Abstract

The Carlile and Niobrara formations were deposited during the regressive phase of the Greenhorn cycle and the transgressive phase of the Niobrara cycle (early Turonian-late Santonian). The formal lithostratigraphic definition and correlation of these units in the Canadian portion of the Western Interior Basin (WIB) is still debatable, due to the paucity of litho- and biostratigraphic data, particularly in the province of Saskatchewan. In this study, we present results from micropaleontological and lithostratigraphic analyses of the Carlile and Niobrara formations based on the logging and sampling of five cores distributed along the southern portion of the province (between Townships 13 and 24). The uppermost part of the Second White Specks Formation and the lowermost part of the Milk River Formation were also included in this analysis to evaluate faunal variations with the underlying and overlying units, respectively.

The Whiteinella aprica Subzone (Muricohedbergella loetterlei Zone) represents the transgressive phase of the Greenhorn Cycle which occurred during the early Turonian. This maximum transgression was followed by a major environmental shift as a result of increased tectonic activity and a drop in eustatic sea level. During this time, agglutinated foraminiferal species of the Pseudoclavulina sp. Zone dwelled at the sea floor, mainly in southwestern Saskatchewan, and were subsequently replaced by calcareous benthic and planktic species of the Gavelinella kansasensis Subzone (Trochammina sp. Zone). The initial stage of the transgressive Niobrara Cycle (early Coniacian) is characterized by increased volcanic activity, the disappearance of foraminifera, and the first occurrence of radiolarians at the base of the Govenlock Member of the Niobrara Formation. After the volcanic event, foraminifera of the Marsonella oxycona Subzone recolonized the sea floor followed by the highly diverse Bullopora laevis microfauna. The late stage of the transgressive Niobrara Cycle is represented by the benthic-dominated Gavelinella henbesti Zone and the planktic-dominated “Heterohelix cf. reussi“ Zone.