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Structural Control on Deposition of Late Permian Reefs in Western Poland


In western Poland, in addition to reefs occurring at the shelf-edge, isolated reefs (usually 1 km wide and a few to more than a dozen km long) have been recorded in the subsurface, in the basinal facies of Upper Permian Zechstein Limestone. These reef bodies are the gas reservoirs, each field being a gas field. Carbonate buildups originated, and were growing, on the topmost edges of tilted blocks and/or on the top of uplifted horsts. The structure of the buildups was controlled by the nature of particular tectonic blocks in their basement. The reefs came into existence after the initial flooding of the Rotliegend basin that happened some 258.3 Ma. During the initial flooding, the strongly elevated parts of the Wolsztyn Palaeo-Ridge became emergent and these were the source of mudstone/claystone extraclasts recorded occasionally in the lower part of the Zechstein Limestone. However, rising seawater eventually reached and covered the tops of these islands. There is a close relationship between the perfectly preserved paleogeomorphology of the uppermost Rotliegend tectonic blocks and the shape and size of the Zechstein Limestone. The inherited relief locally controlled the thickness of the Zechstein Limestone reef sections (from about 10 to 90.5 m thick) with elevated areas accumulating the thinnest sequences and lower-lying areas being overlain by the thickest Zechstein Limestone. The tilting of particular blocks during the Zechstein Limestone deposition that in some areas the rocks characteristic for the upper part of reef sections are lacking, and in such cases they rock packages typical for the lower part of the Zechstein Limestone are overlain by the pisolitic-stromatolitic unit that is the lowstand systems tract deposit; the reefs in turn are highstand systems tract deposits. Some reef complexes host thin (up to several centimeters thick) neptunian dykes that formed very soon after deposition in early-cemented limestones, as the infillings of some of them have been reworked by burrowing organisms. In addition, the common laminar, vertical to obliquely-oriented cement crusts occurring in the Zechstein Limestone in western Poland, equivalent to “tension gashes” described from coeval reefs from NE England where they are up to 0.6 m wide and parallel with the reef trend, also seem to be related to the block-tilting. The block-tilting was also an important control on development of evaporites which blanket the reefs and fill the adjacent basins.