Thrust plate stratigraphy and structure of the Many Glacier area, Glacier National Park, Montana

Harp, Edwin

Roughly 7,000 feet of Precambrian Belt strata are exposed above the Lewis overthrust in the Many Glacier area of Glacier National Park east of the continental divide. Be low the overthrust rocks of Cretaceous age occur. The Pre-Ravalli sequence and Ravalli Group of the Belt Supergroup are subdivided into and mapped as units of member status using criteria and nomenclature similar to that used by the Fentons (1937). The only Pre-Ravalli sequence present is the Hell Roaring Member of the Altyn Formation, which consists of 520 feet of blue-gray, variably sandy dolomite. Overlying the Altyn is the Appekunny Formation of the Ravalli Group, which is divided into a lower-Singleshot, middle-Appistoki, and upper-Scenic Point members. The Appekunny consists of roughly 2,200 feet of green argillite, siltite, and white to gray quartzite. Minor reddish strata occur in the Singleshot Member. The overlying Grinnell Formation also contains three members: lower-Rising Wolf, middle-Red Gap, and upper- Rising Bull. The Grinnell consists of 1,800 feet of red argillite and white quartzite with the Red Gap containing very little quartzite. Minor green beds are present throughout. The overlying Siyeh Formation is the only unit within the Piegan Group and consists of roughly 3,000 feet of dolomite, limestone and calcareous shale. This formation contains many laterally continuous stromatolite zones and is divisible into three stratigraphic units; however, it is mapped as an undivided unit. Underlying the Siyeh is the 150-foot thick Snowslip Formation which here consists of red and green argillite and quartzite. The youngest exposed Belt in the area is the Shepard Formation. It is comprised of dolomite, limestone and calcareous shale, only the basal 150 feet are exposed. The structural features in the map area, besides the Lewis overthrust, are minor thrusts and associated zones of small scale folds. The folds appear to be of two main types: folds with southwest dipping axial planes ("normal") and folds with north or northeast dipping axial planes ("reverse"). Stereonet analysis of the fold orientation indicate that the "normal" folds are related to the Lewis overthrust, and that the "reverse" folds are probably related to structural adjustments after thrusting. Stereonet analysis also reveals that the most probable direction of movement of the Lewis overthrust plate was N 20-30 E. The mineralogy, lithology, thickness of the Belt, and lack of intense deformation of the overthrust plate seem to agree with Hubbert and Rubey's (1959) hypothesis that development of abnormal fluid pore pressure was responsible for thrusting and/or gravity sliding of allocthonous masses of great thickness and displacement. However, apparent porosities' of quartzite units studied indicate that they would have been unsuitable media for development of abnormal fluid pore pressures. Studies of the clay from the Lewis fault zone by R. C. Wilson (personal communication, 1970) also disagree with the "abnormal pore pressure" theory.