THE ORIGIN OF PERICLINES IN THE OZARK PLATEAU, MISSOURI

Periclines in the Early Ordovician Roubidoux sandstone Formation are exposed in road cuts along US Highway 63 in the Salem Plateau, Missouri over a distance of 10 km. These folds are in marked contrast to the more typical horizontal to sub-horizontal Ordovician strata of the Ozark Plateau. The folds outcrop between the trace of two NW trending faults: Bradford Branch fault and the Licking fault.

We present the results of an integrated detailed structural analysis of the folds with geomechanical modeling to constrain their origin. Structural data of 43 folds indicate the following: 1) Two major fold orientations at 059 and 330 are consistent with a “Basin and Dome” pattern that reflects the geometrical nature of the periclines within the transpression zone. 2) Fold geometries vary from upright open to gentle asymmetric verging south. The fold amplitude (from ~2.5m to ~0.5m) appears to decrease towards either end of the fold train. 3) Field observations indicate the folds define a pattern of alternating basin and domes. 4) Fold orientations, based upon attitudes of axial surfaces and hinge lines as determined from stereographic projection, also indicate a pattern consistent with the presence of basins and domes. Stereogram patterns (i.e., π-diagrams) for these folds include: 1) great circles, 2) small circles, and 3) ellipses with “fish-hook” patterns. Based upon π-diagrams, these folds would traditionally be interpreted as conical folds. However, comparison with synthetic stereogram patterns and curvature analyses indicate these folds are best interpreted as periclines.

Formation of the periclines is modeled using finite element analyses with left-lateral strike-slip motion along the bounding faults at 5mm/year with 35% shortening as a result of transpression in a restraining step-over. This results in a localized shortening direction (σ₁) at 338 consistent with the orientation axial surfaces of the periclines. We propose that the Roubidoux periclines formed by as a result of reactivation of basement faults with left-lateral slip during a single deformational event, possibly during the late Paleozoic Alleghenian Orogeny.