Syn- to post-rift alluvial basin fill: seismic stratigraphic analysis of Permian-Triassic deposition in the Horda Platform, Norway

Discrepancies in models of continental rift-basin dynamics and stratigraphic response calls on further investigation on the subject. Geometricand lithological trends between stages of faulting is studied in the PermianTriassic continental rift succession in the Horda Platform. The Horda Platform occupies the northeastern margin of the North Sea aulacogen where Late Permian-Early Triassic faulting shaped the Caledonian pre-rift landscape into a series of N-S trending half-graben basins, filled by Permian-Triassic strata. A tectonostratigraphic model developed from seismicand well-data details the Permian-Triassic basin fill and structural basin development. Regional unconformities mark the top and base of the succession, while internally, six depositional sequences are delineated by erosionaland transgressive surfaces. Thickness maps reveal three syn-rift stages, where strain migrated and concentrated in different parts of the developing rift basin, from disconnected faults and scattered depocentres with varying accommodation space hosting deposits of different thicknesses, to fully linked faults bounding half-graben basins with expanded and connected depocentres. The lithology through the syn-rift stages reflect how sedimentation gradually outpace accommodation space creation. The overlying thick post-rift succession shows no evidence of rifting besides a minor fault-displacement along the Øygarden Fault Zone in the Late Triassic, reflected by slight wedging. Meanwhile distinct interchanging sandand mud-dominated intervals reflect strong climatic fluctuations during the post-rift. The spatio-temporal heterogeneity in fluvial facies makes stratigraphic correlation challenging and is further complicated by discontinuous surfaces caused by basin floor tilting. With this study, classical sequence stratigraphic models are evaluated and revised against the observations from the Horda Platform. The stratigraphic model presented takes into account the dynamics of strain along faults through time and space and the resulting diachronic boundaries between the rift stages. Key-words: Continental rift-basin dynamics, Permian-Triassic, Horda Platform, tectonostratigraphy, fault growth, climate

crust is covered by a sedimentary succession ranging in age from Devonian to Cenozoic (Zanella et al., 2003).
Carboniferous to Early Permian extension (Heeremans and Faleide, 1991) was followed by a significant rifting event in the Late Permian to Early Triassic period where E-W/SE-NW extension resulted in the creation of graben-and half-graben systems (Faerseth, 1996;Odinsen et al., 2000a;Coward et al., 2003;Bell et al., 2014;Whipp et al., 2014;Duffy et al., 2015;Fazlikhani et al., 2017;Phillips et al., 2019). Early Triassic rifting waned throughout the Mid-Triassic to Late Jurassic leading to a broader and larger continuous marine basin. Linkage of the Arctic and Tethys oceans in the early Rhaetian led to transgression within the continental Triassic basins (Ziegler, 1992).
Meanwhile, climate changed gradually through Middle Triassic to Early Jurassic from arid to humid, contemporaneous to the northward drift of southwestern Baltica, including Scandinavia (Røe and Steel, 1985;Steel, 1993;Nystuen et al., 2014) A second Late Jurassic to Early Cretaceous rift phase formed the "North Sea triple-junction", after which subsidence and mild tectonic inversion dominated from the Late Cretaceous and onwards (Faerseth, 1996;Odinsen et al., 2000a;Gabrielsen et al., 2001;Coward et al., 2003).

| Key structures
The Triassic basin is restricted largely to a 170-180 km wide N-S trending depression, flanked to the east by the Øygarden Fault Zone and to the west by the Hutton alignment (Odinsen et al., 2000 a, b). The Triassic rift axis was located below the Horda Platform (Roberts et al., 1995) before migrating westward, resulting in the development of the Viking Graben during the second rift phase (Faerseth et al., 1995;Faerseth, 1996;Ter Voorde et al., 2000).
The Horda Platform forms a ~300x100 km N-S elongated structural high bounded to the west by the northern Viking Graben and to the east by the Øygarden Fault Zone .

| Stratigraphic framework
On the Norwegian Continental Shelf, basement is defined as pre-Devonian bedrock (Coward, 1995;Fazlikhani et al., 2017). The pre-Permian-Triassic rocks in the Horda Platform are generally considered crystalline basement (Norsk Hydro Produksjon, 1984;Phillips Petroleum Company, 1996;Riber et al., 2015). Well 32/4-1 T2 allegedly penetrates Devonian strata, rather than crystalline rocks, but this is uconfirmed by dating techniques. The carbonate and evaporite-rich Zechstein Group, which underlies the Triassic in the central and southern northern North Sea (Lervik, 2006), is absent in the Horda Platform.
The stratigraphic framework for the Triassic succession in the northern North Sea has long been a subject of debate, largely due to poor biostratigraphic control (e.g. Vollset and Doré, 1984;Nystuen et al., 1989;Steel and Ryseth, 1990;Underhill and Partington, 1993;Nystuen and Fält, 1995;Odinsen et al., 2000a;Goldsmith et al., 2003;Lervik, 2006;Nystuen et al., 2014). A review by Lervik (2006) collated available information to systemize and unify the stratigraphic nomenclature schemes, which are applied here. Jarsve et al., (2014) used seismic facies and regionally continuous seismic reflections to subdivide and map the central North Sea Triassic deposits. The same procedure is applied here to the Triassic Hegre Group in the Horda Platform.
The Lomvi Formation (Lower to Middle Triassic): blocky, kaolinitic sandstones, extending laterally for hundreds of metres with thin, interbedded marls and mudstones (Lervik, 2006). A fluvial origin (Vollset and Doré, 1984) as well as aeolian reworking is evidenced by the presence of very well-rounded sand grains and a high maturity (Nystuen et al., 1989).
The Alke Formation (Middle to Upper Triassic): red/brown/grey/green mudstones and subordinate sheet sandstones (Lervik, 2006), laid down in lacustrine and terminal basins with proximal alluvial fans .
Overlaying the Hegre Group is the Rhaetian to Sinemurian Statfjord Group (Lervik, 2006). This is a sandstone-rich coarsening-upwards succession, dominated in the lower part by sinuous to straight stream deposits . The succession is thin (< 50 m) in the eastern Horda Platform and thickens west towards the Viking Graben ( Figure 3). The overlying Jurassic deltaic and marine Dunlin and Brent groups are more or less uniformly thick across the Horda Platform but thicken dramatically in the Viking Graben (Husmo et al., 2003;Fraser et al., 2003).
The Cretaceous succession thickens towards the Øygarden Fault Zone in the Horda Platform, being truncated by overlying Quaternary strata, while west of the Horda Platform, the succession thickens significantly towards the Viking Graben where it is buried by Cenozoic deposits in a large synclise (Faleide et al., 2002;Copestake et al., 2003;Surlyk et al., 2003).

| Seismic analysis
Seismic sequences, representing certain depositional and/or structural stages in basin development, were mapped using Petrel E&P Software Platform. The sequences were defined based on internal reflection signatures (seismic facies; Figure 4) related to geometry, continuity, and frequency, reflecting changes in depositional style and/or lithology (Roksandic, 1978;Strecker et al., 1999). Surfaces defining the base of the sequences were generated from mapped horizons and used to constrain sequence thicknesses. Delineating boundaries of the seismic sequences are commonly manifested as high-amplitude, continuous seismic reflections, coinciding with lithological variations in well-logs. These result from erosion, non-deposition, or regional changes in the physical sedimentary environment, and represent possible time-lines between intervals of specific depositional patterns and/or of differential geometry (Strecker et al., 1999).
The GN1101 (Gassnova AS) and a subset of the NVG (CGG Services AS) Horda post-stack time migrated 3D seismic surveys, supplemented with regional 2D seismic surveys (NSR) provide the primary datasets applied in this study (Figure 1c).

| Well data
Gamma-ray (GR) logs from wells 32/4-1 T2, 31/6-1 and 31/2-4 R has been applied to inform seismic to well correlations and recognise stratigraphic boundaries ( Figure 5). Coupled with sedimentological descriptions (e.g., lithology, grain size and sorting) from well reports, GR-logs were used to infer depositional environments (Figure 4). Since wells only penetrate footwall crests, facies changes in deeper basin parts were inferred from seismic. Depositional environments proposed herein are based on comparative analysis to observations in lithologically equivalent rock formations in the Tampen Spur area (i.e., Nystuen and Fält, 1995;Nystuen et al., 2014). Stratigraphic divisions were assigned with respect to the scheme presented by Lervik (2006) for the northern North Sea province. The diachroneity, however, means that although descriptively useful, this lithostratigraphic sub-division is of little value for correlation purposes. Therefore, rather than trying to correlate sandstones (which are laterally discontinuous in fluvial systems) we connect overall fining-and coarsening-upwards intervals, and if possible unconformities representing flooding or sub-aerial erosional surfaces.
Even though forty wells penetrate Triassic strata on and adjacent to the Horda Platform, very few have been drilled more than a few metres into Triassic rocks, and only wells 32/4-1 T2 and 31/6-1 are drilled into Pre-Triassic strata. Well 31/2-4 R logs 2700 m of Triassic strata without encountering pre-Triassic rocks and offers the only available Triassic core-material in the Horda Platform from the lowermost 8 meter of the well. Two additional wells (31/4-3 and 31/5-3 S)

| Upper and lower boundaries
The Permian-Triassic succession is bounded by and contain internal surfaces, that are mappable across the whole study area. The surfaces are presumed to represent depositional shifts during basin fill. The base Permian-Triassic surface (Base PT) is a major sub-aerial unconformity between clastic deposits and crystalline rocks (i.e., pre-Devonian metamorphic and/or Devonian metasedimentary rocks). Base PT is a strong seismic reflection, displaying a topographic relief in footwalls to the major fault zones ( Figure 6). The relief is most pronounced in the footwall to the Vette Fault Zone where it displays drainage catchments and incisions ( Figure 6b). Base PT is readily mappable in major parts of the study area, except to the northwest where it becomes less pronounced, probably due to large depth combined with overburden stratigraphy, structures, and fluids influencing the imaging quality. Beneath the surface are a series of high-amplitude, shallowdipping, sigmoidal reflections, possibly representing Devonian shear zones (Fazlikhani et al., 2017;Mulrooney et al., 2020).
The upper sequence boundary (Base Statfjord Gp) marks a turnaround from fining-upwards into coarsening-upwards in well-logs   (Figure 5), but is generally poorly resolved in seismic, and thus constrained by careful seismic-well-tie. The surface is occasionally truncated to the northeast in the Smeaheia Fault Block. This may be related to a significant unconformity recognised in the greater North Sea area, resulting from Middle Jurassic to Early Cretaceous erosion, whereas in the northern North Sea the Triassic is conformably overlain by Lower Jurassic marine mudstone and shale (Goldsmith et al., 2003).

| Permian-Triassic seismic sequences
Six seismic sequences (S1-S6) are defined within the Horda Platform area (Figure 3; 5). The sequences are delineated by surfaces resulting from non-deposition, flooding, erosion, or particularly strong lithological contrasts (Strecker et al., 1999). Variations in seismic expressions represent transitions in depositional regime brought about by tectonic and climatic changes, shifts in provenance, or a combination. Overall, the succession thickens northwest and internally towards the hanging-walls of the major fault zones. Well-bores record an average thickness of 1664 m of this succession in the northern North Sea (Goldsmith et al., 2003), but since wells are generally located in footwall crests, they essentially record minimum thicknesses. Seismic data implies thicknesses in excess of 3000 m in the deepest hanging-walls (estimated from well depths). As the study area is outside the Zechstein Salt limit (Figure 1a), the thickness variations are not halokinetic-induced, but instead owing to half-graben accommodation distribution (Goldsmith et al., 2003).

| Sequence 1
Description The base of Sequence 1 (S1) is bounded by the high-frequency Base PT. The sequence is predominantly located in the deep hanging-walls with main depocentres west in the Svartalv and Tusse fault blocks (Figure 3; 7). Except for a few scattered patches, the sequence is absent east in the Smeaheia Fault Block and in the footwall to the Tusse Fault Zone. The patches are mapped as wedges that thickens toward the major faults, but are less prominently wedge-shaped compared to sequences 2 and 3 (described below). Internal reflections are mostly low-frequent, sub-parallel and bifurcating (Facies 5; Figure 4), onlapping onto footwalls and the reliefs in the Base PT surface ( Figure 8). A serrated GR-signature with low values characterise the well-log through S1 ( Figure   5).

Interpretation
Sequence 1 strata developed in a set of isolated basins during the earliest rift development and represents the onset of rifting and initial sedimentation following the exhumation and erosion of pre-Permian-Triassic strata ( Figure 9a) and is labelled an early syn-rift here. The Base PT surface represents a sub-aerial unconformity, and its rugged surface is interpreted as intra-basinal ranges. These affected sediment routing and acted as local sources additional to the uplifted rift shoulders on active fault segments. Sediment was deposited in hanging-wall blocks, with main depocentres west and central-south. The patchy distribution of strata appears to be linked to topographic lows in the Base PT surface. The serrated log-trend with low values suggests dominantly sandstones deposited as thin sand-beds, likely of sheet-flood origin, which is characteristic to arid stream systems where discharge rapidly changes  McKie, 2014).

| Sequence 2
Description Sequence 2 (S2) is bound at its base by the thick, high-frequent Base S2, except in the Tusse and

| Sequence 3
Description Wedge-shapes persist in Sequence 3 (S3) with thickening toward hanging-walls, now also towards the Øygarden fault Zone (Figure 7e.1). The high-frequent Base S3 marks a transition to generally low GR-values and blocky log-motifs, upwards transitioning to more serrated motifs with fining-and coarsening-upwards high-value GR-responses ( Figure 5). Hanging-walls are characterised by down-dipping, contorted, chaotic reflections (Facies 1). Occasionally in footwalls occurs a prograding set of sub-parallel, bifurcating reflections (Facies 3) that flatten out or downlap onto horizontal reflections (Facies 5) in basin centres. This downlap surface coincides with an increase in coarse clastics.
Interpretation S3 represents the third syn-rift stage where fault segments are partially or fully linked to form three (mostly) disconnected half-graben basins as opposed to the isolated basins in S1 and S2 ( Figure 9c). Asymmetry is recorded in equivalent units in the Stord Basin and Tampen Spur, suggesting a regional extensional event (Müller, 2003;Nystuen et al., 2014). The facies variation owes to the asymmetric subsidence creating differential sedimentary conditions across basins (cf. Prosser, 1993). Dipping, aggrading hanging-wall reflections indicate tilted depositional bodies, such as alluvial fans (Moscariello, 2018). Their contorted, diffracted appearance is characteristic to mass flow deposits (Dolson et al., 1999) where high grain-size variability and dis-orderly bedding obscure reflections (Miall, 2010). Towards the fan-apron, mass-flow processes transition into the flow regime, reflected by the channelization in this zone (Moscariello, 2018) (Figure 8).

| Sequence 4
Description The base of Sequence 4 (Base S4) is a medium-strong amplitude reflection with frequent swales ( Figure 8) that marks a change in seismic facies from S3 to S4. The stratigraphic geometry is markedly less wedge-shaped than the older basin fill, appearing as tabular units across the Horda Platform with slight thickening towards the Øygarden Fault Zone. Internal reflections are dominantly sub-parallel with frequent swales and lenses that occasionally concentrate and stack to form larger bowl-shaped, incising features (Facies 2). Close to the Øygarden Fault Zone, reflections are persistently chaotic, contorted and dipping (Facies 1).
In well-logs, the transition from S3 to S4 is marked by a sharp shift to blocky, low-value GR-signals.
Upwards, GR-values increase along with the degree of serratedness ( Figure 5). Well-log reports indicate an increase in carbonaceous horizons upwards in the eastern sections simultaneous to a decrease in western sections.

Interpretation
Extensive sand deposition took place across the basin in S4 (Figure 9d). The coarse-grained lithology, lack of facies-and thickness variations across the basins, and tabular geometry, are all features indicative of faulting ending (Prosser, 1993;Holz et al., 2017). The erosive nature and regional extent of Base S4 suggests this represents a subaerial unconformity. The shift into tabular geometry coincides with the sharp introduction of clean sandstones; a change also recorded in the Tampen Spur (Steel and Ryseth, 1990;Nystuen and Fält, 1995;Nystuen et al., 2014), UK (Vollset and Dore, 1984;Nystuen et al., 1989;Frostick et al., 1992;Goldsmith et al., 2003) and northern Norwegian-Danish Basin (Lervik, 2006), suggesting a regional similarity in depositional characteristics during this time. The GR-signals suggest dominance by amalgamated, homogenous sandstones, becoming less amalgamated upwards ( Figure 5). The amalgamated sandstones indicate high sedimentation rates and low preservation potential for fin-grained deposits (Shanley and McCabe, 1994). These tightly stacked sandstones can be seen in multi-storey channel units (Mitten et al., 2020), where frequent avulsion is commonly generated by braided streams (Walker, 1976;Bourquin et al., 2009), or in aeolian dune deposition, where high sedimentation rates may enhance amalgamation (Kjemperud, 2008;Schomacker, 2008). Meanwhile, alluvial fans characterised the marginal deposition, as suggested by the contorted, dipping reflections here ( Figure 8). Carbonaceous horizons in alluvial successions, reflected by the low peaks, may be of pedogenic origin (Kraus, 1999), a process which requires continuous landscape stability, or aggradation where sedimentation is low, or even non-deposition (Müller et al. 2004).

| Sequence 5
Description The strong Base S5 reflection coincides with a pronounced shift in well-log trend to Sequence 5 (S5). S5 is of uniform thickness across the basin, except local wedges towards central fault  (Müller, 2003).

Interpretation
Parallel, horizontal reflections suggests steady depositional rates on a flat uniformly subsiding surface, such as a basin plain, without significant erosive features, such as channels (Dolson et al., 1999). The low thickness and overall tabular geometry indicates a period of comparative tectonic quiescence with local extension along the Øygarden Fault Zone (Figure 9e). Drainage would have been toward topographic depressions, created in the hanging-wall to zones of strain accumulation, which likely hosted rift lakes, but elsewise moved un-prohibited across the postrift landscape (Gawthorpe and Leeder. 2000). The serrated GR-motif reflects a heterogeneous lithology where high-values represent floodplain and lacustrine deposition (Strecker et al., 1999), while low-values represent sandstones deposited in small channels (bell-shapes) and as sheetfloods or crevasse splays (funnel-shapes)   (Figure 4). The increasing GRvalue suggests more fine-grained deposition, which can reflect either (1) a decrease in sedimentation rates, which may be induced by waning faulting, or (2) a climatic change leading to higher water tables and more extensive lacustrine deposition. The eastward lateral coarsening across the three fault blocks is due to increasing proximity, as alluvial fans dominated the margins, suggested by the dipping and chaotic reflections towards the Øygarden Fault Zone. The low-value peaks in the Smeaheia Fault Block, interpreted as carbonaceous horizons, may be due to pedogenesis.

| Sequence 6
Description At base of Sequence 6 (S6), the strong Base S6 reflection coincides with a sharp decrease in GR- The well-logs display an overall fining-upwards trend, arranged in interchanging intervals dominated by either serrated GR-motifs with bell-shapes or by blocky low-valued GR-signals ( Figure 5). Based on these alternations, S6 can be subdivided into four sub-sequences (S6.1-4).
Proximally, the low-valued intervals are thicker, more consistent, with a blocky responses, while getting more serrated with distality. Pronounced low-peaks in the serrated intervals, most concentrated in S6.1 in the eastern log (32/4-1 T2), stand out from the GR-motif ( Figure 5).

Interpretation
The sudden decrease in GR-signal value across the boundary from S5 to S6 reflect an influx of coarse sediment with an introduction of large quantities of sand, likely due to source denudation during waning rifting (Blair, 1987). The overall fining-upwards reflect gradually filling of the basin, simultaneous to a decrease in sedimentation rate. The differential GR-signals reflect alternating sandstone and mudstone-dominated intervals that overall fine upwards andwestward ( Figure 5). The high-value intervals represent mudstone-rich periods, while the blocky, low-value intervals represent amalgamated sandstones. The amalgamation rate decreases with distality, reflecting how the sandstones split up into several thinner sandstone collections in between mudstone-rich units. The sandstone bodies in the lower part (S6.1 and S6.2) are of braidstream origin, based on their stacked appearance and blocky GR-profile, while higher sinuosity channel-belts prevailed upwards and westward, indicated by the increasing bell-shapes and preservation of mudstones, characteristic to single-storey channels (Miall, 2010) (Figure 9f).
Increased sinuosity through the Upper Triassic was also recorded in the Tampen Spur by Nystuen et al., (2014). The high-frequent low-peaks represent carbonate horizons, which are indicative for low sedimentation rates and pedogenic calcrete production, possibly on interfluves (Müller et al. 2004).
During S6 deposition, the main depocentre shifted west, reflected by the thickness variations ( Figure 7) and divergent reflections (Figure 8), representing westward progradation. The slight wedging in S6.1 reflects the inherited topography from the syn-rift stage, a phenomenon typical to early post-rifts (Prosser, 1993). The truncated reflections in the top indicate erosion, possibly representing a sub-aerial exposure prior to deposition of the Statfjord Group.

| DISCUSSION
The breakdown of the strata that developed in concert with Triassic rifting in the Horda Platform area provides a step-by-step record of the rift phases in the region. Comparative evaluation of the rifting in the Horda Platform area against other models of rift basin evolution (Prosser, 1993;Nøttvedt et al., 1995;Gawthorpe and Leeder, 2000;Morley, 2002;Holz et al., 2017) is possible with the data quality offered here. Generally, the balance between generation of tectonically driven accommodation versus sedimentation rate is reflected in stratigraphic patterns within rift basins, which are commonly considered under-filled in continental settings. Thus, sedimentationrate is the rate-limiting step, outpaced by increasing accommodation, especially in proximity to basin-bounding faults. Sediment delivery is further affected by sediment routing, in turn controlled by catchment areas along with tectonic obstacles (de Almeida et al., 2009;Henstra et al 2016).

| Pre-rift configuration and implications on Permian-Triassic sedimentation
The range-like reliefs in the Base PT ( Figure 6) affected the distribution and routing of sediment through S1 into S2. The origin of the relief is difficult to unravel but it either (1) existed prior to Permian-Triassic rifting or (2) was created as a result of footwall uplift and denudation.
(1) If the landscape pre-dates the rifting, the relief formed as a continuation of the mainland Caledonian topography prior to activation of the Øygarden Fault Zone. The smooth and subdued relief in the hanging-wall was caused by erosion and deposition from stream avulsion during block tilting. The transitions in seismic facies below the Base PT are reminiscent of compositional heterogeneity and varying structural fabric in e.g. dikes and nappe-structures Lenhart et al., 2019), which are characteristic of the crystalline rocks in the onshore Caledonian massif (Boundy et al., 1992).
(2) If the landscape is contemporaneous to rifting, the relief was formed by footwall uplift, and associated development of catchment areas (Elliot et al., 2012), similar to those observed in the Tusse Fault Block ( Figure 5). Commonly, antecedent streams drain away from the crests at a higher slope with limited avulsion (Frostick et al., 1992), incising reliefs as observed here.
Incisions resulting from footwall uplift are expected to show drainage patterns towards the hanging-wall, away from the fault (Trudgill, 2002), which is not the case in the Smeaheia Fault Block where incisions appears to create drainage paths towards the fault (Figure 6b). This indicates that the Smeaheia Fault Block was not tilted until later in the syn-rift so that it formed a palaeo-slope dipping west during the initial movement of the Vette Fault Zone (Figure 9a.2).
The presence of Devonian strata within the denudated area would constrain whether the relief is related to onshore topography. In the Smeaheia Fault Block, Devonian red granitic conglomerates underlies the Base PT according to the 32/4-1 T2 well-report (Phillips Petroleum Company Norway, 1996). A thick succession of distinct seismic facies is indicated below the Base PT ( Figure   3) (Christiansson et al., 2000), which corresponds in thickness to Devonian basins in the Moray Firth and Shetland Platform (Marshall and Hewett, 2003). The strata, now sectioned by faults, exhibits wedging towards the eastern margin, indicating a syn-rift phase. This corresponds with Devonian post-orogenic extension (Faerseth, 1996;Fossen et al., 2017) documented in the surrounding regions (Neumann et al., 2004). Except for this well, pre Permian-Triassic rocks north of the Moray Firth and Horda Platform are, however, Caledonian crystalline bedrock (Marshall and Hewett, 2003) and without dating, the presence of Devonian strata in the area cannot be proven.
With the available constraints, the Base PT relief is linked to erosion during footwall uplift. The wide relief-zone in the Smeaheia Fault Block (Figure 6b) likely combine the two theories, where the first was enforced by the second as the Øygarden Fault Zone was offset; As indicated by thickness maps and the incision patterns in the Base PT, the Vette Fault Zone moved prior to the Øygarden Fault Zone, which accelerated at the late stage of rifting (S3) (Figure 7). At onset of movements of the Øygarden Fault Zone, the Smeaheia Fault Block was tilted and the relief was buried.

| Controlling factors on deposition
The varying thickness in sequences 1-6 ( Figure 7) reflects that accommodation was a strong controlling factor on deposition during most of the Permian-Triassic. The syn-rift succession ( Figure 7e) experienced three rift stages, reflected by strain migration as faults propagated and linked (Figure 7e.1-3). The interchanging mud/sandstone-dominance with decreasing faultcontrol in sequences 4-6 indicate that with waning rifting, accommodation diminished as a depositional control, and the effect was enhanced or even overprinted by climatic variations.

| Sequence 1: tectonics and pre-rift topography
During S1 deposition, the area was immature with several small, heterogeneous basins developed The sandy, homogeneous lithology (recorded in the western fault block basin) reflects a steady supply of coarse sediment at a rate higher than the rate of accommodation so that floodplain fines were unpreserved in between the sheet-like ephemeral stream-and flash flood-sands. Large quantities of immature debris would have been locally available in the high-relief landscape (Lambiase and Bosworth, 1995), further supporting a high sediment supply rate. A high supply of sediment is expected as S1 was deposited on a sub-aerially exposed, topographic surface, meaning that large quantities of immature debris would have been locally available (Lambiase and Bosworth, 1995). In addition, a response to the climate being arid, mechanical weathering would prevail in the source area, favouring the production of sand over clay .

| Sequence 2: tectonics
In the second stage, the epicentre of faulting shifted northward and accommodation zones widened concurrently with faults propagating and strain expanding east (Figure 7e.2). Subsidence outpaced sedimentation (Figure 10) as rifting increased, demonstrated by the fine-grained (lacustrine) lithology and highly serrated GR-response in well-logs. Contemporarily, the proximal area experienced continued non-deposition, suggesting persistently low subsidence and incipient faulting in this zone. Furthermore, remnants of pre-rift topography were probably not buried before termination of S2 deposition. Pronounced reliefs, related to antecedent drainage (Trudgill, 2002;Wise and Noble, 2003;Cowie et al., 2006;Elliot et al., 2012), remained in footwall crests.
Thus, the rift was structurally more developed compared to the stage of S1, but unevenly distributed accommodation persisted.

| Sequence 3: tectonics
By the end of the third stage, strain had migrated east, and linked faults formed major fault zones dividing the Horda Platform into half-graben basins (Figure 7e.1). Fault-junctions locate breached relay zones, commonly between overstepping fault segments (Peacock and Sanderson, 1994). In response, the strain was distributed along the faults, and deposition widened onto the footwalls.
The basin was in a mature stage with deposition from hanging-walls to footwalls in all fault blocks ( Figure 10). During the course of S3, the basin transitioned from rift climax (S3.1) into a waning rift (S3.2), reflected by the fining-into coarsening-upwards trend, separated by a flooding surface.
Stratigraphic boundaries in synrift successions are often discontinuous, as a flooding surface in the hanging-wall may form synchronously to subaerial exposure in the footwall due to concurrent uplift here (Gawthorpe et al., 1994;Withjack et al., 2002;Holz et al., 2017). The recorded lithological trend is related to the development on footwall crests where the influence of lacustrine facies is minimal (Holz et al., 2017). The trends and boundaries may therefore represent other inducing mechanisms than those recorded in hanging-walls by e.g. Prosser (1993) and Martins-Neto and Catuneanu (2010). A flooding surface in the footwall can form under different, possibly interplaying, scenarios: (1) subsidence balancing as faulting wanes; (2) low sedimentation rates; (3) decreasing precipitation in hinterlands and thus less erosion in the source area ; (4) increased precipitation in the basin leading to lake level rise and transgression onto the footwall (Holz et al., 2017). The flooding surface in the middle of the wedge-shaped sequence, present on the footwall crest even, reflects a pause in faulting, which hampered basin floor tilting and balanced subsidence.

| Sequence 4: climate
Sequence 4 represents a period with a low A/S (Figure 10), reflected by the amalgamated sandstone-rich succession. Erosion of basin shoulders may have expanded the catchment area, which increased the sediment supply (Nystuen et al., 1989;Nystuen and Fält, 1995). The removal of syn-rift relief allows for transverse drainage-systems, which enhance the dispersal of coarse sediment across the basin (Prosser, 1993).
The origin of the sandstones is a subject of debate; Steel and Ryseth (1990) documented a pebbly nature and small fining-upwards motifs in equivalent Tampen Spur sandstones, indicating a fluvial origin (Vollset and Doré, 1984). Nystuen et al. (2014) suggested aeolian influence, based on high sorting-degree and -maturity. Aeolian deposits are abundant in the Triassic across Europe (McKie and Williams, 2009) and arid rift basins are important sites for aeolian deposition (Prosser, 1993), as the linear topography creates tunnel-winds that abuts the steep rift slopes, leading to grain fall-out (Howell and Mountney, 1997).
Aeolian sandstone preservation requires sufficient accommodation relative to accumulated sediment (Kocurek and Havholm, 1993), which may be achieved by thermal subsidence, sediment loading, or -compaction (Blakey, 1988;Kocurek and Havholm, 1993;Mountney et al., 1999;Rodríguez-López et al., 2014), the latter two enhanced by increasing sedimentation (Nøttvedt et al., 1995;Kjemperud, 2008;Schomacker, 2008). Aeolian deposition can also form and preserve during general humid climatic conditions due to high water table levels (Kocurek and Havholm, 1993;Mountney et al., 1999;Mountney, 2012;Al-Masrahy and Mountney, 2015). Increasing fluctuating humidity is recorded through the middle and late Triassic in the North Sea area (Goldsmith et al. 2003;Preto et al., 2010;Nystuen et al. 2014). Rising and falling water-table levels, reflecting wet and dry periods, respectively, control dune and inter-dune sizes; by rising watertable, inter-dune environment expands, and aeolian sand dune area shrinks, whereas falling water-table has the opposite effect (Mountney and Thompson, 2002;Wakefield, 2019). This pattern has been documented in the Anisian Helsby Sandstone Formation in the Cheshire Basin, England (Mountney and Thompson, 2002). Wet periods in areas with overall aeolian processes lead to fluvial re-transport and re-deposition (Al-Masrahy and Mountney, 2015), as demonstrated in the Etjo Sandstone in Namibia (Mountney et al., 1999;Masrahy and Mountney, 2015). The upward increase in mudstone beds in S4 ( Figure 5) may reflect increasing frequency of stable fluvial channels and floodplain fines, in accordance with increasing humidity during late middle to early late Triassic. Intercalation of aeolian and fluvial deposits within S4 is here considered to be controlled by alternation between arid and wet periods; the arid periods controlled the dominance of sand production and aeolian transport and wet periods fluvial sand-recycling anddeposition.

| Sequence 5: tectonics and climate
The uniformly low thickness across the western and central Horda Platform (Figure 7c) suggests low accommodation during S5 deposition. Wedging, albeit localized, could results from minor renewed faulting along the eastern margin, but can also be attributed to other factors; For example, Müller (2003) explained similar geometric variations in Tampen Spur as post-depositional erosion, while Prosser (1993) mentioned wedges occurring in post-rift strata of sediment-starved basins. However, as other studies (e.g. Nøttvedt et al., 1995;Nipen, MSc, 2020) report on a minor middle-Late Triassic rift-stage in other parts of the northern North Sea, this was likely no restricted event.
The fining-upwards and frequent carbonaceous horizons link to changing climatic conditions. Besides increasing humidity through the mid-Late Triassic (Preto et al., 2010), a pronounced wet period in the early-Late Triassic, termed the "Carnian Pluvial Event" (CPE) is known (Mueller et al., 2016). The fine-grained trend in S5 is explained by this, as raising precipitation led to increased runoff and river stabilization by increasing vegetation on floodplains (Prosser, 1993, Corenblit et al., 2015. A knick-point near the hanging-wall of the Smeaheia Fault Block indicate possible fluvial incision (Figure 8). The edges of this may have formed terraces where non-deposition allowed pedogenic processes (Mack et al., 2010;Müller et al. 2004).

| Sequence 6: climate
The continuous reflections in S6 indicate a thermally subsiding basin with limited topography or fault segmentation (Figure 10). Thermal subsidence occurred over a wider area with rift axis in (what was later to become) the Viking Graben (Ter Voorde et al., 2000), and the basin took on a saucer-shape, reflected in the westward thickening profile (Figure 3; 7b). This may be a response to the transition from active stretching to thermal cooling and -rebounce, substantiating the common consensus of stratal rotation being redirected towards the basin axis (Nøttvedt et al., 1995). The progradational pattern observed most proximal in S6 may represent fault-scarp derived coarse clastic sediments (Nøttvedt et al., 1995).
Contrasting theories on the lithological changes in S6 include tectonically induced base level changes (e.g. Steel and Ryseth, 1990), and climatic forcing (Frostick et al., 1992;McKie, 2014;Nystuen et al., 2014). Due to lack of growth-features, climatic control is also supported by the present study. Climatic wet/dry fluctuations, like in Eastern Africa during the Plio-Pleistocene (de Boer et al., 2021), can cause significant variations in sedimentation rate and runoff (Strecker et al., 1999). During wet periods, vegetation cover stabilized rivers, increases sinuosity, and promotes fine-grained sediment preservation. In the Sahel-Sahara region, which is at similar latitude to the Triassic North Sea (Coward, 1995), climatic fluctuations are linked to orbital cycles (de Boer et al., 2021), which acted similarly in the Triassic, and may have influenced lake level, as well as sea-level (Vollmer et al., 2008;Chu et al., 2020). Müller et al. (2004) and Nystuen et al. (2014) found that the fluvial style in the Lunde Formation of Tampen Spur indicates increasing humid climate during late Triassic, giving rise to perennial streams and mud-rich floodplains, whereas clay minerals and paleosols in floodplain fines reflect dominating semi-arid conditions, interrupted by more humid intervals. The fluvial style was explained as controlled by high precipitation in upland regions, whereas formation of clay minerals, calcrete and pedogenic features in overbank fines was primarily controlled by arid to semiarid conditions in lowland basin areas. This may also have been the situation during deposition of S6 in the late Triassic in the Horda Platform.

| Rift dynamics
Two sub-aerial unconformities delimit the Permian-Triassic rift infill in the Horda Platform: the syn-rift unconformity and the post-rift unconformity (sensu Falvey, 1974;Bosence, 1998) (Figure   10). The syn-rift unconformity marks the onset of rifting and separates the pre-rift succession from the initial rift strata. Tilting of the depositional surface results in erosional truncation of the prerift strata, producing an (angular) unconformity, as observed in the Base PT surface (Figure 8).
The post-rift unconformity marks the end of rift-constrained basin-fill, here represented by the Base Statfjord Group. While first-order units are bounded by sub-aerial unconformities, lowerorder units are delineated by transgressive surfaces (e.g. Embry, 1995).
Rifting is a progressive and diachronous event, meaning that is may not occur at the same geological time across the rift zone area (Holz et al., 2017). The diachroneity of rift events is reflected in the Base 2 surface, which cannot be defined as an "event-line" as it cuts across riftevent-packages. A potential boundary between an initial-and developed rift is unrecognized and thus negligible when it comes to facies-development and -predictability. A potential rift development initiation surface (sensu Holz et al., 2017)

would be further down in stratigraphy than
Base 2 in the western fault blocks. In its full extension, the Base S2 thus cannot be termed a rift development initiation surface. Therefore, we identify Base 2 as a propagation unconformity, (sensu Bueno, 2004), which is defined as an unconformity created by intra-basinal extension, reflecting the evolutionary diachroneity of a propagating rift.
The drainage trend in the initial rift stage presents another discrepancy in literature. According to Prosser (1993), longitudinal river systems prevail, whereas Gawthorpe and Leeder (2000) and Hemesdaël et al. (2017) states antecedent drainage constitutes the dominating sediment routing.

| Progressive rift (Syn-rift stage 2): Sequence 2
The widening and deepening accommodation (Figure 7e.2) corresponds to the fault-interaction and linkage stage by Gawthorpe and Leeder (2000), the rift development phase by Holz et al. (2017), and the early rift climax by Prosser (1993). Common to these models is that subsidence outpace sedimentation as depocentres expand and deepen with fault-linkage. Parts of the basin remain in an apparent rift initiation stage with low accommodation, isolated depocentres and segmented faults. However, with the dominantly fine-grained lithology and major part of the basins being in a more mature state relative to S1, S2 reflects a more developed rift, and is therefore termed a "progressive rift" (Figure 10).

| Rift climax (Syn-rift stage 3): Sequence 3
The characteristic high variability in seismic facies from footwall to hanging-wall in S3 complies with the Rift climax by Prosser (1993), and the homogeneous depocentre-development and faultlinkage correspond with the Through-going fault stage by Gawthorpe and Leeder (2000) and Fault termination by Holz et al. (2017). S3 is defined rift climax as the stage, relative to the two underlying units, is highly mature in the sense of fault-propagation, depocentre development, and a close-to balanced A/S relationship. The A/S though vary through the succession, reflected by the fining-to coarsening-upwards lithological trend. Continental rift basins often exhibit a pulsating pattern of coarsening-and fining-upwards depositional units in syn-rift successions. To account for this, Nøttvedt et al. (1995) (revised by Holz et al., 2017) proposed two models for single-and multi-rift successions: one dedicated to gradually increasing subsidence, while the other illustrates punctuated subsidence with associated intra-rift unconformities, the last one explaining the trends in S3. Similar basin development has been documented by Morley et al. (2007) and Hemelsdaël et al (2017). The maximum rift surface (sensu Holz et al., 2017) in the middle of S3 may represent a transgressive lacustrine surface.

| Rift termination: Sequence 4
S4 can be sub-divided in two stages based on the geometric-and lithological trend ( Figure 10).
The first, coarsening-upwards, stage started in late S3 into S4, while the second stage covers the fining-upwards succession ( Figure 5). The first stage corresponds to the rift termination systems tract by Holz et al. (2017), with the slight wedging at the base in the eastern fault block being related to remnant topography from the syn-rift stage (Nøttvedt et al., 1995). The uniformly thick second stage matches the Late Post-rift systems tract by Prosser (1993), while the homogeneous, widespread facies' corresponds to the Fault death stage by Gawthorpe and Leeder (2000)

and the
Early post-rift sub-stage by Holz et al. (2017). The bounding surfaces of the sequence are supposedly diachronous, as expansion of the channel belt system from the hinterland would be time-transgressive from east to west.

| Post-rift with local rift initiation: Sequence 5
The overall thin, fine-grained and fining-upwards Sequence 5 with lateral continuous facies ( Figures 4 and 8), reflecting low rate of accommodation and low rate of sedimentation (discussed above), is characteristic of a post-rift phase, as defined by Prosser (1993), Nøttvedt et al. (1995) and Holz et al., (2017). As also discussed above, minor fault movements appear to have taken place, at least locally, during deposition of S5 (Figure 8). Though passive thermal subsidence appears to have been the major mechanism in creation of accommodation for Sequence 5, local fault movements make this stage of the basin evolution a transitional phase from late rift to early post-rift.

| Early post-rift: Sequence 6
Saucer-shaped stratal geometry is common for post-rift basins approaching the passive margin stage (White and McKenzie, 1988), as well as the eastward hanging-wall thickening in the initial part (S6.1; Figure 4), reflecting the passive infill of remaining rift topography (Prosser, 1993). The erosional truncation that caps the prograding reflection package indicates that the Base Statfjord Group represents an unconformity, possibly induced by thermal uplift exceeding extensioninduced isostatic subsidence, resulting in subaerial exposure (Ziegler and Cloetingh, 2004). This boundary is here considered a post-rift unconformity, but different to that of Holz et al. (2017) (Ravnås et al., 2000). The possible thin S6 in the Øygarden Fault Zone footwall indicates that S6 deposition locally reached inland over the eastern flank of the basin but was eroded at the basinhinterland transition by the later uplift.

| CONCLUSIONS
This study demonstrates how continental rift successions can be stratigraphically subdivided and correlated by integrating seismic and well log data, exemplified by the Permian-Triassic succession in the Horda Platform. The succession is characterized by continental deposition in eastward-dipping half-graben basins bounded by major N-S striking fault zones. Periodical fault propagation and -linkage, combined with pre-rift topography, exerted the main control on depocentre development, distribution and routing of sediments during rifting. The basin underwent three stages of rifting: from disconnected heterogeneous depocentres with strain concentrated in the west and migrating north, all while gradually expanding and finally reaching a mature half graben state where strain was equally distributed across the Horda Platform. A change from arid to semi-humid climate with strong fluctuations, reflected in distinct shifts in sand versus mud-dominated intervals, enhanced and even overprinted the effects of tectonics in the post-rift. These allogenic and autogenic factors induced changes in basin configuration, structurally and sedimentologically, reflected in six gross depositional sequences (S1-S6).
Correlating alluvial sequences deposited in rift basins pose a big challenge as stratigraphic concepts were developed for marine systems in passive margins, where sea-level changes produce correlative flooding surfaces. Classical continental sequence stratigraphic models have been evaluated and revised according to the observations in this study and we propose a best-fit model. The Permian-Triassic succession is bounded at the base by a syn-rift unconformity and at the top by a post-rift unconformity. Internal sequences are delineated by erosional-and transgressive surfaces. The classical models appear more applicable to single half-graben basins as they do not take into account that: (1) strain is a dynamic through time and space; (2) strain distribution and degree at the initiation of rifting is highly dependent on pre-existing structures (e.g. structural lineaments, topography etc.); and (3) tectonostratigraphic boundaries between rift stages are diachronically propagating. FIGURES FIGURE 1 a) Map of the northern North Sea showing the location of the study area (outlined in black) with main structural elements. The Permo-Triassic depocenter is highlighted in purple, while the blue shade marks the Jurassic. b) Cross-section through the study area. Modified from Faerseth (1996). c) Quadrant map of the greater study area, including the Horda Platform extending across the northern Viking Graben (Tjalve Terrace, Rungne Sub-Basin and Fensal Sub-Basin) into the Tampen Spur area with hydrocarbon fields and faults. The extend of seismic data coverage is marked along with all wells included in the study.  Stratigraphic correlation panel between the three wells (32/4-1, 31/6-1, and 31/2-4 R) used in this study. The wells are drilled in the footwall to each of the major fault zones ØFZ, VFZ and TFZ and correlated in from west to east. The wells 32/4-1 (ØFB) and 31/6-1 (SMFB) are drilled into basement, while the western well 31/2-4 R (TFB) terminates at 5000 m depth without reaching basement. The Triassic succession thickens from 1300 m in the ØFB and 1700 m in the SMFB to 2600 m in the TFB. S1 is only present in the western well (31/2-4 R), S2 thickens west, while S3 and S4 show similar thicknesses in all wells, and S5 thickens slightly towards east. S6 thickens significantly towards west. shows similar thickness in all three wells, while S3 thickens slightly west. Sub-sequences are correlated between the wells based on lithological trends. Estimated ages to the defined sequences are based on palynological ages by operator in well 31/2-4 R and are lithostratigraphically correlated to the adjacent wells.

FIGURE 7
Thickness maps between base surfaces for the total Permian-Triassic sequence (a), each of the sequences 1-6 (b-d and e.1-e.3), and S1-3 compiled (e). In the colour bars, white reflects 0.00 m thickness while a purple scale darkens with increased thickness.

FIGURE 8
Seismic 3D line GN1101 through the Smeaheia Fault Block with structural features, stratigraphic sequence boundaries, and facies interpretations. Note the annotated features discussed in the text (e.g. topographic relief in Base PT surface, onlap relations, progradational features, dragging reflections, truncation).

FIGURE 10
Stratigraphic comparison panel. The tectonostratigraphic evolution of the Permian-Triassic succession in the Horda Platform is presented through a sequence-stratigraphic sub-division, rift phase distribution in map view through time, A/S, conceptual presentation of seismic cross-sections, and tectonic stages. These features are compared to classic tectonistratigraphic schemes. A proposal for a stratigraphic application and a chronostratigraphic correlation to the equivalents formations in Tampen Spur are presented.