Upper-bar deposits in large Amazon rivers: Occurrence, morphology and internal structure

Abstract

Although large anabranching rivers are a very common element in the greatest modern drainage basins, the recognition of such systems in the rock record is hampered by insufficient sedimentological information from active rivers. That is particularly the case of bar-top depositional environments in large anabranching rivers, motivating the survey of six selected areas of seasonally emergent bars in the Solimões and Japurá rivers (western Brazilian Amazonia) using Ground Penetrating Radar, Sub Bottom Profiler and Multibeam Echosounder, as well as direct observation sediment sampling. GPR surveys and field observations indicate these bars are dominated by 0.5 to 4 m tall subaqueous dunes with the local presence of metric angle-of-repose bar-side surfaces. Downstream accretion components are characterized by radar facies indicating horizontal to low-angle downstream-dipping cosets of dm-scale to cm-scale cross-strata. Lateral accretion components are characterized by radar facies indicating m-scale high-angle cross-strata sets passing laterally to low-angle and inclined cosets composed of dm-scale cross-strata sets. Upstream accretion components are characterized by radar facies indicating upstream-inclined cosets of m- and dm-scale cross-strata sets with local mud drapes. Preservation of accretionary bar forms in these periodically exposed areas is supported by calculation of the coefficient of variation of peak discharge, which indicates low interannual peak discharge variability over the bar tops.