Grain-size variability of point-bar deposits from a fine-grained dryland river terminus, Southern Altiplano, Bolivia

Point-bar deposits exhibiting fining-upward grain-size trends are widely documented from both modern rivers and ancient preserved successions. However, in some mud-dominated sedimentary systems this common pattern may not occur; rather, alternative coarsening-upward trends can develop. We investigated point-bar deposits in the mud-dominated river terminus (median value (D₅₀) < 55 μm) of the meandering Río Colorado in Bolivia's southern Altiplano, a semi-arid endorheic basin. Grain-size trends were investigated using double random grain-size distribution (GSD) measurements and end-member modelling analyses (EMMA), as well as detailed mineralogical analyses (XRD and LOI). These methods revealed an upward-coarsening trend of point-bar deposits in terms of D₅₀ of GSD and in proportions of coarse sediment for two (C2 and C0) of three chronologically different meandering channels in the river terminus. Those of another abandoned channel (C1) were, by contrast, dominated by a fining-upward trend. The deposits in channel C2 were characterised by a lower content of organic matter and carbonate compared with those of the other two channels. A novel conceptual model for the formation of coarsening-upward point-bar deposits is proposed. The model implies the action of low-frequency, high-magnitude floods during an overall hyper-arid period when sediment supply to the river terminus is dominated by clay and fine silt. A temporal trend toward higher magnitude and longer duration flood events allows for the transport coarser-grained sediments further down system toward the river terminus where deposition occurs on the inner bend of meandering channels. Successive accretion layers within the point-bar deposits record a coarsening upward trend. Results demonstrate how a coarsening-upward succession of point-bar deposits in the muddy river terminus of a semi-arid endorheic basin can contribute to improved understanding of mechanisms of deposition in fine-grained fluvial systems. Our results contribute to an improved understanding of the varied processes and sedimentology of very fine-grained meandering river terminus systems in semi-arid or arid endorheic basins; the results additionally provide insight to enable improved interpretations of rock record examples of pre-vegetation rivers on Earth and other planetary bodies.