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Applied Geophysics, Inc., Salt Lake City, Utah

Abstract: The Strike-Slip, Compressional Thrust-Fold Nature of the Nemaha Ridge in Eastern Kansas and Oklahoma

Much has been written about the Nemaha Ridge in Kansas and Oklahoma since geologists first became aware of it from oilwell drilling in the early years of this century. It has been described as extensional, compressional, and strike-slip. In this talk I will present data to show that the Nemaha was formed in the usual manner of non-volcanic, non-intrusive 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 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, the Uinta Mountains in Utah and the Wichita Mountains in Oklahoma, essentially making the ridge a Vshaped "popup 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 the end closures of structures along it, i.e. the formation of petroleum traps, plus additional complexities that have made the Nemaha Ridge difficult to interpret and its origin controversial. Small normal faults indicate that extension played a minor role in post-Permian time.

AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas