Ret i wapień muszlowy w północno-wschodniej Polsce

Anna Szyperko-Śliwczyńska




Roethian and Muschelkalk deposits occurring in the north-eastern area of Poland are formations of the peripheral zone of a Triassic sedimentary basin. Their development differs from that of the similar Triassic members which occur in the remaining area of Polish Lowland.
The Roethian formations, characterized here by the smallest extent (Fig. 2), occur in the, south-western part of the area only, and rest transgressively on various members of the lower part of Buntsandstein. They are developed (Fig. 11) as claystones, siltstones and marly sandstones, locally with plant remains and numerous intercalations of marly and nodular limestones with poorly preserved pelecypod fauna. Among pelecypods, Myophoria costata Z e n k., a form (characteristic of the Roethian, was found in one of, the bore boles made in the area under consideration. Thickness of this member ranges from 5 m to about 40 m.
A detailed comparison of the Roethian sections illustrates that here the Roethian transgression developed gradually and embraced greater and greater areas. Roethian sections investigated in the south-western part of the area are more complete than those found in the north-eastern part.
The Muschelkalk formations are more wide-spread in the north-eastern area than the Roethian ones.

Within the Dower Muschelkalk, three members have been distinguished here and determined as A, B,  and C.
Series A is characterized by a great amount of marly rocks, particularly in its uppermost portion. Intercalations of compact limestones, locally with scarce fauna (Velopecten albertii G o l d f .) occur here subordinately.
Series B is rich in calcareous shale rocks, among which pelecypods of the genera Lima and Enantiostreon (L. striata S c h  l o t h., L. radiata G o l d f., E. difforme S c h l o t h.) are particularly numerous. Marls and marly limestones are less abundant in this series than in the series A.
Series C, contains less intercalations of shale rocks than series B, although it again reveals more marls, marly limestones, claystones and marly clay shales.
It was sometimes difficult to distinguish the series B and C. Thus, in some sections the series bear a common term B–C.
Thickness of the Lower Muschelkalk ranges from 5 m to 30 m. The extent of the series A of the Lower Muschelkalk is greater than that of the Hoethian. Transgression of the Muschelkalk sea, characterized here by the greatest extent, reached its maximum in the series B–C of the Lower Muschelkalk.
The series A of the Lower Muschelkalk of NE Poland corresponds to the entire lower part of the Lower Muschelkalk of the Świętokrzyskie Mts., i.e. to the Wolice, wavy and Luków beds. On the other hand, both series, B and C, correspond to the beds with Lima striata of the Świętokrzyskie Mts.
In the north-eastern areas of Poland, no change in chemical character of rocks, i.e. no transition from dolomites and dolomitic rocks to limestones and calcareous rocks has been observed to appear at the boundary of the Middle and Upper Muschelkalk. A deepening of the Middle Triassic sedimentary basin took place at the Upper Muschelkalk time probably on a small scale only, and could not have been markedly expressed in this peripheral area.
Total thickness of the Middle and Upper Muschelkalk dolomites and dolomitic rocks reaches, in the area under consideration, up to 46 metres. The extent area of these formations is narrower than that of the Lower Muschelkalk deposits (Fig. 2). For the most part, this is due to a narrowing of the sedimentary basin and, to certain degree, owing to denudation processes, as well.

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