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

This paper describes the styles and sedimentation patterns of Holocene avul sions of the Saskatchewan River at the Cumberland Marshes predating the mos t recent (1870's) avulsion. Holocene avulsions involved two major styles: ( 1) progradation of lacustrine deltas and splay complexes, and (2) local reo ccupation or appropriation of preavulsive channels. Most avulsions were pro gradational and occurred in floodplain lakes occupying floodbasin areas bet ween elevated alluvial ridges, The two most recent avulsions (Old Channel a nd 1870's) diverted mainly into inundated peatland. Progradational avulsion s were accompanied by deposition of avulsion-belt sediments associated with multiple channel networks. The characteristics of avulsion-belt deposits i nclude: (1) sediments dominated by poorly sorted silt and very fine sand, ( 2) lateral continuity in transverse and longitudinal directions with subhor izontal upper and lower boundaries, (3) predominantly nonerosional contacts with underlying sediments, (4) coarsening-upward successions, (5) consiste nt radiocarbon dates for the tops of preavulsive substrates, and (6) occurr ence beneath levee and overbank deposits of the associated major channel be lt, indicating that avulsion-belt deposition occurs prior to the developmen t of a new master channel. The width of ancient avulsion belts reaches 18 k m, with typical thicknesses of 2-3 m. Avulsion belt deposition accompanied by many small channels was followed by flow reversion to a few larger chann els and, eventually, to one single channel, accompanied by deposition of le vee and channel deposits. Reoccupational avulsions were not accompanied by deposition of widespread avulsion-belt sediments and led mainly to the modi fication of pre-existing channel belts. The sedimentation patterns of Holoc ene avulsions of the Saskatchewan River reveal that avulsion-belt sediments represent major components of the floodplain deposits, supporting the mode l developed from observations of the 1870's avulsion. (C) 2000 Elsevier Sci ence B.V. All rights reserved.