Allostratigraphic subdivision of the Upper Cretaceous Dunvegan, Shaftesbury, and Kaskapau formations in the northwestern Alberta subsurface

The mid-Cenomanian Dunvegan Formation is a lithostratigraphically defined clastic wedge comprising sandstones, siltstones, and mudstones. Over the 30,000 km2 study area, the Dunvegan Formation has been subdivided into 7 allomembers (from youngest to oldest, A to G). The bounding discontinuities that are used to define the successive allomembers are represented by major marine flooding surfaces (MjF surfaces). The allomembers cut across the conventional lithostratigraphic boundaries and extend into the underlying Shaftesbury and overlying Kaskapau formations. Each allomember is heterolithic and comprises sets of offlapping (regressive) units, here termed shingles, punctuated by a thin transgressive unit. Downdip, the facies successions within the shingles usually coarsen upward. Updip they may be be represented by fining-up.ward channel fills. Each allomember is interpreted as representing a genetically related package of sediment characterized by progradation but including the thin transgressive unit above. A radioactive shale horizon (FSU marker) occurs throughout the study area. It lies between 10 and 40 m above the Fish Scale Marker Horizon within the underlying upper Shaftesbury. The allostratigraphic relationships show that the lower allomembers pass into the upper Shaftesbury Formation and downlap onto this marker, which is interpreted as a major condensed section. The Shaftesbury shales above the FSU marker are interpreted as being genetically related to the prograd-ational phase of the Dunvegan Formation. This progradational phase is erosionally truncated by a widespread transgressive surface of erosion (TSE) in allomember C. The overlying allomembers B and A pass laterally into shales of the Kaskapau Formation and are interpreted as being genetically related to the ensuing Kaskapau transgression. Progradation of the Dunvegan Formation is thought to be driven, in part, by a third-order eustatic lowstand of sea level. The allomember bounding MjF surfaces are interpreted as being a result of superimposed, 4th-order, relative rises of sea level, probably caused by tectonically induced subsidence. The shingles are interpreted as representing the deposits of individual deltaic and shallow marine depositional systems and are largely autocyclic in nature.