Regional correlation and seismic stratigraphy of Triassic Strata in the Greater Barents Sea: Implications for sediment transport in Arctic basins

The Greater Barents Sea Basin (GBSB) in Arctic Russia and Norway is an intracratonic basin that accommodated an enormous amount of sediment during the Triassic. These deposits are up to 4.5 km thick over an area 2,500,000 km2, and consist of marine mudstones and mudstone‐rich fluvio‐deltaic topsets with sandstone‐dominated fluvial channels. The basin is well‐studied and data‐rich, but regional correlation between different parts is lacking. Provenance data from adjacent Arctic basins have been interpreted to imply sediment transport from the Ural orogen across the GBSB, but these are disputed because of great transport distances, poorly constrained sediment‐transport directions and unknown timing of bypass. We integrated data from 3,238 seismic lines, 257 wells and palynostratigraphy, as well as published outcrop data, to create the first unified stratigraphic framework for the Triassic deposits across the entire GBSB. Results show that (1) sediment was transported northwest by one linked sedimentary system stretching across the entire basin; (2) sediment was derived from a source in the east comprising the Urals and West Siberia; (3) the main stratigraphic boundaries are major flooding surfaces which can be traced throughout the basin; and (4) significant amounts of sediment overspilled from the Barents Sea into adjacent sedimentary basins, starting with the Lomonosov Ridge from the Early Triassic, and into basins to the northwest (e.g. Sverdrup, Chukotka) during the late Carnian. These results provide a better understanding of geodynamics and provenance data in the Arctic, to improve the prediction of reservoirs in the area, and indicate a protracted uplift‐history of the northernmost Urals that started in the Carnian ~237 Ma. Furthermore, it shows how large intracratonic basins interact with uplands and subside over tens of millions of years.


| INTRODUCTION
The Greater Barents Sea Basin (GBSB, Figure 1) in Arctic Russia and Norway, was an enormous (c. 1,200 km EW, 1,800 km NS) sedimentary basin which accommodated a sedimentary succession up to 4.5 kilometres thick during Triassic (Glørstad-Clark et al., 2010;Klausen et al., 2015;Norina et al., 2014). With an area of 2 500 000 km 2 , the GBSB was about three times the size of the Northwestern Australian Shelf, five times the size of the North Sea and six times the size of the N Gulf of Mexico shelf. How such enormous sedimentary basins are infilled by sediment through geological time has received little study, and in this contribution we integrate information from the entire GBSB to understand linkages between enormous basins and source-area geodynamics, and to better understand Triassic sediment routing in the Arctic basins.
The main tectonic driver that controlled sediment supply to the GBSB has been assumed to be regional contraction resulting in the Ural and Taimyr orogens east of the basin (Bergan & Knarud, 1993;Puchkov, 2009), but the emplacement of the Siberian Traps Large Igneous Province is also likely to have influenced sediment supply (cf. Reichow et al., 2009, Figure 1c). Analysis of GBSB basin fill provides insights into the longevity of topography and compression in the Urals, the effects of emplacement of the Siberian Traps Large Igneous Province, and the timing of the onset of uplift of the Novaya Zemlya Fold and Thrust Belt (NZFTB) and Pay-Khoy orogens in the northern Urals ( Figure 1). The Barents and Kara seas are also believed to contain large amounts of hydrocarbon resources in the Triassic deposits and understanding the depositional history is critical to consider the location and timing of hydrocarbon formation (Artyushkov et al., 2014;Norwegian Petroleum Directorate Resource report, 2016Stoupakova, 2011).
The GBSB has widespread and continuous Triassic deposits, and can be divided into six different geographic regions that will be referred to in this paper ( Figure 1): (a) The Southwestern Barents Sea (Norwegian Barents Sea) and (b) Timan-Pechora Basin are well-explored areas, with abundant drilled wells and actively producing hydrocarbon fields; (c) The Eastern Barents Sea including Franz-Josef Land (Russian Barents Sea), which has limited coverage of seismic data and a limited number of exploration wells, (d) Northwestern Barents Sea, which has no active exploration but several research wells drilled and abundant 2D seismic data collected by the Norwegian Petroleum Directorate, (e) North Kara Sea, which currently has no wells and 6) the onshore areas of Svalbard, which offer widespread and accessible outcrops of Triassic deposits (e.g. Mørk, Elvebakk, et al., 1999;Vigran et al., 2014).
The sediments filling in the GBSB originated mainly in the east (e.g. Artyushkov et al., 2014;Bergan & Knarud, 1993;Bue & Andresen, 2014;Eide et al., 2018;Fleming et al., 2016;Klausen et al., 2015;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999;Norina et al., 2014;Stoupakova, 2001). Thus, the Russian parts of the basin are proximal and the Norwegian parts of the basin are more distal. The sediments are composed of extensive mudstone-rich linked clinoformdelta-delta plain systems (e.g. Klausen et al., 2018Klausen et al., , 2019 similar to other deltaic successions with continental-scale drainages, such as the Mississippi in the Gulf of Mexico (Blum & Pecha, 2014;Blum & Törnqvist, 2000;Galloway et al., 2011) and the Triassic Mungaroo Formation in the northwest Australia (e.g. Martin et al., 2018). The studied deltaic deposits are interspersed by regional flooding surfaces that serve as marker surfaces in the basin, enabling subdivision of the basin-fill into stratigraphic units (Dalland et al., 1988;Glørstad-Clark et al., 2010;Klausen et al., 2016;Mørk et al., 1989). Flooding surfaces coincide with boundaries in the biostratigraphic framework established for the Highlights • For the first time, the entire Triassic succession is correlated across the Greater Barents Sea Basin in Arctic Norway and Russia, and a common lithostratigraphic framework has been established for this enormous area that has seen little holistic study.
• Triassic sediments were dominantly supplied from source areas in the southeast which included both the Ural Orogen and West Siberia. This source was extremely productive, and created one sedimentary system that supplied mudstone-rich sediment which overspilled the Greater Barents Sea Basin, into adjacent basins in the Arctic towards the late Carnian and Norian.
• Local, more sand-rich and mineralogically mature deposits occur close to basement highs such as N Fennoscandia and Svalbard in the Early and Middle Triassic, but these become diluted or displaced by sediments from the East in the later Triassic.
• A three-stage depocentre shift is documented throughout the Triassic, and this is explained by geodynamic in the basins and source areas.
• Cretaceous igneous intrusions make up an essential part (c. 10%) of the Triassic succession in the Eastern Barents Sea, and this must be taken into account when investigating subsidence history of sedimentary basins.
F I G U R E 1 (a) Overview of the entire Greater Barents Sea Basin, the six geographic regions (see Introduction) and all seismic and well data available in this study. Structural elements modified from (Gabrielsen et al., 1990, Faleide et al., 2008. (b) Chronostratigraphic chart of the study interval and lithology of the well-studied SW Barents Sea (for more detailed description of stratigraphic units see Results). Modified from (Cohen et al., 2013;Eide et al., 2018;Gradstein et al., 2012;Klausen et al., 2016;Rossi et al., 2019). (c and d) Palaeogeographic map showing the regional setting of the study area in the early Induan (Early Triassic) and Carnian (Late Triassic). Based on a variety of sources, including (Nikishin et al., 2002;Cocks & Torsvik, 2006;Reichow et al., 2009;Nikishin et al., 2010;Miller et al., 2013;Sømme et al., 2018)

F I G U R E 2
Comparison of formation and sequence subdivisions in different basins. Colours marked different sources that engaged on sedimentation of marked formations. Light green-Greenland, pink-Fennoscadian shield and blue-Ural, Taimyr, West Siberia and Siberian traps F I G U R E 3 Lithostratigraphic correlation of the Triassic between different parts of the Greater Barents Sea Basin, for location see Figure 1, after Fefilova, 2013b;Klausen et al., 2015;Krajewski & Weitschat, 2015;Morakhovskaja, 2000;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999;Mørk & Worsley, 2006;Norina et al., 2014;Rossi et al., 2019;Stoupakova et al., 2011)  present palynological data that for the first time illustrates the Russian assemblages and verifies the seismic interpretations across the GBSB, (d) to understand how Triassic sediments were distributed and how different parts of the basin were interlinked and (e) provide a framework to understand the growing provenance database in the Triassic Arctic. Together these have important implications on palaeogeographic reconstructions and petroleum prospectivity in the region.

| Basin evolution
In the Western Barents Sea basin, the basement is overlain by post-Caledonian sedimentary rocks, and is shallow compared to the basement in the Eastern Barents Sea (Faleide et al., 1984): generally the depth to basement is c. 8 km but beneath localised salt basins it may reach up to 12 km (Aarseth et al., 2017). In the Eastern Barents Sea, the basement is generally overlain by post-Cambrian sediments (Stoupakova, 2011) and located at up to 20 km depth (Gac et al., 2012). In the western Barents Sea, northeast striking rift basins contain mobile salt, and these developed during Late Devonian-mid-Carboniferous rifting (Gudlaugsson et al., 1998). After post-Caledonian rifting ended in the Late Devonian (Eastern Barents Sea) and Carboniferous (Western Barents Sea), the Western Barents Sea subsided slowly and was infilled with carbonates and mudstones until the latest Permian (e.g. Gac et al., 2014;Worsley, 2008). In the Eastern Barents Sea, the transition from carbonates to siliciclastic deposits happened earlier, during the Artinskian (early Permian), because of supply of clastic sediments from the Ural Orogen (Ustritsky & Tugarova, 2013). Rapid and poorly understood subsidence of the entire GBSB occurred from the start of the Triassic (e.g. Clark et al., 2014).

| Sediment sources and surrounding uplands
The Ural orogen (Figures 1c and 2) is traditionally regarded as the main sediment source for the Triassic of the GBSB (e.g. Bergan & Knarud et al., 1993;Flowerdew et al., 2020;Mørk, Dallmann, et al., 1999;. The Ural orogen is a result of a continent-continent collision between Laurussia and Kazahkstania in the Mississippian-Lopingian, and later with Siberia in the Polar Urals, and the Pay-Khoy-Novozemelian foldbelt in the Late Triassic and Early Jurassic (Puchkov, 2009). In the Permian, the foreland basin close to the Ural orogen filled with siliciclastic deposits, but carbonate and spiculite sedimentation dominated in the West and central parts of the GBSB (Worsley, 2008), showing that subsidence balanced sediment supply during the main orogenic phase. Subsequently, during the Induan, large amounts of sediments prograded into the GBSB from the East (Figure 3) (e.g. Eide et al., 2018;Glørstad-Clark et al., 2010). It is worth pointing out that the most significant phases of compression in the Urals occurred during the Carboniferous to early Permian, with waning tectonism and relief into the Permian and Triassic (Puchkov, 2009), and that the rapid progradation in the Induan happened at a time where large-scale compression in the Uralian orogeny is not expected. The Induan progradation coincides with extensive volcanic activity in the Siberian Traps Large Igneous Province in West Siberia (e.g. Burgess & Bowring, 2015), and it is likely that tectonic uplift of the Urals, Taimyr and West Siberia occurred because of magmatic activity (Puchkov, 2010;Sobolev et al., 2009) and led to the widespread progradation of sedimentary system in the Triassic. The Taimyr orogen (Figure 1c,d), located northeast of the Novaya Zemlya, has also been suggested as a sediment source that can explain differences in sandstone composition and zircon ages between the Southwestern Barents Sea and Svalbard (Fleming et al., 2016). The commonality for all the Eastern sediment sources (Urals, Taimyr, Siberian Traps) is that they provided relatively immature sandstones with abundant feldspar and volcanic fragments (Bergan & Knarud, 1993;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999), and a very large proportion of mud (e.g. Eide et al., 2018;Klausen et al., 2018). Other minor sediment sources were the Fennoscandian Shield to the south, and Greenland (Figures 2 and 3) to the west of the GBSB during the Early until the beginning of Late Triassic; these provided more quartz-rich sands to areas close to the basin margins (e.g. Bue & Andresen, 2014;Eide et al., 2018;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999). The Novaya Zemlya and Pay-Khoy ranges could also have been important sediment sources, but only after these areas were uplifted during the latest Triassic and Early Jurassic (e.g. Drachev, 2016;Müller et al., 2019;Scott et al., 2010). However, the exact time of uplift of these areas is poorly dated.

| Post-depositional evolution
Overlying the Triassic succession, a regional unconformity is developed in the Western Barents Sea (Figure 3), likely caused by contraction-related uplift of Novaya Zemlya (Müller et al., 2019). This unconformity is expressed as an erosive contact between the Triassic and the overlying Rhaetian to Lower-Middle Jurassic deposits. Meanwhile, in the Eastern Barents Sea, an Early Jurassic foreland basin developed in front of the Novaya Zemlya fold-and-thrust belt (e.g. Olaussen et al., 2018;Scott et al., 2010), which reached a thickness of up to 1.2 km (Suslova, 2014). The westernmost extent of studied seismic data. Note the clearly expressed clinoforms, the downlap of the Induan to Anisian deposits onto the Loppa High, the drowning of the Loppa High in the Anisian, the clearly expressed discontinuous reflections in the topsets (fluvial channel deposits) and the erosion thickening and erosion below the Quaternary deposits of the Carnian unit. (c) Seismic data from the formerly disputed zone to the Eastern Barents Sea Basin. Note the well-expressed stratigraphic boundaries (based on well-imaged regional flooding surfaces), the similarity of deposits on both sides of the 5.5 km gap between the Norwegian and Russian data and the clearly expressed igneous intrusions in the east. (d) Seismic data of the most proximal parts of the Kara sea. Note the gradual onlap of the angular unconformity below the Triassic deposits, the gradual onlap of the Induan deposits onto this unconformity and the truncation of Triassic deposits below the Base Jurassic Unconformity F I G U R E 6 A regional composite seismic line from the Finnmark platform in the south and the Victoria High on the north. Note A) the same progradational Carnian units in the north as they are shown in Figure 5; (B) Triassic strata are hosting two types of Cretaceous intrusions-clinoform shape in unit H2 and saucer-shaped in Olenekian and Anisian units. C) Also note the H1 unit in the south, which is the only observable sedimentary unit from Fennoscandia, the rest of the Triassic deposits are sourced from the East The GBSB was later buried beneath kilometres of Cretaceous deposits ( Figure 6). During the Lower Cretaceous the High Arctic Large Igneous Province (HALIP) developed, and a vast quantity of magma intruded the sedimentary succession in the Northern and Eastern Barents Sea (Figures 4-7) (Polteau et al., 2016). During Atlantic breakup in the Paleogene, the Barents Sea was affected by compressional movements leading to the uplift of Svalbard, Novaya Zemlya and the Central Barents Sea (Henriksen, Bjørnseth, et al., 2011). During the Cenozoic, the basin has mainly been under uplift and erosion, and erosion varies from c. 2.5 km on Svalbard and Novaya Zemlya (Figures 4 and 7), to c. 1.5 km in the Southwestern Barents Sea, and c. 400 m in the Eastern Barents Sea Basin ( Figure 8) (Henriksen, Bjørnseth, et al., 2011).

| Palynological studies
Palynological data are critical in large-scale stratigraphic correlation because it provides (1) relative datings on sedimentary  (Crameri, 2018). Thickness maps of the Ladinian (F) and Carnian (G-I) deposits. Note the gradual progradation towards Svalbard. Location of clinoforms-concordant intrusions (K-i), saucer-shaped intrusions (K-ii) and seismically imaged dykes (K-iii) related to the High Arctic Large Igneous Province, the map used from Minakov et al. (2018) across very large distances, and seismic interpretation can be challenging and not necessarily correct in areas that are deeply buried, structurally deformed or intruded.
Until now, no stratigraphic or palynological studies in the GBSB have been conducted with the objective to link the entire area. Rather, palynological studies have focused on smaller regions, such as Svalbard (Bjaerke & Manum, 1977;Smith et al., 1975), the Norwegian Barents Sea and Svalbard (e.g. Hochuli et al., 1989;Paterson & Mangerud, 2020;Vigran et al., 2014) and the Russian Barents Sea (Fefilova, 2001(Fefilova, , 2005(Fefilova, , 2011(Fefilova, , 2013a(Fefilova, , 2013bKlubov, 1965). Mørk et al. (1993) represent the only previous attempt to compare the respective palynofloras in the Norwegian and Russian parts of the Greater Barents Sea. Their work suggested clear differences in the ranges of palynomorph taxa between the two areas, something that would make correlations across the entire GBSB difficult. However, this difference may simply reflect differences in the taxonomic approach applied by different research groups. Currently, this has been difficult to assess because here is a general lack of fundamental documentation of the Russian assemblages, such as an illustration of the age diagnostic palynomorphs taxa recorded. Such issues emphasise the need for holistic studies of the GBSB.

| Seismic
The data coverage in the subsurface of the GBSB varies markedly between areas (Figure 1a). The average distance between individual 2D seismic lines are c 3 km in the Sourhwestern Barents Sea (several 3D seismic surveys are also available here), 14 km in the Northwestern Barents Sea, 125 km in the Eastern Barents Sea and 150 km in the Kara Sea.
This study is based on an analysis of 3,238 2D seismic lines covering 1,700,000 km 2 , 257 wells with logs F I G U R E 9 Interpretation and correlation of seismic, well-logs and biostratigraphic data penetrating the Triassic succession. Note the high gamma-ray values on the top of the low values, indicating maximum flooding surfaces and correlation boundaries across the basin. They might be found in Western and Eastern parts of the GBSB. Palynological zones for well 7228/9-1s after  and Rossi et al. (2019). Palynological data from Krestovaya-1 after Fefilova (2013a), re-interpreted within the zonal framework of Vigran et al. (2014) and Paterson and Mangerud (2020) 1558 | EAGE GILMULLINA et AL.
F I G U R E 1 0 (a) Time-structural map of the base Triassic, (b) A regional overview of the GBSB location from (Scotese & Wright, 2018). (c) Maximum progradational extent of basinward clinoforms breaks in each studied time unit, and the observed progradation directions. Note the linear front of each clinoforms package, the consistent direction of sediment transport through time and the extreme progradational extent of the Induan package. H1 based on Eide et al., 2017 study, the location of Greenland and Sverdrup from (Shephard et al., 2013). Updated version of structural elements can be found in Supplementary materials (Supplement 2) as well as shelf-edge maximum progradation and palynostratigraphic data. Six of the wells are from the Russian Barents Sea. The tops and bottoms of the different prograding seismic units were interpreted in the seismic and matched with constrained age in wells that become a basis for a definition of the thickness of studied units.

| Interpretation of stratigraphic units
Access to seismic data from the earlier disputed area between Norway and Russia (cf. Henriksen & Uflstein, 2011), provided by the Norwegian Petroleum Directorate, made it EAGE GILMULLINA et AL.
possible to trace stratigraphic packages across from the datarich and well-studied Norwegian parts into the more sparsely explored Russian Barents Sea. Based on all available data, reflectors corresponding to main stratigraphic boundaries were traced across the entire GBSB basin. We did not have access to seismic data in the northernmost part of the formerly disputed zone (north of 75°N), which leaves an around 175,000 km 2 gap between interpretations in Norwegian and Russian parts ( Figure 1a). However, the correspondence between the reflectors on both sides shows that the interpretations are reasonable on both sides ( Figure 4b).

| Seismic well tie
Surfaces are defined based on well-ties of stratigraphic markers provided by the NPD based on the well operator's reports, which in turn, rely on biostratigraphy and lithostratigraphic interpretation of wireline logs ( Figure 9). Stratigraphic intervals are calibrated by palynostratigraphy, which is independently calibrated by marine macrofossil occurrences from Svalbard and the Western Barents Sea (Paterson & Mangerud, 2017, data presented herein). Stratigraphic markers have been converted from depth to time by using available checkshot data from wells where these are available ( Figure 9).
The imaged seismic reflectors and associated formation boundaries correspond to lithological boundaries between the deltaic topsets and overlying offshore mudstones in the majority of the basin (Figure 9). In proximal and distal parts of the basin, correlative conformities (reflectors related to varying sandstone-content in alluvial deposits, or varying properties in offshore shales) are recognised in many areas for the flooding surfaces corresponding to the largest transgressions ( Figure 10).

| Sampling, processing and analyses
Thirty-eight samples were collected from core material that span the Lower-Upper Triassic succession from seven Eastern Barents Sea wells (Figures 11 and 12). Palynology samples were processed by Palynological Laboratory Services (PLS) Ltd. using standard hydrochloric acid (HCl) and hydrofluoric acid (HF) maceration techniques, followed by oxidation in nitric acid (HNO 3 ) (see Traverse, 2007, p. 632-647). Analyses were conducted at the Department of Earth Science, University of Bergen, Norway and at the Geological Faculty, Moscow State University, Russia. Semiquantitative palynomorph abundance was determined by counting 300 palynomorph specimens per sample preparation (recovery and preservation permitting). Rarer taxa were recorded as present outside of the count.

| RESULTS
This section describes the stratigraphic boundaries, how they were dated and traced, the thickness trends, the seismic characteristic of each of the Triassic stratigraphic units, how intrusions affected thickness differences and facies variations within stratigraphic units in different parts of the basin. Results presented herein show that more than 90% of the Triassic sediments in the GBSB were deposited by one sedimentary system derived from sources in the east that supplied more than 10 18 tons of sediments. This is demonstrated by: (a) the fact that the sediment transport directions (clinoforms, channels) are correlative across the basin (b) the fact that major stratigraphic surfaces are the same across the basin, and (c) that similar and related lithologies are present over the entire basin.

| Dating of the Eastern Barents Sea wells
Core samples from the selected Eastern Barents wells were analysed for palynology to provide age constraints for the stratigraphic surfaces. A detailed description of the palynostratigraphy is presented in Supplement 1. Most samples yielded moderately well-preserved and diverse palynological assemblages, which were comprised almost exclusively of plant spores and pollen grains (Figures 11 and 12). The scarcity of marine palynomorphs in these wells contrasts with their common occurrence in Triassic assemblages from the Western Barents Sea. Such differences likely reflect the more 1562 | EAGE GILMULLINA et AL.
proximal and alluvial palaeogeographic setting of the Eastern Barents Sea.
Nonetheless, the composition of the terrestrial palynofloras in the Russian wells is remarkably similar to those described from age equivalent formations on Svalbard and in subsurface Western Barents Sea, indicating that Triassic palynofloras from the eastern and western areas differ less than previously imagined (cf. Mørk et al., 1993). These similarities permit correlation with the ammonoid-calibrated palynozonation from the Western Barents Sea outlined by Vigran et al. (2014) and Paterson and Mangerud (2020). The recognition of several palynozones from those schemes indicates an Induan-Norian/ (?) Rhaetian age for the sampled intervals. This enhanced palynological age control for the Eastern Barents Sea wells, combined with seismic interpretations, clearly demonstrates that the different stratigraphic units are correlative across the entire GBSB ( Figure 13), and that the regional seismic reflectors have chronostratigraphic significance across the basin.

Triassic boundary
The base Triassic surface ( Figure 10a) shows different expressions in certain regions of the GBSB. In the majority of the basin, the base Triassic surface occurs as a correlative conformity consistent with uninterrupted deposition from the Permian into the Triassic. The boundary occurs in most of the W Barents Sea between hard, dark, spiculitic shales of the late Permian Ørret formation and the softer, siliciclastic shales of the offshore deposits of the Havert Formation. In the S Barents Sea Basin, the base of the Triassic lies at considerable depth (c. 10 km) and is therefore not penetrated by wells. The Permian-Triassic boundary is likely comfortable here and occurs within mudstone-rich siliciclastic rocks. On the Admiraltey High the boundary is unclear due to few wells and little core material On the Loppa High (near the W Margin) and the St Anna Basin (in the extreme northeast of the GBSB) (Figure 10), the base of the Triassic overlies angular unconformities (Figure 4c). In the Pechora Sea and the Timan-Pechora Basin, the Permian-Triassic boundary is erosive and is overlain by basalt lavas in the southeastern part of the basin. On basement highs in the Western Barents Sea, such as on the Finnmark Platform and distal areas of the Loppa High, the base of the Triassic overlies carbonates of the Røye Formation.

| Long-range correlation of stratigraphic surfaces
Stratigraphic surfaces can be distinguished as flooding surfaces (non-marine interbedded sand-and mudstone sharply overlain by marine mudstones) in core and well data (Figures 4a and 9) over most of the GBSB. These surfaces are recognised as planar seismic reflections (Figure 4a,b), and the most extensive of these flooding surfaces (e.g. top Induan) reflections are clearly recognisable across the entire basin (Figures 6 and 7) and were traced across the entire study area. The hypothesis that these flooding surfaces have chronostratigraphic significance across the entire basin is corroborated by the palynostratigraphic data presented in Appendix/Supplement 1.

| Igneous intrusions and their effect on seismic interpretation
In the northern and eastern parts of the Greater Barents Sea Basin, magmatic intrusions of Cretaceous age (Minakov et al., 2018;Polteau et al., 2016) occur as high-amplitude seismic reflectors (Figures 5, 6b and 8 cont.). Presence of intrusions makes it more challenging to interpret seismic data both because they have complex geometries that complicates tracing of stratigraphy, and because they reflect seismic signals and create difficulties for processing and interpretation of the seismic data below (cf. Eide et al., 2018). Isopach maps show increased thickness in the intruded areas (e.g. the South and North Barents Sea depressions, Figure 8a)

F I G U R E 1 3
The Eastern Barents Sea well correlation panel with palynozones recognized from core samples and Fefilova (2013aFefilova ( , 2013b, Viskunova et al. (2000), Gavrilov et al. (2010) shows the similarity to the terrestrial floras from Western Barents Sea and Svalbard (Paterson & Mangerud, 2020). Combination palynozones with seismic interpretation (see Figure 9) shows that stratigraphic units are correlative across the GBSB 1564 | EAGE GILMULLINA et AL.
Basin: Mark et al. (2019)). For studies modelling basin subsidence (e.g. Gac et al., 2014), this effect has hitherto not been taken into account (cf. Gardiner et al., 2019) in the GBSB. Accounting for the intrusions leads to an ca. 0.5 km decrease of Triassic subsidence, and ca. 0.5 increase in Late Cretaceous subsidence.
In the eastern Barents Sea, igneous intrusions were separated into three groups (Figure 8 cont.): 1) sills in the lower part of Triassic succession that occur mainly between top Permian and top Induan where they are actively controlled by lithology and follow main clinoforms and major flooding surfaces; 2) sills in the upper part, between top Induan and top Triassic where they are less controlled by host rock-lithology and are mainly saucer-shaped (cf. Schofield et al. (2012); Eide et al. (2017)); and 3) as dykes (i.e. subvertical dykes) mainly imaged in the Triassic succession, controlled by tectonic structure. The dykes are mainly observed on uplifted basement highs and continue the trend that could be seen in outcrops (Dymov et al., 2011), and a magnetic map close to Franz-Josef Land (Minakov et al., 2018) and are a part of the HALIP giant radiating dyke swarm (Buchan & Ernst, 2018).

| Induan unit-Havert Formation
In the GBSB, sedimentary wedges of the Havert Formation prograded into the basin from the east. Only minor amounts of sand-rich sediment were supplied from the other basin margins in the south and west (e.g. Eide et al., 2018). The base of this unit has variable expression in different areas of the basin and approximately coincides with the Permian-Triassic boundary. The top Induan reflection occurs as a distinctive high amplitude hard-kick (increased impedance, Figure 9) across the basin except where it is intruded in the Eastern Barents Sea. The formation is dated as latest Changhsingian-early Olenekian (249.8-251.902 Ma) by a combination of palynological (Rossi et al., 2019;Vigran et al., 2014), ammonoid (Vigran et al., 1998) and C-isotope data (Hermann et al., 2010) in the W Barents Sea, and this age is demonstrated to also be valid in the Eastern Barents sea by the palynostratigraphic data presented above.
The Induan deposits can be divided into three subunits. The lowest sub-unit, H1, only occurs along Northern Fennoscandia, from which it prograded . The two other sub-units H2 and H3, prograde from the east, and downlap the lower H1 (Glørstad- Clark et al., 2010;Rossi et al., 2019). The Induan deposits prograded across the Bjarmeland Platform, and pinch out on the Loppa High, the Gardarbanken High and the southeastern part of Kong Karl Platform (Figure 10). In the St Anna Basin, northeastern part of the GBSB, the top and bottom of units H2 and H3 contain erosional features interpreted as paleovalleys (Figure 7) which could have been associated with sediment entry points to the basin.
The Havert Formation is more than 2 km thick on Admiraltey High and Eastern Barents Sea Basin and thins towards basin margins. Critically, the Induan unit does not show any thinning towards Novaya Zemlya, indicating that Novaya Zemlya was not uplifted at this time, but rather was the site of a deep, actively subsiding sedimentary basin. Greatly increased thickness occurs in the salt basins across the GBSB, such as the Nordkapp and Tiddlybanken basins (Rojo et al., 2019). This is likely because the rapid influx of large amounts of sediment led to loading and salt movement across the region. It is worth pointing out that similar thickness anomalies are observed in the Korotaikha Basin and the Varandey-Adzvinskaya structural zone in the Timan-Pechora Basin due to movement of the Dolginsky salt dome (Figure 5c). Deformation observed in these areas was previously interpreted to be the result of tectonic thrusting (Henriksen, Bjørnseth, et al., 2011 (Figure 10.36 i), Ivanova et al., 2011).
The Induan deposits consist mainly of clinoformal siltstones and mudstone-rich topsets with sparse, sandy fluvial channel deposits up to 3.5 km wide and 12 m thick in the western Barents Sea. In the most proximal parts of the GBSB, the Timan-Pechora Basin, the time-equivalent deposits consist of conglomeratic sandstone with volcanic material of the Charkabozhskaya Formation.

| Olenekian unit-Klappmyss Formation
The overlying Klappmyss Formation is assigned to the late Olenekian (Spathian) (249.8-247.2 Ma) by palynology (Rossi et al., 2019;Vigran et al., 2014) and ammonoid faunas (Vigran et al., 1998). Seismic data indicate that the unit prograded in a northwest direction (Klausen et al., 2015;Riis et al., 2008). The top of the formation is defined by a hard kick (negative high amplitude, Figure 10) that can be traced on the Fedynsky High and across major parts of the Western Barents Sea. In the deepest part of the basin, the interpretations are somewhat unclear because of the large number of Cretaceous intrusions. Nonetheless, considering the relatively simple structure of the formation in areas without intrusions, it is generally straightforward to approximate the top and base of the unit.
The Olenekian package is characterised by three third-order clinoform packages which downlap onto the underlying Induan unit (Klausen et al., 2015). Seismic lines and thickness maps (Figure 8) of the Olenekian unit shows that an extensive topset accumulated landward of the clinoforms.
The thickness of the Olenekian unit is c 800 m in the deepest part of the Eastern Barents Sea and thins towards | 1565 EAGE GILMULLINA et AL. the basin margins (Figure 8b). The Olenekian deposits are penetrated by 41 wells in the Southwestern Barents Sea and five wells in the Eastern Barents Sea including wells located in Franz-Josef Land, where it mainly consists of dark shales with thin sandstone beds (Dypvik et al., 1998). Our data shows that the Klappmyss Formation of the Southwestern Barents Sea is a continuation of the Kharaleyskaya Formation (Morakhovskaja, 2000) that occurs throughout the entire Timan-Pechora Basin (Figure 3), except the Korotaikha Basin in the foreland of the Ural Mountains, where it is represented by the upper part of the Lestanshorskaya Formation.

| Anisian unit-Kobbe Formation
The Anisian sedimentary deposits (247.2-242 Ma), corresponding to the Kobbe Formation, prograded northwestwards in the Southwestern Barents Sea, in the same manner as the other pre-Carnian packages. Regionally, the unit is bounded on the top by the marine flooding surface that corresponds to top Anisian ( Figure 4) and represented as a hard-kick (high amplitude negative reflector, Figure 9). The Anisian clinoformal unit downlaps the Loppa High (Glørstad- Clark et al., 2010), Gardarbanken High, Storbanken High and Victoria High.
The Anisian package consists of four third-order clinoforms packages that are best developed in the Southwestern Barents Sea . The thickness of the Anisian package is generally ca. 170 m in the topset deposits on top of the previous clinoform packages, and shows a thickening to 600 m northwest of the previous clinoform break (Figures 5 and 8c). The succession is thicker in the salt basins, up to 1,136 m thick, indicating salt movement during deposition. The fluvial channel belts are observed to be up to 5 km wide and 19 m thick in the Southwestern Barents Sea .
This unit is termed the Kobbe Formation in the Southwestern Barents Sea and is the time equivalent to the lower part of the Anguranskaya Formation in the Timan-Pechora Basin where the thickness is up to 240 m (Morakhovskaja, 2000). No evidence of marine incursions is reported in the Timan-Pechora Basin during this time.
Following the trend of progradation for the previous Triassic units, the Ladinian clinoform package is observed northwest of the Bjarmeland Platform. In more proximal areas, it mainly occurs as a tabular set of reflectors (Figure 4), because the water depth was not sufficiently deep for seismically imageable clinoforms to form (cf. Klausen et al., 2018). The Ladinian package is the first to prograde out of the GBSB and into the deeply buried basins on the Atlantic margin of the Barents Sea (Figures 8d,f and 4a). Because of poor seismic quality, deep burial and complicated tectonic deformation, the sedimentary packages have not been interpreted westward of the GBSB.
It is difficult to distinguish the Ladinian to Norian sedimentary units in the Eastern Barents Sea because the post-Anisian deposits here all occur as parallel reflectors without any candidates for major flooding surfaces (see Figure 3), thus palynological data is particularly useful. In the northwest GBSB, the Ladinian unit is clearly defined between well-developed flooding surfaces.
Considering that the Ladinian succession is developed as a tabular topset and a thick package of clinoforms in front of the previous shelf break (Figure 8 cont.), the increased thickness observed in the Franz-Josef Land wells probably shows that there are similar clinoform packages present there as well. This agrees with the thickness maps of this unit, which show a linear belt of high thicknesses extending to Franz Josef Land (Figure 8 cont.).

| Carnian-Norian unit-Snadd Formation
The Carnian succession (237-227 Ma) can be divided into five sub-units in the distal parts of the system (western GBSB). These units are difficult to trace into the proximal Eastern Barents Sea because the marine shales associated with flooding surfaces become too thin to confidently interpret in seismic data and finally pinch out. This interval contains exceptionally well-imaged amplitude anomalies (units C1-3) that clearly correspond to fluvial channel belts in 3D seismic and core   Figure 4a). In the Timan-Pechora Basin, the most proximal part of the study area, the Carnian succession is entirely terrestrial, but there is a variation of terrestrial and marine deposits in Southwestern and Northwestern Barents Sea. Similarly, no marine sediments occur in this time interval during the middle-late Carnian (C3 and C4) succession in the Eastern Barents Sea and Timan-Pechora basins. In these proximal locations, the Carnian-Norian succession comprises a stacked series of fluvial channels complexes, each up to 50 m thick. On Svalbard, the deposits of the Tschermakfjellet Formation are the onshore equivalents of the Snadd Formations and represent the distal part of the same prograding sedimentary system sourced from the east (Figure 3).

Carnian C1
The top boundary of the Carnian C1 unit is the intra Carnian flooding surface (Klausen et al., 2015). Palynological evidence indicates that the C1 unit was deposited in the early Carnian (early Julian, ca. 237-235.9 Ma) . The thickness map of the interval (Figure 8, cont.) shows the southwest-northeast trend across the Greater Barents Sea Basin; the lower Carnian C1 wedge is more extensive and contains internal clinoforms of smaller order than what has been recorded in the following units. Areas on the Atlantic Margin, west of the GBSB, are buried below ca. 3 km thick packages of the Cretaceous and Paleogene deposits, and are strongly faulted making seismic interpretations difficult. Wells 7220/8-1, 7219/12-1 and 7321/8-1 have penetrated up to 600-m thick deposits of Carnian age in this area. These observations document that large amounts of lower Carnian deposits prograded west and northwest of the present-day Barents Sea Basin, but due to the depth of burial and relatively poor seismic resolution, it is difficult to define the volume of Carnian sediments here with certainty. Clinoformal packages are observed on the Bjarmeland, Kong Karls Land and Svalbard platforms (Figures 4-7). The topset of these clinoforms can be traced into more proximal areas in Finnmark Platform, the eastern part of the Bjarmeland, Kong Karls Land, Svalbard platforms and the whole eastern Barents Sea (Klausen et al., , 2016.

Carnian C2
The C2 unit is observed in seismic data in the western part of the basin; the top boundary is a high amplitude hard kick and easily traceable in platform parts (from Fedynsky High in the east until Western Margin and from Pinegin High to Svalbard) (Figures 5-7). Palynological evidence indicates that it was deposited during the late Julian (c. 235.9-234.8 Ma) . In the same way, as the previous Ladinian L1 and C1 units, the Eastern Barents Sea comprises proximal part of the Upper Triassic succession (Figure 9). The thickness of this unit in the Southwestern Barents Sea is up to 250 m (Figure 8 contn.).

Carnian C3, C4 and Norian N1 (De Geerdalen Formation on Svalbard)
The C3 sub-unit is similar to C1 in that it also contains high amplitude reflectors interpreted as channels (Figure 15), which are easily identified in 3D seismic. The unit thins towards the east and starts to be mappable from the western Nordkapp Basin. It is locally present in the Southwestern Barents Sea where it contains smaller order clinoforms. On Hopen and offshore Kong Karls Land, the C3 unit is dated as approximately late Julian to Tuvalian 2-3 by palynology (Paterson & Mangerud, 2015;, calibrated by Re-Os dating (Xu et al., 2014), and ammonoid faunas (Korchinskaya & Semevskij, 1980;Smith, 1982) from the over-and underlying units respectively.
Clinoforms of the overlying C4 unit are not discernible in the southern Barents Sea (Figure 15). It only occurs as topset deposits which cannot be separated from the underlying C3 unit. In the Northwetsern Barents Sea, however, it is clearly mappable. It prograded northwards towards Hopen, Edgeøya and eastern and central Spitsbergen where both C3 and C4 are exposed in outcrop, and reaches a thickness of 435 m (e.g. Vigran et al., 2014). Paterson and Mangerud (2015),  have assigned a late Tuvalian age for this unit based on palynological evidence, independently constrained by magnetostratigraphy (Lord et al., 2014) and ammonoid occurrences on Hopen (Korchinskaya & Semevskij, 1980;Smith, 1982). This C3 unit is present as a relatively thick package at the most western and northern parts of the basin, and this indicates that great amounts of sediments must have also been deposited northwestwards of the studied GBSB.
The uppermost unit of this package, the early Norian N1 unit has a high amplitude top reflector which is easy traceable east of the Loppa High in Southwestern Barents Sea but is difficult to discern due to strong faulting in the western part. The unit contains third-order small scale clinoforms that have their shelf-break at least 300 km southeast from the minimum position of the C4 package, and therefore represents a backstepping of the system (see Figure 3). The N1 sequence is principally dated by palynological evidence (e.g. Paterson & Mangerud, 2020) calibrated by occurrences of early Norian (Lacian) ammonoids in the Flatsalen Formation on Hopen (Korchinskaya & Semevskij, 1980;Smith, 1982).

N2-R1-Fruholmen Formation
The upper part of the Upper Triassic, corresponding to the Fruholmen Formation in the W Barents Sea, is completely or partly eroded in most parts of the GBSB (apart from the deeper basins in the Eastern Barents Sea, the Atlantic margin and in salt basins), by either the base Quaternary erosion (Upper regional unconformity, URU) or by erosion at the Triassic/ Jurassic Unconformity (Figure 14) (Müller et al., 2019). Rhaetian deposits have also been recorded onshore, on Hopen and Kong Karls Land (Paterson and Mangerud, 2015;Smelror et al., 2019), and offshore in the Hammerfest and Nordkapp basins Vigran et al., 2014). The unit is also relatively thin in most of the study area (ca. 50 m on the Bjarmeland Platform, versus 579 m on the Atlantic margin and in the Eastern Barents Sea), making it difficult to trace on regional seismic data. It is therefore not investigated in the same level of detail as the other stratigraphic units.
Similar to the preceding Triassic units, the N2-R1 deposits contain a lower marine mudstone and an upper progradational deltaic topset with channels ( Figure 15). This unit is generally thinner and has been deposited over a longer time span than the previous units. However, the thickness is poorly constrained due to extensive erosion by the Rhaetian unconformity (Müller et al., 2019;Figures 14 and 3). Because the depositional thickness of the Norian-Rhaetian succession is so poorly constrained by seismic data, defining the thickness of it across the GBSB, is beyond the scope of this study.

| Large-scale thickness trends
The Triassic succession prograded in a northwesterly direction towards the Svalbard Archipelago Lundschien et al., 2014) and is characterised by two different thickness trends: (1) thick deposits in the SE parts and (2) thin deposits in the northwestern parts of the basin for the Early Triassic, a trend that reverses in the Late Triassic (Glørstad-Clark et al., 2010). The results presented here show that the following depositional trends dominated throughout the entire GBSB (also in the Eastern Barents Sea) during the Triassic Period.
The Induan deposits are thickest in South and North Barents depressions in the Eastern Barents Sea. The thickness map (Figure 8a) of the unit shows the general thinning towards the west and northwest (Figure 4). The unit becomes thinner towards Timan-Pechora Basin, as well as east of the St. Anna Basin where it is eroded towards the east close to Novaya Zemlya by the Triassic-Jurassic unconformity (cf. Figure 3). The Induan deposits are eroded by the Upper Regional Unconformity (URU) of Pleistocene age along the basin margins along Northern Fennoscandia and the Kara Sea ( Figure 14). The Olenekian unit follows mainly the same pattern as the Induan, but it is much thinner and does not show the pronounced thickening into the Eastern Barents Sea Basin (Figure 8b). In the westernmost part of the basin, the maximum progradational extent of the Olenekian is ca. 35 km shorter than the underlying Induan package (Figure 4).
The thickness map of the Anisian unit is similar to the Olenekian and thins towards the west and northwest (Figure 8). The thickness increases in the Pay-Khoy foreland, but on the rest of Timan-Pechora Basin territory, the sedimentary package shows a constant thickness. The Anisian unit progrades ca.55 km further than the previous Olenekian unit (Figure 10), filling the accommodation space formed in front of it. Maximum thicknesses are shifted westward towards the Bjarmeland Platform, Loppa High, Hammerfest Basin and Kong Karls Land (Figure 8).
The thickness of the Ladinian, Carnian and early Norian deposits are shown together in Figure 8 because the flooding surfaces between the internal subunits cannot be traced further east than the Norway-Russia border. This sedimentary package shows a different thickness trend from the previous units and is characterised by minimum thickness in the east and a maximum thickness to the west. Typical thicknesses are 0.8 km in the eastern parts of the GBSB and 1.9 km in the western GBSB. The western depocentre is elongated from the Hammerfest Basin in the south to Franz-Josef Land in the north. This elongation is mainly because the deposits filled the accommodation in front of the previous clinoform succession ( Figure 5).
In the western Barents Sea, the Ladinian, Carnian and Norian deposits can be divided into six subunits, defined in Klausen et al. (2015): L1, C1-4 and N1-2. The thickness map of the Ladinian unit (L1) shows a clear trend where the thickest part forms an elongate trend from the southwest in the Hammerfest Basin to northeast to Franz-Josef Land. The thickest part of the Ladinian unit is related to accommodation available in front of the Anisian package and thins towards the west and northwest as the unit pinches out. The topset thins towards the east and becomes indistinguishable from the topset of the Carnian unit just east of the Norway-Russia border.
In the Eastern Barents Sea, the Carnian package is relatively thin (ca. 300 m in the Arcticheskaya-1 well) and increases in | 1569 EAGE GILMULLINA et AL.
thickness towards the Western Barents Sea to 1,395 m in well 7321/8-1. In the western part of the Barents Sea, the Carnian units C1-2 show a progressive progradation towards Svalbard (Figures 8, 10). In Svalbard, deposition of relatively mature sands derived from Greenland and local basinal highs ended with the arrival of the easterly depositional system of the C3 and C4 units in the Carnian (e.g. Bue & Andresen, 2014;Klausen et al., 2015). The transgressive Norian unit N1 is very thin and eroded in the west and northwest (Klausen et al., 2015;Lord et al., 2019). The unit N2, which makes up the Fruholmen Formation, is thin and partly eroded across the GBSB, and shows thickening towards the western margin but is eroded on Svalbard Figure 3

| Main uncertainties and limitations to tracing large-scale flooding surfaces
In this project, we have investigated a vast area (1,200 × 1,800 km, 2,500,000 km 2 , Figure 1a). In some locations, we have good control of the age and architecture of deposits because of an abundance of wells, 3D seismic data (Southwestern Barents Sea) or outcrops (Svalbard).
The greatest uncertainty is in the Eastern Barents Sea where large amounts of the Cretaceous igneous intrusions are present (Figures 4-7). The great depth of the basin (up to 10 km to the base of the Triassic) also causes uncertainties in the mapping of stratigraphic units since no wells are penetrating Induan and Olenekian at these great depths. The northern part of the East Barents Sea and the North Kara Sea have no wells at all, with the nearest wells located in Franz Josef Land (Dypvik et al., 1998) (Figure 1).
The stratigraphic framework in this study (Figure 3) is based on a set of continuous and regionally traceable marine flooding surfaces (Figures 4 and 9). Despite the uncertainties listed above, the Lower Triassic deposits are tabular in all parts of the basin, have excellent biostratigraphic age control, and the flooding surfaces are easy to identify on seismic data. In the Early Triassic, many of these surfaces are traceable all the way into the easternmost parts of the GBSB. The large extent of these surfaces is likely due to the fact that the coastal plain had a very low gradient, as should be expected F I G U R E 1 6 Schematic distribution of thicknesses (corrected for intrusion thickness) and main depocentre shift. The map shows the primary progradational trend (orange) and location of depocentres from southeast to the northwest and the local western source (green) on Svalbard that was mainly active during the Early and Middle Triassic. The Early Triassic trend indicates the decrease of grain size and possibly a channel size as well from proximal to the distal part of the basin; the Late Triassic trend indicates that the average grain size relatively equal across the basin and channel size varies from 25 to 50 m across the basin in subsiding sedimentary basins (Nyberg & Howell, 2015), and that marine transgressions could therefore penetrate far onto the coastal plain. The coastline prograded further basinward in the Middle and Late Triassic, which prevented marine transgressions from reaching the Eastern part of the GBSB. This explains why the marine flooding surfaces, and marine palynomorphs, are not found in Timan-Pechora Basin after Middle and Late Triassic. In addition to the flooding surfaces, the thicknesses and age of units fits excellently with the stratigraphy in the Timan-Pechora Basin (Figures 2 and  3). There is, therefore, a good reason to believe the seismic interpretations in areas without well control are correct.

| Large-scale depocentre change and trends across the basin
From both the seismic cross-sections and the thickness maps presented above (Figures 4-8), a three-stage shift in depocentre in the GBSB is evident ( Figure 16): (A) a Lower Triassic thick depocentre located in Eastern Barents Sea; (B) a Middle Triassic transitional deposits with thin deposits located above and slightly in front of the Lower Triassic package and (C) Upper Triassic thick depocentre located in Western Barents Sea. Each stage is discussed in detail below.

| Lower Triassic southeastern depocentre
During the Early Triassic (Induan and Olenekian), the delta system prograded westwards from the east as far as the middle of the present-day Barents Sea (Figure 8). The depocentre of Induan and Olenekian packages is located in the Eastern part of the basin (South Barents Sea depression, Ludlow Saddle) and these packages thin towards the west and northwest. A marked decrease in channel size and grain size occurs within this package from proximal to distal: In the proximal Timan-Pechora Basin, the maximum observed channel belt thickness is ca. 62 m (Morakhovskaja, 2000); in the more medial Eastern Barents Sea Basin, maximum described channel belt thicknesses are 50 m (in the Krestovaya-1 well, (Norina et al., 2014)). In the Western Barents Sea, for example, in the 7226/2-1 well, the maximum described channel belt thickness is as low as 12 m . Similar trends in channel complex width are difficult to ascertain across the basin due to the lack of available 3D seismic data in the Eastern part of the study area. Grain size shows a similar decrease towards the northeast, with conglomerate being common in fluvial channel deposits in the Timan-Pechora Basin, whereas any grains coarser than fine sand are only very sparsely observed in wells in the southwest Barents Sea. This clearly indicates a proximal-distal trend, and that the sediment source in this time interval must have been very close to the Timan-Pechora Basin. The Lower Triassic sediment package is considerably thicker than the overlying units, (ca. 3.5 km, vs. 0.9 and 2.0 km for the Middle and Upper (Carnian) Triassic packages), despite being deposited over a shorter interval (5.1 Myr vs. 10.1 and 8.6 Myr respectively).

| Middle Triassic transitional depocentre
During the Middle Triassic (Anisian and Ladinian), the main depocentre shifted slightly towards the north-northwest, and the sedimentary package prograded further across Bjarmeland Platform, Loppa High, Sentralbanken High and Kong Karl Platform (Figures 4-8). Middle Triassic clinoform packages are relatively thin in the eastern part and also thinning towards the northwest where they pinch out. Channel belt thicknesses in the Timan-Pechora Basin vary from 40 to 50 m, which is thicker than what is observed in the Southwestern Barents Sea, where it is only up to 19 m thick (Kobbe Formation, (Klausen et al., 2018)).

| Late Triassic northwestern depocentre
In the Late Triassic, progradation rates noticeably increased and the delta front reached at least as far northwest as Svalbard (Figures 10 and 15). How much farther the delta reached is difficult to determine because no exposure exists northwest of central Svalbard. During the Carnian, all structural elements westwards and northwestwards of Bjarmeland Platform, Loppa High, Sentralbanken High and Kong Karl Platform subsided and were buried by tabular deposits sourced from the main sediment source in the southeast. The Carnian package is relatively thin (c. 300 m) in the Eastern Barents Sea but up to 1,400 towards the northwest. Channel thickness throughout Upper Triassic varies from 30 to 50 m. Average grain size is relatively equal across the basin; the content of conglomerate in Upper Triassic deposits is low in the most proximal parts of the GBSB. This implies that the system is much longer than in the Early Triassic, and that the gradients in the source-area close to the most proximal parts of the basin must have been much lower in the Late Triassic than during the Early Triassic.

| Reasons for depocentre shift
This depocentre shift is likely related to two factors: subsidence and sediment supply. Huge amounts of sediment were | 1571 EAGE GILMULLINA et AL.
supplied from the eastern source in the Early Triassic, and the southeastern parts of the basin subsided rapidly to accommodate these sediments ( Figure 16). Much smaller quantities of sediment were supplied during the Middle Triassic, and little subsidence was needed to accommodate these sediments. The Late Triassic package is associated with a renewed increase in sediment supply, but no simultaneous increase in subsidence rates in the southeastern parts of the basin. Very little subsidence occurred in the eastern parts of the basin, but subsidence occurred in the northeastern parts of the basin around Svalbard (Figure 16) in similar amounts to what have been observed for the Lower Triassic. However, the rates of subsidence in the Eastern Barents Sea (3.5 km over 4.6 Myr = 0.8 km/Myr) were four times higher than the rates of subsidence around Svalbard in the Late Triassic (2 km over 9.5 Myr = 0.2 km/Myr).
It is likely that the great supply of mud-rich sediment during the Early Triassic is related to both volcanism in the Siberian Traps Large Igneous Province (c.f. Reichow et al., 2009;Wignall, 2001) and volcanic uplift of the surrounding uplands including the Taimyr. It may also be speculated that the strong subsidence around the Siberian Traps in the Early Triassic is related to the displacement of the mantle due to great magmatic activity in the Siberian Traps at this time (cf. Friedrich et al., 2018).
The decrease in subsidence and sediment supply during the Middle Triassic is likely related to relative quiescence in tectonics in the source areas during this time. It is also worth noting the potential extra accommodation created by the compaction of large amounts of mudstone (e.g. Plint et al., 2009), which would add to the accommodation space created by crustal subsidence.
During the Late Triassic, the main reason for the increased sediment supply is likely to be renewed tectonic activity associated with further contraction of the Ural orogeny and in particular the initial uplift of the Novaya Zemlya orogen (e.g. Klausen et al., 2015;Zhang et al., 2018). The climate also became more humid in the GBSB, and in northern Pangea in general, in part due to northward continental drift out of the arid subtropics and into more humid latitudes (e.g. Francis, 1994;Jarsve et al., 2014). This lead to an increase in both weathering and transport capacity of rivers. A major, worldwide humid episode linked to climate change during the Carnian, the Carnian Pluvial Event (Mueller et al., 2016;Ruffell et al., 2016;Simms & Ruffell, 1989;Xu et al., 2014) might also be partly responsible for greater sediment supply in the Triassic Barents Sea. The subsidence pattern in the Late Triassic likely reflects the fact that the subsidence potential in the Eastern Barents Sea Basin was already exhausted in the Early Triassic. Therefore, sediments likely bypassed in the east, and found accommodation in the Northwestern Barents Sea.

Urals and Novaya Zemlya
The Polar Urals and Novaya Zemlya Fold and Thrust Belt (NZFTB) were uplifted in the Late Triassic-Early Jurassic, but the exact timing and relative magnitude of uplift remains poorly constrained partly due to lack of data. Recent apatite fission track analysis from Novaya Zemlya by Zhang et al. (2018) shows rapid cooling between 220 and 210 Ma, indicating that uplift occurred in the late Norian-Rhaetian. Müller et al. (2019) proposed that the latest Triassic Rhaetian unconformity across the GBSB was caused by the Late Triassic uplift of the Polar Urals and Novaya Zemlya. The present study presents the largest database and best coverage of seismic surveys and wells around the NZFTB, and although it is clear that more data are needed to constrain the timing, magnitude and spatial distribution of uplift, we have made the following observations that relate to the evolution of Novaya Zemlya in the Triassic: 1. There are ca. 3 km thick Lower-MiddleTriassic deposits that can and traced all the way to the NZFTB, without any indications of thinning. This implies that the NZFTB was a basin during the Early Triassic, and that uplift must have occurred later than the Middle Triassic (Figures 4 and 8) 2. There is an angular unconformity at the base Jurassic (Figures 5c and 7) with erosion down to the Anisian (Kobbe Formation) observed both north and south of the NZFTB (Figures 14 and 4).
The base Jurassic/Rhaetian unconformity can be traced on seismic data into the Western Barents Sea, where it corresponds to basin inversion and salt dome reactivation in the Norwegian sector of the Barents Sea, which occurred approximately in the Rhaetian (Müller et al., 2019). It seems safe to assume that the age of uplift in the W Barents corresponds to the culmination of uplift in the NZFTB, and thus the uplift here must have occurred sometime between the Anisian and the Rhaetian (Figures 8 and 4). It is possible that changes in the sedimentary systems in the GBSB in the late Carnian indicate early tectonic uplift in the NZFTB. These changes include the sudden progradational nature of the Carnian C3-4 unit compared to the C1-2 unit (this study), change to more westerly sediment transport directions and change to smaller and more abundant fluvial channels in the C3-4 unit compared to the C1-2 unit (Klausen et al., 2015), the appearance of recycled quartz grains (grains with dustrims and fluid inclusions encased in younger cement) in the Snadd Formation and appearance of young zircons in the Snadd Formation (Fleming at al., 2016). Moreover, the overspill of the Upper Triassic sediments into adjacent basins (discussed below) also indicates strong uplift in the source area, and uplift in the NZFTB is the most likely candidate for that. Thus, the earliest tectonic evidence of uplift might be evident in the late Carnian, giving a protracted uplift of the NZFTB that culminated in the Rhaetian-Early Jurassic.

| Implications for provenance studies in the GBSB
On Svalbard, the Lower and Middle Triassic deposits show great mineralogical maturity and are sourced from the west (Greenland and local sources, Bue & Andresen, 2014;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999). In the Carnian, the mineralogy shows a definite change to less mature sediments, and it is evident that these lithologies are derived from the eastern source (Bue & Andresen, 2014;Mørk, Dallmann, et al., 1999;Mørk, Elvebakk, et al., 1999). However, the Taimyr Orogen has been proposed as a source for the late Carnian age De Geerdalen Formation on Svalbard (Fleming et al., 2016) based on the presence of young zircons in that interval that have not been found in the Southwestern Barents Sea. These authors suggested that the hypothesised Carnian progradation from Taimyr had not been recognised by studies based on seismic data, because of the lack of seismic data from the northern GBSB. In this study, we have now analysed seismic data from the entire northern region of the GBSB, and the northern areas show a persistent northwest progradation direction through the Triassic, including the late Carnian ( Figure 10). Although we do not have access to 3D seismic data from the area, which is needed to determine channel directions, the direction of sediment transport measured from the clinoform directions make a source in Taimyr unlikely. We, therefore, suggest that more detrital zircon ages throughout the GBSB region are needed to better understand the distribution and source of these young zircons, especially from the youngest part of the Carnian deposits where these are abundant. Furthermore, the stratigraphic framework presented in this contribution (Figure 3) will make it possible to design studies in a way that geographical and temporal variations can be isolated in provenance studies.

Arctic basins
In the Triassic, there was a large, semi-enclosed shelf underlain by continental crust north of the GBSB, which comprised the Lomonosov Ridge, the Sverdrup Basin, the Chukchi Borderland and Alaska (Shephard et al., 2013;Sømme et al., 2018). The exact location of the different microcontinents remains controversial (cf. Miller et al., 2018;Nikishin et al., 2019), and it is possible that the locations of these could be constrained by combining detailed provenance work in these regions with the timing of overspill presented in this study.
The western and northern margins of the GBSB were rifted off in the Palaeocene-Eocene and Late Cretaceous, respectively (e.g. Doré et al., 1999;Faleide et al., 2008;Seton et al., 2012). Our analysis of seismic data of the Triassic of the GBSB shows that there are northwest-dipping clinoforms (i.e. indicating sediment transport towards northwest) as far towards the basin margin as there is seismic data, and as far towards the west as until the Triassic is faulted and buried so deeply that it cannot be interpreted confidently, or until the basin margin itself. This strongly suggests that the Triassic sedimentary system prograded beyond the present-day GBSB, and that sediments derived from the eastern sources were deposited into adjacent basins. Furthermore, based on the sediment transport directions that can be deduced from the seismic data, we expect overspill from the easterly sedimentary system (1) throughout the Triassic N of Franz-Josef Land and the Sant Anna Basin, (2) along the Atlantic Margin S of the Loppa High from the Ladinian and (3) along the entire Northern and Western margins of the GBSB and northwest of Svalbard during the Carnian and possibly early Norian (Figures 10 and 15). The Norian-Rhaetian uplift episode described by Müller et al. (2019) likely also made it possible for sediments from the eastern source to reach basins to the north and west of the GBSB. Thus, the seismic data indicates a potential for significant overspill from the GBSB in the (1) late Carnian-earliest Norian and in the (2) late Norian-Rhaetian.
In a study from the Sverdrup Basin, Anfinson et al. (2016) interpreted an Ural source of Carnian and Norian deposits in the northern part of the Sverdrup Basin where, especially in the Norian, large amounts of sediments arrived in the basin. Based on plate reconstructions at ca. 200 Ma (Seton et al., 2012), North Ellesmere Island was located in an area where it was likely reached by the Carnian-Norian sedimentary system sourced from the eastern sources ( Figure 10). A provenance shift in other Arctic basins (e.g. Chukchi basin, Wrangel Island, New Siberian Islands) was documented Miller et al. (2013), Miller et al. (2018) and Ershova et al. (2015). Their studies suggest a close proximity to the Siberian Traps and sediments sourced from West Siberia, Taimyr and Polar Urals during the Triassic and sediment transportation across the GBSB.
The Lomonosov Ridge is a microcontinent to the north of the GBSB and was rifted off in the early Eocene (Doré et al., 2016;Grantz et al., 2001;Nikishin et al., 2017;Pease et al., 2014). Samples from the Lomonosov Ridge are limited, but from the clinoform directions ( Figure 10) and | 1573 EAGE GILMULLINA et AL. thicknesses mapped (Figure 8) in this study, it appears highly likely that great thicknesses of Triassic deposits should exist in the Lomonosov Ridge, with the oldest Triassic deposits towards the southeast and Carnian-Norian deposits towards the northwest. Sediments may also have prograded onwards from the Lomonosov Ridge to basins beyond.
Thick faulted packages of the Upper Triassic are located on the Atlantic (Western) margin of the GBSB, but these are buried by thick packages of the Jurassic to Quaternary sediments and are difficult to interpret in the seismic datasets we have access to. More data are required to determine where the boundary between sediments derived from Greenland (c.f. Bjerager et al., 2019) and the sediments derived from the eastern sources were located in the Carnian.

| Implications for petroleum exploration
Understanding the distribution of different sedimentary systems within the basin makes it possible to extrapolate knowledge from well-studied areas into areas with little data (e.g. Helland-Hansen et al., 2016). The sedimentary system of the large GBSB was fed by a continental scale catchment, and the deposits are relatively well understood due to widespread hydrocarbon exploration in the Timan-Pechora Basin and the Southwestern Barents Sea, with analogues in large, continuous outcrops on Svalbard (Figure 1).
In the southernmost part of the Southwestern Barents Sea, the earliest Triassic H1 unit and also most other stratigraphic units, are relatively sand-rich due to the supply of sand from Fennoscandia Fleming et al., 2016), such as in the Goliat Field (Duran et al., 2013). In the Timan-Pechora Basin, which is the most proximal part of the study area, the Peschanoozerskoe and Tarkskoe fields contain the main reservoir in Lower Triassic channelised deposits (Charkabozhskaya Formation) that despite good porosity contain up to 75% of expansive clay material as smectite and montmorillonite (Toporkov & Denisenko, 2008). Here the location of the fields closest to the source shows both coarser grain size and larger channels, but also more poorly sorted and clay-rich deposits typical of proximal terrestrial deposits. On Svalbard, the majority of sandstone is in the channel belt complexes, and although the proportion of channels is not particularly high (c.10%), in the Carnian De Geerdalen Formation, the sandstone properties are good (porosity ca. 30% in channels; Haile et al., 2018).
Despite the mudstone-dominated nature of the Triassic interval, multiple oil and gas discoveries have been made in this interval in the Southwestern Barents Sea. The discoveries have mainly been in channel belt complexes that are well-imaged in 3D seismic data. It is evident that there is potential for channelised sandstone reservoirs in the northern part of the Barents Sea, and in this distal position the source rock in the Steinkobbe Formation is more favourably positioned directly beneath good reservoir rocks . The present study indicates the limits of terrestrial facies at different times in the Upper Triassic and shows the larger source to sink setting in which these reservoir sandstones must be considered.
Offshore marine deposits that are associated with the best quality of source rock (kerogen types I and II), developed in the western and northwestern part of the GBSB in the Botneheia and Steinkobbe Formation (Figure 3) (Abay et al., 2018;Leith et al., 1993;Vigran et al., 2008). In the more proximal areas, such as the Southwestern and Eastern Barents Sea, the Lower and Middle Triassic deposits are characterised by alternating marine and terrestrial facies and are associated with a mixture of different types of a source rock (kerogen type III and II) (e.g. . The Upper Triassic in the entire GBSB, particularly the De Geerdalen and Svenskøya formations (and their subsurface equivalents, the Snadd and Fruholmen Formations, respectively) are dominated by paralic and fluvio-deltaic facies. These deposits are characterised by the dominance of Type III kerogen . The presence of thin coal beds in the Upper Triassic succession may represent an additional gas-prone source rock (Norina et al., 2014). The intervening Norian deposits, particularly the Flatsalen Formation and its subsurface equivalents in the northwest region, are typified by a high abundance of marine organic matter (Type II Kerogen) . Flooding surfaces are regional and relatively good seals that well developed on a bigger part of the basin during the Lower and Middle Triassic. A local sealing mechanism for channelised deposits is likely to be overbank shales.

| CONCLUSIONS
This study presents a unified stratigraphic framework of the Triassic in the entire Greater Barents Sea Basin, which is a vast (1,200 × 1,800 km) intracratonic basin that includes the Southwestern and Northwestern Barents Sea, Svalbard, Eastern Barents Sea, Timan-Pechora Basin and North Kara Sea. The study is based on analysis of large amounts of seismic data in the region, all available well data and published information from outcrops of Triassic rocks in the region. These have been correlated using new palynological analyses from Russian wells presented herein, and published data for the entire GBSB. The majority of Triassic deposits in the GBSB consists of deposits of a low-gradient deltaic system including clinoformal prodelta mudstones, a mudstone rich deltaic topset and sandstone-dominated fluvial channels, that was fed from a 'continental-scale' catchment in Western Baltica, Urals and West Siberia. This system was active for 1574 | EAGE GILMULLINA et AL.
ca. 50 Myr (Induan to Rhaetian), and is characterised by sediment transport towards the northwest into the Greater Barents Sea Basin.
In the Lower Triassic, many of the flooding surfaces can be traced far into the Eastern parts of the Greater Barents Sea basin, but in the Middle and Late Triassic the sedimentary system progrades so far into the basin that marine flooding is not registered in the more proximal parts of the basin. Enormous amounts of sediments were supplied in the Early Triassic, lower sediment supply occurred during the Middle Triassic, and an increase in sediment supply is observed in the Carnian. These changes largely relate to tectonism in the source area, especially the onset of the Siberian Traps Large Igneous Province in the Early Triassic, relative tectonic quiescence in the Middle Triassic, and incipient uplift of the Novaya Zemlya and Pay-Khoy in the Late Triassic. Large amounts of sediment spilt over into adjacent basins throughout the Triassic in the North (towards the Lomonosov Ridge and possibly Chukchi Borderland) and during the Carnian and Norian towards the Atlantic Margin and Arctic Canada (Sverdrup basin) in the west and northwest, showing that provenance interpretations that require long-distance sediment transport across the GBSB to the wider arctic are reasonable. The results of this study can be used to guide petroleum exploration in the areas of the GBSB with little data because of how the different units correlate and how facies are developed within them. For example, channelised deposits in the distal part of the system well-positioned above the more prolific source rocks in front of the delta, and sandstones are likely better sorted compared to proximal equivalents. Furthermore, the great thickness of igneous intrusions in the Eastern Barents Sea Basin has to be taken into account when modelling basin subsidence and thickness of units.