Cenomanian to Santonian sea-level history from a proximal setting in the Western Canada Sedimentary Basin, southern Rocky Mountain Foothills, Alberta: Sedimentology, micropaleontology and organic geochemistry.

摘要

The Cretaceous (Cenomanian-Santonian) Upper Alberta Group is exposed along the banks of Mill Creek, southwestern Alberta. The outcrop exposes a nearly complete section through the Blackstone, Cardium and Wapiabi formations and as such is proposed as a reference section for the southern Rocky Mountain Foothills.; The Cretaceous interval records sedimentation during the Greenhorn and Niobrara sea-level cycles. The Vimy Member of the Blackstone Formation records maximum highstand during the Greenhorn cycle while the Cardium Formation records sea-level lowstand during the Coniacian. The Thistle Member of the Wapiabi Formation represents the highstand deposits of the Niobrara Cycle and was deposited under slightly shallower conditions than the Vimy Member.; Paleoenvironments of each unit have been reconstructed using paleontological, sedimentological and organic geochemical data. The paleoenvironments range from shoreline during Cardium time to outer shelf (water depth {dollar}sim{dollar} 100 m) during Vimy and Thistle times.; Foraminiferal assemblages are exclusively agglutinated, with the exception of those in the Vimy Member when conditions became deep and approached open marine to allow planktic foraminifera and nannofossils to prosper. Faunal changes through time are less pronounced at the Mill Creek location compared to distal parts of the basin. The more proximal position, characterized by shallow, increased energy conditions, prevented severe periods of bottom water anoxia and allowed shallow water assemblages, including species adaptable to lower salinities, to be sustained. Studies of concretions reveal, however, that anoxia was common within the sediment column and record the presence of fossil components not seen in the surrounding shales. Preferential taphonomy and carbonate dissolution may have also biased the foraminiferal assemblages by eliminating any calcareous components.; Organic geochemical results indicate that most of the organic matter in this system was of terrestrial origin and support a proximal setting. (Abstract shortened by UMI.)