Upcoming Division Events
Structural Geology of the Chief Mountain area, Montana
Schlumberger Palliser One Building, 200, 125 - 9th Ave SE Calgary
Thursday November 1st, 2018, 12.00pm
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The structure and topography of the Rocky Mountain front straddling the International Boundary is dominated by the Lewis thrust sheet. Its outstanding topographic feature is Chief Mountain in northernmost Montana, about 10 km south of the border. Chief Mountain stands as an isolated mass of mid-Proterozoic carbonates of the Belt- Purcell Supergroup (approx. 1.45 Ga), rising 1400m above the adjacent plains with near vertical faces rising 450 metres to the summit ridge.
Despite its prominence and frequent illustration in geological textbooks, there is no definitive description of the geology of Chief Mountain and its surroundings. The existing descriptions and mapping are either incomplete, vague, or inconsistent and are often erroneous. However the spectacular exposure and distinct, colourful Proterozoic stratigraphy permit detailed structural and stratigraphic analysis.
Chief Mountain is one of at least five klippen extending back along a ridge to the main mountain front at Gable Mountain. All are composed of mid-Proterozoic carbonates of the Waterton and Altyn Formations, thrust an estimated 100 km northeast over upper Cretaceous strata. These erosional remnants, and adjacent cliff faces along the main mountain front, display one of the most spectacular duplex structures in the Rockies, up to 250m thick, involving scores of fault slices. This duplex is sandwiched between the Lewis thrust as the floor, and the broadly folded Yellow Mt. thrust as the roof.
The contrast between the near chaos within the duplex structure and the smoothness of the roof and floor thrusts leads us to questions about the mechanism and sequence of duplex development.
Excellent quality Google Earth images combined with the spectacular exposure and distinctive stratigraphic colouration permit us to take a birds-eye geological tour of the area in Google Earth.
Fig. 1. Dr. Bob McMechan stands on Proterozoic carbonate talus on the north side of the Papoose klippe, with the Ninaki (left) and Chief Mountain klippen in the background. Cretaceous strata are exposed on the far side of the saddle below the Ninaki.
Peter is a retired structural geologist who has 35 years of experience in the petroleum industry with Shell, Encana and Devon. He holds a masters in geology from Queen’s University, Kingston, Ontario.