Geological Atlas of the Western Canada Sedimentary Basin - Chapter 22

Chapter 22
Cretaceous Dunvegan Formation of the Western Canada Sedimentary Basin

Author:
J.P. Bhattacharya - ARCO Exploration and Production Research Technology, Plano, Texas

Introduction

General Description

The middle Cenomanian (lowermost Upper Cretaceous) Dunvegan Formation is a lithostratigraphically defined unit that comprises an extensive, southeasterly thinning, sandy clastic wedge confined to northwestern Alberta, northeastern British Columbia, and extending as far north as the Northwest Territories (Fig. 22.1). It presently covers an area of about 300 000 km². This includes undeformed and relatively flat-lying strata, which lie in the subsurface south of the Peace River, and which are exposed along the Peace River valley and farther north (Fig. 22.2). Where the Dunvegan is not structurally deformed, it dips gently to the southwest (Fig. 22.2). To the west, along the central and northern foothills, the Dunvegan Formation becomes incorporated into the tectonically deformed belt, where it is also exposed.

The Dunvegan Formation attains thicknesses of about 350 m, and consists of interbedded mudstone, sandstone, and conglomerate. The conglomeratic facies are confined to the outcrops in the far north (Fig. 22.1b). Thinner beds of coal and shelly limestone also are present in places. The Dunvegan cannot be mapped much farther south than the Athabasca River (Fig. 22.3), although in the subsurface, age-equivalent mudstones can be traced throughout the basin (Fig. 22.4). The Dunvegan Formation is overlain by shales of the Kaskapau Formation and underlain by shales of the Shaftesbury Formation, although the relations are better described as interfingering, since both the upper and lower boundaries are highly diachronous (Fig. 22.4).

Hydrocarbon-bearing intervals are apparently confined to the Alberta subsurface, especially in the area between Twp. 50 and Twp. 70 in ranges west of the 5th meridian (Fig. 22.2). Farther north, Doe Creek sandstones of the lower Kaskapau are also productive (Wallace-Dudley and Leckie, in press).

Tectono-Eustatic Framework

Stott (1984) interpreted the Dunvegan as having been derived from the actively rising Cordillera to the northwest during the waning phases of the Early to mid-Cretaceous Columbian Orogeny. Cant and Stockmal (1989) suggested that this may have related to accretion of the Cascadia terrane in the Western Cordillera in southern British Columbia. The wedge-shaped geometry of the Dunvegan shows that tectonic subsidence was the major control on shaping the foreland basin fill at this time. To the southwest, the Dunvegan was increasingly affected by Tertiary deformation of the Laramide Orogeny, resulting in destruction of the western edge of the Dunvegan depositional basin. The present-day southwest structural dip of the Dunvegan in subsurface (Fig. 22.2) also results from depression of the Alberta Foreland Basin as a consequence of tectonic loading.

The Dunvegan wedge built into the Cretaceous Western Interior Seaway, which is interpreted to have been open all the way to the south during the mid-Cenomanian (Fig. 22.1a). Progradation was controlled, in part, by a third-order eustatic drop of sea level (Bhattacharya, 1988; Bhattacharya and Walker, 1991a), dated at 94 Ma on the global sea-level curves published by Haq et al. (1987; Fig. 22.5).

Previous Work

The name Dunvegan Group was first used by Dawson (1881) to describe his "Lower Sandstones" which cropped out along the Peace River valley. The formation was named after the Dunvegan Trading Post situated by the Peace River (Twp. 80 Rge. 3W6), although only the upper, largely nonmarine portion of the Dunvegan is exposed there. McLearn (1919) subsequently changed the Dunvegan from group to formational status. Biostratigraphically oriented work, centered on the age and correlation of the Dunvegan, Kaskapau, and Shaftesbury formations, was completed by researchers at the Alberta Research Council and the University of Alberta (Stelck and Wall, 1955; Stelck et al., 1958) and has been summarized by Caldwell et al. (1978). More recent work by Singh (1983) details the palynological aspects of the Dunvegan and their implications for Cenomanian biostratigraphy.

Stott (1982) completed a major study of the Dunvegan in outcrop and included a detailed historical summary of early work, including definition and nomenclature of the formation. In much of this previous work, the Dunvegan was interpreted as broadly deltaic, and the term "Dunvegan Delta" was applied to this rather complex sedimentary package. Stott's mapping and paleogeographic interpretations are incorporated into Figures 22.1, 22.2, and 22.3. More detailed sedimentological descriptions of selected Dunvegan outcrops, originally mapped by Stott (1982), have been published by Plint and Hart (1988). The only detailed petrological work is that published by Tater (1964), who examined sandstones in the type area at Dunvegan.

Previous subsurface work is that of Burk (1963). Burk (1963) presented a gross sandstone isolith map of the Dunvegan and established the overall southeast thinning in the subsurface, although he did not make any attempt at further stratigraphic subdivision. Bhattacharya (1988, 1989a,b and 1991) and Bhattacharya and Walker (1991a,b) completed a major study of the Dunvegan in the Alberta subsurface (Fig. 22.1b). Their work involved the application of sequence stratigraphic principles, as outlined by Van Wagoner et al. (1990). In designating the stratigraphic subdivisions they used allostratigraphy (North American Commission on Stratigraphic Nomenclature, 1983). An allostratigraphic unit is defined on the basis of its bounding discontinuities, rather than on gradational facies boundaries. Throughout this chapter, sequence stratigraphic terminology is indicated in brackets as it relates to the interpretations presented.