Anastomosed river deposits: modern and ancient examples in Alberta, Canada

Abstract

The anastomosed fluvial system is proposed as an environment of deposition for some over-bank deposits, coal, and coarse channel sediments (sandstone and conglomerate) in the Rocky Mountain molasse (Mesozoic–Tertiary) of Alberta and British Columbia. The system consists of rapidly aggrading channels and adjacent wetlands, caused by a rising local base level downriver (i.e., alluvial fans) or basin subsidence. Multiple, multi-storied anastomosed channel deposits generally have low gradients, variable sinuosities, and low width/depth ratios.Anastomosed fluvial facies are similar to some river-dominated low energy deltaic facies; therefore, many of the deposits and facies geometries are similar. Similarity occurs because the fluvial processes and morphologies are similar. For example, as with lower deltaic plains, stable anastomosed channels are related to gentle gradients and high amounts of silt, clay, and vegetation in the banks. Channel stability subsequently accounts for a stable deposition environment favourable for the accumulation of organic material. Wetland environments cover the largest area (estimated 60–90%) of anastomosed systems, while channels, levees, and crevasse splays are minor in extent.Modern and ancient examples of anastomosis are described to provide a basic depositional framework. The lower reach (valley fill) of the modern Alexandra River in Banff Park is the modern example described. In east-central Alberta and southwest Saskatchewan, the hydrocarbon-bearing channel sandstones of the upper Mannville subgroup (Vigrass) are interpreted as an ancient (Lower Cretaceous) anastomosed system of very large scale (80 km × 300 km).We believe that the interpretation of environments of deposition in some ancient fluvial sequences of molasse deposits has been limited. This is due to the fact that anastomosed systems have only recently been understood as a unique fluvial style related to specific causal conditions. We suggest that the rapid subsidence along the western margin of the molasse foreland, coupled with high sediment loads from nearby mountain building (i.e., Rocky Mountains), would represent suitable conditions for anastomosis as well as graded meandering or aggrading braided fluvial styles. These same conditions would also apply to molasse deposition in intermontane basins.