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Pochodzenie materiału okruchowego dolnojurajskich warstw połomskich

Kozłowska-Deuszkiewicz M.

Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie

2008

T. 34, z. 3/1

180-181

Clasts of the Liassic gravels (the Polish Uplands) - where do they come from?

Kozłowska-Deuszkiewicz M.

Volumina Jurassica

2006

Vol. 4, no. 1

93-94

The investigated gravels occur in the western part of the Mesosoic margin of the Holy Cross Mountains (MHCM) and in the northeastern part of the Mesosoic margin of the Silesia Upland (MSU), in central and southern Poland. The investigated deposits represent two informal lithostratigraphic units: the Snochowice Beds (in MHCM) - SnB and the Połomia Beds (in MSU) - PmB. The investigated gravels, in both cases, represent the bottom part of the profile of the Early Jurassic deposits. The problem is: where are the source area and the source rocks? Did clasts from MHCM and from MSU Liassic gravels come from the same source rocks or not? Both in SnB and in PmB cases mainly clasts of vein quartz, the quartzites, lydites and jaspers are recognized. Unfortunatelly the main components of the Liassic gravels are represented by the rocks which are insufficient as source indicators. The petrographical analyses have permitted to compare similar rocks from SnB and PmB clasts. The differences between the main groups of similar rocks from PmB and SnB are considerable, especially in the fabric and in the grade of metamorphism of the quartzites. Only few types of rocks - mainly quartz arenites (among the clasts from SnB and PmB) have got similar textural features which were confirmed in the CL analyses. Besides, the author has affirmed that the lithological differentiation of all PmB clastic material is greater than in SnB. Fortunatelly, but only in PmB, fragments of polymictic conglomerates, metamorphic schists, meta-subarcoses, silicified limestones, silicified woods and volcanites were distinguished. Near the southern margins of the Mid-Polish Trough there were 3 areas which could yield clastic material: the Upper Silesia Coal Basin (USCB), the Brno-Upper Silesia Massif (BUSM) and the hypothetical Precarpathian Land. Among the clastic material of PmB and SnB the author hasn't recognized: * any fragments of typical epimetamorphic rocks and Palaeozoic sedimentary rocks which could enable to recognize the BUSM succesion (Bu.a 2000), * any typical clastic material known from the Namurian and Wetphalian conglomerates such as fragments of the gneisses, granitoids, meta-volcanits (Paszkowski et al. 1995; Świerczewska 1995), * any fragments of the black coals and other rocks which could be treated as good indicators for the Carboniferous succesion of the USCB. In the author's opinion only the Precarpathian Land, constituting the Western Outer Carpathian (WOC) basement (Poprawa et al. 2004), could be a source area for the Liassic gravels. This hypothetical land was situated somewhere on the south and consisted of the crystalline basement, covered by a thick succession of the epimetamorphic rocks. During the Early Jurassic these epimetamorphic rocks were the main source of the clastic material for the deposits in the Mid-Polish Trough. Probably since the Callovian, but certainly since the Early Cretaceous, the clastic material deriving from the sea-cordilleras, which were built mainly of epimetamorphic rocks, was deposited in WOC sedimentary basins. Henceforth, the same fragments of rocks (called "exotics") are known from the Liassic gravels in MSU and from the Mesozoic-Palaeogene succession of WOC, especially from the Silesian Unit. In addition, the differences between the SnB and PmB gravels are the effect of some differentiation of source rocks in the Precarpathian Land. The main source rocks for the PmB were metaquartzites with veins of quartz, meta-subarcoses, some detrital limestones, partly silicified and probably a succession of polymictic sandstones and conglomerates, which have got the same components as in PmB. The main source rocks for SnB were only metaquartzites with veins of quartz, but different than in PmB's source. The lydites, jaspers, some quartz arenites which are known from SnB and PmB all come from the same source rocks, and have been eroded during the Early Jurassic time.