ABSTRACT: Avulsion: A Major Control in the Evolution of Fine-Grained Alluvial Floodplains

SMITH, NORMAN D., University of Illinois at Chicago, Chicago, IL, and KATHLEEN M. FARRELL, North Carolina Geological Survey, Raleigh, NC

The conventional model of deposition for high-sinuosity river systems consists basically of a meander belt flanked by a lower-lying floodbasin that accumulates sediment through episodic flooding and crevassing of the master channel. Greater deposition near the master channel creates an alluvial ridge that rises above the floodbasin, producing an unstable topology eventually relieved by avulsion; i.e., the permanent diversion of flow away from the meander belt into the floodbasin. Eventually, a new meander belt forms from the redirected water and sediment. Continuing studies of an ongoing avulsion of the Saskatchewan River (Cumberland Marshes, eastern Saskatchewan) suggest that significant modifications of this conventional model may be required. Here, avulsion (started in 1873) has pr duced extensive deposition (>500 sq. kilometers) in the floodbasin and appears to be evolving a new meander belt. Observations show the following: (1) The avulsion belt is dominated by fine-grained sediments, usually regarded as products of "overbank" flows, but here deposited at virtually all flow stages. (2) A variety of sand bodies ranging from sheets to ribbons are formed by evolving splay complexes and channels within the avulsion belt. (3) Splays are mainly avulsion-belt features and occur only rarely on (now-abandoned) meander belts. (4) Anastomosed channel patterns dominate the avulsion belt.

The evolving avulsion belt illustrates the spatial and temporal continuums of splays, anastomosed channels, and meandering master channels; integrates the origins of assorted sand bodies and contemporaneous fines into a single conceptual model; and provides a framework in which new meander belts are generated.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)