THE SENECA FAULT: A RIGHT-STEPPING DEXTRAL FAULT SYSTEM IN NE OKLAHOMA

The Seneca fault is one of the major structural elements on the western margin of the Ozark Plateau and appears to warrant a re-evaluation of the geometry, sense of displacement, history of formation and tectonic significance of this structure. First discovered by Sibenthal in the early 1900’s by the use of geological field mapping, it was named the Seneca Fault due to its proximity to Seneca, Missouri. The northeast-striking structure was originally identified as a syncline, or as a “double fault," lowering younger strata within a narrow (60-450 m; 200-1,500 ft.) linear trough or graben. With good exposures of the structure in the bluffs of the Grand River described by Siebenthal (1907), he determined that the amount of throw is variable within short distances along the fault, which he traced for more than 150 km. Later works identified the structure as more of a narrow, linear syncline, in places faulted (Ireland, 1930; Weidman, 1932; Huffman, 1958). Many of the exposures described by Siebenthal have been covered due to the construction of the Pensacola Dam and the development of Grand Lake in the late 1930’s. The most recent mapping of the Seneca fault was carried out by Patterson (1986) in the exposed spillways of the Pensacola Dam. Patterson interpreted the fault as a negative flower structure developed on a divergent wrench fault system. Based on relationships in map view and field observations of the entire fault system, we have traced the system for ~200 km, with as many as 5 right-lateral steps spaced every ~30-40 km, suggesting that the fault is a right-stepping dextral slip fault system. Normal drag folds appear to be exposed in these areas along with both steeply and shallowly plunging striations, on steeply dipping NE striking fault segments, suggesting a poly-phase movement history. Geologically recent (post-Paleozoic) activity could be indicated by coincidence of the trace of the Seneca fault with drainage patterns within Terrace deposits of inferred Quaternary age near Strang, OK. Future shallow seismic reflection data along with groundwater well data may help to further constrain the geometry and temporal relationship with other structures in the region, such as the Horse Creek structure, particularly the relative timing of the two structures and potential implications for tectonic evolution of the Ozark Plateau.