Demise of the Jablonna Reef (Zechstein Limestone) and the onset of gypsum deposition (Wuchiapingian, west Poland): carbonate-to-evaporite transition in a saline giant

Microbial deposits commonly occur at the transition between carbonate and sulphate facies, and they also abound in the uppermost part of the middle Wuchiapingian Zechstein Limestone in west Poland. These deposits occur as isolated reefs of the basinal zone and in the condensed sequences in most parts of the study area. The deposits of the latter category reflect evaporative drawdown, and the abrupt boundary between the carbonate and sulphate deposits in the basin suggests the nature of evaporites that start to precipitate as soon as they reach the saturation level. A few-metre-thick unit of mostly brecciated microbial deposits at the top, reefal portion of the Zechstein Limestone records extreme palaeoenvironmental events that occurred at the transition from carbonate to sulphate deposition. These events are related first to subaerial exposure of the reef, which lasted several 105 years and then to the Lower Anhydrite transgression.