Two stratigraphically distinct main lignite seams have been mined in the Konin Basin (central Poland) for decades. These seams originated during two peaks of the Middle Miocene Climatic Optimum, when widespread mires developed as backswamps in the vicinity of river channels. Weather fluctuations likely caused frequent flooding, recorded within the lignite seams as siliciclastic lithosomes or high concentrations of mineral matter in specific lignite layers (e.g., high ash yields). Facies analysis of selected sandy lithosomes in the opencast lignite mines near the town of Konin (Lubstów, Jóźwin IIB, and Tomisławice) illustrates a wide range of sedimentary facies and deformational structures. While some of these facies formed subaerially, others were deposited under subaqueous conditions in shallow, ephemeral lakes on the surface of Middle Miocene mires. Nonetheless, all the studied sediments are typical of crevasse splays or small lacustrine deltas, deposited near inferred meandering and/or anastomosing river channels. Consequently, the subaerial facies are attributed to high-energy, relatively short floods, whereas the subaqueous ones are linked to low-energy, slowly evolving regular floods. This hypothesis relies on the relationship between the rate of rising water levels in the river channel and the adjacent mire area, which is primarily controlled by the permeability of natural levee sediments.
