Problem retyku w zachodniej Polsce na tle profilu w Książu Wielkopolskim

Ryszard Dadlez, Janusz Kopik



In several profiles of deep wells made in Western Poland (Fig. 1) and among them in the base profile at Książ Wielkopolski, a complex of, sediments was investigated. These sediments occur between the sulphate deposits of the Upper Keuper and the mainly limnic clastic deposits of the Lias. The present authors try to correlate this complex with the other profiles of the Jurassic-Triassic boundary in, Western Europe. Moreover, on the basis of palaeogeographic and faunistic analyses they substantiate the stratigraphical position of the Rhaetic in Western Poland.

Four lithological series have been distinguished in this complex, viz.: Drawno, Jarkowo, Zbąszynek and Wielichowo Series (Fig. 2). The fauna found in Drawno, Jarkowo and in the lower part of the Zbąszynek Series (Unionites posterus, Notocythere media tuberosa, N. media excelsa, Darvinula liassica, D. globosa - Tab. 4, allows to regard them as equivalents of the German Steinmergelkeuper, as well as the Konglomeratmergel (Lower Rhaetic incl., according to C. A. ,Wicher 1957 (Tab. 4). The upper, part of the Zbąszynek Series does not contain the Lower Rhaetic brackish species mentioned above, but only scarce marine microfossils (Hyperamminoides sp., Mesoendothyra sp., ?Variostoma sp., Bairdia sp.). This part of the profile is referred to the Middle Rhaetic (?Rhaetavicula contorta Zone). The Wielichowo Series contains a typical association of the Upper Rhaetic mega-spores (Trileites pinquis, T. utilli, Bacutriletes tylotus, Verrutriletes litchi - as identified by T. Marcinkiewicz) and corresponds to the Triletes schichten of H. Will (1953). In the Jarkowo and partly in the Zbąszynek Series, along with the autochtonic brackish fauna, also an allochtonic marine, fauna was ascertained. (Diplotremina sp., Mesoendothyra sp., Trocholina sp., Variostoma sp., Modiulus sp., a.o.). The latter proves evidence of short-lived although frequent marine infiltrations into the brackish basin.

Homogenous character of sedimentation of the four series mentioned above does not allow to draw the lower boundary of the Rhaetic in Western Poland either at the bottom of Rhaetavicula contoria Zone~ or even a little lower (Tab. 4), according to various opinions of Westeuropean authors. On the contrary the Upper Gypsum Series of the Keuper terminating the last sedimentary cycle of the typical Triassic charactet, may be, paralelled with series existing in vast areas of Europe and extending from England to Poland (Tab. 4). After deposition of this series a period of palaeogeographical changes has begun which influenced formation of various facies. As a result of these changes, new conditions of sedimentation have been established in Lias, quite different from those in Trias. Therefore, the authors propose:

  1. To put the Rhaetic lower boundary (in the epicontinental facies), a boundary of wide regional character - at the top of the Upper Gypsum Series of the Keuper.
  2. 2. To consider the Rhaetic in this facies not as a stage, but as an unit of higher order, because of its fundamental transitional character between two quite different epochs. It would be perhaps better to choose another term for this unit, since the boundary mentioned above does not correspond to that of the Rhaetic - Noric in the Alpine stratigraphical scheme.
  3. The analysis of characteristic fauna associations and of changes within them enabled the authors to present a hypothetical scheme of the Rhaetic basin development in the region of Książ (Fig. 3). The presence of marine microfossils containing also forms known till now from, the East-Alpine area, in the Lower Rhaetic of Western Poland, and their absence in the Steinmergelkeuper Series of Germany suggest the possibility of a direct migrations of this microfauna from the Carpathian geosynclinal basin. Nevertheless the existence of a continuous connection of the Western Poland basin with the brackish basin of Germany during the Lower and Middle Rhaetic should be also. taken into consideration (similarities in the brackish fauna association).

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