Morphodynamic Preservation of Fluvial Channel Belts

Abstract

The fluvial sedimentary record is largely composed of deposits from relatively common flow events, rather than more catastrophic scour-and-fill events. At the scales of bedforms, such deposits are preserved within the stratigraphic record because they rapidly accumulate within, and are protected by, morphodynamic topographic depressions that occur naturally in the fluvial system as a result of feedbacks between flow, sediment transport, and topography. Examples include the preservation of ripples in front of dunes, dunes in front of bars, and bars within channels. Here, we used 3D seismic data that images preserved channel belts to test the hypothesis that alluvial-ridge basins, morphodynamic depressions formed between raised channel beds due to decreasing sedimentation rates away from channels in alluvial settings, are a source of topography driving channel-belt-scale preservation. Using the 3D seismic data, we measured the stratigraphic positions of channel belts, as well as their lengths, widths, sinuosities, and centerline orientations in the 3D seismic dataset. Results are consistent with well-preserved channel belts steered by alluvial-ridge-basin topography. Further, the thickness of the channel-belt interval exceeds the relief of any one alluvial-ridge basin, suggesting the volume records the filling of multiple alluvial-ridge basins and that the process is common. Characterizing the stratigraphic signature of alluvial-ridge basins is necessary for understanding contrasting fluvial architectures where external forcings prevented their formation.

Flow depth estimates and avulsion behaviour in alluvial stratigraphy (Willwood Formation, Bighorn Basin, Wyoming, USA)

“…Edmonds et al (2016) proposed scaling laws for avulsion that appear robust even under different boundary conditions of tectonism and prevailing climate. These scaling laws for characteristic avulsion distances, combined with commensurately thick sandbody and overbank cycles, offer a mechanism to explain the stratigraphic 'shingling' of sandbodies observed by Cardenas et al (2023). Overall, it appears morphodynamics play a central role in the construction and organisation of alluvial stratigraphy, imparting non-random behaviours on the system.…”

“…Importantly, the vertical relief of this topography appears to correspond with river flow depths (Swartz et al, 2022). In the subsurface, seismic data sets of Pleistocene sandbodies in the northern Gulf of Mexico coastal plain display orientations and scales consistent with avulsions driven by morphodynamics; preferentially filling associated alluvial‐ridge basins (Cardenas et al, 2023). Pleistocene sandbodies display a ‘shingled’ stratigraphic pattern (i.e.…”

“…Pleistocene sandbodies display a 'shingled' stratigraphic pattern (i.e. partial stratigraphic overlap of laterally offset units) that Cardenas et al (2023) interpreted as compensational in origin. These fluvial systems constructed a sandbody within their channel belt until reaching superelevation, at which point they avulsed into the alluvial-ridge basin and began constructing a new fluvial sandbody, offset and distinct from the previous.…”

“…However, accurate constraints on bank-full flow depth as well as detailed assessments of overbank depositional patterns are needed for these to be recognised. Cardenas et al (2023) suggested major shifts in boundary conditions may disrupt this underlying morphodynamic control on avulsion patterns. Sincavage et al (2022) presents evidence that the Bahmaputra River has preferentially filled Holocene palaeochannels generated by outburst floods rather than the topographically favourable course into the Sylhet Basin of Bangladesh.…”

Lateral river erosion impacts the preservation of Neolithic enclosures in alluvial plains

“…In these areas, river paths change at the centennial-millennial scale with channel avulsions (i.e., the displacement of a river section to a new path) and at the decennial scale with channel lateral erosion 13 , 14 . As rivers migrate, the sedimentary record in the alluvial plain is reworked 15 , 16 . Erosion is also likely to affect the archaeological record, which is highly incomplete, similarly to the underlying geologic record 17 .…”