The structure of the lower terraces of the Dniester River valley (on the example of the Vasyliv-Doroshivtsi section)

Abstract

The whole complex of terraces can be traced in the relief on the Vasyliv-Doroshivtsi area. Well-defined high terraces - XII (jr-kz), XI (bd-sv) and X (bv-br). The middle terraces in this area are narrow, poorly preserved in relief. The inner-canyon part of the Dniester valley is relatively wide, which contributed to the development and preservation of a full complex of lower (late Pleistocene) terraces: the fourth (kd-ts), third (pl-ud; fragmentary), second (vt-bg), first (df-pc) terraces, as well as Holocene high and low floodplains (hl). Researches of features of the structure of low terraces carried out on the basis of a survey of geomorphological surfaces, outcrops of terrace deposits, the analysis of boreholes, fossil vertebrate and mollusc faunas, and also results of long-term observations during archaeological researches of site Doroshivtsi 3. The first above-floodplain terrace (df-pc) is represented by small fragments. It is composed by channel facies of alluvium, represented mainly by pebbles. Above them the floodplain facies - fine- and medium-grained sand, light grey, obliquely and horizontally layered, are bedded. The floodplain alluvium is covered by a low-thickness stratum of loess-like loams (pc). To the west of Doroshivtsi Village, near the Hlynyshche site, the upper layer of sediments is partially destroyed and buried under the sediments of a high floodplain. Only the remains of the lower part of the section are partially present. That is, in this area, the roots of the first terrace serve as the base of the high floodplain. The second terrace (vt-bg) is most clearly manifested morphologically due to the ledge, which stands out quite clearly. It is better developed within the Vasyliv Village. Here it is quite extensive, its surface is planned. In the area of Doroshivtsi, this terrace is narrow, severely damaged, occurs in fragments. The basement of the terrace is located at the height of about 1 m above the current level of the Dniester River. It is composed of dark green-grey argillite and coarse-grained Devonian sandstones. The structure of the terrace is as follows: the channel facies (gravel-pebble deposits with inclusions of boulders and sand aggregate) and powerful floodplain facies of alluvium: rhythmic layers of sands, sands and loams with signs of embryonic soils); complex of fossil soils and forests (forest-like loams) - bg-pc. Numerous remains of molluscs, mega- and microfauna, remains of material culture were found in the terrace deposits (Doroshivtsi 3 site). The third terrace (pl-ud) is weakly expressed and narrow. There are no outcrops of its sediments here and no boreholes. Therefore, currently, its structure in the study area remains unknown. However, in the lower part of the sediments of this terrace during the digging of the well were found fragmented remains of a mammoth (Mammuthus sp.) and a red deer (Cervus elaphus) with traces of disposal (site Vasyliv II). At another location, a mammoth tooth was found, which we identified as Mammuthus primigenius cf. jatzkovi. Fourth terrace (kd-ts). Terrace sediments are exposed to the west of Vasyliv, also discovered by a borehole near the basement of the white-stone temple of the XII-XIII centuries. According to drilling data, the top of gravel here is at the height of 7 m above the boundary level of the Dniester River. Above the gravel are bedded (from bottom to top): a layer of chernozem-like paleosol (pl); the light yellow loess with a columnar structure (ud); brown paleosol (vt); the pale yellow, carbonate loess, with also a columnar structure (bg). According to the terraces' classification by their location in the river valley, low terraces in the study area of the Middle Dniester are incised. According to the genetic classification, they can be considered accumulative, closed- or hidden-socle. Thus, based on the row of data, the peculiarities of the structure of low terraces, the composition of sediments and their approximate geological age have been established. New sections of the second and fourth terraces are described based on the study of outcrops and drilling data. New locations of vertebrate and mollusc fauna have been discovered in the deposits of 2-4th terraces in the village of Vasyliv. The considerable width of the second floodplain terrace in the Vasyliv area in comparison with Doroshivtsi testifies to the prospects of finding here the remains of material culture, similar to the Doroshivtsi-3 site. As a result of research, we established that due to the different history and mechanism of sediment formation, the structure and composition of sediments of the same age terraces may differ. This fact should be considered when allocating terraces and rely not only on the features of the cover part of the sediments but on the set of characteristics of a terrace. The questions of the peculiarities of the fourth and first above-floodplain terraces' structure, as well as the depth of basement of the high floodplain, remain unclear.