Chapter 12 - Devonian Woodbend-Winterburn Strata

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Chapter 12 - Devonian Woodbend-Winterburn Strata

Chapter 12 - Devonian Woodbend-Winterburn Strata of the Western Canada Sedimentary Basin

Chapter Sections Download
  1. Introduction
  2. Geological Framework
  3. Stratigraphy
    1. Subdivision of Woodbend-Winterburn Strata
  4. Woodbend Group
    1. Cooking Lake
    2. Majeau Lake/Lower Leduc
    3. Duvernay/Middle Leduc
    4. Lower Ireton/Upper Leduc
    5. Leduc
      1. Eastern Basin
      2. Western Basin
      3. Peace River Arch
      4. Grosmont Complex
    6. Upper Ireton
  5. Winterburn Group
    1. Nisku
      1. Southern Alberta and Saskatchewan
      2. East-central Alberta
      3. West Pembina - West-central Alberta
      4. Peace River Arch - Northern Alberta
    2. Blue Ridge
    3. Upper Graminia
  6. Summary
  7. Acknowledgements
  8. References

Tables

Table 12.3a Oil production from the Woodbend Group.

Table 12.3b Gas production from the Woodbend Group

Table 12.4a Oil production from the Winterburn Group

Table 12.4b Gas production from the Winterburn Group


Authors:
S.B. Switzer - Chevron Canada Resources, Calgary
W.G. Holland - Consultant, Calgary D.S. Christie - Outtrim Szabo Associates Ltd., Calgary G.C. Graf - Chevron Canada Resources, Calgary A.S. Hedinger - Consultant, Calgary R.J. McAuley - Consultant, Calgary R.A. Wierzbicki - PanCanadian Petroleum Ltd., Calgary J.J. Packard - Consultant, Calgary

Introduction

This chapter deals with the youngest Frasnian and oldest Famennian rocks in the Western Canada Sedimentary Basin, comprising, in succession, the Woodbend and Winterburn groups (Fig. 12.1). These two groups occupy the interval between the Beaverhill Lake and Wabamun groups and contain several distinct chronostratigraphic sequences composed largely of cyclic carbonate-clastic and, to a lesser extent, cyclic carbonate-evaporite deposits.

During early phases of Woodbend Group deposition the Western Canada Sedimentary Basin underwent gradual deepening. This resulted in the development of a thick aggradational succession of carbonates with substantial topographic relief between shelf and basinal areas. By the end of Woodbend deposition, during a period of reduced rate of subsidence and/or sea-level fall, most of the basin was filled by shales. Winterburn Group deposition was characterized by an overall shallowing and filling of the basin.

A number of notable changes occurred during deposition of the Woodbend-Winterburn strata, which resulted in distinct features that set these groups apart from earlier Paleozoic strata. These changes and features include: 1) an apparent increase in the rate of accumulation and preservation of sediment, 2) a dramatic increase in the occurrence of basin-filling shales, 3) the development of thick and extensive reef complexes, 4) the deposition of widespread and prolific source rocks, and finally 5) the significant accumulation of economic reserves of hydrocarbons hosted largely by numerous reefal carbonate reservoirs.

According to Harland et al. (1982) the Frasnian Stage represents approximately seven million years. Considering that the Woodbend and the majority of Winterburn strata were deposited during only part of this time (Fig. 12.1), their maximum thicknesses are extraordinary, in excess of 850 m (Fig. 12.2). The relative contribution of each group is indicated in Figures 12.3 and 12.4. During no other Paleozoic period in the Western Canada Sedimentary Basin was such a thick depositional record accumulated and preserved within such a short time frame. This thick accumulation reflects the interplay between eustatic sea-level changes, rapid sedimentation, and most importantly, a high rate of basin subsidence.

In conjunction with this dramatic increase in sedimentation, Woodbend and Winterburn strata display the lithological complexities of earlier Devonian strata. However, in addition to limestones, dolomites and evaporites, these groups show an increase in the thickness and areal extent of basin-filling shales (Ireton, Fort Simpson, Duvernay and Majeau Lake formations).

The two most distinctive features of these groups are the stacked reef complexes (Leduc Formation) that exceed 275 m in thickness, and the highly bituminous source rocks (Duvernay and Muskwa formations), considered by some (Allan and Creaney, 1991) to have generated the majority of hydrocarbons found within the Upper Devonian reservoirs of the Alberta Basin.

From an economic standpoint, these groups have received a great deal of attention for their significant reserves of conventional hydrocarbons. The Woodbend and Winterburn groups contain approximately 32 and 11 percent, respectively, of the initial established conventional oil plus oil-equivalent gas reserves found within the Paleozoic strata of the Alberta Basin (Energy Resources Conservation Board, 1990). The majority of these oil and gas pools are associated with ancient reef complexes of widely varying size and shape, depositional setting and facies composition (Alberta Society of Petroleum Geologists, 1960, 1966 and 1969). In addition, significant accumulations of heavy oil and bitumen are present at the Grosmont and Winterburn subcrop edges. As a consequence, over the past 45 years a great deal of emphasis has been placed on their study, including reviews of the architectural evolution of reef stages, depositional facies, paleontology, post-depositional alteration, relations between reef development and basin filling and controls on reef localization (see references in Belyea, 1964; Watts, 1987; Moore, 1989; Dix, 1990). Despite this emphasis, there still remains considerable uncertainty in establishing the time equivalence between various reef complexes from different areas of the basin and their relation to basin-filling. This aspect is discussed in some detail in a subsequent section of this chapter.

Considering the economic significance of these strata, it is remarkable that aside from Belyea (1964), little additional regional work has been published on the geology of the Woodbend and Winterburn groups on the scale of the Western Canada Sedimentary Basin. Only Basset and Stout (1967), Ziegler (1967), Nelson (1970), Porter et al. (1982), Burrowes and Krause (1987), Stoakes and Wendte (1987), and Moore (1989) have attempted to enhance the understanding of paleogeography beyond that of Belyea's original work. In an attempt to build upon these previously published works, the aim of this chapter is to illustrate, at a greater level of detail, the sequential basin-wide development and depositional history of the Woodbend and Winterburn groups.

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Geological Framework

Although this chapter contains maps and cross sections confined to the Western Canada Sedimentary Basin, contiguous strata of similar age stretch from the Arctic Archipelago to southern Wyoming. Despite the complexities of combining numerous formations into a single map, Ziegler (1967) succeeded in providing an excellent illustration of the approximate paleogeography and generalized lithofacies distribution of these rocks for the entire northwestern margin of North America. In general, from northwest to southeast, the facies of these groups change from open-marine shales in northeastern British Columbia and the Northwest Territories to shallow-water carbonates in Alberta. These carbonates in turn grade into more restricted dolomites and evaporites in Saskatchewan and Manitoba.

The eastern depositional margin of the basin was undoubtedly farther east than its present-day erosional edge (Fig. 12.2), but the full extent of erosional bevelling is speculative. The western depositional limit of these deposits is unknown, obscured by the presence of younger obducted terranes west of the Rocky Mountain Trench in British Columbia.

On the scale of the Western Canada Sedimentary Basin, it is generally more difficult to trace the bounding surfaces of these groups than many of their internal sequence and cycle boundaries, which show a more laterally persistent borehole log signature. The Woodbend Group is conformable to disconformable with the underlying Beaverhill Lake Group. It typically overlies either shaly limestones (Waterways Formation), clean limestones (Swan Hills Formation), cyclic dolomites and evaporites (Souris River Formation) or arkosic sands (Granite Wash Formation). The top of the Woodbend Group is marked by the cessation of major clastic-carbonate infilling of the basin, with an accompanying marine incursion that is registered at the basin margin by shallow-water shelf carbonate deposition of the Winterburn Group. More basinal areas at this boundary show only slight interruption of shale deposition.

The most chronostratigraphically significant event, the Frasnian-Famennian boundary, lies within the uppermost strata of the Winterburn Group, and is represented by an abrupt disconformable to unconformable surface throughout most of the Western Canada Sedimentary Basin. The top of the Winterburn Group, commonly picked as the uppermost occurrence of quartzose silt, appears to be gradational and conformable over most of the basin with the overlying Wabamun Group carbonates. The uppermost occurrence of silt is not considered an important chronostratigraphic event. The distribution of silt is interpreted as being principally a function of reworking during transgression of the Devonian sea, which led to Wabamun carbonate deposition. Typically, sandstones and siltstones of the Famennian overlie truncated shallow-water dolomites and evaporites of the Frasnian (Blue Ridge Member or older strata).

In contrast to the relative stability of the basin that characterized underlying Beaverhill Lake deposition, Woodbend sedimentation was significantly influenced by one or more major external forces. The major influences were localized movement of basement fault blocks, the generally accentuated differential subsidence that varied on a regional scale, the syndepositional removal of earlier formed salt (primarily Middle Devonian Elk Point salt) and the nature and volume of terrigenous clastics introduced into the basin. The impact and timing of each varies throughout the Western Canada Sedimentary Basin. As a result, the details of basin-filling and their relations to reef and carbonate shelf development are complex.

During Woodbend deposition, the major geological elements that directly or indirectly influenced deposition of the group were: the Peace River-Athabasca Arch, West Alberta Ridge, Tathlina High, Ft. Nelson Uplift, Meadow Lake Escarpment, and several large-scale basement trends such as the Rimbey Arc (Ross and Stephenson, 1989) and Hay River lineament (Fig. 12.6). In general, maximum subsidence occurred over the old central Elk Point Basin north and west of the Meadow Lake Escarpment. Relative subsidence was much less to the south and southeast of the escarpment, which also marks the change from dominantly carbonate-clastic cycles in the northwest to carbonate-evaporite cycles in the southeast. The West Alberta Ridge and Peace River Arch were also areas of relatively lower subsidence rates.

The Peace River Arch, the most topographically prominent element, continued to be an emergent landmass. However, throughout Woodbend and Winterburn deposition the landmass area diminished in size as a result of gradual subsidence and onlapping of the arch by a series of backstepping fringing reef complexes (Dix, 1990). In addition, normal block-fault movements in this area affected both Woodbend and Winterburn deposition (O'Connell et al., 1990).

The West Alberta Ridge, largely inundated by the end of Beaverhill Lake sedimentation, continued to affect deposition, although at a progressively reduced level during Woodbend sedimentation. Initially, however, it was a site of major Leduc reef complex development.

The Tathlina High, although completely buried by the end of Middle Devonian Elk Point deposition, resumed its influence on Woodbend deposition. Relative to adjacent areas, the Tathlina High and Ft. Nelson Uplift remained submarine topographic highs against which basin-fill shales downlapped and thinned. Although block faulting of the basement influenced Woodbend deposition in the vicinity of the Peace River Arch, it was most dramatic in the vicinity of the Tathlina High, especially where fault movements accompanied by Elk Point salt removal produced large increases in accommodation space during deposition of the Fort Simpson Formation (see Fig. 12.28, and related discussion). This feature had little significant effect on Winterburn sedimentation.

The Meadow Lake Escarpment, a pre-Devonian erosional and structural feature well known for its tremendous effect on Middle Devonian sedimentation, also appears to have influenced many of the depositional facies changes in Upper Devonian strata. For example, the southern Alberta Leduc shelf edge closely coincides with both the underlying southern bank edge of the Beaverhill Lake Group (Oldale and Munday, this volume, Chapter 11) and the Meadow Lake Escarpment.

As described by Ross and Stephenson (1989), a large-scale basement trend, known as the Rimbey Arc, coincides with the southern half of the 560 km long Leduc-Homeglen-Rimbey-Meadowbrook reef chain. In addition, this lineament, in part, areally coincides with a change in subsidence and accommodation space during various phases of Woodbend basin-filling (see discussion of subdivisions within Cooking Lake-Majeau Lake deposits).

Both the Woodbend and Winterburn groups were subjected to post-depositional dolomitization and structural movements (Mountjoy, 1980; Machel, 1983; Dix, 1990; O'Connell et al., 1990), with critical influence on reservoir quality and trap configuration (see Walls and Burrowes, 1985).

At a detailed level, basement fault movements and the removal of salt at differing times from both within and below Woodbend-Winterburn strata, created locally complex stratigraphy and structure. Despite these localized complexities, the present-day structure of the Woodbend-Winterburn strata (Fig. 12.5) shows a similar pattern to that of earlier Paleozoic strata. Dips increase both toward the west, due largely to downwarping during the Laramide Orogeny, and toward the centre of the Williston Basin, in Saskatchewan.

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Stratigraphy

Subdivision of Woodbend-Winterburn Strata

To illustrate effectively the depositional history and paleogeography of such a thick and complex stratigraphic succession, the Woodbend and Winterburn groups are herein subdivided into seven major chronostratigraphic intervals. These intervals are designated using central Alberta stratigraphic terms. They are, from youngest to oldest:

(7) Blue Ridge
(6) Nisku
(5) Upper Ireton
(4) Lower Ireton/Upper Leduc
(3) Duvernay/Middle Leduc
(2) Majeau Lake/Lower Leduc
(1) Cooking Lake

With the exceptions of the base of the Cooking Lake, the top of the upper Graminia and the Leduc, each of which require special discussion, these intervals are bounded and defined by correlatable borehole log markers, which are interpreted as chronostratigraphic surfaces throughout the basin. Interval boundaries are shown as heavier lines on all cross sections. Figure 12.7 illustrates the reference and type wells or sections upon which these markers are defined and correlated throughout the basin.

To achieve consistency of treatment, more than 10 000 borehole logs were copied, made into cross sections, correlated and, where appropriate, the major bounding chronostratigraphic markers for the chosen intervals were recorded for inclusion in the Atlas database. Many of the distinct log signatures can be traced confidently over vast areas of the basin, while others will require additional core work, coupled with biostratigraphic or other dating data, before a higher level of confidence in correlation can be attained.

As shown in Figure 12.7, the Woodbend and Winterburn groups are subdivided into intervals that closely approximate most of the well known formations. However, some intervals depart dramatically from the recognized formal lithostratigraphic units. In some examples (e.g., subdivision of Ireton and Fort Simpson formations) departures occur because the formations involved are considered to make up two major depositional sequences.

In areas where the log markers are difficult to pick, the magnitude of error is generally considered to be less than 30 m. Errors of this magnitude are likely confined to areas with thick continuous basin-filling shales, where bounding surfaces do not exhibit a distinctive log character (e.g., in the Fort Simpson Formation of northeastern British Columbia and northern Alberta). Given the possible alternative picks, the shift in resulting isopach values is less than the isopach contour interval of the Atlas maps. Thus, the pattern of deposition inferred from these isopach maps is relatively insensitive to minor miscorrelation. Notwithstanding these uncertainties, the correlation of these bounding surfaces, in most cases, is consistent with published and available (albeit limited) biostratigraphic control (see McLaren 1954, 1959; McLean and Sorauf 1988; Marchant and McLean 1992). In addition, these surfaces also are consistent with some of the distinctive breaks in deposition noted from published and non-proprietary core studies (e.g., see Fig. 12.8).

Designation of these chronostratigraphic events assists in highlighting the transgressive-regressive cyclicity of the Woodbend-Winterburn groups. This cyclicity is schematically shown in Figure 12.9. The basic pattern for each cycle is: 1) initial transgressive stage, 2) maximum transgressive stage, and 3) regressive stage. The initial stage is dominated by carbonate platforms. The maximum transgressive stage has the best developed organic-rich shales and isolated carbonate complexes. The regressive stage is dominated by basin-filling shales and carbonates. In general, the Woodbend Group is characterized by transgressive deposits while the Winterburn Group is dominated by regressive deposits.

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Woodbend Group

The Woodbend Group comprises the first five chronostratigraphic intervals identified in the Woodbend-Winterburn succession (i.e., Cooking Lake to Upper Ireton; Fig. 12.7). Each interval is interpreted as consisting of a relatively rapid upbuilding phase (aggradation) of shelf and reefal sediments, followed by a phase dominated by the progressive infilling of basinal areas by shales and marlstones (progradation). Reef and shelf aggradation occurred during relative sea-level rises, whereas the progradational phase of basin-filling was restricted to periods of relative sea-level fall or stillstand. Figure 12.10 schematically shows the relation between reefal and/or shelf carbonate stages of development and basin-fill deposition for the major Woodbend reef-bearing intervals. It should be noted that for mapping purposes a distinction between post-reef and reef-equivalent basin-fill deposits was generally not made.

The major reef/shelf edges, oil and gas pools, and lines of cross section used to illustrate the stratigraphy and depositional history of these Woodbend intervals are shown in Figure 12.11. The intervals and details of their paleogeography, isopach and lithofacies variations, and areas of problematic correlation, are discussed in stratigraphic succession.

Cooking Lake

The Cooking Lake interval comprises extensive sheet-like shelf carbonates and an equivalent basin-filling shale. During deposition of the Cooking Lake interval a gradual deepening and variable increase in accommodation space occurred in different parts of the basin. This represents a significant change from the more subdued basinal relief observed within the underlying Beaverhill Lake Group. Figure 12.12 shows the paleogeography and isopach of the Cooking Lake carbonate.

Three major paleogeographic realms were established during the Cooking Lake interval. These include: 1) a broad shelf in the east; 2) areas of localized relief such as on the flanks of the Peace River Arch and West Alberta Ridge, overlying Swan Hills reef complexes of the Beaverhill Lake Group, and 3) an extensive basinal area throughout the remainder of the Western Canada Sedimentary Basin.

For mapping purposes (Fig. 12.12), the Cooking Lake interval is defined using the Cooking Lake Formation subsurface type section as a reference point (Fig. 12.7). This type section lies within the eastern shelf area and was designated by the Geological Staff, Imperial Oil Limited (1950) in the Calmont Leduc #3 4-14-51-21W4 well (see inset, Fig. 12.12). The equivalent Saskatoon, Elstow and Wymark members (Duperow Formation) in Saskatchewan are defined in the U.S. Borax and Chem. Corp. Elstow 5-22-34-1W3 type section well (Kent, 1968).

An extensive carbonate shelf developed in the east over southern Alberta, Saskatchewan and Manitoba, where underlying Beaverhill Lake sediments had reached a suitable bathymetry for subsequent prolific carbonate production. As shown in Figure 12.12, the strata of this broad eastern shelf vary in thickness between 65 and 75 m, thinning eastward to 30 m in southwestern Manitoba. The shelf thickens to 90 m directly beneath the Rimbey-Meadowbrook Leduc barrier reef-trend and then thins abruptly to less than 6 m immediately west of this trend (Figs. 12.12, 12.13, 12.14). The Cooking Lake carbonates consist of peloidal and skeletal limestone (brachiopods, crinoids, stromatoporoids, bryozoans). Unlike the majority of the younger Woodbend carbonates within the eastern shelf area, the Cooking Lake interval is relatively undolomitized. Thick, extensively dolomitized Cooking Lake strata are found only beneath the Homeglen-Rimbey-Meadowbrook Leduc reef trend and its southerly extension into the Caroline-Cheddarville area (Figs. 12.7, 12.12) where it has been interpreted as an important migration pathway for petroleum .

The Cooking Lake shelf edge extends in a north to northeast direction from Township 38 to subcrop at Township 92, a distance of 560 km. South of Township 38, the orientation of the shelf edge changes to an east-west direction and persists into the foothills under the Brazeau and Bighorn thrust sheets. Whether this long, linear shelf-edge is controlled by a change in subsidence across a deep-seated hinge line, or is simply a function of antecedent Beaverhill Lake depositional topography, has yet to be resolved. As described by Ross and Stephenson (1989), the southern half of this shelf edge closely coincides with the Rimbey Arc, a major linear basement trend. It is also noteworthy that the northern half of the Cooking Lake shelf edge closely follows the margins of carbonate banks developed within the underlying Mildred and Moberly members of the Beaverhill Lake Group. The position and thickening of the Cooking Lake shelf edge could in fact reflect both structural and inherited topographic controls, which might themselves be genetically related.

Portions of the eastern shelf have been described in detail by Andrichuk (1958), Kent (1968), Wendte (1974) and Dunn (1975). They have shown that shelf strata can be divided into three units, consisting of an upper and lower carbonate cycle separated by an argillaceous carbonate (see for example Fig. 12.15). These units are remarkably consistent in thickness and lithofacies over a broad area of the basin, suggesting that extremely stable conditions persisted during Cooking Lake deposition. Even in the vicinity of the underlying Meadow Lake Escarpment, there is no apparent effect on Cooking Lake deposition.

The lower Cooking Lake cycle represents a westerly (i.e., basinward) tapering carbonate ramp. With only local exceptions, the lower carbonate cycle is conformable with the underlying Beaverhill Lake Group. As such, the lower bounding surface of the Cooking Lake interval is not considered to be a regional chronostratigraphic event, although in parts of eastern Alberta this contact may be abrupt (Wendte, pers. comm., 1992). The separation between the Beaverhill Lake and Cooking Lake becomes less distinct toward the eastern portion of Saskatchewan, where no clear change in log signature is detected (see Fig. 12.15). Despite this limitation, isopach mapping using an approximate Beaverhill Lake top in Saskatchewan can constitute only an insignificant error, given that the transition from Beaverhill Lake to Cooking Lake occurs over a narrow interval. This approximation of the isopach does not significantly affect the interpretation of the Cooking Lake interval map (Fig. 12.12). The pattern of basin-filling observed in the underlying Beaverhill Lake Group continued without any distinct break into the lower cycle of the Cooking Lake Formation. This feature is well noted in Saskatchewan where the upper portion of the Beaverhill Lake and lower Cooking Lake-equivalent are recognized as the Saskatoon Member of the Duperow Formation (see Fig. 12.15; well 14-21-29-18W3). The top of the Saskatoon Member and lower Cooking Lake carbonate cycle appear to represent a basinwide, chronostratigraphically significant event (see Fig. 12.15 ). This event marks the onset of gradual deepening of the basin and areal restriction of subsequent shelf and reef development within the overlying Leduc Formation.

The middle argillaceous unit of the Cooking Lake shelf strata has been designated as the Elstow Member in Saskatchewan (Kent, 1968). It rests sharply and disconformably on the underlying carbonate cycle, and represents the last significant occurrence of shale deposited within the shallow-water carbonate complexes of the Woodbend group until late in Leduc shelf development (see Figs. 12.15, 12.41 left pane, 12.41 right pane, 12.44). The thickness of this shale unit varies from less than 3 m in southern Saskatchewan to as much as 30 m beneath the Bearberry, Ricinus, Strachan and Phoenix Leduc reefs near the mountain front (Fig. 12.11). The source of the shale has yet to be proven but probably was introduced from the northeast. The middle argillaceous unit is largely confined to the extensive Cooking Lake shelf; it does not appear to make up any significant component of basin-filling shale to the west. However, it appears to be contiguous with the lower part of the shaly facies of the upper Cooking Lake cycle in the northeast (Andrichuk, 1958).

The upper Cooking Lake carbonate cycle is transitional with the underlying shale unit, but the top is abruptly terminated and capped by a submarine hardground surface (Wendte, 1974). This cycle correlates with the lower Wymark Member in Saskatchewan (Kent, 1968). The nature of the bounding surface or contact at the top of both the lower and upper Cooking Lake carbonate cycles has not been described in all parts of the basin, particularly toward the eastern subcrop edge.

Unlike the ramp profile of the lower Cooking Lake carbonate cycle, the upper cycle has an abrupt, steeply dipping western shelf edge. This edge is recognized by an abrupt thinning in the Cooking Lake isopach (Fig. 12.12). It is the termination of the upper cycle that actually defines the Cooking Lake shelf edge. The upper Cooking Lake cycle is more areally restricted than the underlying cycle.

Carbonate shoal development within the upper Cooking Lake cycle was the main contributor to thickness variations in the eastern shelf. Andrichuk (1958), Klovan (1964) and Wendte (1974) noted that the configuration of these shoals was important in localizing subsequent Leduc reef growth. The top of the Cooking Lake interval appears to be a synchronous surface throughout the basin. In some areas it can be confidently traced into the basin-filling Majeau Lake shale. Consequently, only the lower Majeau Lake basin-fill belongs within the Cooking Lake interval. Although the lower Majeau Lake shale lies within this interval it appears to postdate the development of the Cooking Lake shelf.

As noted by Stoakes and Wendte (1987), it is difficult to distinguish the Cooking Lake Formation over the Swan Hills reef complexes of western Alberta from younger Leduc reef and platform development. If present, it is likely less than 30 m in thickness. As shown in Figures 12.12 and 12.26, this was an area of thin shallow-water carbonates or nondeposition, possibly as a result of reduced subsidence. More detailed work will be required to map Cooking Lake strata that fringe the Peace River Arch and extend over the Swan Hills reef complexes of the Beaverhill Lake Group before the evolution of carbonate growth in these areas will be satisfactorily resolved.

North of the Peace River-Athabasca Arch the lower Majeau Lake unit either thinned and changed facies into bituminous shales of the Muskwa Formation or was not deposited. To the south of this arch, this unit thins westward onto the Swan Hills bank. Its westerly depositional limit occurs between the Swan Hills bank-margin and the margin of the younger Leduc platform and reefs (see Fig. 12.11). A close relation is implied between the distal depositional limit of the lower Majeau Lake shale and the position of Leduc platform development. Both appear to be controlled by subtle topographic features within the interior of the Swan Hills bank of western Alberta.

Unlike sedimentation of the underlying Waterways Formation, which ultimately resulted in the near complete elimination of basin relief, the Cooking Lake interval reflects a rejuvenation of basin differentiation and a gradual areal restriction of shallow-water carbonate deposition and shelf development. The Cooking Lake interval marks the onset of gradual deepening of the basin.

Majeau Lake/Lower Leduc

Majeau Lake deposition continued the trend of gradual deepening of the basin. The interval is characterized by the development of numerous isolated reef complexes, attached shelves with reefal margins, and a significant cycle-terminating influx of basin-filling shale.

The Majeau Lake basinal shales have been considered the time-equivalent of the Cooking Lake Formation. The type section was established in the Texaco McColl Majeau Lake No. 1 12-1-57-3W5 (Fig. 12.7), 40 km west of the Leduc barrier reef (Glass, 1990). Basinal sediments consist largely of greenish gray and dark brown shales. Unfortunately, attempts to correlate the Majeau type well directly with the Cooking Lake type well (see inset, Fig. 12.12) do not provide a clear representation of the stratigraphic relations. The interpretation adopted here and discussed below is shown in Figure 12.10, and in the inset cross section in Figure 12.16.

The Majeau Lake Formation can be divided into two units (Fig. 12.13). The lower unit is capped by a deep-water limestone that is stratigraphically equivalent to the upper Cooking Lake cycle (Fig. 12.14). This limestone can be observed in wells immediately west of, and along, the full length of the Homeglen-Rimbey-Meadowbrook barrier reef (Fig. 12.13). The upper Majeau Lake unit consists of shales of similar composition and origin to those of the lower unit. Where the limestone disappears, the two shale packages are indistinguishable from one another. This normally occurs where the total Majeau Lake shale is less than 30 m thick, such as in the type well (Fig. 12.13).

The Majeau Lake shales thicken to the north and thin to the south of the type well (see Fig. 12.13). A maximum thickness of 200 m for the combined Majeau Lake units occurs west of the Homeglen-Rimbey-Meadowbrook barrier reef-trend beneath the western extension of the Grosmont shelf-complex (see Fig. 12.16).

The upper Majeau Lake shale unit overlies the northern extension of the Lower Leduc Calling Lake reef complexes, where the reef complexes have an apparent maximum buildup of only 75 m. The Majeau Lake shale effectively fills the basinal areas to the same level as the Lower Leduc reefs in this region. Subsequent carbonate shelves (known as the Grosmont stages) and reefs in this region were able to develop and prograde beyond the limits of the earlier established barrier reef trend.

In the area of thick basin-fill, upper Majeau Lake shales change facies eastward into limestones and dolomites. This upper Majeau Lake carbonate attains thicknesses up to 60 m and forms the lowest unit of the Grosmont shelf complex (see Figs. 12.13, 12.14, 12.25), and is equivalent to the Lower Grosmont stage of Cutler (1983).

Majeau Lake shales and the upper carbonate unit persist northward, beyond the axis of the Athabasca Arch in north-central Alberta, into the Northwest Territories, where they are eroded at the subcrop edge. The only significant deviation from this pattern of deposition occurs along the northern edge of the Athabasca Arch. The upper Majeau Lake carbonate is not a single continuous unit, but rather formed a northern and southern carbonate bank separated by a large, shale-filled embayment (Figs. 12.16, 12.25a, b).

The Majeau Lake shale thins westward to a zero edge. North of the Peace River Arch, there is an area of minimal deposition, undifferentiated from the Muskwa, apparently due to a lack of terrigenous influx. South of the Peace River Arch, however, the depositional limit appears to closely follow the higher relief submarine features created by the underlying Swan Hills Bank (see Fig. 12.11) and thus was more topographically controlled. Southward from the Grosmont shelf area, both the upper and lower Majeau Lake shale units thin. However, it is the upper shale that thins more markedly. In the southernmost extent of the Majeau Lake Basin (Strachan area) only the lower Majeau Lake unit, with its distinct limestone cap, is present (Fig. 12.13).

In portions of the basin, the upper Majeau Lake shale unit engulfs and rests upon the upper Cooking Lake cycle. Over the broad, eastern Cooking Lake shelf, the upper Majeau Lake shale is recognized as a very thin unit, typically less than 7 m thick. It is also found at the base of the Duvernay type section (see inset, Fig. 12.16). The Duvernay and Majeau Lake shales are described in more detail in the next section.

The Majeau Lake interval contains a distinct Lower Leduc reef stage (Figs. 12.7, 12.10). Outlined in map view on Figures 12.16and 12.25a is the distribution of reef and shelf complexes that formed during upper Majeau Lake deposition. For mapping simplicity, the upper and lower Majeau Lake shale units are combined to create the isopach shown in Figure 12.16.

The source of the Majeau Lake clastics has not been determined. However, it does appear that no significant contribution of shale was derived from the west, or Peace River Landmass. The majority of the shale appears to have been introduced into the basin in the vicinity of the Athabasca Arch and deposited along the northeastern edge of the basin adjacent to the Canadian Shield. Stoakes (1980) and Embry and Klovan (1976) suggested that the Upper Devonian shales were derived from the Arctic where there was active mountain building as the result of the Ellesmerian Orogeny. Although the Arctic orogenic highlands may have been the ultimate source, it is reasonable to postulate that a river (or rivers) introduced shale into the basin from the east. Stoakes (1980) suggested that circulation patterns may have been the major factor controlling the dominant east-to-west style of basin-filling during the Late Devonian.

The distribution of reefs and basin-filling sediments of the Majeau Lake-Lower Leduc interval reflects continued gradual deepening of the basin. Although the upper Majeau Lake shale represents a significant and dramatic increase in the supply of basin-filling material, it still is more areally restricted than underlying lower Majeau Lake and Beaverhill Lake basin-fill sediments. This suggests that, for most of the basin, subsidence was greater than sedimentation despite the significance of localized thick upper Majeau Lake shale deposition. Furthermore, isolated Lower Leduc reefs are more areally restricted than the underlying Cooking Lake shelf carbonates. The Majeau Lake interval thus illustrates the continued deepening of the Western Canada Sedimentary Basin which commenced with the upper cycle of the Cooking Lake interval. Only in the area of the Athabasca Arch, where there was relatively reduced subsidence, did shallow-water carbonate production shift basinward.

Duvernay/Middle Leduc

The Duvernay interval is a unique depositional unit within the Woodbend Group. The conditions resulting in its deposition signalled a profound change in the basin. Deposition of the Duvernay sequence is characterized by extensive basinal deposits, synchronous with a significant stage of Leduc reef growth (Middle Leduc) (Figs. 12.10, 12.17), similar to the underlying Majeau Lake interval. In contrast to the greenish-gray and brown shales of the Majeau Lake, the Duvernay Formation consists of dark brown bituminous shale and limestone. The Duvernay represents a period of great accumulation and preservation of organic carbon. This reflects a major change in the stratification and oxygenation of basinal waters during the maximum transgressive stage of the Woodbend (Fig. 12.9). These basinal deposits are considered to be the source rocks for most of the conventional hydrocarbon accumulations found in the Upper Devonian reservoirs of Alberta (Allan and Creaney, 1991).

For the first time since deposition of the Waterways Formation (Beaverhill Lake Group), basinal sediments were deposited extensively throughout the Western Canada Sedimentary Basin (Muskwa, Duvernay and at least the lower part of the Perdrix formations; Figs. 12.18, 12.19, 12.24, 12.27). The basinward depositional limit of these shales and limestones extends far beyond those of both the underlying Majeau Lake and the overlying Lower Ireton basin-fill sediments.

Adjacent to the Leduc reef complexes, the Duvernay is thick and intermixed with reef-derived detritus. This occurs especially within protected Leduc embayments and beneath the Grosmont shelf complex. Basinward from the reefs, Duvernay basin-fill generally thins markedly. Figure 12.17 illustrates Duvernay paleogeography, isopach and distribution of equivalent reefs and associated carbonate shelves.

Duvernay sediments overlie the upper Majeau Lake shale except in the extreme southern end of the East and West Shale basins. In these areas, the Duvernay directly overlies the Cooking Lake interval due to nondeposition of the upper Majeau Lake shale. In addition, west of the depositional limit of the Majeau Lake, in western and northern Alberta (Fig. 12.11), the Duvernay directly overlies either Waterways shale, strata of the Swan Hills bank, or older Slave Point carbonates.

The Duvernay-Leduc interval is well represented in the area of the Duvernay Formation type section. The Duvernay type section (Fig. 12.7 and inset Fig. 12.17) was established in the Anglo Canadian Beaverhill Lake #2 11-11-50-17W4 well (Geological Staff, Imperial Oil Ltd., 1950). Here the Duvernay consists of interbeds of black and brown petroliferous shales, gray and brown dense to fossiliferous limestones with an impoverished fauna of articulate and inarticulate brachiopods and conodonts. It is characterized on borehole logs by a high gamma-ray count and high resistivity.

The Duvernay was described in detail by Andrichuk (1961). He divided it into three units in the Duhamel area. The lowest unit, which varies in thickness from 0.3 to 20 m (average 6 m), consists of brown to greenish-gray argillaceous limestone. Variations in thickness reciprocate topography on the underlying Cooking Lake. This lowest unit appears to correlate in part with the upper Majeau Lake interval. Given that it is present under a portion of some Leduc reefs, such as the Duhamel reef complex, it clearly predates some of the reef growth (Figs. 12.8, 12.23).

The middle Duvernay unit contains reef-derived skeletal debris (brachiopods, crinoids, stromatoporoids, amphiporids and corals). The relation of this unit to reef building is unclear, but it may represent the equivalent of a cycle of reef growth within the Majeau Lake or Lower Leduc interval.

The upper Duvernay unit consists of brown to black shales and dark brown, deep-water limestones that range from 16 to 60 m in thickness in the Duhamel area. This Duvernay unit equates to a distinct Middle Leduc stage of reef and shelf growth and is approximately equivalent to the Duvernay Interval as defined and used in this chapter (Figs. 12.7, 12.18, 12.23).

In the East Shale Basin (Fig 12.17), the Duvernay thickens to over 90 m adjacent to the Killam Barrier reef-edge and within the Ghost Pine, Westerdale and Harmattan embayments (Fig. 12.11). Northward, the Duvernay thickens and grades laterally into shallow-water carbonate shelf equivalents of the Grosmont shelf complex (Fig. 12.18). The thickening coincides with a corresponding thinning of the underlying upper Majeau Lake shale (see Fig. 12.13). The characteristic radioactive shales of the Duvernay disappear and are replaced by dark gray shales similar in origin to the upper Majeau shale (Fig. 12.18). Where the combined upper Majeau Lake and lower Duvernay shales filled this portion of the basin, they were superseded by shallow-water biostromal limestone and dolomite up to 60 m thick. The Duvernay carbonates equate with Grosmont stages 1 and 2 of Cutler (1983), within the Grosmont shelf-complex (Figs. 12.10, 12.18). The Duvernay carbonate cycles prograded farther basinward than the underlying Lower Grosmont carbonate shelf edge (upper Majeau-equivalent) (Figs. 12.13, 12.14, 12.18).

In the West Shale Basin the Duvernay thins away from the reef complexes. It is less than 30 m thick in the basin centre and is dominated by highly radioactive shales. The Duvernay thickens where reef detritus is added adjacent to the Leduc reef complexes in the Wild River Basin and in localized areas abutting the Fairholme and Southesk-Cairn reef complexes in the Front Ranges where it attains a thickness of 125 m.

In central Alberta, the upper Duvernay carbonates include a distinct shale unit with a similar character to that of the Lower Ireton (see Fig. 12.14 and inset in Fig. 12.20). This shale has not been recognized by industry geologists as part of the Duvernay. For the purposes of this Atlas only the recognized radioactive shales and interbedded limestones are included in the Duvernay isopach. The lower Duvernay component maintains a fairly uniform thickness over hundreds of square kilometres in the starved basin. The Duvernay thins gradually to the west and onlaps the Peace River Arch.

North of the Peace River-Athabasca Arch and parallel to the Grosmont carbonate shelf edge, the same sequence of strata occurs, with thick Duvernay carbonates and shales thinning basinward in a fashion similar to the underlying Majeau Lake shales. In northern Alberta, the Duvernay appears to be largely equivalent to the Muskwa Formation. The Muskwa Formation consists of the same characteristic highly radioactive shales. West of the Majeau Lake depositional edge, these shales overlie Waterways shales, Slave Point carbonates or Otter Park shales. In northeastern British Columbia, the Muskwa and Otter Park shales are both highly radioactive and are not readily subdivided. Because the Duvernay approaches its depositional zero edge in this area, the Otter Park shales are not included in the Duvernay isopach.

The end of the Duvernay-Middle Leduc deposition brought to a close a distinct and significant depositional sequence. During this time of maximum transgression, the sea became deep enough to allow for the development of a stratified water column over the entire basin.

Lower Ireton/Upper Leduc

In the Western Canada Sedimentary Basin, particularly northwest of the Meadow Lake Escarpment, the end of apparent deepening and a major shift toward filling and shallowing of the basin began during deposition of the Lower Ireton.

The Ireton Formation, in general, consists of cyclic successions of basin-filling shale clinoforms. However, it is in part equivalent to a significant stage of reef and shelf development (Upper Leduc stage). The Ireton Formation can be divided into an upper and lower sequence (Figs. 12.19, 12.20). It is only the basal portion of the Lower Ireton that embraces the Upper Leduc reef and shelf stage (Figs. 12.18, 12.23). In the east, the upper portion of the Lower Ireton and the Upper Ireton postdates Leduc reef development. In western Alberta, thin cycles of reef growth may have continued to form over the Leduc reefs during late Ireton deposition.

The bounding surface between the Upper and Lower Ireton sequences has been informally referred to as the "Z" marker (Stoakes and Wendte, 1987). In the West Pembina area, where little to no Upper Ireton shale was deposited, the "Z" marker is a sharp, disconformable surface that lies at the top of the lowest cycle of the Lobstick Member (see well 7-4-49-12W5 in Figs. 12.7and 12.35). The complete filling of the entire East Shale Basin by Lower Ireton sediments precluded deposition or accumulation of Upper Ireton shale. As a result, Winterburn carbonates (as originally defined in the B.A. Pyrcz No. 1 12-25-50-26W4 well) rest directly on Lower Ireton in this portion of the basin. Unfortunately the lack of Upper Ireton in this area and the difficulty of correlating the lower Lobstick cycle of the West Pembina area with the Winterburn type well, has misled many workers into the assumption that the "Z" marker represents the boundary between the Woodbend and Winterburn groups. Chevron Exploration Staff (1979) chose to include this lower Lobstick cycle with the Winterburn. However, regional correlations created for this Atlas suggest that the lower cycle more likely belongs to the Woodbend. The base of the middle Lobstick carbonate cycle in the 7-4-49-12W5 well appears to represent the boundary between the Woodbend and the Winterburn groups (see discussion and illustrations in the Winterburn section; e.g., Figs. 12.33, 12.35). The thin shale between the lower and middle Lobstick carbonate cycles is included in the Upper Ireton (Fig. 12.35). For ease of mapping, where no significant thickness of Upper Ireton is present, the boundary between the Lower Ireton and Winterburn is set at the base of the Lobstick Member (Fig. 12.35).

To the north and west of the West Pembina area, the Lower Ireton fondothems are represented by a condensed section of argillaceous deep-water carbonates that are confidently correlated throughout most of the basin. Northwest of the Peace River-Athabasca Arch, the Lower Ireton thins and either becomes an indistinguishable thin unit of the Muskwa Formation, or was not deposited.

The Lower Ireton is divided into two, upper and lower, lithologically distinct units (Figs. 12.7, 12.18, 12.23). The lower unit is dominated by argillaceous carbonate and is separated from the overlying, predominantly shale, unit by a sharp disconformable to conformable contact. This lower carbonate-rich unit has been named the Westerdale Member (Glass, 1990). McCrossan (1957) suggested that this basin-filling carbonate unit is equivalent to deposits of the Upper Leduc reef stage (see Figs. 12.7, 12.10, 12.18). This unit varies in thickness throughout the basin, but is normally less than 15 m. In local embayments between major Leduc reef complexes where reef-derived detritus was protected from strong currents, the Westerdale carbonate thickens to 150 m .

The upper shale unit of the Lower Ireton has been studied in detail by Oliver and Cowper (1963) and Stoakes (1980). They have shown that the Lower Ireton consists of a number of large-scale cyclic stratal packages that exhibit westerward-prograding clinoform geometries. Stoakes (1980) has shown that these internal cycles are easily defined by distinct borehole log kicks that represent carbonate hardgrounds formed during temporary suspensions of clastic influx. The most prominent carbonate to form on these cycles of basin-filling shale was the Camrose Member (Stoakes, 1980). Unlike the Westerdale Member, the upper unit of the Lower Ireton postdates all isolated Leduc reef complexes of eastern Alberta. Reef growth may have continued over parts of the reef complexes of western Alberta (e.g., Windfall, Bigstone).

There has been much conjecture as to the origin of the shale in this interval. Several pertinent observations can be brought forward. Within both the Lower and Upper Ireton sequences a number of shifting lobes or mudbanks were deposited. Successive lobes of infill shifted to areas of least fill, where accommodation space was available. However, the major position and direction of basin-filling is notably different for the Lower versus Upper Ireton intervals. The Lower Ireton shale prograded from east to west and is most significant south of the Peace River-Athabasca Arch (Fig. 12.20), completely filling the East Shale Basin. Although this interval is also significant north of the arch (Fig. 12.27), it did not fill the basin, leaving considerable accommodation space for both Upper Ireton and Nisku basin-filling deposits. The younger Upper Ireton progrades both southward and westward, and is largely confined to the area north of the Peace River Arch (Fig. 12.29). In the intervening area, centred on the more slowly subsiding Peace River-Athabasca Arch, the thick Majeau Lake shale and Grosmont carbonate shelf filled to such a shallow level that very little accommodation space was available for Lower Ireton shale deposition. This resulted in the separation of the basin into northern and southern sub-basins, and restricted the Upper Ireton shale from being transported south of the arch. Following Lower Ireton deposition, the only connection between the northern and southern basins was a narrow passage between the Grosmont Complex and Peace River Arch fringing reef-complex. This narrow passage was likely the site of strong currents that limited the deposition of Lower Ireton shale along the axis of this arch.

The point where most Lower Ireton shale was introduced into the basin was likely between the southern edge of the Grosmont shelf and the northern end of the Killam Barrier. The eastern, proximal portion of what must have been an immense delta complex has all been removed by Mesozoic erosion. Further support to shale introduction south of the Grosmont Complex during Lower Ireton deposition is shown by Figures 12.15, 12.20, 12.22c, 12.41 left pane, 12.41 right pane. In these figures, the Upper Leduc succession is characterized by an extensive shale facies that is restricted to the Saskatchewan part of the southern shelf. The presence of this shale suggests that resumption of significant influx of fine clastic sediment began during the last vestiges of Leduc reef growth. The distribution and thickness of this shale suggests that it also formed from detritus that entered the basin from the east at a point south of the Grosmont shelf. However, the presence of Lower Ireton shale in northern Alberta also suggests an additional point of introduction of shale into the basin north of the Peace River-Athabasca Arch.

The introduction of this enormous amount of Lower Ireton shale (up to 240 m, Fig. 12.20) represents a dramatic change in deposition in the Woodbend Group. For ease of mapping, the Leduc equivalent lower unit and post-Leduc upper shale unit are combined to produce the isopach of Lower Ireton-Upper Leduc interval (Fig. 12.20).

Leduc

The Leduc Formation of the Woodbend Group is well known as a host to prolific reserves of oil and gas within Alberta. Distribution and thicknesses of the Leduc reefs and Grosmont shelf carbonates are shown in Figure 12.21. The axis of greatest subsidence within the basin, highlighted by the 260 to 290 m thickness interval, lies on a northwest-southeast trend between the Peace River Arch and the Killam Barrier. The Leduc thins onto the Peace River Arch in the northwest, the West Alberta Ridge in the southwest, and southeast onto the southern shelf. These areas reflect reduced subsidence and correspond to topographically high areas during Elk Point deposition. The combined Leduc and Grosmont shelf carbonates thin to 210 m along the northeast extension of the Peace River-Athabasca Arch.

Stages of reef growth have been recognized within individual Leduc reef complexes for some time. McCrossan (1957), Klovan (1964), McGillivray and Mountjoy (1975), Stoakes (1980), Andrews (1987), Workum and Hedinger (1987), Dix (1990), McNamara and Wardlaw (1991), Workum and Hedinger (1992), and others, have illustrated the internal stratigraphy of these complexes. Despite this considerable attention and detailed study by industry workers and researchers, little attempt has been made to correlate the stages of Leduc reef growth throughout the basin and relate these stages to the sequences of basin-filling. Workum and Hedinger (1987, 1992) have made many observations and developed depositional models from studies of equivalent strata in the Front Ranges of Alberta, while Stoakes (1987) has provided a conceptual model for the subsurface Woodbend. These studies form an important foundation to the regional perspective provided by this chapter.

No one area of the basin offers all the necessary basin-fill to reef stage relationships that are required to infer with confidence the relative timing of various reef stages. Only through reviewing all the relations throughout the entire basin does the evolution of the Leduc reefs become apparent. However, many of the key inter-relationships required to unravel Leduc history do appear to be present in the region where the Grosmont shelf, the equivalent basin-fill and the Homeglen-Rimbey-Meadowbrook reef chain merge together (Figs. 12.23, 12.25c). Although the reef complexes consist of many cycles of reef growth, only some of these cycle boundaries are significant in the sense that they correlate to the bounding surfaces of distinct and widespread basin-fill sequences (Figs. 12.8, 12.23, 12.26).

Termination of Woodbend reef growth is not well understood. The most serious limitations to continued reef growth appear to have been lack of sufficient accommodation space, and external (as yet unidentified) environmental stresses coincident with or slightly pre-dating the influx of clastics to the basin. Between periods of major clastic influx, reefs and carbonate shelves resumed growth wherever accommodation space and substrate conditions were suitable.

As cited and illustrated in foregoing discussion, the Leduc Formation consists of three major and distinct reef stages, corresponding to Majeau Lake, Duvernay, and Lower Ireton basin-filling depositional intervals. These stages are referred to as the Lower, Middle and Upper Leduc. The distribution of reefs developed during each stage of reef growth, placed in paleogeographic context, is shown in Figures 12.22a, b, c. The evolution and distribution of Woodbend basin-fill deposits are also shown for reference, in Figure 12.22d.

The style and complexity of reef and shelf growth varies for different areas of the basin. For discussion purposes, four paleogeographic areas are differentiated, each with its own stratigraphic framework and facies relations. The four areas are referred to as the Eastern Basin, Western Basin, Grosmont Complex and Peace River Arch fringing reef complex.

Eastern Basin

The Eastern Basin is characterized by isolated reef complexes (Rimbey-Leduc-Meadowbrook chain, Redwater, Bashaw, Stettler, Willingdon, etc.), and a broad carbonate shelf, all of which developed on the Cooking Lake Platform (Fig. 12.11). The regional carbonate shelf is delimited by an abrupt linear reef-edge (lower portion of Killam Barrier), and several reef-fringed intra-shelf basins (e.g., Youngstown; see Fig. 12.22a). The shelf is of extremely large areal extent, covering the southern portions of Alberta, Saskatchewan, Manitoba, and beyond into Montana, Idaho, North Dakota, South Dakota and Wyoming. Within this broad shelf, the Lower Leduc stage consists of two major cycles separated by a thin, discontinuous shale break. In southern Alberta it is called the Cairn Formation, although in a strict sense it does not equate with the Cairn Formation surface type section located in the Front Ranges of Alberta. In Saskatchewan, it correlates with the Middle Wymark Member (Kent, 1968) or Unit 2 (Dunn, 1975) of the Duperow Formation (see Fig. 12.15).

The thickness of the Lower Leduc ranges from approximately 110 m at the southern Alberta shelf edge to less than 55 m in southeastern Saskatchewan and Manitoba. The main Killam Barrier and intra-shelf basin edges appear reefal in nature and are extensively dolomitized. Cyclic, shallow-water carbonates formed within the interior of this shelf complex. Dolomitization of these interior shelf carbonate cycles is variable but increases laterally toward the east, and toward the top of the Lower Leduc stage. Widespread thin anhydrite layers were also deposited, particularly toward the eastern portion of the shelf. Restriction of shelfal waters toward the end of Lower Leduc deposition resulted in evaporite formation within the intra-shelf basins. For example, at least 52 m of halite were deposited in the Youngstown Basin of southeastern Alberta. Two other occurrences of salt are also found in Saskatchewan where they are known as the Etonia (e.g., 4-32-26-24 W3), and Elrose evaporites (e.g., 14-12-26-16 W3) (Meijer Drees, 1986). The halite component of these evaporites is not mapped here.

The remainder of the Eastern Basin consists of variably sized, isolated carbonate buildups (Fig. 12.11). The dimensions of these buildups include those that are hundreds of square kilometres in area (Stettler Complex), tens of square kilometres (Redwater), to buildups that are less than one square kilometre (e.g., Rumsey and Excelsior). Many of the larger reef complexes may have commenced as one or more small, isolated Lower Leduc reefs that coalesced during reef growth. The Lower Leduc reef complexes have gently dipping margins and reach a maximum thickness of approximately 125 m. Figures 12.8, 12.23 and 12.26 summarize the correlations between the reef and inter-reef deposits.

Within the broad Eastern Basin area, Middle Leduc (Duvernay equivalent) deposition (Fig. 12.22b) followed the same basic pattern as that observed for the Lower Leduc interval. Initiated and controlled by the presence of Lower Leduc reefs, the Middle Leduc grew on these lower buildups. Overstepping of the reef or shelf margin beyond that of the underlying reef or shelf margin occurred only locally (Fig. 12.8), such as at Redwater (Klovan, 1964), or across the Youngstown Basin, where sufficient inter-reef debris or basin-filling evaporite accumulated (Figs. 12.15, 12.22b).

Upper Leduc deposition (Fig. 12.22c) continued to follow the pattern of deposition set by the earlier stages, with one important difference. Within the broad shelf area extending throughout Saskatchewan, an extensive occurrence of shale is found (Fig. 12.22c). Unlike earlier stages in Saskatchewan, this stage of shelf growth notably lacks any significant occurrence of evaporites. The Upper Leduc stage correlates with units 1 to 4 of the Seward Member (Duperow Formation) in Saskatchewan (Dunn, 1975). The introduction of shale onto the shelf marks an important change, and represents a shift towards cyclic and reciprocal carbonate/fine clastic deposition. A significant thickness of fine clastics introduced into the southern shelf areas was last observed within the Cooking Lake interval.

Where Majeau Lake basin-fill sediments were absent, the Middle Leduc reef built up considerable relief with respect to the surrounding basin floor. Upper Leduc shelf and reef edges are generally backstepped relative to Middle Leduc reef stages. However, overstepping relative to Lower Leduc edges is common, particularly along the Killam Barrier where sufficient Duvernay basinal sediments are present.

Western Basin

The Western Basin contains reef complexes such as Windfall, Simonette, Miette and Ancient Wall, that show similar reef and shelf development as those of the Eastern Basin (Fig. 12.11). The major difference appears to be a lack of distinguishable Cooking Lake carbonate. Continuous carbonate from Beaverhill Lake through to the top of the Leduc Formation makes recognition of a Cooking Lake equivalent very difficult. Without paleontological data, it is unclear whether Lower Leduc reef-buildups grew upon a thin, presently unrecognized Cooking Lake Platform, or directly upon Beaverhill Lake carbonates. A "Lower Leduc" or "Cairn" Platform extends farther basinward than the main Upper Leduc reef edge (see Figs. 12.11, 12.21). This platform could be partly or entirely equivalent to the Cooking Lake Platform of eastern Alberta.

In these areas, the Lower Leduc comprises backstepping reef-rimmed complexes and isolated reefs, which reach a maximum thickness of approximately 75 m. There may be at least two cycles present; however, due to a lack of core control, the subdivision is subjective and dependent upon correlation of thin gamma-ray log markers.

Figures 12.13, 12.14, and 12.24 illustrate the relation between the reefs of the Western Basin and their inter-reef deposits. The larger reef complexes of the "Deep Basin" (e.g., Windfall, Bigstone, Simonette and Gold Creek) and those exposed in the Front Ranges (Mountjoy, 1980; Andrews, 1987) exhibit a pronounced northeast-southwest elongation that may reflect basement grain and subtle structural influence. This apparent control is particularly notable in the Sturgeon Lake complex (Martin, 1967).

Correlation of the Lower Leduc in the subsurface to the exposures in the Front Ranges of the Rocky Mountains of Alberta is difficult. Andrews (1987) has shown the similarities of the outcrop reef exposures to deeply buried reefs (i.e., Robb) immediately east of the Bighorn Thrust.

Four major Leduc reef complexes outcrop within the Front Ranges of the Rocky Mountains of Alberta (Fig. 12.11). These are (south to north) the Fairholme, Southesk-Cairn, Miette and Ancient Wall complexes (McLaren, 1953, 1956; Mountjoy, 1965; MacKenzie, 1969, Mountjoy and MacKenzie, 1973; Coppold, 1976; Geldsetzer, 1988; Workum and Hedinger, 1988; Mallamo and Geldsetzer, 1991; Workum and Hedinger, 1992). The complexes are rooted on the more areally extensive transgressive shelf carbonates of the Flume Formation (upper Beaverhill Lake equivalent).

In general, only two stages of Leduc reef growth have been recognized in outcrop. These are represented by the upper Cairn Formation, apparently equivalent to a combined Lower and Middle Leduc, and the Peechee Member of the Southesk Formation, roughly equivalent to the Upper Leduc (Fig. 12.24). In a few areas (i.e., bordering the Cline Channel) all three stages can be recognized. demonstrates the relation between the outcropping reefs and their inter-reef deposits.

Peace River Arch

The Peace River Arch fringing reef complex shows a pattern of Leduc reef evolution similar to the Eastern and Western Basin areas. The fringing reef is characterized by a basinward-thickening Leduc shelf edge around a large emergent landmass with an exposed Precambrian core, the Peace River Arch (Figs. 12.21, 12.42). The Lower Leduc stage in the Peace River Arch area approximately corresponds to Stage III of Dix (1990). Again it is difficult to pick a Cooking Lake cycle in this area. The Lower Leduc sediments were deposited (?)disconformably upon siliciclastics and carbonates of the Beaverhill Lake Group or Cooking Lake equivalent, or conformably upon, and interbedded with, arkosic sands of the Granite Wash.

As illustrated in Figure 12.21, the Lower Leduc forms an elliptical halo around the Peace River Arch, varying from 15 to 25 km wide along the northern and southern portions and up to 60 km in the east along the structural nose of the arch (Dix, 1990). The Lower Leduc reaches a maximum thickness of approximately 60 m. The fringing reef margin is generally a dolomite and lacks a clastic component. From the fringing shelf edge towards the emergent landmass, however, the Leduc dolomite has increasingly more interbeds of argillaceous to arenaceous dolomite. The Lower Leduc stage exhibits patchy initial carbonate growth with the patches coalescing in those areas that had limited clastic influx from the arch. Thickness variation for the Lower Leduc and younger stages of reef growth was largely controlled by syndepositional basement fault block movements (Dix, 1990).

Middle and Upper Leduc reef margins are largely stacked upon the Lower Leduc fringing reef complex around the Peace River Landmass. Basinward progradation was inhibited in this area, due to a lack of basin-fill sediment over which the reef could forestep.

Correlation of the boundary between the Middle and Upper Leduc stages from other areas of the basin to the Peace River Arch reef complex is difficult. The correlations shown in Figure 12.26 are based on a widespread and distinct log kick that is thought to correspond to this boundary.

Grosmont Complex

The deposition and evolution of Leduc-equivalent reef and shelf development in the Grosmont shelf-complex is distinctly different from the other areas. Although Lower Leduc reef development was controlled by subtle relief on the Cooking Lake Platform, similar to reef complexes of the Eastern Basin area, the extent of subsequent Grosmont shelf deposits equivalent to the Middle and Upper Leduc was controlled entirely by the level of basin-filling shales that allowed shelf carbonates to prograde basinward (see Fig. 12.25a, b, c).

The Lower Leduc stage lies within the Upper Majeau Lake depositional interval, and in part correlates with the "Lower Grosmont" stage of Cutler (1983). It can be divided into two stages of carbonate deposition, each followed by a reciprocal period of basin filling. The initial cycle consists of isolated reefs growing in areas where the upper Cooking Lake cycle is present as a carbonate. This occurs along the Calling Lake extension of the Rimbey-Leduc-Meadowbrook reef-trend. Grosmont carbonates equivalent to this cycle are not observed but are presumed to have been removed by Mesozoic erosion. Following this stage of reef growth, shale of the Upper Majeau Lake cycle continued to fill part of the basin. With the cessation of clastic influx, reef and shelf growth was resumed. The maximum thickness of carbonate (Lower Leduc plus Lower Grosmont) is 125 m.

In the absence of an initial cycle of the Lower Leduc reef stage, the upper cycle or "Lower Grosmont" stage reaches a maximum thickness of only 45 m. The western margin of the Lower Grosmont shelf is relatively abrupt and steep while the southern margin thins more gradually toward the basin.

Strata equivalent to the Middle and Upper Leduc within the Grosmont follow a similar pattern of deposition. The Middle Leduc equivalent stage (Grosmont l and 2 of Cutler, 1983) exhibits a ramp-like profile and dips gently westward into the basin. To the east, a thick underlying wedge of Majeau Lake shale limited the accommodation space for carbonate accumulation and resulted in marked thinning of the Grosmont in this direction.

The Upper Leduc ("lower cycle of Grosmont 3" of Cutler, 1983) differs from the underlying Leduc strata by the occurrence of significant salt and evaporite (Hondo Member). Unlike the Middle Leduc stage, the western Upper Leduc shelf-edge is abrupt and steep.

Similar evolutionary patterns of reef or shelf stage development are present in various areas of the basin. Distinctive kicks in the borehole gamma-ray logs indicate that the reef and shelf stage boundaries appear to correlate from one reef complex to the next. Figure 12.26 shows log traces for selected reef complexes from different areas of the basin and the interpreted correlations. Additional detailed study of cores and borehole logs will be required to confirm this correlation.

Upper Ireton

The most dramatic and voluminous influx of shale observed during the Paleozoic occurred during Upper Ireton deposition. The Upper Ireton interval represents the last depositional sequence of the Woodbend Group.

Without additional detailed correlation and biostratigraphic control, defining the top of the Woodbend Group will remain difficult. The age of the Upper Ireton interval has not been clearly established. Its assignment to either the Woodbend or Winterburn group depends entirely on which type well is chosen as the basis for correlation. As cited earlier, this study suggests that the defined base of the Lobstick Member (Chevron Exploration Staff, 1979) and the established base of the Nisku Formation are not synchronous. Despite the inclusion of the Upper Ireton cycle within the Lobstick Member (Chevron Exploration Staff, 1979), the interpretation adopted herein has the Upper Ireton within the Woodbend Group. The top of the Upper Ireton marks the end of deposition of a thick interval of fine clastics and the onset of widespread carbonate ramp and localized reef sedimentation of the Winterburn Group.

An isopach and lithofacies map of the Upper Ireton interval is presented in Figure 12.29. Across the Peace River-Athabasca Arch, a marked change is observed in the distribution of Upper Ireton sediments. The Upper Ireton shale constitutes the main basin fill north of the arch. Only in the western parts of northeastern British Columbia and in the Liard Basin did this shale fail to completely fill the basin (with dramatic effects on later Winterburn deposition).

Movement of underlying basement evidently occurred during Upper Ireton deposition. Throughout the northern region the shale is at least 300 m thick. As illustrated in Figures 12.28 and 12.30, block faulting was quite dramatic, especially along the Hay River lineament. In localized downthrown blocks or deepened areas, the shale is over-thickened by as much as 185 m. In addition to localized lows, a broad submarine high remained (Tathlina High) that caused shale deposition to thin to as little as 125 m. Thinning over this feature coincides closely with underlying Slave Point/Keg River shelf development.

For the most part, the style of basin filling was similar to that of the Lower Ireton shales, with large-scale westward-prograding cyclic clinothems (Fig. 12.27). The sedimentology of some of these cycles has been described by Hadley and Jones (1990). In addition to this westward-prograding style of infill, the Upper Ireton also exhibits basin-filling cycles that advanced to the south. This southward advance of basinal sediments was affected by the pattern of sedimentation and level of fill of earlier Woodbend deposits around the Peace River-Athabasca Arch. Southward encroachment of shale was restricted to areas immediately to the east and, possibly, to the west of the Peace River Landmass. A narrow pass between the Leduc reef complex fringing the Peace River Landmass and the Grosmont shelf-edge funnelled clastics into the West Shale Basin south of the Peace River-Athabasca Arch (Figs. 12.29, 12.42). A less well-defined access into the West Shale Basin appears to have been present to the west of the landmass, and is indicated by the presence of a thick easterly-prograding wedge of clastic sediments that locally fills basinal areas between the Simonette reef complex and the Peace River Landmass (Fig. 12.14). Additional support for this westerly access into the West Shale Basin is found in exposures in the adjacent Front Ranges. It is possible to link the thick Ireton shale with basinal shales (part of Mt. Hawk Formation) of the Jasper Basin. Until additional biostratigraphic or detailed lithostratigraphic data are available, the timing and distribution of Upper and Lower Ireton shales in this area will remain interpretive.

The southward advance of shale was limited within the West Shale Basin, resulting in only partial filling of the basin by the close of Woodbend sedimentation. This incomplete filling had a significant effect on subsequent Winterburn deposition (e.g., location of the Meekwap shelf edge discussed in the Nisku section).

The development of the Grosmont shelf complex during Upper Ireton deposition is poorly understood. The most westerly portion of the uppermost Grosmont shelf stage (Fig. 12.14) appears to represent a carbonate shelf equivalent of the basin-filling shales. The "Z" marker clearly lies at the base of this uppermost Grosmont stage (Fig. 12.12). Correlation of this Upper Ireton carbonate from the western shelf edge toward the eastern portions of the Grosmont shelf complex is difficult because of pronounced thickness variations resulting from Hondo evaporite removal. A lowstand shallow-water carbonate wedge may have formed, restricted to the western half of the Grosmont complex where insufficient underlying Lower Ireton fill had left suitable accommodation space. More detailed work is required to unravel the interrelationships between the Grosmont shelf carbonates and Upper Ireton basin-fill.

Figure 12.22d summarizes the evolution of Woodbend basin-fill deposition. As shown, the Upper Ireton was the most significant basin filling event.

By the end of Upper Ireton deposition the Western Canada Sedimentary Basin had received significant volumes of shale. Although the shale did not completely fill the basin, it did produce a shallow, broad basin. This pattern of fill influenced and controlled subsequent basinward shift of Winterburn shallow-water carbonates.

Retun to top

Winterburn Group

Figure 12.4 shows the thickness distribution of the Winterburn sequence. Generally, the succession thickens from less than 20 m in Saskatchewan to more than 380 m in northeastern British Columbia. The significance of local thicks, such as the Cynthia, Karr and Wild River basins, and thins such as over the Peace River Landmass and the Athabasca Arch, are discussed in a subsequent section. The major Winterburn shelf edges, principal lithologies, oil and gas pools and index of key cross sections and other critical features of the Winterburn Group are shown in Figures 12.31 and 12.32. The key formations and members of the Winterburn Group are set out in Figure 12.1.

Winterburn deposition was characterized by continued general shallowing and filling of the basin. This succession represents the culmination of successive Late Frasnian basin-filling events. By latest Frasnian time little or no accommodation space was available over most of the basin (except to the extreme west and northwest). Thus there had been an almost complete filling of the seafloor topography that remained following Woodbend deposition.

The major features that distinguish the Winterburn Group from underlying depositional sequences include: the extensive breadth of carbonate shelf complexes (i.e., Nisku, Meekwap and Jean Marie) which extend much farther basinward than the underlying shelf complexes of either the Woodbend or Beaverhill Lake groups; the development of smaller pinnacle reefs and reef trends (e.g., Zeta Lake Member) similar to those of the Middle Devonian Keg River and Winnipegosis formations; a general lack of more robust stromatoporoid morphotypes and a predominance of corals in open-marine settings (e.g., Zeta Lake Member); the mass extinction of numerous species at the Frasnian-Famennian boundary (top of Blue Ridge chronostratigraphic interval); and finally, the introduction and extensive distribution of larger grain-sized siliciclastics (i.e., Cynthia, Calmar, Graminia, Sassenach and Trout River siltstones and sandstones).

Another notable feature of the Winterburn Group is its significant conventional oil and gas reserves. In addition, an appreciable accumulation of heavy oil is found at the Winterburn subcrop edge in northeastern Alberta.

Despite the economic significance and remaining hydrocarbon potential (see Podruski et al., 1988), the published literature remains relatively meagre. Most recent articles deal with various aspects of the Brazeau to West Pembina pinnacle reef trend of the Nisku Formation (e.g., Machel, 1983; Anderson, 1985; Watts, 1987). Data from other producing pools have been published by Smith and Pullen (1967), Cheshire and Keith (1977), Dolph (1989), Slingsby and Aukes (1989), and Stoakes (1992a, b). Only Stoakes (1987) has provided a further enhancement of Belyea's (1964) original regional synthesis. Furthermore, biostratigraphic control from subsurface samples is extremely limited. Surface work in the Rocky Mountains and southern Northwest Territories is more complete (e.g., Klapper and Lane, 1985, 1988; Marchant and MacLean, 1992), but coverage is still sparse. Despite the lack of biostratigraphic data the Winterburn can be divided into chronostratigraphic intervals based on regionally distinctive borehole log markers reflecting abrupt changes in lithology.

To illustrate the regional paleogeography and evolution of basin filling, the Winterburn sequence is subdivided, in stratigraphic succession, into the Nisku, Blue Ridge and Upper Graminia intervals, respectively. Only the Nisku and Blue Ridge intervals are chronostratigraphic depositional sequences. The Upper Graminia (i.e., Graminia Silt) forms the base of the ensuing Palliser Sequence (Moore, 1989), and is not considered to be a chronostratigraphic interval. It is discussed here because it represents a very significant event in the Devonian evolution of the basin, and has always been treated as the uppermost unit (lithostratigraphic) of the Winterburn Group.

The bounding surfaces used to define the depositional intervals of the Winterburn are shown in Figures 12.7and 12.33. The lower bounding surface of the Winterburn sequence is significant in that it represents an abrupt regional relative sea-level rise coupled with the (probably synchronous) cessation of fine clastic input to the basin. At the beginning of middle Lobstick deposition, carbonate production and accumulation commenced over all but the deepest parts of the inherited Woodbend seafloor topography, resulting in a transgressive (rather than progradational) carbonate unit with a low gradient or ramp profile.

For the purpose of this synthesis, the upper bounding surface chosen for the Winterburn Group closely approximates the Frasnian-Famennian boundary. This event has global as well as regional importance. On a world-wide scale, the Frasnian- Famennian boundary marks one of the three most significant mass extinctions and biotic crises known from the Phanerozoic, and as such has been the subject of much study (see Goodfellow et al., 1988, and references therein). Regionally, this boundary delineates a marked change in depositional facies across the basin. Much of the Graminia Silt and correlative Trout River Formation siliciclastics probably represent detrital material that was originally deposited discontinuously in the basin during a preceding (latest Frasnian) relative sea-level lowstand. The widespread distribution of the clastics is likely the result of reworking associated with the basal transgressive phase of the overlying Wabamun depositional sequence.

As shown by Figures 12.32 and 12.33, the principal lithologies and facies of the Winterburn Group are shallow-water ramp and shelf carbonates, and deeper water ramp to basinal marlstones and shales. South and east of the Peace River-Athabasca Arch, with the exception of the Meekwap shelf trend, most subsurface occurrences of shallow-water carbonates are dolomite, whereas the more basinal carbonates and shallow-water carbonates in northern Alberta are generally limestone. Thin zones of anhydrite and anhydritic carbonate are relatively common across the southern shelf within Alberta, Saskatchewan and Manitoba. A notable occurrence of anhydrite and halite is found within the Karr Basin, just south of the Peace River Arch in west-central Alberta (Fig. 12.31). Also, the Winterburn contains laterally discontinuous thick units of medium-grained subarkosic sandstones distributed principally in a broad belt associated with the Athabasca Arch.

The stratigraphy, depositional history, paleogeography, isopach and lithofacies variations of each Winterburn interval are discussed in stratigraphic succession.

Nisku

By the end of Woodbend deposition the Western Canada Sedimentary basin was almost filled by shale and carbonates. Examination of the isopach of the Woodbend Group (Fig. 12.3) provides for visualization of the relative topography just prior to Winterburn deposition. Uncertainty in inferring relative topography from this map arises due to several factors, such as local differential basin subsidence and differential compaction. Nevertheless, many of the local Winterburn depositional trends can be related to this apparent underlying relative topography (e.g., West Pembina shelf edge- Zeta Lake reef trend). Over most of southern Alberta, Saskatchewan and Manitoba, the pre-Winterburn topography was characterized by very gentle relief. The upper surfaces of the Camrose Member and equivalents, as well as uppermost Woodbend sediments of the southern Alberta shelf complex, formed a relatively flat surface with only local depressions present where the underlying section was thinner as a result of compaction of a shalier facies, or absent through salt dissolution (e.g., Youngstown). In east-central Alberta the surface was more irregular, due to differential compaction between underlying Leduc reefs and Ireton shales (e.g., Bashaw area). In mid- and west-central Alberta a pronounced topographic low, the future Cynthia and Karr basins, was bordered to the south and east by a westward thinning of upper Woodbend clinothems, to the west by Leduc reefs and shelf complexes, and to the north by the Peace River-Athabasca Arch.

The Nisku interval began with an apparent relative rise in sea level. This marine transgression resulted in deposition of a widespread carbonate ramp (middle and upper cycles of the Lobstick Member; see Fig. 12.33) where underlying Ireton shale-fill had reached a shallow enough level for carbonate production. Recognizing this departure from regressive basin-fill deposition of the late Woodbend is difficult in parts of the basin, yet it is critical in identifying the sequence boundary separating the Woodbend from the Winterburn Group.

The Nisku interval consists of a lower transgressive and an upper progradational (regressive) stage (Figs. 12.9, 12.33), that are best defined in the West Pembina area.

The transgressive stage is characterized by widespread carbonate upbuilding in shallower portions of the basin (i.e., Nisku and Grosmont shelves). The equivalent deeper water areas show a set of backstepping carbonate ramps. In these areas, local carbonate production kept pace with deepening and resulted in the growth of downslope isolated reefs (Zeta Lake Member). The lower portions of the Nisku (Lobstick, Bigoray, Zeta Lake and lowermost Cynthia members; Chevron Exploration Staff, 1979) are included in the transgressive stage (Fig. 12.9). In the West Pembina area, each succeeding ramp demonstrates minor to moderate backstepping where carbonate production was outpaced by successive relative sea-level rises. The equivalent shallow-water components of the ramp aggraded to form the Nisku shelf edge (Fig. 12.33) where carbonate production kept pace with relative sea-level rise.

The progradational stage is represented by basinward building of carbonate and fine-grained clastic sediment wedges under conditions of diminishing accommodation space. It culminated locally in the complete filling of parts of the basin by progressively shallower deposits. Elsewhere, isolated, small, highly restricted basins formed (e.g., Karr Basin). Shallow-water carbonates formed relatively thin shelfal deposits. During cessation of clastic influx, shallow-water carbonates were formed upon the shelf area of these prograding wedges. The Jean Marie, Wolf Lake, and Meekwap shelf carbonates are the most notable products of the progradational stage of the Nisku interval.

To facilitate an understanding of Nisku stratigraphy, discussion of the interval is divided into several geographic areas, beginning with the southeastern portion of the Western Canada Sedimentary Basin and progressively shifting northwestward toward northeastern British Columbia.

Southern Alberta and Saskatchewan

In southern Alberta and Saskatchewan, the Nisku Formation and its equivalents generally range in thickness from 10 to 30 m and thicken to the northwest to about 50 m (Fig. 12.34). Lithofacies consist predominantly of dolomites and evaporites deposited on restricted, shelf interior salinas and exposed mudflat environments. For the most part, these strata represent equivalents of the transgressive stage seen in the West Pembina area. The presence of an overlying progradational stage is not obvious and it is probably very thin. In the southern and eastern parts of the basin, the Nisku Formation and its equivalents are erosionally overlain by a coalesced Calmar Formation and Upper Graminia Silt. The merging of these two silt units marks the southern and eastern depositional limit of the overlying Blue Ridge carbonate (Figs. 12.31, 12.41 left pane, 12.41 right pane). Regional stratigraphic relations in the area are illustrated in Figures 12.15, 12.41 left pane, 12.41 right pane and 12.44. In Saskatchewan the Nisku correlates to the upper two thirds of the Birdbear Formation (Kent, 1968). Aspects of the Nisku Formation and its equivalents in Saskatchewan and southeastern Alberta have been documented by Halabura (1983). Slingsby and Aukes (1989) have published a reservoir study of the Nisku in the Enchant area of southern Alberta.

East-central Alberta

In east-central Alberta, the Nisku Formation is also represented by an extensive carbonate shelf (Fig. 12.34). Deposition occurred on an irregular surface that resulted from antecedent Leduc topography, differential compaction, local nondeposition of underlying Ireton shale, and areally restricted deposition of Camrose shallow-water shelf carbonates. Locally the Nisku is difficult to separate from the Woodbend where it was deposited directly on Camrose Member or Leduc carbonate. The Nisku interval thickness increases from about 20 m in the east to 60 m near the underlying Leduc Rimbey-Meadowbrook reef trend (Fig. 12.34). Moderate thickening of the interval occurs along some margins of the underlying Leduc reefs. Examples occur along the east side of the Bashaw and Stettler reef complex, and within the Westerdale and Harmattan embayments between the Bashaw and Cheddarville/Westerose complexes (Fig. 12.11). In addition, the underlying Leduc reefs served as sites of local shelf differentiation during Nisku deposition. As a result, the thickness and nature of depositional facies vary quite markedly over relatively short distances and may include deep-water ramps, open-marine shoals, pinnacle reefs (e.g., Wood River), restricted subtidal to peritidal platform interior carbonates, and subtidal to supratidal anhydrites (Gilhooly, 1987). A generally bipartite division of the Nisku Formation can be recognized over a very broad area from northeast of the Rimbey-Meadowbrook Leduc reef chain to Saskatchewan (see Andrichuk and Wonfor, 1954). The lower part of the Nisku is distinguished by open-marine facies, whereas the upper Nisku is of more restricted aspect and locally has an appreciable evaporite content. Most of the Nisku in this area appears to have accumulated during the transgressive stage. However, the thin upper evaporitic cycles may be equivalent to the progradational stage.

West Pembina - West-central Alberta

Within the Nisku of central Alberta there is a major facies change coincident with a marked thickening of the formation into the Cynthia Basin (Figs. 12.34, 12.35). This thickening is approximated by the 80 m isopach line in Figure 12.34. From shelf to basin, the relatively pure carbonates of the Nisku are replaced by interbedded shale and limestone of ramp aspect (Watts, 1987; Anderson, 1985). These westward-dipping ramps of the Cynthia Basin reflect the gentle slope of underlying Lower Ireton clinoforms at their northwesterly depositional limit. In the deepest parts of the Cynthia Basin, condensed deposits of calcareous shale and marlstone accumulated episodically. The stratigraphic relations between the shelf, ramp and basin at West Pembina are shown in Figures 12.35 and 12.43.

Basinward of the facies change, four successive ramps developed, represented by the Lobstick, Bigoray, Cynthia and Wolf Lake members (Chevron Exploration Staff, 1979). The Lobstick, Bigoray and lower Cynthia were deposited during the Nisku transgressive stage. In proximal portions of the lower Cynthia ramp, the sediments are predominantly carbonate and have been termed the Dismal Creek Member (Machel, 1983). Relative sea-level rise coupled with accelerated subsidence in the vicinity of the Nisku shelf margin, resulted in the successive backstepping of the ramps toward the shelf margin (Watts, 1987). Periodically, increased accommodation, possibly coupled with other environmental stresses (e.g., turbidity), resulted in the regional cessation of carbonate production on the deeper or medial portions of the ramp (base of Bigoray Member, lower Cynthia). At favourable sites (hardgrounds, firmgrounds, minor topographic highs) in situ carbonate production continued, resulting in the growth of small, isolated reefs. More areally extensive biohermal complexes also grew on shallower or more proximal portions of the Lobstick and Bigoray ramps (i.e., Brazeau area).

The Nisku shelf margin is represented by a dramatic thickness change in the Nisku isopach (Fig. 12.34). The margin is complex, and exhibits both facies and slope changes along strike. The southern half of the Nisku shelf margin is less well defined and is represented by a series of isopach thicks between the 60 m and the 80 m contours (Figs. 12.32a, 12.34). These contours converge and the gradient steepens to the northeast where the margin is better constrained. To the north, the margin extends into the Grosmont complex, where again it becomes more irregular and poorly defined (Fig. 12.34).

On the shelf, carbonate production was able to keep pace with relative sea-level rise, and most of the Nisku shelf sediments near the margin accumulated during this transgressive stage. These sediments are largely dolomitized and thicken from approximately 60 m overlying the Leduc Rimbey-Meadowbrook trend to in excess of 100 m at the shelf margin (see Fig. 12.35). The shelf is underlain by Lower Ireton shales and overlain by clastics of the Calmar Formation.

The Zeta Lake pinnacle reef trend at West Pembina is situated approximately between the 80 and 100 m contours on the southeastern side of the Cynthia Basin (Fig. 12.34). The pinnacle reefs occur in a relatively narrow fairway, 180 km long by 65 km wide, on the medial portion of the Lobstick ramp (Fig. 12.32a). They are completely dolomitized along the southwestern part of the trend, becoming progressively less dolomitized to the northeast. Three isolated pinnacles are also present on the distal Lobstick ramp (Fig. 12.32a) northwest of the main reef fairway. The reefs range up to 100 m in penetrated thickness and areally occupy up to 380 hectares. Most pinnacles exhibit a bipartite zonation, and some show a threefold vertical distribution of shallowing-upward biofacies (Watts, 1987) consisting of a basal disphyllid coral zone, followed by a lamellar stromatoporoid zone, and in some cases capped by a coralline-algae biofacies zone (Fig. 12.32a). Apart from this zonation the Zeta Lake reefs show no other lateral facies differentiation or obvious cyclicity, although subtle cyclicity in reservoir quality parameters in fully dolomitized pinnacles has been observed (J.L. Douglas, pers. comm., 1992). The majority of the pinnacle reefs seem to be rooted in the Lobstick Member (mid-Lobstick cycle), although some pinnacle reefs appear to initiate on some overlying ramps such as the Bigoray Member (Watts, 1987). The question of whether the reefs initiated on a single discrete, or multiple, depositional surface(s) is still controversial.

The Cynthia Member can be subdivided informally into a lower and an upper Cynthia. Whereas the lower Cynthia is interpreted as representing the last vestige of the transgressive stage (maximum transgressive cycle) the upper Cynthia and Wolf Lake Member are part of an upward-shallowing basin-fill continuum that makes up the progradational stage. In the West Pembina area the interval is overlain by siltstone and silty carbonate of the Calmar Formation (Fig. 12.35).

The relation between the Zeta Lake reefs and the Cynthia basin-fill is a controversial one. Some workers have stated that the reefs entirely predate the fill and are equivalent to the Lobstick and Bigoray, while others claim that the reefs are in part coeval with the Cynthia basin-fill (Watts, 1987). The reefs are shown herein to have formed during the transgressive stage, and hence were actively growing during lower Cynthia deposition.

The Cynthia Basin trends northeast-southwest and can be separated into 'southern' and 'northern' sub-basins by a basinward-directed thick approximated by the 100 m contour (Fig. 12.34). The fill within the southern Cynthia Basin is commonly less than 120 m thick, while in the northern Cynthia Basin the fill is commonly greater than 120 m and ranges up to 150 m. The geometry of this northern sub-basin suggests a genetic relation to the Karr basin. It appears that the Karr Basin is a westerly extension of the northern Cynthia Basin (Fig. 12.34).

During earliest Nisku deposition, the northern margin of the Karr and Cynthia basins was formed by the Peace River Landmass and the submerged Peace River-Athabasca Arch. During latest Nisku deposition (progradational stage) the basin was progressively filled southward from the Peace River-Athabasca Arch. Where the fill reached a depth suitable for the deposition of shallow-water carbonates, the Meekwap shelf developed. This carbonate shelf extended southward and westward, ultimately isolating the Cynthia and Karr basins from more open-marine conditions (Figs. 12.31, 12.34, 12.35). The configuration of the Cynthia and Karr basins is constrained by the 100 m contour in Figure 12.34. The 100 m contour and the depositional shelf edge are not everywhere coincident, but are demonstrably convergent east of the Leduc Sturgeon Lake reef complex to the eastern edge of the northern Cynthia Basin. An isolated shelf developed over part of the Sturgeon Lake reef complex during the transgressive Nisku phase. In the Sturgeon Lake, Gold Creek and Simonette areas, the basin fill thickens to in excess of 150 m and approximates the Karr Basin depocentre. The southern margin of the Karr and northern Cynthia basins are constrained by the underlying Simonette, Berland, Fir, Pine and Windfall Leduc-age reef complexes. As a result of the relatively elevated subjacent Leduc topography, this southern margin is a mosaic of Nisku-equivalent ramps, shelves and isolated reefs. The stratigraphic relationships developed in the Karr and Cynthia basins are illustrated in Figures 12.35, 12.36 and 12.37.

Figure 12.35 shows the depositional pattern between the West Pembina and Meekwap shelf areas. The Upper Ireton shale continues to thicken in a northerly direction, with the most marked thickening occurring at the southernmost extension of the upper Nisku (Meekwap) shelf edge. The middle and upper Lobstick Member cycles shale out and thicken slightly in the deepest part of the northern Cynthia Basin. Each cycle then thins northward beneath the upper Nisku shelf and across the axis of the Peace River-Athabasca Arch. The Bigoray Member and lower Cynthia follow the same pattern as the middle and upper Lobstick Member cycles and downlap the thickening Upper Ireton shale. The upper Cynthia shale displays a sigmoidal basin-fill pattern and is relatively thin in the West Pembina area. There appears to be a younger cycle of fill that developed in the northern Cynthia Basin which is not represented in the West Pembina area. This upper cycle, in conjunction with Wolf Lake-equivalents, becomes more calcareous (shallow-water facies) to the north and together they form the Meekwap shelf. Thus the Meekwap shelf and associated patch reefs (see Cheshire and Keith, 1977) are apparently younger than the West Pembina pinnacles and the bulk of the southern Nisku shelf sediments (Fig. 12.35).

The southwest-facing Meekwap shelf margin extends from the Sturgeon Lake area through the Meekwap Field (Tp66, R15,16W5) to the Swan Hills area (Cheshire and Keith, 1977). It continues in a southeasterly direction and correlates to the uppermost part (progradational stage) of the Nisku shelf. The entire Nisku interval thins northward to less than 70 m. Where upper Nisku shelf-equivalents (i.e., upper Cynthia and Wolf Lake) thicken there is a corresponding thinning of the lower Winterburn units. However, across the Peace River-Athabasca Arch this thickening is accompanied by a change to highly complex stratigraphic and depositional patterns. This complexity is a result of the introduction of a substantial volume of Nisku-equivalent siliciclastic sediment into numerous isolated carbonate bank complexes. These sands were concentrated in an arcuate area basinward of the Grosmont shelf complex (Figs. 12.31, 12.34, 12.38), but their total distribution is widespread throughout the Athabasca Arch area. The sedimentology of these sandstones has not been broached in the published literature. These sandstones and siltstones reach up to 35 m in thickness. It is speculated that they occurred as marine sandbars that are coeval with, and share a common ultimate sediment source with, the upper Cynthia shale. They were deposited during a stillstand or relative sea-level fall during the Nisku progradational stage. The siliciclastic sediments are typically capped by a thin carbonate unit that may be an equivalent of the Wolf Lake Member. In addition to siliciclastic deposition, complex stratigraphic relationships exist along the axis of the Peace River-Athabasca Arch as a result of the presence of numerous discontinuous carbonate bank complexes. Subtle topography upon the Upper Ireton shale, probably controlled by differential compaction and/or structural movement of basement fault blocks, created sites suitable for localized carbonate bank development. These complexes appear to have initiated at Lobstick and/or Bigoray levels and persisted during deposition of the Meekwap shelf-equivalent cycles.

The Karr Basin also shows unique lithological complexity. Figure 12.36 schematically shows the east-west stratigraphic relations from the Karr Basin to the Grosmont shelf. The Nisku shelf equivalents on the Grosmont shelf are of similar thickness to those just east of West Pembina. Coeval strata of the Lobstick, Bigoray and lower Cynthia members are persistent across the basin, while the upper Cynthia equivalents display an east-to-west progradational depositional pattern. The basin-filling cycles appear to become more calcareous to the west. Depositional fill within the Karr Basin includes anhydrite, dolomite, limestone and locally preserved remnants of halite (Figs. 12.36, 12.39). The 16-29-67-3W6 well in Figure 12.36 illustrates an anomalous thickness of Graminia Silt due to dissolution of underlying evaporites, probably halite. Salt deposition was probably much more extensive than indicated by the few wells that exhibit pronounced solution collapse. Overthickened Graminia Silt intervals are widespread in the Karr Basin area and probably also indicate regional salt dissolution that preceded, or was coeval with, Graminia deposition (Fig. 12.14).

The precise timing of the Karr Basin fill is controversial. Some workers place the restricted fill within the upper Nisku progradational stage while others consider the fill to be of Blue Ridge or possibly Calmar equivalency (Fig. 12.14). Regional correlations indicate that the fill postdates the development of the Meekwap shelf (Figs. 12.14, 12.36, 12.39). However, our correlations depict the continuity of the upper part of the Calmar across the top of this fill. Consequently, the fill is interpreted as representing the final stage of basin filling within the progradational stage. The western margin of the Karr Basin is represented by a dolomitized and undifferentiated upper Nisku shelf edge.

South of the Karr Basin lies the Wild River Basin (Figs. 12.3, 12.34). This basin is surrounded by Nisku reefs that overlie Leduc reefs at Berland, Simonette and Windfall. Leduc reefs also serve to isolate the Wild River Basin from neighbouring Cynthia and Karr basins. Similar to the Karr Basin, this basin was created because of incomplete filling of the Woodbend basinal area by Ireton shale. The Nisku interval within the Wild River Basin reaches a thickness in excess of 140 m (Fig. 12.34). From north to south, the Upper Ireton shale thins into the basin. Unlike the Karr Basin, the Lobstick and Bigoray members on the east side of the Wild River Basin are similar in appearance to their equivalent strata, represented in the 7-4-49-12W5 type well (see inset cross section Fig. 12.34) of the West Pembina area. The upper Nisku shelf-sediments (Wolf Lake or Meekwap equivalent) shale out abruptly to the south. A late-stage basin-fill cycle postdates deposition of the Meekwap shelf sediments and predates Calmar deposition. Along the northern margin of the Wild River Basin, conditions were suitable for development of isolated pinnacle reefs comparable to the West Pembina trend.

Peace River Arch - Northern Alberta

The Peace River Landmass and its eastern extension, the Athabasca Arch, separate the complex sediment patterns of the Upper Devonian in the southern part of the Western Canada Sedimentary Basin from the more simple and less differentiated patterns to the north. Figures 12.38, 12.39, 12.41 left pane, 12.41 right pane and 12.42 show the stratigraphic relationships along and across the Peace River-Athabasca axis. Figures 12.39 and 12.41 left pane, 12.41 right pane show the thinning of the Nisku interval toward the Peace River Landmass, where shallow-water facies are represented by a progradational stage (Meekwap-equivalent) fringing shelf complex. During Nisku deposition the landmass was progressively reduced in size and onlapping Nisku carbonates grade into coeval Granite Wash sands.

Overlying the Leduc Grosmont shelf, a large, approximately 80 m thick, Nisku shelf complex formed (Figs. 12.38, 12.42). Along the Peace River-Athabasca axis, between this shelf complex and the Peace River fringing shelf, the Nisku interval contains several large isolated carbonate banks (Fig. 12.42). These banks are encased by sandstone and siltstone and supported by an extensive basal carbonate platform.

Sedimentation north of the Peace River-Athabasca Arch was less complex. Because of insufficient filling of the basin north of the arch by Ireton-equivalents, no lower Nisku carbonate ramps or shelves could develop. Additional filling by Nisku shale during the progradational stage was required before an upper Nisku carbonate shelf (Jean Marie Member) could form. Figures 12.30, 12.40 and 12.41 left pane, 12.41 right pane illustrate the development of the Jean Marie shelf complex. This shelf is typically only 15 to 20 m in thickness, although there are local thicks in excess of 100 m at the shelf-basin transition, representing downslope reef mounds. The Jean Marie shelf contains cycles that are synchronous with, to possibly younger than, the Meekwap shelf sediments.

The Nisku interval represents the continuation of the gradual process of basin filling, focussed in areas of Woodbend paleotopographic lows. The pattern of Nisku deposition was strongly controlled by inherited topography and is locally complex. Accommodation space increased initially (transgressive stage) but by the end of the Nisku interval, reduced subsidence and/or accelerated sediment influx, possibly coupled with a sea-level fall, led to a net reduction in accommodation space over most of the basin. By the end of the Nisku depositional sequence the vast majority of the Western Canada Sedimentary Basin was filled to a shallow and uniform level with only the westernmost portions of the basin remaining incompletely filled (Jasper, northeastern British Columbia and Liard areas).

Blue Ridge

The Blue Ridge interval is the last widespread Frasnian carbonate cycle in Western Canada. The top of this cycle is considered to approximate the Frasnian/Famennian boundary. This boundary is marked by the mass extinction of many shallow-water invertebrates, including corals (Sorauf and Pedder, 1986), most stromatoporoids (Stearn, 1987) and numerous brachiopod genera. The Blue Ridge interval is unique within the Woodbend-Winterburn strata as it is the only pre-Famennian interval lacking significant reef development. Only small (33 m thick) patch reefs are known within shale of the uppermost Mount Hawk Formation in the Jasper Basin (Hedinger and Workum, 1988), and small shelf edge buildups encased in carbonate are found in the subsurface southeast of the Windfall complex (Wendte, pers. comm., 1992).

Unlike underlying strata, compartmentalization of the Blue Ridge regime into discrete sub-basins is not as evident. Pre-Blue Ridge depositional basin topography was subdued because of significant and uniform filling of the basin by the end of the Nisku interval. The Blue Ridge interval consists of the Calmar Formation and Blue Ridge Member of the Graminia Formation, and their temporal equivalents. Figure 12.7 and the inset cross section in Figure 12.45 indicate the reference well used to define the Blue Ridge interval. The interval thickness ranges from less than 10 m in southern Alberta and Saskatchewan to in excess of 300 m in northeastern British Columbia (Fig. 12.45). Generalized lithofacies are illustrated on the schematic cross sections (Figs. 12.41 left pane, 12.41 right pane, 12.42, 12.43, 12.44). No regional internal correlations are shown although pronounced and correlatable stratal patterns have been identified locally.

No significant hydrocarbon reserves have yet been assigned to this interval although small pools are present, generally overlying Nisku production (Energy Resources Conservation Board, 1990). The present lack of significant reserves has translated into a lack of published articles concerned with the Blue Ridge. The studies available are either of local significance (e.g., Choquette, 1955) or are included as part of more regional compilations (e.g., Burrowes and Krause, 1987; Moore, 1989; Morrow and Geldsetzer, 1989). It is perhaps one of the least understood intervals within the Devonian of the Western Canada Sedimentary Basin.

Similar to the Nisku sequence, the Blue Ridge interval consists of an initial transgressive depositional phase followed by a phase of progradation and basin filling (Fig. 12.9). Sedimentation is considered to have commenced with a relative sea-level rise and reworking of underlying lowstand Nisku siliciclastics. Reworking during this marine incursion may have resulted in the widespread dispersal of silts and clays that form most of the Calmar Formation. For parts of the basin, distinguishing upper Nisku siliciclastics from the Calmar is difficult. This distinction becomes further complicated where an absence of Blue Ridge carbonate results in the Graminia Silt resting directly upon the Calmar or underlying Nisku clastics. Difficulty also arises in the Karr Basin where the distinctive siliciclastics are lacking within either the Nisku or Blue Ridge intervals. Although the Calmar Silt forms a distinctive and commonly employed log marker over much of the basin, little is known about the sedimentology and controls on deposition of this unit. Its regional character shows some variation from area to area.

In the Bashaw area (Fig. 12.11), the Calmar consists of quartz siltstone, green shale and dolomite. It has been interpreted as representing deposition in restricted marginal-marine, coastal- plain and fluvial-deltaic settings (Gilhooly, 1987). These settings were likely typical of those developed throughout eastern and southern Alberta. In the deeper basinal areas to the northwest, the Calmar is represented by shallow-marine carbonates, its base generally identified as a shaly log marker. In the West Pembina area, the Calmar is represented by silty shale and carbonate mudstone, while in the Meekwap area it is characterized by finer grained green shale (Cheshire and Keith, 1977).

Outcrop equivalents of the Calmar Formation consist of a regionally persistent, 10 to 15 m thick, recessive interval at the base of the Alexo Formation (de Wit and McLaren, 1950). It comprises variegated, noncalcareous red and green mudstones, and yellowish, quartzose siltstone. These lithologies are interpreted as representing deposition in shallow-marine and supratidal settings. Within the north-central portion of the outcrop belt, the Calmar is represented by the "lower silty unit" of the Ronde and Simla members of the Southesk Formation (McLaren and Mountjoy, 1962; MacKenzie, 1969; Geldsetzer and Meijer Drees, 1984; Morrow and Geldsetzer, 1989).Within the southern Jasper Basin, the Calmar is recognized within the basinal shales of the Mt. Hawk Formation as a persistent "doublet silt marker" unit (Hedinger and Workum, 1988; Hedinger and Shields, 1990).

In northeastern British Columbia, the Calmar-equivalent is probably the informally designated "upper member" of the Redknife Formation, which consists of limestone and some calcareous quartzose sandstone (Glass, 1990).

As the influx and reworking of clastics waned, carbonate sediments of the Blue Ridge Member began to be deposited. Consequently, the Calmar and its age-equivalents are conformably and largely gradationally overlain by the Blue Ridge Member and its equivalents.

In southern Alberta and Saskatchewan the Blue Ridge carbonate is not present because of nondeposition. As a result, the Nisku interval is directly overlain by the upper Graminia Silt. This Graminia interval is the basal portion of the Torquay Formation.

In central Alberta, the Blue Ridge Member consists of variably silty and anhydritic dolomite with little to no shelly fauna, and a mono-specific ichnofauna, suggesting restricted conditions. It normally displays an overall shallowing-upward profile, terminating in peritidal deposits. In the Karr Basin, the lack of a recognizable Calmar Silt makes definition of the base of the Blue Ridge interval difficult. The top of this locally restricted basin fill is chosen as the base of the Blue Ridge interval (Figs. 12.36, 12.39).

Within the southern and central Rocky Mountains, Blue Ridge-equivalent strata include about 30 m of recessive weathering, variably silty, restricted-marine carbonates and solution breccias assigned to the Alexo Formation (de Wit and McLaren, 1950). These strata thicken and develop more open-marine characteristics to the west and northwest. This change to open-marine facies is observed as the change to the "upper carbonate" unit of the Ronde and Simla members of the Southesk Formation (McLaren and Mountjoy, 1962; MacKenzie, 1969). The Blue Ridge carbonate thins and changes to a shale facies of the upper Mount Hawk Formation within the Jasper Basin (Figure 12.24). In the southern portion of the area (between the Southesk-Cairn and the Miette complexes), small, coral-dominated patch reefs are encased in these shales (MacKenzie, 1967, 1969; Hedinger and Workum, 1988; Hedinger and Shields, 1990).

In northern Alberta, northeastern British Columbia and the southern Northwest Territories, the Blue Ridge interval contains progressively younger, northwesterly-prograding depositional cycles of the Kakisa Formation. The Kakisa Formation generally consists of quartzose, silty, dolomitic limestone that in outcrop includes biohermal and biostromal limestones (McLean and Sorauf, 1988; Glass, 1990).

By the end of Blue Ridge deposition, the Western Canada Sedimentary Basin had been almost completely filled to a very shallow level. Only the northwesterly portions of the basin (northeastern British Columbia) remained incompletely filled. Deposition of this interval terminated abruptly because of a lack of accommodation space. Vastly reduced subsidence of the basin and a possible minor uplifting of the basin marked the end of the Frasnian succession. Uplift and truncation of Frasnian depositional cycles is particularly evident in the vicinity of the Peace River Arch. Accompanying this uplift was the introduction of widespread siliciclastics of the Upper Graminia and the coincidental global mass extinction of numerous Frasnian fauna (no cause and effect relationship for the latter is implied).

Upper Graminia

The Graminia Silt is the upper siltstone of the Graminia Formation. It is briefly discussed in this chapter for reasons of stratigraphic continuity, because it formally is included in the upper part of the Winterburn Group. However, it is more closely related to deposition of the Wabamun Group.

The boundary between the Blue Ridge interval and the upper Graminia Silt-equivalent (Trout River Formation) is marked by microkarst on a subaerial exposure surface developed on upper Frasnian strata in the Northwest Territories (Geldsetzer, 1988). Because of a lack of biostratigraphic data from the lower portion of the Trout River clastics, precise placement of the Frasnian-Famennian boundary has proven to be elusive thus far. The lowest occurrences of clastic sediments with datable biota, however, yield distinctive Famennian-aged conodonts (Orchard, 1989). Because the formational break occurs in close proximity to both the last-known Frasnian and first-known Famennian-aged faunal elements, it is assumed to approximate the Frasnian-Famennian boundary. The faunal break may not always coincide with the siliciclastic-carbonate contact and could vary by a few centimetres, as suggested for a local area of the Rocky Mountains (Shields and Geldsetzer, 1992). Regional biostratigraphic work is required to determine whether the faunal break corresponds to, or deviates from, this regionally persistent stratigraphic and sedimentological break.

This boundary represents one of the most significant global mass extinctions and biotic crises of the Phanerozoic. Regionally, this time boundary or event surface also delineates a marked change in depositional facies across the basin. Much of the Graminia Silt and correlative Trout River siliciclastics are interpreted as representing reworked detrital and/or windblown material, originally deposited in the basin during the preceding relative sea-level lowstand. Their exceptionally widespread distribution may be the result of marine reworking associated with the basal transgressive phase of the overlying Wabamun depositional sequence.

The Upper Graminia silt is present as a thin ( less than10 m) persistent unit in southern Alberta. It thickens slightly in southern Saskatchewan and significantly, to more than 100 m, in northeastern British Columbia. In southern Alberta and Manitoba the base of the Graminia Silt is undifferentiated from the Calmar Formation, where the Blue Ridge Member carbonate is absent either because of nondeposition or change to siliciclastic facies. It is recognized as a thin, persistent siltstone marker in east-central Alberta, although it can be present locally as quartzose sandstone that exceeds 30 m in thickness (e.g., thickening is observed along the eastern margin of the underlying Bashaw Leduc reef complex). In west-central Alberta, the Upper Gramina occurs as either siltstone or sandstone and tends to be over-thickened in response to underlying Winterburn salt solution in the Karr Basin.

In the southern and central Rocky Mountains, the silt break occurs within the upper few metres of the Alexo Formation. Within the Fernie area of southeastern British Columbia, and the Jasper Basin, the Graminia Silt is equivalent to the Sassenach Formation (McLaren and Mountjoy, 1962; Mountjoy, 1980; Raasch, 1989). The Sassenach shows tremendous variation in thickness. It occurs as a thin (les than15 m) interval of sandstone above the Leduc/Nisku reefal buildups of the Fairholme, Southesk-Cairn, Miette and Ancient Wall complexes. Within the Jasper Basin, the Sassenach thickens to about 200 m and consists of siliciclastics and clastic carbonates (McLaren and Mountjoy, 1962). Coppold (1976) suggested that this sand was derived from a westerly source. Uplift at the end of the Frasnian may have been in response to tectonic activity occurring along the continental margin of the Western Canada Sedimentary Basin. Aside from a suggested westerly source for the Sassenach, virtually nothing is known about the source and manner of introduction into the basin of the Upper Graminia clastics.

In northern Alberta, northeastern British Columbia and southern Northwest Territories this unit is equivalent to the Trout River Formation, which consists of siltstone, silty limestone, shale and limestone. With the exception of northeastern British Columbia and the Liard Basin areas, the Western Canada Sedimentary Basin was completely filled, and widely subaerially exposed by the end of the Frasnian. Uplift and erosion marked a dramatic departure from the general deepening and subsidence of the basin observed during deposition of the underlying Woodbend Group. A broad, relatively flat surface was present at the time of the next marine incursion, which led to reworking of the siliciclastics and to Wabamun carbonate deposition. The only significant area of relief that remained was in the vicinity of a small Peace River Landmass.

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Summary

The development of a chronostratigraphic framework for the Woodbend and Winterburn groups provides essential clues to unravelling the history and construction of the Western Canada Sedimentary Basin during the Paleozoic era. The seven key chronostratigraphic intervals represented in this chapter typify the various scales of depositional cyclicity that characterize this basin. A stacked pattern of carbonate platforms (initial transgression), source rock and isolated reef complexes (maximum transgression), followed by extensive filling of the basin by shales and carbonates (regression) is observed repeatedly throughout these groups (Fig. 12.9). The degree or level to which each component of the cycle (platform, reef/source rock, basin-fill) was developed determined the impact and pattern of deposition for subsequent chronostratigraphic intervals.

From the Cooking Lake through to the end of the Duvernay-Middle Leduc depositional interval, the basin gradually deepened, resulting in increasing relief between shelf and basin floor. The Duvernay-Middle Leduc depositional interval marked the attainment of maximum transgression for these groups. This peak transgression formed the best source rock (Duvernay Formation) in the Western Canada Sedimentary Basin. In addition, the greatest relief between shelf and basin was manifest during this interval. Although transgressive components of subsequent cycles (Lower Ireton-Upper Leduc, Upper Ireton, Nisku and Blue Ridge intervals) were important, they were dominated by the regressive parts of the cycles. This dominance resulted in the near complete filling of the basin and the elimination of shelf-to-basin relief. By the end of Woodbend-Winterburn deposition the basin had relatively flat topography. Only a small portion of the Peace River Landmass retained prominent relief, such that it influenced subsequent Wabamun Group deposition. Any shelf-to-basin relief existing by the end of Winterburn deposition had shifted to the west of the West Alberta Ridge and to the west of the Jean Marie shelf edge of northeastern British Columbia. By this time, conditions for relatively stable, uniform deposition of the Wabamun Group were established.

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Acknowledgements

The assembly and organization of a large volume of data has required the assistance of many individuals. We are grateful for the help provided by L. Abbott, P. Whitby, S. Mok, S. Howell and B. Carleton. Brian Edwards and Leo Sorbetti are thanked for their excellent work in deciphering and drafting our rough figures.

We are also indebted to N. Watts, T. Marchant, R. Workum, H. Geldsetzer and J. Wendte for their contributions, made at various phases of the project. In addition, we are very appreciative of the efforts and suggestions made by reviewers J. Wendte and G. Burrowes that have improved the chapter's content and clarity.

Most of all, this project could not have been accomplished without the patience, endurance and support provided by our families and friends. In this regard, special thanks are extended to Lesley Abbott.

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