Litostratygrafia czerwonego spągowca w Wielkopolsce

Abstract

Na tIe zróżnicowania litologicznego utworów czerwonego spągowca w Wielkopolsce zaproponowano, w myśl Zasad polskiej klasyfikacji ... (1975), wyróżniać jednostki formalne: formacje, ogniwa, podgrupy i grupę. Ponadto omówiono niektóre wyróżnione nieformalnie alloformacje (P.H. Karnkowski, 1987).

LITHOSTRATIGRAPHY OF THE ROTLIEGENDES IN WIELKOPOLSKA (WESTERN POLAND)

The author divides the Rotliegendes formations in Wielkopolska into the following formal lithostratigraphical units (Tab. 1): the Dolsk Formation - between the folded, Lower Carboniferous basement and the volcanites; the Wyrzeka Volcanite Formation - where the formation borders are determined by the bottom of the first and the top of the last volcanite occurrence; the Książ Wlkp. Conglomerate Formation - the main coarse·clastic rocks originated from destruction of the Wolsztyn Ridge: and the Siekierki Sandstone Formation - with a dominated sandstone member. Within the Siekierki Sandstone Formation the Polwica Conglomerate Member and the Solec Conglomerate Member are distinguished.

Twenty three borehole profiles (Tab. 2, Figs. 1-5) have been chosen to present the formal litho· stratigraphical units. The Dolsk Formation (Figs. 2, 6) consist of the grey, black and red·brown clastic rock complex, fine· and medium.grained sandstones dominate in its lower part, and mudstones and claystones with accessorial, sandstones and conglomerates in its upper part. This formation can be divided, as an auxiliary into two parts regarding to the colour of sediments (in a cat he gory of allostratigraphical units). In this respect the lower part of grey and black sediments (the Upper Carboniferous) may be distinguished as the Kaczawa Alloformation, and the upper red· brown part as the Kwisa Alloformation. Changes in sediment colour in this case result in mainly palaeoclimatic changes.

The Wyrzeka Volcanite Formation is composed of various volcanic rocks: in the west of the Poznań - Oleśnica Dislocation Zone - mainly acid volcanic rocks and their tuffs, in the east - neutral volcanic rocks and their tuffs (Fig. 7). The volcanites do not form a continuous cover, they occur in separately preserved lobes. It is presumed however that originally they covered much larger area (Fig. 8). Rocks occurring in the Wyrzeka region are mainly trachybasalts. Quartz porphyris have been documented in the west from the Poznań - Oleśnica Dislocation Zone (Figs. 4, 5, 7). All the volcanic rocks in the area show a highrated alteration (e.g. silification). Based on chemical composition the acid volcanic rocks can be classified as rhyolites and rhyodacites.

The second rock formation is composed of quartz porphyries, that may be distinguished macroscopicaly from actual volcanic rocks due to the sedimentary rock debris presence. Mineral composition of tuffs does not differ radically from the extrusive rocks. The sedimentary rock debris found frequently in tuffs originated most commonly from the Lower Carboniferous basement. Besides it has been stated that flowing lavas formed the lava flows what has been emphasized by parallel structure. The Książ Wlkp. Conglomerate Formation form mainly conglomerates and breccias (Figs. 3 - 5). The Wolszyn Ridge peripheries (Fig. 9) is a typical area of the occurrence. In the northern side of the ridge these sediments have a wider range and greater thickness than in the south (Fig. 10). The breccias and conglomerates 'are chiefly composed of debris of the volcanic rocks and Palaeozoic basement rocks. Selection of the coarse·grained material has not been stated in the KsiClZ Wlkp. conglomerate profile. Lithological composition of the coarse·grained material is also variable.

The Siekierki Sandstone Formation is mainly composed of sandstones (Figs. 2, 4, 11). Quartz with a variable degrees of gradation and roundness is a chief lithologic component of the sandstones. Besides, the potash feldspars, plagioclases, sedimentary and volcanic rock chippings occur. The variability of granulation is another factor characterizing the sandstones. The medium- and fine-grained sandstones are most frequent, the coarse- and various-grained are also found. Most sandstones are oblique bedded.

The Polwica Conglomerate Member is separated from the lower Siekierki Sandstone Formation (Figs. 2. 3, 5). It is mainly composed of volcanic rock debris. The Poznań - Kalisz Dislocation Zone and marginal zones of lava covers are typical area of the occurrence (Figs. 12, 13). Four detailed profiles are presented to draw nearer the rock characteristics (Fig. 14). Based on sedimentological and lithological characteristics it can be stated that sedimentary conditions of the Polwica Conglomerate Member and the Książ Wlkp. Conglomerate Formation were similar.

The Solec Conglomerate Member bas been distinguished in the middle Siekierki Sandstone Formation (Figs. 2. 3). This member occupies relatively small area in Wielkopolska (Figs. 15, 16). Based on the lithological differentiation of conglomerates two following regions can be discriminated: the Kaleje-Solec region (mainly trachybasalt debris) and the Klęka region (white quartzitic sandstone debris). To characterize the Solec Conglomerate Member it can be stated that identical features and phenomena occur here and in the other coarse silastic complexes.

The Lower Silesia Subgroup has been distinguished to link the volcanic and subvolcanic sedimentary rocks in one unit (Tab. 1). Such a link has had longlasted tradition in science research.

The Wielkopolska Subgroup links supra volcanic sedimentary rocks of Rotliegendes in one lithostratigraphical unit (Tab. 1). To present the geological structure of the said subgroup the map of the base. the quantitative-lithofacies map (Fig. 17) and the isopachous map (Fig. 10) have been made. The Rotliegendes Group consist of the Lower Silesia Subgroup and Wielkopolska Subgroup Formations (Tab. 1, Fig. 18).

The synthetic geological map presenting all the formations in Wielkopolska is a recapitulation of the Rotliegendes complexity. Mosaic occurrence of particular formations and members has caused the discrimination of 17 various mutual combinations of lithostratigraphical units (Fig. 19).