Avulsion History of a Holocene Semi-arid River System: Outcrop Analogue for Thin-bedded Fluvial Reservoirs in the Rotliegend Feather Edge

Sedimentation at the terminus of low-gradient river systems in a semi-arid climate setting is characterized by thin- but laterally extensive amalgamated sand sheets. Analysis of absolute age dating with Optically Stimulated Luminescence measurements of fluvial sands in the Holocene Río Colorado (Altiplano Basin, Bolivia) aimed to (1) analyze the processes that created this sedimentary architecture, and (2) provide an analogue for thin but laterally-extensive sandstone reservoirs in an overall low net-to-gross setting such as the Rotliegend feather edge. The Río Colorado created a network of laterally-amalgamated fluvial sands by successive river-channel switching as the result of avulsion in a time period of only 4000y. The network formed by successive river avulsions with frequencies from 60 to 910y. The total area covered with these deposits approximates 500 km2; maximum thickness of the deposits is 2m. Fluvial sediment accumulated by vertical aggradation of sand in levees, vertical stacking and lateral amalgamation of crevasse splays, and vertical aggradation of channel-floor sand. This resulted in alluvial ridges with a positive relief on the floodplain. Subsequent river positions avoided the positive relief, and juxtaposition of successive river positions resulted in compensational stacking and amalgamation of fluvial sand deposits to a laterally connected extensive sand sheet.