FOLDED ORDOVICIAN STRATA IN TEXAS COUNTY, MISSOURI: A RECORD OF INTRAPLATE TECTONIC DEFORMATION OR KARST COLLAPSE?

Folded sandstone layers of the Ordovician Roubidoux Fm. are well exposed along a 7 km stretch of Highway 63 in Texas Co, MO. In outcrop, fold heights decrease and fold widths increase from north to south along this traverse. The largest of the folds has a fold height of approximately 6 m and a fold width of approximately 20 m. The folds are upright open asymmetric class 1B folds. Axial planes of the folds strike from 086 to 135 with dips of 85 to 900. Hinge lines plunge 0-150 and typically trend SE, but NW trends are also present. The distribution of oblique- and strike-joints associated with these folds suggest a NE-SW orientation for sigma 1. These folds are remarkable as locally the Paleozoic strata are typically subhorizontal (i.e., undisturbed).

In order to investigate the subsurface extent of these folds a shallow seismic investigation was performed using a 12 channel seismograph, with 40 hz geophones at 5 ft intervals, with a sledge hammer source. Standard processing procedures revealed several prominent reflectors, that were folded in nature and discontinuous laterally across the seismic survey. The folded nature of these reflectors matches the geometry of the folds present in the surface exposures. This indicates that the integrity of the folds is maintained at depth (in dolomites of the underlying Gasconade Fm.) throughout the region imaged. Reflection patterns expected for collapse features or caverns have yet to be observed. This suggests the propagation of folds within this region was produced in a compressional stress regime rather than due to collapse of caverns in the underlying Gasconade Fm. The region investigated is bounded to the north and south by en-echelon “apparent” normal faults. However, the presence of these folds implies an earlier history of motion along these faults that would create a local compressional stress regime leading to shortening and buckling of the Paleozoic strata within the intervening region.