Recognizing the product of concave-bank sedimentary processes in fluvial meander-belt strata |
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| Authors: | Paul R Durkin Stephen M Hubbard John Holbrook Zennon Weleschuk Paul Nesbit Chris Hugenholtz Tania Lyons Derald G Smith |
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| Institution: | 1. Department of Geological Sciences, University of Manitoba, Winnipeg, MB, R3T 2N2 Canada;2. Department of Geoscience, University of Calgary, Calgary, AB, T2N 1N4 Canada;3. Department of Geological Sciences, Texas Christian University, Fort Worth, TX, 76129 USA;4. Department of Geography, University of Calgary, Calgary, AB, T2N 1N4 Canada |
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| Abstract: | Downstream migration of point bars is an important process in meander belts. Inherent to downstream migration is sediment accumulation in concave channel banks, immediately adjacent to and downstream of convex point bars. Despite this, associated concave bank processes are often overlooked, with depositional products sparsely identified in the stratigraphic record. Counter-point-bar deposits are a type of concave-bank deposit that have been positively identified in subsurface three-dimensional seismic datasets, yet outcrop examples are not well-constrained. This study characterizes and establishes recognition criteria of counter-point-bar deposits in outcrop using extensive exposure of Late-Cretaceous meander-belt deposits in eastern Alberta, Canada. Using a combination of traditional field-based sedimentological analyses and three-dimensional outcrop mapping with an Uncrewed Aerial Vehicle and Structure-from-Motion photogrammetry, point bar, counter-point bar, and associated abandoned-channel deposits, as well as adjacent floodplain deposits are identified. Bed-scale characteristics of counter-point-bar deposits include interlaminated and interbedded siltstone and fine-grained sandstone, abundant organic detritus, and evidence of deformation and slumping. At the bend scale, accretion packages bounded by internal erosion surfaces are composed of dipping siltstone and minor sandstone beds that extend from the top to the base of the meander belt. At the belt scale, positive identification relies on concave accretion surface mapping, their orientation relative to the meander-belt edge (i.e. dipping away), and consideration of meander-bend evolution. These results have implications for recognition of counter-point-bar deposits in analogous, less-constrained data sets, which provides a foundation for more complete palaeoenvironmental interpretations. |
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| Keywords: | Counter-point bar fluvial meander belt meandering point bar |
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