Holocene avulsion styles and sedimentation patterns of the Saskatchewan River, Cumberland Marshes, Canada

Abstract

Abstract This paper describes the styles and sedimentation patterns of Holocene avulsions of the Saskatchewan River at the Cumberland Marshes predating the most recent (1870's) avulsion. Holocene avulsions involved two major styles: (1) progradation of lacustrine deltas and splay complexes, and (2) local reoccupation or appropriation of preavulsive channels. Most avulsions were progradational and occurred in floodplain lakes occupying floodbasin areas between elevated alluvial ridges. The two most recent avulsions (Old Channel and 1870's) diverted mainly into inundated peatland. Progradational avulsions were accompanied by deposition of avulsion-belt sediments associated with multiple channel networks. The characteristics of avulsion-belt deposits include: (1) sediments dominated by poorly sorted silt and very fine sand, (2) lateral continuity in transverse and longitudinal directions with subhorizontal upper and lower boundaries, (3) predominantly nonerosional contacts with underlying sediments, (4) coarsening-upward successions, (5) consistent radiocarbon dates for the tops of preavulsive substrates, and (6) occurrence beneath levee and overbank deposits of the associated major channel belt, indicating that avulsion-belt deposition occurs prior to the development of a new master channel. The width of ancient avulsion belts reaches 18 km, with typical thicknesses of 2–3 m. Avulsion belt deposition accompanied by many small channels was followed by flow reversion to a few larger channels and, eventually, to one single channel, accompanied by deposition of levee and channel deposits. Reoccupational avulsions were not accompanied by deposition of widespread avulsion-belt sediments and led mainly to the modification of pre-existing channel belts. The sedimentation patterns of Holocene avulsions of the Saskatchewan River reveal that avulsion-belt sediments represent major components of the floodplain deposits, supporting the model developed from observations of the 1870's avulsion.