Using the tectophase conceptual model to assess late Triassic – Early Jurassic far-ﬁ eld tectonism across the South-central Barents Sea shelf

The Upper Triassic – Lower Jurassic succession of the Barents Sea Shelf (BSS) represents one of Europe ’ s most proli ﬁ c and strategic petroleum systems. This succession re ﬂ ects various depositional environments and tectonostratigraphic events. Even though these strata are considered largely well-understood, connections with far-ﬁ eld stresses triggered by regional tectonics remain a subject of investigation. This study presents new interpretations that focus on relationships between the stratigraphic succession across the south-central BSS and Triassic – Jurassic Novaya Zemlya compressional tectonics. By applying the “ tectophase model, ” developed in the Appalachian Basin, to analyze this succession, the presence of foreland-basin depozones and associated far-ﬁ eld processes related to compressional tectonics in an adjacent orogen are suggested. This model addresses unconformity development, lithostratigraphic succession, and reactivation of structures. Use of this model suggests far-ﬁ eld tectonostratigraphic responses during two episodes of Novaya Zemlya tectonism, re ﬂ ected in the coeval BSS stratigraphy. Overall, this tectonostratigraphic study aligns with other research suggesting a Late Triassic inception for Novaya Zemlya compressional tectonism, which in ﬂ uenced larger parts of the BSS through extensive clastic sedimentation, far-ﬁ eld structural reactivation, and ﬂ exural responses to deformational loading triggered by tectonics.


Introduction
The Barents Sea shelf (BSS) is located between northern Norway, northwestern Russia, Svalbard, Franz Joseph Land and Novaya Zemlya (Figures 1A, B), covers approximately 1.4 million km 2 , and consists of a complex system of sedimentary basins, platforms, and structural highs (Figure 1B), with substantial hydrocarbon resources (e.g., Doré et al., 2022).In this large province, Upper Triassic to Jurassic prolific reservoirs host strategic hydrocarbon fields (e.g., Snøhvit, Albatross, Goliat, Askellad, Ludlovskaya, and Shtokmanovskaya) (Duran et al., 2013;Polyakova, 2015).In the Norwegian part of the shelf (western BSS, NBSS; Figure 1B), hydrocarbon exploration has been conducted since the 1970s with the first wells drilled in the 1980s.Nearly all drilling was done in south-southwestern areas of the shelf, whereas the northern NBSS sector (north of 74 °30') has not yet been opened to exploration; thus, lithologic and stratigraphic information from this sector is currently restricted to the published literature and shallow boreholes (Figure 1B).On the Russian part of the shelf (eastern BSS, RBSS; Figure 1B), most drilling has taken place in the southeast, where correlations with nearby wells from the better-known Timan-Pechora Basin (Figures 1B, C) have been attempted (e.g., Johansen et al., 1993;Mørk, 1999).
essential because the development of the Novaya Zemlya orogen implies BSS crustal loading, flexural responses, and related far-field tectonics.Use of flexural analysis can greatly resolve complex geologic problems ranging from tectonics (Karner and Watts, 1983) to stratigraphy (e.g., Ettensohn et al., 2019).
To evaluate the potential likelihood of Late Triassic-Early Jurassic BSS far-field reactivation, foreland-basin development, and possible ties to Novaya Zemlya tectonic loading, the goal of this study is to see if the "tectophase" model, which is built on traditional, viscoelastic, flexural theory (Quinlan and Beaumont, 1984), can tie BSS stratigraphy to compressional tectonics in the nearby Navaya Zemlya orogen (e.g., Johnson, 1971;Barbeau, 2003;Su et al., 2009;Ettensohn et al., 2019).In this model, presence of sequence-bounding unconformities, occurrence of key lithologies, and regional variations in stratigraphic thicknesses indicate responses to flexurally related structures (e.g., foreland basins and bulges) and far-field structural reactivation across a craton (Ettensohn, 1994).Hence, if Novaya Zemlya compressional tectonism occurred during Late Triassic-Early Jurassic time, coeval BSS structural reactivation and stratigraphic responses should be expected.To achieve this goal, a systematic analysis of the BSS Upper Triassic-Lower Jurassic succession was performed using 85 wells, two shallow cores, and specialized literature (e.g., Ettensohn et al., 2019).
It is important to understand word usage involving the "tectophase" concept.The term "tectophase" was defined as being the series of compressional events occurring during an orogenic event (Johnson, 1971) from the time of cratonward thrusting to the later far-field reactivation of structures across the adjacent craton and foreland basin.In contrast, "tectophase model" is the use of traditional flexural theory (e.g., Quinlan and Beaumont, 1984) to better describe foreland lithostratigraphic composition in terms of simultaneous orogenic events (Ettensohn, 1985).Similarly, the "tectophase cycle" represents a systematic succession of specific, high-resolution, foreland-basin stratigraphic units, the repetition of which reflects repeated smaller-scale compressional pulses during a larger orogeny.This stratigraphic succession is bounded by unconformities and represents lithologic responses to specific and well-defined flexural orogenic events controlled by deformational loading in the orogen (Ettensohn et al., 2019; section 1.3).Such "tectophase" successions are better studied in the Appalachian foreland basin (Ettensohn et al., 2019).Outside the Appalachian area, tectophase sequences have been identified in South China (Su et al., 2009) and the BSS (Martins et al., 2022), and other potential tectophase successions exist in the north Alpine foreland basin (Sinclair et al., 1991;Kempf and Pfiffer, 2004).Also important in this paper is defining the term "structural reactivation," which in this paper is suggested as a means to investigate potential far-field stresses in terms of stratigraphic responses.
The tectophase model is well-known in the Appalachian foreland basin in the eastern U.S.A, and it has been largely calibrated there (e.g., Ettensohn et al., 2019).Furthermore, Appalachian tectonics and geodynamics have been suggested as analogous to processes forming the Uralian-Pai-Khoi-Novaya Zemlya system of basins, which includes the BSS (e.g., Artyushkov and Baer, 1983;Kruse and McNutt, 1988;Puchkov, 2002;2009;Ritzmann and Faleide, 2009;Gac et al., 2013;Martins et al., 2022).In the Appalachian system, however, relationships between stratigraphy and far-field tectonics are relatively wellunderstood (e.g., Klein and Hsui, 1987;Merschat et al., 2007;Hatcher, 2010;Ettensohn et al., 2019), whereas interpretation of BSS tectonic and stratigraphic evolution have at times been conflicting or uncertain.For example, Müller et  Because the tectophase model, as used herein, is largely based on outcrop data, tectonostratigraphic and flexural concepts developed in this model are stratigraphy dependent.In the subsurface, well logs and cores provided datasets for this analysis.Even though seismic analyses can be used to complement basin study, BSS seismic coverage is not homogenous and at times largely inaccessible (e.g., Russian BSS).However, seismically imaged Upper Triassic-Lower Jurassic BSS reflectors used to address structural and tectonostratigraphic aspects in various parts of the BSS can be found in studies like those of Henriksen et al. (2011, 2023), Stoupakova et al. (2011), Müller et al. (2019), Gilmullina et al. (2021), Suslova et al. (2021), andLundschien et al. (2023).

Far-field tectonics and tectophase modelling
Far-field tectonics is a process associated with the propagation of crustal stresses across an intraplate domain (e.g., Cloetingh, 1988;Ziegler et al., 1995;Parizot et al., 2020).Such stresses may cause tensional or compressional reactivation of preexisting structures up to 1700 km from a collisional/subduction zone and are capable of developing rifts, basement uplifts, and pull-apart basins far into the craton (Ziegler et al., 2002;Ussami et al., 2010;Cloetingh et al., 2015;Gianni et al., 2020).These compressional stresses include largescale, low-amplitude undulations of the crust, which can be linked through timing, stratigraphy or structural style to coeval orogeny via supracrustal or subcrustal loading (Klein, 1994;Ettensohn et al., 2002).Structural reactivation during orogeny is for the most part coeval with flexural processes adjacent to the orogen, which are reflected in foreland-basin development.Because flexural theory has been traditionally used to better understand the physical characteristics of foreland basins (e.g., Karner and Watts, 1983;Quinlan and Beaumont, 1984), it can also be used to help resolve many compressional tectonostratigraphic nuances.
Though conceptual in origin, "flexural theory" is used in this study to indicate that a given tectonic process in the orogen (e.g., deformational/tectonic loading) results in a wavelength-like, flexural response (e.g., foreland basin and bulge) of the crust that can affect regional sedimentation (e.g., Quinlan and Beaumont, 1984).These regional responses in the adjacent foreland basin can be subdivided in stages tied to "phases" of flexure triggered by compression in the orogen (Ettensohn et al., 2019).Hence, the tectophase model is used to explain the relationship between stratigraphy and crustal flexure (Johnson, 1971;Ettensohn et al., 2019).Most studies using other methods tend to present orogeny as one, long episode of compression, even though orogenies are widely accepted to occur as multiple pulses that affect different areas of the orogen at different times (e.g., Camacho et al., 2005;DiPietro, 2018).Hence, the tectophase model is useful because it can relate unconformitybound stratigraphic sequences to smaller-order tectonic pulses in the foreland basin and adjacent intracratonic basins reactivated by coeval far-field tectonics.It is important to note, however, that the tectophase model and cycle are used as tools to provide tectonostratigraphic and flexural interpretations that tie lithostratigraphic composition to simultaneous compressional responses in the adjacent orogen.Hence, this study does not aim to provide a traditional structural analysis, as the use of seismic (e.g., Alania et al., 2018) or numerical modeling (e.g., Garcia-Castellanos et al., 1997) would do, but rather to show how the use of tectophase concepts can provide helpful and, most importantly, easy observation of far-field effects in the stratigraphy generated by compressional tectonics.
In tectonostratigraphy, the above relationships may also be explained using tectophase cycles.Inasmuch as orogenies typically progress as a series of pulses (Johnson, 1971;Jamieson and Beaumont, 1988;DiPietro, 2018), tectonostratigraphic responses necessarily manifest as a series of unconformity-bound cycles related to each pulse of crustal loading and relaxation during a few millions to tens of millions of years (Ettensohn et al., 2019).

The tectophase cycle
Ideally, the tectophase cycle consists of a stratigraphic succession consisting of seven distinct and systematic tectonostratigraphic responses, produced as subsurface and surface thrust sheets load the lithosphere (Figure 2) (Ettensohn, 1985).Initially, to isostatically compensate for the load, the lithosphere downwarps into a flexural foreland basin with an uplifted, distal, peripheral bulge that migrates cratonward (Figure 2A).The migrating bulge typically generates a regional, sequence-bounding unconformity that defines the base of the tectophase cycle (Figures 2A, C [part 1]).As deformational loading progresses, rapid foreland-basin subsidence ensues, and the initial response is typically a thin, transgressive, shallow-water carbonate or clastic unit (Figure 2C,[part 2]).Because initial loads are mostly in the subsurface, siliciclastic influx is minor, and subsidence outpaces sedimentation.As a result, organic matter from the water column predominates in starved-basin conditions, generating dark, organic-rich muds (Figures 2A, C [part 3], D) (Ettensohn, 2008;Ettensohn et al., 2019).
As deformation continues, the load eventually becomes subaerial and drainage nets develop.One key assumption of the tectophase model is that once deformation has ceased and the load becomes static, drainage nets and erosion begin the transfer of sediment from the upland load into the adjacent foreland and outlying basins (Johnson, 1971;Ettensohn et al., 2019).However, because the load is now effectively stationary, the lithosphere relaxes in response to the static load and begins to subside while the bulge moves back towards the load (Figure 2E).This process marks the beginning of loading-type relaxation in the tectophase cycle (Figure 2B).Because of crustal relaxation and erosion (Figure 2C [part 4], E), the deepening foreland basin fills with "flysch-like" sediments that might include deeper-water deltaic, turbidites, contourites, and debris flows (Figure 2C [part 4]; 3E) (Martins et al., 2022).
Once the static surface load is eroded and the rate of sediment influx exceeds the rate of basin subsidence, the foreland basin eventually fills or overflows with sediment.At this point, a brief period of elevational equilibrium between the basin and eroded load is established, allowing the deposition of a thin blanket of shallow-water carbonates or shales across the area (Figure 2C, [part 5]).At this phase in the model, compressional tectonism ends and the stationary "equilibrium" stage represents a transition between maximum loading-type relaxation and the succeeding phase of unloading-type relaxation.This phase of equilibrium is short-lived, as parts of the former orogen and adjacent foreland basin begin to rebound in response to the lost load (Figure 2F).Rebound and a compensating "anti-peripheral bulge" result in brief episode of transgression followed by a cratonward progradation of "molasse-like" sediments, including marginalmarine, fluvial-deltaic and, alluvial sediments (Figure 2C,[parts 6 and 7]).Because this flexural stage began at near-equilibrium, sea-level conditions, a single, cratonward paleoslope becomes established (Figure 2F) (Ettensohn, 1994;Ettensohn et al., 2019;Martins et al., 2022).
A bounding unconformity at the top of the sequence (Figure 2C) typically represents a new pulse of tectonism, indicating inception of the next tectophase cycle.The above description represents an ideal depositional cycle, but parts of the succession may be poorly developed, or truncated by an overlying unconformity.
The tectophase model is most applicable to subduction-type orogenies, during which the deformational load must first mount the continental margin during early parts of the margin's convergence history (Ettensohn and Lierman, 2015;Martins et al., 2022).The thick load developed during this process piles up at the margin ramp, generating a relatively narrow, deep foreland basin in which one or more tectophase cycles (Figure 2) are deposited.However, by the time collision is imminent, the deformational load has typically surmounted the continental margin and advanced some distance across the foreland as a surficial load.Because the load is now more expansive and spread out across the foreland, lithospheric flexure generates a broader, shallower foreland basin, in which a typical tectophase cycle will not develop.Instead, the basal unconformity is overlain by a thick sequence of marginal-marine to terrestrial clastic sediments that overflow well beyond the shallow foreland basin, generating a siliciclastic blanket that can spread hundreds of kilometers across the foreland (Ettensohn, 1994;Ettensohn, 2004;Ettensohn et al., 2019).Such a thick clastic blanket at the end of one or more tectophase cycles typically reflects a shallow, overfilled foreland basin, generated during the final, late-stage collisional orogeny at a convergent margin (Ettensohn et al., 2019).

Materials and methods
This study examines Upper Triassic-Lower Jurassic sections in exploration wells from the south-central NBSS (78) and RBSS (seven) (Figure 1C).Lower Jurassic parts of the section are already well-understood (e.g., Olaussen et al., 2018) and were examined largely through the literature.This study will focus on the Upper Triassic parts of the section, which are less wellunderstood and represented by the Fruholmen Formation in the NBSS and equivalent rocks in the RBSS.
NBSS well data for the studied sections are publicly available online from the Norwegian Petroleum Directorate (NPD, 2023), whereas RBSS well data for equivalent deposits were obtained from selected literature (Chirva et al., 1990;Astafiev et al., 2008;Gavrilov et al., 2010;Norina et al., 2014;Burguto et al., 2016;Gilmullina et al., 2021).For the NBSS, only wells that pierced the entire Fruholmen Formation were examined, whereas in the RBSS, approximately equivalent chronostratigraphic intervals were adopted from the literature.Moreover, 14 NBSS well logs were obtained from the NPD (ten are publicly available; NPD, 2023) and used for correlation (Figure 1B).Thicknesses and lithologic changes across the NBSS were measured from gamma-ray (GR), density (DE), and neutron (NP) logs.Well logs were not available for the RBSS.
Minimum and maximum thicknesses of the Fruholmen Formation (~Norian-Rhaetian) were obtained from NBSS wells, whereas equivalent thicknesses from RBSS wells are presented as obtained per individual well.For assessing these thicknesses, gamma-ray (GR), density (DE) and neutron-porosity (NP) well logs were used.The thicknesses in each well are presented as interpreted by the NPD and Russian literature and were used for regional interpretation of thickness patterns.Additionally, two northern NBSS cores from shallow stratigraphic boreholes (7934/ 8-U-1, and 7533/2-U-2) were provided by the NPD (Figure 1B).These cores are equivalent to lower parts of the Fruholmen Formation and were used to illustrate key stratigraphic features.Chronostratigraphy is based on the literature (e.g., Paterson and Mangerud, 2019;Gilmullina et al., 2021).For the NBSS, a west-east section was constructed and correlated with the reference well log section (Dalland et al., 1988) for the Fruholmen Formation (Figure 1B [green symbol]).The tops and bottoms of the Fruholmen Formation are noted as currently adopted by the NPD (NPD, 2023).For the RBSS, however, well logs were not available.Along this section line, key stratigraphic units (see section 3.2) were picked and correlated.

Limitations
In the tectophase model, presence and distribution of unconformable surfaces indicate inception of compressional tectonics.However, these features might also reflect sea-level variations or some combination of eustasy and tectonics, and determining which mechanism predominated is difficult (Embry, 1997).Ideally, tectophase cycles are better analyzed in present or former foreland basins, but that is not possible due to lack of access to RBSS data.However, because every orogenic system comes with onlapping intracratonic sequences that mirror those in the foreland basin (Ettensohn, 1994) and far-field responses (e.g., Ziegler, 1987), evidence from intracratonic areas may reflect developments in the foreland basin (Ettensohn and Lierman, 2015;Ettensohn et al., 2019).
It is also important to remember that seismic data were not available for this study, and that such data are not ideal for determining the lithostratigraphic composition necessary for tectophase analyses.Moreover, the chronostratigraphic analyses presented in this study are also largely based on major lithotypes and on chronologic interpretations from previous literature studies of the area.Because these earlier literature sources pose inherent chronostratigraphic limitations, this study should be scrutinized relative to future work when higher-resolution BSS chronologic data become available.Hence, this study is merely a step toward progress in future BSS foreland studies and an attempt to test the tectophase model in a challenging geologic province where access to datasets for academic research can be of great difficulty (e.g., Russian BSS).
3 Regional setting

Structural development
A simplified framework for this complex structural zone is presented in Figure 1B, which is largely a product of the Timanian (Late Proterozoic-Early Cambrian) and Caledonian orogenies (Early Paleozoic) (Drachev, 2016).These basement structures have been repeatedly reactivated by compressional tectonics and far-field stresses triggered by at least the Timanian, Caledonian, and Uralian-Pai-Khoi-Novaya Zemlya (Late Paleozoic-Middle Mesozoic) orogenies (Nikishin et al., 1996;Petrov et al., 2008;Drachev, 2016;Smelror and Petrov, 2018;Klitzke et al., 2019).Addressing the Timanian, Caledonian, and pre-Novaya Zemlya orogenies is beyond the scope of the study, but detailed descriptions of these tectonic events can be found in McKerrow et al. (2000), Roberts and Siedlecka (2002), and Puchkov (2009).During the Novaya Zemlya phase of the Uralian orogeny, a period of intense reactivation of basement structures was triggered across the entire BSS (Anell et al., 2013;Müller et al., 2019).

Results
During orogenesis, structures may reactivate with different intensity, among other factors (e.g., rheology), depending on the orientation of each structure to orogenic stresses.Availability of space for sediment accumulation will be controlled by these far-field processes and reflected in sedimentary thicknesses and facies.Moreover, the kinematic nature of each structural response to reactivation will invariably influence accumulation at local scales.In the tectophase model (Figure 2), occurrence and distribution of basal black shales and underlying unconformities clearly reflect the far-field effects of orogeny (Ettensohn and Lierman, 2015).Even though black shales and unconformities represent important stratigraphic evidence for structural reactivation, the distribution of overlying clastic wedges can also be important (Ettensohn, 2004).To evaluate far-field effects across the BSS, this study examines the following criteria: 1) the spatial variations in Norian-Rhaetian sedimentary thickness; 2) possible far-field responses from local structures; 3) unconformities; and 4) evidence of a pre-Rhaetian (mid-Late Triassic) orogenic pulse.

Thickness variations
The lithostratigraphic characteristics (e.g., composition) and thicknesses of BSS Norian-Rhaetian sediments were compiled from the 85 wells shown in Figure 1C.In the RBSS, only seven wells were available (Figure 1C; Table 1), and the thicknesses are typically presented in terms of grouped time intervals that were interpreted to represent Norian-Rhaetian thicknesses.However, a few of these reflect incomplete sections (Table 1), which may be the consequence of post-depositional erosion (Figure 3).In these RBSS wells, the recovered succession is thickest (~730 m) in the Ludlovskaya Saddle and thinnest in the west Kola Saddle (~57 m) (Figure 1B; Table 1).

Well-log succession analysis
Use of available NBSS well logs resulted in the construction of a largely west-east section line.For the RBSS, well logs were not available.Along this section line, the tops and bottoms of the Akkar (purple), Reke (yellow) and Krabbe (green) members of the Fruholmen Formation were picked and correlated (Figures 3, 6).The Akkar Member represents a predominantly deep-marine succession with abundant organic-rich muds on top of glauconitic carbonate beds and underlying fluvio-deltaic sediments (Snadd Formation; Figure 3).The Reke Member includes interbedded, shallow-marine to fluvial deposits, whereas the Krabbe Member represents an intercalation of marine and fluvio-deltaic sediments (Figure 3).In the wireline logs (Figure 6) the Akkar Member is characterized by high gamma-ray responses and simultaneous separation between density and neutron curves, whereas the Reke Member is represented by lower gamma-ray responses and minor separation of the density and neutron curves.The Krabbe Member exhibits oscillating, low-to-high, gamma-ray responses and separation of the density and neutron curves Table 3.
In the section, the Fruholmen Formation is thicker in the Hammerfest (well 7120/12-1; 198 m) and Nordkapp (well Thickness values for the Fruholmen Formation in 78 NBSS wells (Figure 1B) relative to structural elements (Figure 1A).BP=Bjarmeland Platform; BFC= Bjørnøyrenna Fault Complex; EL/HA=East Loppa/Hoop area; FB= Fingerdjupet Basin; FP=Finnmark Platform; HB=Hammerfest Basin; LH=Loppa High; MFC= Måsøy Fault Complex; and NB=Nordkapp Basin.See Figure 1B for location of structural elements and Figure 1C for approximate locations of the wells.A detailed description of the structural elements can be found in Gabrielsen et al. (1990).
7227/10-1; 234 m) basins (Figure 6), but thinner on the Loppa High (well 7222/11-1; 47 m).All three members of the Fruholmen Formation are distinguishable in the well logs, except in well 7222/ 11-1 (Loppa High; Figure 1), where the Krabbe Member (Figure 6; green) is absent.Clearly, all members show variations in thickness from well to well (Figures 1, 6), but more important are the largerscale changes in thickness patterns as the section progresses from west to east across various structures.In the four western wells, for example, the entire section is thicker and probably reflects deposition in a basinal setting.The section then dramatically thins in well 7222/11-1 as it moves across the Loppa High, but again thickens moderately on the intervening Bjarmeland Platform.This thickness trend is maintained eastwardly until the Nordkapp Basin (well 7227/10-1; 234 m), where the section again thickens abruptly.South and east of this well, including the southern edges of the basin and adjacent platform areas (Finnmark Platform), thicknesses again become moderate (e.g., Figure 6; ~130 m).It is important to note that the three different members commonly vary in thicknesses independently of each other (Krabbe Mbr., Figure 6).Of the three members, the Akkar Member is probably the most important because variation in accommodation space for these shales represents regional processes associated with the beginning of deformational loading as reflected in the tectophase succession (Figure 2).The thicknesses of the Akkar Member and the sum of the thicknesses of the Reke and Krabbe members are illustrated in a west-to-east graphic format (Figure 7).In Figure 7, thicknesses of the Akkar Member are highly variable, ranging from ~10 m (Well 7132/ 2-2, eastern Finnmark Platform; Figure 1) to 60 m (Well 7228/2-1 S, Nordkapp Basin; Figure 1), but having an average thickness of approximately 40 m (blue line; Figure 7).The graphic presentation in Figure 7 is important because it better illustrates that thickness Maximum and minimum thicknesses for the Fruholmen Formation on the NBSS (blue) and maximum thicknesses for approximately equivalent deposits on the RBSS (black).The positions of the columns represent the approximate well locations (see Figure 1C).

TABLE 2 Maximum and minimum
Fruholmen Formation thicknesses across selected NBSS structural elements (Figure 1).

Well
Location Thickness (m) Well Location Thickness (m) Frontiers in Earth Science frontiersin.org10 variations in the basal Akkar Member are apparent from the beginning of sequence deposition.
erosional truncation of low-angle cross stratification and was interpreted by Lord et al. (2019) to represent subaerial exposure and fluvial erosion.Below this surface, deposits are sand-rich and show hummocky-like to low-angle cross stratification (Figure 8A).Along the truncation surface, brown-colored weathering and sediments containing shale chips and siderite nodules represent a basal lag (Lord et al., 2019).Overlying grey sandstones showing wavy-like bedding that shifts to parallel low-angle cross stratification may represent a progressive decrease in energy.
Presence of a second, older unconformable surface (Early Norian; Figure 3) (e.g., Riis et al., 2008) is less clear.In the shallow core 7533/2-U-2 (Figure 8B), this Early Norian surface separates cross-bedded grey sandstones and grey to dark-grey mudstones from a prominent carbonate-rich lag horizon.The contact between this carbonate lag and underlying siliciclastic sediments is abrupt and shows evidence of scouring.In the shallow core 7934/8-U-1 (Figure 8C), however, the erosional nature of this surface is less clear, though the facies shift is abrupt.In contrast, the contact between the carbonate lag overlying organic-rich shales (Akkar Member equivalent; Figure 3) does not show significant indication of erosion in the shallow core (Figure 8D), but the color and homogeneity of the shales suggest deposition during an apparent transgressive event.

Discussion
In this section, focus is placed on evidence for structural reactivation and foreland-basin development.Moreover, the target stratigraphic succession (Figure 3) and arguments from the literature are used to suggest the occurrence of two episodes of deformational loading in Novaya Zemlya during Late Triassic-Early Jurassic time.

Evidence for structural reactivation
Traditionally, the Fruholmen Formation and equivalent deposits are associated with the Early Norian Pan-Arctic transgression, which reflects a major sea-level rise along the proto-Atlantic seaway that transgressed the BSS from west to east (e.g., Johansen et al., 1993;Worsley, 2008;Olaussen et al., 2018).Assuming tectonic quiescence at the time, the simplest outcome was TABLE 3 Selected wireline logs used to compose the NBSS stratigraphic and correlation section (Figure 1).
for thicker deposits to occur in the western BSS, whereas the eastern BSS basins would have already been filled by the fluvial-deltaic sediments of the developing Uralide clastic wedge.
As observed in the well logs (Figure 6) and the thickness plot (Figure 7), however, Fruholmen sediments are at times thicker in eastern NBSS structures than in western areas (Figure 6).Similarly, erosion of top-Fruholmen deposits is also variable across the shelf (Figures 6, 7).It is feasible to interpret these patterns as a result of differential subsidence and uplift of BSS structures triggered by farfield structural reactivation, which suggests that structures were being simultaneously reactivated at different places across the BSS.During Late Triassic-Early Jurassic time, the Novaya Zemlya orogeny is the best candidate for triggering widespread structural reactivation across the BSS.Variations in sedimentary thicknesses of a formation deposited across different structural elements coeval with orogeny suggests episodes of structural reactivation by far-field tectonics (e.g., Ettensohn et al., 2019).This interpretation aligns well with other studies, such as those of Faleide et al. (2017), Müller et al. (2019), Müller et al. (2022), andGilmullina et al. (2021).

Evidence for foreland-basin development
In the BSS, the North and South Barents basins (Figure 1, SBB, NBB) have been interpreted to represent a Jurassic foreland basin, formed in response to coeval Novaya Zemlya compressional tectonism (e.g., Faleide et al., 2017;Olaussen et al., 2018).Moreover, others have interpreted the orogenic exhumation of Novaya Zemlya to have occurred as early as Norian time (e.g., Klausen et al., 2016;Zhang et al., 2018), suggesting Norian tectonism with uplift and deformation.Because foreland-basin development is Frontiers in Earth Science frontiersin.org13 a flexural response to the inception of orogenic uplift and crustal thickening (Quinlan and Beaumont, 1984), presence of a Jurassic foreland basin implies a Jurassic pulse of orogeny, which, across the BSS, apparently occurred as early as Norian time.
Existence of two Novaya Zemlya episodes of loading and foreland-basin development integrates previous interpretations, including the presence of tectonism around the Carnian-Norian boundary (Bergan and Knarud, 1993;Embry, 1997;Fleming et al., 2016) and its culmination by late Norian time (220-210 Ma) (Zhang et al., 2018), followed by latest Triassic (Rhaetian)-earliest Jurassic tectonism, reflected in bulge and foreland-basin development (Faleide et al., 2017;Müller et al., 2019).Hence, if Upper Triassic-lowest Jurassic BSS deposits indeed represent deposition during two episodes of structural reactivation triggered by the Novaya Zemlya orogeny, then these two orogenic pulses must reflect two distinct tectophases (phases of deformational loading in the Novaya Zemlya orogen).

Tectophase sequences
Understanding the BSS Upper Triassic-Lower Jurassic succession in terms of sedimentary thicknesses, unconformitybound sedimentary sequences, and structural reactivation leads to the presumption of multiple deformational pulses in the Novaya Zemlya orogen.Based on the work of Johnson (1971) and Ettensohn et al. (2019), multiple orogenic pulses and resultant tectophase cycles are common in many orogenic systems.Based on stratigraphic analysis, BSS sedimentary thicknesses and unconformities, two foreland-basin tectophase cycles, reflecting two Novaya Zemlya orogenic pulses during Late Triassic-Early Jurassic time (Figure 9) are suggested in the present work.In particular, the occurrence of unconformities associated with collisional systems is important because these features can reflect orogenic control beyond the foreland basin into adjacent cratonic areas (Ettensohn, 1994).
In this paper, we suggest the presence of an Early Norian unconformity (Lord et al., 2019;Figure 8B), which represents a subtle inception of the first tectophase, whereas the Rhaetian unconformity (Drachev, 2016; Figure 8A) represents the second unconformity (Figure 9).This interpretation does not preclude the possibility of pre-Late Triassic Novaya Zemlya orogenic episodes (Filatova and Khain, 2010;Blakey, 2021), which are beyond the scope of this study.In following sections, we define the two orogenic episodes of deformational loading triggered by tectonics in the Novaya Zemlya orogen in terms of tectophases.Integrative BSS map schematically illustrating the depozones associated with the two Novaya Zemlya tectophases interpreted in this paper.Tectophase 1 (at least Early Norian) represents a moderate thrusting episode, whereas Tectophase 2 (latest Rhaetian) represents a major collisional episode of thrusting.The resultant time-equivalent deposits corresponding to each tectophase is illustrated in Figure 3.
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Tectophase 1 (Norian-Rhaetian)
From the Novaya Zemlya fold belt westward, a thin Norian-Rhaetian succession (~178-m thick, yellow) on the Admiralty High (Figures 1, 3) becomes a much thicker (up to ~500-730-m thick; gray) succession in the central RBSS areas.This region with thick deposits is surrounded by areas containing much thinner accumulations (~57-198 m, orange).From the orange area westward, formation thicknesses increase to ~118-229 m in the central NBSS (Figure 10, blue).Toward the western NBSS, in contrast, a very sharp increase in thicknesses up to ~580 m is present (Figure 10; dark green).Variation in these BSS thicknesses aligns well with the work of DeCelles and Giles (1996) (Figure 10), which suggests that use of thicknesses variations across a tectonically influenced shelf is one means to illustrate geometric patterns associated with the depositional zones of a classic foreland-basin system (DeCelles and Giles, 1996) (Figure 10).
Based on the above assumptions, various stages of an initial tectophase cycle are observed in the Norian-Rhaetian succession.Stage 1 is characterized by unconformity development (Early Norian surface; Figures 2, 8B), which represents the mixed responses to subtle Novaya Zemlya compressional tectonics and eustacy.Stage 2 represents initial subsidence represented by a thin, overlying carbonate-rich unit (Slottet Bed; Figures 2, 8B), whereas Stage 3 represents rapid, regional, subsidence that facilitated widespread deposition of organic-rich shales (Akkar Member; Figures 2, 6, 8D).Stages 4 through 7 represent shallow-marine to fluvio-deltaic deposition (Reke and Krabbe members; Figures 2, 6), related to loading-and unloading-type sediments derived from the Novaya Zemlya orogen.This interpretation fits well with that of Zhang et al. (2018), who interpreted a period of Norian (220-210 Ma) orogenic exhumation.
Even though the cores and logs used to describe this tectophase cycle mostly occur beyond the limits of the RBSS foredeep, tectophase sequences may onlap intracratonic areas, thus providing evidence of foredeep events (Ettensohn, 1994;Ettensohn et al., 2002;Ettensohn et al., 2019).Such an onlap has been indicated by Klausen et al. (2016), who suggested that Novaya Zemlya sediments prograded well-beyond the Norian-Rhaetian RBSS foredeep as far as the western BSS area.Across the NBSS, deposition of the above succession is the result of both widespread structural reactivation by Novaya Zemlya far-field tectonics and extensive transgression.Assessing the causes for early Norian transgression is beyond the scope of this study, but Norian tectonic influence on eustacy has been noted in adjacent Arctic basins (Embry et al., 2018).

Tectophase 2 (Rhaetian-Hettangian)
Across the BSS, the Rhaetian unconformity (Figures 3, 8A) has been confidently tied to a final episode of Novaya Zemlya tectonism and coeval foreland-basin development (e.g., Drachev, 2016;Faleide et al., 2017;Müller et al., 2019).This latest Rhaetian-Hettangian Norian-Rhaetian foreland-basin development and depozones across the southern BSS and adjacent areas.Maximum and minimum thicknesses (numbered columns) for the Fruholmen Formation on the NBSS (blue) and maximum thicknesses for approximately equivalent deposits on the RBSS (black).The positions of the columns represent the approximate well locations.The orange arrow represents thrusting associated with the Novaya Zemlya orogeny.Foreland depozone nomenclature is that of DeCelles and Giles (1996).
stratigraphic feature represents the inception of Tectophase 2 (Figure 9).This event was significant because it represents the final collision of Siberia with Baltica and generated a hiatus of up to 40 million years in some areas and widespread truncation of previous deposits (Figure 3) (e.g., Paterson and Mangerud, 2019), which is easily observed in seismic data (Müller et al., 2019).This unconformity is sequence-bounding and represents the inception of the Lower Jurassic foreland-basin sequence (Faleide et al., 2017;Martins et al., 2022).
This tectophase succession, however, does not contain the basal organic-rich shales and overlying flysch-like sequence that are typical of a tectophase cycle (Figure 2).Rather, this major unconformity is succeeded by a thick accumulation of shallowmarine to terrestrial clastic sediments that blankets nearly the entire BSS under names like the Tubåen and Svenskøya formations (Figure 3) Like the final Alleghanian collisional clastic sequence in the Appalachian Basin (Ettensohn et al., 2019), this widespread blanket of clastic sediments is interpreted to represent a final collisional phase of tectonism (in this case involving Siberia and Baltica) at Novaya Zemlya along with the generation of a broad, shallow foreland basin that allowed sediments to spill out across the BSS.Even though the same flexural stages still operated in development of the foreland basin, the resultant succession and stratigraphic responses differed substantially because of the collisional nature of the orogeny, resulting in a thick sequence of mostly molasse-like sediments, overfilling a shallow foreland basin (e.g., Ettensohn et al., 2019;Martins et al., 2022).Interpretation of a shallow, Jurassic RBSS foreland basin aligns well with interpretations from the studies of Suslova (2014), Gilmullina et al. (2021), andMartins et al. (2023), who noted widespread thicknesses of clastic sediments, up to 1.2 km in thickness, in a foreland basin located in the RBSS area.

Conclusion
Thickness variations, presence of a typical tectophase cycle, and an Early Norian (mid-Late Triassic) unconformable boundary suggest an initial stage of Novaya Zemlya orogeny and deformational loading, which supports an earlier inception of Novaya Zemlya orogeny and deformation, which we interpret here as Tectophase 1.In latest Rhaetian (latest Triassic) time, the Tectophase 1 succession was truncated by another major unconformity with a thick overlying succession of coarse, shallow-marine to terrestrial, clastic sediments, triggered by deformation from a final collisional event, representing the collision of Siberia with Baltica.This event continued into Early Jurassic time and is identified herein as Tectophase 2. Overall, this tectonostratigraphic work aligns with other studies suggesting Late Triassic to Early Jurassic Novaya Zemlya compressional stresses and structural reactivation by far-field tectonics.The conceptual tectophase model used in this study contributes to the understanding of unconformity development, stratigraphic succession, and far-field reactivation of BSS structures in both foreland and intracratonic areas.

FIGURE 2 (
FIGURE 2 (A) and (B) Schematic tectonostratigraphic model illustrating deposition and structural reactivation during phases of active loading (Figure 2A) and relaxation (Figure 2B).(C) Schematic lithologic succession representing an ideal tectophase cycle at outcrop scale with the associated eustatic curve.Complete sequences are typically unconformity-bound and include organic-rich rocks, flysch-like sediments, and molasse-like sediments.Units under the curve on the right are representative of the Upper Triassic-Lower Jurassic NBSS succession that will be discussed later in the paper.(D), (E), and (F) Schematic flexural model illustrating phases of deformational loading, loading-type relaxation, and unloading-type relaxation (modified from Ettensohn et al., 2019).