The Viking Formation and equivalent strata are Albian in age and occur within the Cretaceous Colorado Group (Leckie et al., this volume, Chapter 20). Viking and equivalent units (Bow Island Formation, Paddy Member, Pelican Formation, Newcastle Member) extend over practically the entire Western Canada Sedimentary Basin (Figs. 21.1, 21.2). They represent a wedge-shaped coarse clastic interval that prograded from the Cordilleran orogenic belt eastward into the foreland basin. Interbedded, predominantly marine-influenced sandstones and shales make up the Viking and equivalent units, which range in thickness from a few metres on the eastern margin of the basin to over 175 m in southwestern Alberta. The Viking and Bow Island formations, and Paddy Member have proven to be prolific oil- and gas-bearing units (Fig. 21.3) and, as such, have been the target for exploratory drilling for several decades. Recent estimates indicate that Viking and equivalent strata contain in the order of 5 and 8 percent, respectively, of the oil and gas reserves known to exist in Alberta alone.
This chapter deals in a general sense with regional stratigraphic relations in the Viking clastic wedge, primarily in the subsurface foreland basin of the Western Canada Sedimenary Basin south of 60° latitude. The Dunvegan and Cardium clastic wedges are described in succeeding chapters (Bhattacharya, this volume, Chapter 22, and Krause et al., this volume, Chapter 23, respectively), and basin-wide relations of all units are documented in the preceding chapter (Leckie et al., this volume, Chapter 20). Detailed stratigraphic and sedimentological relations within and between the Viking Formation, Bow Island Formation and Paddy Member are also summarized here. It should be noted, however, that with the current explosive interest in the application of sequence stratigraphic principles, theories on the development and correlation of Viking and equivalent units continue to evolve rapidly.
Much has been written about the Viking Formation because of its historical record as an important hydrocarbon-bearing reservoir in Western Canada. The last basin-wide synthesis of Viking and equivalent units was published in the first Western Canada atlas (Rudkin, 1964). This synthesis was built upon the subsurface investigations of earlier workers such as Glaister (1959), Workman (1959), Stelck (1958) and Hunt (1954). Relevant to Viking and equivalent strata are the papers that focus on the paleogeography of the Colorado Group on a basin-wide scale, including Williams and Stelck (1975), Caldwell (1984), and Smith (this volume, Chapter 17). Regional syntheses of Viking-equivalent strata in Manitoba are those of McNeil and Caldwell (1981) and McNeil (1984). Our understanding of Viking stratigraphy in Saskatchewan is based on the classic paper of Jones (1961), which was followed by a report by Simpson (1982) on the entire Lower Colorado of west-central Saskatchewan.
The need to understand more about where Viking and equivalent hydrocarbon reservoirs can be predicted to occur, and how they can be effectively exploited and developed, led to several, detailed area-specific studies that emphasized depositional controls and environmental interpretation. Linear Viking sandstone reservoirs have been interpreted as deep-water turbidites (Beach, 1956; Roessingh, 1959), nearshore and barrier shoreline deposits (DeWiel, 1956; Shelton, 1973; Tizzard and Lerbekmo, 1975), and offshore shelf sandstone bodies (Evans, 1970; Koldijk, 1976; Boethling, 1977; Beaumont, 1984; Hein et al., 1986; Amajor and Lerbekmo, 1990). Prior to the mid-1980s, Viking sandstone geometries were thought of as relatively long, but narrow, northwest- to southeast-trending bodies less than 5 m in thickness. The documentation of a major unconformity and an estuarine valley-fill deposit within the Viking Formation (Reinson, 1985; Reinson et al., 1988) led to the recognition of similar such deposits elsewhere in equivalent strata (Boreen and Walker, 1991; Leckie and Reinson, in press; Pattison, 1991; Leckie and Singh, 1991).
With the rebirth in the 1980s of transgressive-regressive cycle analysis and the tectonism versus eustasy conundrum in the form of modern "sequence stratigraphy" (Walker, 1990), the above-cited papers have documented unconformities, incised valley-fills and sequence-parasequence boundaries in Viking and equivalent strata, as indicators of basin-wide relative sea-level fall, subsequent transgression, and minor sea-level oscillations. The Viking Formation (along with equivalent units) is now one of the most highly-studied intervals in the subsurface of Western Canada. As such, new studies and re-examinations of older works continue to be published, and our basin-wide interpretations remain in a state of flux.
The Viking Formation and equivalent strata were deposited in the Western Canada foreland basin which, by Late Albian time, was totally flooded by a shallow epicontinental seaway extending from the Arctic Ocean to the Gulf of Mexico (Williams and Stelck, 1975; Caldwell, 1984). The basin fill consists of thick wedges of westerly-derived coarse clastics alternating with thick, marine shale successions (Stott, 1982). The individual clastic wedges, in this case the Viking, record an episode of major tectonic activity along the marginal orogenic belt, resulting in a high rate of sediment availability and the necessary depositional gradient to effect rapid progradational input into the foreland basin. The Viking clastic wedge was terminated by a major, possibly global, rise of sea level, during Late Albian time (Vail et al., 1977; Weimer, 1984), which inundated the entire basin. Details of the major clastic wedges and sequential basin fill are discussed in Chapter 20.
Major structural elements are evident on the structure map (Fig. 21.3). Positive features include the Bow Island (Sweetgrass) Arch situated in southeastern Alberta and the Swift Current Platform in southern Saskatchewan. A positive platform area also occurs in northwestern Alberta-northeastern British Columbia, on the north side of the Peace River Arch. The Peace River Arch began to subside during the Mississippian. It was still subsiding during Paddy-Cadotte deposition (Leckie et al., 1990), and in fact remained a basin from Early Cretaceous to early Tertiary time (Cant, 1988). The isopach map of Viking-equivalent strata in the Peace River Arch area reflects this subsidence, as evidenced by a thickening trend coincident with the arch (Fig. 21.4). In southern Alberta, the thick Bow Island deposits probably reflect high sediment input from the southwest rather than similar collapse of the Sweetgrass Arch during this interval of Cretaceous time.
Last modified: August 20, 2008
