Sedimentary record of the late Neogene avulsion process at the Tomisławice lignite opencast mine in central Poland – a new hypothesis

Abstract

Around the town of Konin in central Poland, lignite deposits are exploited in opencast mines. Surface mining has enabled the discovery of many sediments and structures, both tectonic and sedimentary. However, the greatest research challenge appeared in the Tomisławice lignite opencast mine in 2022, when a so-called lignite-free zone was found during mining activity. Initially, due to limited data, its genesis was associated with syn- or post-depositional tectonics and peat-to-lignite compaction. In 2024, two deeper boreholes were drilled in which no lignite and tectonic denivelations of the Mesozoic bedrock were detected in the mentioned zone, meaning that the above hypotheses were disproved. Therefore, in this paper a new hypothesis was proposed for the creation of the aforementioned lignite-free zone crossing the Tomisławice lignite deposit – palaeochannel I – and as well as marking its NE border – palaeochannel II. The inclusion of data from a larger number of boreholes and the reinterpretation of the depositional architecture of the fills of the lignite-free zones indicate the palaeochannel avulsion of the late Neogene fluvial system. The palaeochannels are filled with fine-grained and multi-coloured Poznań Clays. Unfortunately, they are mainly massive, and the poorly visible sedimentary structures are masked by post-depositional weathering processes. The palaeochannels were incised into the underlying lignite seam and sub-lignite sands during the initial stages of major floods and then filled by the accretion of heterolithics, mainly from suspension, during subsequent flood episodes. Rather than a tectonic/compactional origin, data are consistent with palaeochannel avulsion. Finally, the current paper is the first in Poland devoted exclusively to the effects of avulsion in the rock record.