GEOLOGY OF THE OSAGE SW 7.5-MINUTE QUADRANGLE ADJACENT TO BUFFALO NATIONAL RIVER, NORTHERN ARKANSAS

The Osage SW 7.5-minute quadrangle is situated in the Ozark Plateaus region of northern Arkansas. A topographic watershed divide in the southeast part of the map separates steep slopes proximal to the Buffalo River south of the divide from more gentle slopes and north-flowing streams in the headwaters of the Kings River north of the divide. The southeast portion of the quadrangle captures a portion of the Buffalo National River—administered by the National Park Service—and includes the popular Lost Valley area. The quadrangle resides on the southern flank of the Ozark Dome within the foreland of the Ouachita orogenic belt. Exposed within the quadrangle are approximately 470 m of Middle Ordovician through Middle Pennsylvanian clastic and carbonate rocks that are mildly deformed by faults and folds. Geologic mapping investigations document stratal variations and structural features that include the following: (1) Upper Mississippian Fayetteville Shale, whose thickness beneath conformable Pitkin Limestone varies from 43 m in the southwest to 15 m in the northwest; (2) the northeast pinch-out of the Upper Mississippian Pitkin Limestone beneath a regional unconformity at the base of Lower Pennsylvanian strata; (3) the down-to-south Compton fault and an associated footwall monocline that accommodate as much as 105 m total displacement; (4) a west-trending monocline that accommodates additional down-to-south displacement at the western extent of the Compton fault; (5) a parallel structural zone approximately 2 km south of the Compton fault that includes a steeply dipping monocline and the Logan Mountain fault, which together accommodate down-to-north displacement and, linked with the Compton fault, probably forms an incipient graben; (6) the Pickle Hollow fault and associated monoclines that accommodate as much as 45 m down-to-south displacement in the northern part of the quadrangle; and (7) mixed debris-flow and landslide Quaternary deposits sourced in Pennsylvanian and Upper Mississippian shale and sandstone units, that present potential geologic hazards.