Basin-scale stratigraphic architecture and potential Paleocene distributive fluvial systems of the Cordilleran Foreland Basin, Alberta, Canada

Abstract

The Paleocene Paskapoo Formation in western Canada is composed of several hundred meters of fluvial strata that were deposited in an actively subsiding retro-arc foreland basin adjacent to the southern Canadian Cordillera. Here, we utilize newly available subsurface data from down-hole gamma-ray well logs to reconstruct the large-scale fluvial stratigraphy and depositional history of the Paskapoo Formation in the central portion of the Cordilleran foreland basin in Alberta. Paleocene fluvial strata in this region contain several notable characteristics, including variable degrees of channel sandstone amalgamation, regionally-variable stratigraphic architecture, and a large fan-shaped sandstone unit that emanated from the adjacent fold-thrust belt. Fluvial channel sandstone bodies are abundant and amalgamated near the base of the formation in the southern portion of the basin, but become less apparent in the northernmost 100km of the basin. The middle interval of the Paskapoo Formation, characterized by abundant mudstone with isolated channel sandstone bodies, is prevalent across the entire study area. The upper portion of Paskapoo Formation is dominated by a large (5000km2), north-northwest trending, laterally continuous sandstone in the northern portion of the basin that abuts against the fold-thrust belt and fans outward into the basin. This feature is interpreted as either a distributive fluvial system or a fixed-outlet river system that experienced repeated avulsions. This area of the fold-thrust belt may be the site of a long-lived fluvial outlet. The variable degree of sandstone amalgamation in the Paskapoo Formation suggests two different modes in the relative ratio of accommodation to sediment supply. Periods of rapid subsidence, likely associated with thrust emplacement in the adjacent fold-thrust belt, resulted in mudstone-dominated successions with isolated sandstone bodies, whereas tectonically inactive periods, associated with less subsidence, resulted in amalgamated sandstone units. The principal stratigraphic features reconstructed from the data in this area can be used to better understand and predict fluvial stratigraphy within other foreland basins.