A Subsurface Sedimentology Analysis of Tide-Dominated Deposits in the BlueSky Formation (Early Cretaceous), Peace River Area, West Central Alberta
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The Early Cretaceous Bluesky Formation (Mannville Group) in the Peace River area of west-central Alberta, Canada, is a complexly stratified tide-dominated sediment body composed of monolithic and heterolithic sandstone. The Bluesky Formation is tide-dominated and was deposited during the latter part of a third-order transgressive systems tract. The Bluesky Formation is divisible into two valley-bounded sequences, informally referred to in this study as the “lower Bluesky unit” and the “upper Bluesky unit”. The lower Bluesky unit is composed of tide-dominated deltaic deposits. The upper Bluesky unit is composed of tide-dominated estuarine deposits. The lower Bluesky unit has abundant dynamically deposited mudstone layers (comprising 5-40% of most facies), many of which are interpreted to have been deposited under conditions of moderate to high suspended-sediment concentration (1-1000 g L-1) and appreciable current speeds (> 0.2 ms-1). The upper Bluesky unit, by contrast, has more sparsely distributed mudstone layers (comprising 0-15% of most facies) deposited primarily during slackwater and under conditions of relatively low suspended-sediment concentrations (< 1 g L-1). Both units are composed predominantly of subtidal and lower intertidal channel-bar and tidal-flat deposits. However, the most seaward deposits of the deltaic lower Bluesky unit contain sandstone-dominated heterolithic delta-front and mouth-bar deposits, whereas the most seaward environments of the estuarine upper Bluesky unit contain monolithic tidal sand-ridge deposits. The Bluesky represents deposition in slightly more seaward locations than the underlying Gething Formation, which is composed of lower-energy mudstone-rich facies of a fluvially dominated depositional system. Both the lower and upper Bluesky units are broadly aggradational with several autogenic internal erosional discontinuities, suggesting that high-energy tidal sedimentation generally kept pace with accommodation created by relative sea-level rise. A new and robust method for interpreting tidal facies is presented in this study. The approach uses a broadly applicable process-driven facies classification scheme that ensures a manageable number of facies. Recent improvements in the understanding of tidal systems and their facies models are incorporated into the method, most significantly highlighting the importance of mud and the realization that the set of geomorphic elements that comprise tidal systems is relatively small despite their complex lithofacies.