Sedimentary history and biota of the Zechstein Limestone (Permian, Wuchiapingian) of the Jablonna Reef in Western Poland
The Jablonna Reef, one of the reefs formed in Wuchiapingian time in the western part of the Wolsztyn palaeo High (SW Poland) is characterized by quite irregular outlines and consists of three separate reef bodies (ca. 0.5 1.5 km 2 each; the thickness of the reef complex is usually >60 m) It is penetrated by four boreholes, which show two distinct phases of bryozoan reef development during deposition of the the Zechstein Limestone. The first one occurred early in the depositional history and botryoidal aragonitic cementation played a very important role in reef formation. This phase of bryozoan reef development terminated suddenly; one possible reason was that a relative change of sea level first a fall and then a rise disturbed the upwelling circulation. Consequently, bioclastic deposition predominated for a relatively long time until the second phase of bryozoan reef development occurred, but the latter was not accompanied by dubious early cementation. During this second phase, reticular fenestellid bryozoans were predominant. Subsequently, microbial reefs developed and abound in the upper part of the Zechstein Limestone sections. The general shallowing upward nature of deposition in the Jablonna Reef area resulted in reef flat conditions with ubiquitous, microbial deposits, in the central part of the Jablonna Reef. Then, the reef flat started to prograde and eventually the entire Jablonna Reef area became the site of very shallow, subaqueous deposition. Five biofacies are distinguished in the Jablonna Reef sections: the Acanthocladia biofacies at the base, then mollusc crinoid, brachiopod bryozoan, Rectifenestella and at the top, stromatolite biofacies. They represent a shallowing upward cycle, possibly with some important fluctuation recorded as the distinctive lithofacies boundary, corresponding to the Acanthocladia/mollusc crinoid biofacies boundary. The d 13 C curves of the Jablonna 2 and Jablonna 4 boreholes permit correlation of the trends in the middle parts of both sections and confirm the strong diachroneity of the biofacies boundaries, with the exception of the roughly isochronous Acanthocladia/ mollusc crinoid biofacies boundary. The presence of echinoderms and strophomenid brachiopods indicates that until deposition of the lower part of the Rectifenestella biofacies, conditions were clearly stenohaline. The subsequent elimination of stenohaline organisms and progressively poorer taxonomic differentiation of the faunal assemblage are characteristic for a slight, gradual rise in salinity. The taxonomic composition of organisms forming the Jablonna Reef shows a similarity to reefs described from England and Germany, as well as the marginal carbonate platform of SW Poland. Filled fissures were recorded in the lower part of the Jablonna Reef. The aragonite cementation recorded in some fissure fillings implies that they originated in rocks exposed on the sea floor and are neptunian dykes.