A Study of the Joint Patterns in Gently Dipping Sedimentary Rocks of South-Central Kansas

Abstract

The two most prominent joint sets in Butler and Cowley counties and in parts of Greenwood, Chase and Marion counties, Kans., have consistent regional orientation regardless of the age of the rocks containing them. Set I strikes northeast, whereas Set II strikes northwest. The angle between the sets averages 93°. Both sets occur either as single or compound groups. Set III joints cluster around the two major sets and are subordinate in number with less consistent orientation. Most of the joints studied are vertical. On the basis of physical characteristics, joints of Sets I and II are shear joints, whereas Set III joints are tension joints. The joints are believed to have formed after the major development of the anticlinal folds in the region but may have developed simultaneously with later readjustments of the folds. The joints show no relation to subsurface faults but are closely aligned with the "en echelon" belt of faults of Oklahoma. Because most joints are vertical to the bedding, the jointing and regional tilting may be contemporaneous products of the same deformational period. The age of the joints is determined to be between post-lower Permian and pre-lower Cretaceous time. The joints may have formed as a result of north-West, horizontal, compressive forces generated by wrench-fault tectonics during initial Ouachita Mountain uplift. These forces were coupled with an opposite force, possibly from the Rocky Mountain region. Evidence shows that at least two separate but inter-related joint systems may exist in the midcontinent region.