The complex rock assemblages that form the western margin of the exposed Canadian Shield dip gently beneath the Western Canada Sedimentary Basin (WCSB). Domes and tectonic slices of these crystalline rocks are found still farther west in the Cordillera, indicating a greater areal extent prior to Mesozoic and Cenozoic deformation.
To overcome the problems associated with the study of deeply buried rocks, maps from aeromagnetic, gravity, electrical conductivity and seismic surveys have been used to define the crustal blocks and their boundaries beneath the WCSB (Ross et al., this volume, Chapter 4). Although over 4000 exploratory wells have penetrated the Precambrian in Western Canada, the bulk of the geological information on the basement has been derived from the 400 wells cored. Petrography, whole-rock geochemistry, and isotopic age determinations have been carried out on many of these cores. A major constraint on work with the cores is the uneven distribution of sample localities. A high proportion were obtained from wells in the area of the Peace River Arch in northern Alberta. The Precambrian basement is poorly known under deeper, and several shallower, parts of the basin. However, some of the shallower portions, particularly in northern Manitoba, have been actively explored by mining companies. Well over 2500 diamond-drill holes have penetrated the Precambrian under WCSB cover in northern Manitoba.
The decade of intensive exploration following the Leduc oil discovery of 1947 provided most of the Precambrian core samples used by Burwash et al. (1964) in their synthesis published in the Geologic History of Western Canada. The advent of K-Ar dating was critical in defining the Archean and Proterozoic blocks of the exposed and buried Shield (Burwash et al., 1962; Wanless, 1970). The American Association of Petroleum Geologists completed their Basement Rock Project with the publication of the Basement Map of North America (Flawn, 1967). This incorporated data supplied by Burwash for the Canadian wells then drilled to basement.
Detailed petrography was combined with whole-rock geochemistry by Burwash and Krupicka (1969, 1970) and Burwash and Culbert (1976) to define the Athabasca mobile zone, a belt of dynamic metamorphism and K-metasomatism extending across northern Alberta. Maps for the Decade of North American Geology (DNAG) project (Burwash et al., in press) divide the subsurface Precambrian of the WCSB into five major structural units based on the published geological and geophysical maps available in 1985. Ross et al. (1989) defined much smaller domains based on U-Pb zircon geochronology and the magnetic signatures of the basement in Alberta. Studies of the buried Shield in northeastern Alberta, Saskatchewan and Manitoba incorporated a variety of techniques and they are reported by Wilson (1986), Collerson et al. (1988), McGregor (1986) and McGregor et al. (1990).
Maps of radioactive heat generation in the basement rocks of Alberta are reported by Burwash and Burwash (1989); those for the WCSB as a whole by Bachu and Burwash (this volume, Chapter 30).
Figure 5.1 shows the configuration of the unconformity at the base of the Phanerozoic cover. Regional features, the Tathlina, Peace River and Sweetgrass-North Battleford arches, and the intervening Keg River, West Alberta and Williston basins, serve to interrupt the southwest regional dip of this surface. The history of elevation or subsidence of these features is documented in the chapters which follow.
Prior to burial, differential erosion of the softer Precambrian rock units imparted an etched grain to the unconformity. Differences in basement elevation of up to a hundred metres between wells in adjacent townships were noted during compilation of the database. This local topography is smoothed by the contouring program used. Fault scarps are also minimized. A virtual absence of published seismic reflection data limits direct mapping of basement faults across the basin floor. Notable exceptions are the delineations of the southern Alberta rift (Kanasewich, 1968) and the Churchill-Superior Boundary zone (Hajnal and Fowler, 1982).
The western margin of the Shield adjacent to the WCSB is cut by a number of faults and shear zones of regional extent. At Great Slave Lake, the topographically distinct McDonald fault (MDF, Fig. 5.1) is one of a series of northeast-trending, brittle zone, late Hudsonian lineaments (Henderson et al., 1990). All of these run subparallel to the older Great Slave Lake shear zone (GSLSZ). The dextral movement of the GSLSZ is also characteristic of the Black Bay fault, the Virgin River shear zone and the Needle Falls shear zone (Fig. 5.1). This entire segment of the crust appears to have been subjected to a similar stress field at about 2000 Ma (Byers, 1962). The Churchill-Superior boundary zone (CSBZ) is interpreted by Bleeker (1990) as a zone of collision with sinistral transcurrent movement. The north-trending Allan fault zone is early Hudsonian and has been subjected to recurrent movement after regional metamorphism. The Tabbernor fault defines the east boundary of the Archean Glennie Lake domain, within the Trans-Hudson Orogen (THO).
The northeast-trending faults of the East Arm of Great Slave Lake continue in the subsurface as the Tathlina (TFZ) and Hay River fault zones (HRFZ; Williams, Fig. 2, 1990). The GSLSZ can be traced as far west as the eastern edge of the Cordillera by its aeromagnetic signature (Ross et al., this volume, Chapter 4). Conjugate northeast-northwest-trending faults control the basement topography and sedimentation patterns over the Peace River Arch (Cant, 1988). Vertical movements totalling several hundred metres occurred along these faults in late Paleozoic time. In northeastern Alberta, a series of north-south lineaments, offset by later northeast-trending faults, were inferred by Garland and Bower (1959).
Offsets in linear gravity anomalies were used by Gent (1989) to define fault trends in the subsurface of Saskatchewan. These faults bear little obvious relation to those mapped on the exposed Shield. A set of conjugate faults between 102 and 106°W, 49 and 52°N, may represent the late, brittle stage of deformation of the THO. The position of the CSBZ in the subsurface is now defined (Weber, 1990). This boundary zone may have been active in Paleozoic time as indicated by the disturbance of Paleozoic strata.
The fundamental cause of the differential uplift of arches, especially the Peace River Arch, has been a subject of continued discussion. Burwash and Krupicka (1970) suggest isostatic control. Cant (1988) and Stephenson et al. (1989) favour crustal flexure related to Cordilleran tectonics. Ross (1990) suggests that the Arch reflects a passive, flexural-isostatic feature rather than an area of thermal-extensional tectonics.
Last modified: August 6, 2008
