The Late Quaternary Evolution of the Upper Reaches of Fluvial Systems in the Southern East European Plain

Abstract

Networks of dry valleys (or balkas) and hollows in the upper reaches of fluvial basins in extraglacial areas in the Penultimate Glaciation (Marine Isotope Stage 6—MIS 6) regions of the East European Plain demonstrate clear incision/aggradation rhythms corresponding to global glacial/interglacial climate cycles. The first phase of each incision/aggradation rhythm began after the global glacial maximum and was characterized by a cool and humid climate, permafrost and sparse vegetation, when high surface runoff and active linear erosion formed a dense network of gullies. The second phase occurred at the glacial–interglacial transition and the subsequent interglacial period with its warm and humid climate and dense vegetation. This phase was distinguished by the partial filling of fluvial forms with slopewash deposits, the transformation of gullies into dry valleys (balkas) and the subsequent stabilization of fluvial forms marked by the formation of mature soils on the sides and bottoms of balkas. The third phase of the rapid accumulation of balkas developed during the cold and dry part of the next glacial epoch, resulting in the balkas becoming shallow hollows filled in with sediments. The last full incision/aggradation rhythm occurred in the late MIS 6 to mid-MIS 2. The erosion network formed during the late MIS 6 was almost completely filled by mid-MIS 2, and its manifestation in the modern topography is limited to a network of shallow hollows in the upper parts of the fluvial systems. The modern (incomplete) incision/aggradation rhythm began in the late MIS 2 and caused the formation of the modern erosion landscape in the upper reaches of fluvial systems. This rhythm is now in the stabilization phase, and the main accumulation phase of this rhythm is still far in the future.

Evolution of the Upper Reaches of Fluvial Systems within the Area of the East European Plain Glaciated during MIS 6

“…Most researchers agree that both the formation and major erosion phases in dry valleys (balkas) and old ravines in the Russian Plain occurred within cold phases of the glacial-interglacial climatic rhythms, while during the interglacials the sides and bottoms of small erosion forms were stabilized by soil cover [8,[34][35][36][37][38][39][40][41][42][43][44][45]. Pre-Holocene rise of erosion and subsequent stabilization after the onset of the Holocene has also been reported from different regions of Europe.…”

“…Fluvial sediments of MIS 4 to MIS 2 age were not found in the balkas and lozhbinas in the studied territory, though presumably at that period, slope deposits and mantle loams must have filled the erosion features, at least partly [37,40]. It is most possible that these sediments were carried out of small erosional forms during the next stage of incision in the Late Glacial [8,45]. The erosion event at the end of MIS 2 was less powerful than the one at the end of MIS 6.…”

“…A comparison of the evolution of the upper reaches of the fluvial systems in the areas of the East European Plain covered by the continental icesheet during MIS 6 and those which remained non-glaciated [8] shows both similarities and differences (Figure 14). General sequences of the major erosion events in the two areas are similar.…”

“…This similarity is associated with the common course of hydroclimatic changes over the entire East European Plain during two glacial-interglacial cycles, covering the time interval from MIS 6 to the Holocene (MIS 1). Three main events of high surface runoff are detected from the relief and lithology of the fluvial systems in the East European Plain: (1) at the end of MIS 6 [8,62], (2) at the end of MIS 2 [57,63,64] and (3) in the Late Holocene [65]. All of these events caused intensive erosion at the upper reaches of the fluvial systems both in the glaciated and non-glaciated parts of the plain.…”

“…Here we discuss processes that took place in the headwater elements of fluvial systems-dry valleys and gullies-between the boundaries of the MIS 2 and MIS 6 glaciations, evolution of these erosion features during the last 150,000 years and the main similarities and differences between the evolution of such elements in the glaciated and extraglacial territories [8]. The main information was collected by the authors in the key area situated in the southwestern part of the territory, close to the edge of MIS 6 icesheet, with additional data from published sources.…”