SPATIAL VARIATIONS IN JOINT ORIENTATIONS IN CENTRAL KANSAS: IMPLICATIONS FOR TECTONICS OF THE MIDCONTINENT

A systematic shift in joint orientation in north-central Kansas indicates a shift in the regional state of stress possibly over time. A study of joint patterns exposed in outcrops was conducted in a 9-county area. The joints occur in sandstones and limestones of Cretaceous age, including from oldest to youngest, the Dakota Sandstone, Greenhorn Limestone, and Niobrara Limestone. These units generally dip northward at gentle rates of approximately 1 meter per kilometer. Over 100 joints were mapped in each outcrop, and 28 outcrops were utilized. The joints were subdivided into systematic and non-systematic sets. Systematic joints are longer and continuous at joint intersections; whereas, non-systematic joints terminate or step at joint intersections. All joints in the study area are vertical or nearly vertical. The systematic joints in the western portion of the study area strike approximately east-west (N 83 W). To the east and down section, the systematic joints gradually change to a strike of approximately northwest-southeast (N 63 W). The mean strike of the non-systematic joints is approximately normal to the systematic joints; however, at each outcrop the strike of the non-systematic joints has more variability than the systematic joints. Several possible causes for the shift in orientation of the systematic joints are proposed. The shift may represent changes in the stress field along the boundary between the Central Kansas Uplift and the Salina Basin to the east, possibly influenced by basement faults. A reorientation of the principal stress axis with time is also possible. The underlying Permian Hutchinson Salt pinches out in the study area, which might also create a change in the stress orientation. The systematic joint orientation for the Cretaceous rocks in the study area differs from the dominantly northeast-southwest oriented systematic joints in older Pennsylvanian strata exposed in eastern Kansas. The state of stress that formed the joints in the Pennsylvanian rocks was evidently different from that which formed the joints in the Cretaceous rocks. The joints in Pennsylvanian System were likely influenced by a stress regime set up during the Ouachita Orogeny in Oklahoma; whereas, the joints in the Cretaceous System may have been subject to stresses transmitted from the Laramide Orogeny to the west.