Abstract

Deposits off a Neogene paleovalley provide a geologic datum in southwestern Montana that brackets the age of faulting associated with the Yellowstone hotspot. Ages of paleovalley deposits on top of the modern Blacktail, Ruby, Tendoy, and Centennial ranges and in adjacent valleys range from 16 to 2 Ma. At least 2 km of offset has occurred since emplacement of the Timber Hill basalt (6.0 ±0.1 Ma); much may have occurred after deposition of the 2.0 Ma Huckleberry Ridge Tuff, which is now found in isolated outcrops as far north as the Ruby Range. Gravel clasts within the paleovalley have a northern provenance, whereas volcanic and volcaniclastic rocks were derived from the Snake River Plain to the south. We propose that uplift associated with passage of the hotspot caused the drainage reversal.

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