Gravity flow deposits in Mesozoic sediments of Chukotka microplate (North‐East Russia)

In the Mesozoic succession of the Anyui–Chukotka fold system (North‐East Russia), five stratigraphic intervals were recognised that have an abundance of gravity flow deposits. These are the Olenekian (Lower Triassic), Upper Carnian, Upper Norian, Oxfordian–Kimmeridgian and Valanginian. The Triassic gravity flow deposits formed on the south‐facing, passive margin of the Chukotka microplate and consist of greywackes and lithic arenites. Palaeocurrent data indicate that the flows were directed towards the south‐east. The Olenekian gravity flow units consist of clast‐rich sandstone deposited on the continental slope, and clast‐poor sandstone deposited at the base of the slope. Upper Carnian mud‐poor sandstones were deposited at the base of the slope and the Norian thin‐bedded turbidites were upper to mid‐slope deposits. The continental margin was affected by tectonism and was uplifted in the latest Triassic–earliest Jurassic, possibly due to the initiation of the southward translation of the Arctic Alaska–Chukotka microplate. Following an Early–Middle Jurassic uplift of the area, sedimentation resumed in the Late Jurassic and earliest Cretaceous. Several syn‐orogenic depressions (Rauchua, Pegtymel, Pevek, Myrgovaam and Kytepveem) developed on the south‐western margin of the Chukotka microplate, and deposition in these basins included gravity flow deposits at various times. In both the Oxfordian–Kimmeridgian and Valanginian successions, gravity flow deposits included arkosic and subarkosic sandstones with a northern source area of granitoid complexes and deformed Triassic strata. The intervening Tithonian–Berriasian gravity flow deposits consisted mainly of thin‐bedded turbidites. These sediments had a southern source, which included a volcanic arc that had accreted to the southern margin of the Chukotka microplate.


| INTRODUCTION
Over the last 40 years, the Mesozoic clastic strata of the Chukchi Peninsula of North-East Russia have been used to support various models for the formation of the Amerasian Basin (Til'man, 1980;Embry & Dixon, 1994;Grantz et al., 1990Grantz et al., , 2011;;Miller et al., 2006Miller et al., , 2008;;Laverov et al., 2013;Shepard et al., 2013;Lundin & Dore, 2017;Midwinter et al., 2017;Hadlari et al., 2018).Unfortunately, there are still several questions that are related to the genesis of Mesozoic sediments in this region.For example, regarding the genesis of the Jurassic-Cretaceous sediments, some researchers have interpreted that syn-thrust basins were common in Chukotka at this time (Baranov, 1995;Filatova & Khain, 2007).In contrast, Miller et al. (2008) interpreted that the Upper Jurassic and Lower Cretaceous deposits of the Pevek and Rauchua depressions of Chukotka accumulated in a major syn-orogenic foreland basin that extended to the New Siberian Islands.Another disagreement involves the age of the Jurassic-Cretaceous arkosic sandstones of Chukotka.Gorodinskii and Paraketsov (1960), Paraketsov and Gorodinskii (1966) and Baranov (1995) proposed the existence of two levels of arkosic strata, whereas other authors assigned the arkoses to an undivided Jurassic-Cretaceous formation (Tibilov & Cherepanova, 2001).

SETTING
The study area is part of the Anyui-Chukotka fold belt (Figures 1 and 2), in which the Eastern Chukotka and Western Chukotka terranes can be distinguished (Figure 2).The terranes are composed of Neoproterozoic basement and a sedimentary cover of platform and shelf deposits of Palaeozoic to Mesozoic age (Parfenov et al., 1993;Nokleberg et al., 1994;Sokolov et al., 2001).The Anyui-Chukotka and New Siberian-Wrangel fold belts formed the Chukotka microcontinent, which is part of the Arctic Alaska-Chukotka microplate (AACM) (Grantz et al., 2011;Lawver et al., 2011).
The first stage of Mesozoic deformation in the area occurred in the Jurassic and involved the formation of south-verging folds and thrusts (Katkov et al., 2010;Golionko et al., 2018).The deformation can be related to the early rift stage of the opening of the Amerasia Basin (Grantz et al., 1990(Grantz et al., , 2011)).The second stage of deformation is expressed in north-verging folds and thrusts in both Triassic and Upper Jurassic-Lower Cretaceous strata (Verzhbitskyi et al., 2012;Golionko et al., 2018).This compressive deformation is related to the collision of Siberia with the Chukotka microcontinent in the Neocomian (Parfenov et al., 1993;Sokolov et al., 2001Sokolov et al., , 2015)).
Triassic sediments of Chukotka were deposited on a passive, south-facing continental margin and the Lower Triassic succession is characterised by numerous sills and small hypabyssal bodies of diabase, gabbro and dolerite (252 ± 4 Ma; Ledneva et al., 2011).Jurassic-Cretaceous sediments accumulated in isolated depressions of different ages.The total thickness of the Mesozoic deposits is at least 8 km, with the Triassic deposits being between 4000 and 5000 m thick and the Upper Jurassic-Lower Cretaceous strata varying between 600 and 2360 m (Figure 3).The Mesozoic succession is intruded by Albian post-collision granites yielding dates from 115 to 117 Ma (Katkov et al., 2010).

| Triassic
The Triassic siliciclastic strata occur in NW-SE-directed fold and thrust structures (Figure 2).They consist mainly of thinly interlayered siltstone and mudstone.Sandstone beds are present in most sections, and the ratio and thickness of the sandy units vary greatly from section to section.The palaeontological ages of the strata are based on work by Yegorov (1962), Tibilov et al. (1982), Bychkov (1994), Baranov (1995) and Tibilov and Cherepanova (2001), and are summarised by Tuchkova et al. (2009).Facies studies of the strata established the presence of shelf, prodelta, continental slope and base of slope environments of deposition.Facies distribution indicated a source area to the north with a southward progradation of the facies.(Til'man & Yegorov, 1957;Sadovsky, 1962;Morozov, 2001;Tuchkova et al., 2009Tuchkova et al., , 2014)).
In the Anyui subterrane (Figures 2 and 3), the Triassic deposits unconformably overlie Devonian-Carboniferous strata and thin to the north; this is especially typical for Lower-Middle Triassic strata.Strata of deep marine origin occur in the southern parts of the subterrane.The Lower-Middle Triassic succession is characterised by three lithological associations.The basal section is dominated by interlayered siltstone and mudstone.The middle portion contains numerous interbeds of turbidites, and the upper part is again dominated by siltstone and mudstone with rare thin sandstone beds.The Carnian succession consists almost exclusively of shale and siltstone in the lower portion, with sandstone units becoming common in the upper part of the succession.Norian deposits consist mainly of rhythmically interbedded siltstone and shale, with rare beds of sandstones in local areas (Figure 4).
In the Chaun subterrane, the Triassic succession is similar to that of the Anyui subterrane, and the strata are also thin to the north.In some areas, the succession includes Permian deposits (Yegorov, 1959;Bychkov, 1959; F I G U R E 1 (A) Rotational model for the opening of the Amerasia Basin illustrating the counterclockwise rotation of the Arctic Alaska-Chukotka microplate (AACM) (Grantz et al., 1990).The purple rectangle outlines the study area.(B) Tectonic elements of north-eastern Russia with the Western and Eastern Chukotka terranes, after Sokolov et al. (2010).(C) Location of the Chukotka region (red).Til'man, 1980).In the Velmay River area, the Lower Triassic shelfal deposits are associated with synsedimentary volcanoclastic deposits (Tuchkova et al., 2014).
Oxfordian-Kimmeridgian strata are recognised in the Kytepveem and Myrgovaam depressions, with Tithonian and younger strata in the Rauchua, Pevek and Upper-Pegtymel depressions.These Upper Jurassic-Lower Cretaceous deposits consist of interbedded sandstone, siltstone and mudstone with beds and lenses of conglomerates.The sedimentological features indicate a submarine fan to basin floor origin for the strata (Vatrushkina & Tuchkova, 2014, 2018).
Oxfordian-Kimmeridgian arkosic sandstones are exposed in the Myrgovaam and Kytepveem depressions, where they have been thrust over Norian strata (Baranov, 1995).These strata consist of alternating beds of structureless sandstone with thinner intervals of interbedded sandstone, siltstone and mudstone.An angular unconformity separates Tithonian strata from underlying Triassic rocks.The succession consists of graded rhythms of sandstones, siltstones and mudstones, with lenses and beds of gravels and small-pebble conglomerates in the Rauchua and Upper-Pegtymel depressions; fine-grained deposits dominate the Pevek depression; and the orientation of slump folds indicates the basin shallowed to the south (present-day coordinates).A distinctive feature of the Tithonian deposits is a high proportion of volcanic material and the presence of thin layers of intermediate-composition lavas (Pyankov et al., 1980;Genze, 1990).
Berriasian strata are widespread in the Rauchua and Pevek depressions, where they conformably overlie Tithonian deposits.The strata consist of interbedded, fine-grained sandstones, siltstones and mudstones.Valanginian deposits conformably overlie the Berriasian strata and consist of alternating sandstone units with intervals of rhythmically interlayered sandstone, siltstone and mudstone.

MATERIAL
As noted earlier, this contribution focusses on the gravity flow deposits of the Mesozoic succession of Chukotka.In the field, the description of the strata included the identification of the type of rock, the description of primary sedimentary structures, the thickness of individual beds and their relationship with underlying beds, the presence of inclusions (e.g.rock fragments, fossils, wood) and the identification of rhythms and their thickness.All sedimentological descriptions in the field were accompanied by a determination of the structural geology.This was especially important for determining the direction of palaeocurrent indicators, given the substantial deformation of the strata.
To classify gravity flow deposits, the units were subdivided into thin beds, thick beds with mud-poor sandstone and thick beds with mud-rich sandstone, following the work of Walker (1985), Mulder and Alexander (2001), Haughton et al. (2003) and Talling et al. (2004).Petrographic investigation included microscopic identification of mineral composition.Thin sections were point-counted (200-250 grains) following the petrographic method of Shutov (1967).To avoid the problem of grain-size-dependent compositional variations, only the medium-sand-sized fraction (0.15-0.25 mm) was counted.
An important feature of the arkosic sandstones in the Oxford-Kimmeridgian and Valanginian successions is the presence of mudstone fragments.To determine the composition and origin of the mudstone clasts, they were extracted from sandstones and analysed by inductively coupled plasma-mass spectrometry (ICP-MS).The obtained data were then geochemically compared with Triassic, Oxford-Kimmeridgian and Valanginian shales.

MESOZOIC DEPOSITS OF CHUKOTKA
In the Mesozoic strata of the Western Chukotka terrane, gravity flow deposits are dominant at several levels in the Triassic succession.Thick intervals of gravity flow deposits are the most common in the Olenekian and Upper Carnian successions, while being sporadic in Norian strata (Figure 3).For the Jurassic-Cretaceous succession, notable intervals of gravity flow deposits are present in the Oxfordian-Kimmeridgian and Valanginian strata, with lesser amounts in the Tithonian-Berriasian succession (Figure 5).

| Triassic deposits
Olenekian sandstones were studied in the riverside cliffs of the Enmynveem River as well as the Karalveem River (near Bilibino).Both thin beds and thick beds with mud-rich sandstones are present.Thin-bedded turbidites comprise fine sandstone, siltstone and mudstone, and their thickness is no more than 40 cm, typically 5 to 25 cm (Figure 4А).Flutes and erosional basal contacts are rare.The coarsest grain size is fine, and siltstone is most common.
Thick beds with mud-rich sandstone are found in both areas, and they can be of two types: (i) clast-rich and (ii) clast-poor sandstones.The gravity flow deposits are interpreted to have accumulated in sedimentary depressions on a continental slope and at the base of the slope.
Small, flattened mudstone clasts are common at the base of the clast-rich sandstones (Figure 4D).The mudstone clasts in some interlayers comprise 10 to 30% of the rock.An erosional contact, as well as grading bedding, are observed in the Olenekian thick-bedded sandstones.Flame structures, prolapsed bedding and slumping of sandy material are sometimes present.Sandstone bed thicknesses vary between 0.3 and 0.4 m in the Enmynveem River section and between 0.25 and 0.35 m in other sections.These sandstones are interpreted to have been deposited by high-density turbidites in a proximal prodelta portion of the continental slope (Tuchkova et al., 2009).
Mudstone clasts are rare in clast-poor sandstones and are sometimes found in the middle part of a sandstone bed.Sandstone beds are composed of amalgamated units of alternating fine-grained sandstone, siltstone and mudstone, with an interbed thickness between 4 and 12 cm.These sandstone beds do not have erosional basal contacts and are structureless to thinly laminated.They are interpreted to have been deposited by low-density turbidite flows (Tuchkova et al., 2009).
The Lower Triassic sandstones are classified as lithic greywacke (Figure 6A) and they contain 15 to 30% clay matrix.The sandstones are fine-medium-grained and poorly sorted and they occasionally contain coarse grains with the grains, being subround, subangular and angular.There are two greywacke groups: one is dominated by basic volcanic rock fragments, and the other by metamorphic rock fragments (Tuchkova et al., 2007b).The first corresponds to the clast-rich, high-density turbidites described above, and the second to clast-poor, low-density turbidites.sandstones 5 to 15 m thick, alternating with thin-bedded intervals.The sandstones have erosional contacts and ripple cross-bedding near the base.Cross-beds and wavy lamination also occur.In the shaly intervals, flame structures and sediment slumping are common.Sandstones contain chaotically oriented oval mud clasts (Figure 4H).The strata are interpreted as turbidites deposited at the base of the continental slope (Tuchkova et al., 2009).
The Carnian turbidites are mainly litharenites, with occasional arkosic arenites (Figure 6B).The sandstones are medium to coarse-grained, and grain roundness varies from angular to subrounded.The clay matrix is usually about 5%.Basic volcanic rock fragments are absent, and the mineral composition is uniform.In turbidites without mudstone clasts, rock fragments are not common and are primarily shales, cherts and granite-gneisses; in the turbidites containing mudstone clasts, sedimentary and metasedimentary rocks predominate.
Sandstones are rare in Norian strata and are found in outcrops on the southern and western spurs of Pyrkanay Ridge, the riverhead of the Machvavaam River, the Irguneyveem River and tributaries of the Kytep-Guytenryveem River.They form isolated interbeds in thick intervals of mudstone and siltstone and are categorised as thick beds with mud-poor sandstones.The sandstone beds commonly have erosive lower boundaries, tool marks and soft sediment deformation.The sandstone is fine-grained and not graded, with bed thicknesses of 0.2 to 0.4 m (Figure 4J).The sandstones are interpreted to have been deposited on an outer shelf with the flow of clastic material from small deltas (Tuchkova et al., 2009).
The Norian sandstones are litharenites with a clay matrix of 5 to 7%.They are medium to well-sorted with subangular to angular grains.Disseminated plant debris is common.Mineral composition is uniform, and rock fragments of shales, granites, gneisses, acid and medium plutonic rocks and metavolcanic fragments predominate.
In the Triassic, the sediment supply came from the north-east based on palaeocurrent measurements of cross-beds and current marks in the Olenekian, Carnian and Norian sandstones (Tuchkova et al., 2009(Tuchkova et al., , 2014)).The mineral compositional evolution of Triassic sandstones points to the erosion of a large metamorphic complex made up of metasedimentary, metavolcanic and granitegneiss rocks (Tuchkova et al., 2014).
Mud-poor sandstones have a thickness of 0.4 to 1.2 m.In the lower portion of graded beds, the sandstones are medium to coarse-grained and massive.Higher, they become cross-laminated and planar-laminated, fine-grained sandstones and siltstones.Flame structures are sometimes present at the base of beds (Figure 5D).Mud-poor sandstones often form units of amalgamated beds (15-25 m), separated by thin intervals (no more than 1 m) of thinbedded siltstone-mudstones.
Mud-rich (type clast-rich) sandstones have thickness up to 1.5 m, and erosional contacts and load casts are common.The sandstones are not graded and contain chaotically oriented mudstone fragments of two types: rounded 2 to 3 cm mud clasts and non-rounded angular mud chips up to 0.7 cm (Figure 5A,B).The amount of mud clasts differs significantly from bed to bed.Thick beds with clastrich sandstones that contain only small mud chips are frequent.The geochemical composition of the mudstone fragments indicates that the large mud clasts were from local beds, whereas the small, angular mud clasts were most probably derived from Triassic shales (Figure 7).
The clast-rich and mud-poor sandstones are classified as arkosic arenites with a clay matrix of 5 to 10% (Figure 6C).The sandstones are fine-medium-grained with grains that vary from angular to subangular to subrounded.Feldspars and quartz are dominant, with potassium feldspar being about 20% of the total mineral components.Rock fragments, which rarely exceed 10%, consist of felsic volcanics, quartz-feldspar aggregates, organic rich, cleaved mudstones and quartz-mica schists.The sandstones are interpreted to be slope deposits with a reasonably close granitoid source (Vatrushkina & Tuchkova, 2014).The cleaved and angular mud clasts were derived from Triassic strata.It is difficult to interpret the location of the source area due to a lack of palaeocurrent data or facies changes.A northern source is supposed based on grain composition.
Tithonian-Berriasian deposits were studied in the Rauchua, Pevek and Upper-Pegtymel depressions (Figure 2).Sandstones of gravity flow origin form the bases of thin-bedded turbidites and thick beds with mudrich sandstone intervals (clast-poor type).The thin-bedded turbidites are represented by fine-grained sandstones, siltstones and mudstones, which form graded beds varying from 5 to 40 cm.The thickness of sandstone beds does not exceed 20 cm.Sandstone units are characterised by small bulbous flutes and load casts as well as by wavy, occasionally laminar or convolute lamination in the central and upper parts of the bed (Figure 5F).
Thick beds with mud-rich sandstones are present but not common in the Tithonian-Berriasian succession.Their thickness varies from 0.7 to 1.7 m.They have lower erosional contacts and flame structures, and rip-up clasts occur near the base of beds.The sandstones (clast-poor type) are laminated, and clay content increases upward (Figure 5G).
The Tithonian-Berriasian sandstones are mainly lithic arenites, with occasional arkosic arenites (Figure 6C).The matrix contribution, with a high content of ash material, varies from 5 to 25%.Grains are quartz, plagioclase and rock fragments in different proportions.The Tithonian sandstones are characterised by volcanic rock fragments of mostly mafic and intermediate composition.The Berriasian sandstones are more mature, with a higher percentage of quartz grains, and mudstone fragments dominate (Figure 6I).Volcanic rocks are mainly represented by felsic clasts.The main source areas of the Tithonian-Berriasian deposits are interpreted to be located to the south and include a volcanic arc and Triassic siliciclastics.
Valanginian deposits are widespread in the Pevek and Rauchua depressions to the north of the Oxfordian-Kimmeridgian strata (Figure 2).The Valanginian sandstones form units of thick beds with mud-poor sandstones and sporadic, thick beds of mud-rich (clast-rich type) sandstones, alternating with intervals of thin-bedded turbidites.
Mud-poor sandstones form graded beds up to 1.5 m thick.The basal contact is erosional, with load casts and large flame structures.The sandstones are structureless, with sporadic interlayers of flattened mud clasts.The upper portion is composed of fine-grained sandstones and siltstones with grading bedding.The sandstones often form amalgamated beds without the finer-grained upper portion, and these units alternate with thin-bedded turbidites.
Clast-rich sandstones are present but not common in the Valanginian strata.They have a thickness of 0.6 to 0.8 m and form sporadic interlayers with thin-bedded turbidites.The lowermost part of these sandstones is structureless, and rounded mud clasts (up to 15 cm) often occur in the middle portion (Figure 5I).The upper portion is represented by silty sandstones and siltstones.
The arkosic sandstones are fine-medium-grained with a silt fraction up to 40% and a clay matrix.The sandstones are poorly sorted, with angular to subangular grains.The mineral composition of Valanginian sandstones is similar to that of Oxfordian-Kimmeridgian arkosic arenites, but they have less feldspar, with potassium feldspars being present in single grains.Mud clasts contained in the sandstones are synsedimentary (Figure 7).
It appears that the Valanginian gravity flow sandstones were derived mostly from the same granitoids that sourced Oxfordian-Kimmeridgian sandstones as described above.However, it is also possible that exposed Oxfordian-Kimmeridgian strata were also a source (Vatrushkina & Tuchkova, 2018, 2020).
The location of the varying types of gravity flow deposits for the Mesozoic succession is illustrated in Figure 8.

| SUMMARY OF GRAVITY DEPOSITS OF THE CHUKOTKA ARCTIC MARGIN IN THE MESOZOIC
A passive continental margin existed from the earliest Triassic and possibly even from the latest Permian, as is evidenced by the presence of sills and small hypabyssal diabase bodies, 252 ± 4 Ma in age (Ledneva et al., 2011).Olenekian sandstones were deposited on the continental slope (clastrich sandstone) or at the toe of the slope (clast-poor sandstone) being fed from basin margin deltas (Figure 9A).These deposits included high-density turbidites, suggesting the presence of a narrow shelf at this time (Tuchkova et al., 2009(Tuchkova et al., , 2014)).
In the Late Carnian, the sandstones were deposited at the base of the continental slope (Figure 9B).An underwater fan with a developed channel, several lobes and a levee on the slope is indicated (Tuchkova et al., 2007a(Tuchkova et al., , 2007b(Tuchkova et al., , 2009(Tuchkova et al., , 2014)), where mud-poor sandstones were deposited.In the Norian, thin-bedded turbidites were the most common deep-water deposits, with rare clastpoor sandstones found in local areas.They are interpreted to correspond to the distal zone of a submarine fan (Figure 9C).
The mineral composition of the Triassic sandstones indicates the existence of two sources-metamorphic rock complexes and a metavolcanic source.The metavolcanic source dominates in the Olenekian, and the metamorphic source is represented by low-grade metamorphic rock fragments in Early-Middle Triassic deposits as well as by intermediate-grade metamorphic rock fragments in the Upper Triassic sandstones.
Sedimentation on the passive continental margin ended by latest Triassic with deformation of the Triassic strata close to the Triassic-Jurassic boundary (Tuchkova et al., 2007a(Tuchkova et al., , 2007b)).Lower Jurassic deposits are locally found in small outcrops, and Middle Jurassic deposits are absent.Sedimentation in the region began again in the Oxford-Kimmeridgian in various depressions (Paraketsov & Paraketsova, 1989).
Oxfordian-Kimmeridgian sandstones were deposited on unchannelled slopes in a relatively shallow environment (Figure 10A).The arkosic composition of Oxfordian-Kimmeridgian clast-rich and mud-poor sandstones suggests that a high relief, granitic source area was relatively close to the depositional basin.A high sedimentation rate is indicated by the common presence of flame structures in mud-poor sandstones.Fragments of cleaved Triassic mudstones are also present in the Oxfordian-Kimmeridgian sandstones.Overall, it appears the main source area, with granites and deformed Triassic strata, lay to the north.
Tithonian-Berriasian deposits contain lenticular interbeds of gravelstones and conglomerates (Rauchua depression) and intervals of interbedded sandstones, conglomerates, thinbedded turbidites and pebbly mudstones (Upper-Pegtymel depression).The succession is interpreted to have been deposited in the several deep-water fans.Tithonian sandstones contain mafic and intermediate volcanic rocks and cleaved organic-rich mudstones.The main source terrain for the Tithonian-Berriasian deposits is interpreted to be a volcanic arc located to the south based on the presence of a large amount of pyroclastic material as well as the common occurrence of volcanic rock grains and fragments (Vatrushkina & Tuchkova, 2018;Vatrushkina et al., 2019).
The next stage of deposition was in the Valanginian, when arkosic, mud-poor and clast-rich sandstones were deposited in the Pevek and Rauchua depressions (Figure 10C).As described earlier, amalgamated thick beds with mud-poor sandstones are represented by turbiditelike rhythms, and they are separated by thick units of thin-bedded turbidites (Figure 7).The composition of the sandstones indicates a similar source area as that for the Oxfordian-Kimmeridgian sediments, but probably with lower relief.In the interval between two 'arkose' events, granitic complexes located to the north did not take part in the orogeny, the erosion rate decreased and the weathering process destroyed potassium feldspars.

PALAEO -TECTONIC EVOLUTION OF CHUKOTKA MARGIN IN THE MESOZOIC
In the Triassic, sedimentation occurred on a passive margin that faced the proto-Arctic (South Anyui) Ocean to the south (Zakharov et al., 2002;Lobkovskii et al., 2011, Laverov et al., 2013;Sokolov et al., 2008Sokolov et al., , 2010Sokolov et al., , 2012)).In the Early and Middle Triassic, a narrow shelf zone triggered the accumulation of deep water, clast-rich and clast-poor sandstones fed from basin margin deltas.Due to very high sedimentation rates (Tuchkova et al., 2007a(Tuchkova et al., , 2009(Tuchkova et al., , 2014)), the deltaic, shelf and deep-water environments prograded southward throughout the Triassic (Figure 9B).The composition and size of rock fragments in the Triassic sandstones suggest that the source terrain included low mountains or hills.
During the Triassic, Chukotka's passive continental margin did not undergo any significant tectonic transformations.However, as described herein, in the Triassic section, gravity flow deposits were most common in the Olenekian, late Carnian and Norian, indicating there was an increase in sediment supply at these times.It is suggested here that these times correspond to periods of increased uplift in the source terrain and/or increased runoff due to climate changes.
The development of the passive margin ended with a stage of deformations, as indicated by the presence of cleaved Triassic mud chips in Upper Jurassic deposits.Moreover, the difference in the type of deformation of the Triassic and Jurassic-Cretaceous deposits (Golionko et al., 2018) of this tectonism.This deformation, which involved folding, thrusting and tectonic transport of the Triassic succession to the north, appears to be related to the early phase of the rotational opening of the Amerasian Basin when the AACM moved to the south-west (Grantz et al., 2011;Sokolov et al., 2015;Golionko et al., 2018).
From the Late Jurassic through the Barremian, there was a convergence between the AACM, which includes Chukotka, and the active margin of Siberia.This resulted in the uplift of orogenic structures along the southern margin of the AACM continental block.In the Late Jurassic, the deformation included periods of warping and extension of the continental crust near the orogen.A graben-shaped depression formed since the Oxfordian-Kimmeridgian on the Chukotka's southern margin and arkosic, clast-rich sandstones with cleaved Triassic mud chips were deposited on the basin slope and floor.The sandstones were derived from uplifted, granitoid massifs and deformed Triassic strata, which were located to the north.
In the Tithonian and Berriasian basins, gravity flow deposits of lithic and arkosic arenites were deposited.
The source area lay to the south and included deformed Triassic rocks as well as a continental arc that existed on the southern edge of the Chukotka microcontinent.A new stage of the accumulation of arkosic, mud-poor and clast-rich sandstones occurred in the basin in the Valanginian.The main source area presumably lay to the north and included granites and/or uplifted arkosic Oxfordian-Kimmeridgian sandstones as well as deformed Triassic strata.
The main phase of the collision of Siberia's active margin and the Chukotka microcontinent occurred in the Hauterivian-Barremian time and was completed by the start of the Aptian.At this time, a north-verging fold and thrust belt formed in the region with the closure of the oceanic basin and the formation of the South Anyui suture zone.Following the final stage of the collision (Aptian-Albian), the orogen began to collapse with sublatitudinal stretching accompanied by the formation of granitic and metamorphic domes and post-collisional intrusions (Gel'man, 2000;Bondarenko, 2004;Luchitskaya et al., 2010;Sokolov et al., 2001).changes in the northern source region, which comprised a large continental block.
In the Jurassic-Cretaceous, gravity flow deposits occur in the Oxfordian-Kimmeridgian, Tithonian-Berriasian and Valanginian strata.These Upper Jurassic-Lower Cretaceous deposits accumulated in an active tectonic setting with frequent changes in the main source areas.Local basins were surrounded by mountains consisting mainly of deformed Triassic rocks as well as granitoid complexes.Gravity flow sediments were deposited on the slopes and at the bottom of syn-orogenic depressions.In the Oxfordian-Kimmeridgian, orogenesis occurred to the north of the depressions and exposed granites to erosion.This led to the deposition of thick arkosic sandstones.
The accretion of the Kulpolney island arc and the formation of the Nutesyn continental arc on the southern edge of the Chukotka microcontinent occurred in Tithonian and Berriasian times, resulting in sedimentation from the south in a back arc basin (Figure 11).These strata consist mainly of litharenites with a high content of volcanic material.Subsequently, during the Valanginian, arkosic gravity flow deposits were once again derived from granite complexes and deformed Triassic rocks to the north.
From the perspective of the tectonic evolution of the Amerasia Basin, the data presented here indicate that the basin began to form no later than the earliest Jurassic, when deformation and uplift of the Triassic passive margin were initiated on the southern margin of Chukotka.These findings also indicate that, during the rift phase of the Amerasia Basin (Hettangian-Hauterivian), there were intervals of increased tectonic activity during the Oxfordian-Kimmeridgian and the Valanginian.

ACKNO WLE DGE MENTS
We would like to give special thanks to Ashton Embry, who corrected the English language of this manuscript.This study was supported by the GIN RAS project 'Fundamental problems of tectonic, lithogenetic and magmatic processes in formation of NE Asia fold structures'.

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I G U R E 2 A geological map of the Arctic modified from the Geological Map of the Geological Survey of Canada (2008) showing the location of Jurassic-Cretaceous depressions: Pg, Upper-Pegtymel; Kt, Kytepveem; M, Myrgovaam; R, Rauchua; P, Pevek.On the map: OCVB, Okhotsk-Chukotka Volcanic Belt; An, Anyui subterrane; Ch, Chaun subterrane; SA, South Anyui terrane.

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I G U R E 3 (A) Stratigraphy of the Permian-Triassic and Jurassic-Cretaceous sedimentary succession in the Chukotka region.(B) Photograph of a typical Olenekian sandstone.Lower-Middle Triassic, Enmynveem River.The lower part of the cliff is interpreted as massive, high-density, thick-bedded turbidite; Upper part-rhythmic thin-bedded unit of turbidite.(C) Photograph of thick-bedded turbidites in Upper Carnian sandstone.Maly Anyui River, near Bilibino.(D) A unit of thin-bedded turbidite with one bed of mud-poor sandstone.Norian sandstones, Pyrkanay Ridge.(E) Photograph of Oxfordian-Kimmeridgian massive clast-poor sandstones in Pogynden River.(F) Photograph of Valanginian clast-rich sandstones.Chaun Bay, near Pevek.

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I G U R E 4 Sedimentary structures of Triassic gravity flow deposits in Olenekian, Upper Carnian and Norian Sandstones.Left column: schematic representation of turbiditic sandstone units with a detailed description of the sandstone fragments: facies and turbidite identification.Photographs of typical sandstones are to the right: (A) Thin-bedded turbidite alternating with thick-bedded turbidite in Olenekian succession, Enmynveem River.(B) Mud-rich sandstone with small sulphide concretion, cross-bedding and rare mud clasts, Enmynveem River.(C) Photograph of Olenekian sandstone succession with mud-rich turbidite sandstone (greywackes).Enmynveem River.(D) Thick-beds turbidite with rounded mudclast, Enmynveem River.(E) Photograph of Upper Carnian succession with thin-bedded turbidites, Kytep-Guitenryveem River.(F) Thick turbidite beds with clast-poor sandstones, Kytep-Guitenryveem River.(G) Thin sandstone beds alternating with thick beds of mud-poor sandstone, Maly Anyui River.(H) Clast-rich sandstone, mudclasts in the lower part of the bed are rounded, Kytep-Guitenryveem River.(I) Norian thin-bedded succession with rare thick beds with clast-poor sandstone.Sandstone with a vertical quartz vein.Western part of Pyrkanay Ridge.(J) Clast-poor sandstone with erosive contact, Machvavaam River.(K) Clast-poor sandstone with convolute bedding and erosive contact, Irguneiveem River.Carnian strata with gravity flow deposits were studied in the riverside cliffs of the Maly Anyui, Bolshoy Keperveem, Maly Keperveem, Karalveem and Kytep-Guytenryveem rivers.The sandstones are confined to the upper portion of the Carnian succession and are of two types: (1) thin and (2) thick beds with mud-poor sandstone.Intervals of mud-poor sandstones, with a thickness of 0.25 to 0.6 m, alternate with intervals of thin-bedded sandstone, siltstone and mudstone, with a thickness of 0.1 to 0.4 m.These intervals often form amalgamated F I G U R E 5 Sedimentary structures of Jurassic-Cretaceous gravity flow deposits in Oxfordian-Kimmeridgean, Tithonian-Berriasian and Valanginian sandstones.See Figure 4 for a legend.(A) Oxfordian-Kimmeridgian thick beds with mud-rich (clast-rich type) sandstone interval.(B) Fragment of Oxfordian-Kimmeridgian sandstone with angular mud chips.(C) Oxfordian-Kimmeridgian thick-bedded mudpoor sandstone.(D) Oxfordian-Kimmeridgian clast-poor sandstone with flame structure.(E) Tithonian thin-bedded siltstone and argillite with rock fragments.(F) Laminated sandy siltstone and argillite in the Berriasian Unit.(G) Thick bed with Berriasian mud-poor sandstone.(H) Valanginian thin-bedded turbidites.(I) Valanginian thick beds with clast-rich sandstone (J) Valanginian thick-bedded mud-poor sandstone.

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Geochemical composition (REE) of mudstones and mud chips in sandstones.

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I G U R E 8 Geological map of Chukotka region with location of sections with different types of thick-bedded and thin-bedded intervals for Triassic (A) and Jurassic-Cretaceous (B).
, as well as the absence of sedimentation in the Early and Middle Jurassic, indicate tectonism began near the Triassic-Jurassic boundary and perhaps extended into the Middle Jurassic.The age of newly formed micas developed along cleavage in the Carnian siltstones and argillites (Tuchkova et al., 2007a) adds further support to the timing F I G U R E 9 Schematic sedimentation for the different sandy intervalspalaeogeography with the block diagrams for the Triassic: (A) Norian; (B) Carnian; (C) Olenekian.

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Schematic sedimentation for the different sandy intervals-palaeogeography with the block diagrams for Jurassic-Cretaceous: (A) Valanginian; (B) Tithonian-Berriasian; (C) Oxfordian-Kimmeridgian.In summary, the evolving geodynamic setting of the Chukotka microcontinent during the Mesozoic is reflected in the gravity flow deposits of the Mesozoic succession.In the Triassic, a passive continental margin was present, and gravity flow sediments, which occur mainly in Olenekian, Upper Carnian and Norian strata, were deposited on the continental slope and sourced from basin margin deltas (Figure11).The mineral composition of the sandstones indicates an extensive catchment area with various sources, which were dominated by low-mediumgrade metamorphic complexes.Changes in mineral composition throughout the Triassic reflect relatively minor

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Mesozoic geotectonic evolution of Chukotka.The main source regions for the gravity flow deposits are illustrated.