25-6 New Field Evidence for Multiple Phases of Shortening in the Benton Uplift, Ouachita Mountains, Arkansas

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland (Posters)

Presenting Author:

Authors:

Abstract:

The Ouachita Mountains of southwestern Arkansas are part of a late Paleozoic orogenic system that developed during collision between the southern margin of Laurentia and the northward- advancing South American continental margin. Due to northward tectonic transport from the south, this collision primarily produced north-verging folds and thrust faults. The central uplift regions (i.e., the Benton and Broken Bow uplifts), however, predominantly consist of south- verging structures, leading to ongoing debate regarding their origin. In this study, we conducted fieldwork to examine newly exposed outcrops in the eastern Benton Uplift in central Arkansas to better understand the structural patterns associated with the Ouachita orogeny. Our study area, located in the Zig Zag Mountains fold-and-thrust belt, forms part of the southern limb of the eastern Benton Uplift anticlinorium and consists of southwest-plunging folds of the Ordovician to Early Mississippian strata. Noncompetent, shale-dominated units are common (e.g., the Womble and Stanley shales) and occur among ridge-forming competent units such as the Bigfork Chert, the Arkansas Novaculite, and the Hot Springs Sandstone. Field observations indicate that while some structures in the Zig Zag Mountains verge northward, many display southward vergence. Both upright and asymmetric southward-facing overturned folds are present, with wavelengths on the order of one to two kilometers. Numerous smaller, meter-scale parasitic folds exhibit strong vergence reversals (i.e., vergence opposite to the inferred sense of shear). Steep, north-dipping faults with normal separation are also observed and are interpreted as former low- angle thrust faults that were later rotated to steep orientations. These preliminary results suggest two phases of folding and thrusting during the late Paleozoic Ouachita orogeny. An initial

shortening phase produced north-verging structures, which were subsequently overprinted by a later episode of south-verging folding and thrusting. Possible explanations for this southward vergence include late-stage orogenic inversion, south-directed thrusting, or basement-involved underthrusting beneath the Benton Uplift. The results of this study help constrain the structural evolution of the Ouachita orogeny and have implications for understanding similar orogenic systems elsewhere. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

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