Evolution of the northern Cordilleran foreland basin during the middle Cretaceous

Abstract

A detailed study of the middle–late Cenomanian Belle Fourche Formation in southern Alberta helps to elucidate the evolution of the Cordilleran foreland basin during the middle Cretaceous. By combining isopach data from the study area with previously published research on middle Cretaceous stratigraphy in adjacent areas of northern Alberta and Montana, it is possible to defi ne the position of the proximal foredeep, forebulge, and backbulge depozones from late Albian to Cenomanian time. The foredeep probably lay to the west of the present fold-thrust belt in western Montana from the late Albian to middle Cenomanian, with the forebulge extending northwestward through the northwest corner of Wyoming and curving northeastward through the present-day location of Calgary. Thick Dunvegan deposits (middle Cenomanian) of northwestern Alberta are interpreted as foredeep deposits. 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. At the end of the middle Cenomanian the forebulge retreated westward within Alberta, defi ning 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 middle Cenomanian may refl ect the change of convergence vectors along the western margin of North America during the middle Cretaceous.