Rowy tektoniczne kimeryjskie na tle ewolucji strukturalnej Niżu Polskiego

Władysław Pożaryski

Abstract


CIMMERIAN GRABENS IN THE LIGHT OF STRUCTURAL DEVELOPMENT

OF POLISH LOWLAND

Summary

Grabens produced at the Jurassic-Cretaceous boundary at the Cimmerian tectonic phase, related to Laramie folds, were recognized in 1962 in the Polish Lowland area. The first discovery took place in the vicinity of Płońsk, north-west of Warsaw. Using reflection seismic methods and drillings one has ascertained the presence of several belts of parallel faults, the vertical throw of which amounted to 200-300 m. These are faults restricting this graben (fault trough) filled in with the uppermost Jurassic (Portlandian) deposits, and the Lower Cretaceous (Berriasian, Valanginian, Hauterivian) deposits, absent outside the faults considered. Both the zone of the graben and that of the adjacent (elevated) areas are overlain with the uppermost Lower Cretaceous deposits (Barremian - Albian), and with the Upper Cretaceous deposits non-disturbed with faults. This, allows us to determine precisely the moment of the origin of these faults as Lower Cretaceous. It is possible, however, that the beginning of the faults under consideration should be related to an earlier period, probably Triassic. Generally speaking, we should refer them to the older Cimmerian part of the Alpine tectonic epoch. The Cretaceous deposits overlying the graben, and the underlying beds are concordantly seized in, the folds of Laramie age. It should be accepted that the origin of both faults and grabens is related to a strong subsidence of the perycratonic zone of the East-European platform. At the Upper Permian, Triassic, Jurassic, and Lower Cretaceous times, the platform marginal area was being bent mainly due to a quick subsidence of the area of the central pericratonic depression occupied to-day by the Middle-Polish anticlinorium. As a result of this, stress, tension, fractures, and trough zones (grabens of Płońsk) were characteristic features of the surface strata in the marginal area of the platform. Compression that began at the close of the Lower Cretaceous time, and was responsible, at the latest Upper Cretaceous time, for folding processes in Laramie phase, was here the last act of the strong movements of that time. Folds that, among others, were produced on the previously stretched graben zones, where the compression of the Cretaceous basement was most intensely developed.

The Cimmerian grabens cover an area of the north-eastern limb of the marginal synclinorium of the platform. Approximately, they are parallel to the axis of the synclinorium. Most distinct are they along the Kujawy portion of the pericratonic depression, where the subsidence was of the utmost degree. Consequently, in the marginal areas of this depression a tension must have corresponded to this subsidence.
Analogous symetrically situated tectonic structures are found also on the opposite side of the Middle-Polish anticlinorium, i.e. in the Łódź synclinorium and in the adjacent part of the Fore-Sudetic monocline. These latter were active still in the earlier part of the Upper Cretaceous, since at that time the Łódź synclinorium was an object of a considerably stronger subsidence than the area adjacent to the monocline.
A system of similar grabens, perpendicular, however, to the previous ones, cut the Mesozoic strata in the area of the Fore-Sudetic monocline. Most of them are referred to the Miocene time (Bełchatów graben).

 


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