ABSTRACT

Much has been written about the buried Nemaha uplift in Kansas and Oklahoma since drillers and geologists first became aware of it from oil-well drilling in the early years of the twentieth century. It has been described as extensional, compressional, and strike-slip. In this paper I will present data to show that the Nemaha was formed by compressional or thrust faulting that is rooted deep within the Precambrian crust and extended in listric fashion to the ground surface coincident with the Humboldt fault zone, or east-bounding fault. Compressional effects observed from well data and seismic surveys do not permit an extensional origin.

Two additional effects occurred simultaneously with the thrusting. A back thrust evidently formed locally in a manner similar to that mapped in many compressional environments, for example the Front Range of Colorado (Jacob, 1983), the Uinta Mountains in Utah (Stone, 1993b), and the Wichita Mountains in Oklahoma (Brewer and others, 1983), essentially making the Nemaha uplift a V-shaped "pop-up" block in many places, thus explaining the up-to-the-east fault or fold on the west. A strong component of strike-slip motion resulted in end-closures of structures along the uplift including many petroleum traps, plus additional complexities that have made the Nemaha system difficult to interpret. Small, near-surface normal faults indicate that extension played a minor role in post-Permian time.

The main movement of the Nemaha system began in early Pennsylvanian time or perhaps latest Mississippian, and extended to (or into) Permian time, coincident with the Alleghenian orogeny on the eastern seaboard 800 mi (1,300 km) to the east. Earlier periods of lesser movement occurred in mid-Ordovician and mid-Devonian time, coincident with the less extensive Taconic and Acadian orogenies of the eastern U.S. The relationship in time between these compressional events on the eastern seaboard and the compressional Nemaha system is clear.