Alberta Geological Survey
Alberta Geological Survey |
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Additional Contributors:
B. Power - McMaster University, Hamilton
A.R. Sweet - Geologica Survey of Canada, Calgary
W.A.D. Edwards - Alberta Geological Survey, Edmonton
The uppermost Cretaceous-Tertiary succession is a predominantly clastic prism that thickens from a zero edge in eastern Saskatchewan and southwestern Manitoba to over 4000 m in the foothills of Alberta and northeastern British Columbia. Over one third of the foreland basin stratigraphic thickness is encompassed in this interval. The interval is bounded at the base by the Milk River Formation and equivalents and is unconformably overlain by Quaternary sediments. The rocks range in age from early Late Campanian (83 Ma) to Late Paleocene (57 Ma), with scattered remnant gravels as young as 1.6 Ma (Mack and Jerzykiewicz, 1989).
Sediments were deposited in a series of transgressive/regressive cycles associated with the development of two depocentres, the foreland basin in the west and the Williston Basin in the east, separated by the Bow Island Arch. Tectonic influences, eustatic sea-level changes and regional climatic overprinting were major factors that controlled sedimentary facies. Natural resources in the uppermost Cretaceous-Tertiary strata consist of extensive hydrocarbon accumulations in coarse-grained continental strata near the base of the Belly River wedge, abundant coal deposits and potential coalbed methane from the Belly River Group, and the Horseshoe Canyon and upper Scollard formations, refractory clays from the Whitemud Formation, and extensive aggregate resources and minor metalliferous deposits from the poorly consolidated remnant upper Tertiary gravels.
Stratigraphic data for this chapter reflect, in part, control from oil and gas wells, at depth. In the shallower realm, where conventional borehole casing masks formation boundaries, stratigraphic control is drawn from selected portions of the coal exploration databases maintained by the Alberta Research Council and the Geological Survey of Canada. Information from outcrop sections in the foothills is incorporated into the stratigraphic analysis, but subsurface mapping is limited to the plains region, because foothills strata are tectonically disturbed. Although the upper limit of the subject interval is defined as the Tertiary-Quaternary boundary, Pleistocene Saskatchewan Sands and Gravels are included here for easier use of the integrated Atlas database.
Early geological studies of the uppermost Cretaceous-Tertiary were conducted by Dawson (1886), Dowling (1917), Allan and Rutherford (1934), Russell and Landes (1940) and Crockford (1949). Generally, these studies were regional in scope and concentrated on the near-surface coal resources. Later work by authors such as Williams and Burk (1964), Carrigy (1970), Stelck (1975), Gibson (1977) and Stott (1984) expanded the geological framework. In the 1970s and 80s, detailed sedimentological studies by Caldwell (1968), Shepheard and Hills (1970), McLean (1971), Ogunyomi and Hills (1977), Rahmani (1988), and Jerzykiewicz and Sweet (1988) were undertaken, including some based on subsurface data. Except for the account of Caldwell et al. (1978), which dealt with the marine component of the prism, no recent compilation has addressed the entire sedimentary basin. Detailed paleontological and palynological studies by Wall and Singh (1975), Demchuk (1990), Demchuk and Hills (1991), and Sweet and Braman (1992) have provided age determinations of the various formations within the prism.
The uppermost Cretaceous-Tertiary sediments form an eastward-thinning prism that was deposited during the culminating phases of the Laramide Orogeny and during subsequent (Tertiary) tectonic relaxation. Predominantly non-marine sediments in the west intertongue with marine strata in the east. The stratigraphic interval can be divided into four periods of extensive sandy clastic deposition (represented by the Belly River wedge, Horseshoe Canyon wedge, lower Scollard wedge and the Paskapoo wedge), and four intervals of limited coarse-grained deposition (Pakowki, Bearpaw and Battle shales, and the upper part of the Scollard Formation). In southeastern Alberta, the Bow Island Arch separates the foreland basin from the Williston Basin of southern Saskatchewan. In the Williston Basin, equivalent strata, in general, are more tabular, although local variability does occur because of underlying salt solution structures.
In the southern Alberta Plains, the lower boundary of this stratigraphic slice is marked by a regionally persistent contact between the base of the marine Pakowki Formation, and the underlying Milk River Formation. In the foothills of Alberta and northeastern British Columbia, the equivalent contact is defined by a thin pebble lag separating the Nomad and Chungo members of the Wapiabi Formation. To the north and east, a distinctive geophysical log response in the subsurface represents the contact.
The top of the uppermost Cretaceous-Tertiary prism is a regional unconformity resulting from the removal of between one and three kilometres of sediment during post-Laramide erosion in the middle to late Tertiary (Nurkowski, 1984; Bustin, 1992). The bedrock geology beneath the Quaternary cover is illustrated in Figure 24.1.
The Cretaceous-Tertiary boundary can be traced throughout the basin (Sweet and Braman, 1992) and separates the upper and lower divisions of the Coalspur Formation in the foothills, the upper and lower divisions of the Scollard Formation in the Alberta Plains, and the Ravenscrag and Frenchman formations in Saskatchewan.
Alberta Geological Survey
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