Geologia Karpat przemyskich - "szkic do portretu"

Janusz Kotlarczyk



The Przemyśl Carpathians represent a segment of the Skole Nappe (or so called "Sliced Nappe"), which comprises a bend of Carpathian margin called as Przemyśl sigmoide. In this locality, the Alpine orogene reveals the following structure: the East- European Platform plunges gradually to the South and is covered with the younger autochtonous molasse of foredeep of Badenian - Early Sarmatian age; on those deposits (further outside the molasse may be older, i.e. of Ottnangenian - Carpatienian age) the following overthrusted on each other units are overthrusted: Zgłobice Unit (the folded younger molasse); Stebnice Nappe (folded older molasse with tectogenetic cover of the younger molasse ), Borysław-Pokucie Nappe (folded flysch with older molasse) and Skole Nappe (folded flysch with tectogenic cover of younger molasse). Occurrence of the sigmoide of Borysław-Pokucie Nappe is problematic in the area studied. The tectogen was formed in three orogenic phases: the Skole Nappe with its sigmoide and Borysław-Pokucie Nappe were formed in the Late Styrian phase, moving from their primary position about 50 km to the North (Fig. 4). Those nappes, moving 25-30 km further to the North (in Moldavian phase) to their present position, formed Stebnice Nappe and Zgłobice Unit.
The Skole Nappe is characterized by a scaly - sliced structure, which is strongly developed in the outer part of the nappe (Fig. 1), though also present in the inner part (Fig. 3). The thickness of the Skole Nappe increases gradually from 1,5-3 km near the margin to 6,8 km in Kuźmina, what is confirmed by boreholes.
Flysch deposits of the Przemyśl Carpathians are 4,5 km thick and were formed in a perycratonic trough of the Carpathian geosyncline, mainly on the northern slope of this trough - therefore such phenomena as submarine slides olistostromes, channel facies of submarine fans, conglomerates composed of platform material and olistolites are common.
Most common are turbidite deposits, among them those strongly "diluted" in muddy facies. An average rate of flysch sedimentation (for consolidated sediments) ranges at about 45 mm per one thousand years. In a Hauterivian - Burdigalian vertical sequence one may differentiate four units, of which the first one (Fig. 5) and the third one (Fig. 7) are from the bottom mostly muddy - shaly, while the second one (Fig. 6) and fourth one (Fig. 8) are mostly sandy - shaly, i.e. flysch-like.
Corresponding to that fact, the rate of sedimentation is considerably differentiated and may be estimated in the four above mentioned units as follows (in order from the bottom to the top): 7,5, 62,5, 11,6 and 144 mm per l thoused of years. In the case of the last unit, the rate of sedimentation of the menilite shale is typical for the shales, i. e. l O mm/t thousand years, while the flysch facies of the Krosno Beds reach the rate of sedimentation of above 220 mm/l thousand years.
Palaeontological investigations performed by many scientists (8, 9, 12, 16), as well as presence of izochronous horizons, allowed both to work out a fairly precise stratigraphy of the flysch and to reveal the facial changes (Fig. 5). The presented view, which represents a cross-section of the northern part of the Skole Besin without reconstruction of real thicknesses of the units, explains main zones of clastic sedimentation (axial part of the basin) and carbonaceous sedimentation (northern part of the slope). Two periods of intensive flysh deposition in Upper Cretaceous and Ruppelian - Late Burdigalian time reflect activity of diastrophic phenomena and are connected with a maximum development of the trough and its final filling stage. The apparent difference in the facial development of marginal scales in the final stage (Fig. 8) speaks in advocacy of their separation as a part of the Borysław-Pokucie Unit.

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