ABSTRACT

Proterozoic and Lower Cambrian sedimentary successions are dominantly clastic and virtually absent to the east of the Rocky Mountain Front Ranges, suggesting nondeposition and/or a profound sub-Middle Cambrian period of erosion in the autochthonous region of the Western Canada Sedimentary Basin. The observed stratigraphy consists of a series of stacked passive-margin sequences (separated by unconformities), which record several periods of rifting, thermal cooling, and continental-margin separation.

The initial Purcell sediments (maximum: 20 km thick; dated: 1570-1200(?) ma) are mainly shallow-marine and nonmarine in origin. They overlie fine grained basinal deposits to the west and thinner carbonates and clastics to the east. In Canada, these strata are interpreted mainly as deposits from delta progradation onto a subsiding miogeocline. This contrasts with various interpretations of the Belt Basin in the United States, including a two-sided intracratonic basin setting. The younger Windermere Supergroup (maximum: 9 km thick; dated 730-760 ma) are deep water grits and pelites unconformably overlain by the fluvial/shallow-marine Hamill Group or the shallow-marine Gog Group.

Broadscale correlations at different time-slices show that the proto-Pacific margin was generally oriented northwest-southeast. Sediment transport was mainly to the west-northwest, and sediment was derived both from the east and the west. The western side of the basin is not preserved. Some isolated offshore blocks created smaller half-graben subbasins, which complicated sediment dispersal patterns, particularly during upper Windermere and Hamill time. Syndepositional tectonism (including associated volcanics) occurred within the Hamill Group. Quantitative modelling indicates a cooling period following the bulk of Windermere sedimentation. Latest Proterozoic rifting led directly to the lower Paleozoic subsidence of the proto-Pacific margin.

End_of_Record - Last_Page 166-------