Cenozoic deposits of western Kotel’nyi Island (New Siberian Islands): key insights into the tectonic evolution of the Laptev Sea

ABSTRACT The Arctic sedimentary basins are still poorly studied in comparison with other regions. The lack of deep wells across the eastern Russian Arctic has resulted in numerous contrasting geodynamic models for the geological evolution and age of sedimentary successions within this frontier region, where a modern mid-ocean ridge breaks through the continental crust in the Laptev Sea. The only onshore evidence of rifting processes is a number of small graben-like depressions exposed on the New Siberian Islands and along the Laptev Sea coast. We present U-Pb detrital zircon provenance and palynology study results of the Cenozoic sedimentary rocks filling graben-like depressions across western Kotel’nyi Island. Palynological data indicate that these sedimentary rocks are Early Eocene to Pleistocene in age. Based on U-Pb detrital zircon dating, Early Eocene and Late Oligocene clastic sediments were sourced from underlying deformed Palaeozoic rocks as well as by reworking of Upper Mesozoic rocks outcropping elsewhere on Kotel’nyi Island, which bear Siberian signature. Plio-Pleistocene clastic sediments were not derived from the erosion of deformed Palaeozoic rocks, suggesting the cessation of active uplift by this time and the development of a regional peneplain. Therefore, by extrapolating our onshore observations to the neighbouring offshore, we propose that graben structures imaged by seismic profiles along the eastern flank of the Laptev Rift System are likely to host Eocene and Oligocene sediments. Thus, it implies the Cenozoic extension led to formation of grabens on- and offshore in the eastern portion of Laptev Sea as early as Eocene. Further studies are necessary to evaluate the age of graben-related basins in the central and western part of the Laptev Sea.


Introduction
The eastern Russian Arctic is characterized by a wide and shallow shelf basin with a thick sedimentary cover (Drachev et al. 2010;Moore and Gautier 2017). Due to an absence of deep wells drilled offshore within this region, interpretations of offshore stratigraphy and structures from seismic data can only be based on studies of onshore outcrops around the periphery of the offshore basins. Moreover, Cenozoic history of High Arctic is still enigmatic due to only few wells drilled in the Arctic Ocean. Thus, our study has been focused on Cenozoic rocks of the New Siberian Islands (NSI) and their implication for the Laptev Sea Rift System development.
The NSI archipelago represents an exposed Palaeozoic-Mesozoic crustal domain located in the eastern Russian Arctic between the Laptev and East Siberian seas ( Figure 1). This archipelago comprises three island groups: the Anzhu Islands (Bel'kovsky, Kotel'nyi, Faddeev, and Novaya Sibir) in the centre, the Lyakhovsky Islands in the south, and the small De Long Islands in the far north. Kotel'nyi and Faddeev Islands and separating them tidal lowland of Zemlya Bunge form a larger island that we call informally Greater Kotel'nyi (GK).
Limited rock exposures across the NSI archipelago provide rare glimpses into the geological evolution of the Laptev and East Siberian seas. The western part of Kotel'nyi Island is adjacent to the Laptev Shelf. The latter is dominated by a 350-480 km wide and 750-900 km long rift system developed at the intersection of the modern oceanic spreading ridge (the Gakkel Ridge) with a continental margin (Drachev and Shkarubo 2018 and references therein).
However, due to the lack of deep wells drilled offshore, the age and composition of the sedimentary infill within of this extensive rift system are strongly debated.
As a result, competing geodynamic models exist for the geological evolution of the Laptev Sea. The one model suggests that sedimentary infill of the western and central parts of the Laptev Sea shelf comprises Mesoproterozoic, Palaeozoic, and Mesozoic sedimentary rocks correlative with those of the Siberian Craton to the south (Ivanova et al. 1990;Sekretov 2000). However, we consider this model to be the most unlikely, as there appears to be no direct correlation between onshore and offshore sedimentary successions based on the most recent seismic studies (Drachev et al. 2010;Drachev and Shkarubo 2018). A second model claims that rifting of the Laptev shelf commenced during the Aptian-Albian, with rift basins comprising sediments of latest Early Cretaceous to Late Cenozoic age (Shkarubo and Zavarzina 2011;Shkarubo et al. 2014;Vinogradov et al. 2016;Nikishin et al. 2017Nikishin et al. , 2020Nikishin et al. , 2021. A third model proposes that initial rifting commenced during the latest Cretaceous (Maastrichtian) to Palaeocene, with rift basins mainly comprising sediments of Cenozoic age (Drachev and Savostin 1994;Drachev et al. 1998Drachev et al. , 2010Franke et al. 2000Franke et al. , 2001Franke and Hinz 2009;Drachev 2011Drachev , 2016Drachev and Shkarubo 2018).
The GK was extensively studied in 1970s and 1980s, when first detailed geological maps were constructed (Kos'ko et al. 1985;Trufanov et al. 1986). During that mapping, a few exposures of Cenozoic sediment were found in the NW part of Kotel'nyi Island. Recent studies have mainly focused on the stratigraphy (Danukalova et al. , 2019 and provenance of Palaeozoic deposits (Ershova et al. 2015, stratigraphy of Mesozoic strata (Meledina 1999;Kuzmichev et al. 2009Kuzmichev et al. , 2018Nikitenko et al. 2017Nikitenko et al. , 2018 and the structural and tectonic evolution of the island (Piepjohn et al. 2018;Prokopiev et al. 2018). Thus, Cenozoic deposits of western Kotel'niy Island have not been studied in detail, except for Palaeogene-Neogene sediments of Bel'kovsky Island (Kuzmichev et al. 2013). Meanwhile, the Cenozoic era is crucial for understanding the geological evolution   of the unique Laptev Rift System that is linked to the history of the Eurasia and North America plate interaction and opening of the Eurasia Basin in the Arctic. In this paper we present the results of our study of Cenozoic sediments from the western part of Kotel'niy Island: their stratigraphy and provenance, and discuss their possible relationship with the Laptev Rift System evolution to validate the existing geodynamic models.

Geological background
The GK is mainly composed of compressionally deformed Palaeozoic and Mesozoic carbonate, siliciclastic and volcaniclastic rocks (Figure 2) (Kos'ko et al. 1985;Kos'ko and Korago 2009). According to geological mapping and more recent studies, the rocks are deformed into NWstriking folds, which are associated with reverse faults and thrusts of southwestern and northeastern vergence (Parfenov and Kuz'min 2001;Kos'ko et al. 2013;Piepjohn et al. 2018;Prokopiev et al. 2018).
The Cenozoic deposits are only known in a few small exposures scattered across western part of Kotel'nyi Island: in the northern part along the Reshetnikova River and its tributaries and along Durnaya and Nerpalakh lagoons (Areas 1 and 2 in Figure 2 correspondingly). In the GK, Palaeogene-Neogene deposits also crop out along the eastern coast of Faddeev Island and were penetrated by shallow wells (50-200 m deep) in the south of Zemlya Bunge where they are mostly continuously covered by Quaternary sediments (Kos'ko and Trufanov 2002). A reliable correlation between separate outcrops of Cenozoic sediments is impossible due to lack of fossils in many sections. The oldest Cenozoic strata known from the western part of Kotel'nyi Island are Eocene in age based on palynological results (Kos'ko et al. 1985;Lopatin 1999;Kos'ko and Trufanov 2002). However, a 90 m-thick section of continental brown clays with subordinate layers of sands, silts and coals beds of presumably Latest Palaeocene -Lower Eocene sediments was penetrated by shallow wells on Zemlya Bunge (Lopatin 1999). Previous researchers (Kos'ko et al. 1985) are showed that Eocene deposits are only located in the central part of Kotel'nyi Island where they have been drilled by a few shallow wells (up to 150 m deep) in the Balyktah River valley and comprise 90 m thick alternating non-marine clays and siltstones with subordinate beds of sandstones and coals. Oligocene-Miocene deposits are exposed in the northern part of Faddeev Island and have been cored by a few shallow wells across Zemlya Bunge (Lopatin 1999;Kos'ko and Trufanov 2002). They comprise predominantly non-marine sands, with subordinate layers of silts and clay. Thin coal beds have also been described from the lower part of the Oligocene-Miocene succession (Trufanov et al. 1986). The thin succession of Cenozoic rocks filing small depressions mapped across western part of Kotel'niy Island as undivided Oligocene-Miocene in age, and are comprised of intercalating sands, silts and clays (Kos'ko et al. 1985). The continental Oligocene clastic sediments penetrated by a shallow well on Zemlya Bunge (15 m in thick). The Pliocene to Lower Quaternary sediments have a patchy distribution and mainly crop outs on the small exposures along the western part of Kotel'niy Island comprising up to 6 m thick alternating clays and silts with subordinate beds of sands (Kos'ko et al. 1985).

Study Area 1
Study Area 1 is located in the north-western part of Kotel'nyi Island (Figures 2 and 3), where Middle-Upper Palaeozoic carbonates and subordinate uppermost Palaeozoic and Triassic clastic rocks outcrop at the surface.

Sub-Area 1.1.
Cenozoic deposits are exposed in a few small depressions along the unnamed eastern tributary of the Reshetnikova River (Figure 3), where they have previously been mapped as Oligocene-Miocene in age (Kos'ko et al. 1985). Horizontally bedded Cenozoic deposits crop out here in relatively small exposures, reaching a maximum apparent thickness of 6-8 m.
The studied succession displays significant lateral facies changes over small distances (Attachment 1, Figure 2). Easterly located exposures (localities 19, 20, 21, 22, 23, 24; Attachment 1) comprise mainly fine-to coarse-grained sands, containing coalified plant detritus and occasional lenses of coaly clay and clayey silt. Incised channels also occur here, filled with gravelly conglomerates and coarse-grained sands with multidirectional cross-bedding, while ferruginous silts and sands have also been described from numerous horizons. Meanwhile in the western exposures, 4 m-thick coal beds intercalated with sands and silts crop out.
The palynological composition of samples from the studied sections (Attachment 2) can be correlated with the regional Kengdeyskii and Tastastahskii palynocomplexes (Grinenko et al. 1998), which suggest an Early Eocene age for these sediments (Attachment 2). Furthermore, the occurrence of dinoflagellate cysts in several samples suggests that certain horizons within the Early Eocene succession accumulated in a coastal marine environment, while the occurrence of entirely freshwater palynomorphs elsewhere in the succession suggest deposition within non-marine lacustrine to fluvial environments. Therefore, the studied succession   Figure 2. (a), Geology of the western Anzhu islands and surrounding shelf (based on Kos'ko et al. 1985;Kos'ko and Trufanov 2002;Kos'ko and Korago 2009;Kos'ko et al. 2013;Prokopiev et al. 2018, and our observations). The offshore rift structures are shown after Drachev and Shkarubo (2018). Capital letters denote the following islands: BI, Bel'kovsky, KI, Kotel'nyi, ZB, Zemlya Bunge, and FI, Faddeev. The italic capital letters denote the following structural elements of the rift system: ULR, Ust' Lena Rift, SH, Stolbovoy Horst, and KH, Kigilyakh Horst. (b), interpreted fragment of MAGE multichannel seismic reflection profile A4 (modified from Drachev and Shkarubo 2018). The location of this seismic fragment is shown in Figures 1 and 2a).
probably formed in a coastal margin to non-marine environment, close to or along the margin of a shallow marine basin. Four samples were selected for U-Pb detrital zircon analysis from this locality (14AP15, 14AP18, 14AP21, 14AP24).

Sub-Area 1.2.
This area is located 500 m to the south of the Shlupochnaya River mouth on the northwestern coast of Kotel'nyi Island. Cenozoic deposits are preserved within an incised paleovalley, with a pronounced angular unconformity at the basal contact with underlying deformed Lower Devonian limestones ( Figure 3). The lower part of the succession comprises 70 cm of fine-grained sands with thin lenses of brownish clays and scattered subangular limestone clasts, similar in composition and presumably reworked from the underlying Lower Devonian carbonates. These sands are overlain by 5 metres of interbedded sands and silts (Attachment 2). The lower part of the studied section has been dated as Pliocene in age based on palynological data (Attachment 2). One sample has been selected for U-Pb detrital zircon analysis from this locality (14AP43).

Study Area 2
Stdy Area 2 is located in the western part of Kotel'nyi Island across Tas-Ary Island and the Durnaya lagoon (Figures 1 and 3). Deformed Middle-Upper Palaeozoic carbonates predominantly crop out within this area, while Cenozoic deposits are confined to a few small depressions. We studied three outcrops in detail ( Figure 3).

Sub-Area 2.1. The Cenozoic strata on Tas-Ary
Island unconformably overlie Palaeozoic rocks and occur in two separate locations ( Figure 4; Attachment 1). The underlying Palaeozoic sandstones and limestones, beneath a prominent angular unconformity, are heavily weathered. The weathering crust penetrates from a few decimetres to 10-15 m into underlying Palaeozoic rocks, depending on their composition. Sandstones and siltstones are usually lightly weathered, while limestones are weathered to greater depths of up to 15 m.
Cenozoic deposits of the northern part of Tas-Ary Island occur in small fault-bounded graben-like depressions and crop out in a single 70 m-long exposure. They overlie gently weathered Famennian-Lower Tournaisian  sandstones and siltstones above an angular unconformity ( Figure 4; Attachment 1), the lower unit of Cenozoic succession forms a gentle monocline. The bog ores occur at the base of the lower unit and graded into the brownishgrey clays and silts. The thickness of lower unit reaches 2.5 m. The upper unit (up to 5 m thick) comprise grey to brown massive clays with subordinate thin silty layers. The palynological composition of these strata can be correlated with the regional Nizhnekolimskii and Omolonskii horizons (Fradkina 1995;Grinenko et al. 1998), indicating that these sediments are of Early Oligocene in age (Attachment 2).
Other outcrops of Cenozoic deposits have been studied in the central part of Tas-Ary Island (Figure 4; Attachment 1). The Cenozoic strata are overlain by Quaternary deposits, although the base of the Cenozoic succession is not exposed across central Tas-Ary Island. Cenozoic strata range from 100 to 200 cm in total thickness, comprising fine-grained sands with numerous lenses and thin layers of clay and silt, along with sporadic fossilized wood fragments. Two samples were selected for U-Pb detrital zircon analysis from this locality (14AP56, 14AP59).
The palynological composition of these strata can be correlated with the regional Onkuchahskii palynocomplex (Fradkina 1995;Grinenko et al. 1998), indicating that these sediments are of Late Oligocene age (Attachment 2).

Sub-Area 2.2.
Cenozoic strata also crop out along the eastern coast of the Durnaya lagoon (Figure 4; Figure 5; Attachment 1), where five sections attaining a maximum thickness of 3.2 m were studied. All strata lie horizontally, and sections are located at the same hypsometric level. Cenozoic strata are mainly represented by fine-to medium-grained sands with subordinate thin layers and lenses of clay and silt, along with a few thin layers and lenses of pebbles and gravel with limestone clasts. Medium-to coarse-grained sands often display trough cross-stratification, while fine-grained sands are characterized by wave-ripple cross-lamination. Although these sediments were previously dated as Oligocene-Miocene (Kos'ko et al. 1985), our new palynology data (Attachment 2) suggest that they are actually Pleistocene in age. Two samples have been selected for U-Pb detrital zircon analysis from this locality (14AP66, 14AP65).

Methods
U-Pb detrital zircon dating was carried out for eight samples from Cenozoic strata. Samples were crushed and heavy minerals were concentrated using standard techniques at the Institute of Precambrian Geology RAS (St. Petersburg). The zircon grains were mounted in epoxy and polished.
U-Pb age dating was performed at the Geological Survey of Denmark and Greenland (Copenhagen) using the laser ablation -single collector -magnetic sectorfield -inductively coupled plasma -mass spectrometry (LA-SF-ICP-MS) method, employing a Thermo-Fisher Element 2 mass spectrometer coupled to a New Wave UP213 laser ablation system. Data were acquired by single spot analyses using a frequency of 10 Hz, 3-10 J/cm 2 and a spot diameter of 25 or 30 µm, producing a crater depth of ca. 20-25 µm. The methods employed for analysis and data processing are described in detail by Gerdes and Zeh (2006) and Frei and Gerdes (2009). At least 100 grains were selected for analysis in a random fashion from each sample. 207 Pb/ 206 Pb ages are reported for zircons >1.0 Ga in age and 206 Pb/ 238 U ages for ≤1.0 Ga zircons. Following Gehrels (2012), only analyses with a discordance of less than 30% were used for interpreting the data (except grains younger than 300 Ma). Data tables and a description of analytical procedures are provided in Attachment 3. The diagrams have been constructed in detzrcr program designed by Andersen et al. (2018). The source code is available at https://github.com/magnuskristof fersen/detzrcr.

Sub-Area 1.2, Pliocene deposits near Shlupochnaya River
Sample 14 AP43 was collected from the lower part of the Cenozoic succession filling an incised paleo-valley ( Figure 2). Archaean grains constitute 14% of the dated zircon population and group at ca. 2700 Ma, while Paleoproterozoic grains (31%) form peaks at ca. 1980 and 1850 Ma. Mesoproterozoic grains are not present within the dated sample, while Neoproterozoic grains (7%) do not form significant peaks. Palaeozoic zircons (38%) form peaks at ca. 530, 520, 320, 300б 280 and 255 Ma, while Mesozoic grains (10%) group in peaks at ca. 150 and 120 Ma.

Provenance analysis
The Kernel Density Estimation (KDE) diagrams for all studied samples ( Figure 6) display some similarities in the distribution of detrital zircon ages between different levels within the Cenozoic strata of Kotel'nyi Island (Figure 7).
Zircons of Precambrian age can be correlated with the ages of accreted terranes of the Siberian Craton basement. The oldest Archaean grains correspond to    the oldest Archaean terranes (Rozen 2003;Smelov and Timofeev 2007). Numerous Paleoproterozoic grains dated at ca. 2000 and 1800 Ma can be correlated with accretion of the Archaean terranes within Siberian Craton basement (Parfenov and Kuz'min 2001;Rozen 2003). Furthermore, Precambrian detrital zircon populations from Upper Cretaceous deposits of Novaya Sibir Island (Kostyleva et al. 2022) and Upper Jurassic strata of Stolbovoy Island (Miller et al. 2008) (Figure 7) are characterized by age distributions similar to the Cenozoic strata of Kotel'nyi Island, with populations grouping between 2800-2500 and 2000-1800 Ma.
A tectonically quiescent period known as the Siberian Gap occurred between 1800 and 800 Ma within the Siberian Craton, when no major magmatic or metamorphic events occurred (Gladkochub et al. 2010).
Zircons with Mesoproterozoic ages are also quite rare within the Carboniferous-Cretaceous strata of northern Siberia Ershova et al. 2015Ershova et al. , 2016Ershova et al. , 2020Vereshchagin et al. 2018), identifying a clear fingerprint of the Siberian Gap within detrital zircon age populations of this region (Figure 7). Therefore, predominant zircon age populations of 2800-2500 and 2000-1800 Ma, with sparse or absent Mesoproterozoic zircons, are the diagnostic fingerprint of a Siberian clastic provenance. In contrast, zircons of Mesoproterozoic age dominate Precambrian zircon populations within Cambrian-Carboniferous deposits of the NSI, suggesting a very different non-Siberian clastic sediment provenance across the region during this time (Ershova et al. 2015a(Ershova et al. , 2015bPease et al. 2015) (Figure 6). Within the studied Cenozoic strata, Mesoproterozoic zircons only occur in Early Eocene and Late Oligocene deposits, suggesting proximal erosion and reworking of Palaeozoic deposits of the NSI containing abundant Mesoproterozoic grains. Moreover, the younger Plio-Pleistocene sediments contain significantly fewer grains of Mesoproterozoic age, suggesting a reduction in erosion rates of the Palaeozoic succession during the Late Cenozoic and increasing peneplanation of the study area. Palaeozoic zircons within the Cenozoic strata encompass a broad age range, with the majority of grains yielding ages between 350 and 250 Ma. Palaeozoic strata from eastern Siberia and Mesozoic strata from the NSI contain a similar distribution with numerous Late Palaeozoic zircons to the Cenozoic strata, derived from Siberian Craton or adjacent orogenic belts (Kuzmichev et al. 2018;Kostyleva et al. 2022;Ershova et al. 2013Ershova et al. , 2014Ershova et al. , 2015Ershova et al. , 2020Prokopiev et al. 2013) (Figure 7). A similar provenance for these zircons can therefore be inferred for Cenozoic strata of Kotel'nyi Island.
Mesozoic zircons mainly form two groups with ages of 150-140 and 100-90 Ma. Magmatic events of these ages are not known from the NSI, where Late Mesozoic volcanic or plutonic rocks are only represented by Albian ignimbrites on Kotel'nyi Island (Kuzmichev et al. 2009) and Aptian-Albian granites on Bol'shoy Lyakovsky Island (Kos'ko et al. 2013). However, Mesozoic volcanic and plutonic rocks of these ages are widely distributed across north-east Russia. The Late Mesozoic closure of the Oymyakon Ocean and collision between Siberia and the Kolyma-Omolon Superterrane resulted in the formation of several major granite batholith belts yielding ages spanning 170-140 Ma across north-east Siberia (Akinin et al. 2020;Parfenov 1991;Parfenov and Kuz'min 2001). A 150 Ma detrital zircon population has been reported from the Jurassic strata of Stolbovoy Island (Miller et al. 2008) and Mid-Cretaceous of Kotel'nyi Island (Kuzmichev et al. 2018), while Upper Cretaceous deposits of Novaya Sibir Island contain 150, 100 and 90 Ma detrital zircon populations (Kostyleva et al. 2022). Furthermore, Turonian-Coniacian tuffs have also been reported from Novaya Sibir Island (Kostyleva et al. 2019). Additionally, 100-90 Ma zircons could represent air fall from Okhotsk-Chukotka volcanic belt (Luchitskaya et al. 2018;Pease et al. 2018). Therefore, the Mesozoic detrital zircon population within the Cenozoic strata of Kotel'nyi Island could be sourced from erosion of syn-collisional granite batholiths and subductionrelated volcanics in north-east Russia, and/or alternatively, reworked from Upper Jurassic and Cretaceous strata of the NSI region.
Two zircons with a crystallization age of ca. 60 Ma within the Cenozoic strata cannot be correlated with coeval magmatism across the NSI, Siberia or Russian Far East. These grains could provide the first possible evidence for coeval (middle Palaeocene) rift-related magmatism in the East Siberian Arctic shelf. However, more data are required to confirm this hypothesis.
In summary, the studied Early Eocene and Late Oligocene strata of Kotel'nyi Island were probably predominantly derived from proximal sources. This provenance included deformed Palaeozoic rocks of the NSI, along with either a direct source from Mesozoic orogens framing north-eastern Siberia via a developed valley associated with Laptev Sea rift system, or reworking from Jurassic and Upper Cretaceous rocks of the NSI and surrounding area. In contrast, Plio-Pleistocene strata do not contain evidence for significant reworking of underlying Palaeozoic strata. These deposits contain a comparable detrital zircon distribution to Upper Jurassic and Upper Cretaceous rocks of the NSI, which carry a Siberian detrital zircon signature. We can conclude that deformed Palaeozoic deposits of the NSI were not a significant source of clastics sediments from the Pliocene and on. It is therefore difficult to distinguish whether zircons within Plio-Pleistocene strata were sourced from erosion of orogenic belts framing Siberia or reworked from Upper Mesozoic strata of the NSI. If Upper Mesozoic deposits were the source of clastic grains within Cenozoic strata, Upper Mesozoic sediments should have been more widely distributed across the NSI region compared to present-day scarce occurrence, which can be a result of significant uplift, erosion and redeposition during the Laptev Sea rifting event. The Stolbovoy High and its northern counterpart, the North Laptev High, represent uplifted blocks of the Laptev Sea basin basement covered by a very thin Cenozoic sediments. This uplifted domain is known to comprise Upper Jurassic and lowermost Cretaceous deposits characterized by a Siberian provenance signal (Miller et al. 2008), which outcrop onshore across Stolbovoy Island. The Stolbovoy High with its inherited Siberian provenance signature is likely to have represented an important provenance for clastic strata deposited across the NSI archipelago during the Cenozoic. Our results are in a good agreement with provenance study conducted in the other rift systems worldwide, showed complexity of the source-to-sink pathways and provenance varied spatially and temporally, and sediment routing switched from being locally to regionally sourced (Castro et al. 2019;Hart et al. 2016;Gong et al. 2021;Roberts et al. 2012;Sánchez Martínez et al. 2012;Zawacki et al. 2022 and references therein). Multiple sources of clastic sediments due to complex history of exhumation of source areas are revealed from the others of rift basins (Sánchez Martínez et al. 2012;Hart et al. 2016;Gong et al. 2021).

Implications for the development of the Laptev Sea Rift
Based on potential field data (Gaina et al. 2002, Mazur et al. 2015 claimed the latest Late Cretaceous to present extension created the Laptev Rift systems. Nikishin et al. (2017Nikishin et al. ( , 2020Nikishin et al. ( , 2021 proposed initiation of Laptev Sea Rift system in Aptian and suggested few stages of rift evolution. Drachev et al. (1998) and Franke et al. (2001) used available data on the Cenozoic sediments from the continental rim of the Laptev Shelf to infer stratigraphic age of seismic horizons. In both models, similar timing of the rift onset was inferred varying from the latest Cretaceous to Palaeocene, and the migration of the extension in the eastward direction was proposed, with the Bel'kov-Svyatoi Nos and Anisin rifts (Figure 2) considered as the youngest rift system member initiated either in the Late Eocene (Drachev et al. 1998) or in the Miocene (Franke et al. 2001). Thus, Cenozoic sediments play a crucial role in constraining the time of the rift onset and stages of the rifting in the Laptev Sea.
In this regard, the studied sediments, which are located at the eastern shoulder of the Anisin Rift (Figure 2) may provide a critical constraint on the age of the extension along the eastern flank of the rift system.
A regional multichannel seismic reflection profile A4 acquired by the Marine Arctic Geological Expedition (MAGE, Murmansk, RF) crosses the eastern flank of the rift system c. 80 km north of the Study Area 1 (Figure 2) in an area where two major eastern arms of the rift system, the Bel'kov-Svyatoi Nos and Anisin rifts, merge in one c. 110 km-wide rifted basin. The latter is constrained by two principal high-standing blocks of the pre-rift basement, the North Laptev High in the west, and the Kotel'nyi High in the east. The transect provides a clear image of rift geometry and its sedimentary infill; it consists of two distinct seismic units, a lower one with higher reflectivity and an upper one that is more seismically transparent (Figure 2(b)). Both units are affected by two generations of normal faults and clearly show synrift accumulation of sediment suggesting two phases of extension. The age of these seismic units, as well as the age of the related extension remain unconstrained.
Our study revealed that the oldest Cenozoic deposits of the western part of Kotel'niy Island are Early Eocene in age (Area 1). This is in a good agreement with the data from neighbouring Bel'kovsky Island (Kuzmichev et al. 2013) with Eocene deposits lying at the base of grabenlike small depressions (Figure 8(a,b)). However, the depressions in the Area 2 are filled by Oligocene clastic sediments (Figure 4). Oligocene sediments of the Sub-Area 2.1 are clearly related to a hanging wall of a normal fault (Figure 8(c)), which permits their possible deposition in association with one of the rift phases ( Figure 9). Based on the U-Pb dating of detrital zircons, both Eocene and Oligocene deposits depict Palaeozoic rocks of Kotel'niy Island as possible of local source of recycled clastic material that deposited in these depressions. At the same time, a clear typical 'Siberian' signatures (Proterozoic ages in detrital zircons; Figure 7) of the detrital zircons suggest another source of the siliciclastic sediments rather than the Palaeozoic rocks of the GK. Similar signatures have recently been detected in Cretaceous sediments of Novaya Sibir Island (Kostyleva et al. 2022) and in the Lower Cretaceous deposits of Kotel'nyi Island (Kuzmichev et al. 2018) east of the study areas, and are also documented in the Upper Jurassic to Lower Cretaceous clastics of Stolbovoy Island (Miller et al. 2008) southwest of the studied Cenozoic sections. Thus, we proposed that rift-related structures on the western part of GK formed as early as Early Eocene (Figure 9). The source of these zircons found in the Cenozoic sections of the western Kotel'nyi Island has an important implication to constraining timing of the extension onset along the eastern flank of the rift system.
The structural setting of the Lower Eocene sediments of Area 1 is unclear. Two possible scenarios associated with extension can be considered: (i) pre-rift and (ii) post-rift. The location of the source areas for the 'Siberian' sourced zircon may help distinguish between these two possibilities. A westerly located source, such as the high-standing areas of the North Laptev or Stolbovoy basement highs, would require pre-rift delivery of this detritus since rifting constrains sediment transport and forms sediment traps (e.g. Gong et al. 2021). It would not have been possible to deliver detritus across the rift valleys to Bel'kov and Anisin rifts. However, for easterly located sources, both pre-and syn-rift settings are viable. In either scenario the lower seismic unit in Figure 2(b) will contain Lower Eocene sediment which represents the youngest onset for sedimentation in the Anisin Rift. Thus, we proposed that offshore-onshore (western part NSI-Anisin Rift) correlation claim the Eocene to Oligocene age of rifting events across the Anisin rift System (Figure 9). Following the models proposed by Drachev et al. (1998), Franke et al. (2001) Anisin rift considered as the youngest branch of Laptev Sea rift system. Thus, an open question remains whether the Pre-Cenozoic extensions really occurred in the western portion of Laptev Sea system in Aptian as proposed by Nikishin et al. (2014Nikishin et al. ( , 2020, Shkarubo and Zavarzina (2011), or in latest Late Cretaceous -Palaeocene (Drachev 2016;Drachev and Shkarubo 2018;Franke et al. 2001;Gaina et al. 2002;Mazur et al. 2015, and references therein). Moreover, there is no consensus on the geodynamic settings of assumed Cretaceous extensions. Nikishin et al. (2014Nikishin et al. ( , 2020 proposed that Aptian rifting occurred as a result of orogenic collapse, then extension continued due to initiation of Eurasia basin rifting. The other models claimed that late Latest Cretaceous rifting caused by initial stages of Eurasia basin evolution. Thus, the presence of older Cretaceous sediment within the western part of Laptev Sea rift system remains to be established to improve our understanding the pre-Cenozoic evolution of Laptev Sea and adjacent Artic region.
We propose that Early Eocene and Oligocene sediments were sourced from deformed Palaeozoic rocks of the NSI, along with volcano-plutonic rocks from north-east Russia or Upper Jurassic and Upper Cretaceous rocks of the NSI region. The Plio-Pleistocene deposits are characterized by a similar distribution of detrital zircon ages but lack Mesoproterozoic grains, suggesting that Palaeozoic strata of the NSI were no longer a sediment source during this time due to peneplanation of the region.
These results provide some insight in the history of the crustal extension along the eastern flank of the Laptev Rift System. Oligocene sediments of the Sub-Area 2.1 are associated with a border normal fault, and therefore were deposited in an extensional setting.
'Siberian' detrital zircons could have been sourced to this section either by reworking from NSI region or via longitudinal transport along a rift valley of the Bel'kov-Svyatoi Nos Rift. Early Eocene sediments of the Area 1 provide a youngest constraint of the age of sedimentation onset along the shoulder of the Anisin Rift. However, they do not reveal direct evidence of their synrift deposition, and both pre-rift and syn-rift deposition are possible. If the further studies could succeed proving a westerly or south-westerly located source of the 'Siberian' zircons in the Early Eocene sediments, this would help to constrain their pre-or syn-rift deposition.

Acknowledgments
The fieldwork and analytical analyses were supported by project of Clapton Research Inc. Interpretation of isotopic data was supported by the Russian Science Foundation grant no. 20-17-00169. The study of A.Khudoley was supported by Ministry of Science and Higher Education of the Russian Federation grant 075-15-2022-1100. Valuable reviews by Prof. Victoria Pease, anonymous reviewer and editor Dr. Robert J. Stern significantly improved the manuscript.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was supported by the Russian Science Foundation [no. 20-17-00169].