ANTICIPATING THE SEQUENCE LESSON: K.M. WAAGE'S APPROACH TO THE DAKOTA PROBLEM IN THE BLACK HILLS

Half a century ago, USGS geologists looking for uranium in the Black Hills were befuddled by the variable stratigraphy and inconsistent terminology of the Dakota Sandstone. Karl M. Waage solved the problem via the application of what would now be termed sequence stratigraphy. Waage recognized that the "Dakota" was plagued both by facies changes across the Black Hills uplift and a formal stratigraphic nomenclature based on outmoded concepts (e.g., formations based on geologic age). Formation contacts were hard to recognize or trace laterally. Supposed sandstone units locally were shaly or coal-bearing, lenses of freshwater limestone were present, major thickness changes were observed, and formations had been named, and then renamed, based on their paleontologic content. The word "Dakota" itself had been abandoned, and the entire package of rocks, with all its internal variation, was called the Inyan Kara Group. Waage clarified the stratigraphy of the Inyan Kara by recognizing and tracing a single surface, what he termed the "transgressive disconformity," throughout the region. The surface is planar, locally conglomeratic, commonly overlies a bleached zone, and in places shows a root horizon, or truncation of underlying beds, or a zone of siderite nodules. Strata below the disconformity all have nonmarine affinities, whatever their specific lithologies. Overlying strata are all marine, whether sandstone or shale. Waage broke the Inyan Kara into two formations only, with the disconformity as the formation contact. Today's stratigraphers would speak of depositional sequences and a ravinement surface. In effect, Waage fixed the lithostratigraphy by utilizing the concepts of an approach still decades in the future.