Низькі тераси долини Стиру в середній частині Волинської височини

Abstract

The paper focuses on the description of the low Pleistocene terraces (with the height of up to 20 m above the river level) of the Styr River valley that were identified in the middle part of the upland between Bilche–Krasne villages in the south and Boratyn–Pidhaitsi villages in the north. The main attention is concentrated on the mapping of the terraces and the overview of their outcrops as well as analysis of the boreholes data. The study of the geological structure of the terraces becomes the basis to determine their age. Mapping of river terraces was executed using topographic maps in scale 1:10 000. On the territory where previous researchers identified one or two terraces, we have identified five morphologically distinct cyclic (paired) terraces. The highest low terrace that is widespread territorially is named here the main terrace and the other four that are locally preserved are combined in a complex of minor low terraces. The level of the main Styr terrace has almost constant altitude of about 191–192 m. The average height of the terrace surface is gradually increasing downstream from 12–13 to 14–16 m above river level. Its surface is slightly undulating and complicated by relatively large isometric depressions with the diameter of up to 100 m and more. The slightly meandering forms similar to channels with the plan size comparable with the modern Styr channel occur on the terrace surface. The main terrace is relatively complicated and has mainly two-termed structure – the lower part of sedimentary sequence is composed of normal (perstrative) alluvium of large thickness; the upper part is composed of constrative alluvium accumulated when the river channel was laterally inactive. The terrace is overbuilt by a loess layer a few meters thick that covers floodplain alluvium without a distinct break in accumulation. Geological structure of the minor low terraces is relatively simple and recalls the floodplain structure. Absence of well-expressed paleosols in the alluvial and loess deposits of the low terraces suggests that they were formed during Late Pleniglacial (MIS 2). Key words: river terrace, alluvium, Late Pleistocene, Late Pleniglacial, Volhyn Upland, Styr River valley.