Uwagi o sedymentacji fliszu podhalańskiego

Andrzej Radomski


Remaks on the sedimentation of the Podhale Flysch

Under the name of the Podhale Flysch we understand the beds of th e Upper Eocene — Lower Oligocene age which appear in the tectonic depression between the Tatra Mountains and the Klippen belt of Pieniny. They consist almost exclusively of alternating sandstones and shales. The sandstones are mainly fine — or medium — grained. The kind of the sandstone bedding depends on the size of grain and the thickness of bed. Thick and coarse-grained bedding are: the complete graded bedding, the discontinous one, the pensymetric one, the multiple one 1 and the homogenous one 3. The sorting of the material in the beds is good. Trask sorting coefficients vary within 1.16 — 2.8. In the lower parts of the beds the sorting is usually better than in the top ones. The amount of the coarsest material does not exceed 10— 15 per cent. In graded beds appear shale fragments of one cm up to several cm of diameter. They are either chaotically situated within the bed or grouped near the top or base surface. The last two cases are most often met with. Their situation depends perhaps on the density of the turbidity current forming the bed. In the case of dense currents they are grouped in the top parts of the bed and in case of diluted ones they are grouped in the lower parts. The orientation of quartz grains in the graded beds of the Podhale Flysch is not frequently developed. It is most often met with in the loosely „packed“ sandstones, in which, bigger grains occur in the mass of pelitic material. In this case the grain is exhibit on imbricate arrengement. Longer axes are steeply inclined (70 — 80 degrees). The directions of currents measured on the basis of the orientation of grains were very variable and even so in adjoining beds. Perhaps the relatively fine material owes its arrangement to the turbulences in the turbidity current moving on the bottom. The roundness of grains is slight. The percentage of well rounded grains, even in the fraction over 3 mm does exceed 28 per cent. This points out to short transport and to slight abrasion of the material in the littoral area. It seems that the material was in any case deposited below the wave base. The degree of roundness of grains in individual fractions does not change in the vertical section of th e bed. The laminated bedding is, on account of considerable prevalence of thin-bedded sandstones in the Podhale Flysch, the type very often met with. The diameters of grains in these beds do not exceed 0,14 mm and averagely thery are between 0.07 to 0.05 mm in the diameter. The lamination is caused by the alternate repetition of dark and light laminae. The dark ones consists of fine mica blades, plant detritus and clay particles. In the light ones grain is larger (the demensions given above concern them). The laminae are from a fraction of a mm to 2 — 3 thick. The diagonal bedding appeare also in the pelitic, thin-bedded sandstones. The laminae are convex upwards. They form with the top surface an angle of about 2 0 degrees, while being tangent to the base surface. The currents owing to which these beds were formed were re latively weak; their speed was probably about 20 — 30 cm/sek. In the Podhale Flysch the composite bedding is very frequent. The graded bedding very often combines with the laminated or- diagonal one. In such a case the last two types appear mainly in the top pa rt of the bed; more rarely they occur in the lower parts of it. The laminated and diagonal bedding may combine with each other. Usually the lower part of the bed as then diagonnallv bedded and the upper one is laminated. In the area of Eastern Podhale in over 300 investigated beds there was 11 per cent graded, 50 per cent diagonally beded and 35 per cent of laminated. The direction of diagonal bedding in the great majority of cases are NE and SE which points out that the currents flowed principally from the West. The same direction show also numerous groove casts and flow marks. Exotic rocks found in the sandstones allow to assume that beside the Tatra massif there, was still another area which provided the material for the Podhale Flysch basin. Variously developped submarine movements recorded in the sandstone beds present a very frequent phenomenon. Many beds possess folded laminae, whilst having parallel and smooth top and base surface often truncates the contertions. Sometimes the folding embraced a few beds whereas the ajoining bods siutuated above and below are undisturbed. Sometimes some beds are torn apart and characteristic sandstone balls apear. The structure thus formed are plastic which points out to the slight advance of diagenetic processes in the moment of formation of slides. The third type represent the submarine slides in which not only the individual beds were torn and folding, but the 'material was completely mixed together. The erosion slides reached in many cases so deep that fragments of hard consolidated sandstones torn away together with the shales situated immediately above them were involved in the sliding movement.

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