqueous coarsely crystalline gypsum (selenite) deposition in perennial-to-ephemeral saline pans, and also for fine-grained gypsum deposition (clastic, microbialite and pedogenic) on specific, flat, semi-emerged shoals (majanna-type shoals), commonly flooded by wind-driven brine sheets, in between them. Coarse, crystalline selenites appear to form below the pycnocline and the crystallization of particular well-developed grass-like selenite beds is thought to be connected with the pycnocline highstands. Such beds are both marker beds and ideal datum surfaces. The models presented are used for sedimentological interpretation of the various gypsum facies of the Badenian (Middle Miocene) evaporite basin in the northern Carpathian Foredeep. The observed architecture of these facies suggests that the margin of the basin was occupied by a system of variable saline pans (dominated by selenite deposition) and evaporite shoals (dominated by gypsum microbialite deposition); some of these shoals were majanna-type. Many features of the studied facies can be clearly explained by the models presented, particularly by the rapid and variable fluctuations in water and pycnocline levels incomparable in time scale and size with any global sea-level changes. All these features together consistently suggest that the Badenian gypsum basin was a salina-type basin with a water level below global sea-level, although salts in the basin are/were basically marine in origin.

Active anhydrite hydration during weathering leading to crystallisation of secondary gypsum and significant volume expansion was investigated and documented by the authors at three sites: the environs of Walkenried (Germany), Dingwall (Canada), and Pisky (Ukraine). As a result of these processes, peculiar landscape forms were created: hydration domes and ridges with empty internal chambers, some of them large enough to be called hydration caves (German: Quellungshöhlen). Currently, there are only four recognised sites on Earth featuring such a unique landscape and with a large group of hydration caves in one place (the fourth site is in the Alebastrovyye Islands, Russia). These sites constitute a particularly valuable geological and geomorphological heritage, including potential geosites and geomorphosites which require special protection. Actively growing hydration domes and caves change shape and size within a short time span, on the scale of months, years, or decades. Their study and proper protection require these changes to be monitored. Several different methods of documentation were applied in the field in order to document continuing morphological changes. The practical aspects of the use of each of these methods were assessed, demonstrating that the photogrammetric methods offer the greatest utility; not only are they the most efficient (fast and sufficiently precise) but also, compared with other methods, they yielded the most complete results. The key documentation of outcrops in Canada and Ukraine was executed with the application of terrestrial photogrammetry at Pisky (GoPro camera) and aerial photogrammetry at Dingwall (unmanned aerial vehicle). Application of these methods enabled the recording of the morphology associated with the hydration process in the form of 2.5D and 3D models as well as of orthophotomaps. The maps and the models were created using the Photoscan programme. The authors demonstrate that the photogrammetric models can be used for spatial morphological analysis of hydration forms in the ArcGIS programme. Repetition of this documentation in future will enable analysis of the morphological changes expected to occur during the progressive expansive hydration of anhydrite.

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