Abstract

The maturely dissected area immediately west of Cincinnati, Ohio, is underlain by rhythmically interbedded Upper Ordovician mud-stones and calcarenites that dip less than one-half degree to the northwest. Structure contours drawn on the lower and upper contacts of the Fairview Formation (lower Maysville) show very low and broad, northwest-plunging ridges and troughs that roughly coincide with present topographic highs and lows; these ridges and troughs may also reflect original sea-floor irregularities. Especially interesting are deformational structures visible in outcrops, i.e., small folds, thrust faults, and joints.

Typical structures are small asymmetric anticlinal folds, 1–3 feet high and 10 feet across, which are cut by one or more low-angle thrusts. The thrusts have dislocations up to 1 or 2 feet and terminate in bedding planes. These structures are observed almost exclusively in stream cuts, situated topographically below 700 feet elevation, stratigraphically in the Kope Formation and lower part of the Fairview Formation, and lithologically in thick mudstone beds separated by thin limestone layers. When the structures of the entire area are considered, a compilation of their strike directions produces a slight maximum to the north-northwest. Locally, they are generally transverse to the creek in which they occur although some types are parallel to it.

In decreasing abundance, systematic vertical joints have formed along four main directions–northwest, north-northeast, east-northeast, and north-northwest. The strongest trend follows the direction of a parallel drainage, as well as the ridges and troughs shown in the structure contours of the Fairview contacts, and the northwest prong of the Cincinnati Arch.

The deformational structures, which are of complex origins, have been formed by gentle postdepositional draping of the beds over the regional Arch; unloading by erosion and intrastratal down-dip flow of mudstone by gravitational squeezing; expansion; and sliding and slumping. Broad irregularities of the depositional surfaces, the contrasting physical properties of the lithologies, the arrangement of the lithologies with increasing amounts of mudstone toward the bottom, and the mature topography are responsible for locally controlling the direction and magnitude of the deformational stresses and, hence, the location, attitude, size, and shape of these folds, faults, and joints.

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