Detailed facies analysis of Cenomanian–Turonian organic‐rich mudstones: Implications for depositional controls on source rocks

Understanding mudstone depositional processes, including both traction transport and dynamic mechanisms, requires a re‐assessment of how these processes relate to the accumulation and preservation of organic carbon in fine‐grained successions. This relationship was addressed using facies described in detail from a Cenomanian–Turonian aged, organic‐rich, marine mudstone‐dominated succession from the Albertan Western Interior Seaway. All facies exhibit a high degree of heterogeneity at the millimetre‐to‐decimetre scale, contain evidence of bioturbation and display similar ranges of sedimentary structures including wave ripples and current ripples, sharp‐based graded beds and starved ripples. Described facies vary in grain size as well as relative abundances of biogenic versus siliciclastic grains. Siliciclastic grains are dominantly composed of quartz silt as well as silt‐sized to sand‐sized clay mineral–rich aggregates while the biogenic grains are dominantly composed of silt‐sized to sand‐sized calcareous pellets, bivalve fragments and foraminifera. Although the succession is relatively clay mineral‐rich, the majority of clay minerals occur as mud aggregates that were transported by traction processes. The sedimentary structures present are diagnostic of sea floor reworking by waves and currents, indicating that the basin was relatively shallow (i.e. not hundreds of metres deep) with frequent reworking of the sea floor sediment with suspension settling deposits being rare. The abundance of bioturbation and in situ bivalves indicates that there was sufficient oxygen at the sea floor and that widespread bottom water anoxia was not persistent at the time of deposition of the organic‐rich interval of interest. The accumulation of organic‐rich sediment appears to be a preservational phenomenon caused by sediment being buried more rapidly than the organic carbon can oxidize in the surficial layers due to episodic inputs of siliciclastic material during large storm events.


| INTRODUCTION
The controls on source rock deposition have been a topic of debate amongst geologists for decades. Past literature placed a strong emphasis on the requirement of bottom water anoxia for the preservation of organic matter in marine sediments (Schlanger and Jenkyns, 1976;Fischer and Arthur, 1977;Thiede and van Andel, 1977;Arthur and Schlanger, 1979;Damison and Moore, 1980); or an emphasis on the relationship between settling flux of organic carbon-rich material to the sea floor with increased organic matter preservation (Mϋller and Suess, 1979;Pedersen, 1983;Pedersen and Calvert, 1990;Creaney and Passey, 1993). These past studies were completed with the assumption that fine-grained sediments were homogenous, suspension settling deposits (Potter et al., 1980); however, there has been significant advancement in the understanding of mudstone depositional processes in the past decade. Many laboratory experiments and sedimentological studies of ancient and modern mudstone successions have shown that mud can be transported as bedload at current velocities that are sufficient to transport and deposit sand (Macquaker and Bohacs, 2007;Schieber et al., 2007;Schieber and Southard, 2009;Schieber and Yawar, 2009;Macquaker et al., 2010aMacquaker et al., , 2010bSchieber et al., 2010;Plint et al., 2012a;Denommee et al., 2016). These studies demonstrate that fine-grained sediments can be deposited by a wide variety of depositional processes and have implications for our understanding of source rock deposition. Bohacs et al. (2005) addresses the three main variables that have complex interactions to control the accumulation of organic matter: production, destruction and dilution. Production is a function of nutrient supply; destruction is a function of how long the organic carbon is exposed to oxidants, which in turn are controlled by production and burial rates; and, finally, dilution is a function of clastic sediment input and biogenic silica/carbonate input (Bohacs et al., 2005). This demonstrates that organic-rich deposits can occur through a combination of these factors and recent research has shown that organic-rich mudstones can be deposited under a variety of conditions. These include, but are not limited to: oxic-dysoxic bottom waters in relatively high-energy environments with high productivity controlling organic carbon preservation (Macquaker et al., 2010b); in anoxic, distal environments through suspension settling (Knapp et al., 2016); and under periods of intermittent anoxia in the pore waters due to rapid burial (Bohacs et al., 2005).
The controls on source rock deposition are complicated and this study aims to characterize the processes responsible for organic carbon enrichment in a Cenomanian-Turonian aged, organic-rich succession from the Canadian portion of the Western Interior Seaway.
This will be accomplished through detailed facies analysis to determine the depositional controls on the accumulation and preservation of organic-rich mudstones in a mixed siliciclastic-calcareous system of the Belle Fourche and Second White Specks (2WS) formations. These organic-rich mudstones are a world class natural laboratory due to the rich, diverse dataset of abundant cores, +100,000 wells with well logs and well-exposed outcrops in the Rocky Mountain Foothills. Most mudstone sedimentology studies are limited to investigations of weathered exposures or old core where thin sections are required to discern subtle sedimentary structures within the fine-grained intervals and to determine environments of deposition (Rӧhl and Schmid-Rӧhl, 2005;Schieber et al., 2007;Ghadeer and Macquaker, 2012;Plint et al., 2012a;Trabucho-Alexandre et al., 2012;Li and Schieber, 2018). Although thin section analysis is essential for describing the small-scale sedimentary structures, compositional changes and grain origins, such methods generate point data that do not characterize distinctive facies longevity over a continuous interval. The dataset presented in this study provides a relatively rare opportunity to characterize a mudstone succession in detail due to the pristine nature of the studied core. Sedimentary structures on the millimetre-to-centimetre scale can be discerned even in the finest grained intervals over the entire 100 m length of the studied core. This allows for a greater understanding of the relationships between facies and microfacies (MF) over the entire succession, not just at small select windows from which thin sections were taken.

| Geological setting
The core available for this study is dominated by fine-grained and locally organic carbon-rich units and spans the Late Cenomanian to Turonian Belle Fourche and Second White Specks (2WS) formations. These formations form part of the Colorado Group (Bloch et al., 1993; Figure 1) and were deposited during the ocean anoxic event (OAE) 2 (Schlanger and Jenkyns, 1976;Arthur et al., 1988;Leckie et al., 2002). These deposits are roughly time equivalent to several other major 'black shale' deposits across North America, such as the Eagle Ford and Greenhorn formations. The study interval is a major oil and gas source rock as well as a proven hydrocarbon reservoir within the Alberta Basin (Creaney and Allen 1990).
The Western Canadian Sedimentary Basin during the Cenomanian and Turonian was a retro-arc foreland basin located on the western margin of the North American craton. The transition between the siliciclastic Belle Fourche Formation and calcareous 2WS Formation marks a major, Late Cenomanian to Early Turonian transgression that flooded the Western Interior Seaway (Plint, 2000;Varban and Plint, 2005, 2008a, 2008bKreitner and Plint, 2006; Figure 1). This transgression was a result of two factors: eustatic sea-level rise as part of the Greenhorn cycle that reached its maximum transgressive extent in the Early Turonian (Kauffman and Caldwell, 1993); and the onset of a major phase of flexural subsidence that took place in the adjacent fold and thrust belt (Varban and Plint, 2008b;Plint et al., 2012b). This rise in relative sea level connected cold, nutrient-rich, normal salinity waters from the Boreal Sea with the warm, high salinity waters of the Tethys Sea. The progradation of warm Tethyan sea waters into the Canadian portion of the basin allowed for a change in biogenic production with a relative increase in carbonate secreting forms (Schröder-Adams et al., 1996;Schröder-Adams, 2014). The Belle Fourche and 2WS formations increase in thickness towards the orogen margin due to tectonic subsidence, but water depths for the basin were relatively shallow (< 70 m) across the basin resulting in dominantly storm reworked deposits (Plint et al., 2012a). The core analysed for this study occurs along the eastern margin of the deformation front where the Cenomanian-Turonian foredeep would have been located ( Figure 1); however, the western margin of the palaeo-seaway is no longer preserved due to subsequent uplift and erosion of the fold and thrust belt.

| MATERIALS AND METHODS
Mudstone-dominated successions, including the one of interest, are typically heterogeneous at all scales. To capture this heterogeneity, facies were described at the 10 −3 m scale from core and UV core images using nomenclature guidelines of Lazar et al. (2015) and Macquaker and Adams (2003). Mudstone components are referred to as being 'dominant' (> 90%), 'rich' (50%-90%) and 'bearing' (10%-50%) (Macquaker and Adams, 2003); mudstone grain size is referred to as fine, medium and coarse depending on the proportion of clay-sized and silt-sized grains (Lazar et al., 2015). In this study, facies were identified with the motivation of characterizing depositional processes and described based on composition, grain size, sedimentary structures and degree of bioturbation. Thin sections taken from representative facies were then used to describe the MF components of each facies. Each facies is composed of bedsets of sediment accumulated under repeated depositional events of similar character while MF consists of lamina-sets from a certain depositional process. F I G U R E 1 Location and palaeogeography of the study interval. The core studied is located just north-west of Calgary at the margin of the fold and thrust belt. The red box denotes the study interval that the core covers. The Belle Fourche Formation was deposited in the Cenomanian when the delta complex of the Dunvegan Formation was prograding into the basin north of the study area. The 2WS Formation was deposited during the Turonian when warm sea water from the southern Tethys Sea entered the Canadian portion of the basin. Maps modified from Bhattacharya and MacEachern (2009

Northwest Plains
A cored section of over 100 m covering the upper Belle Fourche and 2WS formations from well 13-06-030-5W6 was used for this study (Figure 2). The described core is slabbed, well-preserved and has a high recovery offering an excellent opportunity to observe subtle bedforms in mudstone not usually resolved in outcrop or older, poorer quality cores. This core allowed for grain size, composition and sedimentary structures to be described in detail over 100 m of vertically continuous mudstone deposits. To illustrate the value of being able to describe a continuous sedimentary record, facies logged at a 5 mm scale were compared to facies logged at point samples at 50 cm intervals to imitate sampling styles typically used in mudstone studies.
Shale Petroleum donated the core studied, along with X-ray diffraction (XRD) and programmed pyrolysis data, to the University of Calgary. A total of 79 samples were taken from the core from representative facies and submitted to Core Lab Petroleum Services for petrology and geochemical analysis ( Figure 2). Total organic carbon (TOC) content was measured using pyrolysis analysis (LECO-TOC and Rock-Eval) on 100 mg of crushed whole rock and un-mineralized samples. Whole rock XRD analyses were performed on crushed, dried and disaggregated random whole rock mounts. A separate split of each sample was used for clay fraction analysis. Quantification of the whole rock composition and phyllosilicate minerals was completed by integrated peak areas and empirical reference intensity ratio (RIR) factors. Total clay mineral abundances were determined from whole rock XRD patterns using combined {001} and {hkl} clay mineral reflections and empirical RIR factors.
Of the 79 samples, 41 were prepared for thin section analyses (note these sections were prepared unusually thin (20 μm) to visualize small-scale sedimentary bedforms, composition and bioturbation styles. All but three of the thin sections were stained with Alizarian Red-S to differentiate calcite (stains red) from dolomite (clear); potassium ferricyanide to differentiate ferroan dolomite (stains blue) and ferroan calcite (stains dark red/purple/blue); and sodium cobaltinitrite to distinguish feldspars (stains potassium feldspar yellow). To obtain details at a finer scale than observable using an optical microscope, the remaining three thin sections were used for imaging and chemical analysis using a scanning electron microscope (SEM) and Energy Dispersive X-Ray Spectrometry (EDS). This allowed for detailed characterization of grain and matrix compositions and distributions.

| Sedimentology and facies descriptions
The cored section covers two formations: the siliciclastic Belle Fourche and the mixed calcareous and siliciclastic 2WS formations. The contact between the two formations in western Alberta is often gradational in core and is denoted by an upwards decrease in quartz content (in the form of quartz silt) and increase in organic carbon and carbonate content (in the form of carbonate grains and cement; Figure 2). The contact is typically overlain by a thick, regionally correlatable bentonite bed (Tyagi et al., 2007). Similar sedimentary structures occur in both formations, although the relative abundance of biogenic and siliciclastic grains described from core and thin sections changes. Biogenic grains are sourced from within the basin and include bivalves, foraminifera, bioclastic debris and calcareous pellets. The siliciclastic component is detrital in nature and is dominantly composed of quartz silt and clay mineral-rich aggregate grains with a significant portion consisting of sediment that has been reworked across the basin.
The 2WS Formation is informally divided into lower, middle and upper units based on composition and stratigraphic surfaces ( Figure 2). The lower 2WS Formation is clay mineral-bearing, organic matter-bearing and carbonate-bearing containing abundant pellets, bivalve fragments and foraminifera tests. The middle 2WS has a gradual basal contact with the lower 2WS and exhibits a greater abundance of quartz-rich siltstone beds. The middle 2WS is topped by a flooding surface that is associated with a bentonite bed and large Fe-carbonate concretions. The upper 2WS is clay mineral-rich and carbonate-poor with an abundance of thin siltstone stringers and massive graded beds.
Systematic analysis of over 100 m of well-preserved core and 41 representative thin sections revealed seven facies, each composed of various MF outlined in Table 1, in photographs ( Figure 3), photomicrographs ( Figure 4) and SEM images ( Figures 5 and 6). Facies are numbered, with subset numbers assigned to MF (e.g. MF X.Y refers to MF Y within facies X; Table 1). This nomenclature is used to refer to specific MF within a certain facies because the same MF can occur in multiple facies (Table 1). The MF represents lamina-sets with varying composition and/or sedimentary structures within a certain bed. Mineralogical compositions of each facies are outlined in Table 2.

| Facies 1: Parallel laminated argillaceous fine to coarse mudstone (f-cMs)
Facies 1 dominantly consists of quartz silt and clay mineral aggregate grains that compose normally graded beds with an erosive base similar to the deposits formed by waveenhanced sediment gravity flows (WESGFs) described in Macquaker et al. (2010a). Facies 1 is typically composed of three different MF that are analogous to the units described in Macquaker et al. (2010a). A complete bed in Facies 1 is composed of a basal erosive contact overlain by cross-laminated coarse mudstone (cMs) of MF 1.3, that normally grades into  Microfacies 1.1 (clay mineral-rich mudstone fMs) appears as structureless, black to dark-grey mudstone lamina-sets (0.5-500 mm; Figure 3A) and is analogous to unit c in Macquaker et al. (2010a). The SEM ( Figure 5) and thin section observations show that this is the finest grained MF and is composed of silt-sized and clay-sized grains dominantly composed of quartz and mud aggregates ( Figure  5E). Microfacies 1.1 has a sharp upper contact and either abruptly overlies MF 1.2 ( Figure 3A), or overlies a sharp, erosive surface ( Figure 3E,F). Silt-filled sub-vertical burrows often cross-cut into this MF ( Figure 3F), or almost completely homogenize laminations (bioturbated c-fMs of MF 1.4; Figure 4A).
Microfacies 1.2 (interlaminated fMs and medium to coarse mudstone m-cMs) consists of normally graded lamina-sets of intercalated clay mineral-rich mudstone and quartz silt. The clay mineral-rich mudstone is dominantly composed of mud aggregates. Microfacies 1.2 is typically overlain by MF 1.1 and either has a gradational lower contact with MF 1.3 or overlies a sharp, erosive surface ( Figure 4A). Microfacies 1.3 (quartz-rich cross-laminated cMs) is quartz silt-rich, has a sharp erosive basal contact and contains cross-laminations or wavy-laminations. Microfacies 1.3 is composed of cross-laminated quartz silt and mud aggregates that overly an erosive contact and grades upwards into MF 1.2 ( Figure 4A).
Facies 1, the most clay mineral-rich of all of the facies, is quartz-rich, carbonate-poor and has relatively low TOC (Table 2). Clay minerals occur in the form of mud aggregates, quartz as detrital quartz silt and carbonate as rare pellets and cement.

Depositional processes
The sharp-based nature of this facies is indicative of relatively high-energy erosive currents preceding the deposition of MF 1.3, 1.2 and/or 1.1. The normal grading of beds within Facies 1 indicates deposition by waning flows. and Chondrites, or as homogenized laminations (MF 4) indicates that after deposition of Facies 1 there was a short period of calm allowing for increased benthic activity before the overlying lamina was deposited.

| Facies 2: Parallel-bedded argillaceous fine and cMs
This facies dominantly consists of alternating thin beds (ca 1-4 mm) of cross-laminated quartz-rich (cMs; MF 2.3) and quartz silt-rich and mud aggregate-rich mMs (MF 2.5;Figures 3B,C and 4B,C). This facies has a sharp basal and upper contact and has a pin stripe appearance from alternating beds of MF 2.3 and 2.5. Facies 2 is different than the interlaminated fMs and m-cMs (MF 2) because it is usually not graded ( Figure  4B). Instead, this facies is similar to the rippled mudstone beds described in Schieber et al. (2007). Additionally, both Facies 1 and Facies 2 contain quartz-rich cMs of MF 3. However, MF 2.3 has a sharp upper contact and often occurs as starved ripples ( Figure 3B,C) while MF 1.3 normally grades into MF 1.2. In thin sections, MF 2.3 contains silt-sized quartz grains, mud aggregates and some calcareous pellets. Microfacies 3.5 contains finer-grained, silt-sized quartz grains dispersed throughout a mud aggregate-rich matrix. Thin section analysis reveals that Facies 2 contains thin beds of MF 2.1 and 2.2 that are interbedded with the bedsets of MF 2.3 and 2.5 ( Figure 4B). Bioturbation occurs as passively filled, unlined, sub-vertical burrows or locally, where the sediments are almost completely homogenized (MF 2.4;Figures 3B and 4B,C). Facies 2 has a comparatively high abundance of quartz, clay minerals and a moderate abundance of TOC (Table 2). Quartz occurs as detrital silt grains, clay minerals as mud aggregates and TOC within biogenic aggregates and pellets.

Depositional processes
Scoured beds, starved ripples and cross-laminations indicate periods of relatively high-energy erosion and deposition under uni-directional currents with limited sediment input. The pin stripe appearance of this facies is probably due to T A B L E 2 X-ray diffraction mineralogy and programmed pyrolysis TOC for each facies.

| Facies 3: Parallel-bedded pellet-rich cMs and argillaceous fMs
Facies 3 is composed of alternating bedsets of interbedded pellets and quartz silt (MF 3.6) and beds of clay mineralrich fMs (MF 3.1, 3.2). This facies appears as a homogeneous, blocky, black mudstone in hand sample ( Figure 3D). However, thin sections reveal the complexity of this facies ( Figure 4D,E,F). Bedsets of interbedded fine sand to siltsized calcareous pellets and mud aggregates with quartz silt have sharp upper and lower contacts with clay mineralrich fMs beds ( Figure 4D,F). Sand to silt-sized calcareous pellets are composed of coccolith fragments ( Figure 6) and denote cross-laminations with clay mineral-rich mud aggregates ( Figure 4D,E). Whole Inoceramus shells that range in size from 5 to 10 cm in length occur on some bedding planes. The cross-laminations are more easily identified in thin sections taken from an early-stage concretion ( Figure 4E). This concretion prevented compaction of the mud intraclasts resulting in preservation of the depositional fabric. Foraminifera, some quartz silt grains and some bivalve fragments also occur cross-laminated with the pellets. This probably indicates that the coarser-grained pellets are hydrodynamically equivalent to the finer-grained quartz silt. Other sedimentary structures include scours, planar laminations, bioturbation and graded beds.
Thin sections reveal the presence of MF 3.1 and 3.2 within Facies 3. These clay mineral-rich fMs beds are normally graded and composed of interlaminated quartz silt and clay mineral-rich aggregates that are fine silt to clay grade in size ( Figure 4F). These beds are similar to the normally graded mudstones of MF 1.1 and 1.2, except they are much thinner in this facies (ca 1 mm) and occur in association with pellet-rich bedsets. Bioturbation occurs within this facies (BI: 0-3) and is identified by cross-cut laminations, homogenization of laminations (MF 4) and sub-vertical unlined burrows.
Facies 3 has the lowest abundance of quartz, a high abundance of carbonate (calcite and dolomite), relatively high clay mineral content and the highest TOC (Table 2). Quartz occurs as detrital silt, calcite dominantly occurs as pellets and foraminifera tests while dolomite is present as a cement. Clay minerals occur as mud aggregates and organic carbon occurs within biogenic aggregates and pellets.

Depositional environment
The pellet-rich bedsets of Facies 3 display the same sedimentary structures as Facies 2, indicating it was deposited under similar, high-energy depositional processes with low sedimentation rates. These bedsets have a different composition than the interbedded argillaceous fMs and cMs of Facies 2, with a greater abundance of biogenic material such as calcareous pellets, foraminifera and bivalve fragments, as well as less detrital siliciclastic material such as quartz silt. This indicates that this facies was potentially deposited further away from a siliciclastic sediment source, or the input of siliciclastic sediment into the basin was decreased at the time of deposition. The occurrence of bioturbation and benthic bivalve fragments indicates an oxygenated sea floor at the time of deposition.
The normally graded fMs of MF 3.1 and 3.2 indicate intermittent periods of rapid sediment input by waning flows. The thinner nature of these MF in comparison with Facies 1 could be due to decreased availability of siliciclastic material for resuspension and redeposition, and/or greater distance from the siliciclastic sediment source.

| Facies 4: Parallel-interbedded bioclast-rich cMs and argillaceous fMs
In hand sample this facies appears similar to Facies 3, but with abundant disarticulated bivalve fragments, mud intraclasts, bioclastic lags and quartz siltstone lamina ( Figure 3D). In thin section, this facies contains the same pellet-rich bedsets and clay mineral-rich fMs beds as Facies 3, except with a greater abundance of bivalve fragments. Bivalve fragments typically occur in association with pellet-rich and quartz siltrich bedsets, with large fragments and whole shells denoting laminations. Bioclastic bone beds composed of fish scales, foraminifera, bivalve fragments and calcareous pellets are sometimes associated with the base of quartz siltstone beds. Similar to Facies 3, the organic-rich grains such as pellets and biogenic aggregates occur in association with the quartz silt rather than in the finer-grained mudstone fraction. The graded fMs of MF 1 and 2 are thin in Facies 4 (ca 1 mm) and occur interbedded with pellet-rich bedsets. Bioturbation (BI: 0-3) occurs in thin beds that can be recognized in thin section ( Figure 4F). Facies 4 has a high relative percentage of carbonate, TOC and clay minerals and a low abundance of quartz (Table 2). Quartz occurs as detrital silt, clay minerals as mud aggregates, calcite as bivalve fragments, pellets and foraminifera. Dolomite occurs as cement while organic carbon is present within biogenic aggregates and pellets.

Depositional processes
The pellet-rich bedsets of Facies 4 contain the same sedimentary structures as Facies 2 and 3, indicating that similar uni-directional and combined-flow currents acted on the sea floor during deposition of this facies. Low sediment input rates and traction transport of grains by currents produced the pin stripe appearance of the pellet-rich bedsets. The concentration of bivalve fragments also indicates current reworking of the sea floor. The abundance of biogenic calcareous material such as bivalve fragments and other bioclastic debris indicates proximity to environments where these organisms would have lived, as well as an oxygenated sea floor. The graded fMs of Facies 4.1 and 4.2 are probably rapid sedimentation events by waning flows. However, these flows probably did not carry a large amount of sediment due to the thinness of Facies 4.1 and 4.2.

| Facies 5: Cross-bedded quartzrich cMs
This facies is typically 1-5 cm thick and is dominantly composed of amalgamated sharp-based, cross-laminated, quartz-rich cMs and fine-grained sandstone beds (MF 5.3). Mud aggregates occur within this facies and often denote the laminations within MF 5.3 ( Figure 4G,H). Thin beds of MF 5.5 (quartz-rich and mud aggregate-rich mMs) are often interbedded with the quartz siltstone beds of MF 5.3. Biogenic grains such as pellets, foraminifera and bivalve fragments often occur within MF 5.3 and 5.5 as well ( Figure 4G). Crosslaminated cMs of MF 5.3 is sometimes sharply overlain by combined-flow laminated cMs (MF 5.8). This MF is composed dominantly of quartz silt with a lesser abundance of mud aggregate lamina. Facies 5 is often sharply overlain by 0.5-5 cm thick beds of clay mineral-rich fMs of Facies 1.1 ( Figure 3E,F). This sharp contact is erosive and often scoured; in many instances only thin beds of Facies 5 are preserved between beds of Facies 1.1 ( Figure 3F). Bioturbation in this facies occurs as discrete, passively filled, unlined, sub-vertical burrows, or as disrupted and homogenized lamina (MF 5.4).
Facies 5 contains the greatest abundance of quartz, a relatively moderate abundance of carbonate, the lowest abundance of clay minerals and the lowest average TOC (Table 2). Quartz silt occurs as detrital grains, clay minerals as mud aggregates and carbonate dominantly as cement or bivalve fragments.

Depositional processes
The association of this facies with Facies 2 indicates that deposition occurred under similar processes. The abrupt base and crosslaminations of MF 5.3 indicate deposition by uni-directional currents that reworked the sea floor, transporting and depositing siliciclastic grains. The amalgamation of quartz cMs beds in this facies is probably due to increased sediment input either into the basin, or into the study area by stronger currents. The combinedflow laminations of MF 5.8 are indicative of wave action on the sea floor. The lack of grading in the beds, as well as the incorporation of biogenic grains, indicates that sediment input rates were low. The occurrence of Planolites and Chondrites bioturbation, together with the high degree of bioturbation seen in some beds, indicates periods of oxic to dysoxic bottom waters.

| Facies 6: Bioturbated argillaceous f-cMs
In hand sample and in thin section this facies is recognizable where laminations and bedding are either disrupted or homogenized by infaunal colonization (Figure 3E,F). This facies has a similar composition to Facies 1, 2, 3 and 5. Similar to all other facies, quartz occurs as quartz silt grains, clay minerals occur as mud aggregates and carbonate occurs dominantly as bivalve fragments, foraminifera tests and pellets. Organic matter occurs dispersed throughout the mudstone matrix, as well as occurring concentrated within some burrows. Homogenized intervals are typically no more than a few millimetres thick.
Facies 6 has a variable composition as indicated by the large range seen in samples taken from this facies (Table 2), with averaged values being comparatively moderate for all components.

Depositional processes
Sedimentary structures in this facies are difficult to discern due to biogenic homogenization of laminations by bioturbation. This indicates periods of high benthic biological activity and oxygenated bottom waters. This probably occurred during short-lived, relatively quiescent periods. However, colonization did not extend more than a few millimetres into the sediment, possibly due to low-oxygen pore waters a few millimetres below the sea floor and insufficient time between bed emplacement events. Burrows are dominantly Planolites. The wide range in composition (Table 2) is probably due to bioturbation occurring in a range of facies that are then homogenized resulting in their classification as Facies 6.

| Facies 7: Bioclastic debris-rich cMs
This facies only occurs in the middle 2WS Formation and appears as sharp-based, white to light-grey beds that are dominantly composed of quartz silt grains and broken bivalve shells ( Figure  3G). Clay minerals occur as mud aggregates and organic material occurs in association with pellets and phosphatized skeletal debris in some laminations. This facies appears similar to Facies 5, but with a greater abundance of bioclastic grains and quartz silt. Laminations within this facies are sometimes hard to discern due to the poor sorting of grains. However, sharp-scoured basal contacts, uni-directional and combined-flow ripple laminations can sometimes be distinguished. Bioturbation typically occurs at the upper bed contacts as discrete, unlined, sub-vertical burrows, or as cross-cut laminations.
Facies 7 has the highest carbonate content, a relatively moderate quartz content, the lowest clay mineral content and a moderate average TOC (Table 2). Quartz occurs as detrital silt, clay minerals as mud aggregates and carbonate as bivalve fragments, foraminifera tests, pellets and cement.

Depositional processes
Similar sedimentary structures as Facies 5 indicate that this facies was deposited by similar depositional processes. Scoured basal contacts indicate periods of erosion under high-energy conditions preceding the deposition of quartz and bioclastic grains under uni-directional and combinedflow currents. The abundance of bioclastic grains may be due to more favourable benthic living conditions due to decreased siliciclastic sediment input and/or increased

| Vertical distribution of facies
Relative abundances of MF vary throughout the study interval and distinct differences were documented for the Belle Fourche, lower, middle and upper 2WS formations. To illustrate the importance of describing a continuous succession, facies logged at a 5 mm scale and at point samples at 50 cm intervals were compared (Figure 7). Additionally, detailed (1 mm resolution) logs of three 50 cm intervals were made from the Belle Fourche, lower 2WS and middle 2WS formations ( Figure 8). The relative abundances of facies measured at each scale were quantified for each unit (Figure 8). The Belle Fourche Formation is relatively heterogeneous but does not contain the carbonate-rich and organic-rich Facies 3, 4 and 7 (Figures 7 and 8). The relative abundances of facies quantified at the three scales for the Belle Fourche Formation are very similar, with a slight overestimation of Facies 1 and underestimation of Facies 6 in the detailed 50 cm log ( Figure  8). The distribution of facies ( Figure 6) does vary, with the point sampled 50 cm log not capturing the abundance of Facies 5 and 6 in the upper portion of the Belle Fourche Formation (Figure 7). This would falsely indicate that crossbedded cMs (Facies 5) and bioturbated f-cMs (Facies 6) do not occur throughout the unit. The lower 2WS Formation is dominantly composed of pellet-rich and bioclast-rich Facies 3 and 4, as well as the clay mineral-rich Facies 1. This corresponds with a greater abundance of carbonate, less quartz, a higher TOC content, but a relatively high clay mineral content (Figure 2). The relative abundances of facies measured at each scale for the lower 2WS Formation vary significantly (Figure 8), with the point sampled 50 cm log underrepresenting Facies 4 and the detailed 50 cm log underestimating Facies 1. The distribution of facies between the 5 mm and point sampled 50 cm logs are relatively similar, except that the point sampled log underestimates Facies 1 at the base of the lower 2WS Formation and does not illustrate the upward increase in the abundance of Facies 2 and 5 (Figure 7). This would falsely indicate that clay mineral-rich waning flow deposits of Facies 1 do not exist in the lower part of the 2WS and that there is not an upwards increase in the abundance of parallel-bedded fMs and cMs (Facies 2) and cross-bedded cMs (Facies 5).

F I G U R E 8
The middle 2WS Formation contains all facies. This unit has a similar abundance of quartz as the Belle Fourche Formation (Figure 2), but a greater proportion of carbonate in the form of abundant bivalve fragments and calcareous pellets, as well as a higher TOC content. The relative abundances of facies measured at the three scales for the middle 2WS Formation are not similar (Figure 7). The detailed 50 cm log underestimates the abundance of bioturbated Facies 6 and overestimates the abundance of pellet-rich Facies 3 and 4 (Figure 8). The relative facies abundances between the 5 mm and point sampled 50 cm log are in better agreement than the detailed 50 cm log. The upwards decrease in abundance of bioclast-rich mudstone (Facies 4) and occurrence of bioclast debris-rich cMs (Facies 7) are not captured using point samples at 50 cm intervals. The point sampled log also falsely indicates an upwards increase in the abundance of bioturbated mudstones (Facies 6) instead of bioturbation being pervasive throughout the unit. Additionally, the presence of waning flow deposits (Facies 1), pellet-rich mudstones (Facies 3) and cross-bedded cMs (Facies 5) does not occur throughout the unit in the point sampled log.
The upper 2WS Formation is dominated by graded fMs of Facies 1 and the interbedded fMs and cMs of Facies 2, corresponding to a low carbonate content and relatively high amounts of clay minerals and quartz.

| Mud aggregate grains
The study interval is a clay mineral-rich succession as indicated by XRD and thin section analysis. However, most clay minerals occur as aggregate grains that are silt to medium sand in size and were deposited as bedload. This agrees with findings from time equivalent formations to the north of the study area where two types of aggregate grains were recognized, biogenic and intraclastic aggregates (Plint et al., 2012a). The biogenic aggregates dominantly include coccolith fragments compacted into pellets ( Figures 4D,E,F and 6), as well as some mixtures of organic matter, pyrite and clay that are probably products of marine snow and phytodetritus (Macquaker et al., 2010b). The clay mineral-rich aggregate grains described in this study are mostly composed of clay minerals ( Figure 4H) and appear similar to those described by Plint et al. (2012a). The occurrence of mud aggregate grains cross-laminated with quartz grains indicates the mud aggregates are hydraulically equivalent and are transported with silt to sand-sized quartz grains by traction processes.
Both aggregate grain types occur in all facies that show evidence of deposition mainly by traction currents. Within these facies, biogenic aggregates typically occur within bioclastic debris beds but also in quartz-rich siltstone beds where they denote laminations. In the pellet-rich Facies 3 and 4, calcareous pellets are up to fine sand in size and cross-laminated, indicating that although the biogenic grains were introduced into the system through suspension settling from the water column, they were reworked and concentrated by waves and currents.
Clay mineral-rich aggregate grains occur in all facies and vary in size from clay to medium sand size. These grains are difficult to decipher in thin section and SEM images due to compaction and a lack of mineralogical differentiation between the grains and matrix in the clay mineral-rich MF. They are probably not benthic faecal pellets as they do not exhibit the 'brick-like' morphology as outlined by Needham et al. (2006). These aggregates probably formed from the erosion and reworking of semi-consolidated mud, however, they could also be reworked lithified mudstone grains sourced from the thrust belt. The lack of compaction of the clay mineral-rich aggregate grains in Figure 4H indicates that the grains had low to no water saturation upon deposition. However, samples taken from an early-stage concretion in Facies 3 show that the clay mineral-rich aggregates were completely cemented by Fe-dolomite and the calcareous pellets remained uncemented (Figures 4E and 5D) indicating that there was enough water-filled porosity in the intraclastic aggregates for significant cement precipitation. The clay mineral-rich aggregates are probably not floccule aggregates as described in Schieber and Southard (2009) because they only show a modest degree of compaction indicating that their water saturation was not as high as 80%-90% at the time of deposition. It is possible that the clay mineral-rich aggregate grains consist of both intraclastic rip-up clasts and thrust belt-derived mudstone fragments.
Elemental mapping using SEM indicates that almost all sulphur occurs as framboidal pyrite ( Figure 6B), implying that there was abundant available reduced iron to sequester any available free sulphide (Aplin and Macquaker, 1993;Macquaker et al., 2014) contributing to the relatively low-sulphur (sweet) oils generated from the 2WS source rock. The source of iron for these reactions was not limiting and was probably introduced into the basin with the siliciclastic material as well as through bentonite ash fallout (Aplin and Macquaker, 1993).
Kaolinite cement infills shelter porosity ( Figure 6A), predating compaction, and solutes for its precipitation were probably derived from the dissolution of poorly crystalline aluminosilicate detritus (Macquaker et al., 2014). The dissolution of clays, rather than carbonates, probably acted as a buffer for acids produced by the bacterial breakdown of organic matter through sulphate reduction. Fein (1994) demonstrated that this process occurs with the dissolution of feldspars and one can assume it would also occur with the dissolution of clays. This is indicated by the lack of dissolution of surrounding foraminifera tests ( Figure 6A), a phenomenon also seen in the Kimmeridge Clay Formation (Macquaker et al., 2014).
The carbonate cement in the study interval occurs as Fe-dolomite and dolomite cement, as well as Fe-dolomite concretions. The dolomite cement dominantly occurs in coarser-grained quartz siltstone beds with inter-particle pore filling and grain replacing textures. It is probably sourced from the recrystallization of calcareous biogenic grains where clay dissolution did not act as a buffer. Dolomite also occurs as concretions within the study interval. However, these concretions do not always occur at flooding surfaces as described in other studies (Klein et al., 1999;Taylor & Macquaker 2011). Instead, these concretions typically occur within coarser-grained beds where fluid permeability would have been greater and around biogenic grains that would act as nuclei (Machent et al., 2012). The largest concretion occurs above a 20 cm thick bentonite bed and within the most organic-rich Facies 3 (Figure 2). The abundance of organic matter could be associated with increased primary production due to nutrient inputs from the bentonite ash (Rigby and Davies 2000;Hamme et al., 2010;Chikamoto et al., 2016). Bacterially mediated reduction in pore water sulphate resulted in available sulphides for pyrite precipitation. The iron initially was incorporated into pyrite, but as the sulphides were used up, the iron could be preferentially incorporated into dolomite due to the removal of sulphide from the system (Kastner, 1984). This is evidenced by dolomite cement with a Fe-poor core with a Fe-rich rim ( Figure 6C), indicating increased incorporation of iron into the dolomite structure as the cement formed. The lack of sulphides could be explained by a switch from bacterially mediated sulphate reduction to methanogenesis. These Fe-dolomite concretions probably occur in areas with rapid sedimentation where the available organic matter quickly passes through the aerobic zones of sulphate reduction, where pore water sulphate is reduced, into the methanogenesis zone where sulphides are no longer being produced (Gautier, 1982). The relatively large grain size (silt to fine sand) of this facies would have allowed for high initial porosity and permeability, allowing for the supply of necessary solutes for dolomite precipitation.
Fractures filled with Fe-calcite, Fe-dolomite and minor gypsum (probably from core weathering) also occur in some facies ( Figure 4H). There is usually organic matter at the edges of these fractures indicating that they may have formed through hydrocarbon generation and acted as conduits for hydrocarbon migration.

| Upscaling
Comparison of MF abundances and distributions with a 5 mm resolution, from point sampled data at 50 cm intervals and from a representative 50 cm interval logged in detail, indicate that results may vary depending on the unit being characterized. The Belle Fourche Formation is dominated by only four facies (Figures 8 and 9) so point sampling this unit has a higher probability of capturing these facies than in the lower and middle 2WS Formation where more facies are present. Point sampling did not capture the facies relationships well in any of the units compared to the 5 mm continuous log. The detailed 50 cm logs did not have strong correlations with the results from the continuous 5 mm log for the lower and middle 2WS Formation and therefore using one 50 cm interval to describe facies relationships for each unit may not give accurate results.
These results indicate that if one simply wants to characterize the bedforms present along with the depositional processes associated with them, a point sampling method may F I G U R E 9 Schematic depositional models for the Belle Fourche Formation, lower 2WS and middle 2WS units demonstrating the relationships between depositional processes and sediment sources  be sufficient. However, if one wants to characterize the depositional processes of the bedforms present and determine their relationship with one another to understand the depositional system as a whole, a more continuous log is required.

| Oxygen levels and water depth
Palaeobathymetric reconstructions of the Turonian WIS suggest water depths ranging from 50 to 200 m for Alberta (Sageman & Arthur, 1994). Plint et al. (2012a) suggest that water depths did not exceed 70 m across Northern Alberta during the Cenomanian-Turonian as this is the effective wave base for mud. This interpretation is based on the occurrence of the equivalent of the described pellet-rich siltstone in this study only 1-20 m above Late Cenomanian sandy nearshore and shoreface deposits. The organic-rich pellet siltstone facies occurs across the Cenomanian-Turonian boundary where eustatic rise is interpreted as being about 20-30 m (Jarvis et al., 2006;Voigt et al., 2006). The deposition of the organic-rich pellet siltstone facies is interpreted as being a product of the drowning of the clastic sediment source rather than a significant deepening of the basin. Findings from Plint et al. (2012a) can be applied to the deposits from this study where the same pellet-rich siltstone facies occurs at the Cenomanian-Turonian boundary. This agrees with the estimate that that annual storms in epeiric seaways can rework sediment down to 40-100 m based on modern analogues and computer modelling (Jönsson et al., 2005;Allison & Wells, 2006;Peters & Loss, 2012). Abundant Inoceramus bivalves and bioturbation of bed tops indicate that bottom waters were not pervasively anoxic. The abundance of pelagic foraminifera increases from the Belle Fourche Formation into the 2WS Formation, however, abundant agglutinated benthic foraminifera are recognized by Stelck and Wall (1954), indicating oxygenated bottom waters. The lack of diversity of bivalves and bioturbation indicates a relatively stressed environment possibly due to sediment input by storms, or low oxygen levels.

| Depositional processes and sediment sources
Throughout the study interval the same suite of sedimentary structures occurs, including wave ripples and current ripples, scours, graded beds and bioturbation. There are two suites of depositional processes among the facies identified, wave current and storm current reworking of the sea floor with minimal sediment input and rapid sedimentation events by sediment-laden waning storm flows.
Facies 2-7 contain similar sedimentary structures indicating deposition by similar processes, however, the composition of each facies varies. Scours, ripple-tail lamination, cross-laminations, combined-flow laminations and starved ripples indicate deposition by waves and currents during times of low sediment input. Continental shelves are often exposed to wind-driven geostrophic flows that have a strong along-shelf and weaker across-shelf flow direction (Nittrouer & Wright, 1994;Ogston et al., 2004).
Alternatively, Facies 1 contains scours, normally graded beds and structureless clay drapes indicative of deposition by waning flows that have high sediment concentrations. The triplet motif of a complete bed of Facies 1.3, 1.2 and 1.1 is similar to what is described by Macquaker et al. (2010a) as WESGFs. The deposits in this study are probably not gravity-driven flows because of the low gradient of the Western Interior Seaway (Schieber, 2016). However, the turbulent flows are probably driven by storm-induced waves and currents re-suspending fluid mud introduced into the basin by rivers during large flood events (Macquaker et al., 2010a;Denommee et al., 2016).
Facies 1.1 is analogous to the fluid mud deposits outlined in Ichaso and Dalrymple (2009). When the clay mineral-rich fMs (Facies 1.1) occurs independently of Facies 1.2 and 1.3, it overlies a sharp erosive contact. This suggests that the isolated deposits of Facies 1.1 could be due to storm wave resuspension of previously deposited mud that was transported into the study area. Alternatively, the clay mineral-rich fMs could sometimes be deposited by high-energy, bypassing flows that scoured the sea floor and only deposited material from their fluid mud tail, similar to those documented in deep-water sediment gravity flows (Stevenson et al., 2015).

| Belle Fourche Formation
The high degree of facies heterogeneity in the Belle Fourche Formation indicates that it was deposited by a complex set of processes with sediment being sourced from the surrounding hinterland and reworked on the sea floor. The facies in the Belle Fourche Formation are dominantly composed of detrital siliciclastic material, with some minor input of biogenic matter such as foraminifera and bivalve shells.
Waves and currents eroded previously deposited sediment and re-suspended it into the bedload, this included eroding semi-consolidated mud into intraclastic aggregates ( Figure  9). These sediments were then transported across the basin by bottom-flowing (probably geostrophic) currents, resulting in pin stripe-bedded mudstones of Facies 2 and cross-laminated quartz siltstones of Facies 5. Waning of storm-induced currents resulted in the sea floor being influenced by the oscillatory flow of waves, resulting in the deposition of combined-flow laminations (MF 5.8). There are usually fewer clay mineral-rich aggregates in the combined-flow deposits because the intraclasts were ripped up and re-deposited by the initial uni-directional current. While these sediments were being reworked across the basin, they were incorporating biogenic sediment that had settled from the water column (pellets, biogenic aggregates) or that already existed on the sea floor (bivalve fragments). Overall, the sediment load in these facies is relatively low, as evidenced by the abundance of starved ripples and thin beds.
During higher-energy storm events, storm currents re-suspended fluid mud from proximal fluvial sources and transported it offshore as a wave-enhanced and current-enhanced flow (Facies 1). Relatively thick beds of Facies 1 occur in the Belle Fourche Formation indicating that large volumes of fluid mud have been introduced to the area, possibly due to proximity to a siliciclastic sediment source.

| Lower 2WS Formation
The same depositional processes that produced the Belle Fourche Formation occur in the lower 2WS Formation, however, there is a greater relative abundance of biogenic sediment (Facies 3, 4 and 7). At this time, oceanographic conditions and possibly a decrease in detrital input allowed for increased carbonate production. A possible decrease in detrital sediment input could be due to increased distance to the siliciclastic source (transgression), or a change in the nature of sediment delivery (i.e. a possible change to a more wave-dominated delta that would keep siliciclastic sediment closer to the shore). Sedimentary structures are indicative of deposition above storm wave base, showing that there was no significant deepening of the basin. Sediment in the lower 2WS Formation dominantly consists of calcareous pellets and biogenic aggregates sourced from the water column, reworked bivalve shells sourced from the sea floor and intraclastic aggregates and quartz grains that are sourced from reworking of previously deposited sediments on the sea floor. The abundance of Inoceramus bivalve shell fragments indicates that although there was extensive storm-reworking of this material, the environment at the time of deposition was favourable for these benthic organisms.
Similar to the Belle Fourche Formation, the facies of the lower 2WS Formation indicate periods of low sediment input (Facies 2, 3, 4, 5 and 7), interrupted by rapid sedimentation events (Facies 1 and MF 1, 2 and 3). The difference is that the wave-enhanced and current-enhanced sediment flow beds in the lower 2WS Formation are much thinner than in the Belle Fourche Formation due to decreased siliciclastic input during deposition of the former.

| Middle 2WS Formation
The middle 2WS Formation contains all of the facies described in this study, indicating that it was deposited by the same processes as the Belle Fourche and lower 2WS formations and has a combination of siliciclastic and biogenic sediment input. Detrital sediment input from the adjacent fold and thrust belt has been reworked into the study area and mixing of Boreal and Tethyan sea waters promoted primary production in the water column and on the sea floor.
Similar to the lower 2WS Formation, alternating periods of deposition by bottom-flowing traction currents with low suspended volumes (Facies 2, 3, 4, 5 and 7) and rapid deposition of thin beds (< 1 cm) by wave-enhanced and current-enhanced flow events (Facies 1, MF 1, 2 and 3) occur throughout this unit. Unlike the other intervals studied, single burrows are hard to identify due to the high degree of bioturbation. This indicates either a more favourable environment for benthic activity during this time, or a greater period of time between high-energy, wave-enhanced and current-enhanced flow events when bioturbation would take place. Some beds show no evidence of bioturbation, indicating that preservation of these beds occurred possibly due to burial by periodic sediment input by waning wave-enhanced and current-enhanced flows.

| Controls on organic matter
The traditional, widely accepted model for organic-rich marine mudstones is that they are deposited by suspension settling in quiescent, deep, anoxic environments (Creaney & Allen, 1992). Instead, the most organic-rich facies in this study are deposited by traction processes in water that is < 70 m deep (Plint et al., 2012a). The occurrence of lowintensity Planolites and Chondrites bioturbation, benthic bivalve and benthic foraminifera (Schrӧder-Adams et al., 1996) throughout the study interval indicates that bottom water anoxia was not long lived. However, the lack in diversity of bioturbation and benthic organisms indicates that it was a stressed environment to live in, possibly due to low oxygen levels, salinity and/or suspended sediment.
Organic-rich grains such as calcareous pellets and biogenic aggregates dominantly occur in Facies 3 and 4 and are most abundant in the lower 2WS Formation where the TOC is highest. These grains were transported and deposited as bedload and are often concentrated within the coarser-grained, cross-laminated beds. The introduction of warm waters from the Tethys Sea into the Canadian portion of the basin at the time of deposition of the lower 2WS Formation probably contributed to an increase in primary productivity in the water column compared to the Belle Fourche Formation, resulting in an increased flux of organic carbon to the sea floor (Schrӧder-Adams et al., 1996). One of the controls on the abundance of preserved organic matter in the study interval appears to be a balance between low dilution by siliciclastic material and periodic, rapid input of sediment to bury and preserve organic carbon-rich beds. A similar phenomenon is documented in the offshore deposits of the Mancos Shale in DeReuil and Birgenheier (2019). In the lower 2WS Formation, bottom currents rework and concentrate organic carbon present in biogenic aggregates and pellets, while periodic wave-enhanced and current-enhanced flow events rapidly bury and preserve these beds. These deposits in the lower 2WS Formation are very thin compared to the other units, resulting in less overall dilution of organic carbon in the unit.
Additionally, the calcareous pellets themselves may enhance the preservation of organic matter in the succession. These aggregate grains are silt to medium sand in size resulting in increased settling times, similar to that documented in Turner (1979), reducing their exposure to oxygenated waters and possible consumption by other organisms. The lack of dolomite cement in the pellets within concretions indicates that they are isolated from the surrounding oxic-dysoxic pore waters ( Figure 6A). This is probably due to the presence of a membrane outside the pellet, commonly recognized in faecal pellets (Turner & Ferrante, 1979), preventing disaggregation of these grains by the bottom-flowing currents.

| CONCLUSIONS
Results from this study challenge the historical view that accumulation and preservation of organic material occurs in low-energy, suspension settling deposits dominated by clay-sized grains. Detailed facies descriptions reveal that grains in the studied organic-rich mudstones range from silt to medium sand in size and are found in sedimentary structures that were formed by traction transport processes, bioturbation and benthic organisms. This described facies heterogeneity has significant implications for the controls on organic matter accumulation, preservation and distribution.
Delivery of organic matter to the sea floor was dominantly in the form of silt to sand-sized biogenic aggregates and pellets, resulting in relatively fast settling times that enhanced preservation. These organic-rich grains are concentrated into beds that are cross and planar-laminated, indicating current and wave reworking of the sea floor sediments. Preservation is also enhanced by intermittent input of clay mineral-rich sediment by periodic waning flows from storm events that rapidly bury underlying sediment. Thick storm event beds dilute the organic material in the system; however, the most organic-rich facies have thin beds resulting in low levels of dilution. There is a lack of persistent bottom water anoxia as indicated by the occurrence of bioturbation, benthic bivalves and benthic foraminifera. This indicates that the OAE-2 was not the main control on the preservation of organic matter in these Cenomanian-Turonian aged sediments, instead the accumulation and preservation of organic matter was probably controlled by periodic, rapid burial of organic-rich sediments by high-energy event beds. Beds in most facies indicate starved sediment deposition and current and wave ripples indicate storm wave and current reworking of the sea floor.