Structural geology of the north-half of the Swift Reservoir culmination, Sawtooth Range, Montana

Abstract

The Sawtooth Range forms a broad salient at the eastern edge of the Sevier fold-and thrust belt along the Rocky Mountain front of northwest Montana. The Swift Reservoir Culmination is a structural high located along the range-front of the Sawtooth Range, just south of Glacier National Park. The range is dominated by steeply-dipping imbricate thrust sheets. The culmination exposes an anomalous suite of Cambrian rocks that are not found elsewhere along the Sawtooth front. The structural complexity of the range is underscored by the high degree of imbrication that has taken place as well as the tight folding. Moreover, units in the SRC are characterized by strike-parallel cutoffs that create a surface expression of a compound eyelid window. The chief objectives of this research were to determine if the structural geometry of the culmination was compatible with an interpretation of a hinterland-dipping duplex in the footwall of the Major Steele Backbone thrust; what factors are responsible for the strike-parallel cutoffs seen in the culmination; and whether the SRC is a viable structural analog to subsurface duplex systems targeted for exploration along the Rocky Mountain front. An in-depth structural investigation of the culmination was conducted through field-based mapping, followed by lab investigations of the data collected in the field. This included geologic mapping followed by the construction of cross-sections, as well as the synthesis of this data with published magnetic and gravity data in order generate an accurate structural model of the culmination from basement to surface. Deformation within the SRC is controlled primarily through the mechanical properties of the units within the culmination, resulting in compartmentalization of the culmination into four distinct lithostratigraphic/structural domains. Moreover, this facilitated the development of duplex fault zones within the culmination. The Heart Butte thrust is the linking fault between three of these domains, and is a reflection of pre-orogenic basement lineaments that controlled the structural development of not only the Sawtooth Range, but the SRC as well.