THE HORSE CREEK STRUCTURE: A NEWLY IDENTIFIED FAULT IN NORTHEASTERN OKLAHOMA?

Evaluation of a new, moderate-resolution (10 m) digital elevation model of Oklahoma reveals a prominent, ENE-trending, ~100-km long, arcuate topographic escarpment through Mayes, Craig, Delaware, and Ottawa Counties in northeastern Oklahoma and adjacent southwestern Missouri. Previously identified as an asymmetric, southeast-vergent fold called the Horse Creek anticline (Siebenthal, 1907), the escarpment attains maximum local relief of ~65 m (200 ft). Geologic mapping of the region shows that the Horse Creek structure is developed within a regionally extensive exposure of Lower Mississippian strata (Keokuk, Reeds Spring, St. Joe Group, and equivalents), but a linear isolated band of Upper Mississippian strata (Pitkin, Fayetteville, Batesville, Hindsville, and Moorefield Formations) marks the base of the escarpment for a distance of ~60 km. At least three distinct segments can be identified along the escarpment, characterized by either a noticeable change in strike, a difference in the sharpness of relief, or both. In particular, the Horse Spring structure intersects the NE-striking, divergent strike-slip(?) Seneca fault near the confluence of the Neosho and Elk Rivers, where both the escarpment and the Upper Mississippian strata appear to be offset by the Seneca fault in an apparent dextral sense. While we cannot discount the possibility that the Horse Creek structure is a regional contractional asymmetric fold, as originally interpreted, we believe the current observations are more consistent with a previously unrecognized normal fault of regional significance on the western margin of the Ozark Plateau. Apparent cross-cutting relationships with other regional structures suggests there may be multiple phases of deformation. Analysis of petroleum exploration and groundwater wells in the region, possibly combined with acquisition of shallow seismic reflection data, should be useful in testing the normal fault origin of the Horse Creek structure.