Chapter 14 - Carboniferous Strata

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Chapter 14 - Carboniferous Strata

Chapter 14 - Carboniferous Strata of the Western Canada Sedimentary Basin

Chapter Sections Download
  1. Introduction
    1. Previous Work
    2. Geological Framework
      1. Prophet Trough
      2. Peace River Embayment
      3. Cratonic Platform and Williston Basin
  2. Stratigraphy
    1. Stratigraphic Nomenclature and Lithostratigraphy
      1. Banff Assemblage
      2. Rundle Assemblage
      3. Mattson Assemblage
      4. Besa River Assemblage
    2. Stratigraphic History and Environmental Interpretations
      1. Banff Assemblage
      2. Rundle Assemblage
      3. Mattson Assemblage
      4. Besa River Assemblage
    3. Regional Cross Sections
      1. Cross Section A-A'
      2. Cross Section B-B*
      3. Cross Section C-C*
      4. Cross Section D-D*
      5. Cross Section F-F'
      6. Cross Section G-G*
      7. Cross Section H-H'
      8. Cross Section J-J'
    4. Reference Logs
      1. Logs for Williston Basin, Saskatchewan
      2. Logs for Southwestern Alberta
      3. Logs for Peace River Embayment
      4. Logs for Northeastern British Columbia
    5. Maps
      1. Map Units and Overview
      2. Total Carboniferous Isopach and Oil and Gas Fields
      3. Map of Banff Assemblage
      4. Map of Rundle Assemblage
      5. Map of the Mattson Assemblage
      6. Carboniferous Structure and Paleogeology Map
    6. Tectonic History
    7. Acknowledgements
    8. References

Tables

Table 14.32a1 Oil production from the Banff/Bakken.

Table 14.32b1 Gas production from the Banff/Bakken

Table 14.32a2 Oil production from the Shunda/Pekisko - Mission Canyon.

Table 14.32b2 Gas production from the Shunda/Pekisko - Mission Canyon.

Table 14.32a3 Oil production from the Debolt/Elkton.

Table 14.32b3 Gas production from the Debolt/Elkton.


Authors: B.C. Richards - Geological Survey of Canada, Calgary J.E. Barclay - Geological Survey of Canada, Calgary D. Bryan - PanCanadian Petroleum Ltd., Calgary A. Hartling - PanCanadian Petroleum Ltd., Calgary C.M. Henderson - University of Calgary, Calgary R.C. Hinds - University of Pretoria Exploration Centre, Pretoria, South Africa

Introduction

The Carboniferous System of the southern and central parts of the Western Canada Sedimentary Basin (WCSB) is represented by a thick succession deposited on the downwarped and downfaulted western margin of the ancestral North American plate and the central to western cratonic platform (Figs. 14.1, 14.2, 14.3 and 14.4). These deposits, which are divided here into three main map units, are preserved in a region that includes much of the eastern Cordillera and southern to western parts of the Interior Platform (Fig. 14.3), extending from southwestern Manitoba into southwestern District of Mackenzie.

The succession comprises two main lithofacies associations. The lower association generally thickens southwestward or basinward and is dominated by shale, spiculite and bedded chert of basin to slope origin. Upward and northeastward, the lower array passes into an upper association of platform and ramp carbonates and sandstone-dominated siliciclastic facies that were deposited in slope to continental settings. Both associations consist of numerous formations, many of which are separated by regional disconformities.

Relatively thin intervals of Upper Devonian strata occurring in the Bakken, Exshaw, Banff and Besa River formations, which span the Devonian/Carboniferous boundary (Fig. 14.2), are mapped and discussed with the Carboniferous. The Devonian strata are included because they are widely separated from underlying Famennian strata by disconformities, and are lithologically indistinguishable from overlying Carboniferous facies that occur in these formations.

Subaerial erosion during the latest Carboniferous, Permian, and subsequent periods removed much of the Carboniferous succession, particularly on the northern Interior Platform and the region west of the Rocky Mountain Front Ranges. Where the Carboniferous remains, it is generally unconformably overlain by either Permian or Mesozoic strata.

The Carboniferous of the WCSB contains substantial accumulations of conventional oil and gas, ranking third in recoverable reserves after the Devonian and Cretaceous (Podruski et al., 1988; Hay, this volume, Chapter 32). Approximately 14 percent (365 x 106 m3, 2295 million barrels) of the proven conventional oil reserves of the Western Canada Sedimentary Basin occur in the Carboniferous, and 16 percent (580 x 109 m3, 20.5 TCF) of the marketable conventional natural gas reserves. Sulphur extracted from sour gas is of significant economic importance, as is limestone, used principally for manufacture of Portland cement and lime. In addition, potentially economic coal seams are present in upper Viséan deltaic deposits of the Mattson Formation in southwestern District of Mackenzie.

Previous Work

Geological studies on the Carboniferous of the WCSB date back to 1887 (McConnell). The current lithostratigraphic nomenclature and the depositional origins of major lithofacies have been established by numerous geologists including: Douglas (1958), Edie (1958), Macauley (1958), McCabe (1959), Halbertsma (1959), Christopher (1961), Scott (1964), Macqueen and Bamber (1968), Macqueen et al. (1972), Fuzesy (1960), Bamber and Mamet (1978), Chatellier (1988), Richards (1989a), O'Connell (1990), Barclay et al. (1990), and Savoy (1992). Regional stratigraphic syntheses were presented by Macauley et al. (1964), Douglas et al. (1970), Bamber et al. (1984), Henderson (1989), Richards (1989b), and Richardset al. (in press).

The biostratigraphy of the Carboniferous in the WCSB is established for several fossil groups (Fig. 14.2). Zonations are available for conodonts (Baxter and von Bitter, 1984; Higgins et al., 1991), foraminifers (Mamet and Skipp, 1970; Mamet, 1976; Mamet and Bamber, 1979; Mamet et al., 1986), corals (Sando and Bamber, 1985); ostracodes (Crasquin, 1984), and brachiopods (Nelson, 1961; Carter, 1987). International correlations have been established mainly by utilizing foraminifers and conodonts.

The concept of the Tournaisian Series used in this chapter (Fig. 14.2) differs substantially from that used in many recent papers about the Carboniferous of the WCSB. The Tournaisian Series was formerly divided into three parts (Tn1, Tn2 and Tn3), and its base was placed in the Upper Devonian (see Richards, 1989a, p. 3).Conil et al. (1976) formally restricted the type Tournaisian of Europe to make its base coincide with the Devonian/Carboniferous boundary. They also divided the Tournaisian into two parts, referred to as TI (= Hastarian Stage) and TII (= Ivorian Stage). These revisions, recently accepted by most European stratigraphers (see Paproth et al. 1983), are used here.

Geological Framework

During the Carboniferous, the principal tectonic elements in the WCSB were the Prophet Trough, the Peace River Embayment, the cratonic platform, the Williston Basin and the Yukon Fold Belt (Fig. 14.1; Richards, 1989b).

Prophet Trough

The Prophet Trough, which contains the thickest Carboniferous sections in Western Canada, developed during the latest Devonian to Early Carboniferous and persisted into the Late Cretaceous. The trough was connected to the Antler Foreland Basin of the western United States (Fig. 14.4), and extended from southeastern British Columbia to the Late Devonian and earliest Carboniferous Yukon Fold Belt (Fig. 14.1).

The western boundary of the trough is poorly defined but was an orogenic belt, extensively exposed from the Famennian into the early Viséan, subsequently largely transgressed. Late Devonian to Early Carboniferous volcanism, granitic plutonism, faulting and folding took place along the western rim (Richards, 1989b, Parrish, 1992; Smith and Gehrels, 1992).

A broad hinge zone, where water depths and sedimentation rates increased markedly basinward, formed the boundary between the cratonic platform and the Prophet Trough (Fig. 14.1). Along much of its length, the hinge was a foreland peripheral bulge during the late Famennian and Tournaisian.

Thickness trends of the Banff assemblage (see Fig. 14.33, and discussions of cross sections A-A' and B-B') demonstrate that the eastern hinge of Prophet Trough in the northwest lay west of the position indicated by Richards (1989b). Figure 14.33 also shows that northern Prophet Trough was separated from northwestern Peace River Embayment by a relatively low and broad, northwest-striking positive belt resembling the Sukunka Uplift (Fig. 14.1), which occurs on the southwestern side of the embayment. The positive belt on the northwestern side of the embayment is here named the Beatton High (Fig. 14.1), after the Beatton River of northeastern British Columbia (NTS 94A, H), where the positive belt was best developed and first recognized. Beatton High developed during the Tournaisian, or slightly earlier, and was episodically high into the Late Permian (Henderson et al., this volume, Chapter 15, Figs. 15.1, 15.14).

In the northwest, the Prophet Trough included the Liard Basin (Liard Trough of Douglas et al., 1970; Richards, 1989a), a northwest-trending feature with a depositional axis coinciding with the Yukon/District of Mackenzie border and the northern Rocky Mountain Foothills of British Columbia.

In southernmost Canada and northern Montana, Prophet Trough included a slightly positive area on the site of the Cambrian landmass Montania (Fig. 14.1). This area, episodically positive into the Permian, is characterized by relatively thin Carboniferous and Permian successions. The positive area was bounded on the north by an unnamed, northeastward-striking trough or half-graben which contains an anomalously thick upper Paleozoic interval (Richards, 1989b). The latter negative belt developed in approximately the same region as the western part of the Precambrian Vulcan Low (Fig. 14.1) of Ross and Stephenson (1989).

Growing evidence indicates that southern Prophet Trough was a compressional foreland basin from the late Famennian to the early Viséan. Central Prophet Trough (from southern Peace River Embayment into the Yukon) was also a foreland basin, but subsidence in that area and on the adjacent cratonic platform was accompanied by widespread block faulting (Richards, 1989b; Smith et al., 1993).

Peace River Embayment

The Peace River Embayment of northwestern Alberta and northeastern British Columbia opened into the Prophet Trough and was a broad, fault-controlled re-entrant into the western cratonic platform (Fig. 14.1). The principal depositional and structural axis of the embayment had an easterly trend and coincided approximately with that of the Late Devonian Peace River Arch; a subordinate northwest-trending axis was also present. Regional subsidence accompanied by extensive block faulting along northeasterly and northwesterly striking normal faults (e.g., Figs.14.5, 14.6) facilitated deposition of a thick Carboniferous succession inthe embayment, which included an extensive central grabensystem (Barclay et al., 1990).

The embayment, bounded on its southwestern side by the northwest-trending Sukunka Uplift and by the slightly elevated eastern hinge of Prophet Trough in the northwest (Beatton High), occupied an extensive region from the earliest Carboniferous into the late Serpukhovian. Higgins et al. (1991) stated that the embayment may have been of minor extent during the Late Carboniferous, based on the lack of documented occurrences of Upper Carboniferous strata. However, conodont data (Chung, pers. comm., 1992) for the upper Taylor Flat Formation of the embayment indicate that Upper Carboniferous marine strata are widely preserved in that formation, thereby proving that the embayment was extensively developed during the Late Carboniferous. The Peace River Embayment was also widely developed in Permian time.

Cratonic Platform and Williston Basin

The western cratonic platform, which extended northward from the United States to the Yukon Fold Belt (Figs. 14.1, 14.4), included the Williston Basin and several broad arches. Williston Basin, centred in western North Dakota, originated during the Ordovician by downdropping along basement faults. It redeveloped in several subsequent periods. The basin was well developed during most of the Carboniferous, but regional mid-Carboniferous epeirogenic uplift and erosion occurred. The substantial Carboniferous subsidence resulted largely from sagging of the Paleozoic successionin response to reactivation of the basement faults. Movement on the faults also produced fault-controlled folds and sub-basins (Gerhard et al., 1991). Normal faults and uplifts (Fig. 14.7), resulting from post-Carboniferous dissolution of salt in underlying Devonian formations, are common in the basin (Fuzesy, 1960).

During the Tournaisian, the Williston Basin constituted part of an extensive embayment that was connected to the Prophet Trough and the Antler Foreland Basin by a broad seaway, extending from southeastern Alberta into Wyoming. This connection had an axial graben system called the Central Montana Trough (Fig. 14.4; Roberts, 1979; Precht and Shepard, 1988). In the early and middle Viséan, the Williston Basin became a topographical basin as abroad, northeast-trending uplift (Sweetgrass Arch of Douglaset al., 1970) developed across the seaway in southeastern Alberta and southwestern Saskatchewan while a karst plain (Sandberg et al. (1982) developed to the south. During the Carboniferous, the axis of the (proto-) Sweetgrass Arch probably was approximately coincident with that of its present-day expression (Figs. 14.1 and 14.8).

Early Carboniferous shelf regions that extended from the Leadville-Redwall Shelf of the southwestern United States to northern Montana were called the Madison Shelf by Sando et al. (1990). The name, Madison Shelf (Fig. 14.1), is herein extended northward to Early Carboniferous shelf areas that formed between southern Alberta and Manitoba, in the seaway connecting the Williston Basin and Prophet Trough. The name Rundle Shelf (Fig. 14.1), named after the Rundle Group, is introduced here for Carboniferous shelf regions that lay north of the seaway and extended into southwestern District of Mackenzie.

The Precambrian Canadian Shield was exposed along the northwest-trending Severn Arch of Manitoba and the north-trending Wisconsin Arch of Ontario (Porter et al., 1982). These arches, along with the Transcontinental Arch of the north-central United States (Figs. 14.1, 14.4), controlled the positions of the eastern and northeastern shorelines of the latest Devonian and Carboniferous seas.

Stratigraphy

Stratigraphic Nomenclature and Lithostratigraphy

The uppermost Devonian and Carboniferous succession from southwestern Manitoba into the District of Mackenzie was divided by Richards (1989b) into a number of mappable lithofacies assemblages (Figs. 14.1, 14.2). Lithofacies that lie west of the Rocky Mountains are called the western assemblage and were deposited in western Prophet Trough and on its western rim. This assemblage, which is not included on the isopach maps and cross sections, includes carbonates, volcanics, and remnants of an easterly thinning clastic wedge that was largely removed by post-Carboniferous erosion.

The succession to the east, which comprises platform to ramp carbonates and deltaic terrigenous clastics, includes the Banff, Rundle and Mattson assemblages (Figs. 14.1, 14.2). These three assemblages, widely preserved in the Rocky Mountain Fold and Thrust Belt and on the Interior Platform, constitute the three main divisions of the Carboniferous that are dealt with in this chapter. Fine-grained siliciclastics and cherty to argillaceous carbonates of the Banff assemblage are widely overlain by carbonates of the Rundle assemblage, which in turn are partly overlain by sandstone and subordinate carbonates of the Mattson assemblage. Following Higgins et al. (1991), the Mattson assemblage of Richards (1989b) is expanded to include the sandstone-dominated Upper Carboniferous succession. From east-central British Columbia into the District of Mackenzie, the three assemblages overlie and pass basinward into the shale-dominated Besa River Assemblage.

Stratigraphic relations, lithology and depositional environments of the lithostratigraphic units that constitute the Banff, Rundle, Mattson, and Besa River assemblages are illustrated in schematic cross sections (Figs. 14.10, 14.11, 14.12, 14.13, 14.14, 14.15 and 14.16) and in detailed stratigraphic cross sections (Figs. 14.19, 14.20, 14.21, 14.22, 14.23, 14.24, 14.25, 14.26, 14.27, 14.28, 14.29, and 14.30).

The lower boundary of the uppermost Devonian and Carboniferous succession in the WCSB is locally conformable, but in most areas it is a minor unconformity caused largely by transgressive ravinement (Fig. 14.2). In western Manitoba and southeastern Saskatchewan and in parts of the central Rocky Mountains (Sukunka Uplift) and foothills, it is a subaerial unconformity (Richards, 1989b).

The Carboniferous succession is unconformably overlain by Permian deposits in most of the eastern Cordillera, and on the western Interior Platform of the Peace River Embayment area and locally to the northwest. Northward and eastward of the erosional zero edge of the Permian deposits, the Carboniferous is unconformably overlain by progressively younger strata of Triassic to Cretaceous age (Figs. 14.2, 14.19, 14.20, 14.21, 14.22, 14.23, 14.24, 14.25, 14.26, 14.27, 14.28, 14.29, and 14.30). The unconformity between the Carboniferous and overlying systems shows pronounced diachroneity, with the top of the Carboniferous becoming gradually older northeastward and northward (Fig. 14.8) as basal strata of the overlying systems become younger. The diachroneity resulted largely from southwestward epeirogenic tilting of the craton accompanied by several periods of erosional bevelling.

Banff Assemblage

Distribution and Regional Lithostratigraphy. The uppermost Famennian to upper Tournaisian Banff assemblage (Fig. 14.2) comprises carbonates and fine-grained siliciclastics of the Bakken, Exshaw, Lodgepole, Banff, and Yohin formations (see Fig. 14.33). An arbitrary nomenclatural boundary along the axis of the Mesozoic expression of the Sweetgrass Arch divides the assemblage into two successions (Figs. 14.8, 14.27, 14.30). The Bakken Formation and overlying Lodgepole Formation lie east of the arch and were deposited on the cratonic platform and in the Williston Basin. The Exshaw, Banff and Yohin formations, deposited on the western cratonic platform and in the Prophet Trough and the Peace River Embayment, lie west of the Sweetgrass Arch and extend into the District of Mackenzie.

In Manitoba, the Lodgepole Formation comprises formal members (Fig. 14.2; McCabe, 1959), whereas in Saskatchewan, it was divided by the Saskatchewan Geological Society (1956) into informal marker-defined units called beds (Fig. 14.9; Fuzesy, 1960). The Banff Formation of Alberta is here divided into several informal members that are similar to those used by Richards (1989b) and Richards et al. (in press).

Assemblage Boundaries and Unconformities. In most areas, the Banff assemblage unconformably overlies Famennian strata, but in some areas the boundary is either conformable or its nature uncertain. In the northwest, the assemblage overlies and passes basinward into the Besa River assemblage. In southeastern British Columbia, the latter assemblage is represented by the Lussier shale (Savoy, 1992) and underlies the Banff assemblage in one syncline.

Unconformities have been recognized at two levels in the Banff assemblage. An unconformity commonly separates the Exshaw Formation and overlying Banff Formation in extensive areas from east-central British Columbia to southern Alberta. An equivalent but minor unconformity may be locally present between the middle and upper members of the Bakken Formation from southeastern Alberta into Manitoba (Macqueen and Sandberg, 1970; Richards, 1989b). Higher in the assemblage in Manitoba, a subaerial unconformity separates the Scallion and Virden members of the Lodgepole Formation (Sereda, 1990).

In most of the Williston Basin, the southern Rocky Mountain Fold and Thrust Belt, and on the Interior Platform of southernmost Alberta, the top of the Banff assemblage becomes younger basinward as it grades into the overlying Rundle assemblage. Elsewhere, the boundary between the Banff and Rundle assemblages is commonly a minor unconformity resulting largely from transgressive ravinement (Richards, 1989b).

Rundle Assemblage

Distribution and Regional Stratigraphy. The Tournaisian to upper Viséan Rundle assemblage comprises the upper Madison Group and all of the Rundle Group except the Etherington Formation (Fig. 14.2; see also Fig. 14.34). In Canada, an arbitrary nomencla- tural boundary along the axis of the Mesozoic Sweetgrass Arch separates the Madison and Rundle groups (Figs. 14.8, 14.27).

The upper Madison Group, comprising the Mission Canyon Formation and overlying Charles Formation, extends from southwestern Manitoba into southeastern Alberta (Figs. 14.10, 14.11, 14.12, 14.13, 14.14, 14.15, 14.16, 14.17, 14.18, 14.19, 14.27, 14.28) and to the south. During 1956, the Mission Canyon and Charles formations of Saskatchewan were divided into informal marker-defined units called beds by the Saskatchewan Geological Society (Fig. 14.9; Fuzesy, 1973).

The Rundle Group, which consists of numerous formations and members (Fig. 14.2), extends from the District of Mackenzie into southeastern British Columbia (Figs. 14.19, 14.23-14.27). South of the Peace River Embayment, the principal formations within the Rundle Group are the Pekisko, Shunda, Turner Valley, Mount Head, and Livingstone (Fig. 14.8). In the southwestern part of this southern region, the Pekisko and commonly most of the lower to middle Shunda pass basinward (generally southwestward) into the Banff Formation, whereas most of the upper Shunda, Turner Valley, and the lower to middle Mount Head grade into the Livingstone Formation (Figs. 14.12, 14.29, 14.30). On the Interior Platform (Madison Shelf) of southernmost Alberta, the Pekisko generally passes southward into the Banff Formation, but locally the upper Pekisko grades basinward into the Livingstone.

In the Peace River Embayment and northward, the principal constituent formations of the Rundle assemblage are the Pekisko, Clausen, Shunda, Debolt, and Flett (Fig. 14.2), which grade basinward (generally southwestward) into the Prophet Formation of the Rundle assemblage. Along the southwestern side of the Peace River Embayment, the Turner Valley Formation is also present (Fig. 14.14).

The Pekisko Formation (Douglas, 1958; Penner, 1958) is here restricted to include only the lower, clean carbonate unit of the type Pekisko. The Pekisko is restricted because its definition, as manifest by its subsurface stratotype (borehole in 2-25-19-3W5), overlaps that of the Shunda Formation of Stern (1956), which has priority of publication. The middle and upper units of the type Pekisko are here included in the Shunda Formation (members D and G, respectively). The concept of the Pekisko utilized here has been widely used by others (see Macauley, 1958; Martin, 1967; Mamet et al., 1986; Richards, 1989b).

The Shunda Formation includes unnamed formation F of Richards (1989a,b), Higgins et al. (1991), and Richards et al. (1991). Earlier, most strata assigned to formation F had been included either in the Shunda Formation (Macauley, 1958; Macauley et al., 1964) or informally called "Shunda" (Bamber and Mamet, 1978; Beauchampet al., 1986). Formation F, dominated by open-marine, skeletal limestone, can be readily differentiated from the restricted-marine Shunda of Richards (1989b) in outcrop but not in the subsurface. The Shunda as used herein is characterized by its lithological heterogenity and by the predominance of argillaceous carbonates and shale.

The Debolt Formation, extending from west-central Alberta into northeastern British Columbia (Figs. 14.8, 14.13, 14.19, 14.23, 14.24), presents several stratigraphic problems. At its type section in central Alberta and over wide areas to the south and northwest, the Debolt includes stratigraphic and lithological equivalents of the Mount Head and Turner Valley formations and is currently separated from them by arbitrary nomenclatural boundaries. Most of the Debolt of British Columbia closely resembles the stratigraphically equivalent Flett Formation of the District of Mackenzie, and is separated from it by a strictly nomenclatural boundary.

The Mount Head and Flett formations are divided into formal members (Douglas, 1958; Richards, 1989a), but most formations within the Rundle assemblage have not been formally divided. The Pekisko Formation of west-central Alberta (Figs. 14.26, 14.31c) is locally divided here into lower, middle and upper members. The Elkton Member constitutes the lower Turner Valley Formation; the middle and upper members of the latter resemble the middle dense and upper porous members of Penner (1958) and the Mt1 and Mt2 of Rupp (1969). The Shunda and Prophet formations are here divided into informal members with letter designations. Members of the Prophet resemble those of Sutherland (1958), but the members in the Shunda are new. The lower Debolt is equivalent to the Turner Valley Formation and comprises the lower carbonate, lower argillaceous, and middle carbonate members of Law (1981), whereas the middle and upper Debolt are equivalent to the upper argillaceous and upper carbonate members of Law, respectively.

Assemblage Boundaries and Unconformities. The Rundle assemblage generally overlies and passes basinward into the Banff - assemblage, but from east-central British Columbia into southwestern District of Mackenzie, its western deposits overlie and pass basinward into the Besa River assemblage.

The contact between the Rundle assemblage and overlying Mattson assemblage is probably conformable in central Williston Basin, where the Kibbey Formation overlies evaporites of the Madison Group (Maughan and Roberts, 1967). On the unstable craton of central Montana, however, the Big Snowy Group and its correlatives unconformably overlie a karst surface developed on the Madison Group (Sando, 1988). A minor subaerial unconformity separates the Rundle assemblage from the Etherington Formation in southeastern British Columbia and southwestern Alberta (Richards, 1989b). The boundary between the Rundle and Mattson assemblages is a subaerial unconformity in southwestern Peace River Embayment and locally near the subcrop edge of the Mattson assemblage elsewhere in the embayment. In the axialregion of the embayment, the contact is mainly conformable.Farther northwestward, the Rundle assemblage is generally conformably overlain by the Mattson asemblage at a contact that becomes older northward.

Most of the numerous unconformities within the Rundle assemblage are at formation and member boundaries and either lie within one foraminiferal zone or form the boundary between consecutive zones (Fig. 14.2). Transgressive ravinement produced at least part of the hiatus below the Turner Valley Formation and those below the Baril and Loomis members. Subaerial unconformities are locally present below the Wileman, Salter, and Marston members.

Mattson Assemblage

Distribution and Formational Composition. In the Williston Basin, the Mattson assemblage (Fig. 14.2; see also Fig. 14.35) is represented by the Kibbey and Otter formations (Big Snowy Group).In the Rocky Mountains and western foothills of southwestern Alberta, it is represented by the Etherington Formation and by the overlying Spray Lakes Group (Tyrwhitt, Storelk, Tobermory and Kananaskis formations; Fig. 14.11). In the Peace River Embayment, the Mattson asssemblage comprises the Golata, Kiskatinaw, and Taylor Flat formations of the Stoddart Group (Figs. 14.2, 14.25). Farther northwestward in northeastern British Columbia and in the District of Mackenzie, the assemblage is represented by the Golata and overlying Mattson Formation (Figs. 14.15, 14.16, 14.22).

New conodont data (Chung, pers. comm., 1992) indicate that a new carbonate and sandstone unit herein informally called the Ksituan member and assigned to the upper Taylor Flat Formation (Figs. 14.2, 14.25) is of late Bashkirian and Moscovian age instead of Permian as indicated by Halbertsma (1959). The conodonts were extracted from core obtained near Ksituan Lake, west-centralAlberta, and the adjacent Dawson Creek region of British Columbia (Fig. 14.6). The new unit, which was also included in the Taylor Flat at some localities by Naqvi (1972, Fig. 4; Higgins et al., 1991), is named after Ksituan Lake (Sec 36, Tp78, R10W6; NTS 83M-14). The interval between 2585.5 m and 2719.5 m in the borehole at 11-18-79-16W6 (Fig. 14.31e) and the extensively cored section between 7316 and 7554 ft at 16-19-77-10W6 are designated as reference sections. Moscovian conodonts were extracted from the lower Ksituan at the latter locality, from its middle unit at 6-18-80-15W6, and from its upper unit at 3-29-83-18W6. Although it is Late Carboniferous in age, it should be emphasized that most of the Ksituan is herein mapped with Permian strata (Henderson et al., this volume, Chapter 15) because its age was recognized after mapping had been completed.

Halbertsma (1959) included the Ksituan member in the Belloy Formation of Peace River Embayment; however, assignment of the member to the Carboniferous Taylor Flat Formation iswarranted. The lower Ksituan of the reference sections is correlative with the upper part of the type Taylor Flat at 3-29-83-18W6, whereas the Ksituan is absent in the Belloy's Permian stratotype at 12-14-78-1W6. The middle and upper Ksituan at 11-18-79-16W6 (Fig. 14.31e) are correlative with most of the succession that Halbertsma (1959) included in the Belloy above the type Taylor Flat. Therefore, the concept of the latter stratotype requires revision, with the top of the Taylor Flat placed at 6278 ft instead of the present 6630 ft.

Assemblage Boundaries and Unconformities. The Mattson assemblage overlies mainly the Rundle assemblage, but northwestern occurrences of the Mattson assemblage overlie and grade basinward into the Besa River assemblage. In most of the eastern Cordillera and in the Peace River Embayment area of the western Interior Platform, the boundary with overlying Permian strata is a subaerial unconformity. Northeastward of the erosional edge of the Permian, Mesozoic deposits unconformably overlie the Mattson assemblage.

Several unconformities are present in the Mattson assemblage. In the southwest, the most important of these are partly of subaerial origin and occur at the base of the Spray Lakes Group, and base of the Tobermory Formation (Fig. 14.11; Scott, 1964; Stewart and Walker, 1980). In the Peace River Embayment, substantial breaks (partly subaerial) occur between the Golata and Kiskatinaw formations and at the base of the Taylor Flat Formation (Figs. 14.21, 14.25). A major subaerial unconformity separates the Upper Carboniferous Ksituan member of the Taylor Flat Formation from the Kiskatinaw, Golata, and Debolt formations (Figs. 14.19, 14.21, 14.25).

Besa River Assemblage

The Besa River assemblage was deposited in the Prophet Trough and western Peace River Embayment (Fig. 14.1). In the southwest, the assemblage is represented by the Famennian to Tournaisian Lussier shale, known only from the Rocky Mountain Main Ranges of southeastern British Columbia (Savoy, 1992). Elsewhere, the assemblage is represented by the Middle Devonian to upper Viséan Besa River Formation, which is widely distributed in the eastern Cordillera from east-central British Columbia to southeastern Yukon Territory (Figs. 14.2, 14.14-14.16; Pelzer, 1966; Bamber and Mamet, 1978). Prior to deep, post-Early Carboniferous erosion, the Besa River Formation was part of a more extensive shale lithosome that included much of the Earn and Black Stuart groups in the western assemblage of Prophet Trough (Richards, 1989b; Gordey et al., 1987). The Besa River Formation, which is up to 1655 m thick, is generally thickest in the foothills.

In most areas, the Besa River assemblage conformably overlies Middle Devonian strata and is conformably overlain by Tournaisian to upper Viséan strata of the Banff, Rundle and Mattson assemblages. In east-central and northeastern British Columbia, the Besa River Formation is locally unconformably overlain by Permian deposits (Fig. 14.14).

Stratigraphic History and Environmental Interpretations

Banff Assemblage

The Bakken and Exshaw formations comprise fine-grained siliciclastics deposited in euxinic-basin to shallow-neritic environments during late Famennian and earliest Tournaisian time (Hays, 1985; Richards and Higgins, 1988). Black shale of the lower Exshaw and its coeval correlatives in the lower member of the Bakken andbasal part of the northern Banff Formation (Figs. 14.10, 14.12, 14.13, 14.14, 14.15 and 14.16) record the culmination of a regional transgression that commenced with deposition of the Famennian Big Valley Formation and correlative strata in the upper Costigan Member of the Palliser Formation (Richards et al., 1991). Subsequent regional shallowing during the early Tournaisian climaxed with the subaerial erosion of most of the Exshaw Formation and part of the underlying Palliser Formation on the Sukunka Uplift (Richards, 1989b).

Lower Tournaisian black shale in the upper member of the Bakken, and correlatives in the basal Banff Formation south of 56°N, record the early phase of a second transgression (Richards, 1989b). The overlying Banff and Lodgepole formations are mainly heterogeneous associations of carbonate ramp deposits (Fig. 14.17) and siliciclastics, but facies deposited on poorly differentiated carbonate platforms (Fig. 14.18) are commonly preserved in the upper Lodgepole Formation and in the middle to upper Banff Formation. Shale, spiculite, and fine-grained carbonates in the lower parts of these suites record the continuation of the second transgression and the establishment of widespread, moderately deep-water basin, distal-ramp, and slope environments. At this time, Waulsortian mounds, consisting largely of lime mudstone and submarine cement and lacking an organic framework, developed in Williston Basin and the Central Montana Trough (Smith, 1977; Precht and Shepard, 1988; Sereda, 1990).

The early Tournaisian transgression was followed by regional shallowing and basinward progradation of slope and distal ramp to supratidal carbonates and siliciclastics of the Lodgepole, Banff, and Yohin formations. On most of the stable cratonic platform and in eastern Peace River Embayment, this trend culminated with deposition of the restricted-shelf lithofacies of the uppermost Banff Formation and its correlatives during the late early Tournaisian. On the unstable craton of southernmost Alberta and in most of central Williston Basin and the Prophet Trough, the trend culminated during the late Tournaisian and Viséan with sedimentation of the Rundle assemblage.

In southern Prophet Trough, progradation and shallowing during the late early Tournaisian to early Viséan regression discussedabove were interrupted by an early late Tournaisian regional transgression and episode of marked water deepening. The latterresulted in deposition of slope deposits of lower member F of the Banff Formation and its coeval correlatives in lower member D of the Shunda Formation (Figs. 14.12, 14.27, 14.29, 14.30).

Rundle Assemblage

The Rundle assemblage was deposited from the late early Tournaisian to the late Viséan and comprises carbonate-platform lithofacies with subordinate carbonate-ramp lithofacies and basinal to supratidal siliciclastics (Figs. 14.10, 14.11, 14.12, 14.13, 14.14, 14.15, 14.16). On the platforms, the slope lithofacies were deposited on relatively low-gradient slopes basinward of rimmed shelves characterized by high-energy shelf-margin sand belts (Fig. 14.18). The ramps (Fig. 14.17) lacked an obvious shelf-slope break and rimmed shelves.

In most of the Prophet Trough, the western graben system of the Peace River Embayment, and on westernmost parts of the cratonic platform, the dominantly shallow-marine platform and ramp carbonates of the Rundle Group prograded basinward (mainly southwestward) over deeper water shale and carbonates of the Banff and Besa River assemblages (Figs. 14.12, 14.15, 14.19, 14.23). In the seaway connecting the Williston Basin and Prophet Trough, the Rundle Group and Mission Canyon Formation prograded southward over the deeper-water deposits of the Banff assemblage (Figs. 14.27, 14.29). In central Williston Basin, carbonate-platform facies of the Mission Canyon and lower Charles Formation prograded basinward (southward to westward) over deeper-water carbonates of the Lodgepole Formation (Figs. 14.10, 14.20, 14.28). Elsewhere, carbonates of the Rundle assemblage were mainly deposited above shallow-marine facies of the upper Banff assemblage.

The Rundle assemblage, particularly in the central Williston Basin, western Peace River Embayment, and the Prophet Trough, is an overall shallowing-upward succession, although it formed during several transgressive/regressive cycles (Richards, 1989b).

Most of the Prophet Trough deposits and those formed in central Williston Basin and the western Fort St. John Graben of Peace River Embayment record continuation of the general progradational, shallowing-upward trend established during sedimentation of the upper Banff assemblage. Elsewhere, the Pekisko Formation and its coeval open-marine correlatives in the Clausen Formation and lower Shunda and Mission Canyon formations record a regional transgression during which Waulsortian mounds developed in the Peace River Embayment (Davies et al., 1988). The latter transgression, which took place in latest early to earliest late Tournaisian time, preceded several regional regressions and transgressions.

The first major regression subsequent to the Pekisko transgression occurred during the late Tournaisian and is recorded by widespread restricted-marine carbonates and anhydrite in the Shunda Formation. Subsequent regional regressions resulted in deposition of the Wileman, Salter, Marston, Carnarvon, and upper Opal members of the Mount Head Formation and their correlatives in the Debolt and Flett formations, and upper Madison Group (Richards, 1989a,b). During the Salter regression, most of thecratonic platform of western Montana was exposed (Sando, 1988), resulting in an extensive karst plain that probably extended into southeastern Alberta.

After the Pekisko transgression, there were three main regional transgressions. The first of these occurred in the latest Tournaisian to earliest Viséan, as recorded by the lower Turner Valley Formation and its coeval correlatives in the lower Debolt and Flett formations and Midale beds of the Mission Canyon Formation. The second transgressive event took place during the late middle Viséan and is represented by the Loomis Member of the Mount Head and by its correlatives in the upper Debolt and Flett formations. A subsequent transgression occurred in the early late Viséan (foraminiferal zone 14). The latter event is recorded by the middle Opal Member of the Mount Head and by the uppermost Debolt Formation and overlying lower Golata Formation (Richards, 1989b).

Subsequent to deposition of the Mount Head and Debolt formations, an extensive region south of the axis of the Peace River Embayment was subaerially exposed. This produced widespread but minor unconformities at the base of the Etherington Formation in southwestern Alberta and the locally substantial hiatus between the Rundle Group and Kiskatinaw Formation in southwestern Peace River Embayment and on the Sukunka Uplift. The unconformity at the top of the Golata Formation in the Peace River Embayment apparently developed at this time as well.

Mattson Assemblage

The Mattson assemblage (Fig. 14.2; see also Fig. 14.35) was deposited from the late Viséan to the Moscovian and possibly into the Kasimovian. This assemblage is dominated by deltaic anddelta-related deposits, but includes non-deltaic marine to continental siliciclastics and carbonate ramp deposits (Figs. 14.11, 14.15, 14.16).

Sandstone-dominated estuarine, deltaic, and delta-related slope to peritidal deposits constitute most of the Kiskatinaw and Mattson formations. The Mattson Formation and western occurrences of the Kiskatinaw Formation that were deposited in Prophet Trough prograded southwestward (basinward) over basinal to marine shelf and prodelta shale of the Golata and upper Besa River formations (Richards, 1989a, b). Within Peace River Embayment and on eastern Sukunka Uplift, estuarine sandstone and related siliciclastic facies of the Kiskatinaw Formation onlapped a regional unconformity developed on the Rundle Group and Golata Formation (Barclay et al., 1990).

Sandstone of neritic to continental origin constitutes most of the Tyrwhitt, Storelk, Tobermory, and Kibbey formations, and the upper part of the Etherington Formation (Stewart and Walker, 1980; Richards, 1989b; Henderson, 1989). Shallow-marine sandstone grading northwestward into outer-shelf and slope facies constitutes part of the Taylor Flat Formation.

In the Mattson assemblage, carbonate lithofacies, which commonly grade into marine sandstone and shale, predominate in the lower Etherington Formation and much of the Taylor Flat Formation. They also form the middle Kibbey Formation and parts of the upper Kiskatinaw and Mattson formations. Carbonates of the lower Etherington Formation, the Kananaskis Formation, the southwestern occurrences of the lower Taylor Flat Formation,and the Ksituan member are probably of carbonate ramp origin (Figs. 14.11, Fig. 14.17), but those preserved elsewhere in the assemblage do not constitute either well-defined carbonate ramps or platforms.

Lower Carboniferous deposits of the Mattson assemblage record the culmination of the general regressive trend that predominated during sedimentation of the Rundle assemblage. They also record numerous minor and some major transgressions and regressions. The Upper Carboniferous lithofacies of this assemblage record two main transgressive/regressive cycles.

In the Big Snowy Group of central Williston Basin, the lower Kibbey Formation represents continental deposition and the culmination of a regional regression, whereas the middle to upper Kibbey Formation and overlying Otter Formation indicate a subsequent transgression. The transgression was followed by late Serpukhovian regression and subaerial erosion (Maughan and Roberts, 1967; Sando et al., 1975).

In the Prophet Trough and Peace River Embayment and on the adjacent stable craton, the first major regression recorded by the Mattson assemblage occurred during the late Viséan (foraminiferal zones 15 and 16I). In the south, the culmination of this regression is recorded by the disconformity between the Mount Head and Etherington formations, and by subaerial unconformities associated with paleosols in the basal Etherington. In the Peace River Embayment/Sukunka Uplift region, the major late Viséan regression followed a late Viséan transgression, recorded by carbonates and black shale of the lower Golata Formation. The regression is indicated by redbeds in the upper Golata Formation (Barclay, 1988) and by the subaerial unconformity between the Kiskatinaw Formation and underlying Debolt and Golata formations. Deltaic and related lithofacies of the lower and middle Mattson Formation (Figs. 14.16, 14.22) record this regression in the Liard Basin. The late Viséan regression was followed by a gradual transgression, interrupted by numerous minor regressions, during deposition of uppermost Viséan to Serpukhovian carbonates and marine to continental sandstone in the Etherington, Kiskatinaw, lower Taylor Flat and upper Mattson formations.

The latest Viséan to early Serpukhovian transgression preceded three principal Carboniferous regressions and two intervening Late Carboniferous transgressions. The first of the regressions took place during the late Serpukhovian to early Bashkirian. In the south it is represented by Serpukhovian sandstone in the upper Todhunter Member of the Etherington Formation and by the overlying sub-Tyrwhitt unconformity (Fig. 14.11). In the Peace River Embayment, the mid-Carboniferous regression resulted in deposition of restricted-shelf carbonates and minor evaporites in the upper part of the lower Taylor Flat Formation and later erosion below the Ksituan member (Fig. 14.25). In the south, a subsequent transgression is recorded by Bashkirian, shallow-marine sandstone of the Tyrwhitt Formation, but subaerial erosion probably continued in most of the Peace River Embayment.

The onset of the second post-early Serpukhovian regression is recorded by aeolian sandstone of the Bashkirian Storelk Formation (Fig. 14.11). The culmination of this event resulted in the unconformity below the overlying upper Bashkirian to lower Moscovian Tobermory Formation. Bashkirian shallow-marine siliciclastics in the Tobermory resulted from the initial phase of the second Late Carboniferous transgression, which climaxed with deposition of Moscovian to lower Kasimovian(?) carbonates of the Kananaskis Formation (Stewart and Walker, 1980; Henderson, 1989). In the Peace River Embayment, the latter transgression is recorded by ramp carbonates of the Ksituan member.

A final regression, which exposed all of the WCSB from the District of Mackenzie southward, took place after the Moscovian and prior to the Early Permian (Richards et al., in press).

Besa River Assemblage

The Besa River assemblage consists mainly of black shale and records prolonged deposition in anaerobic to dysaerobic environments basinward of the slope to shelf and deltaic lithofacies of the Banff, Rundle and Mattson assemblages. Northwestern occurrences were deposited in a deep, starved basin, as indicated by pronounced southwestward depositional thinning (Pelzer, 1966; Richards, 1989a).

Regional Cross Sections

Cross Section A-A'

Cross section A-A' comprises a northwestern segment showing the succession in west-central Alberta and northeastern British Columbia (Fig. 14.19) and a southeastern segment representing southern Saskatchewan (Fig. 14.20). The northwestern segment is subdivided into a lower slice, dominated by the Banff and Rundle assemblages (Fig. 14.19), and an upper slice highlighting the Mattson assemblage (Figs. 14.21, 14.22). Figures 14.31a, 14.31e and f illustrate the relations between the gamma-ray logs and rock types. Figures 14.10, 14.13 and 14.15 provide environmental interpretations, and Figures 14.6 and 14.7 illustrate the seismic expression of the succession.

The succession represented by A-A' is thickest in the Liard Basin (Figs. 14.19, 14.22), but thick, relatively complete sections also occur in central Williston Basin (Fig. 14.20) and the Peace River Embayment (Fig. 14.21). The hinge zone of the Prophet Trough is clearly displayed in the northwest (Fig. 14.19), and thickness and lithofacies trends indicate that the Peace River Embayment, lying between 4-23-70-16W5 and 10-13-94-13W6, was moderately well defined during deposition of the Banff assemblage and well developed during deposition of the Rundle and Mattson assemblages (Fig. 14.19). Major facies changes and several of the normal faults characteristic of the Peace River Embayment and Liard Basin are displayed.

In Saskatchewan (Fig. 14.20), the Lodgepole Formation thickens gradually southeastward toward the center of Williston Basin, but near the southeastern end of A**-A', the formation thins abruptly as slope wackestone in its upper part grades eastward into upper-slope to shelf-margin grainstone of the Mission Canyon Formation. The Lodgepole displays southward-dipping clinoforms that resulted from the progradation of sequences toward the centre of Williston Basin and the Central Montana Trough. Similar clinoforms occur in the Lodgepole of eastern Williston Basin (Sereda, 1990).

In the Banff Formation of northwestern Alberta and northeastern British Columbia (Fig. 14.19), shelf carbonates (members B and G) and shallow-water to slope terrigenous clastics (member D) grade northwestward into shale-dominated lower-slope and basin facies of member A and the Besa River Formation. This resulted in the development of the northwestward offlapping transgressive/ regressive sequences of the Banff Formation.

Toward the axis of the Peace River Embayment, from both the southeast and the northwest, lime grainstone-dominated shelf and shelf-margin lithofacies of the Pekisko Formation thin and grade into argillaceous slope carbonates and shale of the lower Shunda Formation (Fig. 14.19; members A to D). In the Liard Basin, the Pekisko passes northwestward into slope spiculite and cherty wackestone to packstone of an unnamed formation.

Higher in the northwestern panel of A-A' (Fig. 14.19), the inner protected-shelf to restricted-shelf carbonates, evaporites, and siliciclastics of the southern Shunda Formation pass northwestward into open-marine Shunda lithofacies occurring in the axis of Peace River Embayment and on the Rundle Shelf to the northwest. Within the overlying Debolt Formation, which includes a widespread unit of restricted-shelf anhydrite in the Peace River Embayment, the proportion of open-marine carbonates increases northwestward. In the axial region of eastern Peace River Embayment (Figs. 14.19, 14.21), the Golata Formation is moderately widespread, but the overlying Kiskatinaw Formation and Ksituan member of the Taylor Flat Formation are preserved only in the major grabens.

At the northwestern end of A-A' (Figs. 14.19, 14.22), upper-slope and shelf carbonates above the Pekisko Formation grade basinward into spiculite and cherty slope carbonates of the Prophet Formation. Overlying sandstone-dominant deltaic and marine-shelf to shoreline lithofacies of the Mattson Formation grade basinward into prodelta shale of the Golata and upper Besa River formations.

Cross Section B-B*

Cross section B-B* (Fig 14.23) illustrates the Banff, Rundle and Besa River assemblages in the Interior Platform of northeastern British Columbia and northwestern Alberta. Figure 14.31f represents a borehole section (b-86-H; 94-I-13) within B-B* and shows the relations between the gamma-ray logs used in B-B* and rock types. Figure 14.15 shows environmental data for a similar succession that lies to the northwest.

In cross section B-B*, the hinge zone of the Prophet Trough is displayed as a prominent positive belt that resembles a forebulge. The hinge is defined by erosional thinning beneath the Exshaw Formation and thickness trends within the Banff, Pekisko and Shunda formations. The Banff and Shunda thin over the hinge, whereas the grainstone-dominant Pekisko thickens. Thickness trends within the Banff Formation indicate that a northwest-trending topographic low, herein called the Petitot Depression, lay east of the hinge.

Cross section B-B* shows southwestward truncation of the Famennian Kotcho Formation beneath the Exshaw Formation and northeastward truncation of the Lower Carboniferous beneath the Cretaceous Fort St. John Group.

In the Banff Formation, proximal to distal ramp carbonates of members B and G and shelf to slope siltstone, sandstone, and silty limestone of member D pass basinward into basinal shale of member A and the Besa River assemblage (Fig. 14.23). Westward offlapping of shallow-shelf to slope strata produced prominent Banff clinoforms (Chatellier, 1988). Overlying ooid and skeletal grainstones of the Pekisko Formation grade basinward into slope spiculite and chert of an unnamed formation. Eastern occurrences of the Shunda (b-86-H; 94-I-13, and eastward) include protected- to restricted-shelf carbonates. Shelf-margin to middle- and upper-slope carbonates of the western Shunda pass into basinal shale of the Clausen Formation and overlying chert-rich slope deposits of the Prophet Formation.

Cross Section C-C*

Southwest-striking section C-C* (Figs. 14.24, 14.25) shows the Banff, Rundle, and Mattson assemblages in the Peace River Embayment region. The relations between rock types and the mechanical logs used are shown by Figure 14.31d, which represents a borehole (11-14-81-25W5) along C-C*. It is also illustrated by Figure 14.31e, which represents a section (11-18-79-16W6) 40 km north of a-10-A; 93-P-10 along C-C*. Figure 14.14 shows environmental data for a similar succession lying toward the southwest.

Cross section C-C* crosses the northeast-trending structural and depositional axis of the Peace River Embayment. In the northeast, thick slope- to outer-shelf carbonates predominate in the Banff Formation and lower Rundle Group, indicating that Carboniferous paleoshorelines and the northeastern limit of the embayment lay well eastward of the erosional edge of the Banff Formation.

Some of the numerous normal faults in the Peace River region (Figs. 14.5, 14.6, 14.14) are shown in C-C*. Block faulting took place throughout the middle to late Paleozoic, but there appear to have been three main episodes, as suggested by the thickness changes occurring across normal faults. The first phase commenced during the middle to late Famennian and continued into the early Tournaisian, when the lower Banff Formation was deposited. A second period, which climaxed during the late Serpukhovian to earliest Bashkirian, started during deposition of the uppermost Debolt. A third occurred during the latest Carboniferous, after deposition of the Ksituan member, and continued through the Early Permian.

At the northeastern end of C-C*, the Big Valley Member of the Wabamun is truncated beneath the Exshaw Formation (Fig. 14.24), but the Big Valley/Exshaw contact is highly variable in aspect and is at least locally conformable in the Peace River Embayment. Because of pre-Banff erosion, the Exshaw is absent along the southwestern portion of C-C* and in part of the Rocky Mountains farther southwestward (Richards, 1989b; Figs. 14.13, 14.14).

In the Banff Formation along C-C* (Fig. 14.24), shallow-shelf ramp carbonates of upper member B pass basinward (southwestward) into distal ramp carbonates of lower member B, which in turn grade basinward into basinal shale and marlstone of member A. Southwestward offlapping of shallow-shelf to distal-ramp deposits formed the Banff clinoforms. Overlying shale, marlstone and argillaceous carbonates of slope and basin origin in the lower Shunda Formation (members A to D) pass southwestward into slope to shelf-margin carbonates of the Pekisko Formation. The latter facies change records uplift of Sukunka Uplift and concomitant subsidence along the axis of the Peace River Embayment. An anhydrite-dominated unit in the upper Debolt Formation of central Peace River Embayment passes southwestward into shelf carbonates.

Of the several Paleozoic unconformities shown on the lower slice of C-C* (Fig. 14.24), the break below the Kiskatinaw Formation is the most significant. Erosion surfaces (not shown) also occur in the uppermost Debolt Formation. These resulted from ravinement during the early late Viséan (foraminiferal zone 14) transgression that culminated with deposition of the lower Golata. In the northeast, the base of the Golata Formation may be locally erosional. The lower panel of C-C* also shows abrupt northeastward truncation of units within the Rundle Group and the Banff Formation beneath the sub-Cretaceous subaerial unconformity.

Upper C-C* (Fig. 14.25) illustrates marked local changes in the thicknesses of units in the Stoddart Group and overlying Belloy Formation. Most of these thickness variations resulted from deposition across growth faults (Barclay et al., 1990) and differential post-depositional erosion accompanied by block faulting. This cross section also shows northeastward erosional truncation of the Stoddart Group below the Belloy Formation. In the southwest, the Ksituan member thickens abruptly at the Murray River Graben west of the prominent northeasterly striking horst herein called the Pouce Coupe High. Over the axis of the Sukunka Uplift, which lies southwest of the line of cross section, the Stoddart Group is truncated southwestward beneath Permian and Triassic strata (Fig. 15.4).

Cross Section D-D*

Cross section D-D* (Fig. 14.26) illustrates the Banff and Rundle assemblages preserved in the western Interior Platform and in the eastern foothills of west-central Alberta. The upper Turner Valley Formation and overlying Mount Head Formation, widely preserved farther west, were eroded. Figure 14.31c, which represents a borehole section (7-23-46-17W5) near the western end of D-D*, reveals the relations between the gamma-ray logs used in the cross section and rock types. Figure 14.13 provides environmental data for a similar succession preserved north of D-D*.

The eastern flank of the Sukunka Uplift, indicated on Figure 14.26 by westward thinning of the Exshaw and Banff formations, is displayed as a broad, positive region along the southwestern side of the cratonic platform. Thickness trends within the Banff Formation demonstrate that a northwestward-trending topographic low, herein called the Drayton Valley Depression, lay along the northeastern side of the uplift.

Cross section D-D* shows eastward truncation of the Exshaw Formation below the Banff Formation. Higher in the succession, unconformities resulting from transgressive ravinement and possible subaerial erosion occur at the bases of the Pekisko and Turner Valley formations. Eastward truncation of the Lower Carboniferous beneath sub-Jurassic and sub-Cretaceous subaerial unconformities is prominently displayed on D-D*.

Members B, C, and D of the Banff Formation are well developed in the region represented by cross section D-D*. In this region, the Pekisko is divisible into three members (Fig. 14.31c), but along D-D* only the contact between the upper anhydrite-dominated member and the underlying units is illustrated. The overlying Shunda Formation is dominated by the argillaceous deposits of member F. Member E, which is widely preserved elsewhere in the Shunda (Figs. 14.19, 14.24, 14.29), is not developed.

Cross Section F-F'

Cross section F-F' (Fig. 14.27) shows the Banff assemblage and lower to middle parts of the Rundle assemblage of southernmostAlberta and southeastern Saskatchewan. The upper Rundleassemblage and the overlying Mattson assemblage, which are widely exposed in the Rocky Mountains to the west, were eroded from even the extreme western part of the section.

Along cross section F-F', the boundary between the Famennian Big Valley Formation and overlying Exshaw and Bakken formations is probably a minor unconformity, as indicated by southwestward truncation of the Big Valley and units within it. Slightly higher in the succession (Fig. 14.27), the upper boundaries of the Exshaw and middle member of the Bakken are probably unconformities, because both units vary greatly in thickness and are abruptly overlain by transgressive black shale. The Lower Carboniferous thins gradually northeastward to zero beneath the sub-Jurassic subaerial unconformity.

Along F-F', the boundary between the Banff and Rundle assemblages is gradational and lies between chert-rich skeletal lime wackestone and mudstone of the Banff and Lodgepole formations and the overlying cherty, skeletal grainstone and packstone of the Mission Canyon and Livingstone formations. Immediately below the contact, the gamma-ray logs show a slight downward increase in radioactivity. Dolomitization and chertification obliterated allochems and primary fabrics near the boundary and in most of the Rundle assemblage. The Lodgepole, comprising basin and distal ramp to slope deposits, thins northeastward as it grades into overlying upper-slope to shelf-margin deposits of the Mission Canyon Formation.

Cross Section G-G*

Cross section G-G* (Fig. 14.28) shows components of the Banff, Rundle and Mattson assemblages that were deposited in central to eastern Williston Basin. Figure 14.31a, representing a borehole section (6-3-1-19W2) along G-G*, reveals the relations between the gamma-ray logs used in G-G* and rock types. Figure 14.10 illustrates environmental data and Figure 14.7 the seismic expression of the succession.

Black to variegated shale and overlying sandstone of the Bakken Formation unconformably overlie the Famennian Torquay Formation because the Big Valley Formation, which commonly occursbetween these two units (Fig. 14.20), was truncated west of the area represented by cross section G-G*. Most units within the illustrated succession (Fig. 14.28) are truncated northeastward below the regional sub-Triassic subaerial unconformity; a few (basal Lodgepole) are truncated beneath the sub-Jurassic unconformity.

Prominent southwestward-dipping clinoforms, resulting from basinward progradation of transgressive/regressive sequences, are present in the Lodgepole Formation along the line of cross section (Fig. 14.28). The Lodgepole of southeastern Manitoba comprises several members that include shallow-marine carbonates (Fig. 14.2; McCabe, 1959), but these units are not differentiated on the cross section.

In Saskatchewan, spicule lime packstone to mudstone of basin and slope origin in the Lodgepole Formation are overlain by upper-slope to shelf-margin skeletal lime grainstone and packstone of the lower Mission Canyon Formation. In the zone between wells at 3-27-8-8W2 and 9-34-2-15W2 (Fig. 14.28), upper-slope to shelf-margin grainstone of the lower Mission Canyon gives way to deeper water spicule-rich Lodgepole facies, thereby indicating that the margin of the carbonate platform was deeply embayed toward its northern paleoshoreline. Protected- and restricted-shelf carbonates (dolostone, lime wackestone to packstone, algal boundstone) and evaporites (anhydrite and hypersaline dolostone) of the lower Charles Formation grade southwestward into the protected-shelf to shelf-margin skeletal, oolitic, and peloidal carbonates of the upper Mission Canyon Formation.

Cross Section H-H'

Cross section H-H' (Fig. 14.29) extends from the Inglismalde thrust sheet in the eastern Front Ranges to the Interior Plains of southernmost Alberta. The Mattson assemblage is thick and well developed in the northwest, but only its basal part is shown because its overlying deposits (about 90 m thick) have not been measured in detail. Figure 14.31b, representing a borehole section at 6-25-22-6W5 along H-H', reveals the relations between the gamma-ray logs used and rock types. Figure 14.12 shows environmental data.

On cross section H-H', the eastern, early Tournaisian hinge zone of the Prophet Trough is indicated by northwestward (basinward) thickening of the Exshaw and Banff formations between boreholes at 10-22-21-2W5 and 6-25-22-6W5. From the late Tournaisian through the Late Carboniferous, the hinge zone probably lay slightly farther westward, as suggested by lithofacies and thickness changes between 10-13-26-8W5 and the Front Ranges.

In the region represented by H-H', a minor unconformity occurs at the base of the Exshaw Formation (Macqueen and Sandberg, 1970; Richards et al., 1991), and several minor unconformities lie within the Carboniferous succession. Between the Front Ranges and foothills, marked southeastward erosional thinning of the Spray Lakes Group and Etherington Formation is evident below the sub-Triassic and sub-Jurassic subaerial unconformities.

The northwestern part of cross section H-H' shows westward-dipping transgressive/regressive sequences of the Banff Formation. The clinoforms, also recognized by Chatellier (1988), developed as proximal- to distal-ramp carbonates of member B that prograded westward over argillaceous to silty carbonates and shale of lower distal ramp and basin origin. In the region representing the Madison Shelf, H-H' shows shelf-margin to upper-slope grainstone of the Pekisko Formation grading southward (toward Central Montana Trough) into the Banff Formation. The southern Banff contains southward-dipping clinoforms and comprises chert-rich distal-ramp and slope carbonates.

Higher in cross section H-H', fenestral cryptalgal boundstone, anhydrite, and associated restricted-shelf carbonates of member E of the Shunda pass northwestward into cherty slope carbonates of member D through protected-shelf to shelf-margin grainstone of member G. Toward the same direction, restricted-shelf carbonates, anhydrite, and siliclastics of member F grade into member H of the Shunda and overlying shelf-margin grainstone of the lower Livingstone Formation. Protected-shelf to inner shelf-margin carbonates of the Turner Valley Formation overlie the Shunda and pass basinward into grainstone of the middle and upper Livingstone. The northwestern limit of the Turner Valley coincides with the underlying Shunda/Livingstone transition. Near the top of the succession, restricted-marine carbonates and shale of the Marston Member and lower to middle Carnarvon Member grade northwestward into wackestone and overlying grainstone of the lower to middle Opal Member.

Cross Section J-J'

Cross section J-J' (Fig. 14.30) extends from the McConnell thrust sheet in the Rocky Mountains to the Interior Plains of easternSaskatchewan. Figure 14.31b, representing a borehole(6-25-22-6W5) near the western end of the cross section, shows the relations between the gamma-ray logs used and rock types. Figure 14.12 provides environmental data.

On cross section J-J', the early Tournaisian hinge zone of the Prophet Trough is indicated by westward thickening of theExshaw and Banff formations between wells at 13-21-26-25W4 and 10-13-26-8W5. From the late Tournaisian through the Late Carboniferous, the hinge was probably slightly farther westward, as indicated by thickness and lithofacies changes between 10-13-26-8W5 and the McConnell thrust sheet.

A minor submarine unconformity occurs at the base of the Exshaw Formation along the line of cross section J-J', whereas a significant subaerial hiatus (not shown) underlies 2 to 4 m of strata in the upper Palliser Formation that correlate with the Big Valley Formation (Richards et al. 1991). Numerous minor unconformities are present within the succession. In the foothills, prominent eastward erosional thinning of the Mount Head Formation is evident below the sub-Jurassic subaerial unconformity. On the Interior Platform, the Lower Carboniferous is progressively truncated eastward below the sub-Cretaceous unconformity.

Cross section J-J' shows siltstone and argillaceous slope to outer-shelf carbonates of the upper Exshaw Formation grading eastward into shallow-marine shelf sandstone of the middle Bakken Formation. Westward-dipping clinoforms developed as overlying proximal-ramp carbonates of the Banff Formation prograded westward over shale and dolostone of distal ramp and basin origin. Higher in the succession, most of the restricted-shelf carbonates, anhydrite,and terrigenous clastics in members E and F of the ShundaFormation on the northern Madison Shelf pass basinward into protected-shelf to shelf-margin grainstone of Shunda member G and shelf-margin grainstone of the lower Livingstone Formation. Overlying protected-shelf to inner shelf-margin carbonates of the Turner Valley Formation pass basinward into shelf-margin grainstone of the middle and upper Livingstone Formation.

Reference Logs

Logs for Williston Basin, Saskatchewan

Figure 14.31a, showing logs for a borehole at 6-3-1-19W2 along cross section G-G* (Fig, 14.28), represents the Carboniferous succession of central Williston Basin. Highly radioactive black shale of the lower Bakken Formation unconformably overlies the continental to restricted-marine Torquay Formation and is overlain by shallow-marine siltstone and sandstone of the middle Bakken. Highly radioactive, black shale also constitutes the upper Bakken.

Conformably overlying basin and slope deposits of the Lodgepole Formation (Fig. 14.31a) are cherty, argillaceous, and dolomitic spicule lime mudstone to packstone. Upper-slope to shelf-margin pelmatozoan lime grainstone of the lower Mission Canyon Form-ation abruptly overlies the fine-grained upper Lodgepole. At the latter major lithological change, the gamma-ray log records only a slight upward decrease in radioactivity and the resistivity and bulk-density logs indicate a corresponding increase in porosity. Toward the top of the Mission Canyon, the carbonates become finer grained and the relative abundance of ooids, calcareous algae and restricted-marine lithofacies increases.

As shown on Figure 14.31a, the base of the lowest anhydrite bed in the Madison Group marks the boundary between the Mission Canyon Formation and overlying Charles Formation. The latter formation is dominated by evaporites and carbonates of restricted-shelf aspect. The contact coincides with a marked upward increase in bulk density and decrease in porosity.

Anhydritic, red to greenish gray siltstone, sandstone, shale and minor dolostone of the partly continental Kibbey Formation gradationally overlie the Charles (Fig. 14.31a)A. A slight upward increase in radioactivity and a decrease in bulk density and interval travel time occurs across the contact.

Logs for Southwestern Alberta

Figure 14.31b, illustrating logs for a borehole at 6-25-22-6W5 within cross section H-H' (Fig. 14.29), represents the Carboniferous of the southwestern Interior Platform and eastern Rocky Mountain Fold and Thrust Belt. Highly radioactive black shale and overlying slope to outer-shelf siltstone of the Exshaw Formation unconformably overlie thin, transgressive carbonates of the Big Valley Formation. The overlying Banff Formation, dominated by basin and slope deposits (Fig. 14.12), is a shallowing- and coarsening-upward sequence of shale grading upward into grainstone through marlstone and skeletal lime wackestone.

Higher in the section (Fig. 14.31b), pelmatozoan and ooid lime grainstone and cherty packstone of the Pekisko Formation abruptly overlie the Banff Formation and are gradationally overlain by the dominantly argillaceous carbonates, shale, siltstone, and anhydrite of the Shunda Formation. The Shunda, in turn, is unconformably overlain by peloid-skeletal lime grainstone and subordinate dolostone of the Turner Valley, which becomes finer grained and more dolomitic upward.

The sharp contact between the Turner Valley Formation and overlying Wileman Member of the Mount Head Formation (Fig. 14.31b) coincides with a marked increase in radioactivity and marks a change from clean dolostone to restricted-shelf siltstone and argillaceous dolostone. The overlying Baril, Salter and Loomis members of the Mount Head are poorly developed.

Logs for West-central Alberta

Figure 14.31c provides reference logs for a borehole at 7-23-46-17W5 (near 6-3-46-17W5 in cross section D-D*, Fig. 14.26), and represents the Carboniferous succession in the plains and foothills of west-central Alberta. The section resembles that shown in Figure 14.31b, but it lacks the Mount Head Formation and differs significantly in the upper Banff Formation and the Pekisko Formation.

Member D of the Banff Formation is well developed in the section (Fig. 14.31c) and is dominated by sandy siltstone of restricted-shelf origin. This member can be differentiated from underlying carbonate-dominated members of the Banff by its higher radiaoactivity.

The overlying Pekisko Formation comprises three members (Fig. 14.31c) and has its typical blocky gamma-ray/sonic signature. However, the Pekisko comprises mainly restricted-shelf carbonates and anhydrite instead of the skeletal and ooid lime grainstone that characterize it elsewhere (Figs. 14.31b, 14.31f). The upper Pekisko member is differentiated from adjacent units by using the bulk-density log. Samples must be used to pick the boundary between the grainstone-dominant lower member and the middle member, which includes algal wackestone and cryptalgal boundstone.

Figure 14.31c indicates that the Shunda Formation is dominated by restricted-marine lithofacies (anhydrite, argillaceous dolostone and shale) of member F. The Shunda is distinguished from theunderlying Pekisko Formation and overlying Turner ValleyForm-ation by its more highly serrated gamma-ray signature. The Turner Valley contains argillaceous units, but it is mainly dolomitized grainstone and packstone of protected-shelf origin.

Logs for Peace River Embayment

Figure 14.31d comprises logs for a borehole section at 11-14-81-25W5. The well is located along cross section C-C* (Figs. 14.24, 14.25), and it represents the Carboniferous in the axial region of northeastern Peace River Embayment. Highly radioactive black shale of the Exshaw Formation marks the base of the succession, and the upper boundary is defined by the subaerial unconformity between the Golata Formation and overlying Belloy Formation.

In the section shown (Fig. 14.31d), the Banff Formation is dominated by member B, which overlies basinal shale of member A and is overlain by shallow-marine siltstone and sandstone of member D. Member B is a coarsening- and shallowing-upward succession of distal-ramp marlstone and argillaceous limestone grading upward into relatively clean skeletal wackestone and packstone of proximal ramp origin.

The grainstone-dominated Pekisko Formation, which is widely developed along the southern and northern margins of the embayment is not developed at 11-14-81-25W5 (Fig. 14.31d). At this locality, the Pekisko has graded into argillaceous limestone and marlstone that are of basin to slope origin and preserved in the lower Shunda Formation (members A to D). Overlying member E of the Shunda is a shallowing-upward package of proximal-ramp carbonates that includes fenestral cryptalgal boundstone. Argillaceous proximal-ramp carbonates of Shunda member F record deepening.

The Debolt Formation at 11-14-81-25W5 (Fig. 14.31d) comprises three main units (lower, middle and upper Debolt) differentiated principally by using the ratio of clean carbonates to shale andargillaceous carbonates. Both the lower and upper Debolt aredominated by clean platform carbonates of protected- and restricted-shelf origin, dolomitized in the upper Debolt. The middle Debolt is dominated by argillaceous ramp carbonates and shale deposited on a restricted shelf. The middle Debolt is abruptly overlain by the anhydrite-rich lower unit of the upper Debolt, which is differentiated by using the bulk-density log.

Figure 14.31e represents the Carboniferous Stoddart Group of central Peace River Embayment. The figure comprises logs for a borehole section at 11-18-79-16W6 in British Columbia that lies 40 km north of a-10-A; 93-P-10 along C-C* (Fig. 14.25). The Golata Form-ation, dominated by marine shale, is preserved as an erosional remnant below the sub-Kiskatinaw unconformity. The sandstone-dominant Kiskatinaw Formation shows sharp-based channel fills (partly estuarine) and coarsening-upward sequences.

In the section (Fig. 14.31e), ramp carbonates and subordinate siliciclastics of the cyclic, transgressive/regressive lower member of the Taylor Flat Formation unconformably overlie the Kiskatinaw. The base of the Taylor Flat is placed at the base of a unit comprising shale grading upward into carbonates. The lower Taylor Flat is, in turn, unconformably overlain by transgressive sandstone and ramp carbonates of the Ksituan member. An upward change fom restricted-marine carbonates and minor anhydrite to fossiliferous open-marine limestone and sandstone coincides with the contact between the lower Taylor Flat and the Ksituan. Cherty dolostone and subordinate sandstone of the middle Ksituan unconformably overlie the sandstone- and limestone-dominant lower Ksituan and are, in turn, unconformably overlain by dolostone of the upper Ksituan. The lower and upper Ksituan have more highly serrated gamma-ray signatures than the middle Ksituan, and the SP log shows a leftward shift at the base and top of the latter.

Logs for Northeastern British Columbia

Figure 14.31f shows logs for a borehole section at b-86-H; 94-I-13. This locality, which is included in cross section B-B* (Fig. 14.23), represents the Banff and Rundle assemblages deposited on the northwestern part of the cratonic platform. Highly radioactive black shale of the Exshaw Formation constitutes the base of the package, but the Devonian/Carboniferous boundary lies with-in the lower Banff, as indicated by data from the southwestern District of Mackenzie (Richards, 1989a).

The Banff Formation in the section (Fig. 14.31f) is a coarsening- and shallowing-upward sequence dominated by basinal shale of member A and overlying slope to outer-shelf sandy siltstone, shale, and silty limestone of member D. Upper-slope to shelf-margin packstone and grainstone of the Pekisko Formation unconformably overlie the Banff. The overlying Shunda Formation, a transgressive/regressive succession overall, comprises two members of shale and argillaceous limestone (D and F) separated by a relatively clean middle member of shelf-margin to protected-shelf limestone (E). Grainstone and packstone of protected-shelf origin in the lower Debolt Formation abruptly overlie the Shunda.

Maps

Map Units and Overview

The mainly Carboniferous stratigraphic interval embraced in this chapter (Fig. 14.32) is divided into four assemblages. Three of these- the Banff, Rundle and Mattson assemblages (Figs. 14.33, 14.34, 14.35) - are distributed over most of the basin and are mapped separately. Each of these three assemblages contain subdivisions that are recognizable over wide areas but cannot be readily mapped throughout the basin. The fourth assemblage, the Besa River assemblage in the west, is included in the isopach map of the total Caboniferous (Fig. 14.32) but is not mapped separately.

The stratigraphic interval discussed in this chapter lies mainly in the Interior Plains and southern to central foothills, but part of the Front Range succession of west-central Alberta and east-central British Columbia is included. The isopach maps and structure map (Fig. 14.8) also include the broadly deformed and downfaulted succession in eastern Liard Basin, which lies mainly west of the Bovie Lake normal fault (Figs. 14.19, 14.22). Data used to compile the maps are derived principally from wells drilled during exploration for hydrocarbons in the Interior Platform, but thickness data from 57 subsurface and 23 surface sections in the eastern Cordillera are used as well. During the Mesozoic and early Tertiary Columbian and Laramide orogenies, northeastward-directed overthrusting foreshortened the western part of the Carboniferous succession, displacing westernmost deposits to the greatest extent. The Cordilleran localities are palinspastically restored to their pre-orogenic positions using data provided by Shell Canada Limited.

In the south, isopach maps for the total Carboniferous (Fig. 14.32) and the Banff assemblage (Fig. 14.33) include less than 10 m of Upper Famennian strata preserved in the Bakken and Exshaw formations. The Famennian component thickens northwestward from Peace River Embayment, and in the District of Mackenzie it comprises more than 220 m of Exshaw and Banff strata (Richards, 1989a).

In the Hines Creek Graben (Fig. 14.5) and the southwestern Peace River Embayment, the total Carboniferous isopach map (Fig. 14.32) and the Mattson assemblage map (Fig. 14.33) are depicted as thinner than they should be (locally up to 180 m). Thisresulted from late recognition of the Upper Carboniferous age of the Ksituan member (formerly thought to be Permian) and its consequent exclusion from the isopach mapping. The Ksituan member is included in the Permian isopach (Fig. 15.1). Where overlain by Permian strata, the upper surface of the Carboniferous succession is a peneplain resulting from latest Carboniferous (post-Moscovian) and Early Permian erosion. In contrast, areas of the Carboniferous that are overlain by Jurassic and Cretaceous strata are commonly deeply incised by broad Mesozoic paleovalleys and channels (Figs. 14.32, 14.33), also noted by Macauley et al. (1964).

Total Carboniferous Isopach and Oil and Gas Fields

In the areas mapped (Fig. 14.32), the Carboniferous is thickest in the central Liard Basin of northeastern British Columbia and southwestern District of Mackenzie. There, more than 1600 m of strata are preserved. On the Interior Platform to the south, the thickest deposits lie in the axial region of the Peace River Embayment (1200 m or more). Farther south on the platform, central Williston Basin contains about 550 m of strata. In the Cordillera, the succession attains 700 m in the western Front Ranges of east-central British Columbia and in the western foothills of southern Alberta.

Below the Permian map unit and the sub-Mesozoic unconformities, the total Carboniferous interval (Fig. 14.32) generally thins gradually toward its northern and eastern erosional zero edges. Thinning is, however, moderately abrupt along the western, fault-controlled side of Peace River Embayment, from about 54°40' to 57°30'N. Over one fault block on the southwestern side of the embayment, the Carboniferous thins abruptly below the Permian from over 500 m to less than 50 m. Similarly, the succession thins abruptly below Pleistocene deposits to about 66 m on the Celibeta High of southwestern District of Mackenzie (Fig. 14.32), due largely to rejuvenation of basement block faults after the Early Cretaceous (Williams, 1977). Below the sub-Cretaceous unconformity farther northwestward, the Carboniferous is strongly beveled northward along the southern flank of the broad, northeastward-striking La Martre Arch of Meijer Drees (1993).

In the southeast, the 500 m isopach of Figure 14.32 defines the central part of the Canadian component of Williston Basin, whereas the northeast-trending segment of the 300 m isopach marks the approximate western limit of that basin. In southeastern Alberta, the Carboniferous thins gradually toward the Mesozoic axis of the northeast-striking Sweetgrass Arch. Moderate westward thickening in the Cordillera southwest of Calgary defines the eastern limit of the Prophet Trough. The Peace River Embayment, lying between 53°50' and 57°30'N, is indicated by progressive thickening toward the easterly-trending axis of the embayment, where 1300 m of section are attained. Pronounced southwestward thinning on the southwestern side of the embayment records the location of the Sukunka Uplift. Farther north, the Bovie Lake normal fault system (Figs. 14.8, 14.22), a site of recurrent tectonism in the Liard Basin, preserves an anomalously thick succession on its downthrown side, generalized in Figure 14.32 by prominent northwestward thickening in northeastern British Columbia.

Nearly all of the Carboniferous oil and gas reserves in the fields shown on Figure 14.32 are in Lower Carboniferous strata; moreover, most occur in the Rundle assemblage east of the Rocky Mountains. Dolomitized lime grainstone and packstone are the main reservoir rocks, but other carbonate rock types and sandstones contain oil and gas pools. Most of the reserves are in unconformity-related traps along the Carboniferous subcrop edges and in traps that are primarily structural (Podruski et al., 1988; Hay, this volume, Chapter 32).

The insert tables on the Figure 14.32 map enumerate the ten largest oil fields (Table 14.32a) and the ten largest gas fields (Table 14.32b). Further breakdowns of the oil and gas statistics, by stratigraphic subdivision, are set out in additional tables (Tables 14.32a1, 14.32a2 and 14.32a3 for oil and Tables 14.32b1, 14.32b2 and 14.32b3 for gas), as per Hay (this volume, Chapter 32).

The Lower Carboniferous succession is the principal oil exploration target in the Canadian part of Williston Basin, but is of lesser importance elsewhere. East of the Sweetgrass Arch, most oil pools are found in unconformity-related traps in the Williston Basin of Saskatchewan. Most of the recoverable oil reserves in the Carboniferous west of the arch are in southern Alberta, where the Pekisko and Turner Valley formations are the most important producing units.

Most of the proven gas reserves are in carbonates of the Rundle assemblage. The major fields are in southwestern to west-central Alberta, but other fields occur in northwestern Alberta and in northeastern British Columbia.

Map of Banff Assemblage

The maximum thickness shown on the distribution and isopach map of the Banff assemblage (Fig. 14.33) is in the Liard Basin. Western Peace River Embayment preserves a slightly thinner succession.

Cratonward of the erosional edge of the overlying Rundle assemblage, the Banff assemblage is gradually truncated northward in Williston Basin and on the adjacent Madison Shelf (Fig. 14.33). From the southern Rundle Shelf at 51°50'N in southeastern Alberta to the District of Mackenzie, much of the assemblage, particularly in the axial region of Peace River Embayment, is truncated markedly eastward because of Jurassic and Cretaceous erosion accompanied by regional tilting. Below the Permian on the Sukunka Uplift, the Banff assemblage is reduced to less than 50 m because of faulting accompanied by erosion. Similarly, over the Celibeta High in the District of Mackenzie, the Banff assemblage is eroded to about 66 m.

Where the Rundle assemblage is preserved, the map of the Banff assemblage (Fig. 14.33) shows thickness variations that resulted mainly from differential sedimentation rates and penecontemporaneous tectonism; therefore, tectonic features that the other isopach maps lack are evident. Isopachs in south-central Saskatchewan outline the Canadian component of central Williston Basin, and southward thickening trends on the cratonic platform to the west demonstrate that subsidence rates increased southward toward the Central Montana Trough. Farther westward, eastern Prophet Trough is locally represented by moderate rates of southwestward thickening in the southeastern Cordillera. In west-central Alberta, the Banff assemblage thickens gradually toward the easterly-striking axis of Peace River Embayment, where the greatest thicknesses attained are in the western Fort St. John Graben (Fig. 14.5). On the southwestern side of the embayment, gradual southwestward thinning reflects the location of the Sukunka Uplift. Subtle westward thinning in northwestern Peace River Embayment and on the Rundle Shelf farther northwestward represents the Beatton High and hinge zone of the Prophet Trough.

Map of Rundle Assemblage

The maximum thicknesses on the Rundle assemblage map (Fig. 14.34) occur in the Liard Basin and axial region of western Peace River Embayment; central Williston Basin preserves a moderate thickness of strata. Substantially greater thicknesses than those shown are attained in the southern Rocky Mountains. Prominent westward thickening occurs in eastern Liard Basin and in eastern Prophet Trough of southwest Alberta. Because much of the assemblage has been eroded, the paleogeography during deposition of this map unit is not well known.

Cratonward of the erosional edges of the overlying Mattson assemblage and the Permian succession, the Rundle assemblage is generally truncated progressively toward its eastern and northern edges (Fig. 14.34). Along the eastern nose of the Peace River Embayment, however, pronounced thinning takes place below Jurassic and Cretaceous formations. On the southwestern side of the embayment, southwestward erosional and depositional thinning reveals the presence of the Sukunka Uplift, on which the assemblage is locally truncated to zero below the sub-Permian unconformity. In the northwest, abrupt northward truncation occurs along the La Martre Arch, and over the Celibeta High the assemblage is totally removed.

Map of the Mattson Assemblage

The distribution and isopach map of the Mattson assemblage (Fig. 14.35) shows deposits that are widely separated and appear to occur in unrelated depocentres, but the assemblage is widely preserved in the Cordillera west of the area mapped (Figs. 14.1, 14.11). It is also widely developed in the Williston Basin and Antler Foreland Basin of the United States. The thickest succession occurs in the Liard Basin. A substantial thickness is also attained along the axis of western Peace River Embayment, which contains numerous anomalous thickness changes that resulted from block faulting (see Barclay et al., 1990) but cannot be shown at this scale. Along the eastern flank of the Sukunka Uplift, the assemblage is truncated southwestward below the Permian and Triassic.

Carboniferous Structure and Paleogeology Map

The top of the Carboniferous interval (Fig. 14.8) is a vast, broadly undulatory surface that dips southward from western Manitoba into southeastern Alberta and southwestward from southwest Alberta into District of Mackenzie. In the east, it is lowest in central Williston Basin. In the west, it is structurally low and (comparatively) steeply inclined along the western margin of the Interior Platform from southwestern Alberta into the Peace River region of northeastern British Columbia, but from the latter area northward it rises gradually along the disturbed belt front. The Sweetgrass Arch of the present day is displayed as a prominent, northeast-striking high in southeastern Alberta and southwestern Saskatchewan.

The structural patterns shown on the western part of Figure 14.8 resulted mainly from tilting of the craton during subsidence of compressional basins that lay in the foreland of the Columbian and Laramide orogens. Subsidence in Williston Basin and uplift of the Sweetgrass Arch occurred largely in response to intraplate compressional stresses present during the Jurassic to early Tertiary Columbian and Laramide orogenies. Uplift of the Celibeta fault block and the La Martre Arch in the District of Mackenzie elevated the upper surface of the Carboniferous above sea level in the northwest.

Tectonic History

Three principal phases of tectonism are recorded by the uppermost Devonian and Carboniferous succession. The first of these, which commenced during the latest Devonian and continued into the late Tournaisian and early Viséan, followed broad Famennian epeirogenic uplift that is recorded by the unconformity at the base of the Exshaw Formation in the northwest (Figs. 14.19, 14.23) and the subaerial unconformity below the Big Valley Formation in the south (Figs. 14.20, 14.29, 14.30). This phase of tectonism was dominated by episodes of marked regional subsidence. The latter resulted from intraplate compressive stress (Bond and Kominz, 1991) and to lesser extents from block faulting and tectonic loading during the Antler Orogeny of the western United States and the partly contemporaneous Cariboo Orogeny of eastern British Columbia (see Richards, 1988b; Poole and Sandberg, 1991; Smith et al., 1993).

During this initial phase, the major tectonic elements shown on Figure 14.1 became well established. Block faulting was not widespread, but took place in the Central Montana Trough (Fig. 14.4; Smith, 1977), Peace River Embayment (Fig. 14.19), and the Liard Basin. The major northeasterly striking normal faults in the trough and embayment formed at high angles to the southwestern margin of the continent and may therefore have resulted from intraplate shearing. Northwest-striking block faults probably resulted from flexural foreland deformation in the south and at least partly from back-arc extension in the northwest (Richards, 1989b). This initial compression-dominated phase of structural evolution was followed by a transitional period in the late Tournaisian and early Viséan characterized by moderate subsidence rates and basinward progradation of shallow-water carbonates.

The second phase of Carboniferous tectonism, which was dominated by extension in the Peace River Embayment region and northwestward, started during the early late Viséan and continued to the late Serpukhovian (Richards, 1989b). Profound subsidence, accompanied by widespread block faulting, took place in the Liard Basin and the axial region of western Peace River Embayment (Barclay et al., 1990). Substantial subsidence also took place in southern Prophet Trough. In the latter area, subsidence may have resulted from compressional stresses, given that extensional or transtensional structures other than the Central Montana Trough and the trough (Vulcan Low) north of the Montania block have not been documented. In addition, the Antler Orogeny in the United States continued through the Viséan (Poole and Sandberg, 1991) and would have influenced the region. The principal tectonic elements that developed during the previous tectonic phase were still well developed; Sukunka Uplift and the Liard Basin were particularly prominent.

The third phase of tectonism, which started in the late Serpukhovian and continued into the Permian, was characterizedby moderate to slow subsidence rates, local block faulting, and episodesof broad epeirogenic uplift accompanied by deep subaerialerosion. At this time, regional stress patterns in the North American plate were greatly influenced by the collision of Euramerica with Gondwana (Ross, 1991). The dominantly slow subsidence rates are indicated by the presence of several subaerial unconformities and by the relatively thin, shallow-marine to continental nature of the deposits preserved in the Peace River Embayment and Prophet Trough (Figs. 14.11, 14.25). During the latest Early Carboniferous, Williston Basin was subjected to broad epeirogenic uplift accompanied by deep subaerial erosion (Maughan and Roberts, 1967; Sando et al., 1975). Broad, late Serpukhovian to Bashkirian uplift accompanied by subaerial erosion also produced the major unconformity below the Ksituan member in the Peace River Embayment and that below the Tyrwhitt and Tobermory formations of the Prophet Trough (Fig. 14.11). A final phase of epeirogenic uplift, accompanied by a major drop in sea level, produced the unconformity that separates Lower Permian from Upper Devonian to Moscovian strata throughout the basin.

Peace River Embayment and other major tectonic elements of the Early Carboniferous persisted during the third tectonic episode, but extensive sedimentation may not have taken place in the Canadian component of the Williston Basin. At this time in the western United States, numerous transtensional basins and transpressional uplifts developed in response to the sinistral intraplate shearing that gave rise to the ancestral Rocky Mountains during the Ouachita-Marathon Orogeny (Kluth and Coney, 1981; Budnick, 1986; Ross, 1991). The shearing influenced sedimentation and tectonism in Idaho and probably Williston Basin and southern Prophet Trough as well (Richards, 1989b; Gerhard et al., 1991).

Acknowledgements

This chapter would not have been possible without the work of Grant Mossop and Irina Shetsen, who spent numerous hours compiling subsurface data, generating the four isopach maps and the structure map, and guiding preparation of the text. The authors acknowledge Graham R. Davies and Richard D. Sereda for critically reading this manuscript and for providing numerous comments. Mika Madunicky spent many days coordinating the drafting and layout. We also acknowledge the many geologists who discussed aspects of this chapter with us and provided constructive advice and encouragement.

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