The alluvial architecture of a suspended sediment dominated meandering river: the Rio Bermejo, Argentina

The alluvial architecture of fine-grained (silt-bed) meandering rivers remains poorly understood in comparison to the extensive study given to sand-bed and gravel-bed channels. This paucity of knowledge stems, in part, from the difficulty of studying such modern rivers and deriving analogue information from which to inform facies models for ancient sediments. This paper employs a new technique, the parametric echosounder, to quantify the subsurface structure of the Rio Bermejo, Argentina, which is a predominantly silt-bed river with a large suspended sediment load. These results show that the parametric echosounder can provide high-resolution (decimetre) subsurface imaging from fine-grained rivers that is equivalent to the more commonly used ground-penetrating radar that has been shown to work well in coarser-grained rivers. Analysis of the data reveals that the alluvial architecture of the Rio Bermejo is characterized by large-scale inclined heterolithic stratification generated by point-bar evolution, and associated large-scale scour surfaces that result from channel migration. The small-scale and medium-scale structure of the sedimentary architecture is generated by vertical accretion deposits, bed sets associated with small bars, dunes and climbing ripples and the cut and fill from small cross-bar channels. This style of alluvial architecture is very different from other modern fine-grained rivers reported in the literature that emphasize the presence of oblique accretion. The Rio Bermejo differs from these other rivers because it is much more active, with very high rates of bank erosion and channel migration. Modern examples of this type of highly active fine-grained river have been reported rarely in the literature, although ancient examples are more prevalent and show similarities with the alluvial architecture of the Rio Bermejo, which thus represents a useful analogue for their identification and interpretation. Although the full spectrum of the sedimentology of fine-grained rivers has yet to be revealed, meandering rivers dominated by lateral or oblique accretion probably represent end members of such channels, with the specific style of sedimentation being controlled by grain size and sediment load characteristics.