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Galina S. Morozova1,
Norman D. Smith2
(1) University of Illinois at Chicago, Chicago, IL
(2) University of Nebraska, Lincoln, NE
Abstract: Holocene Floodplain evolution of the Lower Saskatchewan River and its implications for models of alluvial successions
Eight ancestral courses of the lower Saskatchewan River were identified in the Cumberland Marshes, Saskatchewan-Manitoba, from aerial photos, borehole data, and radiocarbon dates. These courses are products of repeated avulsions of the Saskatchewan River since 5400 yr BP. Avulsions occurred about every 200-1300 years, with the most recent event occurring in the 1870’s. Periods of channel-belt activity lasted up to 2400 years.
The avulsions took place into floodplain lakes and inundated peatlands. Most of the ancestral courses evolved from multiple channel systems formed during avulsive deposition. Each ancestral course includes two major complexes of sediments: an avulsion belt, reflecting flooding and subsequent progradation of splay complexes and lacustrine deltas following the initial avulsion, and a channel belt ultimately evolving from this avulsion belt. Avulsion belts represent contiguous layers dominated by sandy silt and very fine sand. These belts attain widths of up to 18 km with typical sediment thicknesses of 2-3 m lying conformably on preavulsive substrates. Channel-belt deposits include levee and various channel and point-bar facies. Floodbasin deposits unaffected by avulsion are composed of fine silt/clay and organic-rich deposits representing marsh, fen, bog and lacustrine facies.
Most existing models of alluvial stratigraphy subdivide floodplain sediments into two main groups: coarse channel and fine overbank facies. Floodplain evolution as inferred from the Holocene record of the lower Saskatchewan River suggests that avulsion belts may represent a third major component of some aggrading alluvial floodplains.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana