GEOLOGIC FRAMEWORK OF THE BOXLEY QUADRANGLE ON THE SOUTHERN FLANK OF THE OZARK DOME, NORTHERN ARKANSAS

The Boxley 7.5-minute quadrangle lies within the Boston Mountains Plateau on the southern flank of the Ozark Dome and includes the westernmost part of Buffalo National River, a park administered by the National Park Service. Deep valleys of the Buffalo River and its tributaries expose an approximately 500-m-thick sequence of Ordovician, Mississippian, and Pennsylvanian carbonate and clastic sedimentary rocks that have been mildly deformed by faults and folds.

Twelve Ordovician through Pennsylvanian sedimentary units are mapped in the quadrangle. A stratigraphic contact within a sandstone and shale sequence separating the upper part of the Bloyd Formation (Morrowan) from the overlying Atoka Formation (Atokan) is most problematic. Based on several partly exposed sections, we placed this contact below a laterally continuous interval of shale and very fine grained sandstone and above a laterally variable interval of quartz-pebble-bearing sandstone to siltstone which is often strongly bioturbated. This lower unit is interpreted as a drowned top of paleoriver deposits of the middle Bloyd sandstone.

A N35ºE-trending structural zone of faults and folds, which drops strata 20-60 m to the southeast, transects the quadrangle and coincides with the southwestern part of the regional Ponca lineament. The 4-km-long Edgemon Creek fault, at the southwest end, changes to a faulted monocline in the northeastern part of the zone. Kinematic analysis of small faults indicates that this zone accommodated right-lateral transtension during late Paleozoic N-S extensional flexure of the southern Ozark Dome, and that the zone was locally reactivated in N-S shortening at the close of the Ouachita orogeny.

Geology controls many landscape features of the quadrangle. Valley alignments of part of the Buffalo River and Beech and Edgemon Creeks along the N35°E structural zone probably reflect enhanced erosion of fractured rock concentrated along the zone. Steep hillsides in the quadrangle host both extensive colluvial deposits with fan-like morphologies and large landslides whose basal glide-planes root into the Mississippian Fayetteville Shale. Springs that source a defunct fish hatchery in Boxley Valley are concentrated where Mississippian limestone is folded and faulted down to the base level of the Buffalo River flood plain.