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The present form of the Sweetgrass arch is the cumulative product of a geologic history which began in the late Precambrian. At that time, the area of the arch formed the eastern limit for the Belt strata, which were deposited on the Precambrian continental shelf. These thick (40,000 ft, 12 km) deposits depressed the underlying lithosphere sufficiently to cause a mild upwarping at the adjacent shelf hingeline, the area of the arch. During the Paleozoic, the arch was a relative high between the Williston basin to the east and continued shelf sedimentation to the west, and the arch was mildly uplifted in Walcott's 1970 process of "amplified topography."
With the formation of the Sevier overthrust belt in Late Jurassic time, this ancestral arch provided a susceptible area at an optimum distance for the formation of a forebulge on an elastically flexed lithosphere. This forebulge (the arch) was mildly uplifted in response to the supracrustal loads created to the west by overthrusting. Although uplift events at the arch can be tentatively correlated with thrust events in the eastward-migrating overthrust belt, the arch remained stationary, and the load to forebulge distance did not remain constant as flexural theory would predict. This was probably caused by early curvature at the arch in excess of elastic limits, creating brittle and plastic components in the local lithosphere, which thus became more susceptible to flexure than the adj cent areas, localizing the arch.
With the onset of the Laramide orogeny, involving basement as well as thin-skinned tectonics, horizontal compressive forces tightened and significantly uplifted the existing arch. In addition, sinistral shear along the elements of the Lewis and Clark lineament may have enhanced the arch as a large-scale drag fold feature, as proposed by Thomas in 1974 and 1979.
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