ABSTRACT The geometric, kinematic, and deformational features along the subprovince boundaries of the Archean Superior Province are keys to understanding the tectonic amalgamation of the province. This field trip investigates the structural geology along two of the subprovince boundaries—the Wabigoon-Quetico and Quetico-Vermilion—of the Superior Craton. These boundaries separate the relatively high-grade gneisses of the Quetico Belt from typical low-grade granite-greenstone terranes to its north (Wabigoon) and south (Wawa). Both boundaries are characterized by different styles of transpressional deformation and strike-slip tectonism. Along the Wabigoon-Quetico boundary, deformation is recorded by a variety of structures controlled by competence contrast of heterogeneous lithologies at a variety of scales: from weak greenstones surrounding more competent gneiss domes to deformed polymictic conglomerates. Along the Quetico-Vermilion boundary, we will emphasize the role of plutonism and pluton geometry on subsequent deformation. Lastly, we characterize multiple deformation episodes in the Vermilion district in Minnesota.

In Montana, Wyoming, North and South Dakota, and Minnesota, Cretaceous strata are preserved in the asymmetric Western Interior foreland basin. More than 5,200 m (17,000 ft) of Cretaceous strata are present in southwestern Montana, less than 300 m (1,000 ft) in eastern South Dakota. The asymmetry resulted from varying rates of subsidence due to tectonic and sediment loading. The strata consist primarily of sandstone, siltstone, mudstone, and shale. Conglomerate is locally abundant along the western margin, whereas carbonate is present in most areas of the eastern shelf. Sediment was deposited in both marine and nonmarine environments as the shoreline fluctuated during major tectonic and eustatic cycles. A discussion of Cretaceous strata from southwestern to east-central Montana, the Black Hills, eastern South Dakota, and southwestern Minnesota shows regional stratigraphy and facies relations, sequence, boundaries, and biostratigraphic and radiometric correlations. The thick Cretaceous strata in southwestern Montana typify nonmarine facies of the rapidly subsiding westernmost part of the basin. These strata include more than 3,000 m (10,000 ft) of synorogenic conglomerate of the Upper Cretaceous part of the Beaverhead Group. West of the Madison Range, sequence boundaries bracket the Kootenai (Aptian and Albian), the Blackleaf (Albian and Cenomanian), and the Frontier Formations (Cenomanian and Turonian); sequence boundaries are difficult to recognize because the rocks are dominantly non-marine. Cretaceous strata in east-central Montana (about 1,371 m; 4,500 ft thick) lie at the approximate depositional axis of the basin and are mostly marine terrigenous rocks. Chert-pebble zones in these rocks reflect stratigraphic breaks that may correlate with sequence boundaries to the east and west. Cretaceous rocks of the Black Hills region consist of a predominantly marine clastic sequence averaging approximately 1,524 m (5,000 ft) thick. The Cretaceous System in eastern South Dakota (457 to 610 m; 1,500 to 2,000 ft thick) consists of a marine shelf sequence dominated by shale and limestone. Major sequence boundaries in South Dakota are at the base of the Lower Cretaceous Lakota Formation, Fall River Sandstone, and Muddy Sandstone, and bracket the Upper Cretaceous Niobrara Formation.