Alberta Geological Survey
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The middle Cretaceous Colorado/Alberta Group consists predominantly of mudstone interspersed with relatively thin sandstone and conglomerate beds. Other minor lithotypes include shaly chalk, chalky limestone, bentonite, pelecypod coquinas, hori- zons of fish debris, nodular phosphorite, and siderite, calcite and pyrite concretions. The Colorado Group is of significant economic importance in that it contains about 14 percent of the total Western Canada hydrocarbon reserves and about 80 percent of the reserves within the Middle Jurassic to Cretaceous foreland basin succession (Podruski et al., 1988; Porter, 1992). The largest and historically oldest gas pool in Canada occurs within the Medicine Hat Sandstone of the Colorado Group; this pool was discovered in 1890 during early exploration for coal resources.
This chapter presents an overview of the dominantly shaly Colorado and Alberta groups, hereinafter referred to as the Colorado Group. Other following chapters describe in detail the coarser clastic wedges represented by the Viking, Dunvegan and Cardium formations (Chapters 21, 22 and 23, respectively).
Earlier regional syntheses of the Colorado Group are in Caldwell (1984) and Williams and Burke (1964). Detailed lithostratigraphic and micropaleontological aspects of the Colorado Group in the Manitoba Escarpment and adjacent subsurface were described in McNeil and Caldwell (1981) and McNeil (1984). In Saskatchewan, detailed lithological descriptions were carried out by Simpson (1982). Stott (1963, 1967, 1982) meticulously mapped the foothills of Alberta and British Columbia, providing a comprehensive lithostratigraphy of the Upper Cretaceous strata. This lithostratigraphic framework has served as an excellent starting point for more detailed studies. The biostratigraphic zonation of Cretaceous strata in Western Canada was synthesized by Caldwell et al. (1978).
Summary lithological and stratigraphic descriptions of the units making up the Colorado Group across the basin are in Glass (1990). Highly generalized maps illustrating the extent of marine inundation were presented by Williams and Stelck (1975). Detailed paleogeographic maps illustrating the evolution of the Western Canada Sedimentary Basin are presented in Leckie and Smith (1992).
The Albian to Santonian Colorado Group was deposited within the Western Canada Foreland Basin during an approximately 25 to 30 million year period. Global sea level was high during this time, with specific sea-level maxima in the Late Albian, Early Turonian and Middle Santonian (Caldwell, 1984; Haq et al., 1987). Deposition at this time was also coincident with a regional tectonic downflexing of the North American craton (Lambeck et al., 1987). The major marine inundations were separated by four major regressive pulses represented by the Peace River-Viking, Dunvegan, Cardium-Bad Heart and Milk River formations. During the highstands, warm Tethyan water from the Gulf of Mexico mixed with the cooler boreal water extending south from the Arctic to form a shallow epeiric seaway.
The Colorado Group contains several sandstone and conglomerate units, some of which are prolific hydrocarbon producers (Table 20.1). These include, in ascending order, the Basal Colorado Sandstone, Spinney Hill Sandstone, Viking Formation, St. Walburg Sandstone, Barons Sandstone, Dunvegan Formation, sandstones of the lower Kaskapau Formation (Doe Creek Member), sandstones of the Second White Speckled Shale (the Phillips Sandstone), Cardium Formation, the Medicine Hat Sandstone and the Alderson Member of the Lea Park Formation (Fig. 20.1). Within the Colorado Group, the First and Second White Speckled Shales, the Fish Scales Zone, and shale at the base of the Shaftesbury Formation are more radioactive than overlying and underlying shales, have high total organic carbon contents, and have considerable hydrocarbon generating potential. An interval such as the Second White Speckled Shale is potentially both a source and a reservoir rock for hydrocarbons.
The Colorado Group thins eastward from about 700 m in southwestern Alberta to 200 m in the Manitoba Escarpment (Fig. 20.2). In northwest Alberta, the Colorado Group exceeds 1500 m in thickness where it overlies the Peace River Arch, which was subsiding during much of the Cretaceous. Regional cross sections constructed across the basin show the eastward thinning of the Colorado Group away from the Cordillera, with maximum thickening occurring in the northwest. The distribution of the Harmon, Cadotte and Paddy members is restricted to the general vicinity of the Peace River Arch where the Joli Fou Formation is absent.
Major structural elements affecting Colorado Group deposition in the basin are represented in the structure maps constructed on the Base of Fish Scales Zone (Fig. 20.3) and the top of the Milk River Formation (Fig. 20.4). The major elements identified on Figure 20.3 are similar to those identified by Williams and Burk (1964).
Positive structural elements include the Bow Island Arch (also referred to as the Sweetgrass Arch) in southeastern Alberta, and the Bowdoin Dome and Swift Current Platform in southern Saskatchewan. The Bow Island Arch separates the Alberta Basin from the Williston Basin.
Negative structural elements include the Williston Basin and its northern extension, the Moose Jaw Syncline in southern Saskatchewan and the Eastend Syncline in southwestern Saskatchewan. In southeastern Saskatchewan and southwestern Manitoba, the influence of the Williston Basin is evident in structural lows centred southeast of Regina. In northwestern Alberta and northeastern British Columbia, the Peace River Arch began to subside, with accompanying block faulting, during the Mississippian. It remained a topographic low during the Early Cretaceous and through to the Late Cretaceous (Cant, 1988). The Peace River Arch may have become a subtle high again by at least the late Turonian-early Santonian. Isolated stratigraphic and structural anomalies are attributed to local block-faulting over the Peace River Arch during the deposition of the Colorado Group.
The influence of the dissolution of Devonian-aged salts has been described elsewhere in this atlas (Wright, et al., this volume, Chapter 3). Salt solution during or prior to deposition of the Colorado Group sediments created local anomalous thickening. Salt solution subsequent to deposition of the Colorado Group has resulted in depressions on structural surfaces.
Regional cross-sections (Figs. 20.5 - 20.10), regional maps (Figs. 20.2, 20.17), structure maps (Figs. 20.3, 20.4) and reference logs (Fig. 20.18) make up much of the database for this chapter. Detailed sections and maps are provided for the Milk River/Chungo units and the Muskiki, Marshybank and Bad Heart formations (Figs. 20.20 - 20.27).
Regional cross sections from the top of the Mannville Group to the top of the Milk River Formation are illustrated in Figures 20.5 to 20.10. The datum for all these sections is the Base of Fish Scales Zone, which occurs across most of the basin, although it is sometimes difficult to identify in parts of Saskatchewan. Some difficulties with identification and correlation of the Second White Speckled Shale are evident in Figures 20.5, 20.9 and 20.10 because of changes in log character across the basin. Cross section H-H' (Fig. 20.9) is oriented along the axis of the western Alberta foreland basin and highlights the geometry of the Dunvegan clastic wedge. The northwestern end of the cross section stops where the units come to the surface in the Peace River valley.
The suite of reference logs (Fig. 20.18) illustrates the variability of the different components of the Colorado Group across the basin as well as the markers used for correlation.
Alberta Geological Survey
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