Uwarstwienie spływowe we fliszu karpackim

Marian Książkiewicz


Slip-bedding in the Carpathian Flysch

In the Carpathian Flysch graded bedding is the most prevalent type of stratification. Besides, composite graded bedding, current bedding and slip-bedding are occasionally met with. To the last-named type of bedding the author referred corrugated, fine-grained sandstones of the Sub-Magura beds in the year 1935 (5). Some examples of slipbedding from the Carpathian Flysch at a later date have been described by O. Ganss (9). Basing one’s opinion on numerous papers concerning corrugated beds occurring in geological series and particularly on A. D. Arhangelski’s (1) study of the structures created by slipping of recent sediments on the bottom of the Black Sea, one may assume that corrugated beds occurring in the Carpathians are due to slipping of unconsolidated sediments. However, not all corrugated beds in the Carpathian Flysch may be attributed to sliding. In some calcareous sandstones corrugation is evidently associated with epigenetic concretions formed by the subsequent distribution of calcium carbonate and cementation. There exist two types of structures created by subaqueous sliding. In the first group the continuity of lamination or stratification is preserved and the particular laminae in spite of contortion keep their identity. In another group of structures this continuity is broken and the sediment divided into fragments embedded in another deposit. In several cases the movement must have been spontaneous as fragments of shales and even the sideritic concretions formed originally in shales are embedded in the corrugated sandstones. The present paper discusses the first group of structures, as they are much more frequent and easier for studying than the second group, also present in the Carpathian Flysch. Intraformational contorted beds occur in numerous members of the Flysch sequence; they are developed, although not very frequent, in the Lgota beds (Aptian-Albian), in the Sub-Magura, Beloveza, Magura and Krosno beds (ail members of the Palaeogene). They are especially numerous in the Middle Eocene of the Magura series. Some examples are given in Figs. 1, 3 and 4 of the Polish text. Corrugated beds are often associated with current bedding, which is fairly well developed in beds with contorted layers (Fig. 2). In all the examples the continuity of stratification is well pronounced; if the laminae are broken anywhere, it is an exception rather than a rule; the contortion is thus regular, although its intensity is visible in the pattern of a strongly folded deformation resembling the picture of Alpine tectonics. From the continuity of stratification two conclusions may be drawn. First, the slipping deposit must have been unconsolidated; secondly, the movement has not been spontaneous, but rather slow; therefore, the author presumes in accordance with S. H. Straw (20) that the corrugated beds are due to slow creep on the sea bottom. Slip-bedding in Flysch rocks is exhibited in mudstones or very fine-grained sandstones. It is absent both in coarser and finer grades. Possibly clays soaked with water and highly plastic cannot preserve their former internal lamination when slipping down and become a pasty mass in which any trace of stratification is destroyed, and the results of slipping are not recognizable. Very likely only in beds possessing distinct lamination, which is as a rule well marked in strata possessing composite graded bedding, the results of slumping may be visible. The corrugation may overtake the whole layer, but more frequently only its upper portion is contorted (Fig. 3). In rare instances the lower part is corrugated and the upper portion rests unconformably on truncated folds (Fig. 4). The upper surface of the contorted beds is uneven and cuts distinctly the folded structure. This surface is in most cases erosive, but in one instances it seems that the bevelling of the contorted layer is due to secondary slipping of the plastic material from crests into troughs of the upper surface. Thus may be interpreted the picture visible in the middle part of the lower corrugated layer in Fig. 3. In all observed cases both the underlying and covering layers, constituted of shales, are not deformed, not even to the smallest degree. It must be concluded that the deposition of the covering shales must have taken place after the creep had been completed and the corrugation created. Otherwise it would be unexplicable why the covering shales, much more plastic than the contorted mudstone or sandstone, are not contorted. Therefore, it may safely be assumed that the corrugation is contemporaneous with sedimentation and occurred before consolidation; it is not possible in these instances to accept the interpretation of C. B. Brown (4) that the slipping took place when the bed was to some extent consolidated and covered not solely by water, but by overlying sediments. In the Krosno and in the Magura sandstones the upper surfaces of beds are frequently deformed in regular parallel and wave-like folds resembling ripplemarks. On the cross-sections the folding nature of these forms is evident. Possibly these deformations correspond to pseudo-ripplemarks described by P. H. Kuenen (22, p. 372). The corrugated sediments are mostly calcareous; it seems that calcareous mud, if present in sands, facilitates slipping as much as does clay. This conforms to the numerous described instances of submarine slipping observed in limestones. No special suggestions can be made with regard to the cause of slipping. The most probable seems that the increase in thickness of deposits near the margins of the Flysch geosyncline, causing the increase of the slope, similarly as O. T. Jones (14) accepted for the Caledonian geosyncline, was the main reason of slumping. The association with current bedding points to relatively shallow water environment in which slumping took place. Possibly the action of currents, removing some portions of sediments and thus weakening their support, may also be responsible for sliding. Earthquakes probably evoked spontaneous and violent slides to which the slip-bedding observed in the Flysch cannot be attributed.

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