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Geophysical evidence in the Mid-Continent has led to delineation of a rift system active during the Proterozoic Y Era. The Mid-Continent rift system can be traced by the Mid-Continent gravity high and corresponding aeromagnetic anomaly signature from the surface exposure of the Keweenawan Supergroup in the Lake Superior basin southwest in the subsurface through Wisconsin, Minnesota, Iowa, Nebraska, and Kansas.
The Mid-Continent gravity high, which includes the highest gravity values within the continental United States, has been interpreted historically as a reflection of mafic extrusives, structurally high to the surrounding basement complexes. However, studies of analogous continental rifts and current rift theory call for existence of an anomalous mantle-derived body within the crust. The presence of this anomalous dense body is expressed by the narrow gravity high. The gravity high is imposed upon a broader gravity low which reflects the crust-mantle boundary's isostatic response to injection of mafic mantle material into the crust. Seismic refraction studies support this with findings of an unusually thick crust in the Lake Superior region. The surface response to the crustal attenuati n and subsidence due to mafic loading during the late Proterozoic was the development of a deep rift valley and sedimentary basin.
The aeromagnetic anomaly signature of the rift trend discloses where these sediments have been preserved. Thick accumulations of upper Proterozoic sediments are indicated by both upward continuation of the aeromagnetic profiles across the rift trend and gravity models which incorporate: (1) a deep mafic body to create the narrow gravity high, (2) anomalously thick crust to account for the more regional gravity low, and (3) sedimentary accumulations on the Precambrian surface to explain the small-scale "notches" which occur within the narrow gravity high. Reflection seismic data are virtually unknown in the rift area; however, data recently acquired by COCORP across the southern end of the feature in Kansas provide evidence of thick stratified sequences in the rift valley.
Studies of the East African rift have revealed that the tropical rift valley is an exceptionally fertile environment for deposition and preservation of kerogenous material. Although penetrations of the Keweenaw rift sediments are extremely scarce, the occurrence of indigenous mobile crude oil in the White Pine mine on the Keweenaw Peninsula, Michigan, and bituminous partings in a core at the periphery of the Twin Cities basin, Minnesota, strongly support the extrapolation of a rich algal-fungal community throughout the sediment-filled valley of the Mid-Continent rift. Rift valley basins areally represent only 5% of the world's basins, but they have been determined to contain 10% of the world's present reserves. The Sirte, Suez, Viking, Dnieper-Donetz, and Tsaidam basins are just a few of the rift basins currently classed as "giant" producers. The existence of a rift basin trend with thick accumulations of preserved sediments, demonstrably organic rich, introduces the northern Mid-Continent United States as a new frontier for hydrocarbon exploration.
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