The Ouachita Mountains of Arkansas and Oklahoma are made up of two structural decks. The lower deck of tight to isoclinal folds in pre-Middle Mississippian strata records multiple folding and low-grade metamorphism. The upper deck of open folds in Carboniferous rocks shows no evidence of the multiple folding and metamorphism. Dips of fold limbs in the lower deck are typically more than 60°; dips of fold limbs in the upper deck are generally less than 45°; fold wavelengths in the lower deck are in the range of 0.5 to 3.5 km; fold wavelengths in the upper deck are generally in the range of 12 to 15 km. Estimates of shortening of the folds in the lower deck are five times greater than those of shortening of the upper deck. The change from tight folding to broad folding takes place in the middle part of the Mississippian Stanley Group. The difference in fold style has been attributed to disharmonic folding of a stiff upper deck and a ductile lower deck. However, the boundary between harmonic and disharmonic folds shows no apparent relation to the fold wavelength or the stratigraphic spacing of stiff beds. We hypothesize that the difference in structural style reflects the unconformable deposition of younger folded and faulted foreland-basin strata (the upper deck) over the older lower deck strata, which were stacked in an accretionary wedge.

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