A LATE PALEOZOIC RECORD OF THE OUACHITA OROGENY IN WESTERN ARKANSAS

A record of late Paleozoic orogenesis in the south-central midcontinent region of Laurentia is preserved in a northward transect from Ouachita fold and thrust belt through the Arkoma foreland basin and into the Ozark uplift in western Arkansas. Within the Ouachita belt core, the Benton uplift is comprised of strongly deformed, weakly metamorphosed, pre-orogenic lower Paleozoic strata that were thrust several 10’s of km over the continent margin. Deep-water sediments that surround the Benton uplift record rapid synorogenic accumulation that began during the Late Mississippian (about 328 Ma).

In the adjacent Arkoma basin, high sedimentation rates and associated normal growth faulting began during deposition of the middle Atoka Formation (about 315 Ma) in response to thrust loading and flexural extension of foreland basin crust. Normal faulting continued to affect strata as young as Desmoinesian 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. Shortening directions estimated from fault-slip analyses, anisotropy of magnetic susceptibility data, and regional joint sets demonstrate consistent NNW-directed shortening. Zircon (U-Th)/He dates from lower Atoka Formation from the frontal Ross Creek thrust span 266 - 344 Ma (305 ± 17 Ma average), consistent with rapid burial and subsequent Late Pennsylvanian activity on this thrust.

The distal foreland in the 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 new high-resolution aeromagnetic data. Fault-slip analyses show that several northeast-trending zones were later 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 Early Permian ages for lead-zinc mineralization in the Ozark uplift.