Structure, granulometric and mineralogical composition of unconsolidated sediments in the Sentsa River valley (Eastern Sayan, Russia)

Alekseev Sergei Vladimirovich ORCID: 0000-0002-3853-5022 Doctor of Geology and Mineralogy Head of the Laboratory; Laboratory of Hydrogeology; Institute of the Earth's Crust SB RAS 128 Lermontov str., Irkutsk, 664033, Russia salex@crust.irk.ru Other publications by this author     Alekseeva Liudmila Pavlovna ORCID: 0000-0001-5687-4143 Doctor of Geology and Mineralogy Leading Researcher; Laboratory of Hydrogeology; Institute of the Earth's Crust of the Siberian Branch of the Russian Academy of Sciences 128 Lermontov str., Irkutsk, 664033, Russia lalex@crust.irk.ru Other publications by this author     Vasil'chuk Yurii Kirillovich Doctor of Geology and Mineralogy Professor at the Faculty of Geography of Lomonosov Moscow State University  119991, Russia, Moscow, ul. Leninskie Gory, 1 vasilch_geo@mail.ru Other publications by this author     Pellinen Vadim Aleksandrovich ORCID: 0000-0003-2181-5594 PhD in Geology and Mineralogy Researcher; Laboratory of Hydrogeology; Institute of the Earth's Crust SB RAS 128 Lermontov str., Irkutsk, 664033, Russia vadim.a.pellinen@ya.ru

Svetlakov Artem Aleksandrovich ORCID: 0000-0003-2002-4790 PhD in Geology and Mineralogy Researcher; Laboratory of Hydrogeology; Institute of the Earth's Crust SB RAS 128 Lermontov str., Irkutsk, 664033, Russia sir.swetlackov@yandex.ru Other publications by this author

Introduction

Understanding and correct assessment of current global climate changes should be based on research and reconstruction of natural and climatic conditions of the past. The Late Pleistocene glaciation of the Eastern Sayan and Holocene deglaciation caused the formation of underground lakes on the Okinsky Plateau^[1]. In the periglacial region, there was a special situation of accumulation of fluvioglacial, lacustrine-alluvial, deluvial and proluvial sediments. The dynamics of sedimentation is generally characterized by the granulometric and mineralogical composition of sediments, which is formed under the influence of many factors – climatic and geological conditions, hydrological features, hydrodynamic regime of various water systems. Grain size is an important indicator for identifying the potential environment, movement and dynamic state of loose sediment particles during transportation. In particular, grain size changes in lake sediments usually provide important information about hydrodynamic changes in the catchment area, which corresponds to local climate change^[2,3]. In addition, the grain size of sedimentary rocks, the nature of the rolling and the degree of sorting make it possible to determine the source of demolition, the relative rate of transfer and the conditions of their formation. An increase in the volume of the sand fraction in the sediment and a decrease in the clay fraction indicates an increase in the velocity of water flows; the rolling of particles directly depends on the duration of movement; the degree of sorting is affected by the nature of the movement of the aquatic environment (turbulent or laminar) or the absence of such (lake reservoir). The mineral composition is also important for determining the sources of the pool's supply with detrital material and their changes over time ^[4-6]. The analysis of the structural features and material composition of sediments in combination with geological materials makes it possible to perform paleogeographic constructions for the Late Quaternary period in one of the basins of the Okinsky plateau, to clarify the structure of the Sentsa River valley, including the features of occurrence and distribution of permafrost rocks.

The aim of the work is to reconstruct the sedimentation regime of loose sediments in the Sentsa River valley in the Holocene based on the analysis of the granulometric and mineralogical composition of lacustrine-alluvial, alluvial and fluvioglacial deposits.

Research area

The Sentsa River cuts through the Okin Plateau (with an abs. of 1800-2500 m) within the southeastern part of the Eastern Sayan. Its valley is a trough with a trough-shaped transverse profile, steep slopes and a flat bottom, with an incision depth of up to 700-800 m. The length of the river is small – 55 km – and throughout its course intensively meanders, with the exception of some narrow sections where the riverbed straightens (Fig. 1). The valley contains relief elements formed by the activity of a glacier in the late Holocene and completely degraded 14 thousand years ago^[1]. The Sentsinsky glacier left a terminal moraine (boulder-pebble deposits, coarse-grained unsorted material) with a length of 8 km and a thickness of 90 m 8.5 km from the mouth of the river flowing into the Oka River. During the retreat of the glacier, underground lakes formed in the valley, in which lake sediments accumulated. The Sentsa River, constantly meandering and changing the location of its channel, processed unconsolidated sediments and accumulated fine-grained material (sandy loams, sands). After the descent of the paleozoic, there was a progressive comprehensive freezing of water-saturated loose sediments, the formation of mineral heave mounds over a significant area, and a specific cryogenic landscape of the valley.

In general, the thickness of quaternary loose sediments forming the valley and having different genesis (glacial, fluvioglacial, alluvial, lacustrine, deluvial and proluvial) is represented by clays, loams, sandy loams, sands, pebbles, boulders.

Fig. 1. Location of the research area (red flag on the diagram) and morphology of the landscape of the Sentsa River valley (photo below. The picture was taken by a remote manned DJI Inspire 1 pro aircraft equipped with a Zenmuse 3X camera (resolution 3840×2160 pixels) with a spatial resolution of 5.7–7.8 cm/pixel, height 500 m, March 15, 2022).

Materials and methods of research

Comprehensive studies in the Sentsa Valley were carried out in the period 2011-2021. To analyze the granulometric and mineralogical composition of loose sediments, 18 sand samples were selected in the coastal ledge of the Sentsa River, 28 sediment samples from finite moraine shafts and 15 core samples from the well (the coordinates of the objects are shown in Table 1).

The granulometric analysis of the samples was performed using sieve and laser diffraction methods. The selected samples, dried to an air-dry state, were ground in a porcelain mortar with a pestle with a rubber tip. The separation of soils into fractions without rinsing with water was carried out with sieves with hole sizes of 10; 5.0; 2.0 and 1.0 mm. The weight of the moored sample averaged 100 g. The weighted sample was sifted through a column of sieves with a pallet manually. The sieving losses are distributed over all fractions in proportion to their weight. The results of the analysis are presented in tabular form. The analysis of fractions 2.0–0.00008 mm was performed using a High-End laser diffraction particle size analyzer "ANALYSETTE 22" NanoTec (FRITSCH GmbH, Germany). The sample weight of 2-15 g was placed directly into the dispersion bath on the path of the laser beam. As a result of the deflection of the laser beam, an annular intensity distribution was formed, which was measured by the detector. The particle size is calculated based on the width of the rings and the distance between them. The MaScontrol software displayed the results in the form of a cumulative curve and a tabular particle distribution.

Table 1. Coordinates of clearings and wells

X–ray fluorescence analysis of the sands (9 samples) was performed on the equipment of the Center for Geodynamics and Geochronology of the Institute of the Earth's Crust SB RAS (operator - lead engineer M. N. Rubtsova). To determine the mineral composition, samples ground in an agate mortar with alcohol were examined by powder diffraction on a DRON – 3.0 diffractometer, radiation – SiKa, Ni filter, V= 25 kV, I = 20 mA, in the range 3 – 65 ° 2θ, scanning step – 0.05°. To identify clay minerals, the oriented sample material was prepared by precipitation of the clay fraction on a glass substrate, heating at a temperature of 550 ° for 3 hours and saturation with ethylene glycol^[7]. The phase composition of the samples was deciphered using the EVA program (Diffrac plus, PDF-2, 2007). Data from the semi-quantitative analysis of certain phases in the samples are presented, calculations are carried out by the RIR method using corundum numbers of mineral phases from the crystallographic database PDF-2^[8].

Quantitative mineralogical analysis was carried out by the immersion method^[9] (operator – lead engineer I. A. Kalashnikova). The granulometric fraction 0.25-0.05 mm, previously divided into light and heavy parts in the bromoform, was studied. The preparation, which must contain at least 500 grains, is prepared by quartering. The immersion liquid had a refractive index of 1.540 and 1.530 for the light fraction, and 1.630 for the heavy fraction. The diagnosis and counting of minerals were carried out using a Micromed microscope and a MIN-8 polarization microscope.

The well with a depth of 45 m was drilled using the URB 2A2 drilling rig (INGEO LLC, Irkutsk). Core sampling was carried out after 0.5-0.8 m.

Radiocarbon analysis of 5 samples of loose sediments with plant residues taken from the clearings and wells was performed at the Tomsk Central Research Institute of the Siberian Branch of the Russian Academy of Sciences (IMKES SB RAS) (operator – leading researcher G. V. Simonova). The activity of the carbon radioisotope was analyzed by liquid scintillation method using an ultra-low-background spectrometric radiometer Quantulus 1220, manufactured by Wallac Oy, Finland. The device provided a background of ¹⁴ S – 0.4 CPM (CPM – counts per minutes (counting rate per minute).

Results

Lithological and cryogenic features of loose sediments in the Sentsa River valley according to drilling data

In 2020, for the first time, a deep well Se-20-sq. 5 was drilled on the floodplain terrace 100 m from the Sentsa riverbed (Fig. 2).

Fig. 2. The position of the Se-20-sq.5 well and the Se-14-2 (blue triangle) and Se-20-3 (red triangle) clearings in the coastal ledge of the Sentsa River.

The depth of the well was 45.1 m, the section is composed of sandy loams, bluish-gray loams, clays of the same color and sands (a detailed description of the section is shown in Fig. 3). At a depth of 1.1 m, the permafrost roof is fixed, the sole of permafrost is set at a depth of 44 m. The ice content of rocks decreases in depth from 84-114% in the upper part (in the range of 1.5-4.5 m) to 20-33% in the lower part (in the range of 4.5-44 m) of the section. Cryotextures of frozen rocks are mainly sparsely clad, massive, but there are lenses and ice layers up to 8-10 cm thick. At a depth of 44 m, a pressure aquifer has been uncovered, the water-bearing rocks are represented by sands. The pressure above the roof was 16.7 m, the water level was set at a depth of 27.3 m.

Fig. 3. Lithological composition and cryogenic structure of the section of the Se-20-sq. 5 well drilled in the valley of the Sentsa River. 1 – sand; 2 – sandy loam; 3 – loam; 4 – clay; 5 – slivers and ice layers; 6 – pebbles; 7 – organic matter (plant residues); 8 – depth of water opening and pressure above the roof of the aquifer; 9 – place of sampling for radiocarbon; 10 – place of sampling rocks for granulation.

Granulometric and mineralogical composition of the sands of the above-floodplain terrace of the Sentsa river

The Se-14-2 and Se-20-3 clearings were carried out in the ledge of the left bank of the Sentsa River in the area of a steep (90°) turn of the riverbed to the north (see Fig. 2).

In the Se-14-2 clearing, which is located 50 m upstream from Se-20-3, the permafrost roof was fixed at the end of June at a depth of 1.05 m (Fig. 4). In general, the section is represented by sandy loams and loams of bluish-gray color with layered and obliquely layered cryotexture, with horizontal and oblique lenses of ice (up to 5 cm thick). The apparent volumetric iciness is quite high – up to 50%. At a depth of 3.8 m, loam is replaced by yellowish-gray sand, including rolled pebbles and gravel. The contact is uneven with interlayers and nests of torn and hardened sand. The cryotexture of the sand layer in the range of 3.8-4.1 m is sparsely and incompletely layered, horizontally layered and massive.

Fig. 4. Lithological composition and cryogenic structure of the Se-14-2 section.

Long-term observations of the state of the river ledge in this area have shown that thawing of loose sediments occurs annually, their intensive thermal erosion during periods of floods and the retreat of the terrace edge. In some years, this process is accompanied by the formation of deep cavities in the coastal ledge (Fig. 5).

Fig. 5. Dynamics of retreat of the ledge of the Sentsa River terrace as a result of thawing of strongly icy frozen loams lying on alluvial sands and overlain by sandy loam.

The 2.5 m high Se-20-3 clearing section is composed of thawed alluvial sands. The permafrost roof is fixed at a depth of more than 2.5 m from the earth's surface. Studies of the granulometric composition of alluvial sands (in the section Se-20-3), washed, sorted and accumulated by river water during the active formation of the meander, are based on the analysis of the differential distribution and cumulative curves of 18 samples (Fig. 6) selected from the section interval 0.1-2.25 m .

Fig. 6. Results of granulometric analysis of sediments from the Se-20-3 clearing. Histogram (1, 3, 5 and 7) and cumulative curve (2, 4, 6 and 8) for samples: g41, g42 and g44 (orange); g43 and g45 (black); g40, g46, g47, g48, g50-g55 and g57 (red); g49 and g56 (blue).

Three of the obtained cumulative curves (2, 4 and 6) demonstrate the uniformity of particles, their good and very good sorting – the sorting coefficient So^[10] varies from 1.55 to 1.99. The fourth curve (8) is characterized by a greater blurriness of the fraction distribution and indicates the average degree of sediment sorting – the maximum So coefficient (up to 3.09) The value of the Sk asymmetry coefficient for the entire section is less than one (0.23-0.96, on average 0.75), except for one sample from a depth of 1.23 m, i.e. a large fraction dominates in the section.

The analysis of the granulometric composition of sediments and comparison with the classification of fractions by I. P. Ivanov^[11] allowed us to identify 4 groups of sediments combining different fractions of sands and characterized by varying degrees of sorting (Fig. 7).

The first group includes deposits with a coarse sandy fraction (0.5-0.25 mm), which lie in the intervals of 0.80-1.10 m (mod. g46-g48, So 1.29-1.42), 1.55-2.00 m (g52-g55) and at the base of the clearing (g57), at the water's edge, i.e. they compose most of the section. The sand in these layers is very well sorted (So 1.34-1.40) and has a high degree of uniformity.

The second group includes fine-grained sand with a fraction size of 0.25–0.05 mm (g43 and g45), which can be traced in layers of low thickness - 0.5-0.15 cm. The group is characterized by uniformity and a good degree of sorting (So 1.56-1.71).

The third group includes fractions of 0.05-0.01 mm – fine and fine-grained sand, heterogeneous and medium-graded (g40, g41, g42 and g44), lying in the upper part of the ledge to a depth of 0.2 m and in the range of 0.6-0.75 m. These fractions are characterized by high values of So(2.04-2.32) and low values of the Sk asymmetry coefficient (0.23-0.46).

The fourth group differs sharply from the described three groups both in the flatter shape of the cumulative curve and in the presence of coarse and fine dust particles (up to 0.002 mm), which is also evidenced by the maximum asymmetry coefficient Sk 1.15 and the minimum kurtosis Ku 0.19. It covers layers (interlayers) of fine-grained, heterogeneous and medium-graded sands lying at depths of 1.25 and 2.1 m (g49 and g56). As a rule, increased dustiness is characteristic of rocks of seasonally shallow and seasonally frozen layers, which have undergone cyclic processes of freezing-thawing, which makes it possible to identify layers of the IV group of sands as the position of the paleocover of the frozen strata.

Fig. 7. Lithology of the Se-20-3 section and selected groups of sand deposits by granulometric composition: 1 - group I, 2 – group II, 3 – group III, 4 – group IV, 5 – sampling sites and their names. The characteristics of the groups are given in the text.

X-ray phase analysis of the mineral composition of five samples of alluvial sands made it possible to get an idea of the material composition of each of the selected groups (Table. 2), which serves as an indicator of the areas of demolition of the material. Group I is characterized by a maximum content of quartz (55%) and a minimum content of clay minerals (about 5%). Quartz also prevails in group II, but in smaller quantities (40%), and there are more clay minerals (up to 20%). Groups III and IV are characterized by the predominance of feldspar over quartz and a fairly high content of clay particles (20-35%). Amphibole of the tremolite-actinolite series is present in all samples in an amount of 5-10%.

Table 2. Results of X-ray phase analysis of the mineral composition of alluvial sands

The quantitative analysis of the mineralogical composition of the sands was performed on the basis of the immersion method, the granulometric fraction of 0.25-0.05 mm was studied. The degree of granulation varies from semi–rolled and rolled particles to sharp-angled, angular fragments, which predominate in both heavy and light fractions.

The study of the mineralogical composition of the sands composing the ledge of the left bank of the Sentsa River showed that the heavy fraction is represented by amphibole – on average 44%, diopside and hypersthene – 8-12% (only at a depth of 0.7-1.25 m), biotite – 10% (at a depth of 0.6 m); the remaining minerals (ilmenite, garnet, zircon, rutile, sphene, tremolite, magnetite, epidote, apatite, tourmaline, stavrolite, distene, sillimanite and chlorite) occur either in single grains, or their content does not exceed 1-4% (Fig. 8).

A significant part of the analyzed samples are rock fragments (up to 63% of the total weight of the heavy fraction), which are black-gray semi-shiny particles, relatively soft with gray crumbs (probably an admixture of coal, chlorite, granular epidote and amphibole). Among the minerals of the light fraction, biotite prevails (on average 48.8%), the content of plagioclase and quartz reaches 44 and 34%, respectively, at a depth of 2.25 m, averaging 24 and 16% in the section. The content of K-feldspar and muscovite does not exceed 8%, and chlorite, graphite and carbonates make up tenths of % or appear as rare signs. There are rare signs in the sand at a depth of 0.7 and 1.25 m (<0.001%) carbonified plant residues – brittle, black semi-shiny elongated fragments. Carbonates present in the sands in the form of rare small grains can, apparently, be attributed to diagenetic neoplasms^[12].

Fig. 8. The content of heavy and light fraction minerals in the sands.

Granulometric and mineralogical composition of the sediments of the terminal moraine

The deposits of the terminal moraine were studied by four clearings: Se-17-2- mor 1, Se-17-2- mor 2, Se-17-2- mor 3, Se-17-2- mor 4, made within the shaft of the terminal moraine in the valley of the Sentsa river near Lake. Huhe-Nur (fig. 9).

Fig. 9. Photo and diagram of the position of the terminal moraine near the lake. Huhe-Nur (red square – the location of the clearings)-17-2- mor1, mor2, mor3, mor4).

The height of the shaft above the water's edge is 24 m, four clearings (height from 0.9 to 1.4 m) from top to bottom characterize the section of moraine deposits (Fig. 10 A and B). In general, the shaft of the final moraine in the studied area is composed of medium-coarse block material with a sandy filler. The results of granulometric analysis (sieving) of Se-17-2 moraine deposits are presented in Table 3.

Table 3. Granulometric composition of moraine sediments in the Se-17-2 section

But

B

Fig. 10. Lithology and structure of sediments of the terminal moraine in the Se-17-2 section: A – clearings mor1 and mor2, B – clearings mor3 and mor4. 1 – gravel and pebbles; 2 - sand; 3 – sandy loam; 4 – buried soil; 5 – sampling site for granulation and radiocarbon dating.

The uncovered surface of the shaft is covered with gravel-pebble-boulder material with fragments of rocks of various colors – from dark gray to white – and varying degrees of rolling. The host sands are of various grain sizes, brown and gray in color. In vertical sections at different depths, layers of gravel-pebble deposits with a sandy filler, the size of pebbles and gravel 1-10 cm, sometimes with large rounded boulders (up to 20 cm in diameter) are also fixed. The gravel and pebble material is well and moderately rolled. Sand deposits are characterized by good sorting of the material, sometimes oblique and diagonal layering, which is inherent in sediments of flowing waters, along with the presence of more or less large boulders.

In the middle part of the slope, a layer of buried soil was recorded in a layer of gray fine-medium-grained sand (clearing mor2, see Fig. 10A), which has the shape of a wedge and may have been formed as a result of frost cracking during the period of occurrence close to the surface. In the lowest clearing (mor4, see Fig. 10B), located 5 m above the water edge of the lake, a layer of buried soil 8-10 cm thick was also uncovered in a layer of ochreous coarse-grained sand with inclusions of pebbles up to 20 cm in size.

All sections within the shaft of the terminal moraine are characterized by the presence of a basal layer of gravel-pebble deposits with a sandy filler, often with the inclusion of boulder material, at a depth of 1-1.5 m from the surface.

To determine the mineralogical composition of the deposits, an X-ray phase analysis of the samples was performed (Table. 4) selected from the Se clearance-17-2- mor2 from a depth of 0.37 cm (sample M2-2), from Se clearing-17-2- mor3 from a depth of 0.80 cm (sample M3-3), from Se clearing-17-2- mor4 from a depth of 0.56 cm (sample M4-2) and 0.89 (sample M4-5).

Table 4. Mineral composition of the studied samples

In general, the mineral composition of sands and sandy loam of moraine deposits is mainly represented by feldspar – 40-45% and quartz – 30-40%. Quartz is found in significant amounts in the upper part of the moraine shaft at a depth of about 0.40 m – 40%, amphiboles make up 5-10%. Clay minerals are contained in small amounts – from traces to 5%.

The immersion method showed a similar, but more accurate, distribution of heavy and light fraction minerals in the selected sand samples (Fig. 11).

11. The content of heavy and light fraction minerals in the sands of moraine deposits.

Amphibole dominates in the heavy fraction (up to 42%), the remaining minerals (sphene, hypersthene, epidote, goethite, biotite, carbonates) are contained in an amount of n·10-1, rarely 2-7% (garnet, diopside). Rock fragments representing aggregates of quartz with amphibole, less often with epidote, account for up to 60% of the total weight. The light fraction is characterized by a predominant content of plagioclase (Na-Ca-feldspars) – up to 50% and quartz – up to 40%. The subordinate value of K is feldspars (up to 10%) and biotite (up to 3%).

Granulometric and mineralogical composition of sediments of the stadial moraine

In order to study the structure and composition of sediments of a shaft clearly expressed in the relief, located two kilometers upstream and transversely to the riverbed, 2 sections composed of sandy sediments in the area of the Urda-Khuryelok tract were studied. One of them – Se-19-R 1 – is located on the left bank of the Sentsa River (Fig. 12), the other – Se-20-R 2 – on the right (Fig. 13).

The Se-19-R 1 incision is unfrozen to a depth of 2.50 m. The study of the opened section and the analyzed granulometric composition of 9 soil samples taken from

Fig. 12. Section Se-19-R1 on the left bank of the Sentsa River, within a finite moraine shaft (phase II). 1 – sand; 2 – sandy loam; 3 – gravel, crushed stone; 4 – pebbles; 5 – buried soil; 6 – plant residues; 7 – sampling site for granulation and radiocarbon.

depths 0.15, 0.60, 0.90, 1.10, 1.40, 1.60, 1.70, 1.80 and 2.00 m, allowed us to establish that the thickness is composed of fine- and fine-grained sand with layers of sandy loam, plant residues and humus horizons to a depth of 1.85 m. Below there is a layer of gravel and uncoated rubble measuring 0.5-1.0 cm; under the rubble layer there are pebbles (rolled) and boulders up to 10 cm in size. The filler is sand and sandy loam of ochre color.

Radiocarbon dating of two samples of organic detritus from soil horizons taken from depths of 1.55–1.60 and 1.60–1.70 m showed an age of 4185±110 and 5215±450 years, respectively.

The second section – Se-20-R 2 – is located in the Urda-Khuryelok tract and opens a shaft transverse to the riverbed on the right bank of the Sentsa River, which continues shaft R1 from the left bank. The section is unfrozen, composed of yellow and gray sands, rolled and semi-rolled grains of sand. At a depth of 3.50 there is a layer of pebbles, the average size of the pebbles is 3x2x1.5 cm. Boulders of 0.5-1.2 m in size lie at a depth of 3.95 m.

Fig. 13. Section Se-20-R2 on the right bank of the Sentsa River, within the limits of a finite moraine shaft (phase II). The symbols in Fig. 11.

Discussion

The study of the granulometric composition of the well core showed that in the section of loose sediments (up to a depth of 44 m), loams of medium and poor sorting prevail, which rarely overlap with clays of medium and poor sorting, as well as with sandy loams of good, medium and poor sorting, well-sorted sands. The ratio of soil types in the section of loose sediments of the valley is clearly visible from the Ferre triangle, which is constructed using the Trask sorting coefficient^[13] (Fig. 14). The Trask coefficient is calculated using the formula S 0 = , where Q25 is the 25% quantile for large particles, Q75 is the 75% quantile for smaller particles.

Fig. 14. Triangular diagram of the ratio of particles of different sizes in loose sediments uncovered by the Se-20-sq.5 reference well in the Sentsa River valley. Trask Sediment sorting coefficient: 1 – well sorted (1.00-1.58); 2 – medium sorted (1.58-2.12); 3 – poorly sorted (> 2.12).

The analysis of the granulometric composition of loose sediments uncovered by the Se-20-sq.5 reference well made it possible to determine the sequence of precipitation accumulation in the lacustrine paleovodoem, which existed in the valley of the Sentsa River in the Holocene and was formed as a result of backup by finite moraine sediments^[14]. At the first stage, the accumulation of lake-glacial clays (not sorted or poorly sorted) occurred under conditions of minimal energy of the water flow. Later, when the Sentsa River was cut into a finite moraine complex, a thickness of loam (medium-graded) was formed in an underground lake under conditions of increasing flow energy. At the stage of active cutting of the river into the terminal moraine complex and changing the location of the riverbed, well-sorted sandy loams and sands accumulated.

Judging by the core of the drilled well, it can be stated that during the Holocene, the sedimentation regime in the underground lake repeatedly changed, providing an alternating accumulation of interlayers of clays, loams, sandy loams and sands of various capacities. Periodic fluctuations in the water level in the paleozoic, possibly caused by tectonic factors, caused variations in sedimentation features.

Radiocarbon analysis of the selected core sample (bluish-gray sandy loam, dusty, frozen with plant residues) from a depth of 1.5 m showed an age of 3510±120 years (calendar age -1838 AD) (lab. no. IMKES-14C2215). This result confirms the existence of an open reservoir in the Late Holocene. According to calculations, the average sedimentation rate in the Late Holocene in the lake was 0.4 mm/year. Previous studies have established that the active formation of soil horizons after the drainage of the paleozoic started only 500-200 years ago^{[14, 15]}. At this time, the freezing of loose sediments, active heaving and the formation of lithosis began in the drained areas of the Sentsa River valley.

The composition of rock-forming minerals in the alluvial deposits of the Sentsa River is quartz-plagioclase-biotite, and its characteristic complex of heavy minerals is pyroxene-amphibolite. Such a composition allows us to assume a relatively short-term transfer within the nutrition region, since these minerals (amphiboles and pyroxenes) are easily eroded during mechanical transfer and quickly deteriorate during weathering in a humid climate, and a significant quartz content indicates the erosion of metamorphic complexes ^[16-19].

The mineralogical composition of the sands reflects the petrographic composition of the feeding province: intrusive rocks of the Sayan complex of the Upper Proterozoic γ ₂ PR ₃ and δ ₁ PR ₃ (plagiogranites, granodiorites, diorites) and the Ognite complex of the Paleozoic γξ-ξPZ (grano-syenites, syenites) and γπPZ (granite porphyry), as well as metamorphic and Proterozoic sedimentary deposits PR _1-2 and PR ₃ (marbled, graphitized and dolomitized limestones, with interlayers of gneiss and crystalline shales). These rocks come to the surface at the source of the Sentsa River, which is formed at the confluence of three rivers, and compose the entire transit path of the river for 50 km (to the research site). The average mineralogical composition of rocks (Table. 5) predetermined the composition of the sands that lie in the ledge of the river in the middle reaches.

Thus, the same type of mineralogical composition of all groups of the sand fraction, thin horizontal layering (rarely oblique micro-layering), the presence of carboniferous plant residues and a significant number of angular, angular-rounded and semi-rolled grains indicate a calm water regime, lake and lake-marsh sedimentation conditions. The comparative consistency of the complex of heavy minerals suggests that the demolition area was unified and constant, and the change of four sand groups isolated by granulation was the result of a change in the composition of eroded rocks of local feeding provinces (intrusive, metamorphic or sedimentary rocks). The material was transported by water flows over short distances, since the grains retained an angular shape, and after deposition it underwent the same diagenetic transformations. Coarse-grained gray sand of group I, dominating in the section by thickness, indicates a high energy of the water flow forming it and an increase in the sedimentation rate^[20].

In the clearing of Se-14-2, opened in a ledge 50 m upstream of the river (see Fig. 3), the described alluvial sands from a depth of 3.8 m are overlain by icy bluish-gray and brown-gray loams of lake facies, which were recorded in all previously performed clearings and core wells drilled in the valley of the Sentsa River. As established by the results of palynological analysis, they were formed under conditions of relative cooling and decreasing humidity, as evidenced by the widespread distribution of light coniferous larch-pine forests with minor participation of dark coniferous spruce and fir species^[21].

Table 5. Average mineralogical composition of rocks of the feeding province, % (according to the Map of minerals of the USSR, 1966).

Note. 1 – quartz, 2 – feldspar, 3 – plagioclase, 4 – biotite, 5 – muscovite, 6 – amphibole, 7 – pyroxene, 8 – chlorite, 9 – epidote, 10 – carbonates. The + sign indicates only the presence of a mineral in the average composition of the rock, regardless of its quantity.

Taking into account the location of the uncovered clearings one under the other vertically (see Fig. 10A) and the fact that at the base of each of them lies a layer of gravel-pebble deposits, well and moderately rounded, with sandy aggregate, with large rounded boulders, it is possible to imagine a section of a moraine shaft with a height of more than 20 m as a "layered pie" in which roughly clastic deposits (boulder-pebble), heterogeneous and unsorted, and loose accumulations (sandy loam, sands) alternate, which store traces of the activity of water-glacial flows during the melting of the glacier. These streams washed away the moraine material and re-laid it when exiting from under the glacier, forming a complexly constructed course-moraine shaft. The mineralogical composition of the deposits is inherited from the composition of the rocks composing the mountain ranges surrounding the valley of the Sentsa River (see Table 5).

The study of the morphological features of this section of the Sentsa River valley showed that the ramparts on the left and right banks probably represented a single shaft of the terminal stadial moraine. It was formed as a result of the degradation of the marginal part of the glacier from the boundary of the maximum distribution in 8.5 km from the mouth of the Sentsa River to the considered section of the valley 4-5 thousand years ago during the warming period. During the interglacial period, soils were formed, which currently lie at a depth of 130-170 cm (see Fig. 9, 10), and the vegetation zone expanded. The subsequent cooling, which is confirmed by the wide spread of light coniferous larch-pine forests with minor participation of dark coniferous spruce and fir species (according to the results of palynological analysis of loose valley sediments^[21]), led to a slowdown in the deglaciation process or even to a temporary activation of the development of the valley glacier. During the period of subsequent warming, the final destruction of the ice masses took place, and the formation of a stadium course-moraine shaft across the valley, which was later cut through by the riverbed. Large-block deposits lie at the base of the moraine shaft, and the largest fragments – erratic boulders – are observed everywhere on the earth's surface (Fig. 13). Thawed glacial waters during the degradation and retreat of the glacier formed a powerful thickness of fluvioglacial deposits: pebble and gravel deposits were deposited on top of the block accumulations, and a layer of sand covered them from above. Swirls and inversions of layers of different granularity and color are observed in the sand column, which indicate the movement of water flows.

Fig. 13. Erratic boulder in the sediments of the terminal moraine.

Of course, the moraine shafts created barrages for surface watercourses and conditions for the formation of retaining paleozoic lakes. Attention is drawn to the fact that the straightened sections of the Sentsa riverbed are confined to these accumulative glacial landforms, and intensive meandering is typical for sections of the valley where lake conditions existed and fine-grained sedimentary material accumulated during deglaciation and glacier retreat. It can be assumed (following V. G. Chuwardinsky^[22]) that the formation of the marginal ridge-hilly shaft was also facilitated by tectonic movements along active discontinuous faults operating the Sentsinsky regional fault of the sublatitudinal strike and laid across the valley.

The detrital material of the terminal and lateral moraine partially overlapped the older deluvial deposits of the cones of the outflow from the surrounding mountain ranges (Fig. 14).

The gravel-pebble accumulations with sandy aggregate at the base of the described sections in the Urda-Khuryelok tract with the occurring rounded and semi-rolled boulders may represent mixed deposits of cones of outflow and glacial material, which are quite difficult to separate. The surfaces of the outflow cones are blackened, mostly covered with woody vegetation below the proximal zone, the flow hollows are swampy with small streams, which indicates the stability of these geomorphological forms.

Fig. 14. The layout of finite moraine deposits (solid yellow line), lateral moraine (intermittent yellow line) and outflow cones (intermittent white line) in the valley of the Sentsa river.

Conclusion

The results of the performed studies allowed us to formulate the following conclusions:

1. The sands of the above-floodplain terraces of the Sentsa River are confidently divided into four groups according to their granulometric composition, characterized by varying degrees of sorting and dustiness, which allows us to consider the sand layers at depths of 1.25 and 2.1 m as deposits, probably marking the position of the paleocover of the frozen strata. The mineralogical composition of the sands reflects the petrographic composition of the feeding province. A number of characteristic features indicate a calm water regime of their accumulation, with a rare change in the dynamic regime towards an increase in the flow energy rate and sedimentation rate.

2. The final moraine in the Sentsa River valley is characterized by an alternation of roughly clastic material (boulder-pebble), heterogeneous and unsorted, and loose sediments (sandy loam, sands) with traces of the activity of water flows during the melting of the glacier, which washed the moraine material and re-laid it when leaving the glacier, forming a complexly constructed course- moraine shaft. The mineralogical composition of fluvioglacial deposits is also inherited from the composition of the bedrock composing the mountain ranges surrounding the valley of the Sentsa River.

3. The sand column, composing a well-defined shaft in relief, located two kilometers upstream from the terminal moraine and transversely to the Sentsa riverbed, lies on large-block deposits. The structure of the entire section indicates the formation of this powerful stratum as a result of the activity of thawed glacial waters during the retreat of the glacier and the formation of a stadium-shaped moraine shaft across the valley, which was later cut through by the riverbed. The detrital material of the terminal and lateral moraine later partially overlapped the older deluvial deposits of the removal cones from the surrounding mountain ranges.

4. The granulometric and mineralogical composition of loose sediments of the Sentsa River valley, opened by a well to a depth of 45 m, indicates a repeated change in the sedimentation regime in the subsurface paleozoic in the Holocene, which was accompanied by an alternating accumulation of interlayers of clays, loams, sandy loams and sands of various capacities.