The Williston Basin forms the southeastern extremity of the Western Canada Sedimentary Basin. It is the archetypal intracratonic basin, and its significant reserves of hydrocarbons have made it a major exploration region. Consequently, more than 20 000 exploratory boreholes have been drilled, and these provide a relatively large subsurface geological database. Approximately four fifths of these wells have been drilled in the Canadian part of the basin.
The term "Williston Basin" is arbitrarily applied to the Phanerozoic succession in Manitoba, Saskatchewan, the Dakotas and eastern Montana, but in a structural context it denotes the ellipsoidal depression centered in North Dakota, more or less below the -1500 m contour on the Precambrian basement (Christopher et al., 1973; see Fig. 5.1, this volume). This configuration is a consequence of the Laramide Orogeny. So defined, the basin has a 560 km diameter, an area of some 250 000 km2, and a 4900 m maximum stratal thickness. It extends north into southwestern Manitoba and across southern Saskatchewan as a broad, southeasterly plunging trough, some 3200 m below ground level at the Saskatchewan/U.S. border near longitude 104 °W (see Fig. 3.2, this volume).
The basin is bordered to the east by the Sioux Arch of the Dakotas and southeastern Manitoba, and to the north by the Punnichy Arch fronting the Saskatchewan monocline. The latter rises at a rate of 2 to 4 m/km northeastward onto the Severn Arch of central Manitoba. The western limit is the Sweetgrass Arch of northern Montana and southeastern Alberta. The structural definition notwithstanding, sedimentary strata of the Williston Basin were laid down in greatly expanded versions of the present basin. These expansions were countered by flanking uplifts and peripheral stripping of strata, imprinting the present configuration on the basin (Fig. 27.1).The net effect is that the Phanerozoic isopach map resembles the structural map. The well-log cross section of Figure 27.2 further illustrates the destructional/constructional phases of development of the basin. In particular, it demonstrates that the basin configuration seen in isopach maps of lower Paleozoic strata (Norford et al., this volume, Chapter 9) is due to peripheral thinning by both erosion and depositional onlap.
Historically, the Sweetgrass Arch refers to a large structural complex situated in the region of northwestern Montana, southeastern Alberta and southwestern Saskatchewan. Herbaly (1974) demonstrated that the arch may have been active during several stages of geological time. The most intense periods of activity were during the pre-Devonian (Fig. 27.3) and the pre-Jurassic (Fig. 27.4). In its post-Laramide configuration the arch is a northward-plunging compound antiform made up of the South Dome at Great Falls, Montana and the Kevin-Sunburst Dome at the Montana-Alberta border (Fig. 27.5). North of the border, it elongates into a nose overlooking the broad saddle of the Alberta Platform, and rises beyond onto the south-southwesterly plunging North Battleford Arch. As mapped on the Devonian system by Kent (1968), the Sweetgrass Arch is crowded against the steeply dipping and thrusted Rocky Mountain geosyncline to the west. Toward the east the slope falls across Saskatchewan unevenly but more gently around subsidiary domes, terraces and troughs toward the Williston Basin some 500 km away. However the eastern slope in Montana is relieved by domal forms towering up to four times the height of the Arch. These are Late Cretaceous and Tertiary laccoliths represented by the Bearpaw, Little Rocky and Highwood mountains and the nonintrusive Cypress Hills and Bowdoin Dome. This region fronts the northern side of the easterly trending central Montana uplift comprising the Little Belt and Big Snowy mountains.
From a stratigraphic perspective the eastern expanse of the Sweetgrass Arch is represented by the Swift Current Platform of southwestern Saskatchewan and the adjacent Central Montana Platform, both of which have alternated, by upwarp and downwarp, as extensions of the Arch and of the Williston Basin (Christopher, 1990). A third element of the region is the easterly trending Belt-Big Snowy Trough, terminating the arch to the south as a deep re-entrant to the Williston Basin from the Rocky Mountain geosyncline (Peterson and MacCray, 1987).
In this chapter, the arguments bearing on the origin of the basin are reviewed, as well as its subsidence history and the impact that basin-margin and intrabasin uplifts have had on its sedimentary history. The most influential of these uplifts is the Sweetgrass Arch. Kent (this volume, Chapter 7) shows that the pre-foreland basin phase of the Williston Basin played an integral role in the evolution of the cratonic platform; therefore the chronology of depositional events employed here is the same as that established in Chapter 7.
The early Williston Basin and Sweetgrass Arch were integral components of the western margin of the proto-continent. Two important events in that period were the Antler and Nevadan- Columbian orogenies. Each influenced the pattern of sediment accumulation in the basin. Pre-Antler deposits extended well beyond the present structural limits of the basin. By contrast, post-Antler sedimentation was restricted to the basin and a relatively narrow shelf surrounding it (Kent, this volume, Chapter 7). Antler tectonics may have been responsible for bringing to prominence an ancestral Sweetgrass Arch, over which the depositional strike changed from essentially east-west and parallel to the Belt-Big Snowy aulacogen, to north-northwest along the proto-continental margin (Kent, this volume, Chapter 7). The Nevadan-Columbian Orogeny changed the depositional framework on the western cratonic platform including the Williston Basin (Smith, this volume, Chapter 17) by providing a source for thick successions of siliciclastic sediments, whereas in the pre-Nevadan-Columbian, autochthonous carbonates were dominant.
Last modified: August 20, 2008
