Powstanie lokalnych form tektonicznych w polskiej części syneklizy perybałtyckiej na tle rozwoju geologicznego całej jednostki

Abstract

Formation of local structural forms in the Polish part of the Peri-baltic syneclise in the background of the geological development of the whole unit

The area of the present investigations covers the south-western part of the peribaltic syneclise. The writer's research studies confirm the previously recognized block character in the structural style of the peribaltic syneclise as suggested by the presence of numerous deep dislocations. This is applicable most particularly to the Old Paleozoic structural stage.

The peribaltic depression was marked already in the Cambrian. Its most readily distinguishable formation occurred in the Devonian-Carboniferous. The axis of the sedimentary basin in the Cambrian constituted by the greatest thickness of sediments lay along the line from Kościerzyna to Wilno (Vilnius). Advancing subsequently northwards during the Paleozoic sedimentation it followed the southern peripheries · of Latvia during the· Carboniferous. During the Permian sedimentation it is gradually displaced southwards as far as the northern slope of the Mazury-Suwałki elevation. This is most clearly suggested in comparing the thickness of sediments in Zechstein cyclothems and the occurrence range of the saliniferous horizons.

Comparatively the maximum gradient of changes in the facies and thickness of Old Paleozoic sediments occurs along the line of the Lower Vistula course and may be connected with the existence of a regional southern dislocation zone with a considerable downthrow of the western limb. The oldest Cambrian members, the so-called Zarnowiec series occur to the west of that line. Here the Cambrian deposits are more deep-water in character as is shown by the markedly greater mudstone content when compared with that in the area east of the Vistula. Similarly it is in this zone that occurs the Ordovician boundary of the carbonaceous facies, it being thicker in the east and a thinner mudstone one in the west The sedimentation of Paleozoic rocks was broken up by intermittent periods of the intensity of uplifting block movements in the substratum, the area of the blocks showing the maximum uplifting being also at that time subjected to erosion. This process is fairly well detectable at the turn of the Lower and Middle Cambrian. In order to determine its range the Cambrian has been cut up into smaller complexes on geophysical prospecting diagrams. Their correlation has shown the absence of the youngest Cm₁C member out of those· differentiated in the Lower Cambrian in the neighbourhood of Lidzbark Warmiński and Barciany. Following the Lower Cambrian these regions had been somewhat more uplifted the overlying sediments being partly outwashed.

This process is still more evident after the Middle Cambrian, where the dismembered into blocks syneclise areas are quite distinct. The further east and the nearer to the Mazury-Suwałki elevation the deeper the erosional activities.

The longest discontinuity connected with strongest erosion is Post-Silurian. At that time the erosional processes in the Polish part of the Platform area continued with varying intensity until the Permian.

The bearing of the differentiated structure of the crystalline substratum is observable in the development of the sedimentary cover within the area under consideration. Namely, facing the general tendencies towards the uplifting of the platform, the oldest and simultaneously the lightest of the granite complexes, that is those of Mazowsze, Dobrzyń and Kaszuby, were subjected to the greatest uplifting and the sediments covering them to the deepest erosion. Hence, in the marginal area of the platform, above the granite massifs, there is a lack of deposits of the Podlasie stage (Silurian) in spite of their observed presence farther NE within the axial zone of the peribaltic syneclise.

The development of positive local structural forms in the sedimentary complex started during the initial period of sedimentation. In the Cambrian this was expressed mainly in the decreasing thickenss of particular stratigraphic members in elevated areas. This resulted, on the one hand, from the slower tempo of sedimentation in· elevated areas, or even its complete absence at certain periods, like for example in the Zaręby region, while on the other hand it is due to the more elevated structural elements being eroded during the sedimentary breaks, like for instance in the region of Barciany or Zaręby.

There are also differences in the lithological development of Cambrian sediments on elevated areas and outside of them, these are, however, less readily explained.

In the Lower Silurian, the more important local forms of elevation, such as the ridge of Zaręby or that of Olsztyn, showed an increase in the thickness of nodular limestones. Mudstone sedimentation dominated to the west in deeper zones. Hence, undoubtedly, the tectonic activity of the substratum favoured facial differentiation.

The principal phases of the formation of local tectonic forms are referable to the end of the Silurian (the ardenne phase). This being probably connected with the folding of the Caledonian Chain in the western forefield of the Platform, also to the turn of the. Lower and Upper Carboniferous (the Sudetic phase).

The positive structural forms developed during the older Paleozoic were expressed in an increase in the thickness of the Zechstein anhydrites, mainly the anhydrite of the Lower Z₁ cyclothem.