LITHOSTRATIGRAPHY AND STRUCTURAL CONTROL OF THE ATOKA FORMATION (MIDDLE PENNSYLVANIAN), INTERSTATE 540 ROADCUTS, SOUTHERN OZARKS, NORTHWESTERN ARKANSAS

The Atoka Formation (Middle Pennsylvanian), an alternation of sandstone and shale, forms most of the Boston Mountains Plateau and represents the thickest Paleozoic interval in the southern Ozarks. It reaches nearly 3000 feet at the boundary faults of the adjacent Arkoma foreland basin in west-central Arkansas. Atokan lithostratigraphic subdivision and mapping in the subsurface of the Arkoma Basin has recognized at least 23 named, but informal, sandstone reservoir units in the Arkoma “fairway” of western Arkansas. In contrast, surface mapping of the Atoka has only recognized informal lower, middle and upper divisions restricted to the central Arkoma Basin and Ouachita Mountains. There has been no recognition of named divisions of the Atoka Formation in the southern Ozarks complicating understanding of their geologic history. Specifically, the middle Atoka is the thickest portion of the formation in the subsurface, and generally viewed as representing maximum subsidence of the Arkoma foreland basin. Yet previous diagrammatic cross-sections indicate either no exposed middle or upper Atoka, or only a modest aerial distribution.

Construction of Interstate 540 in northwestern Arkansas has provided excellent outcrops of the Atokan units in a north-south dip section through the south flank of the Ozark Dome. Correlation in outcrop, and to logs of nearby wells, demonstrates that both middle and upper Atokan reservoir units make a significant contribution to exposures in the southern Ozarks. Three key tie points indicate that most of the change in section occurs at the Drakes Creek Fault, a normal fault striking northeast-southwest and downthrown to the southeast. Between West Fork, Washington County, and the Craddick Well at Chester, Crawford County, the base of the Atoka Formation exhibits a dip of only 26.6’, changing elevation by 750 feet in 18.3 miles, although there are at least 10 faults mapped or observed along this portion of the interstate. Most of the section above road level is middle Atokan. From the Craddick Well to the Gilker #1-32 well, a distance of 6.2 miles along the interstate, the Drakes Creek Fault is encountered and the base of the Atoka Formation is 2100 feet below its position at Chester. Because of the fault, most of the section from this point south is upper Atokan on the surface.