The study investigates the origin, age, paleo-depositional environment and formation conditions of gypsum lithofacies deposited in northeastern Mus in Eastern Anatolia using element analysis and sulfur, oxygen and strontium isotope combinations. The correlation diagrams of major ions and trace elements plots in the gypsum lithofacies indicate that carbonates and clastics carried by terrestrial waters (streams, rivers, etc.) significantly contributed to the evaporitic phases. Furthermore, the effects of hydrothermal solutions, increased salinity of the basin and presence of biological activity were associated with high values for major ions and trace elements. While the delta O-18 and delta S-34 isotope contents of some gypsum lithofacies samples show that they are of terrestrial or marine origin, others do not indicate either origin because they are affected by other factors such as water salinity variations, bacterial sulfate reduction, new terrestrial water and detrital inputs carried into the basin. However,Sr-87/Sr-86 isotope concentrations analyzed in gypsum lithofacies in the study area indicate Rupelian (Lower Oligocene) time, which is compatible with the Cenozoic age curve. Based on all these data, gypsum lithofacies in this basin were deposited in a transition zone (shallow sea-lagoon-inland sabkha-mudflats) under the influence of both marine and terrestrial conditions. Therefore, we claim that this basin formed during the Rupelian (Early Oligocene), at a time when the sea connection between the Indian Ocean and the eastern Mediterranean was restricted; that is, the southern branch of the Neotethys was extremely shallow during this period and almost terminated in the study area.