Evolution of late Cretaceous-early Tertiary depositional sequences in the Beaufort-Mackenzie Basin, Canada

Abstract

The Maastrichtian to Lower Eocene rocks of the Fish River and Aklak sequences of Arctic Canada's Beaufort-Mackenzie basin were deposited during the northward migration of a fold and thrust belt across a north-facing passive margin. Detailed outcrop analysis performed in this study has resulted in delineation of the depositional processes, environments and history of systems tracts contained within the Fish River sequence. The Fish River sequence contains Maastrichtian to Lower Paleocene rocks of the Tent Island and Moose Channel Formations of the Fish River Group. This 1900 m thick terrigenous clastic succession consists of channelized conglomerate, sandstone and siltstone overlain by mudstone and capped by a large scale coarsening-upward package of interbedded mudstone, sandstone and conglomerate. Regional unconformities separate the Fish River Group from both the underlying shale of the Cenomanian to Turonian Boundary Creek Formation and the overlying conglomerate, sandstone, mudstone, and coal of the Paleocene to Eocene Aklak Member of the Reindeer Formation. The Fish River Group records a systematic vertical succession of depositional environments including submarine canyon, slope to outer shelf, prodelta, delta front and lower and upper delta plain. The succession of paleoenvironments, regional stratigraphic correlations and relationships, thickness and age control, and bounding unconformities suggest that the Fish River Group records the evolution of a complete type-1, second order, depositional sequence. The Cuesta Creek Member at the base of the Fish River sequence consists of stacked fining-upward channel fills consisting of conglomerate, sandstone and siltstone. The channel fills contain a complex assemblage of high and low density turbidites, current deposits, debris flows, slumps, and slide blocks. These deposits record the complex fill history of a coarse-grained lowstand submarine canyon system. The internal stratigraphic organization of the Fish River sequence records alternating periods of uplift and subsidence in the Beaufort-Mackenzie basin. The scale and timing of these episodes, combined with the environments of deposition and lithologic character of the rocks, suggests that the driving mechanism was alternating episodes of subsidence due to thrusting followed by flexural rebound.