Sedimentology and high-resolution stratigraphy of the Upper Cretaceous (Late Albian to Middle Turonian) Blackstone Formation, Western Interior Basin, Alberta, Canada

The Blackstone Formation (Late Albian--Middle Turonian) of the Upper Cretaceous Alberta Group in central and southern Alberta forms an eastward tapering wedge of more than 600 m of interbedded mudstone, fine-grained sandstone, occasional coarse sandstone, and pebble beds. The Blackstone Formation was deposited in a retro-arc foreland basin during the Greenhorn marine cycle in a storm dominated environment of the Western Interior Seaway. The study interval of the Blackstone Formation defined between two regional markers (base of the Fish Scales marker and the Cardium E2 surface) spans around 6.2-7.1 my. The overall thickness, eastward tapering cross-sectional profile, and dominance of mudstone are suggestive that the Blackstone formation accumulated during a time of overall high accommodation rate. The Blackstone Formation has not been studied in detail since 1967. Significance of this study (encompassing an area of around 200,000 Km2) is that it provides the physical evidence of several regionally mappable hiatuses (> several hundred ky duration each) in a seemingly complete stratigraphic succession.;Southeastward and eastward (cratonward) thinning of the study interval is not only a result of stratigraphic condensation, but is mainly due to stratigraphic truncation resulting in hiatuses of regional extent. Integrating subsurface and outcrop data with previous studies suggests that the Albian/ Cenomanian, and Cenomanian/ Turonian boundaries in the study area are located within the Late Albian---Early Cenomanian and Late Cenomanian Early Turonian hiatuses respectively.;Integrating the data from the study area with regional observations suggests that tectonics, climate, and eustatic mechanisms controlled the deposition of different units. It is interpreted that tectonics and climate were the dominant controlling mechanisms of sedimentation. Further, there are several potential stratigraphic surfaces in the study interval that might have been formed due to eustatic mechanisms.;Keywords: Western Interior Seaway, Blackstone Formation, retro-arc foreland basin, Alberta Group, Greenhorn marine cycle, Upper Cretaceous, Late Albian, Cenomanian, Middle Turonian, allostratigraphy, high-resolution event stratigraphy, hiatuses, subsidence pattern, forebulge depozones.;Combining the principles of high-resolution event stratigraphy and allostratigraphy, the study interval was divided into 14 mappable stratigraphic units. Isopach mapping of individual units permitted interpretation of the regional subsidence patterns over the study area. During unit I to III time, the axis of subsidence was located in northwest Alberta and British Columbia, whereas during unit IV to XIII time, the axis of subsidence was along the strike of the Thrust Belt. As a consequence of this subsidence pattern, forebulge depozones in central and southern Alberta during unit I to III time were oriented in a northeast to southwest direction. In contrast, during unit IV to XIII time, forebulge depozones were oriented in a northwest to southeast direction recognized at a distance of around 300--400 km from the present deformation front in the eastern Alberta Plains.