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Seismic reflection profiling in northeastern Kansas by COCORP was initiated to investigate several prominent features of the midcontinent, including the Midcontinent Geophysical Anomaly (MGA), which has been associated with a buried extension of the Keweenawan rift, and the Nemaha Uplift, part of a crystalline basement block uplifted in Pennsylvanian time (Figure 1). The MGA is characterized by gravity and aeromagnetic highs extending from the Lake Superior region to near the Kansas-Oklahoma border (Figure 1, 2, 3; see Yarger, 1981). Surface exposures in the north, as well as samples drilled along the trend of the anomalies, indicate that mafic volcanic rock and associated clastic sediments were deposited in a narrow trough formed by continental rifting of Keweenawan age (1.1 b.y. ago; see also King and Zietz, 1971; Chase and Gilmer, 1973).

The Nemaha Uplift is a north-south trending feature extending from Omaha, Nebraska to northern Oklahoma (Figure 1). in northeastern Kansas the Nemaha separates the Forest City basin from the Salina basin to the west (Steeples, in press). The origin of the Nemaha Uplift and associated Humboldt border fault (Figure 4) is enigmatic, but their lateral proximity and subparallelism to the MGA suggests that they may represent reactivated rift structures (Yarger, 1981).

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