Tremadok obniżenia perybałtyckiego

Bronisław Szymański




Within the Polish part of the Peribaltic depression a series of conglomerate-sandstone-clay rocks of Lower Tremadocian age (Pakerort) is found to rest on an uneven, erosionally formed surface of the Cambrian basement. The rocks make here a fragment of distinctly separated transgressive sedimentary cycle that closes a long-lasting depositional period of the Wendo-Cambrian clastic formations. As a rule, the Tremadocian rocks are covered with the transgressive glauconite-arenaceous deposits of Lower Arnigian age (Laptorpian).
A marked regional sedimentary unconformity and a considerable stratigraphic gap occur both between the Cambrian and Tremadocian and between Tremadocian and Arenigian (Z. Modliński, B. Szymański, 1972).
The rock complex of Lower Tremdocian age of the Peribaltic depression is naturally subdivided into the following lithostratigraphical complexes: lower – conglomerate complex, middle – sandstone complex, and upper – sandstone-clay complex. The age of the clastic formations of Treadocian age in the Peribaltic depression area is documentated by: Obolus apollinis Eichaw., Obolus cf. apollinis Eichw. and Obolus sp. The thickness of the complex of Tremadocian formations ranges from 0,05 to 2,5 m.
Two distinctly visible elements of higher occur in the early Palaeozoic sedimentary cover within the Peribalyic depression: lower – clastic complex, and upper – glauconite-carbonate-marly complex. In the lower clastic complex of the sedimentary cover at least five individual sedimentary cicles can be distinguished. So far only three younger cycles have examined, i.e. Middle Cambrian cycle, Upper Cambrian cycle and Tremadocian cycle. The distinguished cycles separate four periods of emergence, intensified palaeotectonic activity and epigenetic erosion, which develop at the boundaries Lower Cambrian/Middle Cambrian (pre-Middle Cambrian phase), Middle Cambrian/Upper Cambrian (pre-Upper Cambrian phase), Upper Cambrian/Tremadocian (pre-Tremadocian phase), Tremadocian/Arenigian (pre-Arenigian phase).
The Tremadocian epicontinental basin was situated between the Finnish continent in the north and the Pannonian massif in the south. Eastwards, it reached the vast Sarmatian continent that made the land of Ukrainian shield and the Byelorussian land adjoining to it in the north-east. In the western part the epicontinental basin was restricted by a zone of deep fractures, i.e. by the so-called Teisseyre’s line, along which it bordered on the marginal zone of the Caledonian geosyncline (J. Znosko, 1969; S.Marek, J. Znosko, 1972).
Tremadocian and Arenigian marine transgressions expanded from west to east, i.e. the deep basin of the Caledonian geosynclines was – as during the Upper Carboniferous time – the alimentation basin of their waters. The activity of the zones of deep fractures, which at the beginning of Tremadocian, and then during Arenigian were responsible for their general and considerable plunging, are considered to be the main factor allowing the vast areas of the platform – herein also the Peribaltic depression – to be inundated during the two transgressions.
In the present-day area of the Tremadocian the deposits of this age occur only in the central part of the Paribaltic depression, where they make a number of small, isolated and irregularly distributed patches. The eastern and western parts of the depression lack and Tremadocian sedimentary cover at present. Various members of Cambrian age – Middle Cambrian in the east, and Middle or Upper Cambrian in the west – are overlain with the Arenigian rock complex (Z. Modliński, 1967; F. Stolarczyk, S. Tyski, 1972b).
The structural position of the preserved relics of the primarily uninterrupted and more complete cover of the Tremadocian deposits has been determined by the pre-Arenigian tectonic movements and the accompanying erosional processes. These have dismembered the Tremadocian cover and its substratum into the individual blocks which, due to the diversity of the palaeostructural position, have been affected, with various intensity, by the destroying gradation processes. As a result of this the Tremadocian formations underwent a general destruction and are now preserved fragmentarily only, mainly in the tectonic depressions of the Cambrian substratum.
On account of the lithofacial and biofacial nature and the sequence of rock types the Lower Tremadocian formations (Pakerort) found to occur in the Peribaltic depression strongly resemble the equivalents of the Lithuanian and Estonian zone of the Peribaltic region, also of the same age (R. M. M¬nnil, 1966; W. A. Korkutis, 1963,1965, 1968).


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