ИСТИНА

Borisoglebsk Upland is one of a number of the Moscow glaciation marginal zone uplands in the Russian Plain center. Evolution of its fluvial network began since the last deglaciation. It is generally accepted that the density of glacifluvial network and its penetration into interfluves significantly exceeded those of modern fluvial systems. However, our knowledge on the postglacial incision-infill cycles, their drivers and timing is very limited, especially at the local scale. In this study, we attempt to reconstruct the Northeastern Borisoglebsk Upland fluvial systems development over the last 150 ka and determine its past extents at different stages based on palaeoarchives comparison from presently inactive interfluve headwaters, contemporary gullies and small valleys. Interdisciplinary research within the ≈15 km 2 case study area involved detailed description of cores and sections, analysis of grain size, chemical and organic contents, pedogenic and microstructural properties of associated deposits, dating, investigations of the present-day and buried topography, remote sensing data interpretation, and DGPS survey. Integrating the available results, at least four distinct fluvial incision stages were determined while network extent has significantly changed through the Late Pleistocene. Three – Late Moscow (Saalian), Late Valdai (Weichselian) and Late Holocene – can be generally associated with the Nero Lake level decrease and accompanying isolation of the 130, 100-105 and 95-98 m ASL terraces respectively. For the late incision, human impact has also been an additional trigger. The Late Moscow and Late Valdai incisions mostly determine general outlines of modern fluvial systems while the latter appears to be most expansive. The Late Valdai gullies cut only middle reaches of the initial fluvial network and eastern slopes of highest hills. In contrast, the Early-Middle Valdai incisions of limited distribution and problematic causes were most likely associated with permafrost degradation impacting surface runoff formation. Incision stages were separated by landscape stability or aggradation periods. Upper parts of the Late Moscow depressions and fluvial incisions were gradually infilled and smoothened by Valdai lacustrine sediments and Late Valdai-Holocene colluvial deposits. In contrast, in middle parts of the fluvial network, two complex periods included heterogeneous aggradation interrupted by stabilization and smaller but prominent incisions. For the Mikulino (Eemian) interglacial prolonged lacustrine sedimentation was altered by stabilization and incision in the middle (partly eroded peats with tree logs). For the most of Holocene at least some of superimposed colluvial infills and local incisions can be attributed to short-term events as forest fires. It is particularly important to note that most of the detected incision stages were asynchronous between headwaters and middle reaches and spatially non-uniform. Thalwegs of major Late Moscow incision and local incisions during the Mikulino interglacial generally do not coincide with the following Late Valdai and Late Holocene incisions.