The age of the Horseshoe Canyon Formation in the Edmonton area: refining the timing of the Campanian Western Interior Seaway transgression

The North Saskatchewan River in Edmonton provides excellent exposure of the basal Horseshoe Canyon Formation: characterised by coal-rich paralic successions, these deposits mark the maximum inland extension of the Bearpaw Seaway prior to its eastward regression during the late Campanian. This interval is crucial for stratigraphic correlations across the Western Interior Basin in Canada but has previously been documented only through subsurface data. In this study, we present new U-Pb radiometric dates for bentonite layers and detailed palynological analyses from five stratigraphically constrained sites within the transition between the Bearpaw and Horseshoe Canyon formations in Edmonton. Bentonites adjacent to the #1 coal seam at the Riverbend section and the #6 coal seam near Skunk Hollow yielded high-precision radiometric ages of 74.42 ± 0.07 Ma and 73.19 ± 0.10 Ma, respectively. Furthermore, palynological evidence from the #1-11 coal intervals indicates a stratigraphic range from the Pseudoaquilapollenites parallelus–Parviprojectus leucocephalus palynozone to the Mancicorpus rostratus–Mancicorpus vancampoi palynozone. This refines the depositional framework of the late Campanian to early Maastrichtian. New data facilitate the correlation of Edmonton's exposures with the formally defined members of the Horseshoe Canyon Formation in southern Alberta: coals #1-3 to the Strathmore Member, coals #4-8 to the Drumheller Member, coal #9 to the Horsethief Member, and coal #11 to the Tolman and Morrin members transition. Additionally, the new U-Pb ages from the Edmonton valley support along-strike diachroneity in the Bearpaw transgressive-regressive cycles, constraining the geographic and temporal extent of the third- and fourth-order T-R cycles associated with the Bearpaw Seaway.