Department of Geology, University of Kansas. Lawrence
Scaling relationships developed from modern fluvial deposits provide useful guidance for interpretation of the stratigraphic record. At the system level, fluvial channel depth and the related thicknesses of channel-belt sand bodies scale to contributing drainage area, whereas, at the local to subregional level, characteristic width-thickness ratios exist for muddy abandoned channel fills (10–30:1) and laterally amalgamated channel-belt sand bodies (70–300:1). Moreover, net deposition as the river goes through its backwater reach, and feels the effects of sea level, results in significantly less lateral migration before avulsion: muddy channel-fill dimensions may not change significantly, but channel-belt width-to- thickness ratios are significantly less (20–50:1) due to greatly decreased rates of lateral migration, and sand bodies are encased in muddy flood-basin or delta-plain strata.
These and other relationships provide insight into the paleogeographic significance and scale of fluvial deposits in the Early Cretaceous Mannville Group, Alberta foreland, which house the prolific Alberta Oil Sands. At the system level, thicknesses of Mannville point-bar sand bodies commonly exceed 30 m, which suggests a potential drainage area exceeding that of the modern Mississippi. This view is consistent with detrital-zircon signatures that fingerprint a source terrain that stretched from the Appalachians in the SE US to the SW US, and routed sediments to the Alberta foreland backbulge and eventually to the Boreal Sea. Channel-belt scales and DZ records of drainage basin source terrain converge to indicate that Mannville fluvial-deltaic deposits record North America’s continental-scale river of that time.
At the local to subregional level, the well-imaged Lower Mannville McMurray Formation in Athabasca displays channel-fill dimensions consistent with those from modern rivers and sand body width-to-thickness ratios typical of amalgamated channel belts within the upper limits of, or above, the backwater reach. Backwater lengths in large river systems with deep, low-gradient channels can exceed 500 km, which would suggest contemporaneous McMurray shorelines would have been very far to the north of the type area. It would be rare to see tidal effects all the way through the backwater reach of any river, and very unlikely to see brackish conditions recorded this far upstream. Yet, sedimentological and ichnofacies characteristics in these deposits have historically been, and still are in many cases, interpreted to record brackish and/or tidal influences. Observations of scaling relationships for channel deposits therefore provide alternative interpretations of McMurray strata that are guiding ongoing investigations.