THE RECORD OF THE LATE PALEOZOIC OUACHITA OROGENY IN WESTERN ARKANSAS

Late Paleozoic orogenesis in the south-central midcontinent region of Laurentia is recorded in western Arkansas in a northward transect from Ouachita fold and thrust belt through the Arkoma foreland basin and into the Ozark uplift. Within the Ouachita belt core, the Benton uplift is comprised of strongly deformed, pre-orogenic lower Paleozoic strata that were thrust over the southern shelf margin. Deep-water sediments of the Stanley Shale that surround the Benton uplift record rapid synorogenic accumulation that accelerated at about 324 Ma during the Late Mississippian.

In the adjacent Arkoma basin, high sedimentation rates and associated normal growth faulting began during deposition of the middle Atoka Formation (about 313 Ma) in response to thrust loading and flexural extension of foreland basin crust. Normal faulting continued to affect strata as young as Desmoinesian (< 310 Ma) at the Mulberry fault in northern Arkoma basin. Buried normal faults acted as ramps for subsequent thrusts that migrated northward into the southern Arkoma basin and incorporated Atoka Formation in frontal thrusts and thrust-cored anticlines. Two preliminary zircon (U-Th)/He dates from lower Atoka Formation from the frontal Ross Creek thrust sheet yielded similar ages of 305 ± 17 Ma and 301 ± 13 Ma that indicate a late Pennsylvanian cooling, providing a minimum age of thrusting.

The distal foreland in the southern Ozark uplift responded to flexural extension through coordinated normal and strike-slip faulting of shelf strata over preexisting basement weaknesses, whose presence are supported by high-resolution aeromagnetic data. Fault-slip analyses show that several northeast-trending zones were reactivated with opposite slip sense as Ouachita shortening propagating into the foreland. An associated regional migration of warm mineralizing brines into the forebulge area during Ouachita shortening is shown by a northwest-maxima trend of mineralized faults and fractures in the Northern Arkansas lead-zinc district. Previous paleomagnetic and isotopic studies indicate Late Pennsylvanian to Permian ages for lead-zinc mineralization in the Ozark uplift.