Some Deformations of Southwestern Iowa

Possibilities of oil production have recently revived interest in the structure of southwestern Iowa. Along the Missouri River is a series of low major synclines and of two major anticlines trending east-northeast. The largest, the Brownville syncline, extends from southeastern Nebraska into southwestern Iowa. On the north it is bounded by the Jones Point deformation, consisting of the asymmetric Redfield anticline and the Union fault. Both the Jones Point deformation and the Union fault were originally described in Nebraska, but are now traced across the river into Iowa. Possibly other minor faults occur in the south flank of the arch. North of the Redfield anticline the strata dip clown into the shallow Bartlett syncline and then rise abruptly in the La Platte deformation opposite La Platte, Nebraska. The deformation seems to consist of an asymmetric anticline or of a normal fault similar in nature to the Jones Point deformation. Attempts to connect the Jones Point deformation with anticlines or faults in Iowa near Red Oak, Milford, Redfield, and Ames, either as the Thurman-Wilson fault or as the Redfield anticline are not based on sufficient field evidence. They do not form a single straight structure with approximately the same magnitude the entire distance. At the only place where this projected structure has been crossed by a traverse other than along the Missouri Valley the structure was lacking. This traverse is the Middle River traverse by Condra and Upp. Furthermore, where best exposed (i.e. at Jones Point, Nebraska and near Thurman, Iowa) the strt1cture plunges to the northeast 65 feet in 4½ miles. At this rate it would soon die out. While it is true that there is some deformation of strata in this region, the paucity of well records and of exposures and uncertainties as to the exact correlation of the strata do not reveal the magnitude of the displacement. It is more likely that there is a series of deformations, possibly an echelon, forming a broad zone of faulting and monoclinal or anticlinal folding. Many small faults and folds are exposed in the field, paralleling this structure. Therefore a zone of faults or folds several miles in width would best explain the displacement of the strata