--> Abstract: An Avulsion Origin For High-Sinuosity Fluvial Sandbodies, by N. D. Smith and R. Slingerland; #90928 (1999).

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SMITH, NORMAN D.1 and RUDY SLINGERLAND2
1Dept. of Geology, Univ. of Nebraska, Lincoln, NE
2Dept. of Geosciences, Penn State Univ., University Park, PA

Abstract: An Avulsion Origin for High-Sinuosity Fluvial Sandbodies

The conventional model for high-sinuosity fluvial deposits assumes that the bulk of the sandbodies are produced in the meander belt by lateral migration of the main channel and by periodic overbank and crevasse flooding. Our investigations of present-day processes, recent historical changes as determined by aerial photos and treering dates, and several hundred shallow boreholes of the lower Saskatchewan River (eastern Saskatchewan) where it intersects the Cumberland Marshes, and the Columbia River in the Rocky Mountain Trench of British Columbia suggest to us a different model wherein the bulk of the sandbodies in a high-sinuosity fluvial complex are produced during an avulsion. The Saskatchewan River for example, prior to 1870 consisted of a single meandering channel within 4-6 km wide levees standing 3-4 m above the floodplain ' Since that time an avulsion has created a 500 km2 alluvial cone consisting of active and abandoned splay systems and associated wetlands, connected by systems of anastomosed channels. Thicknesses of the avulsion deposits range from 7 m in proximal channels fills to negligible in distal lake sites. Although the avulsion belt is dominated by fine-grained sediments, a wide array of sand body geometries occurs, including continuous sheets (order 100's of meters), discontinuous and irregular sheets, and isolated shoestrings or ribbons. Proximal portions of the avulsion cone contain three main distributary complexes, each consisting of multiple anastomosed channel sandbodies with average bifurcation/convergence angles of 12 degrees and associated splay sheet sands. Channel fills consist of fine to medium sand; associated levees consist o very fine sand and silt. Levee thicknesses and levee and channel grain sizes decrease linearly downstream, the latter by greater than 1 <phi>. Distal areas consist of finer-grained multiple channel sands with higher bifurcation angles and thinner levee deposits.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas