Budowa geologiczna podłoża retyku obszaru monokliny przedsudeckiej

Zbigniew Deczkowski, Irena Gajewska

Abstract


Podano zarys budowy geologicznej podłoża retyku. W wyniku przeprowadzonej analizy wszystkich otworów wiertniczych przewiercających na obszarze monokliny przedsudeckiej utwory triasu górnego ustalono, że na ukształtowanie tej budowy duży wpływ wywarły ruchy tektoniczne ujawniające się tu na przełomie kajpru i retyku oraz w jurze dolnej. Dane geologiczne oraz wynikli badań sejsmicznych refleksyjnych pozwoliły na odzwierciedlenie głównych zarysów regionalnych stref dyslokacyjnych zaznaczających się w podłożu retyku. Ustalono również, że wyróżnione strefy dyslokacyjne uległy niekiedy w czasie młodszych ruchów tektoniczych znacznej przebudowie.

GEOLOGICAL STRUCTURE OF RBAETIAN BASEMENT IN THE AREA OF THE FORE-SUDETIC MONOCLINE

Geological works conducted in the area of the Fore-Sudetic Monocline for

several years now provided, among other things, valuable data concerning geological structure of Rhaetian basement. Detailed analysis of borehole profiles showed that the area was effected by tectonic movements acting from the turn of the Keuper and Rhaetian till the end of the Early Jurassic. These movements resulted in origin of regional dislocation zones in several parts of the Fore-Sudetic Monocline. The borehole data and the results of the latest reflection seismic surveys made it (possible to trace the following dislocation systems: Poznań-Kalisz, Poznań-Oleśnica, and Middle Odra river (Fig. 2). The Middle Odra river system, found by J. Oberc (1962, 1967), is situated in area of advanced Laramie reconstruction which makes it difficult to decipher chronology of tectonic movements. There was also found a Borzęcin-Złoczew elevation zone (Fig. 2), stretching from Sierardz to the vicinities of Trzebnica and presumably extending as far as the Lubin-Polkowice region.

The analysis of distribution of thickness of the Upper Gypsum Beds of the uppermost Keuper (Fig. 3) and overlying Rhaetian deposits (Fig. 1) suggests that the elevation zones marked in the studied area were developing from the end of sedimentation of the former. The Upper Gypsum Beds are the thickest and their profiles are most complete in areas NW and SE of the Borzęcin-Złoczew elevations Fig. 3). In these areas, it is assumed that there is sedimentary continuity of the Keuper and Bhaetian but a small gap may·be marked in some places. The Upper Gypsum Beds are thinning out towards central parts of these elevation, which is accompanied by cropping out of their successive, older links. In most strongly elevated areas, the Rhaetian is directly underlain by the Reed Sandstone and Lower Gypsum Beds (Fig. 2).

The Poznań-Kalisz and Poznań-Oleśnica dislocation systems are better known. They represent nonuniform arrays of elements shifted in relation to one another. The zones were presumably subjected to horizontal tension which resulted in loosening of layers and origin of framing faults converging at the base of a structure (Fig. 6). The rate of subsidence of a given wedge-shaped block was nonuniform, which is reflected by the geological structure. The formation of such structures began at the turn of the Keuper and Rhaetian. The fault system formed also at that time and active in the Early Jurassic determined the outline of Early Jurassic troughs.

It should be stressed that the dislocation zones traced in the Rhaetian basement were markedly modified by younger tectonic movements in several regions.

The analysis showed that the majority of gas fields discovered in the Rotliegendes and Werra Limestone f the Zechstein in the Fore-Sudetic Monoline are related to the Borzęcin-Złoczew elevation zone (Fig. 4) and the Poznań-Kalisz and Poznań-Oleśnica dislocation systems. It may be concluded, therefore, that the tectonic movements affecting several parts of the Fore-Sudetic Monocline from the turn of the Keuper and Rhaetian till the end of the Early Jurassic have markedly contributed to the origin of the traps in the Rotliegendes and the Werra Limestone of the Zechstein.


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