Post-evaporitic restricted deposition in the Middle Miocene Chokrakian-Karaganian of East Crimea (Ukraine)

In the Middle Miocene of East Crimea, gypsum evaporites formed in a shallow basin from mixed seawater–nonmarine waters are overlain by marl, siltstone and claystone which contain a few horizons of stromatolitic limestone. The thickness and abundance of the stromatolitic horizons increase up the section. In the siliciclastic portion of the section, a very poor and taxonomically impoverished assemblage of benthic foraminifers ( Jadammina, Nonion, Haynesina, Astrononion and Eponides) is recorded. It is typical for a shallow water marsh or lagoon environment with a lowered salinity. Accordingly, the brackish conditions prevailing during gypsum precipitation in East Crimea continued afterwards, although at the end of evaporite deposition the basin became desiccated and then it was rapidly reflooded by brackish water. The impoverished biota and the occurrence of microbialites in the Ptashkino section indicate extremely unfavourable conditions for most living organisms during the deposition of both the terrigenous and carbonate beds. The water salinity thus was probably not only lowered but also anomalous in composition if compared to normal marine water. The occurrence of carbonate stromatolitic horizons is probably related to the periodic shallowing of the basin caused by a drop of the lake water table driven by climatic factors. The resulted changes allowed for a growth of the bizarre microbial-serpulid communities that gave birth to most of the stromatolites. In Karaganian time, the Eastern Paratethys was a huge lake isolated from the Tethys. This lake responded to any climatic fluctuation that in turn might lead to water level oscillations. In humid climate periods, the lake could be open with a surface outflow and, when it was drier, it could be a closed system without surface outflow. Gypsum evaporites and stromatolitic carbonates are clear evidence of strong evaporation in a dry climate that probably induced water level fall in the whole basin. However, all the time the environmental conditions were predominantly brackish, even during gypsum precipitation as suggested by the chemical composition of fluid inclusions in gypsum. Similar conditions may be expected in other evaporite-hosted, predominantly brackish basins, such as some Messinian basins of the eastern Mediterranean, where some evaporites were deposited in the oligohaline to mesohaline conditions typical of the Lago Mare deposits.