The duality of varved clays above the Bug-Narew River

Abstract

The study presents an analysis of geological profiles from the Bug–Narew valley area, where two distinct series of varved clays were identified, separated by congelifluction deposits and a fossil soil horizon. Investigations conducted at the Wierzbica and Orzechowo sites enabled a detailed reconstruction of the sedimentation environments and subsequent periglacial transformations. The lower series of varved clays overlies silty–sandy deposits showing characteristics of fluvial transport under periglacial conditions. Above it lies a congelifluction zone and a fossil soil with low clay fraction and low calcium carbonate content, indicating an interstadial warming phase. The upper series of varved clays contains involutions and shows a disappearance of varves toward the top, caused by modern soil-forming processes. Sedimentological, structural and geochemical analyses confirm the environmental distinctness of both varve series and the presence of a subaerial depositional break between them. A comparison of geochronological diagrams revealed no correlations between the two varve series, demonstrating that they formed in two separate lake basins and are separated by a periglacial period marked by active congelifluction and development of a tundra-type fossil soil. Morphological evidence and correlations with existing literature suggest that the lower varve series corresponds to the Warta glaciation, while the interstadial deposits represent the phase preceding the recessional stage of the Middle Polish Glaciation.