Geological Atlas of the Western Canada Sedimentary Basin - Chapter 2

Chapter 2
Cordilleran Tectonics and the Evolution of the Western Canada Sedimentary Basin

Author:
R.A. Price - Queen's University, Kingston

Introduction

The Cordilleran Connection

The connection between the Western Canada Sedimentary Basin and global plate tectonics lies in the Cordillera, because the origin and evolution of the Western Canada Sedimentary Basin was linked inextricably to the origin and evolution of the Cordillera, and thereby to the global plate tectonic processes that produced the Cordillera. Episodes of epeirogenic subsidence and sediment accumulation in the Western Canada Sedimentary Basin generally can be ascribed directly to the effects of concurrent episodes of orogenic deformation in the Cordillera (Porter et al., 1982), and, thereby, indirectly to the displacements between the North American craton and the adjacent lithospheric plates to the west of it that produced the Cordillera (Monger and Price, 1979). The main purpose of this chapter is the elucidation of the Cordilleran connection.

The Supracrustal Wedge

The Western Canada Sedimentary Basin, as viewed from the perspective of a cross section of the continental lithosphere, is a very thin, northeastward-tapering wedge of supracrustal rocks overlapping the Precambrian crystalline rocks that form the core of the North American craton. The thickness of this supracrustal wedge increases gradually southwestward, over a distance of between 600 and 1200 km, from a zero edge along the exposed margin of the Canadian Shield, to between 3 and 5 km at the northeastern margin of the foreland thrust and fold belt (Wright et al., this volume, Chapter 3), and even there it comprises only 10 to 15 percent of total thickness of about 40 km of continental crust (Richards, 1958).

The thickest and stratigraphically most complete part of the supracrustal wedge occurs farther west, within the eastern part of the Cordillera (Fig. 2.1), in the Rocky Mountain Foreland Thrust and Fold Belt and the eastern part of the Omineca Belt (Bally et al., 1966; Price, 1981; Price and Mountjoy, 1970; Thompson, 1979). Stratigraphic relations in this part of the supracrustal wedge are partly obscured by deformation, regional metamorphism and granitic plutons because this part of the wedge has been incorporated into the accretionary prism that separates the North American craton and its cover of autochthonous supracrustal rocks from the tectonic collage of allochthonous terranes that make up the main mass of the Cordillera. The supracrustal rocks within the accretionary prism have been detached from their basement and displaced northeastward. They were scraped off the North American craton and accreted to the overriding tectonic collage of allochthonous terranes. Although this part of the supracrustal wedge has been horizontally compressed and tectonically thickened by folding and imbricate thrust faulting, palinspastic reconstructions of the deformed rocks in the accretionary prism show that prior to the formation of the accretionary prism, the thickness of the supracrustal wedge increased southwestward relatively abruptly, over a distance of 200 km or less, from between 3 and 5 km at the present position of the northeastern margin of the foreland thrust and fold belt to between 10 and 15 km along the former position of the Paleozoic and early Mesozoic continental margin of North America (Bally et al., 1966; Price and Mountjoy, 1970; Thompson, 1979; Price, 1981; Price and Fermor, 1985). The birth, growth and deformation of this part of the supracrustal wedge is the main focus of this chapter because it holds the key to the understanding of the origin and evolution of the remaining undeformed part of the Western Canada Sedimentary Basin that lies east of the Rocky Mountain Foreland Thrust and Fold Belt, beneath the western plains.

Stages of Tectonic Evolution

Two main stages in the development of the Western Canada Sedimentary Basin are distinguished by a profound change in provenance of the clastic sediment preserved within the supracrustal wedge (Bally et al., 1966; Price and Mountjoy, 1970). A Late Proterozoic to Late Jurassic miogeocline-platform stage, during which the main external source of the sediment was to the northeast on (and beyond?) the present North American craton, has been correlated with the continental rifting and drifting that created the initial Cordilleran continental margin of the North American craton and its adjacent ocean basin, and subsequently, the continental terrace wedge (miogeocline) that was prograded outboard from this "passive margin" (Stewart, 1972; Monger and Price, 1979). A Late Jurassic to Early Eocene foreland basin stage, during which the main source of sediment was to the southwest in the emerging Cordilleran mountain belt, has been correlated with the accretion of a tectonic collage of allochthonous oceanic terranes that occurred following the subduction of intervening oceanic lithosphere, and consequent closure of intervening ocean basins (Davis et al., 1978; Monger and Price, 1979). During the foreland basin stage, as a result of oblique collision between the accreted terranes and the North American craton, the outboard part of the miogeocline-platform component of the supracrustal wedge was detached from its basement, displaced northeastward, compressed and thickened. The weight of the displaced and tectonically thickened supracrustal rocks induced subsidence of the foreland basin (Price, 1973; Beaumont, 1981), and the associated uplift and erosion provided much of the sediment that accumulated in the foreland basin. This cannibalization of the supracrustal wedge continued as some of the older deposits of the foreland basin component were themselves detached from their North American basement, attached to the colliding accreted terranes, and uplifted and eroded to provide the sediment that formed some of the younger foreland basin deposits. The pattern of growth of the foreland thrust and fold belt, and of the foreland basin component of the supracrustal wedge, were effectively terminated by an episode of Early and Middle Eocene crustal extension in the central part of the Cordillera that marked the transition to the present-day plate tectonic regime (Ewing, 1980; Price, 1979; Price, 1986).