The grey, grey-green, grey-brown claystone, silty claystone and silts yield foraminiferal association of Sarmatian (Upper Serravallian 12.7–11.6 Ma). The studied deposits belong to the Holí Formation. The foraminiferal assemblages suggest a very shallow water depositional environment. In such conditions, the environment can change rapidly in dependence on tidal effects causing the repeated drying and inflow of fresh water. Therefore, it was not possible to rely on stratigraphical value of identified ecozones. The Sarmatian fauna was commonly regarded as a brackish-water community suffering from gradually decreasing salinity. However, our results allowed us to interpret very unstable marginal marine conditions, even hypersaline episodes. The foraminiferal associations document sedimentation in a very shallow water with several episodes of reduced oxygenation at the bottom, changing upward to hypo/hypersaline marshes, vegetated swamps and finally to the Glyptostrobus marsh.