Rozwój górnojurajskiego basenu sedymentacyjnego na Niżu Polskim

Abstract

EVOLUTION OF THE UPPER JURASSIC SEDIMENTARY BASIN IN THE POLISH LOWLANDSummaryThe marine Upper Jurassic basin developed within the Zechstein – Mesozoic palaeotectonic unit situated on the south-western slope and foreland of the East-European Platform. In Oxfordian it was the largest basin covering almost the whole Polish Lowlands (Fig. 1). As results from the palaeontological premises it was connected directly with the adjacent Oxfordian seas. In its central part there was a zone of stronger subsidence of the NW – SE direction, that appeared in the older Mesozoic basins, and was generatically connected with the T-T tectonic zone. The maximum subsidence in it, compensated by sedimentation, was about900 m. To the north and to the south of this zone the subsidence decreased gradually, reaching the zero value in the west.Calcareous lithofacies with brachiopods, gastropods, pelecypods and in the Early Oxfordian - ammonites predominated in most area of the Oxfordian basin. Terrigenous lithofacies with rich ammonite fauna developed in the northern and western parts of the basin, and adjacent to eroded elevations.Since the Kimmeridgian the frames of the Upper Jurassic basin began to narrow so that in theUpper Volgianit became a basin limited to the Middle Polish Lowlands (Figs 2–4). Its connections with adjacent basins were limited. During those times the zone of maximum subsidence remained in the central part, and this subsidence compensated by sedimentation was400 min Kimmeridgian and l00 m in Volgian. Positive structures, ridges and domes connected with salt movements appeared in the Kimmeridgian basin. These forms also occurred in the Volgian basin (Figs 2–4).Distribution of lithofacies in the Kimmeridgian basin repeated the distribution in the Oxfordian basin but a remarkably smaller part of the basin was occupied by calcareous lithofacies (Fig. 2). In the Lower and Middle Volgian terrigenous lithofacies dominated and calcareous lithofacies was limited to the south-eastern most part of the basin (Fig. 3). In theUpper Volgianbasin, in its central part there prevailed the calcareous - anhydrite - gypsum lithofacies, in the south - eastern part - oolithiclimestone lithofacies, and in the marginal parts - the terrigenous one.The Upper Jurassic palaeotectonic map (Fig. 5) illustrating the configuration of the Upper Jurassic base at the end of Jurassic shows similarly differentiated subsidence of the basin bottom as it was in the early Upper Jurassic. The maximum subsidence in the central part of the basin caused a depression distinguished as the Mid-Polish Furrow with depressions of Chmielnik, Radomyśl and Kcynia as well as the Kujawy Domes in its central part. Monoclines with a local Suwałki Embayment andSzczecin–Zielona Góra Terrace constituted a border of the furrow.The Upper Jurassic epicontinental sediments represent the late stage of tectonic evolution marked by two following associations: platform marine carbonate association with terrigenous – calcareous, calcareous – terrigenous and calcareous sub-associations, and platform continental-lagoonal association.As a result of the analysis of thickness and facies it can be stated that the area of the Upper Jurassic sedimentary basin in the Polish Lowlands subject to differentiated downwarping movements which were the most intensive in the present areas of thePomerania– Kujawy Swell, Świętokrzyskie Mts and Lower San Swell. Upwarping movements occurred in the Late Jurassic and caused a sedimentary-erosive gap in the north-western part of the basin where the Middle Volgian sediments lay directly upon the Oxfordian sediments.