BEGINNING OF FORELAND SUBSIDENCE IN THE COLUMBIAN-SEVIER BELTS, SOUTHERN CANADA AND NORTHWEST MONTANA

Subsidence analysis and geometry of Jurassic-Cretaceous foreland strat a in northwestern Montana and southern Alberta and British Columbia su ggest that loading by the fold-thrust belt in Canada began as much as 40 m.y. earlier than in Montana, In Canada, early foreland basin depos its are Late Jurassic age, thicken rapidly westward, and are restricte d to a narrow belt within 30 km of the thrust belt. In western Montana , contemporaneous deposits are widespread and do not increase markedly in thickness toward the thrust belt. The unconformity overlying these deposits also changes from Canada, where it is angular, to a disconfo rmity in western Montana near Great Falls. Between these two areas, fo reland geometry is transitional over a distance of <250 km, Beyond the transition zone, early foreland basin geometries are broadly consiste nt, showing Late Jurassic foreland subsidence in southern Canada and E arly Cretaceous initial subsidence in the United States.