JOINTS IN THE MIDDLE-PENNSYLVANIAN MCALESTER FORMATION, FORT SMITH, ARKANSAS

An understanding of joints and joint systems is an essential part of many geologic investigations. Joints conduct the flow of subsurface fluids and serve as potential zones of weakness, and are thus of interest to hydrogeologists, petroleum engineers, and civil engineers. Joints also serve as indicators of the types and orientations of stresses during past deformational events. Preliminary field work in sandstones of the Middle Pennsylvanian McAlester Formation in the vicinity of Fort Smith, Arkansas has revealed a widespread joint system comprising both systematic and nonsystematic joint sets. Data was collected at eight isolated outcrops of thinly bedded sandstone over an area of about 30 km². At the time of this writing, the data set contains 284 systematic joints and 82 nonsystematic cross-joints. For each of the eight outcrops, joint azimuths were measured and plotted on two rose diagrams – one for systematic joints and the other for nonsystematic joints. The systematic joint set has a markedly uniform mean azimuth of 347° ± 1° and spacing ranging from about 0.25 to 1.5 meters. Cross-joints are less abundant and less regularly oriented, with mean azimuth of 80° ± 13°. The systematic joints are interpreted as mode I tensile fractures oriented vertically and approximately parallel to NNW- directed compression associated with the Ouachita Orogeny about 300 million years ago. At that time, as evidenced by the orientations of fold axes associated with the orogeny, the maximum principal stress (σ₁) in the region was oriented slightly west of due north, the least principal stress (σ₃) was oriented just south of due west, and the intermediate principal stress (σ₂) was near-vertical. The systematic joints formed parallel to the vertical σ₁-σ₂ plane, concurrent with and roughly perpendicular to the east-west oriented fold axes that characterize the late Paleozoic sedimentary rocks of the Arkansas River Valley.