Migration and stratigraphic fill of an underfilled foreland basin: Middle–Late Cenomanian Belle Fourche Formation in southern Alberta, Canada

The geometry of a foreland basin is mainly a product of a complex dynamic balance between the orogenic loading, erosion and sedimentation, and lithospheric flexural response. To improve our understanding of the relationships between these processes and the foreland basin stratigraphy, a detailed stratigraphic and sedimentologic study of the Middle–Late Cenomanian Belle Fourche Formation in the subsurface of southern Alberta is presented in this paper. The Middle Cenomanian lower Belle Fourche Formation and the Late Cenomanian upper Belle Fourche Formation were deposited in underfilled conditions, showing prominent forebulge zones. It is demonstrated that during the Late Cenomanian with the cratonward migration of the uplifting forebulge zones, the axes of backbulge zones also migrated cratonwards and a wide uplifted forebulge zone was formed. During the last stage of the underfilled condition, the foreland basin had a relatively flat topography and fine sands were deposited along the flank of the uplifting forebulge. A qualitative model for the migration and stratigraphic fill of an underfilled foreland basin is proposed. This model provides a qualitative pattern of the cratonward migration of the forebulge zone and backbulge zone within a several-million-year tectonically-driven cycle. It is suggested that during the early underfilled period, the basin geometry is mainly controlled by the orogenic loading, and a prominent forebulge zone with an approximately fixed location is formed. During the late underfilled period, the basin geometry is mainly controlled by the sediment loading, and the forebulge zone move very fast along with the rapid cratonward shift of the sediment loading center. Over a longer time period and between different tectonically-driven cycles, along with the cratonward propagation of the thrusting wedge, the orogenic loading plays a more important role than the sediment loading for the migration of the forebulge zone.