CGRG Bibliography of Canadian Geomorphology
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Author : Evans, D.J.A.
Date : 1991
Title : A gravel/diamicton lag on the South Albertan prairies, Canada; evidence of bed armoring in early deglacial sheet-flood/spillway courses.
Publication : Geological Society of America Bulletin
Issue : 103(8):
Page(s) : 975-982
Abstract
A clast-supported, massive diamicton to poorly sorted, massive gravel (Gm/Dcm) bed in the area around Alberta, Canada, provides a means of identifying early deglacial flood courses. This is especially useful where flood and spillway courses have very subtle geomorphic definition and are difficult to map from aerial photographs. The Gm/Dcm bed conformably overlies a brown, late-Wisconsinan till and rarely overlies bedrock and lacustrine sediments. The geographic distribution of the bed is restricted to flat to gently undulating prairie on either side of the Red Deer River where shallow floods and spillway meltwaters were concentrated upon deglaciation. The bed has also been observed on the banks of the South Saskatchewan River at Medicine Hat, 90 km to the south, and is therefore regionally extensive. Because the Gm/Dcm has none of the attributes of supraglacial sediment and possesses a clast fabric and sphericity very similar to those of underlying till, it is interpreted as a winnowed gravel/diamicton lag, which was produced by low-competence meltwater floods along shallow depressions in the prairie surface after the drainage of local proglacial lakes. After enough of the fine-grained matrix of the late Wisconsinan till was removed to create a lag < 0.5 m thick, bed armoring protected the till from further fluvial erosion. This bed armoring also may explain the preservation of fluvially cut bedrock surfaces in the badlands of Dinosaur Provincial Park. Here the erosion of soft Cretaceous bedrock by melt water that did not have the competency to carry coarse bedload may have been largely halted by the lag deposit after all the till had been removed. This explains the paucity of alluvium on the bedrock surfaces today.
Bibliography of Canadian Geomorphology