The history of geomorphological development in the northern part of Western Siberia was a subject of intensive discussion at the end of the 20th century. The stratigraphy of the Yenisey River estuary is a key factor of the Western Siberian lowland Quaternary evolution. Numerous examples of sedimentation alternation induced by various cover glaciations of different ages and thicknesses are presented. This series of sediments was used as a background for geological interpretation of the history of the Western Siberian lowland. The Q-43 national geological map of Russia for this region indicates the dominance of glacial and fluvioglacial types of the surface sediments (Alyavdin and Mokin, 1957). The possible existence of ice sheets and related permafrost sediments was identified as a key issue at the beginning of the systematic geological study of the north of Western Siberia in the 1960s. Some researchers (e.g. Svendsen et al., 2004) suggested that there were extensive glaciations that resulted in blocking the Yenisei, Pur, Taz and Nadym rivers at certain stages, leading to the formation of large glacier dammed lakes (Grosvald, 1999).

Another point of view considers possible glaciation on the plain (e.g. Generalov, 1986). It explains why the landforms are a sequence of terraces formed by marine transgressions of various ages. There is also an opinion that the glaciation was localised in the form of ice caps on separate watersheds and that the river flows of the Ob, Yenisei and other rivers were unblocked (Velichko, 1987; Velichko et al., 1997). Bolshiyanov (2006) challenged this opinion and introduced the “passive glaciation” concept. In this context, it is assumed that the sea level fluctuations might have created extensive abrasion platforms. Another viewpoint suggests that the forms of relief which previously were regarded as glacial and fluvioglacial (moraines and eskers), did not originate from cover glaciations but resulted from erosion, abrasion and thermokarst outcrops associated with permafrost erosion and tectonic processes of the Late Pleistocene. It was suggested that isolated parts of Smarovskoye glaciation (MIS8, 300 000 to 230 000 years ago) existed in some areas of the Tyumen region combined with relics of ancient marine terraces (Lazukov, 1972). Later, there was a heated discussion in the Russian geology community regarding the nature of possible glaciations and sedimentation history of Western Siberia. It was suggested that glaciations extended up to Siberian ridges that continued as the ancient periglacial Mansyiskoye Lake (Grosvald, 1999). Bolshiyanov (2006) suggested that the glaciations were passive, without forming a discontinuous cover or preferential flow blocking in the area topography. At the same time, the abrasion relief with extended ledges was formed in the Late Pleistocene period. Finally, the Q-42-43 national geological map indicates that there is a combination of both terrestrial glacial and marine glacial sediments and numerous lake terraces in Western Siberia. Nowadays, the glacial sediments are excluded from the current version of the national geological map (Babushkin, 1995), which contradicts the interpretations by Astakhov et al. (2016) and Fredin et al. (2012). Currently, there is no uniform concept of the landforms genesis in Western Siberia. The basin of the Nadym River is regarded as most important for the Quaternary interpretation of the regional Pleistocene history. The topography and sediments of the Nadym River provide the most information for the study of glacial landforms. Many field investigations and remote-sensing operations were completed by multiple generations of researchers, providing a valuable baseline for future studies. The results of studying the Nadym River and adjacent areas, combined with other data, served as a basis for a classification of the Quaternary deposits in Western Siberia (Maslennikov, 1998; Sedov et al, 2016; Sheinkman et al, 2016; Rusakov et al, 2018). Nevertheless, the current geological map (Faibusovic and Abakumova, 2015) still has unresolved issues that are highlighted as new geological and geomorphological data are obtained.

The study objective is to summarise the results of detailed lithological, chronostratigraphic, petrographic and geomorphological studies conducted in the Nadym River basin, and to identify the origins of the key factors of sedimentation accumulation and topography.

Fieldwork was conducted in 2016–2018 in the Nadym River basin, including the valleys of its main tributaries: Kheygiyaha, Yarudey, Tanlova, and Left and Right Khetta. The region is characterised by a moderate human-induced burden. There are main gas pipelines (Urengoy–Pomara–Uzhhorod, Nadym–Punga–Lower-Tura, etc.), high-voltage power transmission lines (200, 500 kV), an oil pipeline (Yarudeyskoe field CGS to Puryel OPS) and the Nadym–Yagelskoye asphalt road. The survey covered the natural exposures along riverbanks, walls of dry quarries located at the watersheds and river terrañes as the most informative terrain features. This paper is based on the results of detailed studies of the five most prominent stratigraphy sections of the upper part of Quaternary sediments (Fig. 1; Table 1). For clarity, the section coordinates, their locations, survey dates and the total thickness of the studied deposits are specified.

Samples for bulk chemical composition, grain size distribution, sand quartz grain morphoscopy and morphometry, as well as luminescent analysis of sandy textured particles of feldspars were taken from each specified layer of the studied sections in order to clarify the conditions of the sediment formation.

The bulk content of oxides was determined by the X-ray fluorescence method at the Analytical Center for Multi-Elemental and Isotope Research, Siberian Branch (SB), Russian Academy of Sciences (RAS), Novosibirsk, Russia, and at the laboratory of the Institute for Physical, Chemical and Biological Problems of Soil Science (Pushchino, Russia). The grain size distribution was determined by conventional fractions separation (sieve analysis) of samples with the Fritsch Analysette 3 vibratory sieve shaker. The fractions were weighed with laboratory scales at 0.1 g accuracy. Seventeen samples from the sections NS-6 and NS-13/14 were analysed at the Laboratory of Ground Mechanics, Institute of Cryosphere of the Earth, Tyumen Research Center with the Mastersizer 3000E laser diffraction particle size analyser (Malvern Panalytical, Britain). Since different laboratories measured the granulometric composition, the figures for the lightest fractions are slightly different.

The Altami CM0870-T binocular microscope was used to study the quartz grains (50 grains per sample) taken from the coarse sand fraction. The grain surface morphology was studied with the JEOL JSM-6510LV scanning electron microscope (SEM) using the secondary electron image (SEI) at the Analytical Center for Multi-Elemental and Isotope Research, SB, RAS. According to the technique applied (Velichko and Timireva, 1995), the grain scale was determined using Rukhin (1969, Fig. 2) and Khabakov (1946), where 0 is an angular and IV is a perfectly rounded grain. The coefficients of roundness and the grades of dullness (Velichko and Timireva, 1995) were estimated for each sample. The dullness of the grains was determined visually as glossy (shiny), quarter-matte, half-matte and matte. The grain surface microrelief structure study was based on numerous published diagnostic features found in grains with various genesis and sediment accumulation conditions (e.g. Velichko and Timireva, 1995; Krinsley and Doornkamp, 2011; Vos et al., 2014; Woronko, 2016; Kalinska-Nartisa et al., 2017). The previous studies in Western Siberia that examined sand quartz grain micromorphology covered peat histic sand deposits in the area of the Siberian Uvals, the valleys of the rivers Taz and Pur (Velichko et al., 2011), and aeolian sediments of the southern part of Western Siberia (e.g. Sizikova and Zykina, 2015).

The potassium-feldspar-based infrared optically stimulated luminescence (IR-OSL) dating method was applied to produce an absolute chronology of the deposits from the five sections studied in the present work. The upper limit of the method is normally 300–500 ka, depending on burial conditions and the physical properties of the mineral. The reliability of the dating technique used in the present study is demonstrated by comparative results obtained using several numerical dating methods (mollusc-shell-based electron spin resonance (ESR), quartz-based optically stimulated afterglow (OSA), U–Th, 14C) applied to the same sedimentary samples (Molodkov, 2012). An overview of the IR-OSL dating procedure is presented by Molodkov and Bitinas (2006). All IR-OSL ages reported in this paper were obtained in the Research Laboratory for Quaternary Geochronology (RLQG), Department of Geology, Tallinn University of Technology.

In addition to the analysis at the sampling area, samples were taken for petrographic examination. The samples were cut perpendicular to the bedding direction (if any) and made into transparent sections. The Carl Zeiss AxioScope A1 optical microscope at the Geology and Mineralogy Institute, SB RAS (Novosibirsk) was used.

For the first time for the studied area, digital surface models (DSM) with a spatial resolution of 12 and 26 m/px based on TerraSAR-X and TanDEM-X radar data were used to characterise the geomorphological structure. The mapping is based on the remote features available in the literature (Atkinson et al., 2014; Astakhov et al., 2016, etc.) and the comparison of the field and remote-sensing data (Table 2).

In addition, public multi-spectrum space images from Sentinel-2 (10 m/px.) were used to clarify the location of such natural features as rivers, lakes, swamps and forests (https://scihub.copernicus.eu/, last access: 10 April 2020).

According to current viewpoints, the territory of the north of Western Siberia was exposed to several cover glaciations: Zyryanka (MIS 4), Taz (MIS 6) and Samarovo (MIS 8). Areas at the lower level (up to 40–45 m a.s.l.) could represent serial repeated marine transgressions in Kazantsev (MIS 5) and Karga (MIS 3) time periods in Fig. 13. Directly within the boundaries of the investigated areas the boundaries of Taz (MIS 6) and possibly Zyryanka glaciation periods are identified (Zemtsov, 1976; Babushkin, 1996).

The key natural feature of the glacial genesis of Quaternary strata in northern Western Siberia is the presence of wrecked rock: semi-rounded angular stones, gravel and large boulders with evident glacial striation, carried over by the glacier from the territories outside the Western Siberian Plain (Strelkov et al., 1965; Zemtsov, 1976). The water–glacial sediments in the research area include well-washed grey sand characterised by poor chemical composition (the gravimetric concentration of SiO2 is 94 %–97 %) and also containing amendments of gravel and stones (Chekunova, 1954; Groysman, 1954; Khlebnikov, 1954). The glacial sediments include unsorted coarse-grained sands with an abundance of pebbles, as well as moraine-like bodies of lumped clay, loam and clay sand with gravel and large boulders. The petrographic composition of boulders and pebbles includes quartz, opal, sandstones, quartz porphyries, amphibolites, granitoids, gneisses and trachytes (Chekunova, 1954; Groysman, 1954; Khlebnikov, 1954). However, it was noted that interpreting the exact location of the origin of these rocks from the geological markers representing different territories is so far problematic.

The results of the study of the sections, in general, showed that the youngest of the discussed sediments are those of the second floodplain terrace (section NS-6, K-1). In the top part, this includes aeolian sand formed no later than the beginning of the Holocene (MIS 1); in the middle part there are floodplain series of alluvium; in the lower part there are river streams of grey oblique sand of the late MIS 3–middle MIS 2.

The absolute age of the second floodplain terrace formation of the Nadym and Tanlova rivers (sections K-1 and NS-6) correlates well with radiocarbon and OSL datings of postglacial Pleistocene sediments throughout all of northern Western Siberia (the age ranging from 42 000 to 25 000 years) (Astakhov and Nazarov, 2010). On average, the age of the cover formation is between 20 000 and 12 000 years (Astakhov, 2006; Zemtsov, 1976). Two types of glacial relief areas and extensive sandur surfaces were identified on the surface of the second floodplain terrace in the large-scale field studies on the left bank of the Left Kheta River (Vasilyev, 2007).

At the middle and upper geomorphological level, grey monomineral sand with a similar age to the beginning of the NS-20 stage was also found in the NS-22 and both sections MIS 5 in age. It can be suggested that during the Kazantsev interglacial period in the vast area of the Nadym River basin there were favourable conditions for the erosion of the previously accumulated sandy textured deposits and their transfer downstream the main rivers.

One of the most interesting points of research is the kameform hill on the left bank of the Right Khetta River (NS-13/14); the formation of its middle part corresponds to the Tobol interglacial period (MIS 9–11). It can be suggested that the sediments in the upper part of the hill are not younger than the Taz glaciation (MIS 6), while the pebble layer formed during the degradation of the glacier reinforced the previous sediments and later was resistant to erosion and was not covered by the waters of the Kazantsev and Karga transgressions

The results of the sand quartz grain morphology analysis confirmed the supposed genesis of the studied sections. Thus, for sections NS-6 and K it was shown that in the upper part of sections the sequence is aeolian sediments, alluvial flood plain facies and channel facies of coarse stratified sands. At the base of both sections, there are sediments in which, apart from typical river grains, a large number of grains of various morphologies are found. These are grains of varying degree of roundness and irregular shape, with a smooth surface and smooth faces; often on their surface, there are various grooves and scratches formed under a strong mechanical impact, as well as conchoidal fractures. Their origin could be the result of freezing weathering and cryogenic transformation (Velichko and Timireva, 1995), as well as of high pressure applied to the grain surface (Immonen et al., 2014; Vos et al., 2014). Well-rounded ellipsoid and ball-shaped grains predominate in the top layer sediments. One can associate this distribution with materials coming from two different sources. One source could have been the former glacial sediments eroded by fluvial processes. This type of terrace structure corresponds well with the results of the study by Velichko et al. (2011), who analysed sands with underlay peat deposits in the investigated region.

Quartz grains from sections NS-13/14 and NS-20 are often characterised by low rounding classes, multiple conchoidal fractures, sometimes even conchoidal systems, a deep-pitted surface, scratches, grooves and cleavage surfaces. Such elements could be signs of processes that occur in glacial environments. Often, there are also signs of subsequent water treatment: separate crescentic depressions and smoothed sharp peaks of grains. It indicates the redeposition of the glacial grains by water flows. Along with the grains described above, there are also typical subaquatic grains, i.e. well-rounded with a fine-pitted surface, but their number is inferior to grains with glacial features.

Currently, we lack sufficient evidence to confirm the glacial genesis of these deposits. It is possible that the grains were exposed to the effects of glacial processes, with a final processing phase in their history that included subaquatic processes. In section NS-22, the grain morphology provides evidence that suggests the existence of a quiet subaquatic environment under which quartz grains underwent long-term treatment.

In general, the results of sand quartz grain morphoscopy and morphometry show that most quartz grains from all sections underwent complex multi-stage processing throughout their life.

The petrographic diversity of erratic boulders in Western Siberia helps us distinguish between two or three paleo-glacial regions that combine several dozen distributed provinces. Each is characterised by a specific set of rocks and petrographic features. The first major generalisation in this respect was made by Zemtsov (1976), who identified the guide boulders of the Ural region as ultramafic and mafic rocks of the main (axial) Uralian zone, plagio-granites, and highly metamorphosed rocks (gneisses and shales). In the central Siberian region, the prevailing boulders include dolerites and basalts of the Putorana Plateau, as well as various granitoids, quartzites and Palaeozoic sandstones of the Taimyr Region. These studies were substantially supplemented in detailed work by Sukhorukova et al. (1987).

Despite their small quantity, the petrographic analysis of pebbles and boulders led to the following conclusions. First, high-silica alkaline effusive rocks (sample N-10, quartz trachyte) are indicative of both the northern Taimyr Province (Troitsky and Shumilova, 1974) and many moraines of the Ural paleo-glacial region (Sukhorukova et al., 1987), but they are never found in the Putorana Plateau and the southernmost regions. Moreover, there is only a small relative share of dolerites (sample N-14, dolerite) and other effusive mafic rocks, which is a property of the Putorana and Nizhnyaya Tunguska regions. In contrast, there is no limestone that would be typical of the central Siberian paleo-glacial region (Kulyumbinsk and Sukhaya Tunguska distributive provinces according to Sukhorukova et al., 1987). There is no granite in the samples either, which is a property of the northern Taimyr region.

Second, quartz and quartzite sandstones are typical for the Ural paleo-glacial region, but their share is usually within a few per cent. Quartzitic sandstones also described as Palaeozoic were found 50 km north of Surgut within the tentative central Siberian and middle rock outwash zones (Sukhorukova et al., 1987). The source of the polymictic platy jointing sandstone could be the Palaeozoic bordering of the eastern slope of the Urals (Sukhorukova et al., 1987) or the Mesozoic sandstone of the Western Siberian Plate.

In general, the samples have a significant proportion of terrigenous rocks (sandstones and siltstones) and low content of dolerites. On the one hand, this can be explained by the poor representativeness of the samples. Nevertheless, the key washout zone could be located further north than the Putorana Plateau in the Taimyr area. To substantiate this point of view, further research is planned to determine the trace element composition and absolute dating and to expand the sampling.

Despite the numerous features that make it possible to attribute the thickness of grey monomineral quartz sand (K-1, NS-6, NS-20) to fluvioglacial sediments and the upper pebble strata of section NS-13/14 to glacial sediments, the study did not find typical moraine-like formations of lumped clay, loam and clay sand with gravel and large boulders in this territory. However, detailed descriptions of this type of sediment can be found in some references (Strelkov et al., 1965; Zemtsov, 1976; Sukhorukova et al., 1987; Babushkin, 1996).

Thus, in the middle course of the Left Khetta River at Point 70 (Khlebnikov, 1954) at 2.5 m deep there is a 20 m thick unit of densely clumped loam with interlayers of mica-enriched sand (the layers are up to 25 cm thick) (section AS-1, Fig. 9). The colour of the loam is brown, small glitter mica is visible, and angular debris (granite) is found, up to 25 cm in diameter. In the right part of the section upstream, stripping exposed a layer of fine-grained sand. Below 15 m it is followed by an interlayer of gravel–pebble rock. The prominent colluvium slope is covered by loam-crushed stone, and a cluster of gravel–pebble rock is also found on the towpath.

The huge moraine was described in the watershed of Nadym and Left Khetta rivers (point 2368) (section AS-2, Fig. 9) (Khlebnikov, 1954). It has a wide extension and rises up to 25–30 m above the surrounding plain. The ridge part of the range is convex and consists of individual peaks separated by meso-ridges. On the surface of the ridge, the congestion of pebble and gravel is found. The gravel–pebble, coarse-grained, well-washed and leached sand is traced down to the depth of 1.2 m.

Two esker-like linear elevations and a small kameform hill were discovered in the lower course of the Right Khetta River at well no. 18 (Khlebnikov, 1954) at 1.8 m depth in the gravel–pebble horizon with a total depth of 17.6 m (section AS-3, Fig. 9). The diameters of the pebbles are between 0.5 and 3–4 cm. The pebbles are not rounded and consist mainly of quartz and sandstone.

The moraine hills in the upper part of the Bolshoy Huhu River (right tributary of the Nadym River) have a north-west and a north-east orientation. The length reaches 6–7 km, and the relative height varies from 15 to 60 m (Yevseyev and Reynin, 1958). Morphologically, the steep slopes of the hills have individual smoothed tops separated by small saddles. The upper layer of the hills to a depth of 1–2 m is peeled loam with abundant pebble rock. The pebbles are weak and poorly rounded, and their diameters do not exceed 2–4 cm. Petrographic composition in one of the sections reveals (so-called point 367, Chekunova, 1954) silica, clay shale, arkose sandstones, breccia of clay–quartz rocks and limonite. The results of manual drilling at some small hills (Yevseyev and Reynin, 1958; Andreev, 1960) showed that they are folded with permafrost sediments. The total ice content as determined visually is not less than 30 %. As an example, well no. 10 (Yevseyev and Reynin, 1958), where light grey clay with a yellowish colour, which is light and porous, with aleurite interlayers, is found at a depth of 1.4–10.7 m; it has a wavy and horizontal lamination (section AS-4, Fig. 9). Clay thickness is underlaid with grey clay and fine-grained sands with poor sorting and admixture of gravel grains, quartz and silicon pebbles.

Data from both our studies and previous field ones are in good correspondence to the results of the analyses with the Tandem-X digital terrain models. These models revealed that despite the plain origin of the territory and the high salinity and dominance of erosion processes, various glacial and fluvioglacial relief features preserved to various degrees (kameform hills, proximal moraines, linear elevations, glacial meltwaters, etc.) are evident.

A linearly oriented relief caused by a glacial impact in northern Western Siberia is highlighted on the Map of Quaternary Formations in Russia (1:2500000 scale; Astakhov et al., 2016). At the same time, linear features and glacial remains are identified on geological maps of larger scales (Babushkin, 1995).

Nowadays, due to the increasing availability of digital elevation models, remote mapping of glacial relief features has become the standard method across the world (Clark et al., 2004; Glasser et al., 2008; Sharpe et al., 2010; Fredin et al., 2012; Atkinson et al., 2014; Norris et al., 2017). Based on modern spatial data, a detailed map for the “British Isles territory and coastal zone” (BRITICE-2) is available for digital study and analysis and was updated (Clark et al., 2018). The remote features of most forms of glacial relief for various natural conditions are described in detail and offer numerous pieces of evidence that can be used as standards for remote-sensing data interpretation, including the entire northern area of Western Siberia.

Our results showed a high efficiency of the simultaneous application of field ground and remote methods even with limited raw site rocks. Sediments were identified which can be immediately attributed to fluvioglacial (lower part of section K-1 and NS-6, section NS-20) and glacial (upper layer of section NS-13/14) origins. Traces of glacial treatment were also found as landforms in certain areas such as kameform hills, proximal moraines, linear-bed elevations and depressions of melt glacial water runoffs. Due to low organic substance content and sparse lichen, pine trees are formed over the fluvioglacial sediments on the low-fertility podzolic soils. It is a characteristic landscape feature of the leaching soil condition for the north taiga in Western Siberia. At the same time, the moraine-like layers of aggregated clay, loam and clay sand with gravel and large stone boulders that could not be found in field studies are widely described in sources previously unpublished (particularly the Left Khetta and the upper reaches of the Great Huhu River).

Thus, the development history of the Nadym River lower stream area provides evidence that periods of cover glaciations occurred here in the Pleistocene. At the same time, it is difficult to say whether it was a single glacier with a common front or whether there were several separate centres of ice accumulation. The available data, especially the structure and functional characteristics of the relief, appear to favour the second option, at least in the Late Pleistocene. In the early periods, traces of larger glaciation may represent the vast lake alluvial plains and flood plains, reaching a maximum area in the basin of the Nadym, Pur and Taz rivers. In this case, they can be regarded as the latest erosion formations but have preserved a characteristic structure inherited by modern landscapes.