GEOLOGICAL STRUCTURES AND TIMING CONSTRAINTS ALONG THE MISSOURI RIVER, CENTRAL SOUTH DAKOTA

The newly compiled geological map of South Dakota presents one of the most comprehensive views of the geology and structural features of central South Dakota. Information included in this compilation resulted from structures that were newly mapped by the authors, as well as previously documented structures gleaned from the literature, resulting from over one hundred years of sporadic research in this area. Structures include folds and faults, both normal and reverse faults, and most displacements are within the range of one to ten meters, but some are greater. Compilations of drill data from eastern South Dakota have also illuminated numerous potential structural features beneath the glacial drift, several of which extend across the Missouri River into western South Dakota. Central South Dakota is on the margin of the Williston Basin, and many of these structures are also potential traps for hydrocarbon accumulations, particularly natural gas.

Although geological structures are common along the Missouri River, the timing of their formation has been difficult to ascertain. Nearly all faults, excluding landslides, occur within the Late Cretaceous Niobrara and Pierre Shale units, and most faults have only late Quaternary deposits lying undisturbed above. Previous authors have suggested Laramide, post-Miocene, and even Quaternary times of formation, but noted that faults could not be unequivocally dated as younger than Late Cretaceous because direct evidence was wanting. Now, a normal fault discovered in Brule County, central South Dakota, appears to offset glacial deposits and glacially derived boulders are cemented within the fault gouge. This occurrence constrains the date of faulting to after the initiation of glaciation. Therefore, at least some faulting within central South Dakota occurred relatively recently in the Quaternary, and this fault represents the first direct evidence of the youngest known structural event.