Rozwój basenu sedymentacyjnego i paleotektonika jury środkowej na obszarze Polski

Abstract

 Ekspansywny charakter morza środkowojurajskiego wyraża się m.in. tym, że pełne profile litologiczno-stratygraficzne występują, tylko w strefach obniżonych, obszary wyniesione charakteryzują, się skróconymi profilami stratygraficznymi i redukcjami miąższości. Dominują, osady klastyczne. Od aalenu po baton górny strefa maksymalnej subsydencji kompensowanej osadami przypada na środkową, i południowo-wschodnią, część bruzdy środkowopolskiej i obniżenie sulechowsko-częstochowskie, w keloweju przemieszcza się na północno-zachodnią, część basenu sedymentacyjnego.  EVOLUTION OF SEDIMENTARY BASIN AND PALAEOTECTONICS OF THE MIDDLE JURASSIC IN POLAND Eight palaeothickness and lithofacies maps of consecutive stages, substages and horizons of Middle Jurassic (Tab. 1, Figs 1 - 8), and palaeotectonic map of the Middle Jurassic at the close of Callovian (Fig. 9) are presented. Platform lithological associations are distinguished in the inset map. The shallow and mobile Middle Jurassic sea showed a very conspicuous expansive character during the whole period. Full lithologic-stratigraphical sequences from Aalenian to Late Callovian occur only in two depressed zones. The remaining units or their parts are characterized by a transgression more or less delayed reductions or rapid increases of thickness as well as by erosion and stratigraphical gaps. It points to their mobility in various stages Middle Jurassic. The Middle Jurassic initial transgression advancing the area of Poland from the south-east in the Early Aalenian covered the Sącz-Przemyśl Depression, the Mid-Polish Furrow and the Sulechów Częstochowa Depression (Figs 1, 9). These two narrow lowered zones joining together in the south by a strait were surrounded by eroded continental areas, and separated from each other in the west by a large land which was a source area for the adjacent basins. Such a system existed generally from Aalenian to the beginning of Upper Bajocian (Figs 1 - 6). The exception is a land separating the both arms of the basin in the west. Its western part was subject to periodical downwarping (Figs 2, 4, 6) and uplifting (Figs 1, 3, 5) movements. They are testified by the existence in this zone of the relics of complex accumulated sediments (documented by ammonites) and by numerous erosional gaps. The eastern part of the land however was either more stable or was subject to very slow uplifting movements in the early Middle Jurassic. The transgression conquered the area rather late, and the erosional processes destructing here the Lower Jurassic sediments reached deeper than in the west in some elevated parts as deep as to the Rhaetian sediments. A strong transgressive impu.lse in the beginning of Upper Kuiavian (Fig. 6) caused the whole land to become the accumulation area however with thinner sedimentary cover in comparison with the depressed zones. Sedimentation also appeared in the same time in major north and north-eastern parts of Poland and to some extent in the Małopolska Terrace. A zone of maximum subsidence compensated by sedimentation in all the maps presented occurred in the Kujawy-Świętokrzyskie part of the Mid-Polish Furrow extending periodically to the Sącz Przemyśl Depression. Evident domination of shaly sediments (Figs 1 - 6, 9) is also characteristic of the furrow. However, from the beginning of Middle Kuiavian the zone of the clay domination occurred also in the Sulechów - Częstochowa Depression in the west, with simultanous thickness increase of sediments. Subsidence was here substantially weaker that in the Mid-Polish Furrow. An inclination of the bottom was almost zero but in the most subsided area of the middle part of the furrow it reached 3-4°. The palaeostructural pattern of the Late Kuiavian and Early Bathonian (Fig. 6) remained unchanged until the Late Bathonian transgression and last impulse in Late Callovian that was the Middle Jurassic maximum transgression, continuing in Upper Jurassic (Figs 7,8). Then the final picture of the sedimentary basin was formed. The Callovian transgression advanced from the north-west. The maximum subsidence zone compensated by sedimentation moved to the north-west and reached the western part of the Szczecin - Piotrków Tryb. Uplift together with the north-western part of the Mid-Polish Furrow and the adjacent parts of the Koszalin-Mszczonów Monocline and the Kaszuby-Lublin Terrace (Figs 8, 9). In the Upper Callovian the sedimentary conditions south-east wards of this zone were positively different - generally the so-called nodular bed originated here. It is characterized by an extremely small thickness, remarkable accumulation of broken and rounded fossils characteristic of several ammonite zones which are embedded in a very scarce and highly differentiated cement. It points, among others to the prevalence of submarine erosion over deposition. Similar sedimentary conditions lasted still in the Lower Oxfordian.