THERMOCHRONOLOGY RECORD OF THE LATE PALEOZOIC OUACHITA OROGEN, WESTERN ARKANSAS

The Ouachita Mountains and Arkoma foreland basin record the late Paleozoic collision between Gondwana and southern Laurentia. Although the structure and stratigraphy of the system have been studied extensively, the timing of thrusting, uplift, erosion, and sedimentation are incompletely known. Here, we summarize new and existing thermochronology which we integrate with other thermal indices across the Ouachita orogeny in western Arkansas, USA. Vitrinite reflectance and illite crystallinity indicate heating of pre-tectonic Ordovician through Mississippian deep-water strata to temperatures as high as 300˚ C. Eight zircon fission-track (ZFT) dates from these strata, exposed in the central Benton uplift of the orogen, indicate an average Late Mississippian cooling (~340 Ma; Johnson, 2011). Syntectonic Upper Mississippian and Lower Pennsylvanian strata, flanking the Benton uplift, yield average Late Pennsylvanian ZFT ages (~303 Ma, 6 sites). Middle Pennsylvanian Atoka Formation from frontal northern and southern thrust sheets yield slightly older ZFT ages (~319 Ma, 6 sites), which overlap with depositional age (< 316 Ma). These data suggest either (1) rapid burial (> 6 km) and exhumation in the foreland basin or (2) derivation from an external Appalachian source, whose detrital ZFT ages were consistently reset just prior to deposition. Zircon (U/Th)/He thermochronology dates from 8 sites in the frontal thrust sheets of the Ouachita orogen are latest Pennsylvanian to Triassic in age, ranging from 299 Ma to 215 Ma. Grain ages within these individual sites exhibit high dispersion and eU-age correlations consistent with slow cooling linked to post-orogenic exhumation. Likewise, apatite fission-track dates across the orogen and into the foreland are Cretaceous, recording continued slow, post-orogenic cooling.

Johnson, H.E., 2011, Texas A&M MS thesis