Lithological characteristic of the channel facies of alluvium of Early Pleistocene terrace of the pra-Stryi river in the Bolekhiv section
Authors: Yatsyshyn Andriy , Khrypta Oleksandr 
Published: March 2020

The article highlighted main lithologic characteristics of channel facies alluvium, which is the most ancient within Morshynska hill of pra-Stryi terrace. Upper and central layers of alluvium section are found to have lenticular structure fixing lines of shallow (up to 1.0–1.2 m) palaeochannels 4–5 wide.
Granulometric composition of alluvium of channel facies varies little up the incision. Its main component is the pebble and its content except the first one exceeds 50%. The content of other less spread components of the alluvial layer remains almost unchanged. The fact that there are no vivid changes in the granulometric composition of alluvium testifies the stability of conditions of accumulation of sediments in the bed course of palaeoflow.
Petrographic composition of coarse fraction appeared to be immensely homogeneous due to the dominance of Carpathian sandstones. In particular, all boulders are composed of light-to-dark, fine-to-medium grained sandstones. Siltstones and silicites appear in the composition of pebble and gravel only, their content is insignificant and ranges from 1 to 6 %. Siltstones are yellowish-white and yellowish-brown, and silicites are dark grey and almost black.
Analysis of roundness of coarse fraction alluvium has shown the predominance of fine-to-medium rounded grains in its content that, in its turn, testifies to long-distance transportation of fragmental matter. Besides, better roundness of clast is traced up the section. In particular, the number of poorly rounded clast in the boulders is decreasing and even well-rounded grains are appearing; the content of pebbles shows the decrease of poorly rounded clast and gradual increase of well-rounded pebbles.
The samples of disc-like shape prevail in sampling material. Only boulders of sample 3 do not contain them at all. Samples of other shapes are less spread; first of all, it is spheroid and roller fragments. The lower part of the alluvium thickness contains the smallest amount of them. The content of blade fragments is the most consistent.
Measurements of the imbrication of fragments testified them to have moved in the lower part of the alluvium thickness from northwest to the southeast that coincides with hypothetic flow of palaeo-Stryi during the formation of the terrace under investigation. The predominant direction of the palaeoflow in the upper part of alluvium accumulations has changed; there dominates the direction from southwest to northeast, which is closer to the direction of palaeo-Sukil flow.
Taking into account relatively coarse content of alluvium, particularly, significant content of boulders and big pebbles and predominance of well-rounded fragments practically in all samples, it can be assumed that alluvium deposited in the bed of the pra-Stryi river, that flowed along the Carpathian escarpment to the southeast to the Svicha river during the formation of the terrace. Change in the direction of palaeoflow in the upper part of alluvium stratum may point out a local change in the direction of this river flow.
Key words: terrace, pra-Stryi, alluvium, granulometric composition, petrographic composition, roundness, shape of fragments, Carpathian material.