Abstract

A clast-supported, massive diamicton to poorly sorted, massive gravel (Gm/Dcm) bed in the area around Dinosaur Provincial Park, southern 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 melt waters were concentrated upon deglaciation. The bed has also been observed along the banks of the South Saskatchewan River at Medicine Hat, 90 km to the south, and is therefore regionally extensive.

Because the Gm/Dcm bed 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 melt-water 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 may also 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 bed-load 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.

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