PRE-OLIGOCENE LANDSCAPE DEVELOPMENT ALONG THE EASTERN FLANK OF THE LARAMIDE BLACK HILLS UPLIFT, SOUTH DAKOTA

By the latest Eocene, approximately 29 million years after initiation of the Black Hills Laramide uplift, a continuous erosion surface extended westward from the present-day Badlands National Park area to the crystalline core of the uplift. Across this distance of 110 kilometers, latest Eocene sedimentation produced sequentially a disconformity, an angular unconformity and a nonconformity on the Mesozoic and Paleozoic strata and the Precambrian basement such that the missing rock record varies from ~35 m.y. to ~1,700 m.y.

The elevation difference across this distance is ~875 m, averaging 4.6 m/km for the 89 km of prairie (Cretaceous shale) and 21 m/km within the mountain segment (Mesozoic, Paleozoic and Precambrian lithologies). Valleys within the mountain segment were less than one kilometer wide and 150 m deep: in the prairie segment they were as much as four kilometers wide and 200 m deep. Within the Black Hills the typical mudstone and sandstone of the White River Group grades to coarse river gravels and, locally, lacustrine carbonates.

Localization of the east-southeast-trending channels was controlled, in part, by topography related to Laramide anticline-syncline pairs which, in turn, parallel the grain of the underlying Precambrian basement. Present drainages have exhumed portions of the Late Eocene landscape, following earlier channels. Across the prairies, the former topography is inverted by erosion of Cretaceous shale from the interfluves. The distinctive erosive styles of the “Needles”, carved upon the Precambrian Harney Peak granite, may have evolved as the highest portions of the Black Hills remained above the White River deposition.