THE MIDCONTINENT RIFT SYSTEM AND THE NEMAHA UPLIFT - A TECTONIC TANGLE

The Midcontinent Rift System (MCR) and the Nemaha Uplift (NU) are two prominent tectonic features of the midcontinent. Although much different in age and genesis, they are often discussed as similar features in affecting structure, stratigraphy and environments of deposition. The MCR was early noted on gravity signature and trend as the Greenleaf Anomaly. The NU, based on early drilling, was noted as the Nemaha Granite Ridge. When these two features are more closely examined in space and time, there are significant differences. The MCR is an important chapter in Precambrian history and has had only subtle effects on the later Paleozoic. Conversely, the NU had significant impact on depositional patterns, structure, and preservation of strata during the Phanerozoic. The 1.1 billion-year-old MCR represents a rift that has a subcontinental extent and contains thousands of feet of basalt and sediments. The NU has an origin in the original growth of the continent, consists largely of granite and has undergone a variety of rejuvenations that strongly influenced the Paleozoic record. Both features were influenced in origin by earlier Precambrian history but later had histories that strongly varied in space, origin and time. An important interrelationship has been the influence of the MCR on the location and development of the NU and the resulting patterns in the Paleozoic.