ABSTRACT

The Ouachita Mountains of west central Arkansas consist of thrust-faulted and folded Paleozoic sedimentary and low-rank metamorphic rocks. Detailed structural and stratigraphic mapping was conducted on portions of the Norman and Caddo Gap 7.5 minute Quadrangles to determine the depositional and structural evolution of the area. Lithologies, fossils, sedimentary structures, paleocurrent trends, and trace fossils characteristic of lower to middle Ordovician Mazarn, Blakely and Womble Formations indicate deposition under deep water, reducing conditions, probably on distal lobes of a submarine fan in a starved basin.

Features evaluated to determine structural evolution of the study area include mesoscopic folds, cleavages, planar quartz veins and lineations. Faults were both observed in the field and inferred from surrounding structural styles. Three orogenic fault and/or fold episodes and one post-orogenic uplift phase have been inferred from over-printing relationships. Class 2 (similar) folds of slump origin are common in the Mazarn Shale and preceeded tectonic folds and faults.

The first orogenic phase began in Late Pennsylvanian time with development of Class 1 and 3, east-west oriented mesoscopic and macroscopic folds, with genetically related axial cleavage, synchronous with strong northward transport along low-angle thrust faults. Earlier structures were subsequently overprinted during a second tectonic phase by horizontal, northwest-trending Class 1 and 3 folds. The third deformation phase was characterized by northwest-southeast trending, right-lateral strike-slip cross faults that offset structures developed during orogenic episodes 1 and 2. These faults are genetically related to steeply-plunging mesoscopic folds that developed simultaneously with northeast-southwest macroscopic folds. A northwest transport vector characterizes the third tectonic phase. Finite strain markers such as ellipsoidal pebbles, granules, and planar quartz veins indicate an average extensional direction that plunges shallowly to the southeast.