GEOMETRY, RELATIVE AGES, AND TECTONIC IMPLICATIONS OF JOINTS FROM A REGIONAL ASSESSMENT OF THE BUFFALO NATIONAL RIVER AREA, NORTHERN ARKANSAS

Joints record past stress states, impart fracture permeability, and enhance physical and chemical erosion that contributes to landscape evolution, but their origins are often complex. We use relative field ages, stratigraphic and spatial analyses to characterize a large joint population (n = 4432, 816 sites) measured within an 80-km-long, E-W-trending transect in the Buffalo National River area in northern Arkansas. Lying on the southern flank of the Ozark Dome, this area exposes gently dipping Ordovician, Mississippian, and Pennsylvanian sedimentary rocks deformed by faults and open folds formed during N-S extension and subsequent local N-S shortening in the foreland of the Late Paleozoic Ouachita orogenic belt.

Six steeply dipping joint sets are recognized on the basis of orientation frequency. The highest frequency set strikes N (345-5 azimuth range), with subordinate NNW (325-345) and NNE (5-20) subsets. Other prominent sets strike NE (40-65) and WNW (280-310), and a minor EW set (80-100) is recognized from age relations. Whereas N and NE sets are spatially pervasive, the WNW set and NNE subset are relatively more common in the east and the NNW subset is more common in the west. Abutting relations between joints provide 210 relative ages at sites and indicate mixed ages among the N, NE, WNW, and EW sets. In the west, the EW set is locally oldest and bleached by fluids related to nearby Pb-Zn mineralization in Boxley Valley whereas younger abutting NE and N sets are unbleached. In the east, N, NE, WNW, and EW sets mutually abut and thus are broadly contemporaneous.

Joints of the minor, oldest EW set probably accompanied widespread faulting and N-S extension of the Ozark Dome during the mid- to late Pennsylvanian. The pervasive N set probably formed in late Pennsylvanian to Permian time as compressive stresses propagated into the foreland as the Ouachita orogenic belt docked with Laurentia to the south. Those NE and WNW joints contemporaneous with the N set are parallel to common fault trends, and we speculate they reflect local stress reorientations influenced by reactivation of these preexisting structures. Paradoxically, most joints formed after most faults, implying an evolution to low effective-confining-stress conditions favorable for joint formation, perhaps due to Ozark dome uplift and/or elevated fluid pressures.