The Mio-Pliocene muddy deposits of the Wielkopolska Member (Poznań Formation) cap the Neogene succession in more than 75,000 km2 of Poland. These mud-rich overbank sediments comprise the ribbon-shaped fluvial paleochannels filled with sand, mud, or a mixture of both. The study area covers a shallow Neogene tectonic graben that contains a lignite seam several meters thick overlain by the Wielkopolska Member. Channel-body geometries can generally be defined as broad ribbons with width/thickness (W/T) values ranging from 9.2 to 15.5 (avg. 12.7). Such low W/T values are typical of laterally stable channels and indicate that the riverbanks were stabilized by cohesive deposits (e.g., muds), and possibly vegetation.
The paleochannels are filled by fine-grained sediments consisting, in order of abundance, of fine-grained sands to silty sands and clayey silts (muds). Occasionally, a mixture of gravels, sands, and fossilized wood fragments (xylites) corresponding to a channel lag, are also present. Stratification of the deposits is very diverse in individual paleochannels. In the more sandy channel bodies the sediments tend to be massive, whereas in channels with a higher content of silty and clayey particles sedimentary structures are much more common. Facies analysis clearly shows that the stratified fine-grained deposits (fine sands, muddy sands, and clayey silts) were formed by low-density, tractional turbulent flow during high and low water stages. In contrast, the poorly stratified and massive sediments (pure fine sands and silty sands) were deposited by high-density flows during rising and falling water stages. The increase in suspended particles in the river channel can be linked with erosion of muddy banks during rising stages and/or with floodwaters that eroded freshly deposited fines from the overbank zone flowing back into the channel during the falling stage.
The detailed plan view of the examined paleochannel pattern indicates that individual reaches of the river system were both tributive and distributive on a local scale. This pattern comprises a few sandy-muddy-filled, straight to sinuous channels separated by islands built of strongly cohesive overbank muds. Furthermore, there is evidence that both the planform and facies architecture of the river system were most likely controlled by a combination of graben tectonics and compaction of an underlying peat or lignite seam. The studied fluvial succession represents an anastomosing river system developed ∼ 1.5 My after the Mid-Miocene Climatic Optimum (MMCO), when the climate in the central Poland area began to change from temperate, humid, and wet to predominantly semiarid and cool with evident seasonality.