THE NEMAHA UPLIFT - A CURSOR AND ITS PRECURSORS

Probably the best-known and least understood feature of Midcontinent USA is the Nemaha Uplift. This feature, known early as the Nemaha Mountains or Granite Ridge, is both a cause and effect of some major geologic processes across the area. The basic framework of the area was created from 1.8 to 1.6 billion years ago (Ga) by a series of island arcs sutured onto the Archean continent from both the south and southeast with a N-S intervening boundary zone ancestral to the Nemaha trend. Pre-existing tectonic framework controlled the trend of the Midcontinent Rift System (1.0 Ga). Stress at about 0.5 Ga emplaced the Elk Creek Carbonatite. Seas covered the area in latest Cambrian time depositing marine carbonate and sandstone. Local uplift occurred (Southeast Nebraska Arch) at the end of Lower Ordovician time as a precursor to the Nemaha trend and erosion exposed Precambrian rocks. Downwarping created the North Kansas Basin, a feature that existed from Middle Ordovician though Mississippian time. Regional stress beginning in Late Mississippian time reactivated the major trend of the Nemaha Uplift and its eastern boundary, the Humboldt Fault Zone. Pennsylvanian-age dark shale deposited marginal to the Nemaha Uplift was sourced from erosion of the Paleozoic carbonate section. Arkosic sandstone followed indicating that the Precambrian crystalline rocks were exposed. The highest parts of the Nemaha were not overlapped until Kansas City time in the Middle Pennsylvanian. Cyclic sedimentation and shallow marine deposition continued through the Pennsylvanian and Permian. Post-Permian uplift is evidenced by the Table Rock arch imposed on the Nemaha trend and over which Permian deposits were removed. Cretaceous stress is recorded by kimberlites in northern Kansas. Thus the patterns of both tectonics and stratigraphy record rejuvenation along the Nemaha trend. Historic earthquake records, particularly microseismic, indicate that there is still active stress.