In 1983, leasing agents representing several major exploration corporations appeared in the Lake Superior district of northern Wisconsin, an area best known for its exposures of Duluth gabbro and Mellen "black granite," and not for its hydrocarbon potential. The geologic column of interest is the 7,620 + m (25,000 + ft) thick clastic sequence termed the Keweenawan Supergroup (Precambrian), a basal shale that has been Rb-Sr-age dated at 1,075 ± 50 billion years. Within the past decade, this sequence has been recognized as having been deposited in response to tensional processes resulting in the development of a rift system. This Mid-Continent rift is best outlined by gravity and can be traced from the Keweenawan Peninsula of Michigan across northern Wisconsin, Minneso a, Iowa, and Nebraska into northeastern Kansas, a distance of 1,290 km (800 mi).

An organic-rich shale source unit is known in the Lake Superior area, where it creates intermittent subsurface oil seeps in the White Pine copper mine of Michigan. To the southwest along the trend of the rift, the extent of Keweenawan-equivalent sediments are masked by Paleozoic and younger sediments, increasing from a feather edge in central Minnesota to a reported 1,524 m (5,000 ft) in southwestern Iowa. In the mid-1960s, a natural-gas-storage drilling program centered on Dakota County, Minnesota, cored extensive thicknesses of Precambrian clastics, subsequently named the Solor Church Formation. This subsurface unit, together with surface exposures of the younger Hinckley Sandstone and Fond du Lac Formation, constitutes the Keweenawan section of Minnesota. Less is known of the Kewee awan-equivalent lithology of Iowa, where the Minnesota terminology is being used in geophysical studies of the Mid-Continent gravity high. Here, a late Precambrian "red clastic" sequence is known, apparently similar to that termed the Rice Formation in Kansas--a carbonate subordinate sequence of sandy shales and feldspathic sandstones.

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