Late Pleistocene chronostratigraphy of infills and incisions based on 3D-modelling of a local watershed sediment sink structure (Borisoglebsk Upland, Central European Russia)

3D-modelling based on a series of geological exposures and cores up to 7 m deep allowed to reveal the structure, stratigraphy and chronology of a local sediment sink on the watershed of one of the Late Saalian glacial uplands at the center of the Russian Plain. This local section representing an infill of a kettle hole was initially occupied by the shallow stagnant water body during the Eemian up to the Late Weichselian, which had periodically dried and transformed into the forested bog. The Late Pleistocene to Holocene transition was associated with most dramatic environmental changes and abrupt fluctuations. Response in the local geomorphodynamics first involved activation of mass movements followed by the gully incision reaching the site by regressive head knickpoint retreat. The observed sediment record provides evidences of at least 4 linear erosion incision-infill cycles. The first incision into the gradually undulating surface composed by lacustrine and colluvial deposits and its infill by stratified colluvial silts occurred not later than 6.5 cal. ka BP. It is fixed by the next incision phase infilled by pedo-sediments of reworked humic, eluvial and sub-eluvial horizons of the upper slope soils enriched by pyrogenic charcoal. The third incision stage can be linked to regressive growth of the main gully head and its branches (ca. 1.4 cal. ka BP) triggered by the onset of cut-and-burn agriculture practices determining conditions favorable for active deposition of agrogenic colluvium. Modern gully incision was preceded at least by another infill phase and occurred between 1941-1968 AD.