STRUCTURAL GEOLOGY OF THE NORTH-HALF OF THE SWIFT RESERVOIR CULMINATION, NORTHERN SAWTOOTH RANGE, MONTANA

The Swift Reservoir Culmination (SRC) is located in the Sawtooth Range of northwest Montana, just south of Glacier National Park, where it forms an anomalous outcrop zone of imbricated and tightly folded Cambrian rocks approximately 20 km long x 5 km wide. The atypical nature of the SRC is underscored by the fact that Cambrian strata are not exposed elsewhere along the Sawtooth Front, but are typically found several thrust sheets to the west where, regionally, the basal décollement has cut deeper into the stratigraphic section. The easternmost thrust carrying Cambrian rocks is the Major Steele Backbone (MSB), which has been interpreted to be the roof fault of a hinterland-dipping footwall duplex involving Mississippian and younger strata. The importance of the SRC lies in its potential as an excellent outcrop analogue to subsurface duplex systems along the Rocky Mountain Front, including the Waterton Gas field to the north.

Detailed mapping of the SRC shows that the most dramatic folding within the culmination occurs in the Cambrian succession comprising the MSB thrust sheet. Within this suite of rocks, fold axes typically plunge away from the crest of the culmination. Folding becomes more pronounced to the east where, proximal to the MSB, folds of Cambrian Steamboat Limestone become isoclinal and overturned. Moreover, the MSB thrust sheet is internally imbricated, with the majority of faults rooted into detachments in the Cambrian Switchback Shale. Fracturing within the Steamboat Limestone also increases in intensity proximal to thrust faults, where penetrative fabrics found in association with fold development, such as axial planar cleavage and pencil structures, are observed. Northwest of Swift Reservoir, the décollement along the mountain front climbs up-section in a step-like manner from Cambrian to Mississippian rocks along two lateral ramps. Further northward, the frontal thrust plunges down-section towards Glacier National Park, where the Lewis thrust exposes Proterozoic rocks in the hanging wall along the mountain front. The ramps are evidenced by the strike-parallel fault-cutoffs that characterize the north end of the SRC. These basement-controlled ramps are key to creating the compound eyelid window found adjacent to Feather Woman Mountain in the SRC.