Abstract

The structure of a deformed geosyncline is made up of many inter-related features. When one discusses the structure of only part of a deformed geosyncline it must be realized that even if accurate in itself the discussion cannot be comprehensive. This is true of the Ouachita Mountains. A discussion of the structure of the frontal belt may not satisfy a student of the central Ouachitas. A discussion including the central Ouachitas with both the frontal belt and the known subsurface would still leave much to be desired by the theorist who is aware of the possible significance of the deeper part of the Ouachita structural complex buried beneath the Gulf Coastal plain and not yet opened to study by drilling. This discussion deals with the structure of the frontal belt of the Ouachita Mountains in Oklahoma and is in no sense a complete discussion of the Ouachita Mountain structural province. The discussion will attempt to analyze the known features of the structure, particularly those shown on U. S. Geological Survey Oil and Gas Investigations Preliminary Map 66 (Hendricks and others, 1947) and to deduce the structural history from that analysis.

The structure of the frontal belt of the Ouachitas is dominated by faulting. The faults in general consist of a complex set of reverse faults roughly parallel with the Ouachita Mountain front and a related set of cross faults. Most of the reverse faults appear to dip at high angle and to have a horizontal component of movement that is of the order of magnitude of the vertical movement. Others, such as the Pine Mountain and Windingstair faults appear to have had greater horizontal movement than vertical movement, and at least locally appear to dip at moderate angles. The Ti Valley fault and some minor faults appear to have a low angle of dip. The cross faults are of two types. In the northern part of the area they are characterized by strike-slip movement. South of the Windingstair fault the movement was dominantly upward. The minimum amount of movement on the reverse faults in the frontal belt appears to have been in excess of 50 miles with the greatest part of that movement concentrated on the Ti Valley, Windingstair, and Pine Mountain faults. More or less simultaneous deformation seems to have occurred in an extreme frontal block, the block between the Ti Valley and Windingstair faults, and the block south of the Windingstair fault, with the deformation culminating in movement on the Ti Valley and Windingstair faults. Several lines of evidence suggest that the direction of movement was generally northward and that greater movement occurred in the eastern part of the area than in the western part. Incompetent shale zones constituted gliding planes along which thrust movement took place, with the principal ones being the Womble shale, Springer formation, Caney shale, Stanley shale, and Johns Valley shale. One can postulate from indirect evidence the existence of an early period of faulting along the north margin of a late Mississippian-early Pennsylvanian geosyncline. However, the structural development of the frontal Ouachitas started in Atoka time and continued until middle Pennsylvanian time and possibly as late as early Permian time.