Wykształcenie strefy uskoku Pomorzan i jego hydrogeologiczna charakterystyka

Zbigniew Wilk, Andrzej F. Adamczyk, Józef Biernacki, Jacek Motyka


Geological and hydrogeological characteristics of the Pomorzany fault zone

The Pomorzany faulf creates a border between the southern portion of the ZnPb ore deposit of the Olkusz region and the northern one. The southern portion has been mined since centuries whereas the northern one was discovered only several years ago, and the deposit is beeing developed at the present time. The new deposit was opened by means of underground headings (trough-cuts) which were driven from the existing mines northwards primarily in the impermeable Permian, them, they crossed the fault zone and entered the waterbearing Muschelkalk, in which the ores occur. The 11 mine cross-headings and gangways were geologically surveyed, thus supplying the needed information. One of purposes of this investigation was the hydrogeological characteristics of the fault zone. Special interest was paid to the problem whether it is waterbearing or impermeable, thus creating an insulating screen. On the base of surface and underground drillings, as well as geological survey of the mine workings was found that the Pomorzany dislocation can be classified as longitudinal, Similarly inclined, homotetic, dipslip normal fault of brittle type. The fault zone throw measures from 60 to 80 meters. The structure of the fault zone is complicated. The main fault plane is accompanied by a marginal parallell fault iniclined and throwing southwards. Both mentioned longitudinal dislocations create a local graben about 200 m wide. They are cut with a transversal fault throwing down about 20 m. While approaching the Pomorzany fault several signs of its presence could be observed. First a ca 30 m wide zone of joints and fissures in Permian conglomerate was encountered. In a distance of 20 to 10 meters from the fault plane the colour of the conglomerate changed from reddish- brown to greyey-igreen. The width of the main fault fissure varied from several cm to a few dcm. The fissure was filled with dark tightly pressed, clayey material characterized by polished surfaces and lamination parallel to the fault sides. In the clayey material fragments of carbonate rocks could be observed. In the profiles 2, 4 and 6 (Fig. 4, 5 and 6) the fissure filling clays were of intensive cherry-like red colour at the upthrown side which changed to black at the downthrown one. The surfaces of both sides of the fault mostly were even and parallel to each other. After crossing the main fault fissure several other marginal joints, fissures and shiftings of strata were observed. The limestones of the Lower Muschelkalk in the downthrown side are intensively karstified, the diameter of some caves reaching up to few meters. In one case a cave cut by a fault was found. The main fault fissure was dry and while driving the headings through the disturbed brittle zone composed of Roethian dolomitic marls négligeable groundwater discharges were observed. Only after headingsfaces reached the karstified limestones of the Lower Muschelkalk enormous water discharges and inrushes appeared i(Pl. IV). It was found that as a result of throw-down of the Keuper clay the Pomorzany fault locally reduces the horizontal water transmissivity of the Triasie waterbearing horizon diminishing its hydraulic active thickness even to 10% of the primary one. Resulting of this screening effect the cone of depression produced by drainage activity of the existing mines situated southwards of the Pomorzany fault was limited in its. extension possibilities towards the north. This information was essential for hydrogeological prognoses both from the point of view of the mining industry as well as the local water supply plants.

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