STRUCTURE AND DEVELOPMENT OF THE MIDDLE PLEISTOCENE TERRACE OF THE STYR RIVER IN THE AREA OF THE KHRINNYKY RESERVOIR

Abstract

The article presents new results of the study of the structure of the Middle Pleistocene terrace of the Styr River, probably the oldest among the river terraces preserved in the Styr valley. The main objects of the study were natural outcrops of alluvial and cover deposits, confined to the steep banks of the Khrinnytky Reservoir in the vicinity of the villages of Boremel and Naberezhne. The study was focused on the lower part of the section of terrace sediments, which lies under the Dubno (Vytachiv) soil horizon, which is correlated with the 3rd marine isotope stage (MIS 3). The elevation of the terrace surface above the palaeo-Styr water level is about 20–25 m (16–21 m above the average water level in the reservoir), and the thickness of the loess-palaesoil cover varies from 4–5 m, where the terrace height is minimal, to over 15 m , where the terrace surface is the highest. The main stratigraphic horizons of the Upper Pleistocene are reliably recognized in the section, including two soil horizons of the regional stratigraphic scheme - the widespread Dubno and locally preserved Horohiv (MIS 5). The latter is associated with the base of the loess-palaeosol sequence. The Horohiv pedocomplex is mostly lack in the loess-palaeosol sequences of the coastal cliffs, and if present, it is poorly preserved (truncated), exposed at different altitudes (6–12 m above the reservoir). The poor preservation of the Horohiv pedocomplex is a consequence of intense aeolian (blowing) processes that occurred prior to the formation of the terrace loess cover. The abnormally high altitudinal position of the Horohiv pedocomplex is associated with the accumulation of aeolian sands at the final stage of terrace formation (end of the MIS 6). The presence of paleocryogenic features (syngenetic frost veins and cryoturbations) in the upper part of the alluvium, which can be formed both in permafrost and deep seasonal freezing, indicates its accumulation in periglacial and/or subperiglacial conditions. Key words: river terrace; alluvium; Pleistocene; eolian processes; Volyn’ Upland; Styr River valley.