Niektóre cechy sedymentacji fliszu podhalańskiego

Andrzej J. Krawczyk


Some charactreistics of the sedimentation process of the Podhale Flysch

The purpose of the present paper was to present the results of applying selected mathematical-statistical methods as a main tool in displaying sedimentological characteristics of the Podhale Flysch (Eocene? — Oligocene of the Central Carpathians). The examined deposits are a complex about 4 km thick, beginning with the Zakopane Beds where shales are distinctly prevailing over sandstones. This member passes up into the Chochołów Beds which contain a considerably greater amount of thick sandstone layers; the youngest member are the Ostrysz Beds which, apart from numerous thickbedded sandstones, also include middle - and coarsegrained sanstones, usually not found in older members. The studies were carried out on 15 profiles (Fig. 1) satisfying the conditions of a suitable length (at least a few hundred layers), lack of tectonic dislocations and absence of bigger deformational disturbances. In a detailed description of those profiles five lithological-structural elements were applied; they correspond approximately to the elements A, B, C, D, E, distinguished by Bouma. As a whole, the profiles under examination comprise 20077 layers, and their total thickness is estimated at 463.3 m. The mathematical-statistical analysis employed methods based on a stochastic model of the sedimentation process as a realization of the Markov chain, as well as a method of the Duff-Walton modal cycles. The results obtained for respective profiles were then compared; this procedure provided information about variability of parameters in time and space. At the same time, an average taxonomic distance was a measure of matrix similarity, and the [chi](2) test — a measure of distribution similarity. The carried out analysis permitted a relatively accurate reconstruction of the character and direction of changes in the sedimentation mechanism. It became evident that sedimentation of flysch formations in the Podhale Basin was, on the whole, a tranquil process. Its background was a monotonous silty-argillaceous deposition, occasionally interrupted by episodes of sandy material transport to the basin. This is evidenced, above all, by domination of DE cycles (Fig. 4, 5, 7) persisting, with different intensity, though, in the whole examined profile of the Podhale Flysch. Undoubtedly, turbidity currents having a relatively low energy were one of th e means of transport of the psammitic material. The latter assumption is supported by frequent occurrence of complete tripartite sequences of the sandstone-siltstone-shale type. A conclusion about are latively low (on the average) energy of those currents is drawn from diversified bed thickness distributions and frequent occurrence of sandstones with parallel and cross lamination. A number of data (abundance of current hieroglyphs, frequent discordances between the direction of hieroglyphs and the dip of laminae, cross bedding of some siltstones, etc.,) indicate that redeposition of a freshly accumulated sediment was an important sedimentary factor. That redepositions might have resulted from turbidity or bottom currents, not necessarilly running at a high speed, since the material might have been brought to the bottom currents zone by a down-slope turbidity currents and transported farther on without former deposition. An essential factor retaining the material in the suspension was, most probably, turbulence which occurred in the zone of mixing of turbidity and bottom currents. The deposition of the argillaceous material was of a similar character. It seems that a quiet gravitational subsidence of particles was negligible as a final sedimentation mechanism. Essential differences occurring between deposition periods of respective stratigraphic members connected, above all, with changes in the degree of influence exerted by various sedimentary factors are quite conspicuous on such a general background. Sedimentation of the Zakopane Beds in adjacent to the Tatra Mts. region was characterized by a visible domination of the silty-argillaceous background. This fact is evidenced, first of all, by a considerable prevalence of DE cycles (Fig. 4) over the remainder, as well as by the form of transition probability matrix (Table 1—4) and recurrence probability function for siltstones and shales (Fig. 2). The main factors of the sandy material transport were turbidity currents which seldom reached the bottom of the basin and whose energy was low. In the vicinity of the opposite side of the basin (south of the Pieniny Klippen Belt area) sedimentation proceeded in a similar way, except for less frequent but more vehement animation periods. The energy of suspended load currents was markedly diversified; more often there occurred groups of currents succeeding one another in short time intervals. The latter phenomenon was indicated, among others, by an extremaly high variation coefficient of the sandstone beds thickness distribution, a greater amount of sandstones with complex bedding than in the southern margin of the basin, and the abundance of different sedimentary cycles (Fig. 4, 18). The character of the sedimentation process changed considerably at the end of the Zakopane Beds deposition. The profiles fringing upon the Chochołów Beds differ basically from typical profiles of the Zakopane Beds. The amount of sandy material is considerably bigger; at the same time increase both in the number of sandstone beds and in their thickness can be observed. The character of stratification is changed: sandstones with cross bedding are prevailing and those with complex bedding are considerably more frequent. The domination of the DE cycle is negligible (Fig. 5), the abundance of cycles is considerably bigger (Fig. 18), and the transition probability matrix becomes devoid of surpluses connected with transition from sandstone to siltstone (Table 8). The turbidity currents transporting the psammitic material achieved greater energy and deposited sediment portions bigger than before. Redeposition of a freshly accumulated material took place on a large scale, leading to displacement of an aleuritic fraction and also, possibly, to transformation of the fine-grained sandy material. This assumption is supported mostly by the fact that an increased participation of complete sandstone- siltstone-shale cycles coincided with a still more enhanced participation of sandstone-shale cycles (Fig. 5) and frequent occurence in the profiles of complex sequences of diversely arranged structural elements (Table 11). On the whole, it seems that the transition between the Zakopane and Chochołów Beds was a period of the greatest sedimentary animation in the Podhale Basin. The transition period was followed by a succesive change consisting in a partial reversion to former conditions: the sedimentation process became somewhat more quiet, though the amount of the psammitic material transported to the basin was incomparably bigger than at the beginning of the deposition. During sedimentation of the Chochołów Beds the role of DE cycles became more important again (Fig. 7); besides, among sandstones, parallel lamination was prevailing over cross bedding and in some places of the basin there recurred surpluses of transition from sandstone to siltstone (Table 13). As a result, the profiles of the Chochołów Beds are in many respects intermediary between the Zakopane Beds and the transit zone profiles (Fig. 9, 11).

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