Evaluating Backwater versus Upstream Tectonic Controls on the Gravel-Sand Transition, Cretaceous Dunvegan Formation, Western Canada Sedimentary Basin

Abstract

The Cretaceous Dunvegan Formation in the Western Canada Sedimentary Basin is a well-characterized source-to sink system that extends for about 1000 km from proximal gravelly alluvial systems in the Liard Basin, Northwest Territories, to well-developed sandy deltaic and prodelta muddy shelf systems in Alberta. The mud to sand transition has been well-documented and the sandy deltas have been shown to be fed by tributive incised valley systems driven by cycles of high frequency sea-level change. These incised valleys in turn pass landward into the conglomerate alluvial systems, but the nature of these conglomerates and the controls on the gravel-sand transition have not been well examined.Dunvegan conglomerates in outcrops in the Liard Basin are moderately-sorted and well- rounded reaching up to small cobbles in size. These have previously been interpreted as alluvial fans; however, our investigations show a domination of sharp to scoured based meters-thick fining upward facies successions consisting of dune- and bar- scale cross stratification. The conglomeratic units comprise highly amalgamated channel belt deposits that form cliff exposures that are about 70 m high. There is an absence of debris flows or of sedimentary structures characteristic of supercritical flows, and we thus interpret these as likely deposited by meters-deep lower-gradient gravel-bed streams, rather than steep-gradient, sheet-flood dominated alluvial fans.Trunk channels in the sink area, farther to the southeast are associated with incised valleys and have a mean bankfull depth of 10-15 m, carry medium sand (< 200 microns) with slopes estimated to be on the order of 6x 10-5. Source to sink calculations indicate a back-water length of around 200 kilometers. Based on paleogeographic reconstructions, the conglomerates appear to have been deposited 300 to 500 km from the mapped deltaic shorelines, indicating that the gravel sand transition is not related to the back water and is likely not controlled by sea-level changes, thought to be important in generating the incised valleys in the sink area. The conglomerates appear to be confined within the Liard Basin, which is bounded to the east by the Bovie Fault, expressed as a major kilometer-throw normal fault. The Bovie structure was long-lived and may have been active throughout Dunvegan time. Hence, excess accommodation, driven by movement on the Bovie Fault may have prevented gravel from escaping into the more distal parts of the Western Canada Sedimentary basin.  This is in contrast to other clastic wedges in the Cretaceous Interior Seaway, such as the Frontier and Cardium formations, that contain conglomeratic shoreline and shelf deposits and may indicate steeper gradient S2S systems.Tectonics and climate in the Dunvegan drainage basin may not have been linked to processes downstream because of the Bovie Fault. Despite clear evidence of downstream sea-level controls in the Dunvegan sink, it does not appear that these signals were able to propagate upstream.