THE SENECA FAULT (NE OKLAHOMA) REVISITED: DEFORMATION ON THE WESTERN MARGIN OF THE OZARK PLATEAU

The Seneca fault in northeastern Oklahoma constitutes one of the major structural elements on the western margin of the Ozark Plateau and appears to warrant re-evaluation of the geometry, sense of displacement, history of formation, and tectonic significance of this structure. First recognized based on geologic field mapping in the early 1900’s (Siebenthal, 1907), and named for Seneca in southwest Missouri, the NE-striking structure was originally characterized as a syncline, or as a “double fault” dropping down younger strata within a narrow (60-450 m; 200-1,500 ft) linear trough or graben. Siebenthal described good exposures of the structure in the bluffs of the Grand River, and determined that “the amount of throw is variable within short distances along the fault,” which he traced for more than 150 km. Later workers (Ireland, 1930; Weidman, 1932; Huffman, 1958) characterized the structure as more of a narrow, linear syncline, in places faulted. Construction of the Pensacola dam and creation of Grand Lake in the late 1930’s subsequently obscured many exposures described by Siebenthal. The most recent mapping of the Seneca fault was carried out by Patterson (1986) in the exposed spillways of the Pensacola Dam, who interpreted the fault as a negative flower structure developed on a divergent wrench fault system. Exposures here appear to document normal drag folds, as well as both steeply- and shallowly-plunging striations, on steeply-dipping, NE-striking fault segments, suggesting a poly-phase movement history. Little information is currently available on the age of development of the Seneca fault, which appears to be covered both by Quaternary terrace deposits as well as a thick soil horizon. Outstanding questions for the Seneca fault include (1) what is the geometric and temporal relationship with other structures in the region, (2) what role does relief on the basement surface have on estimating apparent slip, (3) are there multiple episodes of movement, and (4) are there additional constraints on the period(s) of movement? Quantitative analysis of moderate-resolution (10 m) digital elevation data combined with examination of historic petroleum and water wells within the region may help to clarify the nature and origin of this prominent structural feature in Ozark Plateau evolution.