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