Morphodynamics and depositional architecture of mid-channel bars in large Amazonian rivers

Large rivers are characterized by large water discharges, high suspended sediment fluxes and low slope, and typically display multiple channels that are separated by large complex bars. The most common channel style found in these rivers is characterized by the alternating presence of single and multiple channel threads. Mid-channel compound bars separate a main deeper channel from shallower secondary channels, leading to low-order braiding and low to moderate sinuosity. Despite the importance of this type of river for global terrestrial sediment transport and the interpretation of ancient fluvial successions, integrated depositional models for large multi-channel rivers are still not fully developed. This paper interprets the channel morphodynamics and depositional architecture of such large rivers by investigating the distinctive features of their laterally-accreting mid-channel bars in the Solimoes-Amazonas River. This is achieved by examination of temporal series of satellite images, quantification of bedforms using multibeam echosounding surveys, analysis of shallow seismic and ground penetrating radar surveys of selected areas, and on-site field observations and sample collections. Such mid-channel bars produce directional variability in planform scroll bar accretion of up to 180 degrees. As these bars - and their associated channels - evolve, they become progressively shallower, due to the bifurcation of the two channels and the resultant partitioning of discharge that leads to a gradual reduction in transport capacity within the outer channel. The preserved successions of this process in the alluvial plain are characterized by fining-upward trends, with larger cross-strata sets at mid-depths and internal erosional surfaces marking distinct cycles of bar development with potentially differing palaeocurrent trends. The new depositional models proposed for these mid-channel bars and channels constitute a basis for the recognition of large multi-channel river deposits in the ancient rock record. (AU)