Allostratigraphy and River- and Wave-dominated Depositional Systems of the Upper Cretaceous (Cenomanian) Dunvegan Formation, Alberta

Abstract

<p>This study, of 130 cores and about 500 well logs, shows that the Dunvegan Formation (Cenomanian, Cretaceous Western Interior Seaway, Alberta) can be subdivided into seven allomembers, A through G. Each member represents a regression and contains several offlapping shingled sandstones and associated mudstones deposited in about 150,000 to 300,000 years. Members are separated from each other by bounding discontinuities produced by rapid transgressive flooding events.</p> <p>Isolith maps of sand body geometries, along with facies associations seen in core, show that individual shingles represent various types of prograding shallow marine to non-marine environments ranging from highly river-dominated deltas in the lower members (G, F, and E) to storm- and wave-dominated prograding deltas, barriers, and transgressive sheet sands in the upper members (D, C, B, and A). Shingles are autocyclic in nature and are related to river avulsion and delta switching while members are allocyclic in nature and are probably related to episodic thrusting events in the rising Cordillera to the northwest.</p> <p>The Dunvegan Formation was deposited during a time of active tectonism (Columbian Orogen), which caused rapid foreland- type subsidence but which did not allow conglomerate phases to be deposited on the shelf. A global third-order eustatic lowstand of sea-level is also associated with deposition of the Dunvegan.</p>