Sequence stratigraphy of a Late Devonian ramp-situated reef system in the Western Canada Sedimentary Basin: dynamic responses to sea-level change and regressive reef development



The Alexandra Formation, located in the Northwest Territories of Canada, is formed of a Late Devonian (Frasnian) reef system that developed on a gently sloping, epicontinental ramp in the Western Canada Sedimentary Basin. High-resolution sequence stratigraphic analysis of its deposits delineates two reef complexes that are separated by a Type I sequence boundary. The second reef complex developed on the outer ramp, basinward of the first, after sea-level fell ≈17 m. Stratigraphic complexity of the second reef complex was a result of its initiation during forced regression, and its development through an entire cycle of sea-level rise followed by sea-level fall. Its highstand systems tract was not characterized by high rates of carbonate production or sediment shedding. Rather, these features took place as sea-level fell, after its highstand systems tract. The sequence stratigraphic framework of this regressive reef system highlights a number of depositional parameters that differ from high-relief, shelf-situated reef systems with steep, narrow margins. These have implications for understanding the controls on the development of ramp-situated reef systems, and the nature of reef systems with gently sloping profiles. This study demonstrates that the development of stromatoporoid reef systems may be far more complex than generally realized, and that high-resolution sequence stratigraphy may provide the tools for better understanding of complex, often enigmatic, aspects of these systems.