Mioceńska dynamika polskiej części zapadliska przedkarpackiego

Nestor Oszczypko



The Outer Western Carpathians are built up of a stack of nappes and thrust-sheets overthrust onto the Miocene deposits of the Carpathian Foredeep. However, along the frontal
Carpathian thrust, a narrow zone of folded Miocene deposits was developed (Fig. 1). The basement of the Carpathian Foredeep represents the epi-Variscan platform and its cover. The present-day structure of the basement was formed during the Late Alpine continental collision. The depth of the platform basement ranges from few hundred metres up to 3 500 m in the Carpathian Foredeep, whereas in the south of the Carpathian margin, this value oscillates from 500 to 4500 m (Z-1, Fig. 1) in the west, and from 2000 to 7000 m (K-1 borehole, Fig. 1) in the east, respectively. The magneto-telluric soundings in the Polish Carpathians revealed a high resistivity horizon connected with a consolidated-crystalline basement (Ryłko & Tomaś, 1995). The depth of the crystalline basement varies from 3-5 km in the northern part of the Carpathians, dips to approximately 15-20 km at its deepest point, then peaks at about 8-10 km in the southernmost part (Fig. 2). The axis of the basement depression coincides more or less with the axis of the gravimetric minimum. In the Polish part, the width of the outer foredeep (outside the Carpathians) varies between 30-40 km in the western segment up to 90 km in the eastern one. The outer foredeep is filled up with Badenian and Sarmatian marine deposits, from few hundred up to about 3 500 m in thickness. The inner foredeep located beneath the Carpathian nappes is more than 50 km wide (Oszczypko & Ślączka, 1989). The inner foredeep is composed of the Lower to Middle Miocene autochthonous deposits of up to 1500 m thick. The Lower Miocene deposits are mainly terrestrial in origin, whereas the Badenian and Sarmatian deposits are marine. The oldest - Burdigalian (Ottnangian-Karpatian ) deposits, up to 1000 m thick, have been pierced in the drilling Zawoja-l (Fig. 1). These deposits are composed of conglomerates passing upwards into variegated claystones and mudstones and contain a 200 m thick, flysch-derived olisthoplaca (Moryc, 1989). The Badenian deposits in Poland are traditionally subdivided into the lower (sub-evaporatic), middle (evaporatic) and upper (supra-evaporatic) ones, which do not fit (Fig. 3) with new chronology of the Central Paratethys (Steininger et al., 1990).
In this paper the terms: Lower, Middle and Upper Badenian used in lithostratigraphic sence are in brackets. The Lower Badenian begins with conglomerates passing upwards into dark, clayey-sandy sediments. The thickness of the "Lower Badenian" deposits is variable, reaching up to 1000 m in the western inner foredeep, whereas in the remaining part of the inner foredeep rarely exceeds 30-40 m. According to the nannoplankton study, the formation belongs to NN 5 zone, and in the uppermost part to NN 6. The evaporitic horizon (anhydrites and salts), traditionally regarded as "Middle Badenian", overlies these deposits or rests directly upon the platform basement. The age of the evaporatic horizon could be estimated as the uppermost part of the NN 5 up to NN 6/ NN 7 zones (Andreeva-Grigorovich, 1994; Peryt & Peryt, 1994; Gaździcka, 1994) The evaporatic horizon passes upwards into "Upper Badenian"- Sarmatian (NN 8/9 zone, see Gaździcka, 1994) sandy-silty deposits with a thick sandstone complex at the base. Their thickness ranges from a few hundred metres in the Tarnów area up to 3000 meters near Przemyśl. In the Rzeszów area these deposits rest directly on the platform basement. The burial history was constructed on the basis of selected well (Z-1, BIG-1, D-1, Al-11, H-42, M-83; Figs 1,4,5). The computation procedures of Van Hinte (1978), Sclater & Christie (1980), and Angevine et al. (1990) have been used. In the construction of total subsidence curve the decompacted sediment load and paleobathymetry were used. For the wells (Z-1, BIG-1, D-1) located in the Carpathians the post nappe tectonic load was additionally regarded. Backstripped subsidence was corrected using simplified Airy isostasy model and paleobathymetry. The total subsidence was 1,5-1,8 times higher than tectonic subsidence (Tab. 1). The subsidence of the inner foredeep was strongly controlled both by the sediment and nappe load. In the case of Z-1 borehole (Fig. 4) the nappe loading took place at least two times. In the outer foredeep (Al-11) the influence of the nappe load was indirect. The Carpathian Foredeep began to form during the Middle Burdigalian period, simultaneously with the folding, overthrust and inversion of the
Outer Carpathians (Oszczypko & Ślączka, 1985, 1989). The northern edge of the Early Miocene foredeep was located about 20 to 50 km south from present-day position of the frontal Carpathian thrust, whereas the axis of subsidence more or less coincided with axis of the depression of the magneto telluric basement (Figs 1, 2, 6). The basin was dominated by the continental (red beds) sedimentation and filled up mostly with the products of erosion of the emerged platform. Total subsidence in the area of the Zawoja-1 borehole (Fig. 4) reached at least 1500 m (1,0 mm/a). Towards the east, the subsidence increased up to more than 2000 m (Sambor-Rozniatow unit). At that time the Cieszyn-Slavkov Ridge was in foreland bulge position. In the Sucha Beskidzka-Cieszyn area the Lower Miocene deposits consist of two horizons containing flysch olistoplacas derived from the front of the contemporaneous Carpathians (Oszczypko & Tomaś, 1985). The upper olistostroma was probably connected with overthrusting of the Silesian/Subsilesian units onto the Lower Miocene molasses (Early Styrian Phase). As a result the front of the Carpathians was shifted 15 km towards the north (Fig. 6). The Lower Miocene deposits additionally loaded by the flysch nappes caused an extra subsidence about 2000 km (2,0 mm/a). This subsidence enabled transgression of the Early Badenian sea onto the Carpathians. The Early to Middle Badenian subsidence was highly differentiated, from few dozen metres in the outer foredeep up to more than 1000 m in the inner foredeep (Figs 4, 5). The Lower Badenian axis of subsidence was located 20 to 40 km to the south of the present position of the Carpathian frontal thrust (Fig. 1). During the evaporitic sedimentation the rate of subsidence was very low. In the Rzeszów area this quiescent period was probably characterized by viscoelastic relaxation, and uplift of the Rzeszów Paleo-ridge. This uplift was probably connected with erosion of the "Lower Badenian" deposits ("Rzeszów Island"). After the salinity crisis the Carpathians shifted 20--30 km towards northeast. It caused "Upper Badenian" subsidence up to 1500 m, collapse of the Rzeszów Paleo-ridge, and a new sea transgression onto the Carpathians. The Sarmatian depocenter was located in the NE part of the basin that was oblique to the Carpathians. The total subsidence was up to 3 000 m. At the end of Sarmatian the Carpathians moved towards the NE and reached present-day position. It was probably followed by the regional regression of sea. The Miocene subsidence in the Carpathian Foredeep was due to the foredeep sediments and thrust-induced load (Fig. 7). However, Royden & Karner (1984) proved that this load was insufficient to create observed deflexion of the foreland plate, and postulated existence of an additional subsurface load on the subducted plate. During the Lower to Middle Miocene (7,8 ma) mean rate of the frontal Carpathian thrusting, is approximated as 7,7-12,3 mm/a. During that time, the loading effect of thickening Carpathian wedge on foreland plate increased. It caused an increase of total subsidence. During the Late Badenian-Sarmatian time the rate of overthrusting was less than that of pinch-out migration (13,8 mm/a) and this resulted in basin widening (Allen & Allen, 1992). The Early to Middle Miocene shifting of the Carpathian wedge resulted in migration of depocenters and onlapping of the successively younger deposits onto the foreland plate. The Miocene foreland stratigraphy of the Carpathian Foredeep was controlled by deflection of the foreland plate (Figs 1, 3, 7) (Oszczypko & Ślączka, 1989).

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