Evolution of the northern Cordilleran foreland basin during the mid-Cretaceous

Using major flooding surfaces as the boundaries of allomembers, the Cenomanian Belle Fourche Formation is subdivided into five allomembers A-E in southern Alberta. By combining isopach data of the allomembers A-E from the study area with previously published research on the mid-late Cenomanian stratigraphy in adjacent areas of northern Alberta and Montana, it is possible to define the position of the proximal foredeep, forebulge and backbulge depozones in the northern Cordilleran foreland basin.

The foredeep was probably located to the west of the present fold-thrust belt in western Montana during the mid-Cenomanian, with the forebulge extending northwest through the northwest corner of Wyoming and curving northeastward through the present-day location of Calgary. Thick Dunvegan deposits of northwestern Alberta are interpreted as foredeep deposits. At the end of the mid-Cenomanian, the forebulge retreated westward within Alberta, defining a nearly straight northwest-southeast trend through the present position of the Foothills and Front Ranges of Alberta and northwestern Montana. The change in trend of the foreland basin at the end of the mid-Cenomanian may reflect the change of convergence vectors along the western margin of North America during the mid-Cretaceous.

Palinspastic reconstruction of the southern Rocky Mountains of Alberta is consistent with a location of the foredeep within the area of the present fold-thrust belt. However, the foredeep and part of the forebulge of the foreland basin in the present Rocky Mountains in southern Canada and northern United States have been uplifted and cannibalized by post-depositional thrusting and shortening during the Late Cretaceous-Paleocene.