THICKNESS VARIATIONS OF UPPER MISSISSIPPIAN STRATA IN THE WESTERN BUFFALO RIVER REGION OF NORTHERN ARKANSAS: EVIDENCE OF LATE MISSISSIPPIAN TECTONIC ACTIVITY IN THE SOUTHERN OZARK DOME

Geologic mapping in the western part of the Buffalo River watershed documents large variations in thickness of Upper Mississippian (Chesterian) strata that record active tectonics in Late Mississippian-Early Pennsylvanian time. The Chesterian stratigraphic interval comprises, in ascending order, Batesville Sandstone, Fayetteville Shale (with upper Wedington Sandstone Member), and Pitkin Limestone preserved beneath a regional Pennsylvanian unconformity. High topographic relief in the study area allows Chesterian thickness estimates along 125 traverses, as determined from the elevation difference between mapped upper and lower contacts and corrected for any tilt shown by structural contours for the lower contact. Chesterian interval thickness varies greatly across the area, from < 5 m to > 150 m. A strong thickness gradient that trends northwest is sharpest where it coincides with a 1.5-km-long exposure of the Kyles Landing fault (KLF). The KLF has 90 m of top-to-east-northeast reverse throw at its southern exposure where it is buried by undeformed Lower Pennsylvanian Cane Hill Member of the Bloyd Formation. Pitkin Limestone and Wedington Sandstone Member cap a 100-m-thick Chesterian interval in the eastern footwall of the KLF, but are absent in the hanging wall. An elongate isopach zone with Chesterian thickness < 15 m thick lies in the immediate hanging wall 1 km west of the KLF but this “thin zone” can be traced 7 km west-northwest beyond the northern tip of the KLF. In the study area, the basal Pennsylvanian Cane Hill Member lacks major thickness variations like the underlying Chesterian interval. Together these geologic relations document a 10-km-long, northwest-trending uplift that was active in Late Mississippian-Early Pennsylvanian time that at least partly accommodated east-northeast shortening along the Kyles Landing fault with synchronous erosion of uplifted youngest Chesterian strata. Similar thinning of Chesterian strata has also been recognized about 15 km farther north over the Carrolton Dome, showing that the southern Ozark Dome was tectonically active in Late Mississippian time.