Sedimentological and ichnological variations in fluvio‐tidal translating point bars, McMurray Formation, Alberta, Canada

Sedimentological and ichnological descriptions of fluvio‐tidal translating point bars are rare, and complex physico‐chemical processes make highly detailed but concise facies descriptions challenging. Herein, mesofacies are defined to describe and interpret three ancient translating point bars from the Lower Cretaceous McMurray Formation, Alberta, Canada. Twenty‐three mesofacies are defined, based on their recurring sedimentological and ichnological characteristics. These mesofacies form the building blocks of beds and bedsets that make up three depositional facies. Facies 1 reflects sand dune migration at the channel base, which grades into inclined heterolithic stratification of Facies 2 and 3. Facies 2 occurs in the centre and seaward portions of the translating point bars and records tide‐dominated deposition of sand and muddy sand during periods of reduced river discharge. Ichnological suites and bioturbation intensities in these beds reflect persistent but variable brackish‐water conditions, fluctuating deposition rates and the deposition of mud. Mud beds are derived from flows with high suspended‐sediment concentrations. Tidally derived mud beds are typically bioturbated with trace fossil suites indicative of slow deposition rates and brackish‐water conditions. Mud deposited during elevated river discharge is burrowed after the dewatering of the bed. Facies 3 occurs at the landward apex of the translating point bar and is marked by sand‐rich and mud‐rich dune deposits with abundant soft‐sediment deformation, indicative of elevated flow velocities and deposition rates. Bioturbation is rare and sporadically distributed owing to unstable substrates. The distribution of the facies reflect the hydrodynamic variations that occurred vertically and laterally across the bar in response to temporal variations in fluvial and tidal flow interaction, as recorded by their mesofacies. The detailed facies analysis strongly suggests that deposition of the three McMurray Formation translating point bars occurred in proximity to the turbidity maximum zone of a fluvio‐tidal channel system.


INTRODUCTION
Translating point bars are marked by apex migration parallel to the channel belt axis (Fig. 1) (see Daniel, 1971).Seaward translation of point bars is common in modern fluvial, fluvio-tidal and tidalchannels (e.g.Ghinassi et al., 2018;Leuven et al., 2018;Fietz et al., 2021).Landward translation has been observed in tide-dominated environments (Ghinassi et al., 2021).The common identification of fluvio-tidal translating point bars in modern meandering channels contrasts with a rather sparse record in the literature of their sedimentological and ichnological descriptions (see Ghinassi et al., 2018;Fietz et al., 2021).The lack of detailed facies descriptions of fluvio-tidal translating point bars also reflects the notably few examples that have been identified from the rock record (see Hubbard et al., 2011;Musial et al., 2012;Ghinassi et al., 2021).
In this study, mesofacies are defined to provide highly detailed sedimentological and ichnological descriptions of ancient, seawardtranslating point-bar deposits from the Lower Cretaceous McMurray Formation, Alberta, Canada.The results are compared with sedimentological and ichnological descriptions of river-dominated and tide-dominated fluvio-tidal translating point bars.The aim of this paper is to establish facies criteria for the identification of the recurring depositional processes characterizing fluvio-tidal translating point bar deposits in the rock record.
Translating point bars are readily identified in plan-view by their concave-shaped scroll bar patterns, which contrast with the convex scroll-bar patterns typifying expansional point bars (Fig. 1) (Carey, 1969;Woodyer, 1975;Smith et al., 2009;Willis & Sech, 2018).In fluvial settings, translation is associated with flow impingement against erosion-resistant channel banks, leading to partial Fluvio-tidal translating point bars 975 flow deflection into an upstream circulating eddy current (Fig. 1) (Carey, 1969;Nanson & Page, 1983;Smith et al., 2009).The resulting sedimentological expression of fluvial translating point bars in mixed sand-transporting and mud-transporting systems, depends on the current strength of the upstream-directed eddy current.Weak eddy currents enable suspended-sediment settling and result in mud-dominated bar tail deposits, while strong eddy currents form bedload-transportdominated, coarse-grained successions (Fig. 1) (Lewin, 1978;Burge & Smith, 1999;Smith et al., 2011).Seaward-translating tide-dominated point bars are affected by bidirectional flow.Ebb-directed flow dominates sediment transport in the lower part of the channel and enforces seaward accretion of the bar (see Ghinassi et al., 2018;Fietz et al., 2021).Impingement of the ebb-directed flow against the erosion-resistant channel bank creates flow separation with eddy currents forming over the translating point bar.Suspended sediment is advected onto the translating point bar by these eddy currents, and may settle from suspension (see Ghinassi et al., 2018;Fietz et al., 2021).During flow reversal, flood-tidal currents impinge on the seaward side of the translating bars (see Dalrymple & Choi, 2007;Ghinassi et al., 2018;Sandbach et al., 2018;Fietz et al., 2021).The elevated flow velocities of the flood-tidal current permit bedload transport of sediment onto the seaward side of the translating bar (see Ghinassi et al., 2018;Fietz et al., 2021).Hydrodynamic processes forcing landward migration of translating point bars are broadly assigned to tidal-dominance on sediment transport (Ghinassi et al., 2021).Suspended sediment deposition is possibly dictated by flood-tidal flow and bedload sediment transport by ebb-directed currents.The resulting sedimentological and ichnological character of tide-dominated translating point bars contrasts markedly with that of fluvial translating point bars and is expressed by the complex lateral and vertical amalgamation of sand-rich and mud-rich successions of inclined heterolithic stratification (IHS; see Thomas et al., 1987) that display variable and overall higher bioturbation intensities (see Ghinassi et al., 2018Ghinassi et al., , 2021)).On a regional scale, the increasing impact of tides on river discharge in the seaward direction affects flow velocities, the frequency and magnitude of flow velocity fluctuations, flow direction, water levels, sediment supply and salinityinduced flocculation of fine-grained sediment (e.g.Thomas et al., 1987;Dalrymple & Choi, 2007;La Croix & Dashtgard, 2014;Braat et al., 2017;Sandbach et al., 2018).Resulting physicochemical stresses on the brackish-water infaunal community include elevated sedimentation rates, variations in substrate consistency, increased water turbidity, diluted marine nutrient supply, regular subaerial exposure, reduced and/or fluctuating salinity, and temperature variations (e.g.Doerjes & Howard, 1975;Reineck & Singh, 1980;Frey & Howard, 1986;Beynon et al., 1988;Ysebaert et al., 1998;Buatois et al., 2005;MacEachern & Bann, 2008;Gingras et al., 2011;Diez-Canseco et al., 2015, 2016;La Croix et al., 2015).To explore how regional and local processes result in vertical and lateral sedimentological and ichnological variations in fluvio-tidal translating point bars, detailed descriptions at the lamina, laminaset, bed and bedset scales are required.Refining the interpretation of translating point bar deposits will aid in accurately placing them along the fluvio-tidal transition zone, improve the reconstruction of palaeoenvironments, increase the reliability of subsurface identification and modelling, and assist in predicting fluvio-tidal architectures (as well as their reservoir quality).

TRANSLATING POINT BAR TERMINOLOGY
The first description of translating point bars was published by Carey (1969), who applied the term 'eddy accretions' to fine-grained point bar deposits with concave scroll-bar patterns in planview.'Eddy accretion' was meant to highlight the formation by upstream-directed eddy currents on the upstream side of the channel bank (Carey, 1969).Subsequent studies of river-generated, fine-grained translating point-bar deposits by Woodyer (1975), Hickin (1979), Woodyer et al. (1979) and Nanson & Page (1983) popularized the term 'concave-bank benches', which was meant to highlight the concave scroll-bar pattern and bench-like cross-sectional morphology of several studied translating point bars.To facilitate contrasting coarse-grained translating point bars with arcuate scroll-bar morphology, Burge & Smith (1999) used the term 'eddy accretion deposit'.Although similar to the initial 'eddy accretion' terminology defined by Carey (1969), 'eddy accretion deposit' is now widely accepted to refer to translating point bar deposits formed by strong eddy currents (see Smith et al., 2009Smith et al., , 2011;;Ghinassi et al., 2016;Willis & Sech, 2018).The grain-size variations and deviations from benchlike morphologies in translating point bars resulted in the introduction of the overarching term 'counterpoint bar' by Lewin (1983), a synonym to the 'translating point bar' defined by Daniel (1971).In more recent work 'counterpoint bar' has been applied to fine-grained translating point-bar deposits, and ultimately has evolved into a term specifically employed for fine-grained deposits at the bar tail of translating point bars (Makaske & Weerts, 2005;Smith et al., 2009Smith et al., , 2011;;Durkin et al., 2020).
Criticism of the translating point bar nomenclature has mostly been directed at the wording, rather than the loose application and redefinition of existing terms.The fine-grained 'counterpoint bar' is argued to imply the development of a bar opposite to and not a part of the associated point bar (Willis & Sech, 2018).Supporters of the 'concave-bank bench' are criticized for its wordiness and the difficulty of its conceptualization (Makaske & Weerts, 2005;Smith et al., 2009).In this study, the term 'translating point bar' is employed after Daniel (1971), which refers to the downstream migration of the bar, and is not biased towards the depositional environment (for example, fluvial, fluvio-tidal and tidal), is independent of the bar's sedimentological expression, and highlights the bar as a single geomorphological unit rather than enforcing an upstream-downstream differentiation of the bar or implying specific morphological characteristics.

GEOLOGICAL OVERVIEW OF THE MCMURRAY FORMATION
The Lower Cretaceous (Aptian) McMurray Formation of north-eastern Alberta, Canada was deposited as the earliest infill of a roughly northsouth-oriented valley system excavated into predominantly Devonian-aged carbonate rocks (Badgley, 1952;Carrigy, 1959;Mossop & Flach, 1983;Ranger & Pemberton, 1997;Hein et al., 2013).Deposition of the McMurray Formation took place in a nearshore environment with elements of fluvial, fluvio-tidal and estuarine channel deposition, intercalated with bay-margin shoreline and delta successions (Fig. 2) (e.g.Pemberton et al., 1982;Ranger & Pemberton, 1997;Gingras et al., 2016;Hayes et al., 2018;Château et al., 2019).Recent workers have challenged the brackish-water to estuarine interpretation of the channel deposits by focusing on the width-todepth ratios of channel forms and meander belts, point bar architectures and inferred channel belt avulsion characteristics, which some consider to be Fluvio-tidal translating point bars 977 unique to fluvial environments (e.g.Hubbard et al., 2011;Musial et al., 2012;Durkin et al., 2017;Horner et al., 2019).
Most of the McMurray Formation consists of heterolithic bedsets, interpreted to record sandrich to mud-rich inclined heterolithic stratification (IHS; see Thomas et al., 1987) of point bars (e.g.Ranger & Pemberton, 1992;Hein et al., 2013;Durkin et al., 2017;Hayes et al., 2018;Horner et al., 2019).The majority the IHS bedsets dip towards the northern palaeo-basin, which is interpreted as down-valley translation of point bars (Fustic et al., 2012).Previous sedimentological and ichnological assessments of the translating point bars in the study area have concluded a river-dominated depositional setting, predominantly based on lithological similarities to fluvial translating point bar facies (see Hubbard et al., 2011;Smith et al., 2011).However, recent advances in understanding facies variations in fluvio-tidal and tidal translating point bars permit a revised interpretation of these ancient translating point bar deposits.
In fully non-cohesive sediment, bedform growth is linked to the flow directionality, grain size and velocity of the flow (e.g.Guy et al., 1966;Allen, 1982;Saunderson & Lockett, 1983;Bridge & Best, 1988;Southard et al., 1990;Kleinhans, 2005).Bedform diagrams were developed for cohesive sediment in both unidirectional currents (e.g.Rubin & McCulloch, 1980) and oscillatory flow (e.g.Perillo et al., 2014aPerillo et al., , 2014b)).Based on grain size and flow strength, bedforms are generally subdivided into lower and upper flow regime.Sedimentary structures produced by quasi-steady currents in the lower flow regime comprise asymmetrical ripple lamination, lower planar parallel lamination and dune-scale cross-stratification. Unidirectional currents in the upper flow regime generate horizontal planar parallel lamination of the upperstage plane bed and cross-stratification of antidunes (e.g.Rubin & McCulloch, 1980;Southard & Boguchwal, 1990).Oscillatorygenerated orbital flow in the lower flow regime is expressed by symmetrical ripple lamination (e.g.Kleinhans, 2005;Cummings et al., 2009;Perillo et al., 2014aPerillo et al., , 2014b)).The upper flow regime of oscillatory orbital flow generates planar parallel lamination of the upper-stage plane bed, and curvilinear parallel stratification of hummocks (e.g.Southard et al., 1990;Kleinhans, 2005;Cummings et al., 2009).
In flume experiments, increasing suspended sediment concentrations alter near-bed fluid dynamics, resulting in a variety of complex bedforms (e.g.Baas et al., 2011Baas et al., , 2015)).With increasing cohesive sediment concentrations, flows change progressively from turbulent flow (TF) to turbulent-enhanced transitional flow (TETF), lower transitional plug flow (LTPF), upper transitional plug flow (UTPF) and quasi-laminar plug flow (QLPF) (Fig. 3) (see Baas et al., 2009).These flow conditions were interpreted to generate three distinct mudstone types (MacKay & Dalrymple, 2011).Cross-stratified mudstone was interpreted as being deposited under TF and TETF conditions, horizontally laminated mudstone deposition was ascribed to LTPF conditions, and unstratified mudstone beds formed under UTPF and QLPF conditions (MacKay & Dalrymple, 2011).Bedform development from decelerating mixed cohesive and non-cohesive sediment-bearing flows vary as a function of flow deceleration, initial suspended sediment concentration, settling velocity of the sediment fractions and duration of bedform migration (Baas et al., 2015).Flume studies with decelerating mixed sand-laden and mud-laden flows show that ripple lamination with incorporated mud particles in the bedform occurs in the lower flow regime of TF and TETF and LTPF (see Baas et al., 2011Baas et al., , 2015)).With increasing flow velocity, these current ripples pass into starved current ripples (washed-out ripples) and upper stage planar parallel lamination in turbulent and turbulent-enhanced flow (Baas et al., 2011(Baas et al., , 2015)).The LTPF is marked by current ripple lamination in the lower flow regime, and complex amalgamation of long, shallow bedwaves and scour surfaces in the upper flow regime (Baas et al., 2015).The UTPF and QLPF form mud beds with some sand grains dispersed (uniformly disseminated) in the mud (Baas et al., 2011).In the upper flow regime, UTPF develops low-amplitude bed waves, while mud deposits from QLPF do not respond to postdepositional flow velocity changes (Baas et al., 2015).The application of flume experiments to natural environments is limited by the large range of physical variables that control the dynamics of mixed sand-laden and mud-laden flows.Hydrodynamic complexities in fluviotidal channels include variations in tidal deceleration and acceleration, variations in fluvial discharge, and the relative interaction of tidal and fluvial flow (e.g.Dalrymple & Choi, 2007; Sisulak & Dashtgard, 2012;Dalrymple et al., 2015).Sediment volumes vary as a result of fluctuating turbidity levels and the shifting location of the turbidity maximum zone (e.g.Uncles et al., 2006;La Croix & Dashtgard, 2014).Additionally, clays in natural environments typically show a range of mineralogy and grain sizes that impact floccule size and its break-up resistance to turbulence (e.g.Winterwerp, 2002;Uncles et al., 2006;Mietta et al., 2009).Nonetheless, the laminasets of the McMurray Formation, which are observed in some complex bedforms and their stacking patterns, display characteristic sedimentary structures produced by decelerating, sediment-laden flows in flume experiments (Baas et al., 2009(Baas et al., , 2011(Baas et al., , 2015)).
The transport of mud-sized (silt and clay) grains can occur in suspension or as bedloadtransported flocs (Dyer, 1986;Schieber et al., 2007;Mehta et al., 2013).Sedimentary structures in mud deposited from suspension settling are characterized by planar parallel lamination and normal grading (see Sorby, 1908;Schieber, 2011).Bedload transported mud is associated with the formation of fine-grained sediment flocculates.Flocculation of fine-grained sediment is promoted by a variety of factors, including mineralogy of the mud particles, fine-grained sediment concentration in the water column, concomitantly transported organic material, pH, salinity and temperature of the fluid (Dyer, 1986;Mietta et al., 2009;Sutherland et al., 2014).In natural environments, large volumes of mud and bedload transport of mud are commonly found in the turbidity maximum zone (TMZ) of estuaries and deltas, wherein freshwater and saltwater mix (e.g.Allen et al., 1980;Mehta, 1991;Traykovski et al., 2000;Uncles et al., 2006;La Croix & Dashtgard, 2014).High concentrations of mud may form fluid mud, which is a near-bed, highdensity body of flocculated fine-grained sediment with suspended-sediment concentrations of >10 g/l (Krone, 1962;Mehta, 1991).Bedload transport of mud in turbulent flow generates current ripples and oscillatory ripples (Schieber et al., 2007;Plint, 2014;Wilson & Schieber, 2014;Shchepetkina et al., 2018).Suppression of turbulence by very high concentrations of mud (see Baas et al., 2009) results in planar to wavy parallel laminated mud.Wavy parallel laminated mud, sand and silt laminae are thought to be generated by Kelvin-Helmholz instabilities along the shear layer separating lower and upper flow regime in clay-rich flows (see Baas & Best, 2002;Plint, 2014).Dense mud beds can support the migration of silt and sand grains across their top surface (Baas et al., 2015).Collapse of the mud beds surface tension can cause silt and sand grains to become incorporated into the top of the mud bed, and may resemble inverse grading (see Baas et al., 2015).

DATASET AND METHODS
The previously published seismic time slice map from the Long Lake area depicts a horizontal transect through the McMurray Formation (Figs 2 and 3) (Smith et al., 2009;Hubbard et al., 2011).The seismic image was taken at a depth of 400 m, at 8 to 220 Hz bandwidth and has a resolution of better than 5 m (Hubbard et al., 2011).The interpreted palaeo-channels are 390 to 640 m wide and up to 36 m deep (Hubbard et al., 2011).Based on the observed scroll-bar-like morphologies, three translating point bar deposits were interpreted (see Smith et al., 2009;Hubbard et al., 2011).
Core data that support the findings of this study are openly available at the Core Research Centre in Calgary, Alberta, Canada.Eight wells from these three translating point bars were chosen for sedimentological and ichnological analysis in this study.For each translating point bar, two wells were selected that transect the seaward part of the bar.For one translating point bar, a single well from the centre and one well from the landward reach of the bar were employed (Fig. 4).The eight cores were logged with the software AppleCore 10 (Chestermere, Alberta, Canada; donated to SFU by Mike Ranger), and are described on a lamina to bedset scale, with a focus on lithology, texture, fabric, lamina, laminaset and bed thickness, bedding contacts, primary sedimentary structures, lithological accessories, Bioturbation Index (BI; see Reineck, 1963;as cited in table 13, p.160 of Reineck &Singh, 1980, andmodified by Taylor &Goldring, 1993), bioturbation distribution, and trace fossil suites.Laminae (<1 cm) and laminasets form mesofacies, which are herein defined as laminaset to bedset-scale sedimentary layers showing recurring lithological, sedimentological and ichnological characteristics.Heterolithic mesofacies consisting of alternating lithologies at the lamina and laminaset scales are herein referred to as 'heterolithic laminated mesofacies' to differentiate them from classical heterolithic bedsets (for example, wavy bedding, flaser bedding, etc.), which reside at a higher hierarchical Ó 2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022 level.Notably, the individual beds comprising a heterolithic bedset, such as wavy bedding or IHS, may consist of discrete heterolithic laminated mesofacies.Mesofacies make up individual beds (>1 cm; Fig. 5), which form the building blocks of bedsets.Sedimentologically and ichnologically similar bedsets are grouped into facies and are combined to form facies associations.Facies and facies associations are mapped in two crosssections oriented perpendicular to the channel axis and two cross-sections oriented parallel to the channel axis, using the top of the IHS deposits as the datum (Fig. 4).Gamma-ray logs were derived from the software GeoScout (geo-LOGIC, Calgary, Alberta, Canada).
Sand is held together by bitumen and therefore is effectively unconsolidated.Silt and clay have experienced sufficient compaction to form highly friable mudstone, but are referred to here as 'mud', for the sake of simplicity.
Mesofacies that are made up of predominantly sand (>95%) are described in Table 1.Mud-clast breccia and heterolithic cross-stratified mesofacies are detailed in Table 2. Heterolithic laminated mesofacies characterized by current ripple lamination are described in Table 3. Heterolithic laminated mesofacies dominated by horizontal to low-angle curvilinear to planar parallel lamination are defined in Table 4. Heterolithic mesofacies that were exposed to physico-chemical conditions favourable to bioturbation are listed in Table 5.
Sand beds comprise 50 to 90% of F1 and predominantly consist of trough and planar tabular cross-stratified or massive (apparently structureless) (M1) beds, with subordinate current ripple laminated layers (M2; Table 1) (Fig. 14A).In some intervals, cross-stratification displays a progressive increase and then decrease in foreset thicknesses (Fig. 16A), and a progressive increase and then decrease in mud drape abundances and thicknesses, both of which are expressed by combinations of M1, M5 and M6 (Tables 1 and 2).Locally, M5 and M6 pass into and out of wavy heterolithic lamination (M9; Table 3).No rhythmicity is apparent.Mudstone rip-up clasts range in size from pebbles to granules and are common in cross-stratified sand beds (M1).Current rippled sand layers commonly occur with mud drapes on foresets and toesets, and encompass the mesofacies M2, M8, M9, M18 and M19 (Tables 1, 3 and 5).Discrete mud beds are uncommon (typically <20%), but can comprise up to 50% of the facies and range in thickness from 3 to 30 cm.These mud beds constitute M12 (Table 3) and/or horizontal to low-angle curvilinear to planar parallel laminated mud and sand (M15-M17; Table 4) (Fig. 14A).Wavy and lenticular heterolithic laminated mud (M10 and M11; Table 3) occurs rarely.The basal contacts of mud beds are typically sharp, although may be gradational.
Clast-supported or matrix-supported mud-clast breccia (M4; Table 2) up to 3 m in thickness can comprise up to 50% of Facies 1, but generally constitutes less than 30%.The mud-clast breccia grades into and out of cross-stratified sandstone units (M1; Table 1).The matrix consists of wellsorted, fine to medium-grained sand.The clasts vary predominantly from mudstone to muddy siltstone, are angular to subrounded, and range from granule-sized chips to cobbles >7 cm in diameter (> core diameter).
Facies 2 interpretation.The sand to sandy mud beds (M1-M3 and M18-M22) of F2 are interpreted to be tidally derived, whereas mud bed deposition is interpreted to be associated with seasonally elevated river discharge (e.g.Shchepetkina et al., 2016;Melnyk & Gingras, 2020).Current ripple lamination and crossstratification in sand beds were formed under moderate current velocities in the lower flow regime, likely under transitional flow (TF) and turbulence-enhanced transitional flow (TETF) near-bed conditions (Fig. 3) (see Baas et al., 2009Baas et al., , 2011Baas et al., , 2015)).Homogeneously distributed bioturbation intensities and equilibrichnia of Cylindrichnus (Fig. 13B) in the sand to sandy mud beds indicate moderate and persistent rates of sedimentation (Figs 14A to D and 16C) (see MacEachern & Gingras, 2007;Sisulak & Dashtgard, 2012).The less commonly developed thick, cross-stratified and current rippled sand beds (>20 cm) with sparse to moderate bioturbation intensities (BI 0-3) (M1 and M2) indicate periods of elevated flow velocity and deposition rate.Decreasing sand bed thicknesses with increasing mud contents and bioturbation intensities (BI 3-5) are interpreted to record colonization at times of reduced sedimentation rates and flow velocities.
In F2A, sand beds with mud-draped current ripple crests, foresets and toesets (M18 and M19) are interpreted to indicate concomitant transport of sand and mud in the lower flow regime of a low to moderately turbid environment, likely under TF and lower transitional plug flow (LTPF) near-bed conditions (Fig. 3) (see Baas et al., 2009Baas et al., , 2011Baas et al., , 2015)).The variation in the abundance of mud laminae and vertical changes in their distribution within sand beds are interpreted to reflect tidally induced variations in flow velocity (for example, neapspring cycles; Fig. 16C) and the availability of low to moderate amounts of suspended sediment in the water column, which likely induced TF, TETF and LTPF near-bed conditions (Fig. 3).The concentration of mud-lined trace fossils (for example, Cylindrichnus) in proximity to abundant mud laminae and overlying mud beds (Fig. 16C and D) is interpreted Fluvio-tidal translating point bars 1005 to the trace makers' response to mud availability and their ability to mantle their burrows, perhaps by means of removing mud from the burrow entrance.By contrast, sand beds in F2B show only rare mud drapes (M18 and M19) and only rare occurrences of Cylindrichnus (Fig. 16).Instead, sand beds of F2B are dominated by fugichnia and Siphonichnus, which are interpreted to reflect generally higher flow velocities and/or reduced suspended-sediment concentrations in the water column compared to the sand beds of F2A.
Horizontal to low-angle curvilinear to planar parallel laminated and current ripple laminated mud beds (M12) reflect deposition of bedload transported fluid mud (see Schieber et al., 2007).High suspended-sediment concentrations and formation of fluid muds are common in fluvio-tidal channels during seasonally driven elevated fluvial discharge (Rodda, 1969;Sisulak & Dashtgard, 2012;Dalrymple et al., 2015).Mud beds that are unburrowed to weakly bioturbated (BI 0-2) are typical for F2A (Figs 14B to D and 16C) and are interpreted to reflect physico-chemical conditions that impede coloof the bed, such as rapid deposition rates, soupground substrate conditions, high turbidity in the water column and reduced salinity associated with elevated fluvial discharge.Mud beds that experienced moderate to intense (BI 3-4) top-down bioturbation with Planolites, Siphonichnus, Skolithos and Thalassinoides occur locally in F2B (Figs 15A to C and 16B) and are uncommon in F2A.Topdown bioturbation records physico-chemical conditions that post-date mud-bed deposition and its initial dewatering.Top-down bioturbation reflects the physico-chemical conditions that occurred during the temporal gap prior to deposition of the overlying bed (see MacEachern & Bann, 2008;Gingras & MacEachern, 2012), or may be related to deep-tier burrowing of organisms, and record the conditions that were prevalent during deposition of the overlying bed.
Bioturbated mud beds in IHS have been interpreted to record slow to moderate tide-dominated deposition of mud (see Melnyk & Gingras, 2020), which hosts marine nutrients and supports biogenic reworking.In F2B, bioturbated mud beds (M20-M22) (Fig. 16B) commonly display remnant intercalated sand and silt lenses and laminae (M20 and M21) (Fig. 16B), suggesting mud deposition from flows with moderate suspended sediment concentrations or flows of moderate to high velocity (for example, LTPF and UTPF near-bed conditions) (Fig. 3) (see Baas et al., 2009Baas et al., , 2011)).Elevated flow velocities that transport both sediment calibres impede the development of thick fluid mud and associated soupground substrates, and support bioturbation of muddominated beds (M20-M22).Moderately to pervasively bioturbated mud beds (BI 2-4) (M20-M22) in F2B are interpreted as either neap-spring induced mud laminae in sand beds, or deposition from fluvial discharge with elevated suspendedsediment concentrations that generate LTPF and UTPF near-bed conditions (Fig. 3).The interpretation of neap-spring-induced mud deposition is favoured where trace fossils are dominated by Cylindrichnus, Siphonichnus and Teichichnus, and is roughly equivalent to the neap-springinduced mud laminae in sand beds of F2A (Fig. 16B).
Cross-stratified beds (M1 and M5-M7; Tables 1  and 2) make up 40 to 80% of the facies, with bed thicknesses varying from 5 to 100 cm.Cross-stratified beds have a sharp depositional or erosional basal contact, and are made up of sand and mud (M5 and M6) with locally occurring sand-dominated cross-stratification (M1) and mud-dominated cross-stratification (M7).In some intervals, a progressive increase or decrease in mud content occurs (M5-M7) (Fig. 15E).In some intervals, cross-stratified beds alternate with flaser to wavy heterolithic lamination (M8 and M9; Table 3) and discrete mud beds (M12; Table 3) (Fig. 15D and E).Decreasing sand bed thicknesses and increasing mud contents are expressed by mud-dominated cross-stratification (M7), wavy or lenticular heterolithic lamination (M9 and M10), and planar parallel lamination of sand with mud (M14 and M15; Table 4) (Fig. 16G).The basal contacts of these heterolithic laminated sand beds are typically sharp and depositional or gradational.Gradational contacts of sand beds, with a progressive increase or decrease of content, can be observed in mud-dominated intervals and reach up to 30 cm in thickness (Fig. 15E).Sand beds alternate with mud beds that are typically 2 to 10 cm thick, but can reach up to 20 cm.Mud beds show horizontal or low-angle curvilinear to planar parallel lamination and current ripple cross-lamination (M11-M12; Table 3), lenticular heterolithic lamination (M10; Table 3), and planar parallel laminated mud with sand (M15-M17; Table 4).Mud beds typically have sharp depositional or erosional contacts (Fig. 16E).Gradational basal contacts are rare.Mud-clast breccia (M4; Table 2) occurs locally and may comprise up to 10% of the facies.The mud-clast breccia of F4 is sedimentologically similar to that described in F1 but generally forms thinner (2 to 20 cm) layers.Soft-sediment deformation is abundant at the lamina, laminaset, bed and bedset scales.
Facies 3 interpretation.F3 is interpreted to reflect rapid sediment deposition from mixed sand-laden and mud-laden flows, with moderate to high near-bed flow velocities and elevated suspended-sediment concentrations.
Crossstratified and heterolithic cross-stratified beds (M1 and M5-M7) are interpreted to record migrating dunes formed in the upper part of the lower flow regime.Variations in mud content in dunes is interpreted to reflect variations in flow energy and/or phases of increased suspended sediment, likely in turbulent flow (TF) and lower transitional plug flow (LTPF) near-bed conditions (Fig. 3) exposed to tidal modulation.Decreasing bed thicknesses of sand beds are interpreted to record decreasing sedimentation rates and possibly declining flow velocities.However, abundant low-angle planar parallel laminated beds suggest that elevated flow energies may also occur.Moderate to pervasive bioturbation associated with Gyrolithes, Siphonichnus, Planolites and Teichichnus is restricted to sandier beds in mud-dominated intervals (Fig. 15E), and supports the interpretation of brackish-water conditions during sand deposition.Mud beds were deposited during periods of reduced flow velocity or phases of increased suspended sediment load, likely under LTPF and UTPF near-bed conditions (Fig. 3), interpreted to reflect fluvial discharge.Gradational contacts from mud to sand beds (Fig. 15D) are interpreted to record decreasing suspended sediment concentrations in the water column as river discharge decreased (Fig. 15E) (see Baas et al., 2009;MacKay & Dalrymple, 2011).Abundant soft-sediment deformation (SSD) on lamina, laminaset, bed and bedset scales, in addition to local occurrences of mudclast breccia, may record recurring mass-wasting events, likely associated with erosion and rapid sedimentation.High sedimentation rates and unstable substrates tend to impede colonization and subsequent bioturbation of the sediment, leading to low bioturbation intensities.Rare beds with elevated bioturbation intensities are interpreted to record more stable substrate conditions.

Vertical and lateral facies trends
Facies 1 is well-developed at the bases of all cores (Fig. 17) and reaches up to 30 m in thickness.F1 grades into overlying IHS deposits made up of F2 and F3, which reach thicknesses of up to 35 m (Fig. 17).Complex amalgamation of F2 and F3 is common on a vertical and lateral scale.The IHS deposits in wells A, G and H are dominated by F2A.The IHS deposits in wells B, C and F are dominated by F2B.Well D is located at the landward meander apex of a translating point bar and is dominated by F3, which transitions into F2B at the top.F2B is present at the bases of the IHS deposits of all wells except for wells E and F.
All wells display a fining-upward trend.The IHS deposits in wells F, G and H are muddominated and this is expressed on gamma-ray logs by values greater than 75 API.Sandier IHS deposits occur in wells A, B, C, D and F and are reflected on gamma-ray logs by API values <75.The IHS successions display two to six stacked fining-upward cycles and rare coarseningupward cycles, each of which range in thickness from 5 to 20 m (Figs 17 and 18).These finingupward and coarsening-upward cycles are accretion packages.On the seismic time slice, these Fluvio-tidal translating point bars 1011 accretion packages mimic scroll-bar patterns (Fig. 4).The thickness of accretion packages (5 to 20 m) correlates well with the spacing of the mimicked scroll-bars (45 to 60 m) on the seismic time slice when applying reported slope angles of ca 10 to 20°in the fluvio-tidal point bars (Fig. 18) (see Pearson & Gingras, 2006;Fustic, 2007;Smith et al., 2009;Brekke et al., 2017;Ghinassi et al., 2018).In fluvial channels, large flood events, accompanied by erosion of the cutbank, have been observed to initiate the development of new scroll-bars (Ghinassi et al., 2019).In flume experiments, widening of a channel initiates sediment supply to the point bar (van de Lageweg et al., 2014).The sediment supply to the point bar immediately after widening of the channel is likely to decrease as a new equilibrium in the channel is achieved.As a result, grain sizes and bedset thicknesses decrease leading to the fining-upward cycles observed in the cores.

DISCUSSION
Sedimentological and ichnological variability in landward-migrating fluvio-tidal translating point bars Facies in the translating point bars of the Long Lake area demonstrate significant lateral and vertical variations, interpreted to be the result of spatial and temporal changes in flow velocity, sedimentation rates and suspended-sediment concentrations.Previous work generally subdivided inclined heterolithic stratification (IHS) deposits into fluvially dominated IHS positioned landward of the turbidity maximum zone (TMZ), IHS deposits positioned centrally in the turbidity maximum zone (TMZ) and tidally dominated IHS deposits positioned seaward of the TMZ (see Dalrymple & Choi, 2007;La Croix & Dashtgard, 2014;Melnyk & Gingras, 2020).Sedimentological and ichnological descriptions that are assigned to fluvially dominated IHS are roughly similar to those of F3.In fluvially dominated IHS, sand deposition is thought to occur rapidly during elevated river discharge, whereas deposits reflecting low river discharge are finer-grained (see Hubbard et al., 2011;La Croix et al., 2015;Jablonski & Dalrymple, 2016;Durkin et al., 2020).Ichnological suites in fluvially dominated IHS display largely unburrowed (BI 0-1) sand beds that otherwise contain low numbers of fugichnia and Skolithos (La Croix et al., 2015).The finer-grained deposits display slightly greater degrees of burrowing (BI 0-2), and feature traces such as Arenicolites, Planolites, Skolithos and Teichichnus (Sisulak & Dashtgard, 2012;La Croix et al., 2015).
The close association of F2A, F2B and F3 in translating point bars in the Long Lake area requires a depositional model that accounts for marked vertical and lateral IHS facies variations at the bar-scale (see Fietz et al., 2021).Hydrodynamically, fluvio-tidal channels are ebbdominated and experience the highest flow velocities during elevated periods of river discharge accompanying ebb flow (Pritchard & Hogg, 2003;Dalrymple & Choi, 2007;Ghinassi et al., 2018;Sandbach et al., 2018).Vertically, tidal impact on bar deposition increases from the base to the top of the bar (tidal flat) and is strongest during spring tide coupled with low river discharge (see Dalrymple & Choi, 2007;La Croix & Dashtgard, 2014;Ghinassi et al., 2018;Sandbach et al., 2018).Laterally, translating point bars experience strong flow velocities on the seaward bar tail, which decrease towards the centre of the bar and then increase into the landward meander apex (see Carey, 1969;Nanson & Page, 1983;Ghinassi et al., 2018;Fietz et al., 2021).This relative flow velocity pattern is maintained independent of fluvial or tidal dominance (e.g.Nanson & Page, 1983;Ghinassi et al., 2018).The seaward bar-tail deposits experience the strongest flow velocities during tidal dominance coupled with low river discharge (see Ghinassi et al., 2018;Fietz et al., 2021), while fluvial discharge leads to the advection of large quantities of suspended sediment onto the middle and upper bar (see Nanson & Page, 1983;Vietz et al., 2012;Ghinassi et al., 2018;Durkin et al., 2020;Fietz et al., 2021).The middle part of fluvio-tidal translating point bar facies successions is therefore thought to record seasonally alternating tidal and fluvial deposition, resulting in the interstratification of subfacies F2A and F2B.The elevated flow velocities at the meander apex during tidal and fluvial discharge leads to deposition of F3, which is marked by high deposition rates and unstable substrates.
Hydrodynamic changes in the channel system serve to influence bar-scale flow velocity variations.Changes in meander migration direction can occur by flow piracy, meander bend cut-off (see Ikeda et al., 1981;Howard & Knutson, 1984;Durkin et al., 2020) or after reorganization following large flood events (see Ghinassi et al., 2019).During large floods, erosion of the outer channel bank downstream of the bend apex takes place and results in readjustment of the point bar (Ghinassi et al., 2019).Upstream and downstream changes to the channel system potentially can affect meander apex migration direction, angle of flow impingement, and ultimately lead to stronger or weaker currents passing over the bar, generating facies variations (for example, F2A and F2B) within a single bar (see well A and well B; Figs 17 and 19).
In fluvial translating point bars, the angle of impingement is thought to determine the strength of the eddy current (see Smith et al., 2009;Durkin et al., 2020).At steep and intermediate angles of impingement, lower to middle point bar facies successions are dominated by bedload sediment transport, with mud-dominated sediment largely confined to the middle and upper parts of the bar (Taylor & Woodyer, 1977;Nanson & Page, 1983).By contrast, shallow angles of impingement result in mud-rich deposits from the lower through to the upper translating point bar (Smith et al., 2009;Durkin et al., 2020).The angle of impingement is also likely to influence deposition in the fluvio-tidal translating point bars.A gentle angle of impingement may cause reduced thicknesses of F1, and more fine-grained expressions of F2 to develop in the centre and seaward portions of the bar (for example, Fig. 17).By contrast, a steeper angle may be reflected in a relatively thicker interval of F1 overlain by coarser endmembers of F2 in the centre and seaward portions of the bar (Fig. 17).Steepening and shallowing of the angle may occur during the growth of the bar and cause variations in facies expressions as observed in well A and well B (see Figs 17 and 19).

Facies comparison of river-dominated, tidedominated and fluvio-tidal translating point bars
The translating point bars in the Long Lake area form upstream of wide meander bends (Fig. 4).In fluvial environments, translating point bars of wide meander bends produce mud-dominated successions at the bar tail, which contrasts with the coarser-grained upstream part of the bar (Fig. 1) (Page & Nanson, 1982;Hickin, 1986;Makaske & Weerts, 2005;Smith et al., 2009).Vertical fining-upward trends are observed in fluvial translating point bars, although they can be very subtle in mud-dominated successions and interrupted by coarser-grained crevasse splay deposits in the upper part of the succession (Hickin, 1986;Smith et al., 2009;Durkin et al., 2020).The mud beds at the bar tail are typically horizontally laminated and record deposition from suspension owing to weak eddy currents.Uncommon beds of massive and current rippled fine-grained sand are deposited during elevated river discharge events (Hickin, 1986;Vietz et al., 2006Vietz et al., , 2012;;Durkin et al., 2020).Infaunal bioturbation in modern fluvial and river-dominated translating point bar deposits has not been reported.
Tide-dominated, seaward-migrating translating point bars are marked by sand-rich and mudrich IHS (Ghinassi et al., 2018).The vertical grain-size trends are largely expressed by finingupward profiles in the subtidal bar, and coarsening-upward successions in the intertidal zone (Ghinassi et al., 2018;Fietz et al., 2021) Fluvio-tidal translating point bars 1013 direction, and a coarsening trend in the intertidal bar towards flood direction, but with a fine-grained meander apex (see Ghinassi et al., 2018;Fietz et al., 2021).Bioturbation ranges from moderate intensities to pervasive, and locally preserved sedimentary structures in sand beds display dune-scale cross-stratification and current ripple lamination.The trace assemblage comprises diminutive, simple structures such as Gyrolithes, Polykladichnus, Siphonichnus and Skolithos constructed by opportunistic species (see Fietz et al., 2021).
In the McMurray Formation of the Long Lake area, the IHS deposits in the translating point bars are dominated by fining-upward successions and no distinct coarsening-upward trends are observed (Figs 17 and 18).Contrary to riverdominated and tide-dominated translating point bars, lateral grain-size trends are not observed in the studied translating point bars.Wells D, E, G and H at the ebb-directed part of the bar display mud-dominated IHS, whereas wells A, B, C and D contain both sand-dominated and muddominated IHS (Fig. 17).Bioturbation intensities in the middle and upper successions of the translating point bar vary from moderate to pervasive (BI 3-5), but display a general absence at the meander apex (for example, well D) (Figs 18 and 20).The trace fossil suites are dominated by Cylindrichnus, Gyrolithes, Planolites, Siphonichnus and Skolithos, with subordinate Teichichnus and rare Thalassinoides.The studied translating point bar is similar to published tide-dominated translating point bars with respect to its IHS deposition as well as its neoichnological suite (see Ghinassi et al., 2018;Fietz et al., 2021).The large quantities of mud required to produce the interpreted high suspended-sediment-laden flows suggests deposition in a more muddominated fluvio-tidal system and/or the bars' position in close proximity to the TMZ during fluvial discharge compared with those of previously published tide-dominated translating point bars (see Ghinassi et al., 2018Ghinassi et al., , 2021;;Fietz et al., 2021).

Dynamic and stationary mud
The interpreted proximity of the turbidity maximum zone of the studied translating point bars may suggest regular salinity fluctuations induced tidally or by seasonal variations in fluvial discharge strength.Primary sedimentary features that are typically assigned to salinity fluctuations are synaeresis cracks, of which the mud beds in the studied IHS deposits are notably devoid.The formation of synaeresis cracks is a function of clay composition, sedimentation rates and water chemistry (e.g.Juengst, 1934;Burst, 1965).Rapid deposition of flocculated clay in saline environments causes synaeresis cracks to form as a result of dewatering (see Juengst, 1934).In muds that contain >2% swelling clay and are deposited from freshwater, shrinkage of the crystal lattice is thought to cause synaeresis cracks as salinity increases in the surrounding water (Burst, 1965).
In fluvio-tidal channels, brackish-water conditions may not provide sufficient salinity contrast to develop synaeresis cracks.In deltaic settings, a notable discrepancy exists in the welldeveloped synaeresis cracks in river-dominated deltaic deposits, compared to the rare occurrence of synaeresis cracks in tide-dominated and wave-dominated deltaic deposits (see MacEachern et al., 2005;MacEachern & Bann, 2008).Tidal-action and wave-action may mix freshwater with marine water and reduce salinity contrasts, but also rework fluid mud.
In fluvio-tidal channels, tidal flux induces bedload transport and repeated partial resuspension of fluid mud (see Manning & Bass, 2006;van Leussen, 2011;Winterwerp et al., 2017).Tidal reworking inhibits abrupt deposition of large quantities of mud and may reduce the formation of synaeresis cracks.Additionally, tidal reworking of fluid muds prior to deposition may dilute the freshwater in the fluid mud with the more saline surrounding water.As a result, the contrast of pore water salinity in the fluid mud may not suffice to develop abundant and pronounced synaeresis cracks.Fluid mud that settled rapidly from suspension and did not experience much subsequent transport may favour synaeresis crack development.
Mud deposits may therefore be subdivided into dynamic mud and stationary mud.Fluvio-tidal translating point bars 1015 pore water salinities that may favour the formation of synaeresis cracks.However, more research is needed with respect to synaeresis crack formation and fluid mud deposition to strengthen the subdivision of dynamically and stationary deposited fluid mud.

Palaeodepositional controversy of the McMurray Formation
The palaeodepositional environment in which the bulk of the point bar deposits of the McMurray Formation accumulated is alternatively regarded to have occurred either predominantly in the river-dominated reaches (i.e.uppermost backwater zone) or in the brackish-water reaches of the fluvio-tidal transition zone.The river-dominated depositional interpretation is supported by the scale of channel forms and channel belts, as well as the large-scale point bar architecture, which is consistent with those found in modern and ancient fluvial systems (Hubbard et al., 2011;Musial et al., 2012;Durkin et al., 2017;Horner et al., 2019).However, the sedimentological, ichnological and palynological data of the point bars in the McMurray Formation are consistent with those found in brackish-water reaches of modern fluvio-tidal systems (Pemberton et al., 1982;Frey & Howard, 1986;Gingras et al., 1999Gingras et al., , 2011Gingras et al., , 2016;;Buatois et al., 2005;MacEachern & Gingras, 2007;Hauck et al., 2009;Sisulak & Dashtgard, 2012;Dolby et al., 2013;Czarnecki et al., 2014;Diez-Canseco et al., 2015, 2016;Fietz et al., 2021;this study).
In an attempt to reconcile the contrasting fluvial and fluvio-tidal palaeodepositional interpretations of the McMurray Formation, Broughton (2018) suggested that dissolution of underlying salt deposits (for example, Prairie Evaporite Formation) allowed saline brines to leak into the continental drainage system of the McMurray basin.This leakage of brine into the fluvial system is envisaged to have caused elevated salinities (brackish-water) that permitted marine organisms to colonize the sediment substrate (Broughton, 2018).However, the volume of subsurface salt would have been vastly insufficient to permit persistent salinity in a large-scale continental drainage system, as has been inferred for the McMurray Formation (Blum & Pecha, 2014).Further, increasing the salinity in a continental river system does not mimic a marine habitat.Brackish-water trace fossil assemblages are made by a depauperate marine benthic community (Pemberton et al., 1982;Buatois et al., 2005;MacEachern & Gingras, 2007;Diez-Canseco et al., 2015).Freshwater organisms have been shown to be sensitive to brackish-water salinities (Remane & Schlieper, 1971), and have yet to show marine behaviour when exposed to saline water.To introduce and maintain marine organisms, connectivity to marine water by tidal currents would have been necessary to enable a migration path for marine larvae and marine nutrients.The woefully insufficient volume of salt and the biological constraints on marine organisms do not permit the theory proposed by Broughton (2018) to be viable.

Implications for palaeo-depositional interpretations of IHS deposits
The IHS in the middle to upper part of the translating point bar succession is interpreted to reflect sand transport by tides during periods of low river discharge and the settling of suspended sediment during periods of elevated river discharge as well as from tides.Seaward sides of expansional point bars and in-channel bars of comparable depositional settings are also expected to experience sand deposition during periods of low fluvial discharge (see Sandbach et al., 2018).However, neither expansional point bars nor in-channel bars are likely to capture suspended sediment as efficiently as do the translating point bars (see Hickin, 1986;Smith et al., 2009).Facies in the translating point bars in the distributary channels of wave-dominated and river-dominated deltas may show subtle differences from the fluvio-tidal translating point bars of estuaries.
In river-dominated deltas, which experience periods of tidal influence during reduced river discharge and a TMZ that forms landward of the coast, distributary channels experience generally low salinities, marked fluctuations of salinity, higher deposition rates of mud, as well as sporadically distributed and generally reduced bioturbation intensities (see MacEachern et al., 2005;Bhattacharya, 2006;La Croix & Dashtgard, 2014).However, low river discharge may also be accompanied by less suspended sediment in the river (e.g.Sisulak & Dashtgard, 2012)  of bedload mud by longshore drift, and reduced salinity-induced stresses on infauna (see Mac-Eachern et al., 2005;Bhattacharya, 2006;Mac-Eachern & Bann, 2020).However, wave propagation into the narrow channels is unlikely (see Dalrymple & Choi, 2007), and translating point bar expressions may be similar to fluvial translating point bars.With careful analysis of mesofacies and the integration of sedimentological and ichnological characteristics, physicochemical stresses acting on an environment can be assessed in high levels of detail, and refinement of depositional interpretations can be achieved.Many more case studies of modern and ancient fluvio-tidal translating point bars are required to test and refine facies trends in translating point bars across the fluvio-tidal transition zone.In combination with insights from field data, flume experiments and numerical modelling, a robust process-response based model and improved facies predictability in fluvio-tidal translating point bars can be developed (see Yan et al., 2017;Leuven et al., 2018;Parquer et al., 2020).

CONCLUSIONS
Mesofacies analysis of sedimentary and biogenic structures at the lamina-scale enable the assessment of processes and animal-sediment responses that act during their deposition.A total of 23 recurring mesofacies are identified, with each defining a unique set of physico-chemical conditions that prevailed during deposition.Sedimentological delineation of mesofacies is based on insights from flume experiments on non-cohesive, mixed sand-mud and cohesive sediment laden flows (e.g.Southard & Boguchwal, 1990;Baas & Best, 2002;Schieber et al., 2007;Baas et al., 2015).Ichnological assessment of the mesofacies is applied to refine the palaeo-depositional interpretation.The 23 mesofacies are organized into five mesofacies groups based on their sedimentological and ichnological characteristics.Sanddominated mesofacies (>95%) are listed in Table 1 and heterolithic laminated mesofacies are found in Tables 2 to 5. Heterolithic laminated mesofacies are subdivided based on crossstratification (Table 2), current-ripple lamination (Table 3) and horizontal to low-angle curvilinear to planar parallel lamination (Table 4).Table 5 comprises mesofacies that express physicochemical conditions favourable for bioturbation.
Mesofacies analysis permits the description and process interpretations of beds, bedsets and facies in the studied translating point bar deposits of the Long Lake area, Alberta, Canada.Three distinct facies expressions (Facies 1 to Facies 3 -F1 to F3) are identified, and include channel deposits (F1), centre to seaward inclined heterolithic stratification (IHS) (F2) and landward IHS (F3).Channels and the lower parts of translating point bars (F1) are marked by high flow velocities and are dominated by the deposits of tidally influenced migrating sand dunes with current ripples forming in the toesets, and mud-rich interdune deposits.The overlying IHS in the centre and seaward positions of the translating point bar (F2) are marked by tidally derived sand and muddy sand beds alternating with tidally derived mud beds and fluvially deposited mud beds.Tidally derived mud beds are dominated by trace fossils indicative of brackish-water conditions, such as Cylindrichnus, Gyrolithes, Planolites, Siphonichnus, Skolithos and Teichichnus.Mud beds that were deposited rapidly from flow with very high suspended sediment concentrations are assigned to elevated fluvial discharge.Tidally derived sand and muddy sand bed deposition is consistent with previous studies that suggested that the strongest currents in the centre and to seaward translating point bars occur during tidal flood flow combined with low river discharge (see Ghinassi et al., 2018;Fietz et al., 2021).Weak tidal currents (for example, neap-tide) may enable the deposition of mud laminae and mud beds.During strong fluvial discharge, high suspended sediment concentrations in the water column are advected by weak eddy-currents onto the bar and deposited as fluid mud beds.The landward part of the translating point bar (F3) is affected by high flow velocities; elevated deposition rates and availability of suspended sediment in F3 are expressed by sand to muddominated trough and planar-cross stratification, and abundant soft-sediment deformation.The resulting physico-chemical conditions are unfavourable for colonization and bioturbation is generally absent.
Despite distinctly different facies expressions found in the studied translating point bars, they corroborate the contention that deposition of the studied fluvio-tidal translating point bars occurred in proximity to the turbidity maximum zone.However, additional descriptions of modern and ancient translating point bar deposits are required to understand the complex physico- Fluvio-tidal translating point bars 1017 chemical processes that affect fluvio-tidal deposition.The mesofacies analysis employed in this study provides a useful tool in which sedimentological and ichnological observations are described, and process-oriented interpretations are provided.

Fig. 1 .
Fig. 1.Schematic representation of scroll-bar patterns and lithological expressions of translating point bars (TPBs) and expansional point bars in fluvial meandering channels.(A) Plan view that highlights the concave scroll-bar morphology of TPBs, which contrasts with convex-shaped scroll-bar morphologies of expansional point bars.Note the subdivision of seaward, centre and landward parts of translating and expansional point bars.(B) Schematic lithologs of expansional point bars, the bar-tail end of a low-energy TPB, and a high-energy TPB.Grain-sizes are indicated for: mud (z); very fine-grained sand (v); fine-grained sand (f); medium-grained sand (m); and coarsegrained sand (c).Modified after Smith et al. (2009, 2011), Durkin et al. (2020) and Fietz et al. (2021).Ó 2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

Fig. 2 .
Fig. 2. The geographic location of the study area and the geological context of the McMurray Formation.(A) The geographic position of Alberta and Saskatchewan within Canada.(B) The position of the major oil sands deposits and location of the study area.(C) The most recent sequence stratigraphic framework for the McMurray Formation and its relative correlation to the informally designated lower, middle and upper McMurray Formation.Parasequences (LM1, Regional C, B2, B1, A2 and A1) are locally transected by laterally migrating fluvial and fluvio-tidal channels (REG C, B2C, B1C, A2C and A1C).Modified after Château et al. (2021) and Fietz et al. (2023).Ó 2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

Fig. 3 .
Fig. 3. Schematic models for the five flow types referred to as turbulent, transitional and quasi-laminar clay flows over a smooth, flat bed.The vertical profile of the characteristic velocity time series is displayed for the dimensionless downstream flow velocity (U/U max ), root mean square of downstream flow velocity (RMS(u 0 )) and dimensionless turbulence intensity (RMS (u 0 ) 0 ).Modified after Baas et al. (2009).(B) Proposed bedform phase diagram for rapidly decelerated cohesive sand-mud flows.Modified after Baas et al. (2015).(C) Schematic drawing illustrating backflow ripple formation.Modified after Baas et al. (2011).

Fig. 4 .
Fig. 4. Seismic time slice displaying three translating point bars (dashed lines) and the wells that were analysed (wells A to H) in this study.Modified after Smith et al. (2009) and Hubbard et al. (2011).

Fig. 5 .
Fig. 5. Schematic representation of the building blocks of inclined heterolithic stratification (IHS) in fluvio-tidal point bars.Accretion packages consist of metre-scale stacked bedsets, with each bedset consisting of centimetre to decimetre-scale sand and mud beds.Sand beds and mud beds can be made up of one or more mesofacies, with each mesofacies defined by its specific sedimentological and ichnological characteristics expressed at the millimetre to centimetre lamina and laminaset scales.

Fluvio-tidal translating
point bars 995 some interdune deposits (M18 and M19) reflect intervals of reduced sedimentation.The predominantly unburrowed mud beds are interpreted to record deposition of fluid mud in interdune areas that were sheltered from stronger flow and resuspension.Fluid mud is difficult for infauna to colonize owing to its rapid deposition and generation of soupground substrates.Burrowing during fluid mud deposition was generally absent with the exception of navichnia (sediment-swimming structures) (e.g.Lobza & Schieber, 1999;Gingras et al., 2007;MacEachern & Bann, 2020).Top-down burrowing is interpreted to have taken place after dewatering of the mud bed(MacEachern & Bann, 2008;Gingras & MacEachern, 2012).Parallel laminated mud and sand (M15-M17) together are interpreted to record concomitant transport of both sediment calibres in an elevated flow regime, likely under upper LTPF and upper transitional plug flow (UTPF) conditions (Fig. 3), especially in close association with traction transported fluid mud.Strongly fluctuating flow regimes that would have ranged from upper flow regime (to form parallel laminated sand) to very low flow velocities (required to deposit mud from suspension) appear unlikely but may have taken place in response to tidally induced flow velocity fluctuations.Mud-clast breccia (M4) is interpreted to have been derived from channel-bank collapse during channel migration.The sand matrix is lithologically similar to the overlying and underlying sand beds, and the mud-clasts are of the same composition as the mud layers found in associated point bar successions of F2 and F3.The poor sorting and angularity of the mud-clasts support local derivation and short transport distances.Bioturbation intensity of the mud-clasts reflects the physico-chemical conditions prior to their incorporation into the mud-clast breccia.The rare occurrence of moderately to intensely bioturbated (BI 3-5) mud clasts may result from their reduced preservation potential (i.e.low resistance to reworking and faster disintegration) and/or the derivation of most clasts from mud layers that generally lack bioturbation (for example, lower point bar facies).

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2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

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2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

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2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

ÓFluvioFig. 17 .Fig. 18 .
Fig. 17.Stratigraphic cross-sections through the three studied translating point bars, using the top of the bar as the datum.Cross-section (A) and cross-section (B) are oriented along depositional strike, whereas cross-section (C) is oriented down depositional dip.Locations of the cross-sections are shown in Figs 3 and 12.The vertical depth is in metres.
. Similar to fluvial translating point bars, a lateral grain-size fining trend is observed in the ebb-Ó 2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

19.
Line drawing of interpreted point bars and channels displayed on the seismic time slice map of Fig. 3. Migration trajectories of accretion packages are indicated with arrows.Dashed red line outlines the translating point bars intersected by wells employed in this study.Note that the migration direction of the accretion package in well A is oriented north-east, whereas the accretion package in well B indicates migration towards the north.Modified after Hubbard et al. (2011).

Fig. 20 .
Fig. 20.Schematic plan-view and lithological expression of fluvio-tidal, seaward-migrating translating point bars (TPBs).(A) Facies distribution observed at the studied translating point bar deposits and hydrodynamic setting during ebb-tidal flow and flood-tidal flow.(B) Schematic lithologs from the central part and seaward-located part of the fluvio-tidal translating point bar (TPB), and the landward-located fluvio-tidal TPB.Grain-sizes are indicated for: mud (z); very fine-grained sand (v); fine-grained sand (f); medium-grained sand (m); and coarse-grained sand (c).Note the differences in heterolithic bedding style, bioturbation pattern, trace fossil suite and abundance of soft-sediment deformation.F1 is present at the base of the TPB.F2 is well-developed in the middle part of the seaward and centre TPB, whereas F3 dominates the middle part of the landward TPB.F2 is present at the top of the seaward, centre and landward TPB.
and result in more sand-dominated facies than is typical in fluvio-tidal translating point bars.Distributary channels of wave-dominated deltas record the mixing of fluvial and marine water by wave action, resulting in slower deposition rates of flocculated mud, reworking and transport Ó 2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022

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2023 The Authors.Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists, Sedimentology, 71, 974-1022