High-Resolution Sequence Stratigraphic Study of the Blackstone Formation, Western Interior Basin, Canada- Implications for Interpreting the Stratigraphic Record of Marine-Shale Successions

Tyagi, Aditya¹, A.G Plint¹ (1) University of Western Ontario, London, ON

Sedimentological and stratigraphic concepts as applied to marine-shale dominated successions are still evolving. This study addresses some of the questions pertaining to the high-resolution sequence stratigraphic studies of such successions on a regional scale (200,000 Km²). Blackstone Formation (Cenomanian-Turonian) constitutes an important source rock in the Western Interior Basin, is a clastic wedge (40-500 m thick) deposited over a period of more than 3.5 my during the Greenhorn transgressive-regressive cycle. High-resolution sequence stratigraphic analysis has been carried out with a view to test the applicability of the existing models of sequence stratigraphy to marine-shale successions. Correlation of the regional flooding surfaces, discontinuity surfaces, bentonites, and foraminiferal biozones permits to divide the study interval into 14 time-stratigraphic mappable units. Based on lapout patterns, and biostratigraphic data 5 surfaces have been interpreted as Maximum Flooding Surfaces. Basinal area towards the east is riddled with hiatal surfaces, indicating that significant part of the stratigraphic record is missing. Thus, interpretation of sedimentation rates, and climatic cycles in the study interval needs to be approached with caution. Stratigraphic succession is punctuated by two regional unconformities, one of which is Middle Cenomanian in age, erosional in nature, and dies out towards the west. This unconformity may have implications for hydrocarbon exploration in the eastern part of the basin. Isopach maps of individual units suggests that the subsidence pattern in the basin was changing dramatically over a time span of 500 thousand years, indicating lateral shifts in thrust loading along the deformation front.