Uwagi o rozwoju tektonicznym złóż soli kamiennej w Wieliczce i Baryczu

Stefan Połtowicz


Tectonic evolution of the rock salt deposits in Wieliczka and Barycz (near Cracow)

In the vicinity of Wieliczka and Barycz near Kraków, in front of the border of the Flysch Carpathians, folded Miocene rocks containing a saltbearing formation have been noted. They are thrust over the autochthonous Badenian sediments of the Carpathian Foreland, containing also evaporites. Nearly all the investigators who studied the tectonics of the Miocene before the front of the Carpathians were of the opinion that folding took place on the turn of the Middle and Upper Badenian. It was generally held that the transgressive Grabowiec Formation (Upper Badenian) overlies discordantly the folded Chodenice Formation, evaporites (Middle Badenian) and the infraevaporitic series (Lower Badenian). The above conclusions were drawn from tectonic investigations carried out over a short section of the Carpathian arch and based on a small number of natural and artificial exposures. A study of the geological structure of the zone of folded Miocene rocks in the area (between Kraków and Przemyśl indicates that the tectonic movements occurred after the Lower Sarmatian. A recent interpretation of the geological section through the marginal part of the Flysch Carpathians, the salt deposit in Wieliczka and the autochthonous Miocene sediments of the Carpathian foredeep (fig. 1A) points to the normal stratigraphie sequence of the Badenian rocks. No Lower Sarmatian sediments have been recorded in the vicinity of Wieliczka. An analysis of the evolution of the Carpathian foredeep implies that the zone of most intensive sedimentation caused by the increased subsidence of the 'basin floor moved gradually to the north and north-east.. The essential change in the position of the axis of the Badenian basin took place after the Middle Badenian. In the Lower Badenian, the zone of greatest subsidence was in the area situated south of the present-day border of the Flysch Carpathians whereas in the Upper Badenian it wasalready north of that border. The Middle Badenian evaporates were deposited in the -bays of the :salt basin. The southern parts of those bays lay in places in the area of the contemporary Carpathians, the border of which was at that time aibout 50 km south of Wieliczka (fig. 2). The salt basin showed variations in the rate of bottom subsidence. In its northern part, situated on the slope of the Carpathian foreland, the subsidence was not very fast. Carbonate rocks and, at a greater distance from the littoral zone, sulphate rocks (gypsum and anhydrite) formed there, In the southern part of the basin, within the bays, the subsidence rate was relatively high, causing the solution enriched in NaCl after partial precipitation of calcium sulphates to flow southwards. In the central parts of the bays gypsum, anhydrite and rock salt were deposited. The successive orogenetic phase in the Carpathians began on the turn of the Middle and Upper Badenian, ending after the Sarmatian. In the Carpathian foredeep it brought about the displacement of the axis of the sedimentary basin to the north and the deepening of the basin, which resulted in a gradual increase of the inclination of the top of evaporates. The exceeding of the shear strength of the rooks produced detachment dislocations and gravitational slides of the rock masses (fig. 3). Plasticized under the pressure of the overburden rocks, the salt facilitated the sliding movements of the Miocene rocks and then of the Carpathian flysch. The Miocene rocks containing evaporates formed before the front of the Flysch Carpathians a rocks mass of complex tectonic pattern, consisting of several tectonic elements thrust one over another and folded. Near Wieliczka and Barycz, the following tectonic elements may be distinguished from the bottom to the top: 1. The element of Krzyszkowice consisting of gypsum and anhydrite and the barren wall irocks; it forms a slice thrust over the autochthonous Upper Badenian sediments of the Carpathian foredeep. 2. The lower element of Wieliczka built of three slices of salt-bearing rocks thrust one over another, and the element of Barycz. 3. The upper element of Wieliczka imade up of salt blocks randomly distributed among the barren rocks; it forms a slide megabreccia and is overfolded with the rocks of the lower tectonic element. The tectonic evolution of the rocks salt deposits of Wieliczka and Bochnia proceeded by four stages (fig. 3). Stage ,,a” (lower part of the Middle Badenian): The Lower Badenian sediments (infraevaporitic series) show substantial variations in thickness, from about 50 m up to more than 700 m. This is due to syntectonic sedimentation in grabens bordered by faults that formed during the deposition of the infraevaporitic series. The evaporate series originated in the period relatively quiet from the tectonic point of view. Stage ,,b” (upper part of the Middle Badenian): The axis of the sedimentary basin was considerably displaced to the north due to the tectonic reconstruction of the Carpathian foredeep. The clayey-mudstone Chodenice Formation was deposited over the evajporites. Stage ,,c” (Upper Badenian): Further northward displacement of the zone of greatest subsidence of the basin floor, accompanied simultaneously by the uplifting of the Flysch Carpathians, intensified the erosion of the flysch and the Badenian rocks. Coarsegrained material forming conglomerates and sandstones in the clayey-mudstone rocks began to penetrate into the basin. At the extension of the larger Carpathian rivers submarine fans (Bogucice sands) were formed. The increasing northward inclination of the top of evaporates produced stresses within the Miocene rocks, which were relieved by detachment dislocations. This initiated slow gravitational sliding of the Miocene rocks, probably synchronous with the sedimentation of the Grabowiec Formation. Stage ,,d” (Lower Sarmatian): Further development of .gravitational sliding, facilitated by the plasticization of salt under the pressure of the overburden rocks, resulted in overthrusting and folding of thrust slices built of Badenian sediments that contained a salt-bearing formation mined in Wieliczka and Barycz. The folding of the Miocene rocks continued presumably over a considerable length of time, most likely throughout the Lower Sarmatian, until equilibrium was achieved in the rock masses.

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