PERICLINAL FOLDING IN THE OZARK PLATEAU: A RECORD OF LOCAL KARST COLLAPSE OR REGIONAL TECTONIC FORCES?

The Ordovician Rubidoux Formation of the Ozark Plateau, Missouri is typically identified as mechanically competent sub-horizontal beds of medium grained sandstone. North of Licking, MO, the Rubidoux Formation was deformed into a series of dome-and-basin shaped buckle folds, exposed in road cuts over a distance of ~10 km. Such folds are highly unusual within the Ozark Plateau and their origin remains controversial.

Three major hypotheses have been proposed based on field observations and stereographic analysis. Given the pervasive distribution of karst collapse throughout Missouri, one hypothesis to explain these folds is that the Roubidoux Formation sagged into collapsed caverns. A second hypothesis is that these folds are the result of compression induced in a possible transpressive zone between two left-lateral strike-slip faults. A combination of the two processes is also possible.

Field measurements of 37 fold structures including more than 300 recordings of bedding attitudes and stereographic projections confirm that the Roubidoux folds are periclinal folds. They elongate along N59E to S59W, with the maximum shortening occurring along N60W to S60E. The uniformity of the fold orientations over this large distance is inconsistent with an origin by sag folding related to karst collapse. While the orientation of the shortening direction is inconsistent with the general trend of midcontinent deformation structures (i.e. Cox, 2009 reports a NE-SW shortening direction), two possible left-lateral strike-slip faults mapped to the north and south of the folding zone may have locally reoriented the stress field. This is supported by a simple 2D finite element model indicating that transpressional deformation in a restraining jog results in localized shortening direction consistent with the distribution of the observed periclines.

It is concluded that a regional tectonic event induced localized buckle folding whereby local karst effects may have contributed in forming the initial perturbation.