The Lewis thrust fault and related structures in the Disturbed Belt, northwestern Montana

Abstract

The classical Lewis thrust fault in Glacier National Park has now been mapped 125 km south of the park to Steamboat Mountain, where the trace dies out in folded middle Paleozoic rocks. The known length of the fault is 452 km, extending northward from Steamboat Mountain to a point 225 km into Canada, where the fault also dies out in Paleozoic rocks.At the south end, the surface expression of the Lewis thrust begins in a shear zone in folded Mississippian rocks. To the north, the thrust progressively cuts downsection into Proterozoic Y (Belt) rocks near Glacier National Park.Displacement on the Lewis plate increases northward from approximately 3 km on an easterly trending hinge line at the West Fork of the Sun River to a postulated 65 km at the southern edge of the park, where the stratigraphic throw is about 6,500 m. Present data indicate the thrust formed during very late Paleocene to very early Eocene time.The Lewis thrust and related structures, the Hoadley thrust and the Continental Divide syncline, probably formed concurrently under the same stress field. The northern limit of the trace of the Hoadley thrust is within the lower portion of the Lewis plate, about 28 km north of where the Lewis thrust develops, and the Hoadley extends for at least 125 km to the south. Displacement of the Hoadley increases southward from about 1 km at the hinge line to an inferred 70 km near its known southern extent. If our inference is correct, the Hoadley is nearly the southern mirror image of the Lewis to the north. The Continental Divide syncline, a doubly plunging, broad, northerly trending open fold that is about 120 km long, is a major fold within the Lewis plate.

Electrical resistivity structure of the Flathead Basin in southeastern British Columbia, Canada

“…The Lewis thrust sheet has been reported to be older than the underlying structure in the footwall plate (Bally et al 1966). In this regard, Constenius (1982) states that the Lewis thrust fault, which extends for about 457 km (Mudge and Earhart 1980) and formed in very late Paleocene to very early Eocene, placed a coherent slab of Proterozoic and lower Paleozoic rocks, 6-7 km thick (Childers 1964), onto a complexly deformed footwall of Paleozoic and Mesozoic strata. The Mesozoic strata are mainly Early Cretaceous in age and include rocks of the Crowsnest Formation and Blairemore Group, which are composed of volcanic breccia, sandstone, mudstone, siltstone, and pebble and cobble conglomerates.…”

Tectonic Klippe Served the Needs of Cult Worship, Sanctuary of Zeus, Mount Lykaion, Peloponnese, Greece

“…Yet another example of klippen influencing history and culture is Chief Mountain, which is located along the eastern border of Glacier National Park, Montana, USA (Mudge and Earhart, 1980) (Fig. 3B).…”

Petroleum potential of wilderness lands in Montana

“…The total horizontal contraction or shortening (west to east) of the sedimentary wedge is estimated by many workers to exceed 100 miles. The disturbed belt, the northern subdivision of the Montana thrust belt, is an extension of the Foothills and Front Range structural subdivisions of the Cordilleran thrust belt in southern Alberta (Willis, 1902;Mudge and Earhart, 1980;Mudge, 1983). The Foothills region of Canada has yielded prolific amounts of gas and condensate (Rice, 1~)77).…”

“…Oil and gas exploration of the disturbed belt, particularly west of the mountain front, has been restricted from exploration. Glacier National Park is located chiefly on the major Lewis thrust sheet in the northern part of the disturbed belt (Mudge and Earhart, 1980) and is probably underlain by imbricate thrust slices of Mississippian carbonate rocks, beneath the Lewis thrust sheet, similar to and coextensive with hydrocarbon-producing structures in southwestern Alberta (Kulik, 1983). The Bob Marshall Wilderness and associated Wilderness Study Areas occupy much of the disturbed belt south of Glacier Park, and these areas are considered to have significant hydrocarbon potential (Mudge and others, 1977;Rice, 1977;Mudge and others, 1978).…”