High‐resolution sequence stratigraphy of the Middle Triassic Sunset Prairie Formation, Western Canada Sedimentary Basin, north‐eastern British Columbia

The Middle Triassic Sunset Prairie Formation has been recently identified between the Lower Triassic Montney Formation and the Middle Triassic Doig Formation in the Western Canada Sedimentary Basin. Due to its recent recognition, the Sunset Prairie Formation has yet to be incorporated into sequence stratigraphic frameworks of the Triassic. Through the investigation of 25 cored wells, facies characteristics, vertical facies stacking and lateral facies distributions have been identified and described. Sequence stratigraphic surfaces were identified in core and extrapolated to geophysical wireline log signatures of 248 wells within the basin. The Sunset Prairie Formation can be divided into three, upward‐coarsening parasequences that exhibit a retrogradational stacking pattern. All parasequences of the Sunset Prairie Formation are truncated at their tops by the Doig phosphate zone. The Sunset Prairie Formation truncates the underlying Montney Formation, suggesting that the stratigraphic interval is unconformity bound by sequence boundaries and their correlative conformities. The addition of the Sunset Prairie Formation reveals a discrete sequence of transgressive deposits previously unaccounted for within the Triassic sequence stratigraphic framework of the Western Canada Sedimentary Basin.


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and Doig formations (Davies et al., 1997;Embry, 1997;Golding et al., 2014b;Crombez et al., 2016;Davies et al., 2018;Proverbs et al., 2018). The Sunset Prairie Formation, however, was not recognized by these studies, and the interval was either included in the Montney or Doig formations (Doig phosphate zone), or straddled both. More recent publications have acknowledged the presence of the Sunset Prairie Formation (Davies et al., 2018;Euzen et al., 2018;Zonneveld and Moslow, 2018), but little work has been done to integrate the interval into the overall Triassic sequence stratigraphic framework. A revised stratigraphic framework is needed to better understand the transition between the Montney, Sunset Prairie and Doig formations.
The objectives of this study include: (a) providing an overview of the lithofacies and facies associations present in the Sunset Prairie Formation; (b) describing facies distributions and lateral variability; (c) defining a sequence stratigraphic framework internal to the Sunset Prairie Formation; and (d) interpreting a sequence stratigraphic relationship between the Montney, Sunset Prairie and Doig formations. The purpose of this investigation is to provide a better understanding of the depositional models, stratigraphy and basin evolution of Western Canada during the Lower to Middle Triassic.

| Lithostratigraphic framework
The Western Canada Sedimentary Basin consists of a westward-thickening wedge of Phanerozoic strata above the Precambrian crystalline basement (Mossop and Shetsen, 1994). Triassic subsurface strata of the Western Canada Sedimentary Basin consist of the Lower Triassic Montney Formation, the Middle Triassic Sunset Prairie, Doig and Halfway formations, and the Upper Triassic Charlie Lake, Baldonnel, Pardonet and Bocock formations (Figure 1; Clarke, 1957;Hunt and Ratcliffe, 1959;Armitage, 1962;Furlong et al., 2018a). Throughout Alberta and British Columbia, Triassic strata sit unconformably above Permian or Carboniferous strata and are unconformably overlain by Jurassic or Lower Cretaceous strata (Edwards et al., 1994).
Stratigraphic nomenclatural discrepancies occur between Alberta and British Columbia for the Montney and Doig formations Zonneveld and Moslow, 2018). Herein, the authors refer to the Montney Formation from a British Columbian perspective. Therefore, the 'Montney Formation' as used in this paper, includes intervals referred to as both the Montney Formation and Basal Doig Siltstone within the Albertan stratigraphic nomenclature Zonneveld and Moslow, 2018).

| Palaeoenvironmental setting
By the end of the Permian, the supercontinent of Pangea had formed from the amalgamation of the world's major landmasses (Ziegler, 1988;Lawyer et al., 2002Lawyer et al., , 2011. On the western margin of Pangea, Triassic sediments were deposited within a large, central sub-basin, called the Peace River Embayment, and these consisted of marine and marginalmarine siliciclastic and carbonate sediments, and lesser volumes of evaporite deposits (Figure 2 FURLONG et aL. et al., 1990;O'Connell et al., 1990;Edwards et al., 1994;Evoy and Moslow, 1995). These deposits accumulated under arid, mid-latitudinal (ca 32 to 34°N) conditions (Wilson et al., 1991;Davies et al., 1997;Davies, 1997a;Dixon, 2000;Moslow, 2000;Golonka, 2007;Zonneveld and Moslow, 2018). During this time, Canada was rotated approximately 30° clockwise from its current orientation and the regional shoreline approximately parallels the axis of the modernday Rocky Mountains (Golonka et al., 1994;Davies, 1997a;1997b;Golonka and Ford, 2000). Many have suggested that the Triassic of Canada represents an arid coastline, mainly through evidence of aeolian dunes, regionally extensive anhydrite beds and solution-collapse breccia (resulting from evaporite dissolution) associated with latest Middle and early Upper Triassic marginal marine and non-marine strata (Gibson and Barclay, 1989;Zonneveld et al., 1997;Zonneveld, 2001). Although minimal direct evidence has been presented on the palaeoclimate of Lower to early Middle Triassic strata, palynology (e.g. taeniate bisaccate and polyplicate palynomorphs; Utting, 2001;Utting et al., 2005;Zonneveld et al., 2010b) and mineralogy (abundance of detrital dolomite, low clay proportions and reworked anhydrite nodules; Davies et al., 1997) are consistent with arid conditions. Palaeoshoreline reconstructions and coastal facies distributions are difficult to determine during the Lower and early Middle Triassic, owing to the presence of unconformities between many of the formations that cannibalized marginal marine and terrestrial deposits.

| Tectonic Setting
Multiple structural features should be taken into consideration when trying to resolve depositional dynamics of Triassic strata in the Western Canada Sedimentary Basin. Prior to the Triassic, the Peace River Arch was a basement-uplifted topographic high, which persisted from the Precambrian to Devonian and greatly controlled deposition and preservation of sediment (de Mille, 1958;Cant, 1988;Stephenson et al., 1989;Barclay et al., 1990;O'Connell et al., 1990). During the Carboniferous, the Peace River Arch began to subside and collapse, which eventually formed the Dawson Creek Graben Complex (Barclay et al., 1990;Gibson and Edwards, 1990;O'Connell et al., 1990). The graben complex consists of three major elements, which comprise the Fort St. John Graben, the Hudson Hope Low and a number of smaller satellite grabens (Hines Creek, Whitelaw and Cindy grabens; Barclay et al., 1990;O'Connell, 1994). These features formed a system of high-angle normal faults that subsided due to syndepositional and post-depositional growth-type block faulting (Barclay et al., 1990;Edwards et al., 1994). Subsidence associated with the Peace River Arch and Dawson Creek Graben Complex significantly influenced deposition of Carboniferous, Permian and Triassic strata, and this region ultimately became the main sediment depocentre within the Peace River Embayment (Cant, 1988;Barclay et al., 1990;O'Connell et al., 1990;Wittenberg, 1992Wittenberg, , 1993Qi, 1995;Davies, 1997a). Other structural features influencing Triassic deposition include: the Hay River Fault Zone, Laurier Embayment, Monias High, Beaton High, Grassy High, Sukunka Uplift and faulting associated with Devonian reef trends (Leduc Reef and Swan Hills Reef; Richards, 1989;Davies and Majid, 1993;Henderson et al., 1994;Davies, 1997a;1997b).
Traditionally, the north-western margin of Pangea during the Cambrian to the Late Triassic has been considered to be a tectonically stable, passive margin with a single north-eastern-derived sediment source (Dickinson, 1977;Monger and Price, 1979;Coney et al., 1980;Gibson and Barclay, 1989;Price, 1994;Davies, 1997a). A passive margin would have fostered the development of a marine ramp/shelf setting (Edwards et al., 1994;Dixon, 2009a;2009b;Onoue et al., 2016). It has been suggested that subduction and collision of island arcs occurred in the Panthalassic Ocean, offshore of the margin of Pangea, with terrane accretion occurring only during the late Jurassic and early Cretaceous; thus, suggesting that the Triassic margin was passive (Gibson and Barclay, 1989;Davies, 1997a). Multidisciplinary evidence, including stratigraphic architecture, geochemical models and zircon dating suggests, however, that basin evolution, margin development and sediment provenance are more complex than previously interpreted (Ferri and Zonneveld, 2008;Berenak et al., 2010;Golding et al., 2016;Rohais et al., 2016;Morris et al., 2018). Terrane accretion likely occurred as early as the Early Triassic (Beranek and Mortensen, 2011;Golding et al., 2016;Rohais, et al., 2018;Zonneveld and Moslow, 2018). These terranes would have provided a minor, but significant, source of sediment to the Peace River Embayment, in addition to the primary sediment source from the Laurentian craton to the east (Ferri and Zonneveld, 2008;Berenak et al., 2010;Golding et al., 2016;Morris et al., 2018). Geodynamic settings and palaeogeographic reconstructions along an active margin have led to multiple hypothesized basin models for Triassic deposition, and include a back-arc-basin (Ferri and Zonneveld, 2008;Miall and Blakey, 2008;Zonneveld et al., 2010a;Schiarizza, 2013;Morris et al., 2014;, fore-arc-basin (Nelson et al., 2006;Colpron et al., 2007;Rohais et al., 2016) and early foreland basin Rohais et al., 2018;Zonneveld and Moslow, 2018). The evolution of Early Triassic basin architecture in Western Canada and its implications on depositional processes, palaeoenvironmental dynamics and oceanic circulation remain debateable.

GENERAL CONCEPTS
Sequence stratigraphy represents a markedly different approach than lithostratigraphy. Lithostratigraphic units are mappable intervals of rocks that share similar sedimentary characteristics, where facies are correlated and boundaries between units typically represent highly diachronous contacts (Hedberg, 1976). Sequence stratigraphy is the study of cyclic sedimentation patterns that have developed in response to variations in accommodation and sedimentation conditions (Catuneanu, 2019). A relative chronostratigraphic relationship can be interpreted from the relationship between coeval stratal units and bounding surfaces, in that strata lying above the discontinuity are younger than strata lying below it (Catuneanu, 2006). The Sunset Prairie Formation is an unconformity-bound lithostratigraphic formation; however, a sequence stratigraphic framework internal to the formation can be established.
This study predominantly utilizes the Exxon model of depositional cycles defined on the basis of bounding subaerial unconformities and their correlative conformities Van Wagner et al., 1988;. Stratigraphic geometries and stratal pattern relationships within the Exxon model were based on the hypothesis of eustatic controls producing systems tracts . For the past few decades, however, it has been increasingly recognized that sequence stratigraphic architectures are a result of 'relative sea level', which is a blend of eustasy, tectonism and climate forcing functions that affect base level and therefore accommodation space (Hunt and Tucker, 1992;Posamentier and James, 1993;Posamentier and Allen, 1999;Catuneanu, 2006;Cataneanu et al., 2009). Within the Peace River Basin, it has been suggested that there was a strong tectonic influence resulting in fault reactivation, which overprinted the higher-order eustatic signature (Embry, 1997;Kendall, 1999).
Due to the fact that sequence stratigraphic terminology is ever evolving, a brief summary of terms and concepts is outlined here. A 'sequence' is a package of genetically related strata that are bound by regionally extensive subaerial unconformities and their correlative conformities (Sloss, 1963;Mitchum, 1977). A sequence is the fundamental stratal unit of sequence stratigraphy and corresponds to depositional processes and products recording a full cycle of base-level change (Catuneanu, 2006;Cataneanu et al., 2009). Sequences are subdivided into systems tracts, which are defined by the types of bounding surfaces, origin of bounding surfaces, their position within a sequence, and the stacking pattern of parasequences and parasequence sets (Van Wagoner et al., 1988;Posamentier and Allen, 1999;Catuneanu, 2019). Parasequences are genetically related beds or bedsets recording progradational regression that have classically been described to be bound by marine flooding surfaces produced by an abrupt increase in water depth (Van Wagoner et al., 1988Posamentier and Allen, 1999). The concept of a parasequence has been deemed obsolete by some (Catuneau, 2019) based on the inconsistency, ambiguity and confusion surrounding the identification and formation of flooding surfaces produced in different depositional settings. Transgression is required at the basal surface of a parasequence and it is likely that a thin transgressive package (likely <1 m thick) would be deposited at the base, which is then followed by regressive deposits; this suggests that parasequences consist of small-scale transgressive-regressive cycles with an identifiable internal maximum flooding surface and maximum regressive surface. Such small-scale (metre-scale) features, however, are difficult to reliably map using petrophysical datasets. Therefore, the authors have decided to retain the concept of parasequences within this study to represent packages of strata that record overall progradational deposition, but may also exhibit thin transgressive deposits at the base associated with an abrupt increase in relative sea level. Genetically related parasequences can be grouped into parasequence sets, which form distinctive stacking patterns, bound by major marine flooding surfaces (Van Wagoner et al., 1988;Posamentier and Allen, 1999). The distinct stacking patterns of parasequences and parasequence sets designate subdivisions within each sequence and are described as the lowstand, transgressive, highstand and falling-stage systems tracts (Brown and Fisher, 1977;Van Wagoner et al., 1988;Catuneanu, 2006;Catuneanu et al., 2011). Three different sequence stratigraphic models have described the interplay between systems tracts and the timing of sequence boundaries (i.e. Depositional Sequence, Genetic Sequence and Transgressive-Regressive Sequence). A thorough description and discussion of the models are outlined by Catuneanu (2006;2019) and Catuneanu et al. (2009Catuneanu et al. ( , 2011.

| STUDY AREA AND DATASET
This investigation of the Sunset Prairie Formation was undertaken on a regional scale within the Western Canada Sedimentary Basin. Stratigraphic correlations between drill cores were produced using geophysical well log data from GeoScout. A total of 248 vertical wells were used to determine the stratigraphic architecture and distribution of the Sunset Prairie Formation. Geophysical data were supplemented with slabbed core from 25 wells. Cores were logged and described for sedimentological, ichnological and palaeontological characteristics. Detailed observations of lithology, grain size, nature of contacts, physical sedimentary structures, biogenic sedimentary structures and body fossils were recorded for each core. Bioturbation intensity was quantified using a bioturbation index (BI), which ranges from non-bioturbated (BI = 0) to pervasively bioturbated/completely biogenically homogenized (BI = 6) (Reineck, 1963;1967;Taylor and Goldring, 1993). Trace fossils were identified down to the ichnogenus level. Core characteristics were categorized into facies and combined into recurring facies associations to interpret their depositional settings. The openmarine environmental subdivisions used in this study are based on Elliott (1986) and Reading and Collinson (1996), which place the offshore below storm wave base, the offshore transition between storm wave base and fair-weather wave base, and the shoreface between the fair-weather wave base and low-tide line. A thorough investigation of facies within the Sunset Prairie Formation and their interpreted depositional palaeoenvironments has been described by Furlong et al. (2018b).
Stratigraphic relationships within the Sunset Prairie Formation were established through the evaluation and interpretation of facies distribution and stacking patterns. Parasequences and sequence stratigraphic surfaces were interpreted in core and tied to geophysical well data. These surfaces were then correlated between cored wells. A grid of depositional dip-oriented and strike-oriented, regional cross-sections was constructed using gamma, resistivity and density porosity logs. Regional isopach maps were produced for each parasequence of the Sunset Prairie Formation to visualize the distribution and thickness of each package. All of the 248 vertical wells were included in the regional grid to ensure the integrity of the depositional sequence stratigraphic framework.

| Summary of lithofacies
Seven lithofacies were identified in the Sunset Prairie Formation and have been described in detail by Furlong et al. (2018b). Sedimentological, ichnological and palaeontological characteristics of each lithofacies are outlined in Figure 3. Overall, the facies can be subdivided into three categories: (a) diminutively bioturbated (Facies 1 and 2), where physical sedimentary structures are observable; (b) pervasively bioturbated (Facies 3, 4 and 5), where burrowing organisms have homogenized the sediment and destroyed most primary physical sedimentary structures; and (c) facies that are associated with regional or local stratigraphic surfaces (Facies 6 = Glossifungites Ichnofacies; Facies 7 = conglomeratic lag deposit). Collectively, the facies are associated with deposition in the offshore, offshore transition and lower shoreface environments (Furlong et al., 2018b).

| Facies associations
Facies associations record the recurrence of lithofacies in a predictable vertical distribution. Collectively, the lithofacies of the Sunset Prairie Formation are interpreted to represent deposition along a wave-dominated, shallow-marine setting, within the offshore, offshore transitions and lower shoreface. The shallowing-upward pattern of lithofacies within the facies associations suggests a small-scale regression of relative sea level (Figure 4).
A variety of different shoreface models have been suggested over the past few decades and use slightly variable terminology for environmental subdivision (Elliott, 1986;Reading and Collinson, 1996;MacEachern and Bann, 2008;Buatois and Mangano, 2011). Within this paper, the open-marine environmental subdivisions are based on Elliott (1986) and Reading and Collinson (1996), which place the offshore below the storm wave base, the offshore transition between the storm wave base and the fair-weather wave base, and the shoreface between the fair-weather wave base and the low-tide line. Ichnological characteristics observed within the Sunset Prairie Formation vary slightly from those that are classically associated with shoreface successions (Reading and Collinson, 1996;Buatois and Mangano, 2011), due to the stressed marine ecosystem attributed to the end-Permian mass extinction and the faunal recovery period that followed (Benton and Twitchett, 2003;Heydari and Hassanzadeh, 2003;Black et al., 2012;Hinojaosa et al., 2012;Payne and Clapham, 2012). General trends observed in each setting are described below.
Offshore deposits (Facies 1) consist of fine-grained to coarse-grained siltstone. Sedimentary structures include faint horizontal planar-parallel laminae and horizontal wavy-parallel laminae, which are indicative of deposition within a low-energy environment. Ichnological assemblages are characterized by a low trace fossil diversity (4 ichnogenera), low trace fossil abundance (BI = 0-2) and small trace fossil size (<1 mm in diameter).
Offshore transition deposits are associated with a large range of sedimentological and ichnological characteristics. The offshore transition setting has been divided into a distal expression (lower offshore transition) and a proximal expression (upper offshore transition). The lower offshore transition is characterized by both diminutively bioturbated facies (Facies 1 and 2) and pervasively bioturbated facies (Facies 3 and 4). Lithologically, deposits consist of fine-grained to coarse-grained siltstone. Within bioturbated facies, trace fossil diversity is moderate (7 ichnogenera), trace fossil abundance is high (BI = 4-6) and trace fossil size is variable (0.5-12 mm in diameter). Fair-weather wave deposits are mainly associated with bioturbated intervals; however, non-bioturbated intervals exhibit horizontal planar, wavy and pinstripe-parallel laminae, which can also be associated with fair-weather deposition when physico-chemical stresses reduce/hinder biotic colonization. Rare low-angle and high-angle planar cross-stratification, and asymmetric ripples are indicative of periodic increased energy, possibly associated with storm deposition. But the lack of strongly storm-influenced primary sedimentary structures and the presence of intensely bioturbated intervals within the offshore transition suggest that a sheltered coastal setting that was protected from storm erosion existed during deposition of the Sunset Prairie Formation. Deposition within the upper offshore transition (Facies 3, 4 and 5) is characterized by a moderately high trace fossil diversity (11 ichnogenera), large trace fossil sizes (up to 15 mm in diameter) and intense bioturbation (BI = 4-6). Lithologically, deposits consist of fine-grained to coarse-grained siltstone. The lack of non-bioturbated facies suggests that bioturbation rates outpaced sedimentation rates, resulting in infaunal organisms completely homogenizing the sediment.
Lower shoreface deposits (Facies 5) are associated with a moderately high trace fossil diversity (10 ichnogenera), large trace fossil sizes (up to 15 mm in diameter) and intense bioturbation (BI = 4-6). Lithologically, deposits consist of fine-grained siltstone to fine-grained sandstone. The coarsegrained sandstone material suggests that these intervals are more proximally located than the other facies of the Sunset Prairie Formation. Facies associated with more proximal settings, like the middle shoreface, upper shoreface and foreshore  are not observed within the Sunset Prairie Formation and have been erosionally removed.

| Vertical distribution of facies
The reoccurring stacking pattern of facies show upwardcoarsening packages that preserve deposits from distal (offshore) to proximal (lower shoreface) settings. Upwardcoarsening successions have a variety of origins within the rock record. Increased hydrodynamic energy resulting from relative sea-level fall leading to depositional shallowing can generate upward-coarsening intervals (Van Wagoner et al., 1990;Catuneanu et al., 2009). However, when independent indicators of palaeoshoreline position or water depth are absent, coarsening-upward successions may reflect bedsets that form without relative changes in sea level . Such conditions include an increase in sand influx due to river flow, or increase in storm waves and/or currents driven by variations in climate, ocean circulation or shoreline palaeogeography (Storms and Hampson, 2005;Somme et al., 2008;Mitchell et al., 2012). Although the exact palaeoshoreline position during Sunset Prairie Formation deposition has not been preserved, it is most likely that the upward-coarsening successions are a result of changes in relative sea level due to: (a) the high bioturbation intensity suggesting prolonged periods of low intensity storms (Howard, 1975;Gani et al., 2007;Furlong et al., 2018b) less frequently); and (c) spatial migration of the lower shoreface deposit capping the top of coarsening-upward packages (parasequences) through time can infer palaeoshoreline trajectory. Facies stacking patterns within the Sunset Prairie Formation preserve shoaling-upward successions, which are interpreted here as parasequences. The base of the parasequence is marked by a series of conglomerate lag deposits (Facies 7), which are interpreted to represent transgressive lags. These surfaces are allogenic and can be mapped across the basin. Burrowed firmgrounds of the Glossifungites Ichnofacies (Facies 6) are also commonly associated with the boundaries of parasequences, but can also be interformational, suggesting both autogenic and allogenic origins. Overlying the transgressive lag deposits is a shoaling upward succession, which records offshore deposits overlain by offshore transition deposits, which are then capped by lower shoreface deposits (Figures 4 and 5). Parasequences coarsen upward in grain size from fine-grained silt to very fine-grained sand and bioturbation increases upwards. Small-scale interbedding of diminutively bioturbated facies and pervasively bioturbated facies occur throughout the parasequence, but an overall shoaling upward trend occurs. Up to three parasequences are observed within the Sunset Prairie Formation.

| Lateral distribution of facies
The Sunset Prairie Formation total thickness increases to the west and thins to an erosional edge to the east (Figures 6 and  7). Eastern-located wells do not preserve all three parasequences, and commonly only preserve one or two of the lower parasequences in thinned accumulations. The parasequences commonly exhibit more proximal facies associated with the upper offshore transition and lower shoreface, and contain abundant coarse-grained lag deposits (Figures 5 and 6). The decrease in total number and thickness of parasequences ( Figure 6) is likely a result of autogenic erosion associated with shallow-marine conditions redistributing sediment due to the lack of accommodation space.  Wells located in western or distal locations within the basin preserve thicker successions of all three parasequences, which preserve offshore, offshore transition and lower shoreface deposits. Over-thickening in some regions might be due to structural features. The formation is thickest (66.5 m in the 03-06-078-22W6 cored well), within the Fort St. John Graben system and the Hudson Hope Low, in the western region of the study area ( Figure 6). The interval thins across the Hudson-Monias High, where the formation is <25 m in total thickness (Figure 6). In the Laurier Embayment, in the northern portion of the study area, the formation thickens to approximately 30 m. Localized, detailed studies would provide a better understanding of how facies, facies associations and parasequence distributions change across these structural features.

| Parasequence geometry
Regional correlation suggests that three upward-coarsening parasequences are recognized within the Sunset Prairie Formation throughout the basin in core and geophysical wireline data (Figures 6 through 8). Commonly, the base of the parasequence is identifiable by high, commonly offscale, gamma ray log deflections, which are frequently associated with a lag deposit (Facies 7) and/or Glossifungites Ichnofacies-demarcated discontinuity surface (Facies 6; Figure 5; Furlong et al., 2018a, Figures 5 through 7). Where a lag deposit or Glossifungites Ichnofacies-demarcated discontinuity surface is not present, an abrupt change from proximal facies overlain by distal facies is observed in core; this corresponds to a sharp decrease in gamma ray log     signature, which may or may not be off-scale. Additionally, in more westward locations where the Sunset Prairie Formation is at its thickest, a correlative conformity can be interpreted when a subtle change in facies occurs ( Figure 5). Spatial distribution of the three parasequences is shown in the isopach maps of Figure 7. The first parasequence (P1) extends across the entire area where the formation is preserved. Compared to P1, Parasequence 2 (P2) is preserved in a more distal/western location. Parasequence 3 (P3) is preserved in the most western location when compared to the other two parasequences.
All parasequences are observed to thicken to the west and are erosionally removed to the east (Figures 7 and 8). Although there is a westward migration of the preserved parasequences, which would lead one to interpret progradation, facies associations within the parasequences exhibit a back-stepping stacking pattern and each subsequent parasequence preserves relatively less proximal associated facies (lower shoreface) at their tops ( Figure 5). Significant erosion would have been needed to truncate the Sunset Prairie Formation and remove the eastern parts of the parasequences. Therefore, the top of the Sunset Prairie is interpreted to represent an unconformity associated with a transgressively modified sequence boundary (FS/SB).

| Sequence stratigraphic models for the Sunset Prairie Formation
The Sunset Prairie Formation has been interpreted as representing a variety of different sequence stratigraphic systems tracts. The interval has previously been interpreted to represent deposition under falling stage and lowstand conditions (Proverbs et al., 2018) and a shelf-margin wedge building out from the underlying highstand unit of the Montney Formation (Davies et al., 2018). The sedimentological characteristics of the interval provide evidence supporting different sequence stratigraphic frameworks, leading to complex interpretations of stratal relationship with the Montney and Doig formations. Here, two different sequence stratigraphic interpretations are made for the Sunset Prairie Formation.

| Transgressive systems tract
Based on facies associations, parasequence stacking patterns, sequence and stratal boundaries, the Sunset Prairie Formation can be interpreted as a transgressive systems tract. Each parasequence exhibits a back-stepping or retrogradational stacking pattern with respect to the underlying parasequence (Figures 7 and 9). The thickest packages of the most proximal facies (lower shoreface deposits) are preserved at the base of the Sunset Prairie Formation (in the first parasequence), and progressively thin in succeeding parasequences ( Figures 5 and 6). Additionally, the average grain size of each parasequence decreases moving stratigraphically upward.
The base and top of the Sunset Prairie Formation are unconformities that are interpreted as transgressively modified sequence boundaries (FS/SB) due to the unconformity truncating the underlying formation (Furlong et al., 2018a;2018b). Little or no sedimentological evidence (e.g. root casts, palaeosols, karsting, etc.) has been observed at the boundary between the Montney Formation and the Sunset Prairie Formation to suggest prolonged subaerial exposure; however, transgressive modification would have been capable of eliminating evidence of subaerial exposure along the surface. Subaerial erosion of the underlying Montney Formation would have also been amplified by regional tectonic uplift.
In this scenario, the lowstand systems tract would be predicted to be spatially detached and the boundary between the Montney Formation and Sunset Prairie Formation would represent a significant drop in relative sea level with subsequent base-level rise. However, no observations have yet to suggest that detached, distally located lowstand deposits occur in the Rocky Mountain outcrop belt (see Orchard and Although interpreting the Sunset Prairie Formation as a transgressive systems tract follows classic sequence stratigraphic concepts, this interpretation does face some challenges. First, the Sunset Prairie Formation consists of more proximal facies (lower shoreface) with coarsergrained material (fine-grained sandstone) than the underlying Montney Formation (siltstone deposited within the offshore transition; Crombez et al., 2016). Although these changes are subtle, the transgressive systems tract has commonly been described as exhibiting the finest grained material in the system (Catuneanu, 2006). Using  Legend classic sequence stratigraphic models, the location of the lowstand systems tract below the Sunset Prairie Formation and a lowstand and highstand system tract above the Sunset Prairie Formation are predicted to be located more distally, or basinward. However, more evidence has suggested an active margin during Early Triassic deposition, which would influence the deposition of lowstand and highstand deposits. Back-arc-basin, fore-arc-basin and early foreland basin types have been previously ascribed to the Montney Formation (Ferri and Zonneveld, 2008;Morris et al., 2014;Rohais et al., 2016;. If these basin types persisted through deposition of the Sunset Prairie Formation, then the positioning of these island arcs would have influenced the distribution of accommodation space and areas of exposure. The basal or top boundaries of the Sunset Prairie Formation do not exhibit direct evidence supporting subaerial exposure or nonmarine deposition. Although transgression could cannibalize these deposits, the subtle changes in facies at the boundaries above and below the Sunset Prairie Formation may be more reflective of diastems (short interruption in sedimentation) associated with a correlative conformity and would reduce the amount of geological time attributed to those surfaces. Understanding the evolution of the basin, proximity of the island arcs/accreted terranes to the continent and available accommodation of the unaccounted systems tracts would better render a more complete sequence stratigraphic model. Although these ideas are speculative, the complex contributions and effects of tectonic controls undoubtedly influence deposition of Early and Middle Triassic deposits.

| Alternative interpretation: falling stage/lowstand systems tract and transgressive systems tract
Due to some pitfalls in the previous interpretation, a secondary interpretation is provided as an alternative means to deposit and preserve the Sunset Prairie Formation. Within the previous interpretation, the lowstand systems tract (and falling stage systems tract) is not accounted for within the cored dataset and is interpreted to be located more basinward. The proposed alternative sequence stratigraphic model interprets the Sunset Prairie Formation as a falling stage/lowstand systems tract (preserved as P1) and a transgressive systems tract (preserved as P2 and P3; Figure 10). Parasequence 1 exhibits the thickest accumulation of proximal (lower shoreface) facies ( Figure 5). These facies are more proximal and consist of coarser-grained material (up to very fine-grained sandstone) than the underlying Upper Montney Formation facies, which mainly consist of siltstone interpreted as offshore transition deposits (Zonneveld and Moslow, 2018). A decrease in relative sea level cut the unconformity and led to a slight basin shift in facies, supplying fine-grained sand to lower shoreface environments. Wave reworking produced an erosional surface at the base of the Sunset Prairie Formation as sea-level fell, producing a regressive surface of marine erosion (RSME). Distal locations would preserve a basal surface of marine regression, or the correlative conformity. A regressive stacking pattern of parasequences within the falling stage/lowstand systems tract cannot be determined due to the systems tract being composed of a single parasequence, rendering the interpretation of both falling stage and lowstand systems tracts for the basal parasequence. Capping P1 is a transgressive surface of erosion (or RSME), which records the most basinward migration of the lower shoreface and marks the onset of transgression. The transgressive systems tract consists of the upper two parasequences, each showing an increased abundance in distal facies (offshore transition) compared to the first parasequence and an overall back-stepping of the palaeoshoreline. All three parasequences are truncated by an overlying coplanar sequence boundary and transgressive surface of erosion associated with the base of the Doig phosphate zone. Although the alternative sequence stratigraphic interpretation of the Sunset Prairie Formation consisting of falling stage/ lowstand and transgressive deposits is more speculative, the absence of the lowstand systems tract down-dip and the occurrence of sharp, erosionally based coarse-grained (fine-grained sandstone) lower shoreface deposits located in a more distal locations, compared to underlying Montney Formation lithology, make this interpretation plausible. 6 | DISCUSSION

| Stratigraphic architecture and sequence stratigraphy of the Lower and Middle Triassic of Western Canada
Many workers agree that the Montney Formation is composed of three unconformity-bound, third-order depositional sequences, corresponding to the Lower Montney (Griesbachian to Dienerian age), Middle Montney (Smithian-age) and Upper Montney (Spathian-age; Embry and Gibson, 1995;Davies et al., 1997;Embry, 1997;Golding et al., 2014a;Henderson and Schoepfer, 2017;Davies et al., 2018;Henderson et al., 2018). However, internal sequence stratigraphic frameworks vary greatly between workers ( Figure  11). The majority of studies have focused on the proximal parts of the formation (Davies et al., 1997;Markhasin, 1997;Moslow and Davies, 1997;Kendall, 1999;Moslow, 2000;Panek, 2000), isolated to localized spatial areas (Evoy and Moslow, 1995;Evoy, 1997;Harris, 2000;Dixon, 2002;2010;2011;Golding et al., 2014b;Proverbs et al., 2018;Zonneveld and Moslow, 2018) or isolated stratigraphic intervals Prenoslo et al., 2018), with only a few studies looking at sequence stratigraphic correlations on a larger, basin-wide scale Davies and Humes, 2016;Davies et al., 2018). Although the scope of each publication differs, no unanimous sequence stratigraphic framework has emerged for the Lower and Middle Triassic, and the sequence stratigraphic surfaces identified are varied (Figure 11). To provide evidence for how the Sunset Prairie Formation ties into the sequence stratigraphic architecture of the Lower to Middle Triassic, stratigraphic surfaces were identified in the Upper Montney Formation, Sunset Prairie Formation and Doig phosphate zone in the Fort St. John Graben system. This area was chosen because it corresponds to the region where the Sunset Prairie Formation is most thickly preserved. Facies and stratigraphic surfaces were interpreted by the author in core and correlated across the area using petrophysical wireline data. Detailed description of facies, facies associations and basin-scale correlation of parasequences within the Upper Montney Formation and Doig phosphate zone were outside the scope of this paper.   Willis and Wittenberg (2000) Formation Seq. Strat.

Davies and
Humes (  Within the Fort St. John Graben system, the Upper Montney Formation consists of a lowstand systems tract at the base, with a thin (<5 m thick) transgressive systems tract overlying it ( Figure 12). The highstand systems tract makes up the majority of the Upper Montney Formation. Three parasequences were identified in the lowstand systems tract, one parasequence in the transgressive systems tract, and up to eight parasequences are identified in the highstand systems tract. Thickness of parasequences in the topset and bottomset areas are typically 10 m or less, whereas foreset thicknesses can reach up to 55 m. Similar results were obtained by Euzen et al. (2018) when mapping the Upper Montney basin wide. They, however, described the packages as parasequence sets, instead of parasequences. They suggested that the parasequence sets were made up of multiple coarsening-upward parasequences but lack further description of facies distributions in the parasequences themselves, or the distribution of the parasequences within the parasequence sets.
Here the packages are considered to be more representative of parasequences and to record a single, overall shallowing of relative sea level.
The top of the Montney Formation was truncated by the unconformity underlying the Sunset Prairie Formation. The Sunset Prairie Formation was deposited as a transgressive systems tract. The overlying Doig phosphate zone is classically considered a transgressive, condensed section (Gibson and Barclay, 1989). Within the Fort St. John Graben system, the Doig phosphate zone erosionally truncates and onlaps the top of the Sunset Prairie Formation, which suggests that the Sunset Prairie-Doig boundary is a coplanar sequence boundary and transgressive surface of erosion. Figure 9 provides a schematic depicting the interplay between relative sea-level change and the deposition of the Upper Montney Formation, Sunset Prairie Formation and the Doig phosphate zone.
With the identification of the Sunset Prairie Formation in the Triassic strata of western Canada, the presence of another unconformity-bound sequence must be accommodated. The Montney Formation exhibits three third-order sequences (Douglas, 1970;Barclay et al., 1990;O'Connell et al., 1990;Edwards et al., 1994;Evoy and Moslow, 1995), and the Sunset Prairie Formation constitutes a portion of a fourth Triassic third-order sequence. The addition of another third-order sequence modifies the timing and processes associated with the evolution of the basin.

Triassic Deposition
The Fort St. John Graben system and the Monias High are two regional structural features that have influenced deposition of both the Montney and Sunset Prairie formations. Studies of the Montney Formation have suggested that differential subsidence across the basin due to tectonic movement and/ or differential compaction of pre-Triassic sedimentary successions produced depositional palaeorelief, which influenced and switched the position of preserved thick intervals (Davies et al., 2018;Euzen et al., 2018;Rohais et al., 2018). The Hudson Hope Low experienced subsidence beginning in the Devonian and continued to be a palaeolow throughout the Triassic (Barclay et al., 1990). The Monias High was a palaeohigh, which can be seen in Belloy Formation structures map (Dunn, 2003;Davies et al., 2018). Within the Montney, regions that exhibit thinned intervals generally correspond to zones where clinoforms (parasequences) change orientation as a result of being deflected around syndepositional highs .
Evidence for syndepositional tectonism is present within the Sunset Prairie Formation. The strike-oriented cross-section ( Figure 6; cross-section D-D′) suggests that structural elements correlate to and likely influenced parasequence set thicknesses. Note that the cross-section intersects the major palaeohighs and palaeolows, which impact parasequence distributions and stratigraphic architectures ( Figure 6). The Hudson Hope Low within the Fort St. John Graben system preserves the thickest interval of the Sunset Prairie Formation. Thickening that is wider than the area of the Fort St. John Graben system, as described by Davies (1997a;1997b) and Davies et al. (1997), is likely due to the graben being active during deposition of the Sunset Prairie Formation. Thinning across the Monias High suggests that the structure remained a palaeohigh during the Middle Triassic.
Other structural features have been identified and discussed within the Western Canada Sedimentary Basin to have influenced Triassic deposition (Barclay et al., 1990;Davies et al., 2018;Euzen et al., 2018;Rohais et al., 2018). Many of the small-scale features occur east of the erosional edge of the Sunset Prairie Formation and are located outside of the study area and/or are of too small a scale to resolve at the scale of this study.
The active margin associated with the western edge of Pangea during Sunset Prairie Formation deposition likely controlled the reactivation and displacement of structural features. The fluctuations in relative sea level, which formed the parasequences, may have a strong correlation with the tectonic pulses associated with the movement of the island arcs along the coast. More research is needed to assess the structural controls on this and other Triassic strata.

Prairie Formation
Understanding the sequence stratigraphic framework for the Sunset Prairie Formation and its stratigraphic relationship with underlying and overlying formations can lead to a better prediction of reservoir targets. Generally speaking, the pervasively bioturbated, very fine-grained sandstone facies have core permeability measurements that are an order of magnitude higher than the minimally bioturbated siltstone-rich facies 2016a;2016b). Porosity doubles between non-bioturbated facies (1%-3%) to bioturbated facies (4%-6%), regionally. This difference in permeability and porosity in facies is based on the complex interplay between grain size variability, diagenetic features (e.g. secondary porosity, cementation) and biogenic modification of grain distribution. This study does not elaborate on petrographic observations (e.g. grain size, diagenetic features) or specific reservoir properties (e.g. permeability, porosity, TOC distribution) due to the local variability of these characteristics; however, the discussion herein provides an overview of lithological distribution related to net-to-gross sand ratios to better predict where sandrich reservoir intervals would be located.
Due to the retrogradational nature of the parasequences, the lowermost parasequence is where the thickest packages (up to 7 m) of pervasively bioturbated, very-fine grained sandstone intervals would be observed. Fine-grained sand content, thickness of coarse-grained beds and bioturbation intensity decrease with each following parasequence. The highest ratio of net-to-gross sand is observed at the base of the Sunset Prairie Formation and decreases stratigraphically upwards (Figure 9; also see lithologs from Furlong et al., 2018a, Figures 4 through 7).
If the Sunset Prairie Formation were actually a continuation of the highstand systems tract associated with the uppermost part of the Upper Montney Formation, facies distribution and stratigraphic architecture would appear differently. Within the highstand systems tract, progradation of parasequences produce coarsening-upward parasequence sets, with the most proximal, coarse-grained facies being observed stratigraphically at the top of the formation (Catuneanu, 2006;Catuneanu et al., 2009). This is the opposite of what is observed in the Sunset Prairie Formation. Additionally, the Sunset Prairie Formation truncates the underlying Montney Formation (Figure 9), which would further suggest against the Sunset Prairie Formation being deposited during the highstand conditions associated with Montney Formation deposition.
This basin wide study provides the overall facies distribution and sequence stratigraphic architecture of the Sunset Prairie Formation, and suggests that the highest net-to-gross sand ratios are observed at the base of the formation. These intervals have the potential to be favourable reservoir targets, when other reservoir characteristics are optimal. Localized studies and detailed hydrocarbon investigations will provide more insight into reservoir characterization of the Sunset Prairie Formation and its potential producibility.
The stratigraphic architecture of the Sunset Prairie Formation consists of three parasequences constituting the transgressive systems tract. These parasequences are retrogradational and suggest that the palaeoshoreline moved eastward during deposition of the formation as a transgressive systems tract. Parasequences are truncated by the overlying Doig phosphate zone, indicating that the boundary represents a transgressively modified sequence boundary (FS/SB). The Sunset Prairie Formation directly overlies an unconformity that truncates the underlying Montney Formation, suggesting a transgressively modified sequence boundary (FS/SB) at the contact. Correspondingly, the Sunset Prairie Formation is an unconformity-bound sequence discrete from sequences of the underlying Montney Formation and those in the overlying Doig Formation. With the addition of the Sunset Prairie Formation into the Western Canada Sedimentary Basin, a revision of the sequence stratigraphic model associated with Triassic deposits is necessary.