Poziomy litostratygraficzne wapienia cechsztyńskiego w zachodniej Polsce

Stefan Witold Alexandrowicz


Lithostratigraphic units of Zechstein limestone in Western Poland

Limestones and dolomites of first Zechstein cyclothem (Z1—Werra) are widely spread in Western Poland. Their thickness ranges from a few to over 80 m. The fauna of these formations (lamellibranchs, branchiopods, bryoza, and foraminifers) gives no grounds for differentiating biostratigraphic zones (S. Alexandrowicz, W. Barwicz, 1970; J. Kłapciński, 1971). However, it is possible to distinguish lithostratlgraphic units. Such a division has been defined for Germany (G. Jankowski, W. Jung , 1962; W. Jung , 1968; H. Helmuth, 1968) and subsequently recognized on the Fore-Sudetic monocline (S. W. Alexandrowicz, 1971; S. W. Alexandrowicz, M. Preidl, 1971, 1972 a, b). On the basis of analisis of the cores from Western Poland it was possible to determine the possibility of regional application of this division (Fig. 1). One can distinguish four lithostratigraphic units in the profile of Zechstein limestone, each of them referred to by a Greek letter: Unit [alfa] — limestones and marly dolomites; massive, or indistinctly bedded. Unit [beta] — limestones and dolomites distinctly bedded, intercalated with thin layers of marly; shales. In limestones and dolomites abundant stylolites are present. Unit [gamma] — limestones or detritic, oolitic, or oncolitic dolomites; indistinctly bedded, intercalated with layers rich in clay material (marlystones or marly dolomites) . Unit [delta] — massive or indistinctly bedded limetones and dolomites, rich in aggregations of anhydrite crystals and laminated anhydrite. In some places, secondary diagenetic processes: recrystalization and dolomitization of the calcareous sediment, obliterate the original sedimentary structures. In these cases, the division cannot be accomplished. In the Southern part of the Fore-Sudetic monocline, and in the North- -Sudetic depression, limestones and dolomites of the Z1 cyclothem range from 30—80 m thickness. The replacement of limestones by dolomites is here irregular, so that within a particular lithostratigraphic unit limestones may intercalate with dolomites (fig. 2). In the central and the northern parts of the Fore-Sudetic monocline, the complex of the sediments in question is remarkably thinner. From West to East, one can trace a gradual transition of limestones into dolomites, with the continuity and the individuality of lithostratigraphic units preserved (fig. 3), A similar phenomenon is to be observed in Pomerania, between Wolin and Łeba (fig- 4). Microscopic examinations emphatically bring out the differences between the foresaid types of rocks. There are two lithologie types in unit a. In the lower parts of the profiles, calcareous or dolomitic marls of shaly structure are most common. They contain thin, pinching out laminae consisting of carbonate miçrite and laminae rich in clay material. In the upper part of the unit, limestones and marly dolomites usually have homogenous micrite structure with irregularly distributed clay material. Fauna is quite rich here consisting chiefly of foraminifers and bryozoa. In unit [beta], micrite limestones can be observed, and their banding is emphasized by the presence of thin laminae rich in clay-material. Along marly layers, stylolites may develop. The limestones described are gradually replaced by fine — and even crystalline — dolomites with stylolites. Unit [gamma] is remarkable for the presence of detritai limestones and dolomites with intradasts, ooids and oncoids of different sizes. The groundmass is micrite, or sparite in places. Lithological différenciation of the rocks is based on the presence of fine-grained and coarse-grained sediments, which can be classified as micrite limestones or dolomites with occasional ooids, oncoids, and intraclasts, limestones corresponding to calcirudites and calcarenites, as oncoide limestones with sparite cement, and also as micrite limestones with a scarce contribution of detrital material. In unit [delta], there, appear dolomites or micro- and fine-crystalline limestones, often uneven-crystalline, with large quantities of big anhydrite crystals. The previously described lithostratigraphic units of Zechstein limestone can be observed throughout vast parts of Central Europe, East Germany and in Poland and Lithuania. On the other hand, they have not been found in the marginal facies of Zechstein (e. g. in the Świętokrzyskie Mountains and in NE Poland). The presence of those units in profiles of various total thickness of limestones and Z1 dolomites is evidence for different rate of accumulating of the limestone sediments of the first Zechstein cyclothem. The sediments were subsequently partly dolomitized. The process operated on different units of the stratigraphie profile in particular regions, thus partly effacing the original structural features of the sediment.

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