Amagnetostratigraphic analysis was carried out on the Bearpaw (225 m) and Blood Reserve (30 m) formations, based upon 58 sampling levels in exposures along the St. Mary River and Jensen Reservoir south of Lethbridge, Alberta.

Comparison to the coeval section below the Oxarart Member of the Bearpaw Formation in the Cypress Hills, across the Sweetgrass Arch, shows significant hiatuses in the St. Mary River section (SMRS). The 80 m-thick magnetozone 32r (representing approximately 0.65 m.y.) plus some thin magnetosubzones are missing on the St. Mary River at the erosional top of the shaly Kipp and Magrath sandstone members, representing a total of approximately 0.85 m.y. This difference between the two areas is attributed to sporadic uplift of the Sweetgrass Arch, affecting deposition at least 100 km west of the axis. Uplift of the Arch is interpreted to have caused the early advance of the Bearpaw Sea to have been diverted around the southern end of the Arch, causing the Sea to enter extreme southwestern Alberta from Montana and southeastern Alberta from the Saskatchewan Williston Basin in early Bearpaw time.

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