The Sweetgrass Arch is a gentle fold that extends from Great Falls in central Montana to Sweetgrass near the Canadian border and northwards into Alberta. The Groundwater Atlas of the U.S. Geological Survey has a map that shows the Sweetgrass Arch. The axis of the fold is shown on the geologic map for MT, ND, SD, and WY by the anticline symbol parallel to and to the left of the section line D-D'. [To bring up the geologic map, click here and then scroll down to image 1010.GIF and click.] The anticline symbol on the map is in the Colorado group, which is labeled "Kcg". The cross section A-A' on this map passes through the Sweetgrass Arch. [To bring up the cross section, click here, scroll to 1011.GIF and click again.] The arch appears in this cross section as a gentle fold between the "Continental Divide" and the "Northern Great Plains".
The Sweetgrass Arch formed as a result of the repeated reactivation of structures in the underlying basement rocks during the last 2.8 billion years. The Sweetgrass Arch is located on the Medicine Hat Block of the Archean-age (i.e., greater than 2.5 billion years) Hearne Province. The internal structure of Hearne Province is exposed in Northern Saskatchewan where long narrow zones show evidence of continental growth by accretion. The zones are well illustrated in aeromagnetic maps. The aeromagnetic maps for Alberta and Montana show similar long narrow zones (oriented north-northwest - south-southeast). [See the low resolution aeromagnetic map of western Canada in Lithoprobe's Slide Set or the high resolution aeromag map in the Geological Atlas of the Western Canada Sedimentary Basin and the aeromagnetic map of Montana, complied by the U.S. Geological Survey.] One of these zones underlies the Sweetgrass Arch. The rocks (most likely granites) underlying the arch evidently have lower densities than those adjacent. In the 2.8 billion years since this zone formed, the Sweetgrass Arch has consistently risen with respect to adjacent zones, forming a topographic high or long ridge. The Sweetgrass Arch is underlain by a large fold in the middle of the crust, which was revealed by COCORP seismic profiles. (See discussion of Precambrian plate tectonics by Baker and Johnson or go to the plate tectonics pages.)
The Sweetgrass Arch has long been of interest to petroleum geologists because domes formed along the crest of the arch are suitable traps for oil and gas. The Kevin-Sunburst field is a notable example.
Giant Springs is located on the crest of the Sweetgrass Arch and the small stratigraphic thickness of beds there from the Kootenai sandstone down to the Madison limestone--less than 400 feet--is a consequence of "thinning" of beds on the Sweetgrass Arch. Flexing or bending of the Kootenai sandstone over the arch has caused the systematic fractures or joints to open, allowing groundwater in the Madison limestone to escape easily to the surface.
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