Budowa geologiczna obszaru między Brzeskiem a Wojniczem

Abstract

Geological structures of the area between Brzesko and Wojnicz (Southern Poland) The area under study is situated within the Carpathian foredeep, directly at the front of flysch tectonic units overthrusted from the south. In the Miocene deposits filling the foredeep, a zone of folded Miocene (Badenian) has been distinguished, which consists of several scales thrusted over autochthonous deposits of the same age. The paper discusse the geology of the Mesozoic and Palaeozoic basement as well as the tectonics of the Miocene deposits. LITHOSTRATIGRAPHY In the basement of the Miocene deposits, formations from the Precambrian to the Upper Cretaceous have been recorded. Precambrian The oldest deposits are grey and grey-greenish claystones with mudstone intercalations, partly phyllitized. Dips of the strata are very high, amounting up to 70°. The deposits belong to the Rhiphean and were folded in the Late Preeambrian. Lower Carboniferous There is a tectonic unconformity between the Lower Carboniferous deposits and the older rocks that they cover. They were formed in the Kulm and Kohlenkalk faciès. The former consists of dark-grey claystones with thin intercalations of fine-grained sandstones with abundant plant detritus. The Kulm faciès deposits are underlain by Kohlenkalk sediments that are made up of hard, light microcrystalline limestones. Locally reef like structures are found. The sediments contain Visean fauna. Lower Triassic Lower Triassic deposits overlie directly the older rocks. They are composed of a соnglomerate-sandstone series with sandy mudstone and claystone intercalations. The pebbles of Lower Carboniferous limestones, cherts and quartzites are cemented with clay matrix with a red colouring. Mudstone and claystone intercalations are red as well. Both the lack of material segregation in the lower part of the profile and the colour of rocks indicate a rapid sedimentation in semi-arid conditions in the piedmont zone. Middle Jurassic Fine-, medium- and coarse-grained sandstones have been recorded. They are grey light-grey, locally containing clay with an admixture of gravel, and conglomerate and mudstone intercalations. In the lower part the sandstones are calciumfree, becoming calciferous towards the top. In the uppermost part of the profile there occur mudstones with intercalations of dark-grey marly micro- and fine-crystalline limestones. Upper Jurassic Upper Jurassic deposits are made up of carbonate sediments. They can be divided into three lithological units: lower, limestone-dolomitic, locally reef-like in character; midle, marly-limestone; and upper, consisting of organodetritic limestones with marl and dolomite intercalations. Upper Cretaceous The well marked abrasion surface of the Jurassic rocks is covered by Upper Cretaceous sediments, beginning with a series of quartz sandstones, locally containing clay and calcium and locally conglomerate intercalations. They fill erosional depressions and belong to the Cenomanien. They are overlain by a complex of sediments consisting of Turonian limestones and Senonian marls. Miocene Miocene deposits begin with basal conglomerate appearing on the eroded basin slopes. The conglomerates are composed of pebbles of limestones and Upper Cretaceous marls. They are overlain by a series of claystones with mudstone and sandstone interstratifications and thin intercalations of clayey dolomites and tuffites. These deposits are assigned to the Lower Badenian. They underlie a series of Middle Badenian evaporites that appears as anhydrites, gypsums and rock salt interbedded with the above-mentioned sulphates. In the upper part of the profile there is a complex of clay-muidstone-sanidstone deposits belonging to the so-called Chodenice and Grabowiec “beds” (Middle and Upper Badenian). TECTONICS OF THE MIOCENE DEPOSITS In the area between Brzesko and Wojnicz, two zones of Miocene deposits are noted: autochtonous and paraautochtonous, the latter being folded and thrusted over autochtonous sediments. In the folded zone three scales have been distinguished: 1. the scale of Dębno (southern) — upper; 2. the scale of Wojnicz — middle, and 3. the scale of Biadoliny (northern) — lower. All the three scales are strongly folded and have lower limbs squeezed out. The overthrusted zone is 3—10 km wide. The lithology of evaporites in the Miocene scales shows that the scale of Dębno is made up of rocks deposited in the southermost part of the basin (clastic and clastic-carbonate facies), the deposits of the scale of Wojnicz cover the area situated in the intermediate zone (carbonate-sulphate facies), whereas the scale of Biadolmy contains sediments of the central part of the basin (sulphate- chloride facies). If the deposits making up the scales were unfolded back to their original breadth, the width of the zone of sedimentation would be at least 20 km. At that distance the original shore of the sedimentary basin should be sought. The lack of flysch units in the cores of folds suggests that the folded Miocene deposits in the area Brzesko-Wojnicz were laid down on the foreland of the Flysch Carpathians. There is an angular and sedimentary conformity between all the stratigraphical units of the Miocene deposits. The earlier geological observations indicating a discordance between the Grabowiec and Chodenice “beds” resulted from erroneous interpretation of tectonic contacts. Overthrusting of the Miocene deposits at distances of the order of 10 km from the edge of the Carpathians was due to the occurence of rock salt, which had become plasticized as an effect of lateral compression (stress) and pressure of the Badenian overburden. Zones weakened by salt plasticization played a role of ”leather-coat” the facilitated the movement of the folded deposits. There is a strict correlation between the width of the folded zone and the lithology of evaporite series and the thickness of rock salt. Tectonic movements that resulted in the folding and overthrusting of the Miocene deposits occured in the upper part of the Lower Sarmatian, which is shown by the tectonic deformation of Lower Sarmatian deposits in the area situated east of Tarnów.