A typical point bar with atypical strata in the McMurray Formation, Alberta, Canada: Floods, tides and high suspended sediment concentrations

Fustic M. Plink-Björklund P. Shchepetkina A. Strobl R.
October 2024 John Wiley and Sons Inc

Sedimentology
2024 #71 Issue 6 1922 - 1952 pp.

Stratigraphic positions and the nature of some breccias and mudstones observed in the Lower Cretaceous McMurray Formation type section (Alberta, Canada) challenge the existing fluvial point bar facies models. The currently applied large-scale, tidally influenced, sandy bedload facies model suggests that point bars fine upward, where breccias and cross-strata occur at the base of the succession and mud content increases upward. Instead, this study documents that: (i) mudstone clast breccias are not exclusively associated with basal channel-lag deposits and can occur throughout the point bar; and (ii) stratified mudstones are not limited to the top of the fining-upward succession and can occur at or near the channel base. This outcrop data, including lithology, sedimentary structures, trace fossils, bed thickness and boundaries, and the nature of stratal packages, further suggests dynamic interplay between fluvial and tidal processes. This article discusses the potential role of high-magnitude river floods in temporarily elevated suspended sediment concentrations and highlights that fluid mud processes played a crucial role in mud deposition in both tide-influenced and fluvial parts of the system. Fluid mud processes allowed mud deposition along the channel base and across the point bar accretion surfaces. The breccias predominantly consist of broken pieces of inclined heterolithic strata and mostly occur on accretion sets of point bars, suggesting erosion and collapse of the point bar rather than of the cutbank. This work introduces a novel perspective to the existing models of point bar sedimentation, offering a new end member and concepts for the interpretation of subsurface data. The authors hope that this research encourages further investigations into similar phenomena elsewhere in the world.

Facies model , fluvial floods , point bar , suspended sediment concentration , tidally influenced meander belts

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School of Mining and Geosciences, Nazarbayev University, 53 Kabanbay Batyr Ave., Astana, Kazakhstan
Department of Earth, Energy, and Environment, University of Calgary, 2500 University Drive NW, Calgary, T2N 1N4, AB, Canada
Colorado School of Mines, 1500 Illinois St., Golden, 80401, CO, United States
Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N, London, N6A 5B7, ON, Canada

School of Mining and Geosciences
Department of Earth
Colorado School of Mines
Department of Earth Sciences

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