Sedimentology and geologic history of the Upper Devonian (Frasnian) uppermost Ireton and Nisku Formations, Bashaw area, Alberta

Abstract

The Camrose Member of the Ireton Formation, and the Nisku Formation (Frasnian) of the subsurface of the Bashaw area, Alberta represent episodes of shallow water carbonate and evaporite sedimentation, influenced by antecedent topography of the underlying Leduc Formation Bashaw carbonate complex. Fifteen lithofacies can be discerned, and range environmentally from deep, open carbonate ramp settings, through ramp-platform transition tabular stromatoporoid-coral communities to restricted platform interior subaqueous to peritidal carbonates and evaporites. In addition, terrigenous elastic sediments become increasingly common in the upper part of the Nisku Formation. Two orders of cyclicity are present within the Camrose Member and Nisku Formation in the Bashaw area. The first is on the scale of greater than 10 m and consists of shallowing upwards sequences ranging from deeper ramp to peritidal carbonates and evaporites. These cycles are best expressed on the west and north side of the underlying Bashaw complex at the ramp-platform transition. Smaller scale cycles less than 5 m thick typify platform interior settings and range from restricted subtidal carbonates to peritidal carbonates and evaporites. Fluctuations in basin subsidence and eustatic sea level were the ultimate control on sedimentation in the Upper Frasnian of the Bashaw area, with carbonate sedimentation dominating during periods of rapid relative sea level rise, and terrigenous elastic sediments prograding into the area from northeastly sources during periods of lower relative sea level rise or stillstands in sea level. Autocyclic mechanisms, particularly in platform interior settings, exerted a strong control on observed cyclicity, and depend upon high sediment production and accumulation rates associated with shallow water carbonate sedimentation. The underlying Leduc Formation Bashaw complex exerted a strong control on sedimentation of the Camrose Member and Nisku Formation. North and west sides of the Bashaw complex acted as relict highs and localized tabular stromatoporoid communities during Camrose Member and Nisku Formation sedimentation. Growth of ramp-platform transition stromatoporoid communities acted to restrict circulation in a platformward direction, leading to establishment of locally complex restricted subtidal to peritidal environments. Camrose Member and Nisku Formation sediments can be traced regionally westward through the Leduc area and to the West Pembina area, where downslope 'pinnacle' reefs comprised of Disphyllid corals and tabular stromatoporoids occur, and are time equivalent to Nisku Formation ramp and platform sediments in the Bashaw area.