Paleoenvironmental and Paleogeographic Reconstructions of the Turonian to Santonian Western Interior Sea, Canada

The Upper Cretaceous upper Colorado Group, including the Carlile and Niobrara formations, reveals a complex and dynamic marine episode of the Western Interior Sea. Relative sea-level fluctuations played a major role in the facies distribution, paleogeography and sediment infill history. This study generates paleogeographic maps for the Late Turonian to Santonian that are based on data from southern Alberta and southwestern Saskatchewan integrated with time-equivalent strata in neighboring regions in the Canadian Foothills and northwestern U.S. The Interior Seaway reached its maximum extent during the peak transgression of the Greenhorn Cycle (Lower to Middle Turonian) and the basin was gradually infilled starting in the Late Turonian with the Carlile Formation. The boundary between the Carlile and Niobrara formations is an angular unconformity where up to 20 m of the upper Carlile Formation was eroded. In the late Late Turonian, the Interior Seaway was a narrow strait of a few hundred kilometers with marginal marine sandstones deposition in western Alberta (Cardium Formation) and central Saskatchewan/north central Montana (Bowdoin sandstone).

The transgressive part of the Niobrara Cycle (Coniacian Verger Member) shows multiple dramatic basin changes. Two lowstand events indicate complete basin drainage interrupting periods of marine mudstone deposition under shelfal to offshore conditions. The Medicine Hat Member (Early to Middle Santonian) represents a change from a shaly to a sandy shelf. Nine sandstones in the vicinity of the Bow Island Arch in southern Alberta were mapped. During lowstand periods with low accommodation space, the Bow Island Arch affected sedimentation, whereas during high accommodation space, the arch had no influence. The maximum extension of the Interior Seaway during the Niobrara Cycle occurred in the Late Santonian with the deposition of the First White Specks Member. This study identifies 13 sequences based on major flooding surfaces, their stratal pattern and sediment source directions.