Wgłębna tektonika brzegu Karpat w okolicy Tamowa i Pilzna

Abstract

Tectonic structures of the Carpathian border in the Tarnów and Pilzno area (Polish Middle Carpathians) To the north of the Carpathians there is a foreland basin filled with thick sequence of Miocene sediments. The outer Carpathian nappes made up of the Cretaceous and Tertiary flysch are thrust over the Miocene so that a considerable part of it is now beneath the flysch rocks. This paper discusses the relationship between the allochthonous flysch and the Miocene sediments. THE STRATIGRAPHY AND LITHOLOGY OF THE MIOCENE The Miocene sea transgression in the foreland of the Middle Carpathians started in the Early Badenian. The sea covered the area, intensively eroded after the Laramide Revolution, with well developed relief, at places with clearly recognizable river valleys (S. Połtowicz, 1964; A. Łapinkiewicz, H. Nowicki , 1967). The Lower Badenian (the so called „sub-evaporite series”) is to be found in two sedimentary zones (Fig. 2). In the northern zone, clay stones and shales with thin intercalations of. mudstones and sandstones prevail (the Tegel facies). The thickness of these deposits seldom exceeds 50 m. In the southern zone, the Lower Badenian deposits consist of mudstones and clayey sandstones with some intercalations of claystones and shales (the Schlieren facies). The Schlieren facies is of a considerable thickness, which is likely to exceed 500 m of the Carpathians south border. Both parts of the basin are separated by an area devoid of Lower Badenian deposits and Middle Badenian evaporites (P. Karnkowski, A. Łapinkiewicz, 1965). This area is referred to as the „Rzeszów Island” (M. Komorowska-Błaszczyńska, 1965). The differentiation of both zones of the Early Badenian basin resulted from their different development. The northern part was situated at the slope of a stable area, which included the South Polish Upland. Epicontinental conditions of sedimentation without any intensive movements of the basement are typical of the northern part. The southern part was situated in a mobile zone, with relatively quick subsidence of the bottom of the basin. The Middle Badenian deposits start with an „evaporite series”. The evaporites of the Carpathian foreland are developed in three facies: detrital facies in the coastal zone, and sulphate and chloride facies in the central part of the basin (A. Garlicki, 1968, 1971). The thickness of the sulphate facies is rather small; it ranges from few up to twenty meters. The facies consists of anhydrite and gypsum layers, intercalated with claystones and shales. The deposits of the chloride facies are more varied as regards their lithology. One can observe here: salt (often mixed with clay material and quartz sand and intercalated with gypsum and anhydrite), gypsum and anhydrite layers, salt mudstones, salt sandstones, and claystones. After the deposition of the evaporites, the barrier that had been separating the two zones of the basin vanished and sedimentary conditions were made uniform. During the sedimentation of the younger sediments of the Middle Badenian (Chodenice beds), considerable palaeogeographical changes took place. The sea covered the whole area of the Carpathian foreland, including the „Rzeszów Island”. Subsequently, there came about the sedimentation of claystones and mudstones, alternately layered with sandstones. The deposits are reminiscent of the „sub-evaporite series” underlying the salt rocks in the southern part of the Early Badenian basin. Similar sediments are to be found in large quantities in the Upper Badenian (Grabowiec beds). The lithology of the rocks of the Lower Sarmatian is similar to that of the deposits situated below. Claystones and mudstones prevail here. with sandstones. Layers of tuffites, connected genetically with rhyolite eruptions in Hungary (W. Parachoniak, 1961), are to be found in the whole profile of the Miocene, especially in the Badenian deposits. The Miocene in the area situated in the front part of the Flysch Carpathians falls into two geological units: an autochthonous one, including most of the Carpathian foreland, and an allochthonous one (Figs. 3, 4, 5). In places, the Miocene deposits occur on the flysch rocks, with which they were displaced towards the north. Between Tarnów and Pilzno, the border of the Flysch Carpathians consists of two tectonic units: of the Silesian unit built up exclusively of flysch rocks and of a complicated subsurface flysch fold. The fold is surrounded by Badenian deposits and it consitutes the front of the Skole unit. Two geological cross-sections have been made to illustrate the geological structure of the Carpathian border in the area under discussion (Figs. 4, 5). In the A—A cross-section (Fig. 4) the flysch rocks form the core of the fold. Both limbs of the fold are built up of the Badenian salt-bearing deposits. The evaporites consist of anhydrites with clay layers in the southern limb and of salt intercalated with anhydrite in the northern limb. The fold is overturned towards the north and overthrusted on autochthonous Miocene. The lower limb of the fold has been squeezed out to a great extent. An analogous situation appears in the В—В cross-section (Fig. 5). There is also a flysch core enveloped by the Miocene deposits. In the southern part of the cross section W. Porębska-Szotowa (1960) proved the presence of the Lower, Middle, and Upper Badenian. Salt-bearing rocks are here absent from the Miocene deposits. In places, however, there are no Lower Badenian deposits or evaporites. The profile starts with the Upper Badenian. Both facts prove, that the deposits were originally situated in the coastal zone of the sedimentary basin, where the chemical facies of evaporites was replaced by detritic deposits, or where there were no Lower or Middle Badenian sediments. In the next zone to the north evaporites are developed in the detrital facies. Detritic rocks are here intercalated with gypsum, anhydrite, and salt. In the central part of the basin the detrital facies is replaced by more or less pure evaporite facies. Anhydrites are interbedded with salt, what is characteristic of the transitory zone between the sulphate and chloride facies. In the В—В cross-section, there can be seen a gradual transition from the area devoid of evaporites horizon throughout the detrital facies of evaporites, up to the chemical facies. THE PALAEOGEOGRAPHY OF THE BADENIAN AND THE SARMATIAN A palinspastic map makes it possible to estimate the minimal width of the zone, which was originally occupied by the Miocene deposits. The stretching-out of both cross-sections in the evaporite horizon makes the width of the sedimentary zone 10—15 km. The total range of the Carpathian overthrust is, however, unknown. One can expect that it should reach 30—40 km. At this distance from the present Carpathian border, the southern limit of the Badenian-Sarmatian basin is to be looked for. The Carpathian border was then appropriately further south, as compared with its present localization. The salt basin, extended partly over the Carpathian flysch, the flysch cores of the sub-surface fold being evidence for this. Among the folded Miocene deposits in the front part of the Carpathians, there are some Lower Sarmatian deposits, and the flysch and Miocene rocks are overthrusted on the autochthonous Badenian and Lower Sarmatian deposits. The tectonic involvement of the Lower Sarmatian indicates the age of the last orogenic paroxysme as the post-Early Sarmatian one. Some geological premises in the Carpathian area indicate that the tectonic movements have occurred also in the Quaternary. The development of the valley of the Biała River between Tuchów and Tarnów also points in this direction (Fig. 7). The width of fluvial sediments in the Tuchów area ranges from 1,0 to 1,5 km, and about 5 km away from Tuchów down-river, the zone suddenly narrows to approximately 100 m. In the place where it does, there is a fault that separates the Silesian unit from the Skole unit, which fault can also be traced in the basement by means of seismic investigations. From the fault, going down-river, the Biała flows along a breach, sharply cut into the flysch rocks. The slopes of the valley are steep, unlike the gentle fall of the slopes in the area up-river from the fault (Fig. 8 ). It seems likely that the river was barred by an uplifted limb of the fault, which must have made a lake, in which there are deposited fluvial sediments of considerable thickness. After having reached the level of the edge of the fault, the river again started flowing along its ancient valley, probably antecedently cutting through flysch. The younger relief in the area north-west of the fault is proof that such must been the case.