Middle and Late Holocene landslides on the northern slope of the Aibga ridge

Slyshkina Elena Head of the Laboratory of JSC Atomenergoproekt 119234, Russia, g. Moscow, ul. Podol'skikh Kursantov, 1, of. 250 lena.slyshkina@gmail.com

Vasil'chuk Yurii Kirillovich Doctor of Geology and Mineralogy Professor, Department of Landscape Geochemistry and Soil Geography, Faculty of Geography, Lomonosov Moscow State University 119234, Russia, g. Moscow, ul. Leninskie Gory, 1, of. 2009 vasilch_geo@mail.ru

Introduction

 When studying landslide processes, researchers pay great attention not only to the geometric dimensions of landslide bodies, displacement mechanisms, geomorphological features and reasons for their activation, but also to the time of their activation. The data obtained by radiocarbon dating of organic material selected from various geomorphological elements of landslides with a large sample of samples ^[1-9] allow us to confidently say about the age of the event, or several episodes of landslide formation, the frequency of repetition of the process, as well as to establish the trigger causes that triggered the landslide.

To establish the history of landslide activity in the Middle and Late Holocene, the authors performed radiocarbon dating of organic material accumulated in lake-marsh drainage basins in the rear part of sliding landslides on the northern slope of the Aibga.

Materials and methods of research. Field work was carried out in the upper reaches of the Mzymta river basin (Western Caucasus), in the central part of the northern slope of the Aibga ridge (Fig. 1), approximately 7.0–11.0 km east of the village. Krasnaya Polyana, on the territory of the Sochi National Park, in the immediate vicinity of the Aibginsky fault. During field research, 4 sliding landslides 17-SS6, 17-SS10, 17-SS11, 17-SS15 were studied, described and tested.

Fig. 1. Overview map of the study area with the main faults and tested landslides in the valley of the Mzymta river: black circles – the locations of dated landslides and their number; active faults: A – Aibginsky, B – Bekisheysky, Mz – Mzymta, E – Estosadoksky; line with bergstreaks – discharge (bergstreaks are directed towards the omitted wings), discontinuous lines – mainly shifts

            Methods of work. The methodical approach to the dating of landslides is that each landslide is considered as original and its testing is carried out according to an individual scheme. For example, when dating a sliding landslide, material is taken from the rear part of the landslide, landslides are dated by the material from the body of the landslide, and landslides flow through the humus of blocked soils or buried roots. The selection varied when dating sliding landslides, currents, and landslides.

In cases where the age of a landslide is represented by a single sample, radiocarbon dating very approximately reflects the time of the collapse. Dating of landslide events from single samples reduces the degree of confidence in the obtained age due to the possible redeposition of the dated material ^[10].

Ancient material of different ages may be involved inside the landslide: the youngest dates are closest to the age of the landslide. This is especially true for mountainous areas, where there is a constant transfer of material downhill and its re-deposition, and, consequently, a decrease in confidence in the age obtained from single samples. The similarity of the ages of different independent samples increases confidence in the dating of the landslide ^{[1, 2]}.

The method of performing serial selection of organic material from different elements of the landslide allows us to obtain more reliable definitions of the age of the landslide. New dating of samples of organic material obtained in different parts of landslides: 1) from sections of depressions in the rear part of the landslide, 2) from sections of depressions on the main body of the landslide, 3) from the humus horizon buried under the linguistic part of the landslide, allow us to reconstruct with a high degree of reliability the chronology of the development of landslides of different scales and different ages.

Research results and their discussion

Table 1. Radiocarbon age of samples of organic material from landslides in the upper reaches of the Mzymta river basin (Western Caucasus), in the central part of the northern slope of the Aibga ridge

Note. Calendar age values are given based on the calibration program "OxCal 4.2" (calibration curve "IntCal 13" ^[10], Christopher Bronk Ramsey ^[11].

The consequent landslide of shift 17-SS8 is located on a slope with a steepness of 35-40 °, at absolute elevations of 1201-1161 m and have an oval shape in plan. The length of the landslide in the direction of displacement is ~ 225 m, width ~ 210 m, sediment thickness ~ 25-30 m. The total volume of the soil involved in the displacement is 1.8 million ^m3.

Fig. 2. Sequential landslide of shift 17-SS8. Yellow shows the body of the landslide

The entire delapsive and accumulative part of the landslide is covered with grassy vegetation, undergrowth, aspen and tall century-old oaks. The depressions on the body of the landslide and the lingual part are swampy, fern grows on them. In the sides and the central part, the landslide is opened by temporary watercourses. The presence of normally developing vegetation and the absence of surface deformations and cracks outlining the landslide indicate a phase of temporary stabilization of the landslide process.

The basis of the displacement of the landslide at the time of its formation was the valley of the Mzymta River. The rocks of the main deformable horizon are represented by a lumpy soil with a loamy filler. Blocks of sandstone, limestone, and igneous rocks reach sizes 15-20 m across. The separation niche is technogenically altered and planned, covered with herbaceous and shrubby vegetation, poorly expressed in relief.

In the section of the depression in the rear part of the landslide, organic residues are represented by concentrated wood residues and scattered organic matter – in the form of soil humus. The passed pit in the lake-marsh drainage basin in the rear part of the landslide exposed from top to bottom: 1) brown soils (up to 0.7 m) of dark gray color, with a high content of dispersed organic matter; 2) lumpy soil with a loamy filler of light brown solid consistency (0.7-1.5 m) - landslide deposits. Samples of LU-8901, LU-8902, LU-8903 and at the contact of the borozems and landslide deposits (wood embers – LU-8904) were selected from the borozems.

The block landslide of shift 17-SS9 is localized at absolute elevations of 1591-1553 m, on a slope with a steepness of 45-50 °. It has an oval shape in plan. The length of the landslide body in the direction of displacement is ~160 m, width ~ 105 m, sediment thickness ~ 35 m (Fig. 3). The total volume of the soil involved in the displacement is approximately 0.6 million ^m3.

Fig. 3. Block landslide of shift 17-SS9

The delapsive part of the landslide is blocked from above by talus deposits in the form of small detrital material and large blocks that have moved from the separation niche to its surface. Huge landslide boulders on the surface of the landslide, covered with a thin soil layer and moss. Young trees grow on the surface of the blocks. The surface of the landslide is overgrown with shrubs, woody vegetation and ferns, the demolition of colluvial material is observed. Many trees are broken by landslide boulders that fall out of the root niche of the separation and are located near the wall on the surface of the landslide (which indicates a single landslide event). The drainless basin in the rear part of the landslide is heavily waterlogged. The breakaway niche is a steep (70-90 °) cliff with a height of ~ 20 m and a width of ~ 130 m, composed of strongly mature, strongly cracked rocks. The age of the landslide event is indicated by the powerful (0.45 m), well-defined horizon of brown soils, as well as the absence of modern stretching cracks and stabs. The phase of the landslide process (to date) is a long–term stabilization.

To determine the age of the landslide, organic residues were selected in the section of the depression in its rear watered part. In the context of the zapadina, the selected organic residues are represented by concentrated wood residues and scattered organic material – in the form of soil humus. The pit passed in the rear part of the landslide body (Fig. 2) exposed from top to bottom: 1) brown soils (up to 0.8 m) of dark gray color, with a high content of scattered organic matter (according to the classification of soils of Russia, 2004); 2) colluvial deposits (0.8–1.2 m); 3) humus horizon buried under colluvial deposits (1.2–1.4 m). Sampling of organic material was carried out from the borozems LU-8905, LU-8907 and the humus horizon LU-8908.

The consequent landslide of shift 17-SS13 is located in the side of the Sulimovsky Creek, a left–bank tributary of the Mzymta River at absolute elevations of 938-910 m. The length of the landslide in the direction of displacement is ~90 m, width ~120 m, sediment thickness ~12-15 m (Fig. 3). The exposed sediment thickness in the tongue of the landslide by the erosion cut is 7.6 m. The total volume of the soil involved in the displacement approximately reaches 160 thousand ^m3.

Fig. 4. Sequential landslide of shift 17-SS13

The entire delapsive and accumulative part of the landslide is covered with grass vegetation, undergrowth and tall woody vegetation. The thickness of the soil layer on the inclined surface of the body is 0.5 m. The body of the landslide is crossed by two active watercourses with a small flow. The accumulative part of the landslide is flooded. The surface of the landslide is morphologically altered by slope processes, eroded by the drainage system and covered with dense vegetation similar to vegetation on adjacent landscapes. The edge of the breakdown and the delapsive part of the landslide is washed out and is poorly readable in relief. The absence of modern signs of activation of landslide processes, the presence of streams that open the landslide, the accumulated soil layer (0.5 m) on the surface of the landslide indicate a phase of long-term stabilization (to date).

The formation of the 17-CC8 landslide body most likely belongs to the Middle Holocene. This is evidenced by the date 6310 ± 150 cal. years ago (LU-8904). obtained from the coals of wood from a depth of 0.8 m from the upper part of the body of the landslide in contact with the soil from the section of the depression in its rear part. Soil formation in this part of the landslide resumed around 890 cal. years ago, due to an increase in moisture in the rear part of the landslide, probably due to deterioration of drainage and intense precipitation, it lasted up to 370 cal. years ago.

The formation of the 17-SS13 landslide body also belongs to the Middle Holocene. This is evidenced by the date 5380 ±170 cal. years ago (LU-8924). obtained from coals of wood from a depth of 1.33 m from the upper part of the body of the landslide on contact with the soil from the section of the depression in its rear part. However, almost immediately about 4500 cal. years ago, soil formation began here, associated with the formation of an underground swampy reservoir in the rear part of this landslide shortly after the landslide formation.

Active waterlogging is indicated by the bluish color of dated sediments. Then up to 1180 cal. years ago, soil formation took place in more drained conditions, the bluish color of the sediments was replaced by dark brown.

The consequent landslide of shift 17-SS5 is located on a slope with a steepness of 30-35 °, at absolute elevations of 960-901 m and has an oval shape in plan. The length of the landslide body in the direction of displacement is ~ 130 m, width ~ 240 m. The thickness of sediments exposed by the erosion cut-in in the side part of the landslide is 11 m, the estimated thickness in the body of the landslide is 12-15 m. The total volume of the soil involved in the displacement reaches 470 thousand ^m3.

The delapsive part of the landslide is covered with rare woody vegetation, sometimes having a saber-shaped shape. The thickness of the soil layer on the inclined surface of the landslide body reaches 35-40 cm. In the sides, the landslide is opened by large active streams with predominant bottom erosion. The absence of modern signs of activation of landslide processes, the presence of constantly operating streams that open the landslide in the sides, the accumulated soil layer (0.3-0.40 cm) on the surface of the landslide indicate the phase of its long-term stabilization.

The basis of the displacement of the landslide at the time of its formation and activation was the valley of the Mzymta River. The movement of the language part did not reach the river's edge and presumably stopped in the central part of the slope. Rocks of the main deformable horizon are represented by coarse–grained, poorly rolled, unsorted material with loamy aggregate. The cirque-shaped breakaway niche is a steep (50-55°) cliff ~40 m high and ~250 m wide, covered with a layer of deluvial-talus deposits, sparse woody vegetation and cut by small temporary watercourses. In the south-western part, the separation niche has been technogenically changed, secured with a nagel field and a counter-basement grid, in the south-eastern part technological roads have been cut on its surface, cable networks have been laid, surface drainage has been equipped and an anti-basement grid has been fixed.

To determine the age of the 17-SS5 landslide, organic residues were selected in the section of the depression in the rear part. Organic material is represented by concentrated wood residues and dispersed – in the form of humus of soils. The pit laid in the lake-marsh drainage basin in the rear part of the landslide (Fig. 1) exposed from top to bottom: 1) brown soils (according to the classification of soils of Russia ^[12]), up to 1.6 m thick, dark gray, brown in color, rich in humus, with the inclusion of colluvial material; 2) landslide deposits - brown loam, semi–solid consistency, with a large number of wood embers, an open capacity of 2.0 m. Sampling of organic material was carried out from the first layer (LU-8890, LU-8891, LU-8892, LU-8894) and at the contact of brown soils and landslide deposits (wood embers – LU-8893).

The interpretation of the distribution of radiocarbon dates in the landslide body 17-SS5 is complicated. Most likely, the formation of a landslide occurred about 3 thousand cal. years ago and immediately formed a swampy underground depression in the rear part of the landslide, where the process of soil formation began, as evidenced by the date 2930 cal. years ago at a depth of 1.6 m. And here are the embers of wood dated 2240 cal. years ago, in a dark layer with traces of fire, they indicate arid conditions that arose 700 years after the landslide formation. The resumed process of soil formation here went on continuously up to 590 cal. years ago.

Almost synchronously with the landslide 17-SS5, a landslide body 17-SS9 - 2820 thousand cal was formed. years ago, judging by the dating of humus in the rear depression. At the same time, the process of soil formation here went on continuously up to 210 cal. years ago.

Conclusions

1. Landslides located on the northern slope of the Aibga ridge were formed during two major stages of landslide formation.

2. The older stage of landslide formation dates from the Middle Holocene from 6310 to 5380 cal. years ago.

3. The relatively younger stage of landslide formation dates from the Middle Holocene from 2930 to 2820 cal. years ago.