Palaeogeographical conditions of the formation of the channel alluvium of the high (Susidovychi) terrace of the Dnister River in the Kulakivtsi section (Podillia-Dnister region)
Authors: Yatsyshyn Andriy , Bogucki Andriy , Olszewska-Nejbert Danuta , Bąbel Maciej 
Published: July 2019

The main lithological characteristics of the channel facies of the Susidovychi terrace, which correspond to the high (situated over the canyon) groups of Dnister terraces, were given. The investigations in the gravel pit at Kulakivtsi proved that the alluvium of the terrace has been formed in two stages. During the first stage, the lower 1.5 thick bed of the alluvium was deposited. The upper one, nearly 4 m thick alluvium bed was formed in the next, second one depositional stage. The stages of the alluvium formation were recorded in the changes of the granulometric and petrographical composition of the alluvium, and of the roundness of the coarse-grained clasts. The transition between these two alluvial beds is outlined by the voluminous intercalations of the sand lenses, and also by a change in the colour of the alluvial deposits. Granulometric composition of the alluvium of the channel facies, in general, changes little in the section. Only in the transition zone from the lower to the upper part of the alluvial deposits, there is a sharp, abrupt increase in the content of gravels and a sharp reduction in the content of boulders. Generally, in the composition of the alluvium two dominant and two subordinate components are clearly identified. The pebble grains and the sandy-clay matrix are the dominant components, whereas the gravel grains and the boulders are less common. Petrographic composition of the coarse-grained fraction of the terrace alluvium proved to be the richest of all the so far investigated sections of the high (situated over the canyon) terraces of the Dnister River (at Kunysivtsi, Ivane-Puste, Repuzhyntsi, and Lysychnyky). The pebbles 40–100 mm in diameter show particularly diverse petrographic composition which includes fragments of nine types of rocks: the four of the Carpathian provenance (sandstone, aleurite, cherts, and quartzite), and the five of the Podillian provenance (red-coloured Devonian sandstone, Albian cherts and sandstone, and lithothamnian and cryptocrystalline chemogenic limestone). More precisely, the richest is the lower part of the alluvial deposits where the fragments of all the nine rock types occur. The upper part of the alluvium is markedly poorer because only the five rock types occur there. Diversity of the petrographic composition of the alluvium decreased by reducing the local Podillian types of rocks, which are represented only by the red-coloured Devonian sandstones. The composition of the Carpathian types of rocks remained unchanged. The detected changes in the granulometric and petrographic composition of the alluvium of the investigated terrace permitted to show that the principal providers of the local (Podillian) debris of rocks was played by the Podillian tributaries of the Dnister River. The Dnister alone transported mainly the Carpathian material and only the small volume of Podillian rocks represented by the debris of the red-coloured Devonian sandstones. It was also found that the accumulation of the alluvial deposits of the Susidovychi terrace in the Kulakivtsi section took place in the conditions of restructuring of the Dnister palaeodrainage system. In the initial stages of this terrace formation, the palaeo-Dnister was directed from the village Dobryvliany further north than today, and it entered into the present-day Tupa River valley at environs of the village Bedrykivtsi. In the vicinity of the village Bedrykivtsi, the palaeo-Dnister was turning sharply eastward and proceeded along the present-day river valleys of Tupa and Seret. In the later stages of the Susidovychi terrace formation, the palaeo-Dnister left the portion of its valley stretching between the villages Bedrykivtsi and Schytivtsi, and it shifted several hundred meters to the south and has stopped practically within its current canyon valley. The desolate portion of its palaeo-valley located between the villages Bedrykivtsy and Kasperivtsi has been inherited by the Tupa River and the lower portion of this palaeo-valley located between the villages Kasperivtsi and Schytivtsi – by the Seret River. Key words: palaeo-Dnister, over the canyon terraces, Susidovychi terrace, alluvium, granulometric composition, petrographical composition, roundness, Carpathian material, Podillian material.