Piaskowce dolnotriasowe w północnej części Niecki Śródsudeckiej

Jerzy Mroczkowski


Lower Triassic sandstones in the northern part of the Intra-Sudetic trough

Lower Triassic deposits in the Sudety Mts occurs in two separate geological units: the North-Sudetic Trough and the Intra-Sudetic one. The subject of the present paper are Lower Triassic sandstones occurring in the northern part of the Intra-Sudetic Trough, in the territory of Poland. Lower Triassic sediments in the Sudety Mts are developed in the Central European facies; they are often known under the name of Buntsandstein. Due to the lack of fossils, the age of the described sandstones, which are up to 100 metres thick in the Intra-Sudetic Trough, is estimated on the grounds of their lithological similarity to Lower Triassic sediments of the North-Sudetic Trough (Zimmermann and Berg, 1904). This idea was accepted and shared by numerous scientists: Dathe and Berg (1912), Dathe and Petrascheck (1933), Tasler (1961, 1966), Valin (1961, 1964), Dziedzic (1973). Müller (1930) and Scupin (1937) differed in their opinions: they claimed that the sandstones under description were of Late Permian age. At the top of the described Lower Triassic sandstones and below grey Cretaceous sandstones containing fauna there can be sometimes observed white kaolin sandstones devoid any distinct sedimentary structures. They are classified by numerous authors as belonging to the Triassic, too (Zimmermann & Berg 1904; Berg, 1905, 1938; Dathe and Berg, 1912; Dathe and Petrascheck, 1933; Tasler 1961, 1966; Valin , 1961, 1964). According to the author, white, structureless kaolin sandstones differ very distinctly from typical Buntsandstein in the region under description. They might have been formed during the whole time interval from Early Triassic to Cenomanian. It is very likely that they constitute fossil weathering waste of Bundsandstein, developed during the interval mentioned above. In the territory of Czechoslovakia Valin (1964) distinguished a few different facies in the Buntsandstein of the Intra-Sudetic Trough. The author of the present paper is of the opinion that there are no grounds for such a division in the Polish part of the Trough. Monotonous development of the Buntsandstein and its small thickness raised an opinion that only the Lower Buntsandstein is represented in the Intra-Sudetic Trough (Berg 1938). It seems, however, that in the region under description the whole Lower Triassic except of Roethian might have developed. At the same time the reduction in thickness, which is 1:5 in relation to the North-Sudetic Trough, results from the „cannibalism” occurring under conditions of intensive redeposition. Among the Lower Traissic sandstones the author has observed the occurrence of the following types of rocks: feldspathic and arkosis arenites, as well as quartz, feldspathic and arkosic wackes. Their composition has been presented in Gilbert’s classifying diagrams (Fig. 2). The scarcity of micas is a noticeable characteristic of sandstones. This fact is quite surprising, since pebbles occurring in the sandstone often come from rocks abundant in micas. The composition of pebbles is shown in Figure 3. They do not form beds but are scattered in the sandy background. Their size is illustrated in Figure 6. Source rocks of the pebbles have not been defined, so far, but the divergences in their composition and in the pétrographie composition of sandstones suggest that the source of sandy material might have been different from the source of pebbles alone. Apart from pebbles of solid rocks, redeposited fragments of mudclay beds can be observed in the sandstone. Their number is disproportionately bilg in comparison with rarely observed in situ beds and mud-clay lenticles. The Lower Triassic sandstone is generally middle-grained (median diameters from 0,35 mm to 0,60 mm; mean diameters from 0,74 0 to 1,53 0), and in most samples fairly well sorted (Table 1). On the grounds of structural characteristics diagrams have been worked out (Figs. 8, 9, 10). The characteristic feature of Buntsandstein is the presence of occasionally horizontal but mainly cross-bedding in it, most frequently developed on a small scale (20—40 cms), only seldom amounting to 3—4 metres. The gradients of cross-beddings change from almost horizontal to very steep. Inclination directions of cross-beddings are presented on the map (Fig. 4). Among other sedimentary structures there occur erosive channels and very seldom small ripple marks. The erosive channels are usually filled with disorderly deposited sand, sand and gravel, and pebbles. Erosive channels with bottoms covered with a thin mud-clay layer, overlaid by a coarser material, are less frequently found. The second type of channels might have been formed as a result of abandoning of the eroded channel by a current or a stream. In consequence, the abandoned channel was filled with fine sediments not by a strong current but by seeping and flowing down waters, similarly as in the case described by Williams and Rust (1969). It is not unlikely that the majority of recently observed mud-clay fragments in the Lower Triassic sandstone come from erosion of structures described above. The analysis of transport directions of clastic material has been based upon: measurements of directional structures cross-bedding and erosive channels (Fig. 4), changes in the pétrographie composition of pebbles (Fig. 5), changes in size of pebbles (Fig. 6), changes in percentage of feldspar participating in the formation of sandstone (Fig. 7). It appears from the above analysis that the principal direction of transport of the clastic material — from the south — does not differ from the one prevailing in that part of Europe in Lower Triassic (cf. Grumbt , 1974). Local distributions of transport directions point out that the outline of sedimentary basin in Lower Triassic was not different, in the main, from the present outline of the Intra-Sudetic Trough. The environment in which the Lower Triassic sandstone was being formed could be reconstructed for the area under description only in general outline, since the inventary of sedimentary structures is poorly diversified, and no fossils are present there. The character of cross-bedding, erosive channels and granulometric data (cf. the diagrams, Figs 9, 10) lead us to believe that those sediments developed in the fluvial environment where at least their part was being deposited in the upper flow régime. Certain characteristic, such as the appearance of some erosive channels and the distribution of directions of paleocurrents, resemble those found in streams of the braided river type (cf. Williams and Rust, 1969). The resemblance between Buntsandstein in the Intra-Sudetic Trough and in the North-Sudetic one is unquestionable. Sedimentary basins corresponding to those units most likely never joined, but were only about 40 kms apart. The comparison of development of the Lower Triassic sediments in those two units can be resolved into a few essential problems: 1° —The thickness of Buntsandstein in the Intra-Sudetic Trough is about 100 metres, and in the North-Sudetic one — above 500 metres; 2° — The inventary of sedimentary structures is much richer in the North-Sudetic Trough than in the Intra-Sudetic one; moreover, in the former there occur biogenic structures. As it appears from the analysis of sedimentary structures Buntsandstein was being deposited in the Intra-Sudetic Trough with the help of higher energy currents than in the North-Sudetic one; also the redeposition of sediments developed on a larger scale there. 3° — Having a similar textural maturity, Buntsandstein of the Intra-Sudetic Trough displays smaller maturity of the pétrographie compositon. The difference is probably due to the fact that in the North-Sudetic Trough a considerable part of the material was being transported periodically, at longer intervals, during which chemical weathering was proceeding. Moreover, in the sandstone of the Intra-Sudetic Trough, a larger amount of pebbles with metastable rocks among them can be observed. Scattering of pebbles in the sandy background is similar in both cases. 4° — Transport directions of the clastic material are, on the whole, very much alike in both units and not very different from the principal direction of transport prevailing in Central Europe in the Lower Triassic (cf. Mroczkowski, 1972; Grumbt , 1974, and Fig. 4 of the present paper). 5° — The sedimentary environment in which Buntsandstein of the North-Sudetic Trough had beed formed, was reconstructed as the fluvial one, of the braided river type with periodical Stillwater reservoirs, in the moderately humid to semi-arid and not very hot climate with periodical rainfalls (Mroczkowski, 1972). In the Intra-Sudetic Trough the environment was probably similar: fluvial, maybe of the braided river type, but with no traces of water reservoirs. The climate might have resembled that of the North-Sudetic Trough (40 kms away), but apart from general resemblance between sediments there are no direct evidences of it.

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