Abstract

Outliers of Cretaceous strata at the west edge of the Canadian Rocky Mountains, deposited 110 km west of the Western Canada foreland basin foredeep, provide the first direct evidence of a piggyback basin in the Canadian Cordillera and impart new insights on the stratigraphy, U-Pb detrital zircon provenance, burial history, and tectonic setting of this area. Three successions are preserved in down-dropped, fault-bounded slices along the Redwall fault: late Middle Albian to Upper Albian Blairmore Group mixed clastics and minor limestone deposited in floodplain and swamp environments; unnamed Cenomanian mixed clastic floodplain deposits; and an unnamed alluvial/fluvial conglomeratic unit dominated by proximal eastern-sourced pebble to cobble conglomerate of probable Coniacian to Santonian age. All units are in the oil window. Estimated maximum temperatures for Cenomanian strata are 115–135 °C. U-Pb detrital zircon ages indicate that Upper Cretaceous units were primarily recycled from Mesozoic and Upper Paleozoic strata. with minimal input from both older sedimentary and western sources. Blairmore strata overlapped an eroded Upper Jurassic–Lower Cretaceous orogenic wedge and appear to have been contiguous with the foredeep succession. Upper Cretaceous conglomerates were derived from highlands to the east formed by uplift along thrust faults. These sediments were deposited in a piggyback basin on the hinterland side of the Simpson Pass and Bourgeau thrusts and were carried >70 km toward the foreland while being buried by an additional 4–5 km of sediment. The presence of this piggyback basin indicates a very low taper wedge with an extremely weak basal detachment.

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