Triassic strata of the Western Canada Sedimentary Basin (Figs. 16.1-16.3) occur in four main physiographic provinces: the Rocky Mountains, the Rocky Mountain Foothills and the Interior Plains of northeastern British Columbia and northwestern Alberta (Gibson and Barclay, 1989), and the Interior Plains of southern Saskatchewan and Manitoba. The Alberta-British Columbia rocks of the Alberta Basin (Fig. 16.1) are separated from the Williston Basin rocks by the Sweetgrass Arch (see Porter et al., 1982). This chapter treats these two basins separately because their strata are quite different and also because Williston Basin strata may or may not be Triassic in age (Fig. 16.4a,b). The Williston Basin in Canada comprises two connected sub-basins, the Watrous and Amaranth sub-basins (Figs. 16.1, 16.5).
In the Alberta Basin, Triassic strata were deposited in one large, central sub-basin, the Peace River Embayment, which extended eastward from the western ocean onto the North American craton. The Embayment was connected to the Liard sub-basin in the north and to deposits in the Rocky Mountains and foothills in the south.
In the Alberta Basin, Triassic strata are up to 1200 m thick and thin eastward to an eroded zero edge (Fig. 16.2). They consist of marine to marginal-marine siliciclastic and carbonate rocks and lesser amounts of evaporites. These strata form a sedimentary wedge deposited on a westward-deepening stable continental shelf and shoreline, inherited from the Carboniferous and Permian of the western passive margin of the North American craton. The strata range in age from Early Triassic Griesbachian to Late Triassic Norian (Fig. 16.6). Triassic rocks in the Alberta Basin extend from the United States border to the Liard River area of northeastern British Columbia and southern Yukon. The Liard sub-basin contains a thick section of Lower Triassic shales overlain by Cretaceous shales.
Throughout most of the Alberta Basin the Triassic is overlain unconformably by marine strata of Jurassic age (Figs. 16.7 - 16.9). In the extreme northwestern foothills of British Columbia and along the northern erosional edge of Triassic deposition in both Alberta and British Columbia, the Triassic is overlain by Lower Cretaceous strata (Figs. 16.7, 16.8). Triassic rocks are underlain unconformably by marine strata of Permian or Carboniferous age (Figs. 16.6 - 16.8).
Alberta Basin Triassic sediments were deposited as a series of three major transgressive-regressive ("T-R") third- or fourth-order cycles (Figs. 16.6, 16.8; Podruski et al., 1988; Gibson and Barclay, 1989).
The first (lowermost) cycle involves sediments deposited along a tidally influenced, deltaic coastline with corresponding deep-marine and distal shelf deposits. Depositional environments of the second cycle show similarities to barrier island/tidal coastlines such as those along the modern Texas Gulf Coast, the Persian Gulf and the New Brunswick coast (e.g., Fig. 16.10). The Western Canada Sedimentary Basin was situated at approximately 30°ree; N during Triassic time and the paleoclimate probably ranged from mid-temperate to sub-tropical (Gibson and Barclay, 1989), similar to the modern Persian Gulf. The region was arid and dominated by winds from the west (Habicht, 1979). The third major cycle is dominated by shallow-water carbonate deposits.
Triassic strata contain substantial proven and potential hydrocarbon reserves in 105 oil fields and 217 gas fields. Triassic gas was discovered in 1950 within the Peace River Embayment at Whitelaw, Alberta. Exploration accelerated as a result of oil discoveries from 1952 to 1957 at Fort St. John, Sturgeon Lake South, Boundary Lake and Milligan Creek (Fig. 16.2). Alberta Basin recoverable oil reserves are 127.4 x 106 m3, and marketable gas reserves are 278.7 x 109 m3 (Tables 16.2a and 16.2b). These fields account for about 4 percent of Western Canadian oil reserves (Podruski et al., 1988) and about 8 percent of the gas reserves. The Triassic remains an active exploration target as shown by drilling activity and recent discoveries at Spirit River (Aukes and Webb, 1986), Brassey (Higgs, 1990a,b) and Ring-Border (Sturrock and Dawson, 1990).
Most of the early Triassic stratigraphic and paleontological studies undertaken in the Rocky Mountain Foothills of northeastern British Columbia were summarized by McLearn and Kindle (1950). This paper reviewed a substantial body of work (see reference list in Barss et al., 1964), which was done principally by McLearn from 1918 onward, and provided a stratigraphic and paleontological framework for the Triassic outcrop belt. Subsequent surface investigations of Triassic rocks in Alberta and British Columbia were made by Pelletier (1960, 1961, 1963, 1964, 1965), Tozer (1961, 1963a,b, 1967, 1982a,b, 1984) and Gibson (1970, 1971a, b, 1972, 1975).
Prompted by the gas and oil discoveries in the Peace River region, subsurface studies were published by such workers as Hunt and Ratcliffe (1959), Clark (1961), Armitage (1962), Fulton (1966), Fitzgerald and Peterson (1967), Mothersill (1968), Sharma (1969), Roy (1972) and Miall (1976). An important synthesis of Triassic surface and subsurface information was published in 1964 by Barss, Best and Meyers in the original geological atlas of Western Canada. This paper still serves as the fundamental reference to the Western Canada Triassic. A more recent synthesis of Triassic stratigraphy in the Peace River area was published by Gibson and Edwards (1990a, b).
In the foothills and Rocky Mountain Front Ranges between Jasper and the United States border, reports concerning Triassic rocks have been prepared by Kindle (1944), Warren (1945) and Gibson (1968a,b, 1969, 1971b, 1974). More recent papers concerning mainly the subsurface Triassic include McAdam (1979), Barss and Montandon (1981), Halton (1981), Cant (1984, 1986), Aukes and Webb (1986), Campbell and Horne (1986), Podruski et al. (1988), Brack et al. (1989), Gibson and Barclay (1989), Bever (1990), Gibson and Edwards (1990a,b), Higgs (1990a,b), Shell Canada Limited (1990), Sturrock and Dawson (1990) and Gibson (in press). Early Triassic fish have been described by Schaeffer and Mangus (1976), from the Wapiti Lake area of British Columbia. The fish assemblages of Wapiti Lake are of significance in evolutionary terms because of the appearance of Parasemionotiformes, a group that is intermediate between the primative bony fish (Paleonisciformes), which are typical of Paleozoic assemblages, and the advanced groups (holosteans), which dominate in the Late Triassic. Work on the Western Canada Ichthyosaur fauna has been published by Callaway and Brinkman (1989).
The Peace River Embayment of Alberta and British Columbia developed during Early Carboniferous and Permian time, coinciding with the area occupied by the Precambrian to Devonian Peace River Arch and present Peace River Arch. The Carboniferous-Permian embayment subsided as a broad downwarp with a large central half-graben complex along the axis of the arch, with subsidence accompanied by intense block faulting (Richards, 1989; Henderson, 1989; Barclay et al., 1990; O'Connell et al., 1990). During Triassic time, the Embayment persisted and subsided as a broad downwarp associated with minor block fault rejuvenation, particularly in the Monias area and southwest of Fort St. John. The rejuvenated faults influenced local sedimentation patterns.
Locally, sediment loading, indicated by deformed bedding and slump structures, and also localized small-scale (10-20 m) faulting normally formed in subparallel sets, were factors in Triassic deposition (see Cant, 1984; 1986; Wittenberg and Moslow, 1991). The fault sets also commonly control fluid distributions within reservoirs (e.g., Ring-Border, Wembley and Cecil fields; Figs. 16.2, 16.25).
To date there is no convincing evidence of a major highland or sediment source area for Triassic sediments west of the Alberta Basin. Triassic rocks in the present British Columbia Cordillera represent islands of the western ocean known as Panthalassa. The islands may have existed off the edge of the craton but these rocks are interpreted to have been accreted to North America during Jurassic to Cretaceous orogeny (Tozer, 1982a, b; Gibson and Barclay, 1989). Swelling claystones in the Montney Formation, Ring-Border Field area (Fig. 16.2; northeast British Columbia and western Alberta), might be bentonitic and thus volcanically-derived, and hint perhaps at western volcanism and thus possible highlands west of the Western Canada Sedimentary Basin (D.L. Sturrock, pers. comm., 1990).
In the western foothills and Rocky Mountain Front Ranges, Triassic rocks have been subjected to the Jura-Cretaceous Columbian and Upper Cretaceous-lower Tertiary Laramide orogenies, resulting in a series of imbricate thrust faults and folds. In northeast British Columbia, some of these structures provide large gas traps, such as at Sukunka, Jedney, Bubbles and Beg fields (Fig. 16.2; Fitzgerald and Peterson, 1967; Barss and Montandon, 1981; Podruski et al., 1988; Brack et al., 1989).
In the Peace River Embayment, tectonic activity during the Paleozoic resulted in numerous block faults, which displayed a rejuvenated but diminished activity during Triassic time. Faulting was much reduced by the time of Halfway deposition and is most evident in the Fort St. John/Monias area near the end of Triassic time (Fig. 16.3). Triassic strata drape over underlying Paleozoic faults in a number of areas (e.g., Pouce Coupe, Tp 78 R12W6). Some of these rejuvenated faults are important in hydrocarbon production (e.g., Cecil and Ring-Border fields). Small-scale "load or gravity faults" occur in the Halfway and Doig interval (Cant, 1984, 1986; Wittenberg and Moslow, 1991) but are too small to appear on regional-scale structure and isopach maps. Tectonic activity associated with the Cretaceous Laramide Orogeny produced a number of thrust faults and folds seen in the west and southwest of the Peace River Embayment. These faults and folds act as important anticlinal hydrocarbon traps, such as at Sukunka, Beg and Jedney fields in British Columbia. Structure on top of the Triassic sediments forms a peneplain surface except along the subcrop edge, where the surface is influenced by Lower Cretaceous channeling (Fig. 16.3).
Last modified: August 14, 2008
