Rozwój basenu sedymentacyjnego pstrego piaskowca na obszarze Polski

Abstract

 Na podstawie ponad 1000 otworów wiertniczych i nielicznych naturalnych odsłonięć wykonano mapy litofacji i paleomiąższości dolnego, środkowego i górnego pstrego piaskowca. Przeanalizowano podobieństwa i zmienności rozkładu litofacji oraz zarysów zbiornika sedymentacyjnego w poszczególnych etapach. Wykonano mapę paleotektoniczną, przedstawiającą, położenie spągowej powierzchni pstrego piaskowca u schyłku sedymentacji środkowego pstrego piaskowca. Wyróżniono jednostki paleotektoniczne różnego rzędu. Stwierdzono, że większość z nich, w tym najbardziej obniżona bruzda środkowopolska, miała wcześniejsze, przedtriasowe założenia. EVOLUTION OF THE BUNTSANDSTEIN SEDIMENTARY BASIN IN POLAND  Palaeothickness and lithofacies maps of Buntsandstein have been elaborated on the basis of over1000 profiles of boreholes and few natural outcrops. Traditional subdivision of the lower epicontinental Triassic into Lower, Middle and Upper Buntsandstein is accepted, with regard results from palaeontologie investigations (Figs 1, 2).The Lower Buntsandstein sedimentary basin (Fig. 3) covered about twice as large an area thanremanant basin of Upper Zechstein. The largest expansion of the basin took place in the PrecambrianPlatform area and in the southern area of the Carpathian foreland. The basin was probably connected with a boreal part of the World Ocean and was closed in the south.The lithofacies distribution points to a dominant role of the southern source areas. Coarse-clastic sediments making over 50% of the profile were deposited only in the southern part of the basin. Monotonous fine-clastic rocks with few limestones intercalations originated in the remaining area.Distribution of subsidence in the basin was similar to this in Permian. The largest thickness, up to 400 m occur in the most subsident Mid-Polish Furrow. Stable areas with a small differentiation of subsidence were situated to the north-east and south-west of the furrow.The Middle Buntsandstein sedimentary basin (Fig. 4) was transgressive in the first phase. Expansion of the basin took place especially within the Precambrian Platform and in the southern part, where the connection with the southern Thetyan seas took place. The lithofacies distribution was changing during the Middle Buntsandstein sedimentation. In the beginning the sandstone lithofacies dominated in the whole southern part of the basin, latter on it was limited to its western part. In this late phase the sandstone complexes with great thickness originated also in the north-western part of the basin.In the, first transgressive phase of evolution great amounts of carbonate rocks also originated.In its closing phase the Middle Buntsandstein sedimentary basin evaluated into the stagnant phase and in the area of lowlands probably narrowed to the axial part of the Mid-Polish Furrow.There was no such a narrowing in the southern part of the basin. The subsidence distribution was similar to that existing before. A contrast in subsidence between the Mid-Polish Furrow, where sediments of about a 1200 m thickness deposited and adjacent areas was intensified during the Middle Buntsandstein.The Lower and Middle Buntsandstein formations developed as a red bed continental terrigenous association were subject to the palaeotectonic analysis (Fig. 6). A bottom of this sedimentary cover was structurally differentiated at the decline of sedimentation of Middle Buntsandstein. Its depth within the basin, ranged from 0 to over 1600 m, its inclination in the prevailing areas did not exceed 1°, reaching 2-3° only on the slopes of Mid-Polish Furrow with a maximum value of 4-5° on the slopes of the Cracow - Tarnów Depression. Majority of the distinguished palaeotectonic units had the pre-Triassic foundations, including mainly the first order units such as the Mid-Polish Furrow, the Pomerania Mazowsze Elevation and the Szczecin - Kalisz Elevation.The Upper Buntsandstein sedimentary basin (Fig. 5) was developed as a result of the Thetys transgression which advanced from the south probably mainly through the Eastern Carpathian gate. The basin was developing gradually reaching maximum in the late sedimentary phase of Upper Buntsandstein. The Rhaetian carbonate lithofacies deposited in the greater area of the basin, terrigenous deposits with high share of coarse-clastic material occured only in the north-western and eastern part of the basin.The Upper Buntsandstein thickness (maximum of about 200 m) shows lower differentiation of subsidence and some changes in its distribution in comparison to earlier Triassic. The Mid-Polish Furrow was then segmented into several smaller units of stronger subsidency. A equivalent unit originated in the Fore-Sudetic area. Location of the most elevated units was similar to the location which existed earlier.The Upper Buntsandstein deposits were analysed palaeotectonically together with the younger deposits of Muschelkalk and Lower Keuper (I. Gajewska, 1988).