ABSTRACT

Glacier National Park has a spectacular glaciated, mountainous terrain of Precambrian Belt sedimentary rocks which in early Tertiary time were displaced eastward onto Cretaceous rocks by the Lewis thrust fault. The park has been the setting for at least ten periods of glaciation. This paper summarizes the pre-Quaternary geology in and adjacent to the park.

The pre-Quaternary rocks range from Precambrian Belt to Tertiary in age. The Belt Supergroup strata range in age from about 1,325 m.y. to about 900 m.y. They contain the Purcell Lava, which is a basalt flow (1,075 m.y.), as well as gabbroic sills and some dikes of the same age.

The Precambrian strata are mostly reddish-brown and greenish-gray argillite and siltite with some quartzite (Greyson, Spokane and Snowslip Formations). The Empire Formation is recognized in the park, and it is a transitional unit as much as 1,170 ft (375 m) thick between the underlying Spokane Formation and the overlying Helena Formation. The Altyn, Helena, and Shepard Formations compose the Belt carbonate units.

Parts of Glacier National Park and the adjacent areas are in the northern disturbed belt of Montana. The area east of the mountains contains thrust-faulted and folded Upper Cretaceous strata; it is equivalent to the Foothills structural province in southern Alberta. The area southeast of the park contains thrust-faulted and folded Jurassic and Cretaceous rocks, which locally are transected by northeasterly trending normal faults. These strata plunge northwest beneath the Lewis thrust plate and are not exposed in southern Alberta and British Columbia, except possibly in the Haig Brook and Cate Creek windows in the Lewis plate in British Columbia.

The Lewis thrust plate is deformed by numerous folds and small normal and thrust faults. The major structure in the plate is a northwesterly trending, doubly plunging syncline. The largest normal fault is the Blacktail fault, which extends northwestward into British Columbia as the Flathead fault. West of it are other northwesterly trending normal faults. The measured minimum easterly translation of the Lewis is 15 mi (24 km), but it may have moved at least 40 mi (64.4 km).

The park is in a southwesterly trending, structurally low area that is bounded on the north and south by southwest-trending structures. The structural configuration of the basement that influenced the present setting of the park was very likely established by the end of Cretaceous time, and it probably controlled the present structural pattern in the area, a pattern resulting from the Laramide (early Tertiary) orogeny.