SULFATE PLATFORM-BASIN TRANSITION OF THE LOWER WERRA ANHYDRITE (ZECHSTEIN, UPPER PERMIAN), WESTERN POLAND - FACIES AND PETROGRAPHY

The peripheral evaporite platform of the Zechstein (Upper Permian) Wer ra Anhydrite of western Poland comprises a series of shoals (with thic k sulfate deposits) and lows (with thin sulfate and thick halite depos its). Three cores were selected to examine facies variations from a su lfate platform, slope, and basin. The basal unit of the Lower Werra An hydrite is represented by different varieties of nodular, bedded-nodul ar, fluidal-nodular, and brecciated facies. Some nodular fabrics proba bly originated within sabkha and/or gypsumpond environments, and the d eposit was then mechanically redistributed. The absence of nodular str uctures in units other than the lower one in two of the wells suggests that the origin of these structures cannot be related to deep burial. Other units of the Lower Werra Anhydrite represent subaqueous deposit s. The facies in the middle and upper units have no recent analog in c oastal salina environments. Turbidity currents displaced sulfates from the platform to the adjacent basin. The origin of lenticular-bedded a nhydrite may be related to deformation by compaction or by gravitation al instability and slumping, which may have initiated crystallization or recrystallization of anhydrite. The vertical sequence of facies sho ws a distinct deepening-upward trend starting from the top of the basa l nodular units. Deposition in shallow (salina) environments dominated the early history of the Lower Werra Anhydrite basin, although it was probably preceded by transgressive sabkha deposits. Relatively thick nodular anhydrites probably formed by syndepositional diagenesis of pr obable salina deposits and minor sabkha sediments. Gypsum was the orig inal sedimentary mineral; it was later transformed into anhydrite, dur ing either early or late (burial) diagenesis. We show that in deeply b uried evaporites not only some macroscopic primary textures remain but also even microscopic details, which allow reconstruction of depositi onal environments and primary mineralogy of ancient anhydrites.