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A depositional model for organic-rich Duvernay Formation mudstones
A depositional model for organic-rich Duvernay Formation mudstones Levi J. Knapp, Julia M. McMillan, Nicholas B. Harris PII: DOI: Reference:
S0037-0738(16)30293-7 doi:10.1016/j.sedgeo.2016.11.012 SEDGEO 5139
To appear in:
Sedimentary Geology
Received date: Revised date: Accepted date:
8 August 2016 21 November 2016 22 November 2016
Please cite this article as: Knapp, Levi J., McMillan, Julia M., Harris, Nicholas B., A depositional model for organic-rich Duvernay Formation mudstones, Sedimentary Geology (2016), doi:10.1016/j.sedgeo.2016.11.012
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A DEPOSITIONAL MODEL FOR ORGANIC-RICH
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DUVERNAY FORMATION MUDSTONES
: University of Alberta, Department of Earth and Atmospheric
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a
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Levi J. Knappa, Julia M. McMillana, Nicholas B. Harrisa
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Sciences
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1-26 Earth Sciences Building
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University of Alberta
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Edmonton, Alberta Canada T6G 2E3
Corresponding author: Levi J. Knapp KnappLeviJ@gmail,com
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ACCEPTED MANUSCRIPT
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Abstract:
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The Upper Devonian Duvernay Formation of western Canada is an organic-rich shale formation now targeted as a hydrocarbon
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reservoir. We present a detailed sedimentological analysis of the Duvernay Formation in order to better understand organic-rich
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mudstone depositional processes and conditions and to characterize the vertical and lateral heterogeneity of mudstone
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lithofacies that affect petrophysical and geomechanical rock properties. Organic-rich mudstone facies of the Duvernay
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Formation were deposited in a dynamic depositional environment by a variety of sediment transport mechanisms, including suspension settling, turbidity currents, and bottom water currents in variably oxygenated bottom waters. Suspension settling dominated in distal relatively deep areas of the basin, but evidence for weak turbidity currents and bottom water currents was observed in the form of graded beds and thin grain-supported siltstone laminae. Organic enrichment primarily occurred in distal areas as a result of bottom water anoxia and low depositional rates of inorganic sediment. In deep water locations near platform margins, alternating silty-sandy contourite beds and organic-rich mudstone
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ACCEPTED MANUSCRIPT beds are present, the former interpreted to have been deposited and reworked by bottom water currents flowing parallel to slope. In
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shallower, more oxygenated settings, mudstone lithologies vary
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from calcareous to argillaceous. These sediments were deposited from suspension settling, turbidity currents, and bottom water
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currents, although primary sedimentary structures are often
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obscured by extensive bioturbation. Locally, organic enrichment in dysoxic rather than anoxic bottom waters was driven by a slightly
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increased sedimentation rate and possibly also by aggregation of sedimentary particles in the water column due to interaction
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between organic matter and clay minerals. Large variations observed in sediment composition, from siliceous to calcareous to
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argillaceous, reflect multiple biogenic, carbonate, and clastic sediment sources. Sediment composition is influenced by basin morphology, circulation patterns, sea level variation, and proximity to sediment sources.
Keywords: Devonian, Duvernay, sedimentology, mudstone, black shale, organicrich
1 Introduction: 3
ACCEPTED MANUSCRIPT Recent sedimentological studies of mudstone successions, including examinations of both core and petrographic thin sections,
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have identified characteristic fabrics that are attributed to a wide
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variety of depositional processes. Processes such as gravity flows (e.g. Macquaker et al., 2010a), storm dispersal and reworking (e.g.
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Schieber et al., 2010), and contour currents (e.g. Stow and Lovell,
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1979) have been proposed to explain mudstone fabrics, contrasting with traditional views that mud deposition was dominated by
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settling of particles from suspension (e.g. Potter et al, 1980). High energy deposition of clays as a result of flocculation, pelletization
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and traction currents has been demonstrated in laboratory flume experiments (Schieber et al., 2007b; Schieber and Southard, 2009;
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Schieber, 2011) and ancient deposits (e.g. Macquaker and Keller, 2005; Macquaker et al., 2010b; Aplin and Macquaker, 2011). The presence of bioturbation challenges interpretations that infer persistent anoxia during organic-rich mudstone deposition (Macquaker and Gawthorpe, 1993; Macquaker et al., 2007; Schieber, 1999, 2003; Macquaker et al., 2010a; Egenhoff and Fishman, 2013). Recognition of these processes has important implications for hydrocarbon exploration, providing a mechanism for describing and predicting heterogeneity of rock properties in mudstones. Small-scale variations in components such as biogenic silica and
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ACCEPTED MANUSCRIPT carbonate minerals have been demonstrated to significantly influence petrophysical and geomechanical properties of the rock
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(Schieber, 1996; Schröder-Adams et al., 1996; Aoudia et al., 2010;
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Harris et al., 2011). Additionally, organic matter distribution may vary both vertically and laterally (Hemmesch et al., 2014) and is
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associated with variations in textural characteristics (Bohacs, 1998;
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Macquaker and Howell 1999; Passey et al., 2010) and organic
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matter porosity (Dong et al., 2015) in mudstones. This study characterizes lithofacies of the organic-rich
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Duvernay Formation – a prolific source rock (Fowler et al., 2001)
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and recent target for liquids-rich gas in the Western Canadian Sedimentary Basin (Rokosh et al., 2012). A core-scale
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sedimentological study of the Duvernay Formation has not been published since Stoakes (1980), and no detailed petrographic study exists. The Duvernay Formation represents an outstanding opportunity to study the depositional processes and conditions of fine-grained, organic-rich mudstones within a stratigraphic and paleogeographic context, with a wealth of long cores and log data representing a wide range of depositional settings, acquired as the result of intense exploration since 2009. The presence of wellstudied shallow water carbonates of the Leduc Formation (e.g. Mountjoy, 1980; Weissenberger, 1994; Van Buchem, 1996a, 2000a; Whalen et al., 2000; Potma et al., 2001) and Grosmont
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ACCEPTED MANUSCRIPT Formation (e.g. Cutler, 1983) allows for correlation of basinal mudstones to their shallow water equivalents, an advantage not
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available in many other mudstone successions.
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2 Geologic Setting:
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Duvernay Formation sediments were deposited during the Frasnian in Western Canada (Fig. 1, 2) and are roughly time-
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equivalent to several other major black shale deposits across North America. During the Middle to Late Devonian, global sea level
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was significantly higher than present day (Johnson et al., 1985; Savoy and Mountjoy, 1995; Haq and Schutter, 2008), resulting in
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widespread flooding of continents. Many organic-rich Devonian successions were deposited within these epicontinental settings, including the Duvernay, Horn River, Bakken, Marcellus, Chattanooga, Ohio, New Albany, and Woodford shales. The Western Canadian Sedimentary Basin during the Frasnian was a passive margin at the western edge of North America. The northwest margin of North America was dominated by deposition of open marine shales in British Columbia and the Northwest Territories, transitioning to shallow water carbonates in Alberta, and restricted dolomites and evaporites in Saskatchewan and Manitoba to the southeast (Ziegler, 1967; Switzer et al., 1994). 6
ACCEPTED MANUSCRIPT During deposition of Cooking Lake Formation through Ireton Formation sediments (collectively the Woodbend Group; Fig. 2),
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the rate of accumulation and preservation of sediment increased
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dramatically (Switzer et al., 1994). In Alberta, the section is characterized by thick, extensive reef complexes, separated by
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source rocks (Switzer et al., 1994).
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thick accumulations of basin-filling shales, including hydrocarbon
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Tectonic features that influenced sedimentation during deposition of Duvernay Formation sediments include the Peace
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River Arch, West Alberta Ridge, Rimbey Arc (overlain by the
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Rimbey-Meadowbrook reef trend), and Meadow Lake Escarpment (overlain by the Killiam Barrier Reef) (Fig. 1). The Peace River
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Arch was an emergent landmass on the northwest side of the basin, fringed by Leduc Formation reefs (Dix, 1990; O’Connell et al., 1990). The West Alberta Ridge was flooded, but formed a base for extensive Leduc reef complexes during and preceding deposition of Duvernay Formation sediments (Switzer et al., 1994). The Rimbey Arc was a southwest-northeast trending basement lineament that exerted a strong control on accommodation space during Woodbend Group deposition (Ross and Stephenson, 1989). During deposition of Duvernay Formation sediments, the lineament was marked by a chain of Leduc Formation reefs called the Rimbey-Meadowbrook trend, dividing the basin into a West
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ACCEPTED MANUSCRIPT Shale Basin and East Shale Basin. Accommodation space was more limited in the East Shale Basin than the West Shale Basin
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due to differences in tectonic subsidence across the Rimbey Arc
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(Switzer et al., 1994). This effect was compounded by differential compaction, because the West Shale Basin was the site of largely
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shale deposition, whereas the East Shale Basin was underlain by
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the Cooking Lake Carbonate Platform (Switzer et al., 1994). The Meadow Lake Escarpment is a pre-Devonian erosional and
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structural feature (Oldale and Munday, 1994). During deposition of Duvernay Formation sediments, the Killiam Barrier Reef roughly
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coincided with the underlying Meadow Lake Escarpment and marks the furthest eastward extent of Duvernay Formation shales
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and argillaceous limestones (Swtizer et al., 1994). The Duvernay Formation conformably overlies the Majeau
Lake Formation throughout most of the basin (Fig. 2). In the far south and west, Majeau Lake Formation sediments are absent, and Duvernay Formation strata onlap Swan Hills Formation platform carbonates and older Devonian strata (Switzer et al., 1994). The Ireton Formation conformably overlies the Duvernay Formation across much of the basin. Reef-margin and platform carbonates of the Leduc Formation and Grosmont Formation overlie the Duvernay Formation at the margins of the basin. An informal stratigraphy, introduced by Andrichuk (1961) and used by
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ACCEPTED MANUSCRIPT industry, separates the Duvernay Formation into lower, middle, and upper members based on the recognition of lithologically
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distinct calcareous strata over- and underlain by less calcareous
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mudstones.
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The Duvernay Formation is predominantly composed of organic-rich, siliceous to calcareous mudstones over much of the
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basin. East Shale Basin deposits are notably more calcareous than
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equivalent West Shale Basin sediments (Rokosh et al., 2012). Towards the northeast, adjacent to the Grosmont Platform,
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mudstones become argillaceous and organic-lean (Stoakes, 1980).
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Limestones and dolostones occur proximally to reef complexes. Organic-rich siliceous-calcareous mudstones are common within
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the lower and upper Duvernay members, while organic-lean limestones dominate the middle Duvernay member (Andrichuk, 1961). Member boundaries are lithostratigraphic. The Duvernay Formation dips to the southwest, with a thermal maturity range from immature in the southeast to dry gas in the southwest (Rokosh et al., 2012)
3 Methods: Eight drill cores (Table 1), were selected for detailed description. Cores were described at a scale of 1:10, paying special 9
ACCEPTED MANUSCRIPT attention to lithology, grain size, sedimentary structure, bioturbation, and presence of cements. Core selection was
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primarily based on total thickness of formation cored, core quality,
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and geographic distribution across the basin. An additional 16 cores were described in less detail to observe facies variations and
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stratigraphic surfaces over a greater extent of the basin. Few of the
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16 additional cores covered the entire Duvernay Formation interval. Total length of described Duvernay core was 628m. A
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total of 108 thin sections were cut from 4 cores, covering the major lithofacies and intervals of interest. Thin sections were ground to a
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thickness of 20 µm rather than the standard 30 µm, so that more detail could be observed in fine-grained facies (e.g. Macquaker et
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al., 2007). Most thin sections were cut perpendicular to bedding but a subset was selected for bed-parallel thin sections to further examine the nature of potential bioturbation features. Thin sections were scanned using a Nikon Super Coolscan 5000 ED scanner to observe centimeter- to millimeter-scale features. Millimeter- to micrometer-scale features were analyzed under transmitted and reflected white light using a Zeiss Axio Scope.A1 petrographic microscope. Samples for geochemical analysis were cut every 1 meter from 5 of the 8 cores. These 5 cores were chosen for wide geographic coverage of the basin and to represent a substantial
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ACCEPTED MANUSCRIPT range of thermal maturity, from immature to wet gas. Detailed geochemical analysis of these samples is presented in McMillan
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(2016). A 10 cm long by 2 cm thick slab was cut from the back of
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the core at each sample location. Vertical splits were cut along the length of the slabs for separate analyses. Weatherford Geochemical
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Services Group in Shanandoah, TX performed Leco-TOC and
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RockEval analysis. Total organic carbon values reported here are averages for each sampled facies from the SCL Kaybob 02-22,
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ECA Cecilia 11-04, GuideX Gvillee 09-06, and EOG Cygnet 0820 cores. Esso Redwater 16-28 was also sampled for TOC but was
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removed from average TOC calculations because it is much less thermally mature than the other sampled wells and, as such, has
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much higher TOC values for all facies. Samples from the Esso Redwater 16-28 well have average Tmax = 421 °C, while the other wells have average Tmax from 444 to 473 °C. Maturity in ECA Cecilia 11-04 is likely even higher but Tmax data was unreliable).
4 Results: 4.1 Lithofacies Descriptions Ten lithofacies comprise Duvernay Formation sediments over most of the basin. Lithofacies were characterized on the basis of composition, grain size, sedimentary structures, bioturbation, 11
ACCEPTED MANUSCRIPT and presence of cements and are described here in order of
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LF1: Planar laminated siliceous mudstone
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decreasing TOC.
LF1 is dark grey-brown, faintly planar-laminated siliceous-
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calcareous mudstone. Lamination is commonly difficult to recognize in core samples of the finest-grained occurrences of LF1
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(Fig. 3A) but in thin section may be seen as sub-millimeter-thick alternating silt-rich and silt-poor mud-supported laminae (Fig. 3B).
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Locally, lamination is defined by semicontinuous pyrite-rich laminae (Fig. 3C-E) or by grain-supported siltstone laminae (Fig.
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4). Pyritic laminae are typically less than 1mm thick, a few millimeters to several centimeters long and commonly pinch and swell. Pyrite in these laminae occurs as both framboidal and euhedral types, with varying abundance of pyrite-replaced carbonate silt grains and fossil fragments. Grading is generally absent, except in grain-supported siltstone laminae in which case both normal and inverse grading is observed. Lamina tops and bases are sharp to gradational. Silt grains are primarily carbonate, with some quartz, and rare chert. Laminae locally contain calcareous styliolinid and tentaculitid tests. Siltstone laminae are commonly cemented with calcite or silica cement. Bioturbation is
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ACCEPTED MANUSCRIPT typically not observed, but in silty sections, rare horizontal burrows with silty infill were present.
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Except for the silty laminae, LF1 is composed of an
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unstructured mixture of clay- and silt-sized detrital grains and
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organic matter. Detrital grains include calcite, dolomite, and quartz, with very minor abundance of petrographically-observable
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clay minerals. Low visual clay abundance is supported by
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geochemical results in McMillan, 2016. Clay minerals are locally observed as small aggregates, generally between 100 and 300 µm
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long and ~20 µm thick (Fig. 5A-B). Fragments or complete tests of
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calcareous styliolinids and tentaculitids are present but uncommon. Siliceous planktonic radiolaria are uncommon to common, are
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crushed and recrystallized, and appear as cherty masses (~50x10
µm). Amorphous organic matter occurs intimately mixed with matrix grains. Individual organic matter aggregates appear as dark brown stringers (commonly 50-200 µm long and less than 10 µm
thick) in bed-normal thin sections (Fig. 5C) and appear equidimensional to ovoid with diffuse or wispy edges in bedparallel thin sections (Fig. 5D). The average TOC for LF1 is 3.4 wt.% (SD=1.6, n=57).
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LF2: Wavy-laminated siltstones and silty mudstones LF2 consists of light to medium grey wavy-laminated silty-
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sandy laminae interlaminated and interbedded with dark grey-
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brown mudstone equivalent to LF1 (Fig. 6A). Wavy to lenticular
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lamination is most common, but sedimentary structures also include planar parallel lamination, low angle cross-lamination,
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starved ripples, loading structures, and flame structures. Laminae are typically grain-supported and show both normal and inverse
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grading (Fig. 6B). Normally graded laminae commonly have sharp
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bases while inversely graded laminae typically have sharp upper
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contacts. Grains are primarily carbonate with minor quartz and rare chert clasts and are typically fine silt to medium sand in size. Laminae locally contain bioclast fragments including styliolinids and tentaculitids. In cores where LF2 is abundant (e.g. Chevron Chickadee 03-05; Fig. 7), the facies is distinctly coarser-grained, and more fossiliferous beds are locally present, compared to cores in which LF2 is a more minor facies (e.g. Shell Kaybob 02-22). These fossiliferous beds are 1-6 cm thick and are always normally graded, with sharp, erosive bases. Fossil fragments commonly include brachiopods and crinoids. Bioturbation is minor to moderate in LF2 and is typically seen as silt-filled horizontal burrows, and homogenization of silt-rich and silt-poor laminae. 14
ACCEPTED MANUSCRIPT The average TOC for LF2 is 2.4 wt.% (SD=0.5, n=10), however the geochemical analysis was performed on vertical strips
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of sample 10cm long, which include both silty-sandy laminae
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described above and mudstones equivalent to LF1. Based on petrographic observation, TOC is insignificant in grain-supported
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becoming greatest in the mudstones.
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laminae and increases as laminae become mud-supported,
LF3: Siliceous-calcareous styliolinid wackestones
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LF3 is composed of siliceous-calcareous wackestone, forming beds a few millimeters to several centimeters thick. LF3 is
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usually poorly-bedded to structureless (Fig.8A), although uncommon sharp bases and normal grading can be observed. Bioclasts include whole or fragmented styliolinids, tentaculitids (Fig. 8B-C), and radiolaria. Fragments of brachiopods, bivalves (Fig. 8B), and crinoids are less commonly observed. Bioturbation is minor to intense. The matrix of LF3 is composed dominantly of clay- to silt-
sized carbonate grains and amorphous organic matter, with minor quartz and clay minerals. Amorphous organic matter occurs intimately mixed with matrix grains. The morphology of organic matter aggregates is similar to LF1. Organic matter abundance is 15
ACCEPTED MANUSCRIPT greatest in fine-grained, less calcareous beds. The average TOC for
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LF4: Bioturbated siliceous, pyritic mudstone
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LF3 is 2.3 wt.% (SD=1.2, n=19).
LF4 is composed of dark grey-brown poorly-bedded
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mudstone with pyritized burrows (Fig.9A-B). Bedding is uncommonly preserved, appearing as wispy, wavy laminae.
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Laminae are manifested by subtle variations in color due to varying silt abundance. Bedding is disrupted by subhorizontal to
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subvertical burrows that are quite easily recognized due to common filling or lining of burrows with pyrite (Fig. 9C). Trace
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fossil diversity is low.
Clay- and silt-sized quartz, carbonate, clay minerals, and
organic matter are primary components of the rock. Clay minerals are more abundant than previously described lithofacies, based on petrographic observation and major element geochemistry (McMillan, 2016).Clay minerals and amorphous organic matter are sometimes observed as aggregates. Aggregate morphology is as described above. The average TOC for LF4 is 2.1 wt.%. (SD=1.3, n=69). Calcareous radiolaria are uncommon to common. In-situ benthic macrofossils such as brachiopods and gastropods are common (Fig. 10) and in-situ benthic agglutinated foraminifera are 16
ACCEPTED MANUSCRIPT uncommon to common. Fossils were identified as in-situ based on preservation, position, and presence within mudstone beds that
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lack traction structures (i.e. no evidence for transport of
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grains/clasts or fossil fragments).
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LF5: Bioturbated calcareous mudstone
LF5 is composed of medium grey-brown poorly-bedded
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mudstone which is distinctly more calcareous than LF4. Where present, bedding appears as wispy, wavy laminae that are typically
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less than 8 mm thick (Fig. 11A). Subtly lighter-colored laminae contain more fine-grained calcite than darker laminae. LF5 is
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moderately to intensely bioturbated with low trace fossil diversity and burrows are horizontal to subhorizontal. Clay- and silt-sized calcite comprises most of LF5 (Fig.
11B-C). Fine calcareous fossil fragments are common. Dark brown, amorphous organic matter fills space between grains. Rarely, organic matter appears as dark brown stringers in bednormal thin sections (~50-200 µm long, <10 µm thick). In bedparallel thin sections, uncommon organic matter masses appear approximately equidimensional. The average TOC for LF5 is 1.8 wt.% (SD=1.6, n=24). No in-situ benthic macrofossils were
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ACCEPTED MANUSCRIPT observed but in-situ benthic agglutinated foraminifera are
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moderately common (Fig. 11D).
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LF6: Nodular wackestone
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LF6 is composed of nodular to burrow-mottled lime
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mudstone and wackestone. Light grey calcite wackestone nodules are hosted in dark grey mudstones to wackestones (Fig. 12).
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Nodules range from very irregular and burrow-mottled to ovoid to near-planar, and commonly contain spar-filled, wedge-shaped
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fractures. Burrows are preferentially cemented with calcite.
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Detrital grains within nodules are predominantly clay- and silt-
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sized calcite and calcareous fossil fragments, most commonly styliolinids, tentaculitids, and brachiopods. Nodules are surrounded by more ductile, uncemented
mudstone, which deforms around nodules. Dark grey mudstone is structureless to poorly bedded, and is calcareous to argillaceous. Where calcareous, mudstones may be compositionally similar to nodules, aside from a lack of cementation. Where clay content is high, mudstones are fissile and tend to physically separate from nodules. The average TOC for LF6 is 1.1 wt.% (SD=1.2, n=34), measured as a combination of nodule and mudstone.
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ACCEPTED MANUSCRIPT LF7: Thinly bedded argillaceous-dolomitic mudstone LF7 is composed of organic-lean, typically non-
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fossiliferous argillaceous-dolomitic mudstone, with laminated to
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thinly-bedded to burrow-mottled bedding style (Fig. 13).
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Millimeter- to centimeter-scale light grey-brown organic lean laminae/beds alternate with 1-2mm thick dark grey-brown, more
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organic-rich laminae, or blue-grey, anhydrite- rich laminae.
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Laminae/beds are most commonly ungraded or normally graded, and are less commonly inversely graded. Bedding contacts are
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typically gradational over 1-2 mm, and are usually irregular.
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Bioturbation ranges from moderate to intense. Horizontal to subhorizontal burrows are more common than subvertical burrows.
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Subvertical burrows are rarely more than a few millimeters to one centimeter long.
LF8: Dolowackestone and evaporite LF8 consists of nodular to burrow-mottled buff-colored
dolomitic to evaporitic mudstone-wackestone (Fig. 14A). Poorly bedded dolomitic-argillaceous mudstone equivalent to LF7 is deformed around dolomitic mudstone-wackestone nodules (Fig. 14B). Uncommon fossils include brachiopod and crinoid fragments. Anhydrite is very common as burrow-filling cement or 19
ACCEPTED MANUSCRIPT less commonly as anhydrite nodules (Fig. 14C). Uncommonly, dolomite nodules contain wedge-shaped fractures filled with
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calcite, dolomite and/or anhydrite. LF8 is organic-lean.
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LF9: Intraclastic packstone
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LF9 is composed of light grey intraclastic packstone. Bedding is structureless to planar-laminated and ungraded to
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normally-graded (Fig. 15A). Cross lamination is uncommon. Grains are most commonly sand-sized calcite clasts and fossil
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fragments, generally with minor quartz and dolomite silt, and siliceous, phosphatic, or pyritic clasts and fossil fragments (Fig.
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15B). Angular mud clasts are common. Fossil fragments include crinoids, gastropods, brachiopods, bivalves, styliolinids, tentaculitids and uncommon amphipora coral. Locally beds are significantly homogenized although specific ichno-taxa are generally not recognizable. LF9 is very organic-lean, with samples only containing
notable organic matter when other organic-rich facies were included. For this reason, TOC values are not reported here. Only very rare wispy organic matter is observed in thin section between grains.
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ACCEPTED MANUSCRIPT LF10: Limestone breccia LF10 consists of angular limestone clasts in a calcareous
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mudstone to packstone matrix (Fig. 15C-D). Beds are generally
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10-20cm thick, fine upwards or, less commonly, are ungraded.
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LF10 intervals may contain several stacked fining upwards beds. Bases of beds are erosive and commonly contain dark to medium
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grey, angular mudstone clasts. Limestone clasts are angular to
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poorly rounded and range from pebble- to boulder-sized. The matrix of LF10 is composed of calcareous mudstone to
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packstone. Coarse-grained matrix contains grains up to granule-
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sized (above which grains were not considered matrix), consisting
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of calcite detritus.
LF10 appears organic-lean based on scarcity of dark-
brown, bituminous mudstone matrix, however no geochemical samples were analyzed.
4.2 Depositional Architecture: Geographic and stratigraphic distribution of lithofacies varies considerably. The western and southern West Shale Basin is dominated by laminated, organic-rich mudstones of LF1, for example, in the SCL Kaybob 02-22 core (Fig. 16A). At GuideX
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ACCEPTED MANUSCRIPT Gvillee 09-06, near the Peace River Arch (Fig. 16B), planarlaminated mudstones are more argillaceous and clay mineral
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aggregates are more commonly observed. In the Wild River
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Subbasin, on the far west side of the West Shale Basin, LF1 mudstones commonly contain pyritic laminae and are frequently
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associated with LF4 bioturbated pyritic mudstones (e.g. the Encana
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Cecilia well; Fig. 17). Towards reef complexes, calcareous siltstone laminae become frequent within LF1 mudstones. Siltstone
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laminae have a higher proportion of quartz in the northwestern portion of the West Shale Basin, compared to anywhere else in the
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basin, although carbonate grains are everywhere the dominant component. LF1 laminated mudstones of the East Shale Basin,
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compared to the West Shale Basin, contain more common peloidal calcite silt, and calcareous styliolinid and tentaculitid tests, but siliceous planktonic radiolaria are not observed. Silty-sandy mudstones of LF2 are most common in the
West Shale Basin and are especially common and coarse-grained towards the northeast (Fig. 7, 18). LF2 is uncommon in the East Shale Basin, and where present, primary sedimentary structures are commonly disrupted by bioturbation. LF3 is more common in the East Shale Basin than the West Shale Basin, although it is consistently a minor facies. In the East Shale Basin, LF3 beds are more fossiliferous, more commonly 22
ACCEPTED MANUSCRIPT containing larger taxa such as brachiopods and bivalves, and contain more abundant clay- and silt-sized carbonate grains.
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Siliceous, pyritic, bioturbated mudstones of LF4 are most
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common in the western and southern West Shale Basin and are
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particularly thick in the Wild River Sub Basin (Fig. 17). Bioturbated mudstones are much more calcareous and less pyritic
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EOG 08-20 core; Fig. 19)
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in the East Shale Basin, where LF5 is a dominant lithofacies (e.g.
LF6 is the most stratigraphically-defined lithofacies as it is
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the dominant lithofacies within the middle Duvernay member (Fig.
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16A). The abundance of LF6 in the middle Duvernay member of
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the West Shale Basin consistently decreases to the south and west margins of the basin. Outside of the middle Duvernay member, LF6 is minor in the West Shale Basin but can be observed throughout the stratigraphic section in the East Shale Basin. LF7 and LF8 were only observed in cores in the
northeastern and eastern West Shale Basin and northern to northeastern East Shale Basin. These two lithofacies are generally observed together in core (Fig. 20, 21). LF9 was only observed in the East Shale Basin (Fig. 19) and proximal to reef complexes in the West Shale Basin. LF10 limestone breccias were only observed in the AOSC Grizzly 01-24 23
ACCEPTED MANUSCRIPT core, located adjacent to the northern margin of a reef complex in the West Shale Basin. No LF10 breccias were observed in East
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Shale Basin cores. LF10 intervals in the Duvernay Formation are
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immediately over- and underlain by organic-rich mudstones of
5 Interpretation:
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5.1 Lithofacies Interpretations
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LF1.
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LF1 planar laminated mudstones
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LF1 was interpreted primarily as the product of
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hemipelagic suspension settling, based on the scarcity of sedimentary structures, the mud-supported fabric, and the morphology of organic matter and clay mineral aggregates. Amorphous organic matter is present as organic matter aggregates, and is interpreted to be “marine snow” (Macquaker et al., 2010b), based on aggregates’ ovoid shape, wispy to diffuse edges and random distribution throughout the mudstone matrix. Such aggregates form in the upper water column due to random collision or the activities of organisms (e.g., Alldredge, 1976; McCave, 1984; Alldredge and Silver, 1988; Macquaker et al., 2010b) and settle through the water column. Clay minerals are present in low abundance in LF1, but in areas where clay content is greater, such 24
ACCEPTED MANUSCRIPT as the Wild River Subbasin and near the Peace River Arch, clay mineral aggregates are observed. These aggregates have similar
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morphology to the organic matter, suggesting a similar origin as
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floccules or fecal pellets in the upper water column. Based on their ovoid shape, some with diffuse to wispy edges, they are interpreted
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erosion and transport of mud clasts.
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to be a product of suspension settling of aggregates rather than
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Of secondary importance are turbidity currents and/or bottom water currents that deposited normally- and inversely-
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graded planar-parallel siltstone laminae that make up a smaller
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proportion of the rock. Grain-supported carbonate and quartz silt laminae are interpreted to be lag deposits. Similar silty laminae are
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found within LF2, interbedded with silty-sandy laminae that contain a variety of traction-type sedimentary structures, most of which are attributed to contour currents (see discussion below). Because of this association, the grain-supported laminae of LF1 are interpreted to be the result of uncommon very weak contour currents that winnow out clay-sized sediment (e.g. Shanmuggam, 2000; Martin-Chìvelet et al., 2008). Locally LF1 contains laminae enriched in framboidal and
euhedral pyrite, and varying amounts of pyrite-replaced carbonate silt and fossil fragments, largely formed during early diagenesis. Similar features have been noted in other Upper Devonian black 25
ACCEPTED MANUSCRIPT shales such as the Woodford Shale in the Permian Basin (N. Harris, personal communication) and Muskwa Formation in the
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Horn River Basin (Ayranci et al., 2016). Limiting factors in pyrite
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formation in marine sediments are the availability of metabolizable organic matter, the rate of diffusion of sulfate into sediments, and
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the concentration and reactivity of iron minerals, especially
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terrigenous clay (Berner, 1970). In LF1-4 of the Duvernay Formation, terrigenous mineral supply is low, based on
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petrographic observation and geochemical analysis (McMillan, 2016). Pyritic laminae of LF1 are interpreted to be the result of
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slightly increased clay mineral abundance. Most commonly, pyritic laminae are seen overlying flooding surfaces, in which case the
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clay minerals are interpreted to be the result of transgressive reworking of more clay mineral-rich regressive deposits, and within intervals of abundant and more argillaceous LF4 bioturbated pyritic mudstone (Fig. 17).
LF2 wavy laminated siltstones and silty mudstones Sedimentary structures in LF2 provide evidence of deposition from bottom water currents. These include planar parallel lamination, low angle cross-lamination, lenticular- to wavy-bedding, starved ripples, gradational to sharp erosive bases, both normal and inverse grading, sharp (non-erosional) upper 26
ACCEPTED MANUSCRIPT contacts, flame structures and soft sediment loading deformation. Shanmugam (2000) identified many of these features and others as
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evidence for reworking and transport by bottom water currents
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when several are observed in combination. Critically, most siltysandy beds and laminae of LF2 show no systematic vertical
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stacking of structures such as those recognized in the Bouma
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(1962) sequence. This observation, combined with the abovementioned sedimentary structures led to the interpretation that
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bottom water currents were a more significant process than turbidity currents for LF2 deposition. Contourite beds show minor
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to moderate bioturbation, likely reflecting variations in bottom water oxygenation and sedimentation rate, consistent with other
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ancient and modern contourite deposits (Dalrymple and Narbonne, 1996; Ito, 1996).
Turbidite beds are also present in LF2, identified by sharp,
erosive bases, normal grading, and the presence of coarser fossil fragments than in surrounding beds. Contourites are differentiated from turbidites by finer grain size and the presence of a distinctive set of sedimentary structures (described in previous paragraph).
LF3 bioturbated styliolinid wackestones
27
ACCEPTED MANUSCRIPT LF3 beds contain fossils and fossil fragments that are often much coarser than LF3 matrix grains and the clay to silt grain size
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of the under- and overlying beds. The presence of sharp bases and
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normal grading, although commonly disrupted or destroyed during bioturbation, leads to the interpretation of LF3 beds as turbidites.
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The fossiliferous nature of LF3 beds is likely due to grain size
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sorting within turbidity currents. Additionally, LF3 beds are consistently more bioturbated than over- and under-lying beds,
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which is likely to be result of temporary oxygenation due to
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downslope transport.
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LF4 bioturbated siliceous pyritic mudstone, LF5 bioturbated calcareous mudstone and LF6 nodular wackestone LF4, LF5 and LF6 are moderately to intensely bioturbated,
and LF6 commonly contains in-situ benthic macrofossils. Bottom
waters are interpreted to have been dysoxic based on moderate to intense bioturbation but low trace fossil diversity, as demonstrated in studies of modern sediments (e.g. Bromley and Ekdale, 1984; Ekdale and Mason, 1988). Additionally, low-oxygen-tolerant
benthic agglutinated foraminifera are observed in LF4 and LF5. In other organic-rich mudstone formations, the presence of benthic agglutinated foraminifera has been used to infer the presence of dysoxic rather than anoxic bottom waters (Milliken et al., 2007; 28
ACCEPTED MANUSCRIPT Schieber, 2009). Benthic agglutinated foraminifera have a high tolerance for low oxygen conditions but cannot survive in anoxic
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bottom waters (Bernhard and Reimers, 1991; Bernhard et al.,
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2003).
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Total organic carbon values are lower in these lithofacies than in LF1-4, which is likely due to an increase in bottom water
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oxygenation and increased aeration of sediment due to burrowing
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activity (e.g. Rhoads, 1974; Aller, 1978; Demaison and Moore, 1980). However LF4 is relatively enriched in organic matter
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(average 2.1% TOC) compared to similarly bioturbated LF5 and
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LF6. One factor in organic matter enrichment in LF4 may be increased sedimentation rates. A slight increase in sedimentation
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rate due to a greater influx of clay minerals in LF4 with respect to LF5 could have more rapidly buried organic matter below the reach of oxidants, which has been shown to result in enhanced organic carbon preservation in depositional settings that are not persistently anoxic (Tyson, 2005; Bohacs et al., 2005; Macquaker et al., 2010b). The higher clay content in LF4 may also be linked to increased organic matter preservation (Macquaker et al., 2010b). Kennedy et al. (2002) demonstrated that increased preservation of organic matter can be associated with the high surface area in clay minerals.
29
ACCEPTED MANUSCRIPT The absence of primary sedimentary structures in LF5 suggests that episodes of rapid sedimentation were uncommon.
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Interlaminated layers of variable calcite silt content may be the
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remnants of thin turbidite or contourite beds, although it is difficult to characterize sediment transport mechanisms here with
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confidence, since primary sedimentary structures are destroyed by
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bioturbation. Low oxygen concentration rather than soft substrate is interpreted to be the cause of the absence of benthic
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macrofossils, as soft substrate reduces benthic diversity but does not eliminate it (Rhoads and Morse, 1971; Byers, 1977; Wignall,
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1993).
In LF6, calcareous nodules contain abundant fine
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calcareous fossil fragments, which may be the result of event bed deposition, although any sedimentary structures diagnostic of turbidity currents have largely been destroyed by bioturbation. Where LF6 is not considerably burrow-mottled, calcite nodules form semi-continuous beds that alternate with uncemented mudstone beds, possibly reflecting changes in the concentration of carbonate detritus. Mudstone-wackestone beds between nodular horizons likely accumulated during prolonged periods of lower sedimentation rates and dysoxic bottom waters. These beds are variably bioturbated but sometimes contain crude lamination and are more organic-rich than calcite nodules.
30
ACCEPTED MANUSCRIPT Uncemented mudstones and wackestones are deformed around calcite nodules, suggesting that nodule formation predates
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complete compaction. Additionally, wedge-shaped, calcite spar-
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filled fractures in calcite nodules suggest that brittle fracturing of nodules occurred, likely due to compaction post-dating early
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lithification. Stoakes (1980) came to the same conclusion, as have
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other authors studying similar deposits (Tucker, 1973; Noble and
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Howells, 1974; Kennedy and Garrison, 1975; Mullins et al., 1980).
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LF7 thinly-bedded argillaceous-dolomitic mudstone LF7 is largely composed of light brown mud-rich laminae
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that are interpreted to be deposited from sediment-gravity flows, based on the abundance of sharp-based, normally graded beds. Similarly graded mud beds have been identified in other studies (e.g. Bhattacharya and MacEachern, 2009; Macquaker et al., 2010a; Macquaker et al., 2010b; Aplin and Macquaker, 2011). Other light brown beds in the lithofacies show inverse grading or both inverse and normal grading, suggesting deposition during waxing and waning flow energies. These conditions can be created by density-driven deep-water contour currents (e.g. Rebesco et al., 2014) or currents driven by climatic factors such as wind, runoff, precipitation and evaporation rates, temperature gradients, and
31
ACCEPTED MANUSCRIPT mixing of water bodies (e.g. Kump and Slingerland, 1999). Darker brown laminae are interpreted to be more organic-rich, based on
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color, and may represent periods of reduced inorganic sediment
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supply and increased concentration of preserved organic matter. Grey-blue, anhydrite-rich laminae that alternate with light brown
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laminae may represent periods of evaporite precipitation that
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occurred between increased sedimentation events. Salinity fluctuations are indicated by anhydrite-enriched laminae which are
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more common in upslope positions where LF7 is interbedded with
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LF8.
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The above described sedimentary structures are locally disturbed or destroyed due to moderate to intense bioturbation,
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indicating dysoxic to oxic conditions and varying sedimentation rates. A lack of organic matter is consistent with oxic conditions, although the absence of in-situ benthic macrofossils suggests that oxygen levels may have varied, and rapid sedimentation may also have limited colonization and diluted organic matter. Variability in oxygen conditions has been suggested to deter colonization of shelly benthic fauna, and act to preserve lamination from disruption by burrowers (Savrda et al., 1984).
LF8 dolowackestone and evaporite
32
ACCEPTED MANUSCRIPT The nodular fabric of LF8, similar to LF6 nodular wackestones, results from early cementation. Ductile, more
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argillaceous mudstone between nodules is deformed around
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nodules, indicating that nodules had formed before significant compaction and lithification of the mudstone. Rare wedge-shaped
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fractures in nodules are filled with calcite, dolomite and/or
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anhydrite and are the result of brittle fracture – another indicator of early cementation (Tucker, 1973; Noble and Howells, 1974;
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Kennedy and Garrison, 1975; Mullins et al., 1980; Stoakes, 1980). The presence of nodules may indicate that overall sedimentation
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rates were low, allowing time for nodule formation before significant burial and compaction.
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Intense bioturbation and scarcity of organic carbon indicate
that LF8 was deposited in an oxic to dysoxic environment. Anhydrite is very common as burrow fills and uncommon as nodules, which may indicate that salinity levels were high. High or fluctuating salinity may explain the scarcity of benthic macrofossils. LF8 is interpreted to have been deposited on the carbonate
platform margin or upper foreslope in relatively oxic bottom waters. Deposition of clay minerals was limited by increased wave energy and increased carbonate sediment supply (Stoakes, 1980).
33
ACCEPTED MANUSCRIPT
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LF9-10 packstones and breccias LF9-10 packstones and breccias contain angular clasts and
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display poor sorting and normal grading, suggesting deposition
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from turbidity currents and debris flows. Debris flows were likely
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caused by slope instability of reef margins (e.g. Braithwaite, 2014). McLean and Mountjoy (1993) suggested that northeastern sides of
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reef complexes in the Alberta Basin had steeper slopes due to the northeast to southwest direction of wind and surface water
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currents. Southwestern sides of reef complexes and areas protected
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from wave energy had more gently dipping reef margins due to the
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accumulation of fine-grained carbonate detritus. LF10 breccias were only observed in the AOSC Grizzly 01-24 core which is located adjacent to the northern margin of the Bigstone reef complex. LF10 breccias likely occur only very close to reef margins. Additionally, authors such as Mountjoy (1980), McLean and Mountjoy (1993), Van Buchem et al. (1996a), and Whalen et al. (2000) have documented in outcrops of time-equivalent reef margin strata that limestone breccias and conglomerates extend a few hundred meters to a few kilometers into the basin from reef margins.
34
ACCEPTED MANUSCRIPT LF9 packstones are more widespread and were deposited further from reef complexes than LF10 breccias (i.e. EOG Cygnet
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08-20 core). However, LF9 packstones are not observed in the vast
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majority of the West Shale Basin due to longer transport distances from reef complexes to our cores. This lithofacies was also likely
SC
generated by slope failure events, as suggested by the wide range
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of clasts, and presence of shallower-water taxa than in organic-rich
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mudstones.
Breccias and packstones are commonly under- and overlain
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by dark grey, organic rich mudstones of LF1, indicating that they
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were deposited as event beds in an environment that was suitable
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for the accumulation of fine-grained, organic rich sediment.
5.2 Depositional Model Described here is a depositional model created from the
summation of the observations and interpretations outlined above. Analysis of lithofacies distribution and characterization of depositional processes and conditions enable us to understand the drivers that controlled the deposition and character of Duvernay Formation sediments. The depositional model is separated into two phases: an initial period of platform-building (Fig. 22A), which largely describes lower and middle members of Duvernay 35
ACCEPTED MANUSCRIPT Formation strata, and a later period when the platform was flooded (Fig. 22B), which generally describes the upper Duvernay
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member. These two phases fall into the sequence stratigraphic
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framework defined by Knapp (2016), and agree with the reciprocal style of sedimentation described by Stoakes (1980) and Cutler
NU
SC
(1983), among others.
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Platform Construction Stage (lower and middle Duvernay) In the northeast part of the basin, adjacent to the Grosmont
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Platform, LF7 argillaceous-dolomitic mudstone and LF8 dolowackestones and evaporites are dominant in the lower and
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middle Duvernay members; these represent a well-oxygenated platform-margin and oxic to dysoxic slope environment, and their great thickness (Knapp, 2016) reflects aggradation of the carbonate platform during a prolonged period of platform-building (Fig. 22A). LF7 is enriched in terrigenous clay minerals, and represents slope deposition. Clastic deposition was a result of deposition of clays on the slope, transported by currents from the north, parallel to the Grosmont Platform edge (Stoakes, 1980; Knapp, 2016). These currents may have been driven by climatic factors such as wind, runoff, precipitation and evaporation rates, temperature gradients, and mixing of water bodies (e.g. Slingerland et al., 1996;
36
ACCEPTED MANUSCRIPT Kump and Slingerland, 1999). Drivers for basin circulation are not the focus of this paper, and little work has been published on the
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subject for the Alberta Basin (Mallamo, 1995), so patterns of
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sediment distribution are currently the strongest evidence for the character of basin circulation (Andrichuk and Wonfor, 1954;
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Newland, 1954; Andrichuk, 1958, 1961; Staplin, 1961;
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McCrossan, 1961; Klovan, 1964; Wendte, 1974; Stoakes, 1980;
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McLean and Mountjoy, 1993; Knapp, 2016). In the West Shale Basin, LF6 nodular wackestones were
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the dominant lithofacies immediately basinward of the major
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platform-building sediments (LF7 and LF8). Nodular wackestones of LF6 are moderately to intensely bioturbated, suggesting dysoxic
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rather than anoxic bottom waters, and the variable abundance of fragmented shell material suggests periodic downslope transport of coarser carbonates.
Further westward and southward into the West Shale Basin,
bottom water anoxia was much more persistent and extensive, and organic-rich sediments were dominant. In these areas, much of the clay-sized sediment was deposited from suspension, but silt- and sand-sized detritus shows evidence for transport by bottom water currents and turbidity currents. Organic matter formed aggregates in the upper water column, settling to an anoxic sediment-water interface, which inhibited the breakdown of organic matter. 37
ACCEPTED MANUSCRIPT Dilution of organic-rich sediments by northeast-derived clastics was minimal.
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In the East Shale Basin, the lower and middle Duvernay
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members are dominated by bioturbated lithofacies, especially LF5
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bioturbated calcareous mudstones and LF6 nodular wackestones, indicating that bottom water anoxia was rare at this time. This was
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probably due to shallow water depths. Organic carbon
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accumulation in East Shale Basin sediments was hampered by higher oxygen levels, and diluted by increased carbonate sediment
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supply.
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Flooded Platform Stage
The upper Duvernay member is best characterized by a
flooded platform model (Fig. 22B). The platform-to-basin sequence of facies described above still generally applies during the flooded platform phase, but distinct differences exist that are important to an understanding of the lithofacies distribution. The oxygenated platform margin in the northeast part of the basin is again dominated by LF7 and LF8; however, the thickness of these deposits is greatly reduced during the flooded platform stage, and the areal extent of these lithofacies is shifted to the north and east (Knapp, 2016). This backstepping resulted in the 38
ACCEPTED MANUSCRIPT formation of a zone of non-deposition over older platform margin and slope sediments deposited during the earlier platform-building
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phase. Non-deposition was probably supported by strong NW-SE-
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flowing currents, which encouraged NE-derived sediments to prograde parallel to the Grosmont Platform edge, rather than
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perpendicular to it (Knapp, 2016). This relationship significantly
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limited the supply of NE-derived sediment in deeper areas of the
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basin.
The areal extent of siliceous, organic-rich mudstones of
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LF1 greatly increased during the flooded platform phase (Fig. 22B;
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Knapp, 2016), and limited input of NE-derived sediments is responsible for the clay-poor nature of these facies. Dilution of
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organic-rich sediment by carbonate detritus was only significant near Leduc reef complexes and at the base of the Grosmont Platform. At the base of the Grosmont Platform, contour currents transported silt- and sand-sized carbonate detritus (LF2) that was originally sourced from reefs and/or the platform margin. LF2 records interbedded black shale and silty-sandy contourite deposition in a variably energetic, but oxygen-deprived environment. In the far west of the West Shale Basin, the Wild River
subbasin is dominated by LF4 bioturbated pyritic mudstones. The dominance of a more clastic-rich facies so far removed from the 39
ACCEPTED MANUSCRIPT northeasterly-derived clastic sediment source, suggests that a second minor source of clastics existed (Fig. 22B), possibly the
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subaerially-exposed Peace River Arch on the northwestern corner
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of the basin. Organic carbon values in LF4 are unexpectedly high, given that the sediments are thoroughly bioturbated. Increased
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sedimentation rates may have more rapidly buried organic matter
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below the sediment-water interface, which was clearly not anoxic. Additionally, the aggregation of organic matter and clay minerals
MA
(e.g. Macquaker et al., 2010b) and/or adsorption of organic matter onto clay minerals (e.g. Kennedy et al., 2002) seems to be
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enhanced with increased clay mineral supply.
6 Discussion
6.1 Bottom Water Currents in Organic-Rich Facies The recognition of event beds and traction-derived
sedimentary structures in black shale sequences (e.g. Schieber, 1999; Macquaker and Bohacs, 2007; Macquaker et al., 2010b; Ghadeer and Macquaker, 2011, 2012; Abouelresh and Slatt, 2012; Hemmesch et al., 2014) indicates that bottom waters were not persistently stagnant. In the Duvernay Formation, the presence of contourite and turbidite beds within black shale units of LF1 and LF2 indicates that quiet water deposition was not persistent and that bottom water currents commonly transported larger grains and 40
ACCEPTED MANUSCRIPT reworked sediment. Contourite and turbidite beds are commonly interbedded in continental slope, rise, and abyssal plain settings
PT
(Rebesco and Camerlenghi, 2008; He et al., 2008; Mulder et al.,
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2008; Gao et al., 1995; Stow et al., 2002d; Moraes et al., 2007; Stow et al., 2002f; Viana and Rebesco, 2007); in these settings,
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bottom water currents may rework turbidite beds or more rarely,
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pirate sediment directly from turbidity currents (Rebesco et al., 2002; 2007). Turbidites likely provided a source of sediment for
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contourites in LF1 and LF2.
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Uncommon turbidite beds within LF1 and LF2 represent
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very short-term events that introduce fresh sediment. However, the majority of the coarser grained material was reworked and
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deposited by contour currents, which represent longer-term fluctuations in bottom water energy. Stow et al. (2008) noted that bottom water currents show variability in strength over periods of tens of years to thousands of years, which can be linked to climatic cycles and sea level cycles. The recognition of bottom water current deposits depends on a diagnostic set of primary sedimentary structures and lack of systematic vertical stacking of structures such as those recognized in the Bouma (1962) sequence (see section 4.1 Lithofacies Interpretation; Martìn-Chivelet et al., 2008; Shanmugam, 2008). However, some authors have suggested that pervasive bioturbation 41
ACCEPTED MANUSCRIPT that eliminates primary sedimentary structures is more common (Stow and Faugères, 2008; Wetzel et al., 2008). Modern examples
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in which bioturbation is associated with oxygen-rich bottom
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currents include the Faro Drift in the Gulf of Cadiz (Gonthier et al., 1984; Stow and Holbrook, 1984; Stow and Piper, 1984). Studies of
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contourites from recent and ancient successions show that
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bioturbation in contourites can be highly variable and locally nonexistent (Dalrymple and Narbonne, 1996; Ito, 1996), dependent on
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bottom water oxygenation and sedimentation rate.
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Bioturbation in LF1 and LF2 is absent to moderate,
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indicating that bottom water currents carried low but variable concentrations of dissolved oxygen or that sedimentation rate was
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too rapid for significant bioturbation of contourite beds. While many bottom water currents in modern oceans occur in oxygenated environments, paleoceanographic conditions were likely much different during the Late Devonian (e.g. Caplan and Bustin, 2001). Eustatic sea level was roughly 150m higher during the Frasnian than today (Johnson et al., 1985; Savoy and Mountjoy, 1995; Haq and Schutter, 2008), an epicontinental seaway covered much of North America, and black shale deposition was widespread. Bottom water oxygen concentration seems to have been minimal (although not necessarily anoxic) during deposition of many Devonian black shales (e.g. Harris et al., 2013), even though
42
ACCEPTED MANUSCRIPT evidence for traction currents is now commonly recognized (e.g. Schieber, 1999; Macquaker and Bohacs, 2007; Macquaker et al.,
PT
2010b; Ghadeer and Macquaker, 2011, 2012; Abouelresh and
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Slatt, 2012; Hemmesch et al., 2014). While it may be that contour currents during times of high sea level do not necessarily introduce
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oxygen into deep parts of the basin, bioturbation in contourite beds
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may also be limited due to increased sedimentation rates.
6.2 Aggregate Grains and Suspension Settling
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Flume experiments by Schieber et al. (2007b) and Schieber
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and Southard (2009) showed that flocculated clay minerals can be
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transported as bedload without disaggregating and can form sedimentary structures such as ripples. Schieber et al. (2010) detailed the transport of mud rip-up clasts in flume experiments, but noted that these rip-up clasts show irregular outlines in plan view and are poorly sorted. Ripple cross-lamination containing clay mineral aggregates or clasts has not been observed in Duvernay Formation mudstones. Additionally, clay mineral aggregates in Duvernay Formation sediments tend to have ovoid rather than irregular outlines in plan view, diffuse rather than sharp edges, and a relatively narrow size range as opposed to the poorly sorted clasts described above. Könitzer et al. (2014) identified similar features in late Mississippian mudstones of the Widmer 43
ACCEPTED MANUSCRIPT Gulf, UK, attributing them to suspension settling of aggregates. They also recognized slightly larger clay mineral aggregates with
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sharper edges and tapered ends, which they attributed to erosion
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and transport of mud clasts. Clay mineral aggregates and organic matter aggregates observed in Duvernay Formation sediments
SC
were deposited through suspension, and any subsequent reworking
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by bottom water currents should have caused disaggregation and resuspension of clay-size particles, rather than traction transport of
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mud clasts or floccules. Abundance of aggregates decreases with
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sediment agitation.
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increasing bioturbation, which suggests disaggregation during
In modern environments, aggregation of organic matter
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increases settling velocity (Shanks, 2002), more rapidly removing aggregates from the water column. Marine snow is most common in modern oceans where primary productivity in the upper column is high (Alldredge, 1976; Billett et al., 1983; Lampitt, 1985; Thiel, 1995; Grimm et al., 1997; Fortier et al., 2002), and some authors have interpreted ancient beds rich in organic matter aggregates to be the result of enhanced productivity or algal blooms (Macquaker et al., 2010b). Organic matter aggregates in Duvernay Formation mudstones are randomly dispersed in the matrix rather than concentrated in layers, suggesting that enhanced productivity events, such as algal blooms, were not frequent. Chow et al. (1995)
44
ACCEPTED MANUSCRIPT reached a similar conclusion by mapping organic facies and depositional facies in Duvernay Formation sediments near the
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Redwater reef complex, concluding that evidence for algal bloom
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facies (defined by algal akinete cells and large, thick-walled Prasinophyte phycomas) in Duvernay Formation sediments was
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generally absent, indicating that organic accumulation was a
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function of normal productivity and low oxygen conditions.
MA
The dual roles of suspension settling and bottom current/turbidity current transport are important to note, as suspension-deposited
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sediment in some organic-rich mudstones represents a very minor
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component of the rock (e.g. Ghadeer and Macquaker, 2011, 2012), but is a more significant component of Duvernay Formation
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deposits.
7 Conclusion:
The detailed lithofacies analysis presented in this study describes the fine-scale variability of lithofacies in Duvernay Formation mudstones, and is the foundation for a basin-scale depositional model. While suspension-deposited sediment is significant in the most organic-rich Duvernay Formation lithofacies, we present evidence of other processes based on the variety of sedimentary structures. 45
ACCEPTED MANUSCRIPT Unlike some other organic-rich mudstone successions (e.g. Ghadeer and Macquaker, 2011, 2012), suspension settling is
PT
locally significant, although it was never the sole depositional
RI
mechanism. A combination of suspension settling, sedimentgravity flows, and bottom water currents distributed sediment
SC
throughout the basin. Near reef complexes, downslope sediment-
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gravity deposits are common, represented by limestone breccias and packstones. Turbidite beds are much more minor further into
MA
the basin but are present as thin fossiliferous beds. Siliciclastic sediment was transported into the basin from the north by slope-
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parallel currents (Stoakes, 1980; Knapp, 2016), whose position and strength were in part determined by basin morphology. Contour
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currents also acted within areas of the basin where organic-rich mudstones were dominant, creating interbedded muddy and silty/sandy successions which still contain significant TOC values. Organic matter and clay minerals deposited from suspension are commonly observed as aggregates. Total organic carbon varies systematically within a lithofacies framework, and organic-enrichment is attributed to a variety of factors. The highest TOC values are observed in planarlaminated, siliceous mudstones, deposited in generally anoxic bottom waters, where dilution of organic matter by clastic or carbonate sediment was minimal. However, significant organic-
46
ACCEPTED MANUSCRIPT rich deposits also occur where contourites are interbedded with siliceous, organic-rich mudstone. Periodic fluctuations in current
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energy and bottom water oxygenation generally acted to reduce
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TOC values, but not necessarily enough to exclude the strata from being prospective for hydrocarbon exploration. Organic
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enrichment also occurred in dysoxic bottom waters where sediment
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composition and increased sedimentation rate were key to organic matter preservation. Clay minerals may have aided in the
MA
formation of organo-minerallic aggregates which decreased the time organic matter was exposed to oxygenated water. Where
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bottom water anoxia was not present, an optimal sedimentation rate may have been key to preserving but not significantly diluting
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organic matter. Organic enrichment in a variety of depositional settings illustrates the interplay of bottom water oxygen concentrations, sedimentation rates, and sediment composition. The relative importance of each factor varies, even within a single succession of mudstones. The character and distribution of lithofacies depended heavily on type and proximity of sediment sources that varied throughout depositional period of the Duvernay Fornation. The relative importance of carbonate, clastic, and biogenic sediment varied through time and with geographic location. Changes in basin morphology, water circulation, and sea level acted to increase or
47
ACCEPTED MANUSCRIPT reduce the significance of each sediment source, and thus the character of sediments deposited.
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Recent literature has broken down the paradigm of organic-rich
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mudstone deposition as the sole product of suspension settling in
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quiet-water, often stratified, anoxic conditions. Depositional conditions are often dynamic, as our results support. However each
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mudstone succession is different. While some recent studies
MA
provide evidence for traction transport of mud clasts and floccules, our study shows no evidence for this phenomenon. There is
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abundant evidence for bottom water currents in mud-dominated
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settings, but clay-sized particles were resuspended, rather than transported in traction with the silt- and sand- sized sediment. We
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have shown that the mechanisms for organic-enrichment are variable, even between contemporaneous deposits in different areas of a basin, reflecting the dynamic interplay of bottom water oxygen concentration, sedimentation rate, and sediment composition. The effect of basin circulation patterns on mudstone composition is significant – a subject which seems to be underrepresented in the literature. The effect of current direction in shallower parts of a basin can strongly control the amount and type of sediment supplied to the deeper basin, affecting both composition and organic-richness of the deeper water mudstones.
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ACCEPTED MANUSCRIPT Acknowledgements: This work was supported by Imperial Oil, Shell Canada,
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ConocoPhillips, Nexen, Devon Energy, Husky Energy, and the
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Natural Sciences and Engineering Research Council of Canada
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(grant number CRD 445064-12).
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Throughout the project, technical discussions with a number of individuals helped form and evaluate the interpretations
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presented here. These people are Korhan Ayranci, Tian Dong, Noga Vaisblat, Chris Schnieder, Murray Gingras, Matthew Fay,
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Stow, D.A.V., Pudsey, C.J., Howe, J.A., Faugères, J.-C., Viana, A.R. (Eds.), Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics. Geol. Soc. London Mem. 22, 443–455.
Stow, D.A.V., Pudsey, C.J., Howe, J.A., Faugères, J.-C., Viana, A.R. (Eds.), 2002f. Deep-Water Contourite Systems: Modern Drifts and Ancient Series, Seismic and Sedimentary Characteristics. Geol. Soc. London Mem., Vol. 22, 464 pp.
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ACCEPTED MANUSCRIPT Stow, D.A.V., Hunter, S., Wilkinson, D., Herna´ndez-Molina, J., 2008. The nature of contourite deposition. In: Rebesco, M.,
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A.S., McAuley, R.J., Wierzbicki, R.A., and Packard, J.J., 1994. The Woodbend-Winterburn strata of the Western Canada
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Pollution Bulletin 30, 490–491. Tucker, M. E., 1973. Sedimentology and diagenesis of Devonian pelagic limestones (Cephalopodenkalk) and associated sediments of the Rheno-Hercynian Geosyncline, West Germany: Neues Jahrbuch f. Geologie u. Palaontologie Abhandlungen Bd. 142, 320350.
Tyson, R V., 2005. The "productivity versus preservation" controversy; cause, flaws, and resolution. In: Harris, N.B. (Ed.), The deposition of organic-carbon-rich sediments: models,
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Van Buchem, F.S.P., Eberli, G.P., Whalen, M.T., Mountjoy, E.W., and Homewood, P.W., 1996a. Basinal geochemical cycles and
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platform margin geometries in the Upper Devonian carbonate
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Wendte, J. C., 1974, Sedimentation and diagenesis of the Cooking Lake platform and lower Leduc reef facies, upper Devonian, Redwater, Alberta. Ph.D. dissertation, University of California, Santa Cruz, 222p.
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Wetzel, A., 2010. Deep-sea ichnology: Observations in modern
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Whalen, M.T., Eberli, G.P., Van Buchem, F.S.P., Mountjoy, E.W.,
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Homewood, P.W., 2000. Bypass margins, basin-restricted wedges, and platform-to-basin correlation, Upper Devonian, Canadian
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Rocky Mountains: Implications for sequence stratigraphy of carbonate platform systems. Journal of Sedimentary Research 70, 913-936.
Wignall, P.B., 1991. Dysaerobic trace fossils and ichnofabric in the Upper Jurassic Kimmeridge Clay of southern England. Palaios 6, 264–270.
Wignall, P.B., 1993. Distinguishing between oxygen and substrate control in fossil benthic assemblages. Journal of the Geological Society of London 150, 193-196.
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Wignall, P.B., and Pickering, K.T., 1993. Paleoecology and
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sedimentology across a Jurassic fault scarp, northeast Scotland.
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Journal of the Geological Society of London 150, 323–340.
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Figure Captions:
Table 1: Name and location of described cores. Length is described
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Duvernay section.
Figure 1: Map of study area. Duvernay Formation organic-rich
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mudstones are the basinal equivalent of Leduc Formation reefs. Organic-lean mudstones are deposited adjacent to the Grosmont
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Platform. The Peace River Arch was an emergent land mass during the Late Devonian, and was fringed my Leduc Formation reefs. Data from Switzer et al., 1994.
Figure 2: Late Devonian stratigraphy of Central Alberta. The Duvernay Formation is informally divided into lower, middle, and upper members. Modified from Switzer et al., 1994.
Figure 3: LF1 planar-laminated mudstone. A) Planar laminae are very subtle in core. Core photo, SCL Kaybob 02-22. B) Laminae 75
ACCEPTED MANUSCRIPT are faintly defined by increases in silt (arrow). Most silty laminae are mud-supported. Thin section scan, SCL Kaybob 02-22. C)
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Discontinuous planar laminae are enriched in framboidal and
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euhedral pyrite as well as pyritized fossil fragments and carbonate silt. Core photo, CNRL Edson 01-10. D) Lamination is defined by
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discontinuous pyrite laminae (arrow) and continuous silt-rich and
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silt-poor laminae. Discontinuous pyrite laminae pinch and swell. Thin section photomicrograph, plane polarized light, SCL Kaybob
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02-22. E) Pyrite (bright reflectance) in laminae is present most commonly as euhedral crystals and framboids, but also as full and
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partial replacement of calcareous grains and fossil fragments. Thin section photomicrograph, reflected white light, SCL Kaybob 02-
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22.
Figure 4: Silty laminae in LF1. A) Silty laminae are clearly visible in hand sample. Core photo, ECA Cecilia 11-04. B) Silt is dominantly calcite and dolomite with lesser amounts of quartz and rare chert clasts. Thin section photomicrograph, cross-polarized light, SCL Kaybob 02-22. C). Silty laminae show both normal and inverse grading. Grain-supported laminae are more common and prominent than LF1-2. Thin section scan, SCL Kaybob 02-22.
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ACCEPTED MANUSCRIPT Figure 5: Types of sedimentary aggregates in LF1. A) Clay mineral aggregate in bed-normal thin section. Thin section
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photomicrograph. B) Clay mineral aggregate in bed-parallel thin
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section. Clay mineral aggregates are round to ovoid in plan view and contain much less silt than surrounding sediment. Thin section
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photomicrograph. C) Organic matter aggregate in bed-normal thin
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section. Thin section photomicrograph. D) Organic matter aggregate in bed-parallel thin section. Organic matter aggregates
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are round to ovoid in plan view and have wispy edges. Thin
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section photomicrograph. All samples from GuideX Gvillee 09-06.
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Figure 6: LF2 wavy-laminated mudstone. A) Lenticular silt laminae pinch and swell and contain starved ripples (arrow). Core photo, Chevron Chickadee 03-05. B) Silty laminae show normal and inverse grading. Thin section scan, SCL Kaybob 02-22. C) Silt is a mixture of calcite, dolomite, quartz, and uncommon chert clasts. Dark, silt-poor laminae are rich in clay-sized grains and organic matter. Thin section photomicrograph, cross-polarized light, SCL Kaybob 02-22
Figure 7: LF2 in the NE West Shale Basin. A) Silt-sand beds are coarser, more prominent, often with sharp, erosive bases, and 77
ACCEPTED MANUSCRIPT sharp, non-erosive tops. Contourite bed shows low angle cross lamination near base (white arrow) and mud drapes (yellow arrow)
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near the top of the bed. Thin section photograph. B) Sharp-based,
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normally graded turbidite beds have especially coarse, fossiliferous bases and become more common. Core photograph. Both images
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from Chevron Chickadee 03-05.
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Figure 8: LF3 siliceous-calcareous wackestone-floatstone. A) Poorly bedded styliolinid wackestone. Calcareous styliolinid
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fossils in medium grey, organic-rich calcareous-siliceous
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mudstone. Core photo, EOG Cygnet 08-20. B) Intensely
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bioturbated styliolinid wackestone. Uncommon larger fossils include brachiopods. Pyrite preferentially precipitates in traces (yellow). Thin section scan, SCL Kaybob 02-22. C) Styliolinid and tentaculitid fossils are unevenly distributed. Matrix contains abundant fine grain calcite debris. EOG Cygnet 08-20.
Figure 9: LF4 bioturbated siliceous pyritic mudstone. A) Intensely bioturbated mudstone. Pyrite preferentially precipitates in traces. Core photo, ECA Cecilia 11-04. B) Most of the sample is structureless but a faint bedding contact (yellow arrow) can be seen in the lower half of the image. White arrows point to pyrite-rich 78
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lined with pyrite (black). Burrow fill is less silty then surrounding
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sediment. Silt grains are dolomite, calcite and quartz. Brown
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photomicrograph, ECA Cecilia 11-04.
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matrix contains clay minerals and organic matter. Thin section
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Figure 10: Benthic fossils in LF4. A) In-situ bivalve in bedding surface of core. Core photo. B) In-situ gastropod on bedding
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surface of core. Gastropod fossils larger than core diameter are
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Cecilia 11-04.
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common in LF4 of ECA Cecilia 11-04. Both photos from ECA
Figure 11: LF5 bioturbated calcareous mudstone. A) Poorly
bedded, moderately bioturbated calcareous mudstone. Lighter colored beds have greater calcite abundance. Core photo. B) Structureless calcareous mudstone. Clay- and silt-sized calcite is abundant. A lesser amount of quartz silt is present. Thin section scan. C) A large percentage of the rock is composed of calcite silt and calcareous fossil fragments. Minor organic matter is present between grains. Thin section photomicrograph, plane-polarized light, calcite is stained pink. D) Agglutinated benthic foraminifera 79
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light. All samples from EOG Cygnet 08-20.
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Figure 12: LF6 nodular wackestone. A) Nodular limestone in
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Perdrix Formation outcrop (outcrop nomenclature for Duvernay
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Formation). Nodule size and morphology is variable and ranges from ovoid to irregular. Outcrop location: Nigel Peak / Wilcox
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Creek, Alberta, Canada. Lense cap dimeter = 5cm. B) Uncemented dark grey mudstone is plastically deformed around
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calcite nodules. Calcite nodules contain spar-filled, wedge-shaped
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fractures. Burrows are preferentially cemented. Core photo, EOG
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Cygnet 08-20. C) Intensely bioturbated nodular wackestone. Fine calcite fossil fragments are abundant. Thin section scan, SCL Kaybob 02-22.
Figure 13: LF7 thinly bedded argillaceous-dolomitic mudstone. Mudstones are laminated to thinly-bedded and moderately to intensely bioturbated. Core photo, Imperial Virginia Hills 06-36.
Figure 14: LF8 dolowackestone and evaporite. Nodules range from near-planar (A), to ovoid (B), to irregular (C), with varying
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ACCEPTED MANUSCRIPT abundance of anhydrite. All images are core photos. A) Sun IOE
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10-09. B) Sarcee et al. Pibroch 10-16. C) Sun IOE 10-09
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Figure 15: LF9 intraclastic packstone and LF10 limestone breccia.
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A) LF9 packstones are light grey and composed dominantly of
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sand-sized calcite with common mud rip up clasts. Core photo. B) Calcite clasts and fossil fragments are dominant but phosphatic,
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siliceous, and pyritic clasts and fossil fragments are present as well. Thin section photomicrograph. Both samples are from EOG
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Cygnet 08-20. C) Subangular limestone clasts are surrounded by
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packstone matrix. Bed has a sharp base and includes dark grey
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mudstone rip up clasts. Core photograph, AOSC Grizzly 01-24. D) Limestone clasts are poorly sorted in mudstone-packstone matrix. This core photo of a LF10 breccia is in the Swan Hills Formation underlying the Duvernay Formation in the Enermax Panther SturLs 14-02 core.
Figure 16: A) Core description of the SCL Kaybob 02-22 core. Note the dominance of LF1 (dark grey) organic-rich mudstones. LF6 nodular wackestone is dominant in the middle Duvernay member. B) Core description of the GuideX Gvillee core. LF1 is less common and more argillaceous near the Peace River Arch. 81
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Figure 17: Core description of the ECA Cecilia 11-04 core. Note
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the tendency of LF1 pyritic laminae to occur in association with
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bioturbated pyritic mudstones of LF4. LF4 is especially dominant
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in the upper Duvernay member in wells in the Wild River Sub
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Basin.
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Figure 18: Core description of the Chevron Chickadee 03-05 core.
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LF2 is dominant in the upper Duvernay member.
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Figure 19: EOG Cygnet 08-20 core description. The core is dominated by calcareous facies LF5 and LF6. LF5 is especially dominant in the upper Duvernay member.
Figure 20: Core description of the Imperial Virginia Hills 06-36 core. LF7 and LF8 dominate the lower and middle Duvernay.
Figure 21: Core description of the Sarcee et al. Pibroch 10-16 core. LF8 is the dominant lithofacies.
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ACCEPTED MANUSCRIPT Figure 22: Depositional models. A) During deposition of the lower and middle Duvernay members lithofacies distribution is best
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explained by a platform construction model. A continuous
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dolomitic-argillaceous-calcareous-siliceous transition of lithofacies occurs basinward from the Grosmont Platform. Sediment is
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primarily sourced from the northeast. Clastic sediment was
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transported parallel to slope by currents (Stoakes, 1980). Organicrich mudstones of LF1 are only prominent in distal parts of the
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basin. B) Deposition of upper Duvernay member sediments is best explained by a flooded platform model. LF7 and LF8 argillaceous-
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dolomitic-evaporitic facies occur further to the northeast, and are separated from basinal organic-rich facies by a zone of non-
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deposition. Contour currents flow parallel to the previously developed platform slope, depositing silty contourites of LF2. Organic-rich mudstones of LF1 are prominent over much of the basin. Clay mineral supply increases to the west (reverse of platform construction model) as a result of NE-derived clastics being trapped upslope, and the Peace River Arch becoming a more significant supply of clastics.
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Well Name
UWI
Sampled
Detailed Description
Length in Duvernay (m)
GuideX Gvillee 09-06
100/09-06-076-23W5/00
Y
Y
14.4
BPC et al. Smoky HT 04-36
100/05-36-072-01W6/00
N
N
11.4
3
Xerex SturLks 07-22
100/07-22-069-21W5/00
N
N
12.6
4
Enermax Panther SturLs 14-02
100/14-02-069-21W5/00
N
N
9.6
5
AOSC Grizzly 01-24
100/01-24-061-23W5/00
N
Y
34
6
SCL HZ Kaybob 02-22
100/02-22-063-20W5/00
Y
Y
54
7
Chevron Chickadee 03-05
100/03-05-062-16W5/00
N
N
34
8
Imperial Virginia Hills 06-36
100/06-36-063-12W5/00
N
N
17
9
ECA Cecilia 11-04
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1 2
100/11-04-058-23W5/00
Y
Y
59.5
100/01-10-052-17W5/00
N
N
25
100/09-06-052-11W5/00
N
N
12.6
100/03-21-040-07W5/00
N
Y
42.5
100/10-17-045-06W5/00
N
Y
27
100/02-06-047-04W5/00
N
N
19.5
Imp Cdn-Sup Tomahawk 16-18
100/16-18-052-05W5/00
N
N
14
16
Forgotson Burk SGSpike 10-04
100/10-04-051-27W4/00
N
N
11.8
17
Sarcee et al. Pibroch 10-16
100/10-16-061-26W4/00
N
N
8.6
18
Imperial Deep Creek 04-33
100/04-33-068-22W4/00
N
N
28.5
19
Imperial Figure Lake 11-19
100/11-19-062-18W4/00
N
N
13.4
CNRL HZ Edson 01-10
11
Imperial Cynthia No. 09-06
12
SCL HZ Ferrier 03-21
13
Penn West Pembina 10-17
14
Imp Cdn-Sup Norbuck 02-06
15
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10
20
Tex et al. Lucky 09-09
100/09-09-061-18W4/00
N
N
9.7
21
Esso Redwater 10-27
102/10-27-057-21W4/00
N
N
55.4
22
Esso Redwater 16-28
102/16-28-057-21W4/00
Y
Y
57.1
23
Nexxtep 07-05
102/07-05-050-25W4/00
N
N
9.6
24
EOG Cygnet 08-20
100/08-20-038-28W4/00
Y
Y
46.8 Total = 628
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S0037-0738(16)30293-7 doi:10.1016/j.sedgeo.2016.11.012 SEDGEO 5139
To appear in:
Sedimentary Geology
Received date: Revised date: Accepted date:
8 August 2016 21 November 2016 22 November 2016
Please cite this article as: Knapp, Levi J., McMillan, Julia M., Harris, Nicholas B., A depositional model for organic-rich Duvernay Formation mudstones, Sedimentary Geology (2016), doi:10.1016/j.sedgeo.2016.11.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A DEPOSITIONAL MODEL FOR ORGANIC-RICH
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DUVERNAY FORMATION MUDSTONES
: University of Alberta, Department of Earth and Atmospheric
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a
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Levi J. Knappa, Julia M. McMillana, Nicholas B. Harrisa
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Sciences
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1-26 Earth Sciences Building
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University of Alberta
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Edmonton, Alberta Canada T6G 2E3
Corresponding author: Levi J. Knapp KnappLeviJ@gmail,com
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Abstract:
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The Upper Devonian Duvernay Formation of western Canada is an organic-rich shale formation now targeted as a hydrocarbon
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reservoir. We present a detailed sedimentological analysis of the Duvernay Formation in order to better understand organic-rich
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mudstone depositional processes and conditions and to characterize the vertical and lateral heterogeneity of mudstone
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lithofacies that affect petrophysical and geomechanical rock properties. Organic-rich mudstone facies of the Duvernay
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Formation were deposited in a dynamic depositional environment by a variety of sediment transport mechanisms, including suspension settling, turbidity currents, and bottom water currents in variably oxygenated bottom waters. Suspension settling dominated in distal relatively deep areas of the basin, but evidence for weak turbidity currents and bottom water currents was observed in the form of graded beds and thin grain-supported siltstone laminae. Organic enrichment primarily occurred in distal areas as a result of bottom water anoxia and low depositional rates of inorganic sediment. In deep water locations near platform margins, alternating silty-sandy contourite beds and organic-rich mudstone
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ACCEPTED MANUSCRIPT beds are present, the former interpreted to have been deposited and reworked by bottom water currents flowing parallel to slope. In
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shallower, more oxygenated settings, mudstone lithologies vary
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from calcareous to argillaceous. These sediments were deposited from suspension settling, turbidity currents, and bottom water
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currents, although primary sedimentary structures are often
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obscured by extensive bioturbation. Locally, organic enrichment in dysoxic rather than anoxic bottom waters was driven by a slightly
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increased sedimentation rate and possibly also by aggregation of sedimentary particles in the water column due to interaction
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between organic matter and clay minerals. Large variations observed in sediment composition, from siliceous to calcareous to
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argillaceous, reflect multiple biogenic, carbonate, and clastic sediment sources. Sediment composition is influenced by basin morphology, circulation patterns, sea level variation, and proximity to sediment sources.
Keywords: Devonian, Duvernay, sedimentology, mudstone, black shale, organicrich
1 Introduction: 3
ACCEPTED MANUSCRIPT Recent sedimentological studies of mudstone successions, including examinations of both core and petrographic thin sections,
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have identified characteristic fabrics that are attributed to a wide
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variety of depositional processes. Processes such as gravity flows (e.g. Macquaker et al., 2010a), storm dispersal and reworking (e.g.
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Schieber et al., 2010), and contour currents (e.g. Stow and Lovell,
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1979) have been proposed to explain mudstone fabrics, contrasting with traditional views that mud deposition was dominated by
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settling of particles from suspension (e.g. Potter et al, 1980). High energy deposition of clays as a result of flocculation, pelletization
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and traction currents has been demonstrated in laboratory flume experiments (Schieber et al., 2007b; Schieber and Southard, 2009;
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Schieber, 2011) and ancient deposits (e.g. Macquaker and Keller, 2005; Macquaker et al., 2010b; Aplin and Macquaker, 2011). The presence of bioturbation challenges interpretations that infer persistent anoxia during organic-rich mudstone deposition (Macquaker and Gawthorpe, 1993; Macquaker et al., 2007; Schieber, 1999, 2003; Macquaker et al., 2010a; Egenhoff and Fishman, 2013). Recognition of these processes has important implications for hydrocarbon exploration, providing a mechanism for describing and predicting heterogeneity of rock properties in mudstones. Small-scale variations in components such as biogenic silica and
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(Schieber, 1996; Schröder-Adams et al., 1996; Aoudia et al., 2010;
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Harris et al., 2011). Additionally, organic matter distribution may vary both vertically and laterally (Hemmesch et al., 2014) and is
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associated with variations in textural characteristics (Bohacs, 1998;
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Macquaker and Howell 1999; Passey et al., 2010) and organic
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matter porosity (Dong et al., 2015) in mudstones. This study characterizes lithofacies of the organic-rich
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Duvernay Formation – a prolific source rock (Fowler et al., 2001)
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and recent target for liquids-rich gas in the Western Canadian Sedimentary Basin (Rokosh et al., 2012). A core-scale
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sedimentological study of the Duvernay Formation has not been published since Stoakes (1980), and no detailed petrographic study exists. The Duvernay Formation represents an outstanding opportunity to study the depositional processes and conditions of fine-grained, organic-rich mudstones within a stratigraphic and paleogeographic context, with a wealth of long cores and log data representing a wide range of depositional settings, acquired as the result of intense exploration since 2009. The presence of wellstudied shallow water carbonates of the Leduc Formation (e.g. Mountjoy, 1980; Weissenberger, 1994; Van Buchem, 1996a, 2000a; Whalen et al., 2000; Potma et al., 2001) and Grosmont
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available in many other mudstone successions.
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2 Geologic Setting:
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Duvernay Formation sediments were deposited during the Frasnian in Western Canada (Fig. 1, 2) and are roughly time-
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equivalent to several other major black shale deposits across North America. During the Middle to Late Devonian, global sea level
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was significantly higher than present day (Johnson et al., 1985; Savoy and Mountjoy, 1995; Haq and Schutter, 2008), resulting in
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widespread flooding of continents. Many organic-rich Devonian successions were deposited within these epicontinental settings, including the Duvernay, Horn River, Bakken, Marcellus, Chattanooga, Ohio, New Albany, and Woodford shales. The Western Canadian Sedimentary Basin during the Frasnian was a passive margin at the western edge of North America. The northwest margin of North America was dominated by deposition of open marine shales in British Columbia and the Northwest Territories, transitioning to shallow water carbonates in Alberta, and restricted dolomites and evaporites in Saskatchewan and Manitoba to the southeast (Ziegler, 1967; Switzer et al., 1994). 6
ACCEPTED MANUSCRIPT During deposition of Cooking Lake Formation through Ireton Formation sediments (collectively the Woodbend Group; Fig. 2),
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the rate of accumulation and preservation of sediment increased
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dramatically (Switzer et al., 1994). In Alberta, the section is characterized by thick, extensive reef complexes, separated by
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source rocks (Switzer et al., 1994).
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thick accumulations of basin-filling shales, including hydrocarbon
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Tectonic features that influenced sedimentation during deposition of Duvernay Formation sediments include the Peace
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River Arch, West Alberta Ridge, Rimbey Arc (overlain by the
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Rimbey-Meadowbrook reef trend), and Meadow Lake Escarpment (overlain by the Killiam Barrier Reef) (Fig. 1). The Peace River
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Arch was an emergent landmass on the northwest side of the basin, fringed by Leduc Formation reefs (Dix, 1990; O’Connell et al., 1990). The West Alberta Ridge was flooded, but formed a base for extensive Leduc reef complexes during and preceding deposition of Duvernay Formation sediments (Switzer et al., 1994). The Rimbey Arc was a southwest-northeast trending basement lineament that exerted a strong control on accommodation space during Woodbend Group deposition (Ross and Stephenson, 1989). During deposition of Duvernay Formation sediments, the lineament was marked by a chain of Leduc Formation reefs called the Rimbey-Meadowbrook trend, dividing the basin into a West
7
ACCEPTED MANUSCRIPT Shale Basin and East Shale Basin. Accommodation space was more limited in the East Shale Basin than the West Shale Basin
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due to differences in tectonic subsidence across the Rimbey Arc
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(Switzer et al., 1994). This effect was compounded by differential compaction, because the West Shale Basin was the site of largely
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shale deposition, whereas the East Shale Basin was underlain by
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the Cooking Lake Carbonate Platform (Switzer et al., 1994). The Meadow Lake Escarpment is a pre-Devonian erosional and
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structural feature (Oldale and Munday, 1994). During deposition of Duvernay Formation sediments, the Killiam Barrier Reef roughly
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coincided with the underlying Meadow Lake Escarpment and marks the furthest eastward extent of Duvernay Formation shales
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and argillaceous limestones (Swtizer et al., 1994). The Duvernay Formation conformably overlies the Majeau
Lake Formation throughout most of the basin (Fig. 2). In the far south and west, Majeau Lake Formation sediments are absent, and Duvernay Formation strata onlap Swan Hills Formation platform carbonates and older Devonian strata (Switzer et al., 1994). The Ireton Formation conformably overlies the Duvernay Formation across much of the basin. Reef-margin and platform carbonates of the Leduc Formation and Grosmont Formation overlie the Duvernay Formation at the margins of the basin. An informal stratigraphy, introduced by Andrichuk (1961) and used by
8
ACCEPTED MANUSCRIPT industry, separates the Duvernay Formation into lower, middle, and upper members based on the recognition of lithologically
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distinct calcareous strata over- and underlain by less calcareous
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mudstones.
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The Duvernay Formation is predominantly composed of organic-rich, siliceous to calcareous mudstones over much of the
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basin. East Shale Basin deposits are notably more calcareous than
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equivalent West Shale Basin sediments (Rokosh et al., 2012). Towards the northeast, adjacent to the Grosmont Platform,
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mudstones become argillaceous and organic-lean (Stoakes, 1980).
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Limestones and dolostones occur proximally to reef complexes. Organic-rich siliceous-calcareous mudstones are common within
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the lower and upper Duvernay members, while organic-lean limestones dominate the middle Duvernay member (Andrichuk, 1961). Member boundaries are lithostratigraphic. The Duvernay Formation dips to the southwest, with a thermal maturity range from immature in the southeast to dry gas in the southwest (Rokosh et al., 2012)
3 Methods: Eight drill cores (Table 1), were selected for detailed description. Cores were described at a scale of 1:10, paying special 9
ACCEPTED MANUSCRIPT attention to lithology, grain size, sedimentary structure, bioturbation, and presence of cements. Core selection was
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primarily based on total thickness of formation cored, core quality,
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and geographic distribution across the basin. An additional 16 cores were described in less detail to observe facies variations and
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stratigraphic surfaces over a greater extent of the basin. Few of the
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16 additional cores covered the entire Duvernay Formation interval. Total length of described Duvernay core was 628m. A
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total of 108 thin sections were cut from 4 cores, covering the major lithofacies and intervals of interest. Thin sections were ground to a
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thickness of 20 µm rather than the standard 30 µm, so that more detail could be observed in fine-grained facies (e.g. Macquaker et
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al., 2007). Most thin sections were cut perpendicular to bedding but a subset was selected for bed-parallel thin sections to further examine the nature of potential bioturbation features. Thin sections were scanned using a Nikon Super Coolscan 5000 ED scanner to observe centimeter- to millimeter-scale features. Millimeter- to micrometer-scale features were analyzed under transmitted and reflected white light using a Zeiss Axio Scope.A1 petrographic microscope. Samples for geochemical analysis were cut every 1 meter from 5 of the 8 cores. These 5 cores were chosen for wide geographic coverage of the basin and to represent a substantial
10
ACCEPTED MANUSCRIPT range of thermal maturity, from immature to wet gas. Detailed geochemical analysis of these samples is presented in McMillan
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(2016). A 10 cm long by 2 cm thick slab was cut from the back of
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the core at each sample location. Vertical splits were cut along the length of the slabs for separate analyses. Weatherford Geochemical
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Services Group in Shanandoah, TX performed Leco-TOC and
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RockEval analysis. Total organic carbon values reported here are averages for each sampled facies from the SCL Kaybob 02-22,
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ECA Cecilia 11-04, GuideX Gvillee 09-06, and EOG Cygnet 0820 cores. Esso Redwater 16-28 was also sampled for TOC but was
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removed from average TOC calculations because it is much less thermally mature than the other sampled wells and, as such, has
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much higher TOC values for all facies. Samples from the Esso Redwater 16-28 well have average Tmax = 421 °C, while the other wells have average Tmax from 444 to 473 °C. Maturity in ECA Cecilia 11-04 is likely even higher but Tmax data was unreliable).
4 Results: 4.1 Lithofacies Descriptions Ten lithofacies comprise Duvernay Formation sediments over most of the basin. Lithofacies were characterized on the basis of composition, grain size, sedimentary structures, bioturbation, 11
ACCEPTED MANUSCRIPT and presence of cements and are described here in order of
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LF1: Planar laminated siliceous mudstone
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decreasing TOC.
LF1 is dark grey-brown, faintly planar-laminated siliceous-
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calcareous mudstone. Lamination is commonly difficult to recognize in core samples of the finest-grained occurrences of LF1
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(Fig. 3A) but in thin section may be seen as sub-millimeter-thick alternating silt-rich and silt-poor mud-supported laminae (Fig. 3B).
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Locally, lamination is defined by semicontinuous pyrite-rich laminae (Fig. 3C-E) or by grain-supported siltstone laminae (Fig.
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4). Pyritic laminae are typically less than 1mm thick, a few millimeters to several centimeters long and commonly pinch and swell. Pyrite in these laminae occurs as both framboidal and euhedral types, with varying abundance of pyrite-replaced carbonate silt grains and fossil fragments. Grading is generally absent, except in grain-supported siltstone laminae in which case both normal and inverse grading is observed. Lamina tops and bases are sharp to gradational. Silt grains are primarily carbonate, with some quartz, and rare chert. Laminae locally contain calcareous styliolinid and tentaculitid tests. Siltstone laminae are commonly cemented with calcite or silica cement. Bioturbation is
12
ACCEPTED MANUSCRIPT typically not observed, but in silty sections, rare horizontal burrows with silty infill were present.
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Except for the silty laminae, LF1 is composed of an
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unstructured mixture of clay- and silt-sized detrital grains and
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organic matter. Detrital grains include calcite, dolomite, and quartz, with very minor abundance of petrographically-observable
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clay minerals. Low visual clay abundance is supported by
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geochemical results in McMillan, 2016. Clay minerals are locally observed as small aggregates, generally between 100 and 300 µm
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long and ~20 µm thick (Fig. 5A-B). Fragments or complete tests of
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calcareous styliolinids and tentaculitids are present but uncommon. Siliceous planktonic radiolaria are uncommon to common, are
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crushed and recrystallized, and appear as cherty masses (~50x10
µm). Amorphous organic matter occurs intimately mixed with matrix grains. Individual organic matter aggregates appear as dark brown stringers (commonly 50-200 µm long and less than 10 µm
thick) in bed-normal thin sections (Fig. 5C) and appear equidimensional to ovoid with diffuse or wispy edges in bedparallel thin sections (Fig. 5D). The average TOC for LF1 is 3.4 wt.% (SD=1.6, n=57).
13
ACCEPTED MANUSCRIPT
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LF2: Wavy-laminated siltstones and silty mudstones LF2 consists of light to medium grey wavy-laminated silty-
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sandy laminae interlaminated and interbedded with dark grey-
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brown mudstone equivalent to LF1 (Fig. 6A). Wavy to lenticular
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lamination is most common, but sedimentary structures also include planar parallel lamination, low angle cross-lamination,
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starved ripples, loading structures, and flame structures. Laminae are typically grain-supported and show both normal and inverse
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grading (Fig. 6B). Normally graded laminae commonly have sharp
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bases while inversely graded laminae typically have sharp upper
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contacts. Grains are primarily carbonate with minor quartz and rare chert clasts and are typically fine silt to medium sand in size. Laminae locally contain bioclast fragments including styliolinids and tentaculitids. In cores where LF2 is abundant (e.g. Chevron Chickadee 03-05; Fig. 7), the facies is distinctly coarser-grained, and more fossiliferous beds are locally present, compared to cores in which LF2 is a more minor facies (e.g. Shell Kaybob 02-22). These fossiliferous beds are 1-6 cm thick and are always normally graded, with sharp, erosive bases. Fossil fragments commonly include brachiopods and crinoids. Bioturbation is minor to moderate in LF2 and is typically seen as silt-filled horizontal burrows, and homogenization of silt-rich and silt-poor laminae. 14
ACCEPTED MANUSCRIPT The average TOC for LF2 is 2.4 wt.% (SD=0.5, n=10), however the geochemical analysis was performed on vertical strips
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of sample 10cm long, which include both silty-sandy laminae
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described above and mudstones equivalent to LF1. Based on petrographic observation, TOC is insignificant in grain-supported
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becoming greatest in the mudstones.
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laminae and increases as laminae become mud-supported,
LF3: Siliceous-calcareous styliolinid wackestones
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LF3 is composed of siliceous-calcareous wackestone, forming beds a few millimeters to several centimeters thick. LF3 is
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usually poorly-bedded to structureless (Fig.8A), although uncommon sharp bases and normal grading can be observed. Bioclasts include whole or fragmented styliolinids, tentaculitids (Fig. 8B-C), and radiolaria. Fragments of brachiopods, bivalves (Fig. 8B), and crinoids are less commonly observed. Bioturbation is minor to intense. The matrix of LF3 is composed dominantly of clay- to silt-
sized carbonate grains and amorphous organic matter, with minor quartz and clay minerals. Amorphous organic matter occurs intimately mixed with matrix grains. The morphology of organic matter aggregates is similar to LF1. Organic matter abundance is 15
ACCEPTED MANUSCRIPT greatest in fine-grained, less calcareous beds. The average TOC for
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LF4: Bioturbated siliceous, pyritic mudstone
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LF3 is 2.3 wt.% (SD=1.2, n=19).
LF4 is composed of dark grey-brown poorly-bedded
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mudstone with pyritized burrows (Fig.9A-B). Bedding is uncommonly preserved, appearing as wispy, wavy laminae.
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Laminae are manifested by subtle variations in color due to varying silt abundance. Bedding is disrupted by subhorizontal to
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subvertical burrows that are quite easily recognized due to common filling or lining of burrows with pyrite (Fig. 9C). Trace
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fossil diversity is low.
Clay- and silt-sized quartz, carbonate, clay minerals, and
organic matter are primary components of the rock. Clay minerals are more abundant than previously described lithofacies, based on petrographic observation and major element geochemistry (McMillan, 2016).Clay minerals and amorphous organic matter are sometimes observed as aggregates. Aggregate morphology is as described above. The average TOC for LF4 is 2.1 wt.%. (SD=1.3, n=69). Calcareous radiolaria are uncommon to common. In-situ benthic macrofossils such as brachiopods and gastropods are common (Fig. 10) and in-situ benthic agglutinated foraminifera are 16
ACCEPTED MANUSCRIPT uncommon to common. Fossils were identified as in-situ based on preservation, position, and presence within mudstone beds that
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lack traction structures (i.e. no evidence for transport of
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grains/clasts or fossil fragments).
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LF5: Bioturbated calcareous mudstone
LF5 is composed of medium grey-brown poorly-bedded
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mudstone which is distinctly more calcareous than LF4. Where present, bedding appears as wispy, wavy laminae that are typically
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less than 8 mm thick (Fig. 11A). Subtly lighter-colored laminae contain more fine-grained calcite than darker laminae. LF5 is
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moderately to intensely bioturbated with low trace fossil diversity and burrows are horizontal to subhorizontal. Clay- and silt-sized calcite comprises most of LF5 (Fig.
11B-C). Fine calcareous fossil fragments are common. Dark brown, amorphous organic matter fills space between grains. Rarely, organic matter appears as dark brown stringers in bednormal thin sections (~50-200 µm long, <10 µm thick). In bedparallel thin sections, uncommon organic matter masses appear approximately equidimensional. The average TOC for LF5 is 1.8 wt.% (SD=1.6, n=24). No in-situ benthic macrofossils were
17
ACCEPTED MANUSCRIPT observed but in-situ benthic agglutinated foraminifera are
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moderately common (Fig. 11D).
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LF6: Nodular wackestone
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LF6 is composed of nodular to burrow-mottled lime
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mudstone and wackestone. Light grey calcite wackestone nodules are hosted in dark grey mudstones to wackestones (Fig. 12).
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Nodules range from very irregular and burrow-mottled to ovoid to near-planar, and commonly contain spar-filled, wedge-shaped
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fractures. Burrows are preferentially cemented with calcite.
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Detrital grains within nodules are predominantly clay- and silt-
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sized calcite and calcareous fossil fragments, most commonly styliolinids, tentaculitids, and brachiopods. Nodules are surrounded by more ductile, uncemented
mudstone, which deforms around nodules. Dark grey mudstone is structureless to poorly bedded, and is calcareous to argillaceous. Where calcareous, mudstones may be compositionally similar to nodules, aside from a lack of cementation. Where clay content is high, mudstones are fissile and tend to physically separate from nodules. The average TOC for LF6 is 1.1 wt.% (SD=1.2, n=34), measured as a combination of nodule and mudstone.
18
ACCEPTED MANUSCRIPT LF7: Thinly bedded argillaceous-dolomitic mudstone LF7 is composed of organic-lean, typically non-
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fossiliferous argillaceous-dolomitic mudstone, with laminated to
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thinly-bedded to burrow-mottled bedding style (Fig. 13).
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Millimeter- to centimeter-scale light grey-brown organic lean laminae/beds alternate with 1-2mm thick dark grey-brown, more
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organic-rich laminae, or blue-grey, anhydrite- rich laminae.
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Laminae/beds are most commonly ungraded or normally graded, and are less commonly inversely graded. Bedding contacts are
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typically gradational over 1-2 mm, and are usually irregular.
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Bioturbation ranges from moderate to intense. Horizontal to subhorizontal burrows are more common than subvertical burrows.
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Subvertical burrows are rarely more than a few millimeters to one centimeter long.
LF8: Dolowackestone and evaporite LF8 consists of nodular to burrow-mottled buff-colored
dolomitic to evaporitic mudstone-wackestone (Fig. 14A). Poorly bedded dolomitic-argillaceous mudstone equivalent to LF7 is deformed around dolomitic mudstone-wackestone nodules (Fig. 14B). Uncommon fossils include brachiopod and crinoid fragments. Anhydrite is very common as burrow-filling cement or 19
ACCEPTED MANUSCRIPT less commonly as anhydrite nodules (Fig. 14C). Uncommonly, dolomite nodules contain wedge-shaped fractures filled with
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calcite, dolomite and/or anhydrite. LF8 is organic-lean.
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LF9: Intraclastic packstone
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LF9 is composed of light grey intraclastic packstone. Bedding is structureless to planar-laminated and ungraded to
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normally-graded (Fig. 15A). Cross lamination is uncommon. Grains are most commonly sand-sized calcite clasts and fossil
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fragments, generally with minor quartz and dolomite silt, and siliceous, phosphatic, or pyritic clasts and fossil fragments (Fig.
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15B). Angular mud clasts are common. Fossil fragments include crinoids, gastropods, brachiopods, bivalves, styliolinids, tentaculitids and uncommon amphipora coral. Locally beds are significantly homogenized although specific ichno-taxa are generally not recognizable. LF9 is very organic-lean, with samples only containing
notable organic matter when other organic-rich facies were included. For this reason, TOC values are not reported here. Only very rare wispy organic matter is observed in thin section between grains.
20
ACCEPTED MANUSCRIPT LF10: Limestone breccia LF10 consists of angular limestone clasts in a calcareous
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mudstone to packstone matrix (Fig. 15C-D). Beds are generally
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10-20cm thick, fine upwards or, less commonly, are ungraded.
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LF10 intervals may contain several stacked fining upwards beds. Bases of beds are erosive and commonly contain dark to medium
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grey, angular mudstone clasts. Limestone clasts are angular to
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poorly rounded and range from pebble- to boulder-sized. The matrix of LF10 is composed of calcareous mudstone to
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packstone. Coarse-grained matrix contains grains up to granule-
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sized (above which grains were not considered matrix), consisting
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of calcite detritus.
LF10 appears organic-lean based on scarcity of dark-
brown, bituminous mudstone matrix, however no geochemical samples were analyzed.
4.2 Depositional Architecture: Geographic and stratigraphic distribution of lithofacies varies considerably. The western and southern West Shale Basin is dominated by laminated, organic-rich mudstones of LF1, for example, in the SCL Kaybob 02-22 core (Fig. 16A). At GuideX
21
ACCEPTED MANUSCRIPT Gvillee 09-06, near the Peace River Arch (Fig. 16B), planarlaminated mudstones are more argillaceous and clay mineral
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aggregates are more commonly observed. In the Wild River
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Subbasin, on the far west side of the West Shale Basin, LF1 mudstones commonly contain pyritic laminae and are frequently
SC
associated with LF4 bioturbated pyritic mudstones (e.g. the Encana
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Cecilia well; Fig. 17). Towards reef complexes, calcareous siltstone laminae become frequent within LF1 mudstones. Siltstone
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laminae have a higher proportion of quartz in the northwestern portion of the West Shale Basin, compared to anywhere else in the
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basin, although carbonate grains are everywhere the dominant component. LF1 laminated mudstones of the East Shale Basin,
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compared to the West Shale Basin, contain more common peloidal calcite silt, and calcareous styliolinid and tentaculitid tests, but siliceous planktonic radiolaria are not observed. Silty-sandy mudstones of LF2 are most common in the
West Shale Basin and are especially common and coarse-grained towards the northeast (Fig. 7, 18). LF2 is uncommon in the East Shale Basin, and where present, primary sedimentary structures are commonly disrupted by bioturbation. LF3 is more common in the East Shale Basin than the West Shale Basin, although it is consistently a minor facies. In the East Shale Basin, LF3 beds are more fossiliferous, more commonly 22
ACCEPTED MANUSCRIPT containing larger taxa such as brachiopods and bivalves, and contain more abundant clay- and silt-sized carbonate grains.
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Siliceous, pyritic, bioturbated mudstones of LF4 are most
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common in the western and southern West Shale Basin and are
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particularly thick in the Wild River Sub Basin (Fig. 17). Bioturbated mudstones are much more calcareous and less pyritic
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EOG 08-20 core; Fig. 19)
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in the East Shale Basin, where LF5 is a dominant lithofacies (e.g.
LF6 is the most stratigraphically-defined lithofacies as it is
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the dominant lithofacies within the middle Duvernay member (Fig.
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16A). The abundance of LF6 in the middle Duvernay member of
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the West Shale Basin consistently decreases to the south and west margins of the basin. Outside of the middle Duvernay member, LF6 is minor in the West Shale Basin but can be observed throughout the stratigraphic section in the East Shale Basin. LF7 and LF8 were only observed in cores in the
northeastern and eastern West Shale Basin and northern to northeastern East Shale Basin. These two lithofacies are generally observed together in core (Fig. 20, 21). LF9 was only observed in the East Shale Basin (Fig. 19) and proximal to reef complexes in the West Shale Basin. LF10 limestone breccias were only observed in the AOSC Grizzly 01-24 23
ACCEPTED MANUSCRIPT core, located adjacent to the northern margin of a reef complex in the West Shale Basin. No LF10 breccias were observed in East
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Shale Basin cores. LF10 intervals in the Duvernay Formation are
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immediately over- and underlain by organic-rich mudstones of
5 Interpretation:
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5.1 Lithofacies Interpretations
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LF1.
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LF1 planar laminated mudstones
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LF1 was interpreted primarily as the product of
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hemipelagic suspension settling, based on the scarcity of sedimentary structures, the mud-supported fabric, and the morphology of organic matter and clay mineral aggregates. Amorphous organic matter is present as organic matter aggregates, and is interpreted to be “marine snow” (Macquaker et al., 2010b), based on aggregates’ ovoid shape, wispy to diffuse edges and random distribution throughout the mudstone matrix. Such aggregates form in the upper water column due to random collision or the activities of organisms (e.g., Alldredge, 1976; McCave, 1984; Alldredge and Silver, 1988; Macquaker et al., 2010b) and settle through the water column. Clay minerals are present in low abundance in LF1, but in areas where clay content is greater, such 24
ACCEPTED MANUSCRIPT as the Wild River Subbasin and near the Peace River Arch, clay mineral aggregates are observed. These aggregates have similar
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morphology to the organic matter, suggesting a similar origin as
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floccules or fecal pellets in the upper water column. Based on their ovoid shape, some with diffuse to wispy edges, they are interpreted
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erosion and transport of mud clasts.
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to be a product of suspension settling of aggregates rather than
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Of secondary importance are turbidity currents and/or bottom water currents that deposited normally- and inversely-
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graded planar-parallel siltstone laminae that make up a smaller
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proportion of the rock. Grain-supported carbonate and quartz silt laminae are interpreted to be lag deposits. Similar silty laminae are
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found within LF2, interbedded with silty-sandy laminae that contain a variety of traction-type sedimentary structures, most of which are attributed to contour currents (see discussion below). Because of this association, the grain-supported laminae of LF1 are interpreted to be the result of uncommon very weak contour currents that winnow out clay-sized sediment (e.g. Shanmuggam, 2000; Martin-Chìvelet et al., 2008). Locally LF1 contains laminae enriched in framboidal and
euhedral pyrite, and varying amounts of pyrite-replaced carbonate silt and fossil fragments, largely formed during early diagenesis. Similar features have been noted in other Upper Devonian black 25
ACCEPTED MANUSCRIPT shales such as the Woodford Shale in the Permian Basin (N. Harris, personal communication) and Muskwa Formation in the
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Horn River Basin (Ayranci et al., 2016). Limiting factors in pyrite
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formation in marine sediments are the availability of metabolizable organic matter, the rate of diffusion of sulfate into sediments, and
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the concentration and reactivity of iron minerals, especially
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terrigenous clay (Berner, 1970). In LF1-4 of the Duvernay Formation, terrigenous mineral supply is low, based on
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petrographic observation and geochemical analysis (McMillan, 2016). Pyritic laminae of LF1 are interpreted to be the result of
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slightly increased clay mineral abundance. Most commonly, pyritic laminae are seen overlying flooding surfaces, in which case the
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clay minerals are interpreted to be the result of transgressive reworking of more clay mineral-rich regressive deposits, and within intervals of abundant and more argillaceous LF4 bioturbated pyritic mudstone (Fig. 17).
LF2 wavy laminated siltstones and silty mudstones Sedimentary structures in LF2 provide evidence of deposition from bottom water currents. These include planar parallel lamination, low angle cross-lamination, lenticular- to wavy-bedding, starved ripples, gradational to sharp erosive bases, both normal and inverse grading, sharp (non-erosional) upper 26
ACCEPTED MANUSCRIPT contacts, flame structures and soft sediment loading deformation. Shanmugam (2000) identified many of these features and others as
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evidence for reworking and transport by bottom water currents
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when several are observed in combination. Critically, most siltysandy beds and laminae of LF2 show no systematic vertical
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stacking of structures such as those recognized in the Bouma
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(1962) sequence. This observation, combined with the abovementioned sedimentary structures led to the interpretation that
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bottom water currents were a more significant process than turbidity currents for LF2 deposition. Contourite beds show minor
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to moderate bioturbation, likely reflecting variations in bottom water oxygenation and sedimentation rate, consistent with other
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ancient and modern contourite deposits (Dalrymple and Narbonne, 1996; Ito, 1996).
Turbidite beds are also present in LF2, identified by sharp,
erosive bases, normal grading, and the presence of coarser fossil fragments than in surrounding beds. Contourites are differentiated from turbidites by finer grain size and the presence of a distinctive set of sedimentary structures (described in previous paragraph).
LF3 bioturbated styliolinid wackestones
27
ACCEPTED MANUSCRIPT LF3 beds contain fossils and fossil fragments that are often much coarser than LF3 matrix grains and the clay to silt grain size
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of the under- and overlying beds. The presence of sharp bases and
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normal grading, although commonly disrupted or destroyed during bioturbation, leads to the interpretation of LF3 beds as turbidites.
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The fossiliferous nature of LF3 beds is likely due to grain size
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sorting within turbidity currents. Additionally, LF3 beds are consistently more bioturbated than over- and under-lying beds,
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which is likely to be result of temporary oxygenation due to
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downslope transport.
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LF4 bioturbated siliceous pyritic mudstone, LF5 bioturbated calcareous mudstone and LF6 nodular wackestone LF4, LF5 and LF6 are moderately to intensely bioturbated,
and LF6 commonly contains in-situ benthic macrofossils. Bottom
waters are interpreted to have been dysoxic based on moderate to intense bioturbation but low trace fossil diversity, as demonstrated in studies of modern sediments (e.g. Bromley and Ekdale, 1984; Ekdale and Mason, 1988). Additionally, low-oxygen-tolerant
benthic agglutinated foraminifera are observed in LF4 and LF5. In other organic-rich mudstone formations, the presence of benthic agglutinated foraminifera has been used to infer the presence of dysoxic rather than anoxic bottom waters (Milliken et al., 2007; 28
ACCEPTED MANUSCRIPT Schieber, 2009). Benthic agglutinated foraminifera have a high tolerance for low oxygen conditions but cannot survive in anoxic
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bottom waters (Bernhard and Reimers, 1991; Bernhard et al.,
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2003).
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Total organic carbon values are lower in these lithofacies than in LF1-4, which is likely due to an increase in bottom water
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oxygenation and increased aeration of sediment due to burrowing
MA
activity (e.g. Rhoads, 1974; Aller, 1978; Demaison and Moore, 1980). However LF4 is relatively enriched in organic matter
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(average 2.1% TOC) compared to similarly bioturbated LF5 and
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LF6. One factor in organic matter enrichment in LF4 may be increased sedimentation rates. A slight increase in sedimentation
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rate due to a greater influx of clay minerals in LF4 with respect to LF5 could have more rapidly buried organic matter below the reach of oxidants, which has been shown to result in enhanced organic carbon preservation in depositional settings that are not persistently anoxic (Tyson, 2005; Bohacs et al., 2005; Macquaker et al., 2010b). The higher clay content in LF4 may also be linked to increased organic matter preservation (Macquaker et al., 2010b). Kennedy et al. (2002) demonstrated that increased preservation of organic matter can be associated with the high surface area in clay minerals.
29
ACCEPTED MANUSCRIPT The absence of primary sedimentary structures in LF5 suggests that episodes of rapid sedimentation were uncommon.
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Interlaminated layers of variable calcite silt content may be the
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remnants of thin turbidite or contourite beds, although it is difficult to characterize sediment transport mechanisms here with
SC
confidence, since primary sedimentary structures are destroyed by
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bioturbation. Low oxygen concentration rather than soft substrate is interpreted to be the cause of the absence of benthic
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macrofossils, as soft substrate reduces benthic diversity but does not eliminate it (Rhoads and Morse, 1971; Byers, 1977; Wignall,
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1993).
In LF6, calcareous nodules contain abundant fine
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calcareous fossil fragments, which may be the result of event bed deposition, although any sedimentary structures diagnostic of turbidity currents have largely been destroyed by bioturbation. Where LF6 is not considerably burrow-mottled, calcite nodules form semi-continuous beds that alternate with uncemented mudstone beds, possibly reflecting changes in the concentration of carbonate detritus. Mudstone-wackestone beds between nodular horizons likely accumulated during prolonged periods of lower sedimentation rates and dysoxic bottom waters. These beds are variably bioturbated but sometimes contain crude lamination and are more organic-rich than calcite nodules.
30
ACCEPTED MANUSCRIPT Uncemented mudstones and wackestones are deformed around calcite nodules, suggesting that nodule formation predates
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complete compaction. Additionally, wedge-shaped, calcite spar-
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filled fractures in calcite nodules suggest that brittle fracturing of nodules occurred, likely due to compaction post-dating early
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lithification. Stoakes (1980) came to the same conclusion, as have
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other authors studying similar deposits (Tucker, 1973; Noble and
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Howells, 1974; Kennedy and Garrison, 1975; Mullins et al., 1980).
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LF7 thinly-bedded argillaceous-dolomitic mudstone LF7 is largely composed of light brown mud-rich laminae
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that are interpreted to be deposited from sediment-gravity flows, based on the abundance of sharp-based, normally graded beds. Similarly graded mud beds have been identified in other studies (e.g. Bhattacharya and MacEachern, 2009; Macquaker et al., 2010a; Macquaker et al., 2010b; Aplin and Macquaker, 2011). Other light brown beds in the lithofacies show inverse grading or both inverse and normal grading, suggesting deposition during waxing and waning flow energies. These conditions can be created by density-driven deep-water contour currents (e.g. Rebesco et al., 2014) or currents driven by climatic factors such as wind, runoff, precipitation and evaporation rates, temperature gradients, and
31
ACCEPTED MANUSCRIPT mixing of water bodies (e.g. Kump and Slingerland, 1999). Darker brown laminae are interpreted to be more organic-rich, based on
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color, and may represent periods of reduced inorganic sediment
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supply and increased concentration of preserved organic matter. Grey-blue, anhydrite-rich laminae that alternate with light brown
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laminae may represent periods of evaporite precipitation that
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occurred between increased sedimentation events. Salinity fluctuations are indicated by anhydrite-enriched laminae which are
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more common in upslope positions where LF7 is interbedded with
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LF8.
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The above described sedimentary structures are locally disturbed or destroyed due to moderate to intense bioturbation,
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indicating dysoxic to oxic conditions and varying sedimentation rates. A lack of organic matter is consistent with oxic conditions, although the absence of in-situ benthic macrofossils suggests that oxygen levels may have varied, and rapid sedimentation may also have limited colonization and diluted organic matter. Variability in oxygen conditions has been suggested to deter colonization of shelly benthic fauna, and act to preserve lamination from disruption by burrowers (Savrda et al., 1984).
LF8 dolowackestone and evaporite
32
ACCEPTED MANUSCRIPT The nodular fabric of LF8, similar to LF6 nodular wackestones, results from early cementation. Ductile, more
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argillaceous mudstone between nodules is deformed around
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nodules, indicating that nodules had formed before significant compaction and lithification of the mudstone. Rare wedge-shaped
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fractures in nodules are filled with calcite, dolomite and/or
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anhydrite and are the result of brittle fracture – another indicator of early cementation (Tucker, 1973; Noble and Howells, 1974;
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Kennedy and Garrison, 1975; Mullins et al., 1980; Stoakes, 1980). The presence of nodules may indicate that overall sedimentation
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rates were low, allowing time for nodule formation before significant burial and compaction.
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Intense bioturbation and scarcity of organic carbon indicate
that LF8 was deposited in an oxic to dysoxic environment. Anhydrite is very common as burrow fills and uncommon as nodules, which may indicate that salinity levels were high. High or fluctuating salinity may explain the scarcity of benthic macrofossils. LF8 is interpreted to have been deposited on the carbonate
platform margin or upper foreslope in relatively oxic bottom waters. Deposition of clay minerals was limited by increased wave energy and increased carbonate sediment supply (Stoakes, 1980).
33
ACCEPTED MANUSCRIPT
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LF9-10 packstones and breccias LF9-10 packstones and breccias contain angular clasts and
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display poor sorting and normal grading, suggesting deposition
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from turbidity currents and debris flows. Debris flows were likely
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caused by slope instability of reef margins (e.g. Braithwaite, 2014). McLean and Mountjoy (1993) suggested that northeastern sides of
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reef complexes in the Alberta Basin had steeper slopes due to the northeast to southwest direction of wind and surface water
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currents. Southwestern sides of reef complexes and areas protected
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from wave energy had more gently dipping reef margins due to the
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accumulation of fine-grained carbonate detritus. LF10 breccias were only observed in the AOSC Grizzly 01-24 core which is located adjacent to the northern margin of the Bigstone reef complex. LF10 breccias likely occur only very close to reef margins. Additionally, authors such as Mountjoy (1980), McLean and Mountjoy (1993), Van Buchem et al. (1996a), and Whalen et al. (2000) have documented in outcrops of time-equivalent reef margin strata that limestone breccias and conglomerates extend a few hundred meters to a few kilometers into the basin from reef margins.
34
ACCEPTED MANUSCRIPT LF9 packstones are more widespread and were deposited further from reef complexes than LF10 breccias (i.e. EOG Cygnet
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08-20 core). However, LF9 packstones are not observed in the vast
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majority of the West Shale Basin due to longer transport distances from reef complexes to our cores. This lithofacies was also likely
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generated by slope failure events, as suggested by the wide range
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of clasts, and presence of shallower-water taxa than in organic-rich
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mudstones.
Breccias and packstones are commonly under- and overlain
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by dark grey, organic rich mudstones of LF1, indicating that they
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were deposited as event beds in an environment that was suitable
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for the accumulation of fine-grained, organic rich sediment.
5.2 Depositional Model Described here is a depositional model created from the
summation of the observations and interpretations outlined above. Analysis of lithofacies distribution and characterization of depositional processes and conditions enable us to understand the drivers that controlled the deposition and character of Duvernay Formation sediments. The depositional model is separated into two phases: an initial period of platform-building (Fig. 22A), which largely describes lower and middle members of Duvernay 35
ACCEPTED MANUSCRIPT Formation strata, and a later period when the platform was flooded (Fig. 22B), which generally describes the upper Duvernay
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member. These two phases fall into the sequence stratigraphic
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framework defined by Knapp (2016), and agree with the reciprocal style of sedimentation described by Stoakes (1980) and Cutler
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(1983), among others.
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Platform Construction Stage (lower and middle Duvernay) In the northeast part of the basin, adjacent to the Grosmont
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Platform, LF7 argillaceous-dolomitic mudstone and LF8 dolowackestones and evaporites are dominant in the lower and
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middle Duvernay members; these represent a well-oxygenated platform-margin and oxic to dysoxic slope environment, and their great thickness (Knapp, 2016) reflects aggradation of the carbonate platform during a prolonged period of platform-building (Fig. 22A). LF7 is enriched in terrigenous clay minerals, and represents slope deposition. Clastic deposition was a result of deposition of clays on the slope, transported by currents from the north, parallel to the Grosmont Platform edge (Stoakes, 1980; Knapp, 2016). These currents may have been driven by climatic factors such as wind, runoff, precipitation and evaporation rates, temperature gradients, and mixing of water bodies (e.g. Slingerland et al., 1996;
36
ACCEPTED MANUSCRIPT Kump and Slingerland, 1999). Drivers for basin circulation are not the focus of this paper, and little work has been published on the
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subject for the Alberta Basin (Mallamo, 1995), so patterns of
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sediment distribution are currently the strongest evidence for the character of basin circulation (Andrichuk and Wonfor, 1954;
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Newland, 1954; Andrichuk, 1958, 1961; Staplin, 1961;
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McCrossan, 1961; Klovan, 1964; Wendte, 1974; Stoakes, 1980;
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McLean and Mountjoy, 1993; Knapp, 2016). In the West Shale Basin, LF6 nodular wackestones were
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the dominant lithofacies immediately basinward of the major
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platform-building sediments (LF7 and LF8). Nodular wackestones of LF6 are moderately to intensely bioturbated, suggesting dysoxic
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rather than anoxic bottom waters, and the variable abundance of fragmented shell material suggests periodic downslope transport of coarser carbonates.
Further westward and southward into the West Shale Basin,
bottom water anoxia was much more persistent and extensive, and organic-rich sediments were dominant. In these areas, much of the clay-sized sediment was deposited from suspension, but silt- and sand-sized detritus shows evidence for transport by bottom water currents and turbidity currents. Organic matter formed aggregates in the upper water column, settling to an anoxic sediment-water interface, which inhibited the breakdown of organic matter. 37
ACCEPTED MANUSCRIPT Dilution of organic-rich sediments by northeast-derived clastics was minimal.
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In the East Shale Basin, the lower and middle Duvernay
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members are dominated by bioturbated lithofacies, especially LF5
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bioturbated calcareous mudstones and LF6 nodular wackestones, indicating that bottom water anoxia was rare at this time. This was
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probably due to shallow water depths. Organic carbon
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accumulation in East Shale Basin sediments was hampered by higher oxygen levels, and diluted by increased carbonate sediment
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supply.
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Flooded Platform Stage
The upper Duvernay member is best characterized by a
flooded platform model (Fig. 22B). The platform-to-basin sequence of facies described above still generally applies during the flooded platform phase, but distinct differences exist that are important to an understanding of the lithofacies distribution. The oxygenated platform margin in the northeast part of the basin is again dominated by LF7 and LF8; however, the thickness of these deposits is greatly reduced during the flooded platform stage, and the areal extent of these lithofacies is shifted to the north and east (Knapp, 2016). This backstepping resulted in the 38
ACCEPTED MANUSCRIPT formation of a zone of non-deposition over older platform margin and slope sediments deposited during the earlier platform-building
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phase. Non-deposition was probably supported by strong NW-SE-
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flowing currents, which encouraged NE-derived sediments to prograde parallel to the Grosmont Platform edge, rather than
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perpendicular to it (Knapp, 2016). This relationship significantly
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limited the supply of NE-derived sediment in deeper areas of the
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basin.
The areal extent of siliceous, organic-rich mudstones of
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LF1 greatly increased during the flooded platform phase (Fig. 22B;
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Knapp, 2016), and limited input of NE-derived sediments is responsible for the clay-poor nature of these facies. Dilution of
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organic-rich sediment by carbonate detritus was only significant near Leduc reef complexes and at the base of the Grosmont Platform. At the base of the Grosmont Platform, contour currents transported silt- and sand-sized carbonate detritus (LF2) that was originally sourced from reefs and/or the platform margin. LF2 records interbedded black shale and silty-sandy contourite deposition in a variably energetic, but oxygen-deprived environment. In the far west of the West Shale Basin, the Wild River
subbasin is dominated by LF4 bioturbated pyritic mudstones. The dominance of a more clastic-rich facies so far removed from the 39
ACCEPTED MANUSCRIPT northeasterly-derived clastic sediment source, suggests that a second minor source of clastics existed (Fig. 22B), possibly the
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subaerially-exposed Peace River Arch on the northwestern corner
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of the basin. Organic carbon values in LF4 are unexpectedly high, given that the sediments are thoroughly bioturbated. Increased
SC
sedimentation rates may have more rapidly buried organic matter
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below the sediment-water interface, which was clearly not anoxic. Additionally, the aggregation of organic matter and clay minerals
MA
(e.g. Macquaker et al., 2010b) and/or adsorption of organic matter onto clay minerals (e.g. Kennedy et al., 2002) seems to be
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enhanced with increased clay mineral supply.
6 Discussion
6.1 Bottom Water Currents in Organic-Rich Facies The recognition of event beds and traction-derived
sedimentary structures in black shale sequences (e.g. Schieber, 1999; Macquaker and Bohacs, 2007; Macquaker et al., 2010b; Ghadeer and Macquaker, 2011, 2012; Abouelresh and Slatt, 2012; Hemmesch et al., 2014) indicates that bottom waters were not persistently stagnant. In the Duvernay Formation, the presence of contourite and turbidite beds within black shale units of LF1 and LF2 indicates that quiet water deposition was not persistent and that bottom water currents commonly transported larger grains and 40
ACCEPTED MANUSCRIPT reworked sediment. Contourite and turbidite beds are commonly interbedded in continental slope, rise, and abyssal plain settings
PT
(Rebesco and Camerlenghi, 2008; He et al., 2008; Mulder et al.,
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2008; Gao et al., 1995; Stow et al., 2002d; Moraes et al., 2007; Stow et al., 2002f; Viana and Rebesco, 2007); in these settings,
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bottom water currents may rework turbidite beds or more rarely,
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pirate sediment directly from turbidity currents (Rebesco et al., 2002; 2007). Turbidites likely provided a source of sediment for
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contourites in LF1 and LF2.
D
Uncommon turbidite beds within LF1 and LF2 represent
TE
very short-term events that introduce fresh sediment. However, the majority of the coarser grained material was reworked and
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deposited by contour currents, which represent longer-term fluctuations in bottom water energy. Stow et al. (2008) noted that bottom water currents show variability in strength over periods of tens of years to thousands of years, which can be linked to climatic cycles and sea level cycles. The recognition of bottom water current deposits depends on a diagnostic set of primary sedimentary structures and lack of systematic vertical stacking of structures such as those recognized in the Bouma (1962) sequence (see section 4.1 Lithofacies Interpretation; Martìn-Chivelet et al., 2008; Shanmugam, 2008). However, some authors have suggested that pervasive bioturbation 41
ACCEPTED MANUSCRIPT that eliminates primary sedimentary structures is more common (Stow and Faugères, 2008; Wetzel et al., 2008). Modern examples
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in which bioturbation is associated with oxygen-rich bottom
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currents include the Faro Drift in the Gulf of Cadiz (Gonthier et al., 1984; Stow and Holbrook, 1984; Stow and Piper, 1984). Studies of
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contourites from recent and ancient successions show that
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bioturbation in contourites can be highly variable and locally nonexistent (Dalrymple and Narbonne, 1996; Ito, 1996), dependent on
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bottom water oxygenation and sedimentation rate.
D
Bioturbation in LF1 and LF2 is absent to moderate,
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indicating that bottom water currents carried low but variable concentrations of dissolved oxygen or that sedimentation rate was
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too rapid for significant bioturbation of contourite beds. While many bottom water currents in modern oceans occur in oxygenated environments, paleoceanographic conditions were likely much different during the Late Devonian (e.g. Caplan and Bustin, 2001). Eustatic sea level was roughly 150m higher during the Frasnian than today (Johnson et al., 1985; Savoy and Mountjoy, 1995; Haq and Schutter, 2008), an epicontinental seaway covered much of North America, and black shale deposition was widespread. Bottom water oxygen concentration seems to have been minimal (although not necessarily anoxic) during deposition of many Devonian black shales (e.g. Harris et al., 2013), even though
42
ACCEPTED MANUSCRIPT evidence for traction currents is now commonly recognized (e.g. Schieber, 1999; Macquaker and Bohacs, 2007; Macquaker et al.,
PT
2010b; Ghadeer and Macquaker, 2011, 2012; Abouelresh and
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Slatt, 2012; Hemmesch et al., 2014). While it may be that contour currents during times of high sea level do not necessarily introduce
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oxygen into deep parts of the basin, bioturbation in contourite beds
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may also be limited due to increased sedimentation rates.
6.2 Aggregate Grains and Suspension Settling
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Flume experiments by Schieber et al. (2007b) and Schieber
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and Southard (2009) showed that flocculated clay minerals can be
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transported as bedload without disaggregating and can form sedimentary structures such as ripples. Schieber et al. (2010) detailed the transport of mud rip-up clasts in flume experiments, but noted that these rip-up clasts show irregular outlines in plan view and are poorly sorted. Ripple cross-lamination containing clay mineral aggregates or clasts has not been observed in Duvernay Formation mudstones. Additionally, clay mineral aggregates in Duvernay Formation sediments tend to have ovoid rather than irregular outlines in plan view, diffuse rather than sharp edges, and a relatively narrow size range as opposed to the poorly sorted clasts described above. Könitzer et al. (2014) identified similar features in late Mississippian mudstones of the Widmer 43
ACCEPTED MANUSCRIPT Gulf, UK, attributing them to suspension settling of aggregates. They also recognized slightly larger clay mineral aggregates with
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sharper edges and tapered ends, which they attributed to erosion
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and transport of mud clasts. Clay mineral aggregates and organic matter aggregates observed in Duvernay Formation sediments
SC
were deposited through suspension, and any subsequent reworking
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by bottom water currents should have caused disaggregation and resuspension of clay-size particles, rather than traction transport of
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mud clasts or floccules. Abundance of aggregates decreases with
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sediment agitation.
D
increasing bioturbation, which suggests disaggregation during
In modern environments, aggregation of organic matter
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increases settling velocity (Shanks, 2002), more rapidly removing aggregates from the water column. Marine snow is most common in modern oceans where primary productivity in the upper column is high (Alldredge, 1976; Billett et al., 1983; Lampitt, 1985; Thiel, 1995; Grimm et al., 1997; Fortier et al., 2002), and some authors have interpreted ancient beds rich in organic matter aggregates to be the result of enhanced productivity or algal blooms (Macquaker et al., 2010b). Organic matter aggregates in Duvernay Formation mudstones are randomly dispersed in the matrix rather than concentrated in layers, suggesting that enhanced productivity events, such as algal blooms, were not frequent. Chow et al. (1995)
44
ACCEPTED MANUSCRIPT reached a similar conclusion by mapping organic facies and depositional facies in Duvernay Formation sediments near the
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Redwater reef complex, concluding that evidence for algal bloom
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facies (defined by algal akinete cells and large, thick-walled Prasinophyte phycomas) in Duvernay Formation sediments was
SC
generally absent, indicating that organic accumulation was a
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function of normal productivity and low oxygen conditions.
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The dual roles of suspension settling and bottom current/turbidity current transport are important to note, as suspension-deposited
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sediment in some organic-rich mudstones represents a very minor
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component of the rock (e.g. Ghadeer and Macquaker, 2011, 2012), but is a more significant component of Duvernay Formation
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deposits.
7 Conclusion:
The detailed lithofacies analysis presented in this study describes the fine-scale variability of lithofacies in Duvernay Formation mudstones, and is the foundation for a basin-scale depositional model. While suspension-deposited sediment is significant in the most organic-rich Duvernay Formation lithofacies, we present evidence of other processes based on the variety of sedimentary structures. 45
ACCEPTED MANUSCRIPT Unlike some other organic-rich mudstone successions (e.g. Ghadeer and Macquaker, 2011, 2012), suspension settling is
PT
locally significant, although it was never the sole depositional
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mechanism. A combination of suspension settling, sedimentgravity flows, and bottom water currents distributed sediment
SC
throughout the basin. Near reef complexes, downslope sediment-
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gravity deposits are common, represented by limestone breccias and packstones. Turbidite beds are much more minor further into
MA
the basin but are present as thin fossiliferous beds. Siliciclastic sediment was transported into the basin from the north by slope-
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D
parallel currents (Stoakes, 1980; Knapp, 2016), whose position and strength were in part determined by basin morphology. Contour
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currents also acted within areas of the basin where organic-rich mudstones were dominant, creating interbedded muddy and silty/sandy successions which still contain significant TOC values. Organic matter and clay minerals deposited from suspension are commonly observed as aggregates. Total organic carbon varies systematically within a lithofacies framework, and organic-enrichment is attributed to a variety of factors. The highest TOC values are observed in planarlaminated, siliceous mudstones, deposited in generally anoxic bottom waters, where dilution of organic matter by clastic or carbonate sediment was minimal. However, significant organic-
46
ACCEPTED MANUSCRIPT rich deposits also occur where contourites are interbedded with siliceous, organic-rich mudstone. Periodic fluctuations in current
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energy and bottom water oxygenation generally acted to reduce
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TOC values, but not necessarily enough to exclude the strata from being prospective for hydrocarbon exploration. Organic
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enrichment also occurred in dysoxic bottom waters where sediment
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composition and increased sedimentation rate were key to organic matter preservation. Clay minerals may have aided in the
MA
formation of organo-minerallic aggregates which decreased the time organic matter was exposed to oxygenated water. Where
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bottom water anoxia was not present, an optimal sedimentation rate may have been key to preserving but not significantly diluting
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organic matter. Organic enrichment in a variety of depositional settings illustrates the interplay of bottom water oxygen concentrations, sedimentation rates, and sediment composition. The relative importance of each factor varies, even within a single succession of mudstones. The character and distribution of lithofacies depended heavily on type and proximity of sediment sources that varied throughout depositional period of the Duvernay Fornation. The relative importance of carbonate, clastic, and biogenic sediment varied through time and with geographic location. Changes in basin morphology, water circulation, and sea level acted to increase or
47
ACCEPTED MANUSCRIPT reduce the significance of each sediment source, and thus the character of sediments deposited.
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Recent literature has broken down the paradigm of organic-rich
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mudstone deposition as the sole product of suspension settling in
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quiet-water, often stratified, anoxic conditions. Depositional conditions are often dynamic, as our results support. However each
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mudstone succession is different. While some recent studies
MA
provide evidence for traction transport of mud clasts and floccules, our study shows no evidence for this phenomenon. There is
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abundant evidence for bottom water currents in mud-dominated
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settings, but clay-sized particles were resuspended, rather than transported in traction with the silt- and sand- sized sediment. We
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have shown that the mechanisms for organic-enrichment are variable, even between contemporaneous deposits in different areas of a basin, reflecting the dynamic interplay of bottom water oxygen concentration, sedimentation rate, and sediment composition. The effect of basin circulation patterns on mudstone composition is significant – a subject which seems to be underrepresented in the literature. The effect of current direction in shallower parts of a basin can strongly control the amount and type of sediment supplied to the deeper basin, affecting both composition and organic-richness of the deeper water mudstones.
48
ACCEPTED MANUSCRIPT Acknowledgements: This work was supported by Imperial Oil, Shell Canada,
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ConocoPhillips, Nexen, Devon Energy, Husky Energy, and the
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Natural Sciences and Engineering Research Council of Canada
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(grant number CRD 445064-12).
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Throughout the project, technical discussions with a number of individuals helped form and evaluate the interpretations
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presented here. These people are Korhan Ayranci, Tian Dong, Noga Vaisblat, Chris Schnieder, Murray Gingras, Matthew Fay,
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Figure Captions:
Table 1: Name and location of described cores. Length is described
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Duvernay section.
Figure 1: Map of study area. Duvernay Formation organic-rich
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mudstones are the basinal equivalent of Leduc Formation reefs. Organic-lean mudstones are deposited adjacent to the Grosmont
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Platform. The Peace River Arch was an emergent land mass during the Late Devonian, and was fringed my Leduc Formation reefs. Data from Switzer et al., 1994.
Figure 2: Late Devonian stratigraphy of Central Alberta. The Duvernay Formation is informally divided into lower, middle, and upper members. Modified from Switzer et al., 1994.
Figure 3: LF1 planar-laminated mudstone. A) Planar laminae are very subtle in core. Core photo, SCL Kaybob 02-22. B) Laminae 75
ACCEPTED MANUSCRIPT are faintly defined by increases in silt (arrow). Most silty laminae are mud-supported. Thin section scan, SCL Kaybob 02-22. C)
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Discontinuous planar laminae are enriched in framboidal and
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euhedral pyrite as well as pyritized fossil fragments and carbonate silt. Core photo, CNRL Edson 01-10. D) Lamination is defined by
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discontinuous pyrite laminae (arrow) and continuous silt-rich and
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silt-poor laminae. Discontinuous pyrite laminae pinch and swell. Thin section photomicrograph, plane polarized light, SCL Kaybob
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02-22. E) Pyrite (bright reflectance) in laminae is present most commonly as euhedral crystals and framboids, but also as full and
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partial replacement of calcareous grains and fossil fragments. Thin section photomicrograph, reflected white light, SCL Kaybob 02-
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22.
Figure 4: Silty laminae in LF1. A) Silty laminae are clearly visible in hand sample. Core photo, ECA Cecilia 11-04. B) Silt is dominantly calcite and dolomite with lesser amounts of quartz and rare chert clasts. Thin section photomicrograph, cross-polarized light, SCL Kaybob 02-22. C). Silty laminae show both normal and inverse grading. Grain-supported laminae are more common and prominent than LF1-2. Thin section scan, SCL Kaybob 02-22.
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ACCEPTED MANUSCRIPT Figure 5: Types of sedimentary aggregates in LF1. A) Clay mineral aggregate in bed-normal thin section. Thin section
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photomicrograph. B) Clay mineral aggregate in bed-parallel thin
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section. Clay mineral aggregates are round to ovoid in plan view and contain much less silt than surrounding sediment. Thin section
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photomicrograph. C) Organic matter aggregate in bed-normal thin
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section. Thin section photomicrograph. D) Organic matter aggregate in bed-parallel thin section. Organic matter aggregates
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are round to ovoid in plan view and have wispy edges. Thin
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section photomicrograph. All samples from GuideX Gvillee 09-06.
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Figure 6: LF2 wavy-laminated mudstone. A) Lenticular silt laminae pinch and swell and contain starved ripples (arrow). Core photo, Chevron Chickadee 03-05. B) Silty laminae show normal and inverse grading. Thin section scan, SCL Kaybob 02-22. C) Silt is a mixture of calcite, dolomite, quartz, and uncommon chert clasts. Dark, silt-poor laminae are rich in clay-sized grains and organic matter. Thin section photomicrograph, cross-polarized light, SCL Kaybob 02-22
Figure 7: LF2 in the NE West Shale Basin. A) Silt-sand beds are coarser, more prominent, often with sharp, erosive bases, and 77
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near the top of the bed. Thin section photograph. B) Sharp-based,
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normally graded turbidite beds have especially coarse, fossiliferous bases and become more common. Core photograph. Both images
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from Chevron Chickadee 03-05.
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Figure 8: LF3 siliceous-calcareous wackestone-floatstone. A) Poorly bedded styliolinid wackestone. Calcareous styliolinid
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fossils in medium grey, organic-rich calcareous-siliceous
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mudstone. Core photo, EOG Cygnet 08-20. B) Intensely
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bioturbated styliolinid wackestone. Uncommon larger fossils include brachiopods. Pyrite preferentially precipitates in traces (yellow). Thin section scan, SCL Kaybob 02-22. C) Styliolinid and tentaculitid fossils are unevenly distributed. Matrix contains abundant fine grain calcite debris. EOG Cygnet 08-20.
Figure 9: LF4 bioturbated siliceous pyritic mudstone. A) Intensely bioturbated mudstone. Pyrite preferentially precipitates in traces. Core photo, ECA Cecilia 11-04. B) Most of the sample is structureless but a faint bedding contact (yellow arrow) can be seen in the lower half of the image. White arrows point to pyrite-rich 78
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lined with pyrite (black). Burrow fill is less silty then surrounding
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sediment. Silt grains are dolomite, calcite and quartz. Brown
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photomicrograph, ECA Cecilia 11-04.
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matrix contains clay minerals and organic matter. Thin section
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Figure 10: Benthic fossils in LF4. A) In-situ bivalve in bedding surface of core. Core photo. B) In-situ gastropod on bedding
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surface of core. Gastropod fossils larger than core diameter are
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Cecilia 11-04.
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common in LF4 of ECA Cecilia 11-04. Both photos from ECA
Figure 11: LF5 bioturbated calcareous mudstone. A) Poorly
bedded, moderately bioturbated calcareous mudstone. Lighter colored beds have greater calcite abundance. Core photo. B) Structureless calcareous mudstone. Clay- and silt-sized calcite is abundant. A lesser amount of quartz silt is present. Thin section scan. C) A large percentage of the rock is composed of calcite silt and calcareous fossil fragments. Minor organic matter is present between grains. Thin section photomicrograph, plane-polarized light, calcite is stained pink. D) Agglutinated benthic foraminifera 79
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light. All samples from EOG Cygnet 08-20.
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Figure 12: LF6 nodular wackestone. A) Nodular limestone in
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Perdrix Formation outcrop (outcrop nomenclature for Duvernay
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Formation). Nodule size and morphology is variable and ranges from ovoid to irregular. Outcrop location: Nigel Peak / Wilcox
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Creek, Alberta, Canada. Lense cap dimeter = 5cm. B) Uncemented dark grey mudstone is plastically deformed around
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calcite nodules. Calcite nodules contain spar-filled, wedge-shaped
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fractures. Burrows are preferentially cemented. Core photo, EOG
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Cygnet 08-20. C) Intensely bioturbated nodular wackestone. Fine calcite fossil fragments are abundant. Thin section scan, SCL Kaybob 02-22.
Figure 13: LF7 thinly bedded argillaceous-dolomitic mudstone. Mudstones are laminated to thinly-bedded and moderately to intensely bioturbated. Core photo, Imperial Virginia Hills 06-36.
Figure 14: LF8 dolowackestone and evaporite. Nodules range from near-planar (A), to ovoid (B), to irregular (C), with varying
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ACCEPTED MANUSCRIPT abundance of anhydrite. All images are core photos. A) Sun IOE
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10-09. B) Sarcee et al. Pibroch 10-16. C) Sun IOE 10-09
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Figure 15: LF9 intraclastic packstone and LF10 limestone breccia.
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A) LF9 packstones are light grey and composed dominantly of
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sand-sized calcite with common mud rip up clasts. Core photo. B) Calcite clasts and fossil fragments are dominant but phosphatic,
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siliceous, and pyritic clasts and fossil fragments are present as well. Thin section photomicrograph. Both samples are from EOG
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Cygnet 08-20. C) Subangular limestone clasts are surrounded by
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packstone matrix. Bed has a sharp base and includes dark grey
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mudstone rip up clasts. Core photograph, AOSC Grizzly 01-24. D) Limestone clasts are poorly sorted in mudstone-packstone matrix. This core photo of a LF10 breccia is in the Swan Hills Formation underlying the Duvernay Formation in the Enermax Panther SturLs 14-02 core.
Figure 16: A) Core description of the SCL Kaybob 02-22 core. Note the dominance of LF1 (dark grey) organic-rich mudstones. LF6 nodular wackestone is dominant in the middle Duvernay member. B) Core description of the GuideX Gvillee core. LF1 is less common and more argillaceous near the Peace River Arch. 81
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Figure 17: Core description of the ECA Cecilia 11-04 core. Note
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the tendency of LF1 pyritic laminae to occur in association with
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bioturbated pyritic mudstones of LF4. LF4 is especially dominant
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in the upper Duvernay member in wells in the Wild River Sub
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Basin.
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Figure 18: Core description of the Chevron Chickadee 03-05 core.
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LF2 is dominant in the upper Duvernay member.
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Figure 19: EOG Cygnet 08-20 core description. The core is dominated by calcareous facies LF5 and LF6. LF5 is especially dominant in the upper Duvernay member.
Figure 20: Core description of the Imperial Virginia Hills 06-36 core. LF7 and LF8 dominate the lower and middle Duvernay.
Figure 21: Core description of the Sarcee et al. Pibroch 10-16 core. LF8 is the dominant lithofacies.
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ACCEPTED MANUSCRIPT Figure 22: Depositional models. A) During deposition of the lower and middle Duvernay members lithofacies distribution is best
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explained by a platform construction model. A continuous
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dolomitic-argillaceous-calcareous-siliceous transition of lithofacies occurs basinward from the Grosmont Platform. Sediment is
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primarily sourced from the northeast. Clastic sediment was
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transported parallel to slope by currents (Stoakes, 1980). Organicrich mudstones of LF1 are only prominent in distal parts of the
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basin. B) Deposition of upper Duvernay member sediments is best explained by a flooded platform model. LF7 and LF8 argillaceous-
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dolomitic-evaporitic facies occur further to the northeast, and are separated from basinal organic-rich facies by a zone of non-
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deposition. Contour currents flow parallel to the previously developed platform slope, depositing silty contourites of LF2. Organic-rich mudstones of LF1 are prominent over much of the basin. Clay mineral supply increases to the west (reverse of platform construction model) as a result of NE-derived clastics being trapped upslope, and the Peace River Arch becoming a more significant supply of clastics.
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ACCEPTED MANUSCRIPT Table 1 Map #
Well Name
UWI
Sampled
Detailed Description
Length in Duvernay (m)
GuideX Gvillee 09-06
100/09-06-076-23W5/00
Y
Y
14.4
BPC et al. Smoky HT 04-36
100/05-36-072-01W6/00
N
N
11.4
3
Xerex SturLks 07-22
100/07-22-069-21W5/00
N
N
12.6
4
Enermax Panther SturLs 14-02
100/14-02-069-21W5/00
N
N
9.6
5
AOSC Grizzly 01-24
100/01-24-061-23W5/00
N
Y
34
6
SCL HZ Kaybob 02-22
100/02-22-063-20W5/00
Y
Y
54
7
Chevron Chickadee 03-05
100/03-05-062-16W5/00
N
N
34
8
Imperial Virginia Hills 06-36
100/06-36-063-12W5/00
N
N
17
9
ECA Cecilia 11-04
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SC
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1 2
100/11-04-058-23W5/00
Y
Y
59.5
100/01-10-052-17W5/00
N
N
25
100/09-06-052-11W5/00
N
N
12.6
100/03-21-040-07W5/00
N
Y
42.5
100/10-17-045-06W5/00
N
Y
27
100/02-06-047-04W5/00
N
N
19.5
Imp Cdn-Sup Tomahawk 16-18
100/16-18-052-05W5/00
N
N
14
16
Forgotson Burk SGSpike 10-04
100/10-04-051-27W4/00
N
N
11.8
17
Sarcee et al. Pibroch 10-16
100/10-16-061-26W4/00
N
N
8.6
18
Imperial Deep Creek 04-33
100/04-33-068-22W4/00
N
N
28.5
19
Imperial Figure Lake 11-19
100/11-19-062-18W4/00
N
N
13.4
CNRL HZ Edson 01-10
11
Imperial Cynthia No. 09-06
12
SCL HZ Ferrier 03-21
13
Penn West Pembina 10-17
14
Imp Cdn-Sup Norbuck 02-06
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10
20
Tex et al. Lucky 09-09
100/09-09-061-18W4/00
N
N
9.7
21
Esso Redwater 10-27
102/10-27-057-21W4/00
N
N
55.4
22
Esso Redwater 16-28
102/16-28-057-21W4/00
Y
Y
57.1
23
Nexxtep 07-05
102/07-05-050-25W4/00
N
N
9.6
24
EOG Cygnet 08-20
100/08-20-038-28W4/00
Y
Y
46.8 Total = 628
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