Understanding channel-overbank interactions in deep-marine slope settings: Architectural analysis of channel and levee elements in the Neoproterozoic Isaac Formation (Windermere supergroup), Southern Canadian Cordillera

Abstract

Well-exposed, periglacial slope deposits in the Neoproterozoic Isaac Formation allow channel-fill and genetically-related overbank elements to be studied in detail. The key to genetically linking these stratal elements is the occurrence of muddy turbidites interbedded with discontinuous thin granule conglomerate and dune-cross-stratified sandstone within overbank deposits. These strata drape a low-relief erosion surface (<1.5 m deep) that occurs at a slightly higher stratigraphic level than the base of the adjacent channel-fill. Both these surfaces are interpreted to represent erosion and bypass by energetic flows during channel initiation. Once initiated, most flows bypassed the channel but also spilled out onto the surrounding overbank area, depositing incomplete and complete Bouma sequences (levee element). The upward-fining and -thinning trend observed in many levee elements suggests that levee aggradation progressively increased channel confinement, restricting overspilling to the uppermost dilute parts of flows. Changes in flow conditions triggered early filling of the channel by sand/granule-rich, high-concentration flows (amalgamated Ta divisions) with minor overspill onto the levee. Latest stage of channel infill is always characterized by thin-bedded deposits suggesting deposition from low-energy, dilute flows (abandonment element). This evolution is repeated several times in Isaac Formation strata producing vertically stacked channel-fill elements separated by abandonment elements, and surrounded by genetically-related levee elements. The fact that the abandonment element described here is not traceable into the adjacent levee element, but onlaps it, differs from previous outcrop-based examples and resembles more the configuration observed in several well-imaged, subsurface examples in which inner levee elements onlap interpreted outer levee elements.