--> The McMurray Conundrum: Conflicting Interpretations of Environment of Deposition and Paleogeography

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The McMurray Conundrum: Conflicting Interpretations of Environment of Deposition and Paleogeography

Abstract

Scaling relationships in modern fluvial systems provide useful guidance for interpretation of the stratigraphic record of the McMurray Formation, within the eastern margins of the Alberta foreland basin. Important scaling relationships include width:thickness of channel-belt sand bodies that form under normal flow conditions (∼70–300:1), and the width:thickness of associated abandoned channel fills (∼10–30:1). In addition, predictable transformations occur as a river system goes from normal flow through its backwater reach and begins to feel the effects of sea level. Rates of lateral migration and resultant channel-belt width decrease significantly, the channel aggrades, and avulsion is common: channel-belt width-to- thickness ratios within the backwater reach are therefore significantly less (20–50:1), and sand bodies are encased in muddy flood-basin or delta-plain strata. The backwater reach for large systems can be hundreds of kilometers upstream from a coeval shoreline. Detrital zircon signatures show the McMurray represents a true continental-scale fluvial system, the Mississippi or Amazon of its time, with a source terrain that stretched from the Appalachian Cordillera in the SE US to the Western Cordillera: thicknesses of McMurray point-bar sand bodies commonly exceed 30-35 m, which is consistent with a continental-scale system. The well-imaged McMurray in Athabasca displays channel-fill width:thickness ratios consistent with those from modern rivers, and sand body width:thickness ratios >100:1, typical of amalgamated channel belts within that part of a river system that is characterized by normal flow through the uppermost limits of the backwater reach. With channel depths that exceed 30 m, backwater lengths would have been in the range of 4-600 km, and, by this reasoning, contemporaneous shorelines would have been very far to the north. It would be rare to see significant tidal influences on sedimentation this far upstream in any large, inherently low gradient river, and it would be perhaps unprecedented to see brackish conditions extend upstream more than 100 km or so from a large river's mouth. There is, as a result, something of a conundrum with the interpretation of McMurray environments of deposition and paleogeography, because sedimentological and ichnofacies characteristics in these deposits have historically been, and still are in many cases, interpreted to record brackish and/or tidal influences, and a tidally-influenced fluvial system.