Abstract

Much has been written about the Nemaha Ridge in Kansas and Oklahoma since geologists first became aware of it from oil-well drilling in the early years of this century. It has been described as extensional, compressional, and strike-slip. Data are presented to show that the Nemaha was formed in the usual manner of nonvolcanic, nonintrusive uplifts, that is, by compressional, or thrust, faulting that began deep within the Precambrian crust to the west and extended in listric fashion to the ground surface coincident with the Humboldt fault zone, or an east-bounding fault. Compressional effects observed from oil-well drilling and seismic surveys along its entire length are too numerous to ignore and to permit of an extensional origin, if it is even possible to consider an extensional origin for an uplift.

Two additional effects occurred simultaneously with the thrusting. A backthrust evidently formed in a manner similar to that mapped in many compressional environments, for example the Front Range of Colorado, Uinta Mountains in Utah, and Wichita Mountains in Oklahoma, essentially making the ridge a V-shaped "pop-up block," thus explaining the up-to-the-east fault or fold on the west. Additionally, the thrusting had a strong component of strike-slip motion which resulted in end closures of structures along it, that is, the formation of petroleum traps, plus additional complexities that have made the Nemaha system difficult to interpret and its origin controversial. Small, near-surface normal faults indicate that extension played a minor role in post-Permian time.