Polski basen cechsztyński
Abstract
POLISH ZECHSTEIN BASIN
Summary
In Polish part of the Zechstein Basin, similarly as in other European subbasins, 4 transgressive-regressive sedimentary cycles may be differentiated. Lithological horizons and cycles successively disappear here shorewards in result of periodical regression related to cyclic and reciprocal sedimentation. Changes in the rate of subsidence and sedimentation influenced thickness and lithological variability of Zechstein deposits.
The main tectonic framework of the basin resulted from Late Variscan movements of the Saalian phase. The subdivision into two paleogeographic provinces: eastern and western, corresponding to the Precambrian and Paleozoic Platforms, respectively, may be dated at these times. The former province is characterized by very stable development. Slow but steady subsidence resulted in formation of Zechstein complex up to 600-700 m thick in depressed parts of the Platform (Peribaltic syneclize and Podlasie Depression). In the Paleozoic Platform area, the zone of maximum subsidence corresponded to the forefield of the Variscan fold front, delineated by the Teisseyre-Tornquist line from NE. The basement of SW part of the Zechstein Basin (Variscan fold zone) was highly mobile almost throughout the Permian and vertical movements resulted in formation of numerous depressions and elevations.
The Zechstein Limestone (Cal) was deposited in shallow shelf sea and the microfacies succession (laminated micrite -> micrite -> biomicrite -> oncolite -> stromatolite) is clearly regressive in character. This is further supported by the presence of vadose deposits and carbonate and anhydrite structures typical of the sabkha environment in the uppermost part of Cal. The rate of subsidence was locally very high in times of deposition of A1d, giving rise to numerous depressions in the marginal zone. The depressions became the centers of deposition of rock salts (Na1) and, occasionally, potassium salts (Kl), being separated by anhydrite barriers. The deposits of the PZ1 were fully compensating low-rate subsidence in the marginal zone whereas in central parts of the Basin the subsidence was not compensated already during the sedimentation of PZ1 evaporites.
The sea somewhat retreated at the beginning of sedimentation of the second cycle (PZ2). The morphology of basin floor was shaped almost exclusively by synsedimentary vertical movements of the basin. A marked regression of the sea may be traced in areas of the Precambrian Platform and central Pomerania. An intense, uncompensated subsidence from times of deposition of the Main Dolomite (Ca2) and Basal Anhydrite (A2) resulted in origin of deep sea basin in central parts of the Basin. The deep basin was subsequently almost completely infilled with the Older Salt (Na2 and K2). Gradual deepening of the sea in central parts of the Basin is reflected by the Ca2 microfacies sequence (micrite -> laminated micrite and, at the boundary of shallow and deep shelf, stromatolite/oncolite -> pelmicrite ->laminated micrite) as well as anhydrite (A2) textures typical of deep water environment. The Ca2 nearshore sequence displays reciprocal, typically regressive microfacies sequence, and the A2 and A3 anhydrites are of shallow water origin.
The Platy Dolomite (Ca3) sea was of the shallow shelf type. This horizon is characterized by predominance of biomicrite microfacies. Regressive microfacies sequence is recorded in all the paleogeographic zones, evidencing gradual shallowing of the Basin. The PZ3 evaporites almost completely infilled depression in central parts of the Basin.
The time of deposition of the PZ4 cycle was characterized by changes in environment of the Zechstein Basin. Marine influences were ceasing in result of isolation from the World Ocean and subsidence zones were narrowing which resulted in origin of relict evaporitic basin. Two lithofacies may be differentiated: one, typical of the German Zechstein Z4 and Z5 cycles in western part of the Basin (Fig. 1), and the other, zuber (zuber means rock salt deposits with large, 15 to 85%, admixture of claystones in the form of irregular concentrations or thin intercalations) in the eastern part.
Summary
In Polish part of the Zechstein Basin, similarly as in other European subbasins, 4 transgressive-regressive sedimentary cycles may be differentiated. Lithological horizons and cycles successively disappear here shorewards in result of periodical regression related to cyclic and reciprocal sedimentation. Changes in the rate of subsidence and sedimentation influenced thickness and lithological variability of Zechstein deposits.
The main tectonic framework of the basin resulted from Late Variscan movements of the Saalian phase. The subdivision into two paleogeographic provinces: eastern and western, corresponding to the Precambrian and Paleozoic Platforms, respectively, may be dated at these times. The former province is characterized by very stable development. Slow but steady subsidence resulted in formation of Zechstein complex up to 600-700 m thick in depressed parts of the Platform (Peribaltic syneclize and Podlasie Depression). In the Paleozoic Platform area, the zone of maximum subsidence corresponded to the forefield of the Variscan fold front, delineated by the Teisseyre-Tornquist line from NE. The basement of SW part of the Zechstein Basin (Variscan fold zone) was highly mobile almost throughout the Permian and vertical movements resulted in formation of numerous depressions and elevations.
The Zechstein Limestone (Cal) was deposited in shallow shelf sea and the microfacies succession (laminated micrite -> micrite -> biomicrite -> oncolite -> stromatolite) is clearly regressive in character. This is further supported by the presence of vadose deposits and carbonate and anhydrite structures typical of the sabkha environment in the uppermost part of Cal. The rate of subsidence was locally very high in times of deposition of A1d, giving rise to numerous depressions in the marginal zone. The depressions became the centers of deposition of rock salts (Na1) and, occasionally, potassium salts (Kl), being separated by anhydrite barriers. The deposits of the PZ1 were fully compensating low-rate subsidence in the marginal zone whereas in central parts of the Basin the subsidence was not compensated already during the sedimentation of PZ1 evaporites.
The sea somewhat retreated at the beginning of sedimentation of the second cycle (PZ2). The morphology of basin floor was shaped almost exclusively by synsedimentary vertical movements of the basin. A marked regression of the sea may be traced in areas of the Precambrian Platform and central Pomerania. An intense, uncompensated subsidence from times of deposition of the Main Dolomite (Ca2) and Basal Anhydrite (A2) resulted in origin of deep sea basin in central parts of the Basin. The deep basin was subsequently almost completely infilled with the Older Salt (Na2 and K2). Gradual deepening of the sea in central parts of the Basin is reflected by the Ca2 microfacies sequence (micrite -> laminated micrite and, at the boundary of shallow and deep shelf, stromatolite/oncolite -> pelmicrite ->laminated micrite) as well as anhydrite (A2) textures typical of deep water environment. The Ca2 nearshore sequence displays reciprocal, typically regressive microfacies sequence, and the A2 and A3 anhydrites are of shallow water origin.
The Platy Dolomite (Ca3) sea was of the shallow shelf type. This horizon is characterized by predominance of biomicrite microfacies. Regressive microfacies sequence is recorded in all the paleogeographic zones, evidencing gradual shallowing of the Basin. The PZ3 evaporites almost completely infilled depression in central parts of the Basin.
The time of deposition of the PZ4 cycle was characterized by changes in environment of the Zechstein Basin. Marine influences were ceasing in result of isolation from the World Ocean and subsidence zones were narrowing which resulted in origin of relict evaporitic basin. Two lithofacies may be differentiated: one, typical of the German Zechstein Z4 and Z5 cycles in western part of the Basin (Fig. 1), and the other, zuber (zuber means rock salt deposits with large, 15 to 85%, admixture of claystones in the form of irregular concentrations or thin intercalations) in the eastern part.