Analiza facjalna utworów czerwonego spągowca w północnej części monokliny przedsudeckiej (rejon Poznań - Śrem)

Paweł Henryk Karnkowski


Facies analysis of the Rotliegendes in the northern part of the Fore-Sudetic Monocline (Poznań-Śrem region)

On data obtained from the boreholes . in the northern part of the Fore-Sudetic monocline the lithology, lithostratigraphy and. sedimentary conditions are here presented of the Rotliegendes together with suggestions respecting the synsedimentary tectonic movements during the Lower Permian. Differences in the facies and thickness of the clastic deposits are due to the synsedimentary Lower Permian block·tectonics. During the Rotliegendes the stresses of the vertical movements of the substratal blocks varied, favouring redeposition, many times repeated of the sediments

The considered area covers the northern part of the Fore-Sudetic monocline in the Poznań-Śrem region. The Permian substrate is built of strongly diagenized sandstones and muddy shales varying in dip. The spores they have yielded (Krawczyńska-Grocholska & Grocholski 1976) suggest their age as the Lower Carboniferousand the lower part of the Namurian. The Lower Permian deposits supply no paleontological data so that the stratigraphic division has been based on their lithological characters. On the basis of borehole profiles (Figs 1-3) and on correlation with other regions (cf. Kłapciński 1971, Głowacki 1973) the Rotliegendes here may be divided into two sub-groups, and an informal lithostratigraphic division may be suggested for the Poznań-Śrem region (Table 1).

The oldest Rotliegendes deposits in the Poznań-Śrem region are those of the Dolsk formation. The lithological variability of this .formation, ranging from conglomerates in the bottom (the Kaleje conglomerate member) through conglomeratic sandstones and sandstones to muddy sediments in the top, reasonably suggests one only sedimentary cycle during which· the sediments were deposited in a flowing-water environment. The wide expansion and the facies variability of sediments in the Dolsk formation indicate their deposition to have taken place within an area of well developed morphology, and their transport direction from the NE to the SW (Fig. 4).

The trachybasalt formation of Wyrzeka has probably developed along the Środa-Donatowo fault. The maximum thickness of volcanic rocks has been observed in the Wyrzeka region and it is there that was probably situated the centre of the volcano disgorging the lava flows, mostly in the north-eastern direction (Fig. 5). The quartzitic porphyries formation of Młodasko is confined to the north-eastern margin of the Poznań-Śrem region. It has been observed only in a few boreholes and probably occurs as a continuous lava cover. These porphyries overlie a Lower Carboniferous substrate.

The volcanic activity was followed by a period of erosion due to the Saale phase. The conglomerates encountered in some of the boreholes, mostly overlying the Autunian rocks, may be reasonably regarded as the main conglomerates of the Saxonian sedimentary cycle. The presence of these conglomerates, differentiated as a member of the Polwica conglomerates, is connected - similarly as the Autunian. deposits - with the Środa-Donatowo dislocation zone. The variable thickness and lithology is probably due to the existence of different alimentary areas. A similar picture of the basin whose outline is traceable in the Kaleje region can be concluded upon the facies analysis of the Solec conglomerate member or that of the muddy layer within the Saxonian sands tones (Figs 6-7).

The sandstone formation of Siekierki occurs practically throughout the area, the southern part excepted where there are no Rotliegendes sediments. The thickness of the sandstones here varies strongly from almost a zero figure to 1000 m in the Kórnik 1 borehole. The great differences in thickness may be explained by the existence of several dislocation zones confining the particular substratal blocks along which the vertical movements had taken place (Figs 8-9). A picture of the extent of translocations is shown in the thickness map where the increase in thickness of the sediments may reasonably be supposed to correspond to the extent of trans-locations (Fig. 8). The vertical movements were gentle in character and did not lead up to the uncovering of the Lower Carboniferous substrate. Nontheless, such sedimentary conditions favoured a many-times repeated redeposition of the sediments accompanied by changes in their transport direction.

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