Wpływ budowy podłoża na wykształcenie osadów permu w okolicach Radomska

Abstract

PERMIAN DEPOSITS AND THEIR RELATlON TO THE SYNSEDYMENTARY TECTONlC IN THE SUBSURFACE OF THE RADOMSKO REGION (CENTRAL POLAND) 

SUMMARY

The lithology and thickness of the Permian sediments has been analysed in detail on the basis at a structural map of the top of the Sub-Permian deposits of the Radomsko region. The lithofacial development had been determined by the area here analysed being situated in a zone of strong tectonic activities at the crossing of several important dislocation zones. In the substratum there is a distinct boundary in the Permian, step-like in character, separating the sulphareous carbonaceous sedimentation type of the Miechów part here considered from the chloric sedimentation of salt structures in the Łódź part. This step, affected by subsequent synorogenous Alpine movements, was modelled into an elevation, today expressed by the presence of the Jurassic anticlines of Chełm, Smotryszów and Łękińsko.
In the region between Radomsko and Bełchatów there is a distinct boundary between the pattern of the distribution of the Permian facies, due to the differentiation of sediments in the northern (Łódź) part from those in the southern (Miechów) part. Figure 1 shows the deep bore hole profiles which had pierced or only reached the Permian deposits. Four boreholes: Gomunice 2, 5, 7 and 8 are in the northern part of our area, south of the Bełchatów graben, while bore holes Gidle 5, Gidle 1, also Pągów IG-1 and Granice 2 are in the southern part.
In the northern part of our area the fossil fragments of the Rotliegendes are overlaid by rather thick (370–560 m) Zechstein deposits developed as salt cyclothems.
The oldest of these, the Werra cyclothem, is that best developed, while they are gradually reduced in the younger profiles. They represent the marine basin zone with a chloritic type of sedimentation, the thick salt beds here present being engaged in the process of halokinesis and forming salt domes (Fig. 2). The Permian profiles observed in boreholes of the southern part of the region here considered differ. They are of relatively small thickness (193–287 m), and the cyclic sequence of beds is observable sporadically and only in the oldest cyclothem. Generally they are carbonaceous sulphareous deposits (the cement) representing the lagoonal peripheries of the Permian basin (Poborski 1964). This distinct change in the thickness and lithofacies of the Permian deposits in the two parts of our area was due to synsedimentary movements, modifications in the configuration of the basin and to complications in the substratum structure (Dadlez & Marek 1969). The substratum here, built by sediments of the Lower Carboniferous was definitely modelled during the tectonic movements of all the successive Post-Visean and Pre-Zechstein phases (Jurkiewicz 1975). Moreover, they had been cut up by a system of dislocations with general NW-SE and SW-NE direction trends mutually overthrust and forming the frames of the blocks. An analysis of the distribution of the thickness and facies in the Permian reliably suggests that these blocks constituted a step-like pattern of depressions along the both direction trends.
During the final phase of Zechstein sedimentation, the lowermost position along the SW-NE direction was that of the blocks directly adjacent in the south to the Bełchatów graben where conditions favoured the development of complete salt cycles. A zone situated higher up was that of the “Kodrąb” dislocation (Fig. 3) with a transitional type of sediments oscillating from the chloritic to the carbonaceous-sulphareous facies. It predominated in the southern Miechów part of our region situated highermost. In this zone the boundary between the Łódź and the Miechów basins runs in the substratum. A similar step-like pattern was also produced in the NW-SE direction with its maximum substratal depression underlying the present Kamieńsk–Chełm–Granice structures rising to the SW (Fig. 4). The greatest thickness of the Permian deposits, agreeing with the here considered pattern of the substratal modelling and with data from boreholes and the seismic sections, occur in the lowermost zones (Fig. 5).
A study of the substratal structure in the vicinity of Radomsko has shown the part played by this zone during the Permian. An analysis of the distribution of the thickness and facies of the deposits suggest strong synsedimentary movements of the substratum along the old Variscan dislocations. This activity was responsible for the step-like formation of the substratum, affecting the sedimentation of this region. Higher up the sea invasion penetrated farther south into the Miechów region (Fig. 6). The step is the southern boundary of the occurrence range of the salt structures in this part of the Polish Lowland. In result of the activity of subsequent synorogenous Alpine movements the step took on the present form of an elevation.