Alberta Geological Survey
Alberta Geological Survey |
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Additional Contributors:
K. Ma - Canada Stratigraphic Services Ltd., Calgary
N,C, Meijer Drees - Geological Survey of Canada, Calgary
The Upper Givetian to Lower Frasnian Beaverhill Lake Group (Fig. 11.1 and 11.2) covers a major part of the Western Canada Sedimentary Basin, attaining a maximum thickness of approximately 240 m (Fig. 11.3). This stratigraphic interval is bounded below by the post-Elk Point unconformity (Fig. 11.2) and above, conformably to disconformably, by the Woodbend Group. The succession can be subdivided into two stratigraphic phases: a transgressive "reefal" phase (Fig. 11.4), dominated by restricted- to open-marine carbonates of the Slave Point and Swan Hills formations (Fig. 11.2), and a regressive "basin-fill" phase (Fig. 11.5), dominated by shales and argillaceous carbonates of the Waterways Formation (Fig. 11.2). Selected reference wells (Fig. 11.6) illustrate the regional stratigraphy and lithological character of the strata that comprise the Beaverhill Lake Group and its Manitoba Group correlatives in Saskatchewan and Manitoba.
The banks and reefs of the transgressive phase are host to significant hydrocarbon reserves, and remain a focus of exploration activity, with recent discoveries at the Caroline Field (Tp 34, R 4 W6M) in southern Alberta and at Hamburg (Tp 96, R 11 W6M) in northern Alberta (Fig. 11.3).
The previous atlas (Committee on the Slave Point and Beaverhill Lake formations, 1964) set out the regional distribution of Beaverhill Lake and equivalent strata. The subsurface stratigraphy has since become much better known because of the increased drilling activity associated with this economically important depositional sequence.
The sequence stratigraphy of the Beaverhill Lake Group is discussed by Moore (1989) for Western Canada and by Stoakes (1988) for west-central Alberta. The biostratigraphy is outlined by Braun et al. (1988) and Braun and Mathison (1986). Fischbuch (1968) presents a detailed geological description and stratigraphic framework for the Swan Hills Reef Complex, and individual reefs are discussed in Wendte and Stoakes (1982) and Kaufman and Meyers (1989). A depositional model for the Watt Mountain, Fort Vermilion and Swan Hills formations in the Swan Hills area of central Alberta is provided by Jansa and Fischbuch (1974). Craig (1987) provides a regional depositional model for the Slave Point fringing reef complex flanking the Peace River Arch. Individual reefs are described by Dunham et al. (1983), Gosselin et al. (1989) and Tooth and Davies (1989).
Beaverhill Lake strata, in conjunction with the Elk Point Group, are described for the northern region by Meijer Drees (1990), Williams (1981, 1984), Griffin (1967) and Norris (1965). The Clarke Lake Field is described in detail by Gray and Kassube (1963). Equivalent strata in Saskatchewan and Manitoba are dealt with by Lane (1964) and Dunn (1982 a,b). The stratigraphic significance of the Watt Mountain Formation and its relation to the Beaverhill Lake Group is discussed by Meijer Drees (1988) and Williams (1984). Braun and Mathison (1986) and Williams (1984) review the stratigraphic relation between the Dawson Bay Formation and the Beaverhill Lake Group.
The post-Elk Point relative sea-level rise allowed seas to return to the intracratonic Elk Point Basin, transgressing the older basin margins. The marine incursion deposited open-marine carbonates that formed an extensive carbonate platform. Continued relative sea-level rise segregated the interior basin into several bank complexes and intraplatform "basins" (Fig. 11.1). A carbonate bank, the Hay River Bank, developed in the northern part of the basin with the seaward margin roughly coincident with the underlying Elk Point barrier, forming a continuous "barrier reef" known as the Presqu'ile Barrier. Along the western margin of the basin a fringing reef complex developed flanking the Peace River Arch, and the Swan Hills Complex developed marginal to the West Alberta Ridge (Fig. 11.1).
Following the transgressive phase, westward- and northward-prograding shale-carbonate clinothem cycles of the regressive phase infilled the Waterways Basin and downlapped the reef complexes. These clinothems correlate with shallowing-upward carbonate-evaporite cycles that were stacked aggradationally in the Souris River Shelf (Fig. 11.1). The basin is presumed to have extended farther eastward but is not preserved because of post-Devonian tectonics and erosion. The basin also extended farther south into Montana and North and South Dakota.
The Peace River Arch and West Alberta Ridge (Fig. 11.1) were paleotopographic high features that strongly influenced the Beaverhill Lake stratigraphy and depositional (facies) pattern. Their origin predates the Middle Devonian (O'Connell et al., 1990) and, except for minor tectonic activity associated with the Peace River Arch, they remained relatively inactive during Late Givetian-Early Frasnian time.
Normal block faulting (post-Elk Point, pre-Beaverhill Lake) was a result of possible readjustment of the Peace River Arch. The evidence includes local erosion of the Elk Point surface (Utikuma area, Tp 82, R 10 W5M), local dissolution of Elk Point salts (Kidney area, Tp 91, R 6 W5M) and the presence of coarse, arkosic, clastic sediment. Arkosic clastic debris was shed off the arch during the post-Elk Point hiatus, as a result of tectonic uplift and erosion. The sediment was reworked and deposited as a fluvial-deltaic complex (Gilwood Member; Fig. 11.2) during the ensuing relative sea-level rise. Arkosic, clastic sediment is also present within the Waterways Formation in the Girouxville area (Tp 78, R 21 W5M) along the southern flank of the arch, indicating a later tectonic pulse.
Givetian-Frasnian tectonic activity associated with the West Alberta Ridge and Tathlina High (an Elk Point paleotopographic feature in southern N.W.T.) is not documented or strongly exhibited, but may exist, as the tectonic history of these two features is thought to be similar to that of the Peace River Arch.
Several stages of post-Beaverhill Lake salt solution in southern Saskatchewan overprinted earlier dissolution events, creating complex stratigraphic and structural relations. The isopach map (Fig. 11.3) depicts the solution-influenced geometries in only a very generalized way. Post-Beaverhill Lake solution removal of Elk Point evaporites along the subcrop edge near the Alberta-Saskatchewan border is best reflected in the Beaverhill Lake structure map (Fig. 11.7).
Alberta Geological Survey
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