The formation of a Paleozoic passive margin on the western side of the North American proto-continent played an integral part in the growth of the Western Canada Sedimentary Basin. To consider the entire 4000 km length of this western trailing edge as a nontectonic margin would be an oversimplification, given that at least the United States portion was subjected to intense tectonic activity. The end product of this activity was the Antler Orogeny. This event had a significant influence on the late Paleozoic growth of the cratonic margin and platform.
The stratigraphy of the cratonic platform and margin is shown in Figure 7.1, in simplified form. Twelve maps depicting the interpreted paleogeography at selected lithostratigraphic levels illustrate the evolution of the cratonic platform and margin. Each interval is highlighted on the correlation charts accompanying the maps. The maps are a synthesis of the most recent literature on the depositional environments and distribution of facies for the stratigraphic units that they represent. Where there is a conflict of interpretation, the majority opinion is accepted; where there is no majority opinion, a compromise is presented. Because this is a synthesis, accuracy in the location of facies boundaries, erosional edges and stratigraphic equivalence is slightly distorted, for improved clarity. On all of the maps, areas of solid colour depict the preserved distribution of facies belts, the hachured areas their inferred original extent prior to erosional removal.
The North American proto-continent was isolated from a Late Proterozoic supercontinent by multi-phase rifting. The rifting created eastern and western continental margins, in the period between 730 and 555 Ma.
Ross et al.(1989) inferred that continental margin sedimentation began on the western side from about 730 to 726 Ma. According to Bond et al. (1984), initiation of the Iapetus Sea, creating the eastern continental margin, occurred probably between 625 Ma and 555 Ma.
Prior to the rifting that created the proto-Pacific passive margin, thick sequences of siliciclastics and carbonates belonging to the Purcell and Belt supergroups were deposited in intracratonic basins, created by early partial rifting (Winston et al., 1984). The thickness of this Upper Proterozoic sequence, some 15 km, and the presence of basaltic flows at various stratigraphic levels, implies a possible aulacogen as the site of sedimentation.
Ross et al. (1989) suggested that the rocks of the Windermere Supergroup that overlie the Purcell may represent, in part, initial continenal margin sediments. However, they suggest that paleocurrent evidence indicates that the sediments have an eastern and western source and that deposition appears to have been in a northwesterly trending deep trough, interpreted as being an indication of a rifted valley setting. The basal deposits of the supergroup are thought to be diamictites of glacial origin. The remainder of the succession appears to represent a shallowing-upward sequence that includes: graded pebbly and coarse-grained sandstone, and laminated mudstone passing upward into sandstone, siltstone, mudstone and carbonate. The lower strata are thought to represent turbidite and submarine-fan deposits and the upper, some of which contain trace and body fossils, are thought to be of shallow-water shelf origin. Readers are referred to Hein and McMechan (this volume, Chapter 6) for details of Proterozoic tectonics and sedimentation.
Last modified: August 7, 2008
