Upper Permian Reef Complex in the Basinal Facies of the Zechstein Limestone (Ca1) (Wolsztyn High, Western Poland)
In West Poland, isolated reefs have originated, and were growing, on the topmost edges of tilted blocks and/or on the top of uplifted horsts in the basin facies. They are sealed by evaporites and are important gas reservoirs. The reef biota are typical bryonoderm associations indicating cool-water and cold-water environments. The Zechstein Limestone basin was sourced by temperate to cool water from the Boreal Sea when the Rotliegend basin located well below the contemporaneous sea level was flooded in mid or late Wuchiapingian times. The sequence of lithofacies and biofacies includes deposition of a coarse coquina and subsequently establishment of a diverse, stenohaline bryozoan community followed by start of reef growth with aggradational geometries by a stenohaline fauna including bryozoans, brachiopods , and various encrusters. Later on, bryozoan community changed to encrusting reefbuilders, and the depositional geometries changed from aggradation to progradation until sea level falls resulted in subaerial exposure of the reefs. The main part of reefs is built of rudstones, and only stromatolitic crusts form massive construction. Astonishing is the mechanism of origin of clearly separated morphologically reef constructions from remains of relatively low potential of fossilization. Zones built of crushed remains clearly dominate over parts representing massive constructions. The Zechstein Limestone reefs abound in the hemispheroid aragonitic cement what is otherwise common for the reefs elsewhere in the Zechstein basin. The abundance of the cement recorded in Permian reefs is interpreted as the result of an unusually high saturation state of surface seawater because of number of factors. The occurrence of reefs at the base of the Zechstein Limestone indicates that locally very quickly after the onset of the Zechstein deposition the environmental conditions were favorable for the local carbonate production. However, over the large area in the region the regime was hostile for net carbonate production and hence condensed sequences were deposited.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California