THE ANATOMY OF A SULFATE PLATFORM AND ADJACENT BASIN SYSTEM IN THE LEBA SUBBASIN OF THE LOWER WERRA ANHYDRITE (ZECHSTEIN, UPPER PERMIAN), NORTHERN POLAND

The Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the L eba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fl uctuating boundaries between well defined basins and platforms. This p attern of local platforms and adjacent basins developed during deposit ion. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial t ransgression. Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydri te with pseudomorphs after upright-growth gypsum crystals suggest rapi d precipitation in a subaqueous environment and/or fluctuating, but ge nerally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to bre cciation possibly connected with salinity fluctuations and the dissolu tion of halite. Massive, textureless anhydrite is locally porous and p asses upward into breccia, indicating a strongly saline environment. B edded anhydrite is considered to form in shallow water environments an d laminated anhydrite in deep water. Bedded anhydrites contain portion s which are graded. Intercalations of sulphate turbidites and upright- growth gypsum suggest fluctuating water depths, with comparatively dee p water during turbidite deposition, but shallower conditions during u pright-growth gypsum deposition. The sequence observed in slope zones at platform-basin margins, detrital (parautochthonous) sulphate sand t o graded beds to basinal laminites, indicates that redistribution proc esses were important. At the onset of the Lower Werra Anhydrite deposi tion bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially expose d. Formation of local platforms and adjacent basins required a relativ ely high subsidence rate, as pre-existing relief cannot account for th e total accumulated thickness of the Lower Werra Anhydrite deposits. O ne implication of this is that the main argument against 'the shallow water - shallow basin' evaporite basin model, i.e. a very fast rate of subsidence, may not be valid for the Leba Lower Werra Anhydrite basin .