LOWER JURASSIC FORELAND BASIN IN WESTERN CANADA

The earliest stages of the Cordilleran foreland basin in western Canada, namely the question of how and when the region evolved from a stable margin to a retro-arc foreland basin, remain enigmatic. Here we integrate subsidence analysis with existing datasets to shed light on the early stages of foreland basin development, which in turn aids our understanding of the nature and timing of tectonic mechanisms that controlled deposition during Jurassic times. A more detailed understanding of the initiation of the foreland basin in Western Canada provides the basis for a comparison of timing and initiation of the foreland basins along the western margin of North America. In particular, we can begin to understand proposed differences in timing across the Canada-United States border.

Jurassic strata preserved in Alberta are interpreted to be part of a foreland basin system. The uppermost Jurassic deposits are generally agreed to be the first foredeep deposits. We propose that Lower Jurassic strata were deposited in a backbulge depozone associated with an earlier foredeep which has since been reworked by the advancing fold-thrust belt. The unconformities overlying these backbulge deposits are interpreted to be the result of low accommodation in a forebulge setting.

Several lines of evidence support the interpretation that the Lower Jurassic strata were deposited in a backbulge setting. Isopach patterns show thinning of the strata toward both the craton to the east and the orogen to the west, suggesting the presence of a bathymetric high between these deposits and the orogen. Geochemical data suggests these strata were deposited in a restricted marine environment, consistent with a backbulge setting. Subsidence curves from over 100 subsurface wells in western Alberta show low subsidence rates during deposition of the Lower Jurassic units, further evidence that the initiation of the foreland basin in Western Canada occurred earlier than previously suggested. A backbulge depozone is further supported by structural data that indicate deformation and tectonic loading along the western margin of North America had begun by Early Jurassic time in southern Canada.

These new data support the hypothesis that the initiation of the foreland basin in Canada differed from that in the United States.