Basin-Scale Stratigraphic Architecture of Upstream Fluvial Deposits: Jurassic-Cretaceous Foredeep, Alberta Basin, Canada

Abstract

The fluvial stratigraphic architecture of the Late Jurassic–Early Cretaceous Monteith Formation, specifically the Monteith A, in the Alberta Basin is documented with a series of maps, cross sections, and core descriptions from 550 wells over an area of > 17,500 km2. The extensive study area provides a unique perspective on basin-scale stratigraphic architecture and sediment distribution of a sandy fluvial system in a foredeep. The stratum, > 150 m thick in places, is divided into three mappable intervals. The proportion of channel sandstone relative to overbank sediment is highest in the basal unit and progressively decreases upward, likely as a result of increasing accommodation associated with orogen growth. Conglomeratic lenses in outcrop indicate deposition from a persistent point source that tapped a drainage area > 100,000 km2. Along-strike sandstone distribution within the units is predictable as the thickest, most amalgamated, and coarsest sandstone deposits are concentrated nearest to the point source. Distal to the point source, isolated channel belts and overbank material are more commonly preserved. The maximum channel-belt dimensions are calculated to range from 4.1 to 12.6 m thick and from 514 to 2851 m wide with an average of 8.5 m and 1527 m, respectively. Sediment distribution patterns, stratigraphic architecture, and tectonic setting suggest that the deposit is a fluvial fan, or more generally a distributive fluvial system (DFS). The stratigraphic architecture observed in the Monteith Formation is inferred to be characteristic of DFSs in an upstream position and provides one of the first subsurface examples of such a system. The overall increase of preservation of overbank deposits upward is consistent with previous models of basin-scale upstream fluvial stratigraphic architecture, but the along-strike persistence and stacking of channel belts directly adjacent to a point source provides unique insight into the basin-scale stratigraphic evolution of upstream fluvial systems.