Additional Contributor:
M.E. Holter - Consultant, Calgary
This chapter provides an overview of the structural framework and overall architecture of the Western Canada Sedimentary Basin (WCSB), on a regional scale, using reflection seismic sections, geological sections, isopach maps and observed subsidence profiles (burial history curves or plots), and on a more local scale, highlighting distinct geological relations such as variations in facies, thickness, porosity and diagenetic characteristics as they relate to the boundaries of individual structural blocks.
The WCSB comprises the eastern Canadian Cordillera and two major sedimentary basins: a northwest-trending trough in front of the Cordilleran Fold and Thrust Belt (extending eastward to the Canadian Shield) called the Alberta Basin; and the cratonic Williston Basin, centered in North Dakota and extending into southern Saskatchewan and southwest Manitoba (Fig. 3.1). These two features are separated by a broad northeast-trending positive element, which includes the Bow Island Arch. The arch was a subtle, mildly positive structural element in the late Paleozoic, and became more clearly defined in the Mesozoic and Cenozoic. At the southwestern end of the Bow Island Arch is the Kevin-Sunburst Dome, within which there are Tertiary intrusives.
The western boundary of the WCSB is here defined by the western limits of the exposed and deformed sediments of the ancestral North American margin, equating to the eastern limits of the allochthonous terranes, and normally located near the boundary between the Omineca and the Intermontane belts of the Cordillera. The boundary between the Omineca Belt and the Rocky Mountain Foreland Fold and Thrust Belt is the Rocky Mountain Trench, which continues northward into the Tintina Fault. For mapping convenience the southern limit of the WCSB in this chapter is normally taken to be at the Canada-U.S.A. border. The northern limit of the WCSB is defined by the Tathlina High in the Northwest Territories.
Strata of Middle Proterozoic to Cenozoic age thicken from an erosional zero edge in the northeast to more than 20 km within the Cordillera. Within this wedge, the Peace River Arch was a prominent east-northeast-trending topographic high in Cambrian to Late Devonian time that subsequently became, in part, the site of a faulted basin (the Peace River Embayment) in Mississippian to Permian time.
The Middle Jurassic to Eocene compressive deformation of the western edge of the WCSB formed the Cordilleran structural elements - the Foreland Fold and Thrust Belt and the Omineca Belt - deforming Middle Proterozoic to Eocene strata. Compressive deformation was followed by regional extension, and consequent deposition of Oligocene strata in the Flathead Valley Graben (southeastern British Columbia). The loading of the North American craton and the creation of western source areas during formation of the Cordilleran Foreland Fold and Thrust Belt greatly affected the Mesozoic and Cenozoic evolution of the entire WCSB.
Estimated post-mid-Jurassic shortening of 170 km across the Rocky Mountain Foreland Fold and Thrust Belt in southern British Columbia and Alberta includes 150 km since the mid-Cretaceous (Price and Fermor, 1984). Tectonic style and grain, thrust sheet thickness, and the nature of the displacement all change to the north in response to the following factors: changes in the lithological character of the deforming sedimentary prism; greater shortening of supercrustal rocks in the Bowser and Sustut basins of the Intermontane Belt (Evenchick, 1991); and increased lateral displacement along the Northern Rocky Mountain Trench fault system.
Structural features are, however, not restricted to the mountains. For example, in the Fort Macleod area of southernmost Alberta, there are a series of horsts and grabens. At the north end of the Alberta Basin, the Liard Basin is a dramatic feature in northeastern British Columbia and the Northwest Territories, bounded on the east by the Bovie Lake fault and fold complex. Also in the north, between the Peace River Arch and the Tathlina Arch, is the northeast-trending Hay River Fault and coincident Great Slave Lake Shear Zone (Fig. 3.1), which has up to 700 km of dextral displacement in Lower Proterozoic basement rocks. Vertical displacement and (possibly) horizontal offset are present in Phanerozoic strata near the Hay River Fault, but these are difficult to substantiate without geophysical data.
Apart from compaction and drape structures over reefs or other competent rock bodies, which are not discussed in any detail here, other "structural features" in the plains component of the WCSB include meteorite impact craters, and salt and carbonate dissolution structures. Several probable impact craters have been studied in the WCSB; all have faults or fractures near their sub-circular, raised perimeters and some have a central uplift. At Viewfield in Saskatchewan (Tp 7 R 7 W2M), there is a probable impact location which is 6 km wide and contains fractured Mississippian carbonates (Sawatzky, 1975). Other possible astroblemes are discussed in Burwash et al. and Norford et al. (Chapters 5 and 9, this volume).
A variety of solution features are present in the basin. For example, dissolution of Devonian Upper Elk Point salt has occurred around reefs of similar age. The timing of dissolution varies and has been interpreted from seismic and subsurface well information. Dissolution of Devonian salt is also evident at the sub-Mesozoic unconformity, controlling to some extent the variation in the thickness of overlying Cretaceous strata as well as the relief on post-Cretaceous surfaces in the eastern and northeastern WCSB. Karst topography is present on several erosional surfaces.
In terms of present-day structural features, adjustment along pre-existing fault planes is undoubtedly occurring and is expressed as geomorphological trends: lineaments on the plains commonly reflect salt solution trends and deep faults. In the Cordillera, the amount of seismic (earthquake) activity recorded throughout this century is low - between 54 and 60°N (Milne et al., 1978).
The Phanerozoic sedimentary wedge (Fig. 3.2) thickens southwestward from the exposed Canadian Shield to a preserved thickness of over 6 km east of the deformed belt in the Liard Basin, and southward to over 3 km in the Canadian portion of the Williston Basin. The latter was centered in North Dakota, but individual systems had different geographic depocentres, commonly within a circle of radius 110 km. In contrast to other intracratonic basins in North America, (Hudson Bay, Michigan and Illinois basins), the Williston Basin was periodically connected to the proto-Pacific ocean to the west through the northern United States, but became isolated in the Late Jurassic during the Cordilleran Orogeny.
Anomalies in the Phanerozoic wedge include the Peace River Arch, the Swift Current Platform and the Bow Island Arch (Fig. 3.1). One of the less obvious isopach anomalies is the thinning around Fort Nelson, in British Columbia, which was a tectonically positive element throughout the Phanerozoic (Fig 3.2).
Periodically separating the Alberta and Williston basins (Fig. 3.1) was a positive area comprising the Bow Island Arch (Williams and Burk, 1964) and the Swift Current Platform, which formed the locus of intermittent, broad, low-relief topographic highs throughout the Phanerozoic (Kent, 1987). The northeast-trending Bow Island Arch is structurally distinct from the more complex Sweetgrass Arch of Montana, which has a northwest trend and consists of a South Arch and a North Arch (the Kevin-Sunburst Dome) separated by the possibly dextral northeast-trending Pendroy Fault. The Sweetgrass Arch may have been contiguous with "Montania" of the early Paleozoic.
Deep well control becomes sparse toward the mountains, but in the Canadian Rockies the Cambrian to Cretaceous interval thickens to the west. Within the fold and thrust belt, much of the Phanerozoic section has been removed by uplift and erosion, yet Paleozoic strata attain thicknesses of over 8 km. West of the Rocky Mountain Trench, subsurface data are minimal but surface stratigraphic data suggest that there may have been a number of margin-parallel sub-basins along the western edge of the North American Craton (see for example Struick, 1987).
Last modified: August 6, 2008
