Rozwój diastroficzno-sedymentacyjny basenu Morawsko-Śląskiego

Rafał Unrug, Zdzisław Dembowski


Diastrophic and sedimentary evolution of the Moravia-Silesia Basin

The Variscan Moravia-Silesia Basin is filled by a nearly continuous sedimentary series ranging from the Lower Devonian to Lower Permian. The integration of recent istratigraphic, facial and sedimenitological studies permits to present a new account of its evolution. For earlier opinions the reader is referred to monographs by S . Czarnocki (1935), S. Siedlecki (1954), V. Havlena (1964) and J. Svoboda (1966). Boundaries and substratum of the basin The boundaries of the Moravia-Silesia Basin can be defined with a varying degree of exactitude. Their individual sectors were formed at different times and played various roles during the sedimentary evolution of the basin (Fig. 1). The best defined western boundary is formed by the contact of sedimentary and parametamorplhic Devonian rociks witlh the Aligonkian crystalline rocks of the plutonie massif of Brno and of the metamorphic zones of Moravia and of Eastern Sudetes. The southern boundary extends from the area of Brno eastward to Kraków. On the stretch between Cieszyn and Kraków the approximate position of this boundary is placed south of a line of bore-holes which pierced Devonian beds underlain here by metamorphic and plutonie rocks (K. Konior and A. Tokarski, 1959; K. Konior, 1969). The north-eastern boundary is formed by the Caledonian fold belt of Kraków (J. Znosko, 1962), where the Devonian overlies sedimentary rocks of Early Palaeozoic and Algonkian age (S. Siedlecki, 1962; S. Bukowy, 1964). The position of the northern boundary of the basin is defined least accurately. Most probably irt should 'be placed along the western prolongation of the Kraków fold belt (A. Siedlecka, 1964). However, the position of the contact of the sedimentary rocks of the Variscan cycle with the area neighbouring from the north, which was eroded during the major part of the Late Carboniferous is not clear. The unconformable contact of the Devonian sedimentary rocks with Early Palaeozoic and older rocks known from various parts of the Moravia-Silesia Basin indicate, that its basement is formed by a Caledonian fold zone. Phases of evolution of the basin. Four phases are distinguished in the course of evolution of the Moravia- Silesia Basin, viz. the phase of transgression, the phase of bathyal and neritic sedimentation, the phase of filling by detrital sediments, the la tte r followed by folding of the basin, and the subsequent phase of deposition of the sedimentary cover of the folded basin. The asymmetry of the development of the Moravia-Silesia Basin is expressed by the diachronous character of some of these phases. The detrital sediments associated with the phase of transgression range in age from the Zigenian in the northern p a rt of the basin to the Frasnian in the southern part. The phase of neritic and bathyal sedimentation which followed the former, ended in the Early Visean in the western part of the basin, while it lasted till the late Visean in the eastern part. The following phase of filling the basin with detrital sediments lasted till the end of Westphalian. The sedimentary cover of the folded txasin was deposited during the Stephanian and Early Permian. The phase of transgression The Early Devonian transgression came from the north, and covered first the western part of the Moravia-Silesia Basin; quartzitie sandstones and conglomerates of the Vrbno beds deposited there contain a Zigenian and Emsian marine fauna known since a long time. At the southern extremity of the basin, in the region of Brno, the detrital sediments beginning the Variscan sedimentary cycle are overlain directly by limestones of Frasnian age. The presence of variegated sandstones, siltstones and claystones was established by bore-holes along the southern and eastern margin of the basin. These deposits containing a psylophyte flora (Konior , 1965; S. Alexandrowicz , 1970), were assigned to the Emsian. They were deposited in a non-marine environment. Their thickness is variable, ranging from a few m up to 200 m (Fig. 2). The phase of neritic and bathyal sedimentation The carbonate series : Middle Devonian — Upper Visean The carbonate series known from surface exposures and bore-holes along the southern and eastern margin of the basin overlies the basal clastic rocks, but locally extends farther, resting directly over crystalline rocks of the basement of the basin. The carbonate series consists of dolomites and limestones. The major part of the latter contains a rich fauna of corals, thick-shelled brachiopods, amiphipora and stromatopora, indicating a shallow-marine sedimentary environment. In the Moravian Karst situated near Brno at the south-western extremity of the basin, the carbonate series begins with coiral-amphipora- -stromatopora limestones of Givetian and Frasnian age, while in the upper part of the series the Fammenian, Tournaisian and lowermost Visean consist of th in bioclastic and nodular limestones which may represent sediments of deeper water (I. Chlupać, 1964). The thickness of the carbonate series amounts in this region to 1000 m. In the area of Cieszyn and Bielsko the carbonate series, known only from bore-holes, begins with dolomites assigned to the Eifelian, overlain by amphipora-braciopod limestones of Givetian age which are followed by Frasnian and Fammenian limestones. The top surface of the limestone series is erosional, and the overlying Upper Visean claystones are resting upon various Devonian members. The thickness of the preserved part of the carbonate series ranges up to c. 700 m. Carbonate sedimentation lasted' without interruption since the Eifelian till the end of the Upper Visean in the eastern marginal part of the basin. In the region of Krzeszowice the carbonate series begins with Eifelian and Lower Givetian dolomites, followed by Upper Givetian, Frasnian and Fammenian amphipora-stromatopora limestones, and Tournaisian and Visean coral-brachiopod limestones. The thickness of the carbonate series in this region exceeds 1000 m. Only one bore-hole at Maczki (east of Katowice) reached the carbonate series under the Silesian Coal Basin. The thidkness of the Fammenian, Frasnian and Givetian carbonate rocks drilled through was c. 700 m, but the base of the series was not reached. The Devonian carbonate rocks are overlain by Upper Visean shales with a thin limestone at the base. The shale-radiolarite series: Emsian — Lower Visean Shales of Emsian and Eifelian age with intercalations of volcanic rocks ovelie the basal clastic rocks in the western part of the Moravia-Silesia Basin. The volcanic rocks consist of spillites, diabases, agglomerates and tuffites, accompanied by rare keratophyres (V. Barth , 1964). The shales c. 100 m thick, pass upwards into an impersistent limestone c. 50 m thick of Givetian-Frasnian age, followed by shales and radiolarites up to 100 m thick. The age of the latter, established on conodont faunas ranges from Upper Frasnian to the lower part of Lower Visean (I. Chlupać, J. Zikmundova, V. Zukalova, 1968). The facies distribution indicates, that a geosyndinal trough trending north-south formed in the western part of the Moravia-Silesia Basin in the Early Devonian. The deep-water shale-radiolarite series and the associated products of submarine volcanism were deposited in this trough, while the carbonate series was deposited in shallow sea in the remaining part of the basin (Fig. 3). The sediments formed during the phase of neritic and bathyal sedimentation are devoid of clastic material coarser than pelite It seems that the boundaries of the basin were non-existent during the discussed phase, and the facial regions described above (especially the region of sedimentation of the carbonate series) extended far beyond their present boundaries. This is suggested also by faunistic affinities with the Rhine region and the Palaeothetys (I. Chlupać , 1964). The lack of detrital material coarser than pelite in the Toumaisian and lowermost Viséan rocks of the shale-radiolarite series suggests that the absence of Tournaisian and Lower Viséan rocks in a large part of the basin is caused by a post-lowermost Viséan erosion. The phase of filling the basin by detrital sediments. The flysch series: upper part of Lower Viséan — Lower Namurian A. The flysch series c. 4000 m thick overies the carbonate series and the shale-radiolarite series in the western part of the basin. It consists of conglomerates, graded sandstones, siltstones and shales, locally metamorphosed into slates. Fine-grained clastic rocks prevail in the uppermost part of the series. The stratigraphie position of the flysch series was discussed for a long time. Upper Viséan goniatite faunas have been known since a long time in the upper part of the flysch series. Upper Devonian fossils have been found in the lower part of the series (H. Schmidt, 1927; J. Dvorak, G. Freyer and L. Slezak , 1959), but recently it has been concluded that these fossils were redeposited (B. Koverdński, 1964). The establishment of the age of the top of the carbonate series and of the shale-radiolarite series directly underlying the flysch series, as lowermost Viséan, permits to assign the base of the Flysch series to the upper p a rt of the Lower Viséan. The upper boundary of the flysch series is formed by the fossiliferous Stur marine horizon of lower Namurian A age, which marks the 'base of the paralic coal-bearing series. The flysch series was regarded for a long time as a shallow marine or deltaic deposit (K. Patteisky , 1929), and M. Elias (1956) was the first to call attention to the flysch character of these rocks. Later O. Kumpera (1959 and later papers) and R. Unrug (1964) proved that the features of the discussed series correspond exactly to the definition of the flysch -faciès. The detrital material of the flysch sandstones and conglomerates was transported by turbidity currents northward (Fig. 4) as indicated by the distribution of grain size in the basin and by directional sedimentary structures. The principal source of the clastic material derived from the Bohemian massif was situated at the south-west extremity of the basin, while the inner part of the Eastern Sudetes also supplied some material (Fig. 4). It is concluded therefore, that the western boundary of the Moravia- -Silesia Basin 'has been formed at the beginning of the sedimentation of the flysch series. The Lower Carboniferous flysch rocks found in bore-holes in the region of Opole, Ostrzeszów and Rawicz suggest, that the1 sedimentary basin of the flysch series extended far to the north. The flysch series has been deposited in the trough formed during the preceding phase of evolution of the basin. This trough was bordered in the east by the sedimentary basin of the claystone series, which is coeval with the flysch series. The boundary between these two series is concealed under the Upper Carboniferous rocks of the Upper Silesia Coal Basin. The claystone series: Upper Viséan — lower Namurian A The claystone series is known from the southern and eastern part of the Moravia-Silesia Basin. In the south the Upper Viséan claystone series overlies various stages of the Devonian. In the east the bore-holes did not reach beds older than the Upper Visean. The claysitone series consists of darkngrey claystones and siltstones, with infrequent intercalations of fine-grained sandstones. Hare intercalations of detrital and nodular limestones, dolomites, conglomerates and tuffites occur at the margins of the basin. The thickness of the claystone series ranges from c. 100 m in the southern part of the basin, to c. 1000 m in the north-eastern part. The Visean — Namurian boundary lies within the claystone series and can be determined only on the basis of fossils. The coal-bearing series. The coal-bearing series overlies the flysch series in the western part of the basin, and the claystone series in the eastern part. The lower boundary of the coal-bearing series is placed at the fossiliferous Stur marine horizon in the lower Namurian A, while the top of the Westphalian forms the upper boundary. The coalj bearing series consists of the lower paralic series (Namurian A), and the upper limnic series (Namurian B, C and Westphalian). The area of occurrence of these series forms the Upper Silesia Coal Basin. The paralic coal-bearing series The paralic series overlies conformably the marine rocks of the lowermost Namurian A without break in sedimentation. Its upper boundary is formed by the ‘base of the lowermost coal seam of the limnic series (seam 510 in Poland, equivalent to the seam Prokop in Czechoslovakia). This seam is absent locally in the western part of the coal basin, and the boundary between the paralic series and the overlying limnic series is erosional in these areas. A large hiatus embracing the upper part of the paralic series and the lower part of the limnic series exists in the south-eastern part of the coal basin. The thickness of the paralic series amounts to c. 3500 m in the western p a rt of the coal basin, decreasing to a few hundred m in the south-eastern part. The paralic series consists chiefly of claystones, siltstones and fine- -grained sandstones, while coarse-grained sandstones and conglomerates occur rarely. Thin marl beds are present occasionally. Pyroclastic material is common, occurring both dispersed in the terrigenous rooks and forming intercalations of tuffogeinic rocks. Phytogenic rocks are represented by coals, coal shales and sapropelites, the la tte r occurring chiefly at the roof of coal seams. Marine rocks are determined on the basis of the presence of marine fossils. Petrographic criteria enable to assign a marine origin also to unfossiliferous marls, marly varieties of claystones and siltstones, sandstones with calcareous cement, and some well sorted quartz sandstones. The marine rocks form therefore a larger proportion of the paralic series than it is estimated on the basis of occurrence of fossils. Yet, the distribution of the unfossiliferous marine rocks is still poorly known. Two types of marine bands are present in the paralic series. One comprise widespread marine intercalations which can be traced over a major part of the entire coal basin and have considerable thicknesses usually exceeding 100 m and ranging up to 270 m. In most profiles the facial development suggest a regular transgressive-regressive cycle. The second type comprise minor thin marine intercalations occurring over relatively small areas, usually in the roof of coal seams. The widespread marine intercalations are used as markers for the delimitation of lithostratigraphic members in the paralic series. Some fossiliferous horizons contain only species of the genera Lingula and Orbiculoidea. They are usually referred to as „Lingula bands” and regarded as indicators of brackish lagoonal conditions of sedimentation. Fossiliferous horizons containing only assemblages of fresh-water lamellibranchs occur usually at the roof of icoal seams. The distribution of the marine bands in the coal basin is extremely characteristic. The largest number of marine bands is present in the1 north-western part of the basin. Towards the south and east the number of marine bands decreases, as they become replaced by „Lingula bands” and fresh-water fossiliferous horizons (K. Bojkowski, 1967; Ł. Musiał, 1967; Ł. Musiał and A. Tabor , 1964). The proportion of sandstones increases generally in the southern part of the coal basin, especially in the Cieszyn area. Some members of the paralic series do not contain coal seams in this region. Directions of cross-stratification in the sandstones of the paralic series indicate that the detrital material was transported by palaeo currants flowing north and north-east (R. Gradziński, A. Radomski and R. Unrug , 1959, 1961). The grain size of a major conglomerate tongue (the Zamek conglomerate), present in the uppermost member of the paralic series in the western part of the coal basin, decreases to the north. All these data indicate, that the source area of the clastic material of the paralic series was situated south and possibly also east of the coal basin, while the marine ingressions advanced from the north (Fig. 5). The paralic series is interpreted as a deltaic complex, composed of rocks deposited in fluvial, limnic, lagoonal and marine environments. This complex expanded to the north-west. In the lower members of the paralic series the coal disappear in the north-west part of the coal basin, where the proportion of the marine rocks increases; this area corresponds probably to the delta front. During the deposition of the higher members of the paralic series which contain a normal proportion of coal, the delta front migrated probably farther to the north-west, and was situated beyond the present erosional boundary of the coal basin. Evidence of the original extent of the paralic coal-bearing beds north of the present boundary of the coal basin is provided by the presence of pebbles of Lower Namurian coals in sandstones of the limnic series of namurian C and Westphalian age. The detrital material of these sandstones was transported from the north. The disappearance of the flysch facies and claystone fades, and the transition to deltaic conditions of deposition of the coal-bearing paralic series was associated with an uplift of the southern and south-eastern border of the Moravia-Silesia Basin. At first the uplift did not affect the area of the basin proper, as there is a continuous transition from the marine beds to the paralic series, but later the south-eastern part of the coal basin was also uplifted, and the uppermost member of the paralic series is missing in that area. The presence of tuffogenic rocks and of dispersed tuffogenic material in the paralic series indicates that the diastrophic activity in the border of the basin was associated with volcanism. The limnic coal-bearing series The limnic coal-bearing series overlies concordantly the paralic series in the western part of the coal basin. Erosional channels truncating the uppermost part of the paralic series are present locally in the region of Rybnik. In the south-eastern part of the coal basin the limnic series is over-stepping the erosional surface truncating the upper part of the paralic series. The upper boundary of the limnic series is formed by the erosional surface truncating the uppermost Westphalian beds, This surface developped after the Asturian folding of the basin. The maximum aggregate thickness of the limnic series amounts to c. 4000 m. The thickness preserved after the folding of the basin and subsequent erosion amounts to c. 2900 m in the west, while in the east it is inferior to 1000 m. The limnic series consists of quartz, polymictic and intraclast conglomerates, fine- to coarse-grained sandstones, siltstones and claystones, accompanied by coals, coal shales and sapropelites. Tuffogenic rocks and siderites form minor constituents. Lithology provide a basis for a tripartite division of the limnic series (S. Stopa , 1967). The lower member called the Upper Silesia sandstone series (Namurian B-C) is marked by a predominance of sandstones, a high proportion of conglomerates, and the presence of thick coal seams. The middle member called the siltstone series (Westphalian A-B), consists chiefly of siltstones and claystones, and the coal seams are thin and relatively impersistent. The upper member is formed by the Kraków sandstone series (Westphalian C-D) composed of coarse grained sandstones, conglomerates and thick coal seams. The Upper Silesia sandstone series is absent in the south-eastern part of the coal basin. During the deposition of the siltstone series the sedimentary basin o fthe limnic series gradually expanded to the south-east. The Kraków sandstone series is present mainly in the eastern part of the basin, but the limits of its occurrence are erosional and were formed after the folding of the basin. The limnic series is interpreted as a continental accumulation cover deposited on an alluvial plain. The directions of cross-stratification of the sandstones show a consistent palaeocurrent system transporting the material to the south and south-east (Fig. 6). The individual cyclothems of the Kraków sandstone series contain elongated sandstone bodies with long axes trending NW-SE (Z. Dembowski, 1967; Z. Dembowski and R. Unrug, 1970). It is concluded therefore, that the source area of the detrital material of the limnic series was situated north of the Moravia-Silesia Basin. The uplift of this area after the Namurian A was associated with a definite withdrawal of the sea, and the transformation of the paralic coal basin into a limnic one. The northern boundary of the Moravia-Silesia Basin was formed at that time. The litihologic development of the limnic series suggests that the source area was vigorously uplifted in two phases. During the deposition of the Kraków sandstone series erosion unroofed large areas of crystalline rocks in the source area, as indicated by the high proportion of feldspars in the sandstones, and the presence of pebbles of granites and gneisses (W. Hef1ik and R. Unrug, 1965). There are no data suggesting the proximity of the southern margin of the area of deposition of the limnic series. It is concluded tentatively, that this area extended far south of the present erosional boundary of the Moravia-Silesia Basin. Folding of the basin. The sedimentary series filling the Moravia-Silesia Basin are folded. The youngest beds affected by the folding are belonging to the Westphalian D, and therefore the folding is assigned to the Asturian phase. The intensity of folding much greater in the western part of the basin than in its eastern part, and the presence of two systems of folds-one striking NNE-SSW with transverse undulations, the other striking WNW-ESE, pose the problem of the number of folding phases, discussed in the literature since a long time. Three opinions were expressed hitherto, one regarding the NNE-SSW fold system as older, and the transverse undulations and folds trending WNW-ESE as younger, the second holding that the reverse is true, the third claiming that only one phase of folding was active in the Moravia-Silesia Basin, and that the two fold systems reflect an adjustment of the folded series to rigid frames of the basin. The latter view was supported recently by results of studies of microtectonic structures (V. Havlena, 1964). The present state of knowledge of the sedimentary evolution of the Moravia-Silesia basin suggests that various sectors of the areas bordering the basin were subject to uplift, possibly associated with folding, in various phases of the Variscan orogeny. The basin itself was folded probably only once in the Asturian phase. The sedimentary cover of the folded basin The sedimentary cover of the folded basin as defined here consists of rocks belonging to the Variscan diastrophic-sedimentary cycle, formed after the Asturian folding of the basin. This cover is present on small areas in the eastern and northern part of the basin, and consists of two accumulation covers, overlying unconformably the older rooks. The lower cover, assigned to the Middle Stephanian (S. Siedlecki, 1951) consists of the pebbly Kwaczała arkose containing intercalations of red clays in the northern part of its area of occurrence. Cross-stratification indicates palaeocurrents flowing to the north-east. The deposition of the Kwaczała arkose was associated with an uplift of the southern border of »the Moravia-Silesia Basin. It is possible that the present southern boundary of the basin was formed at that time. The second accumulation cover, assigned to the Lower Permian, occurs in a narrow belt along the eastern and northern margin of the basin, filling grabens formed in the final stages of deformation of the basin (Fig. 7). It consists of coarse-grained conglomerates composed of limestone and porphyry pebbles, sandstones, red clays with intercalations of gypsum and sandy limestones, tuffs, tuffites, effusive ¡porphyries and melaphyres. A fresh-water limestone occurs locally. The detrital material of these rocks was derived from local source areas, built of older series filling the Moravia-Silesia Basin (A. Siedlecka , 1964). The centers of volcanic eruptions and effusions are associated with faults limiting the grabens containg the Lower Permian rocks. The relation of diastrophism and sedimentation The sedimentary evolution of the Moravia-Silesia Basin was controlled by two factors: the differentiated subsidence of the area lying between the Bohemian Massif and the Caledonian Kraków fold belt, and the uplift of various sectors bordering the basin. The first two phases of evolution of the basin i.e. the phase of transgression and the phase of neritic and bathyal sedimentation were associated with subsidence over a large area. The limits of the basin were not yet formed, and only the geosynclinal furrow extending along the eastern margin of the Bohemian Massif was individualized. This furrow was the site of accumulation of bathyal sediments and products of submarine volcanism. Uniform neritic conditions of sedimentation existed on the remaining part of the basin and there was no supply of terrigenic material coarser than pelite. The next phase, that of filling the basin by detrital sediments, was marked by intense diastrophism in areas bordering the basin. These movements led to the formation of the boundaries of the Moravia-Silesia basin, and the facial changes which occurred during this phase were associated with uplift of various segments of the basin border. The last movements in the southern border of the basin occurred after the Asturian folding, and uplifted the source area of the material of the lower sedimentary cover of the folded basin. The upper cover consists of material derived from local source areas lying within the basin.

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