The geomorphic evolution of the Black Hills of South Dakota and adjacent High Plains during the late Cenozoic: Knickzone propagation

Geomorphic research in the Black Hills and northern High Plains provides new insight into the late Cenozoic evolution of this salient of the Laramide Rockies. Traditionally, geologists have appealed to epeirogenic uplift or climate change to explain the post-Laramide unroofing of the Rockies. On the basis of field mapping and the interpretation of long-valley profiles, I conclude that the propagation of knickzones generated by base level fall, is the primary mechanism responsible for exhumation in the Black Hills. Long profiles of major drainages show discrete breaks in the slope of the channel gradient that are not coincident with changes in rock-type. I use the term knickzones to describe these features because their profiles are broadly convex over tens of kilometers. At and below the knickzone, the channel is incising into bedrock, abandoning a flood plain, and forming a terrace. Above the knickzone, the channel is much less incised, resulting in a broad valley bottom. Numerous examples of stream piracy are documented, and in each case, the capture is recorded in the same terrace units. These observations are consistent with migrating knickzones that are believed to have swept through Black Hills streams, rearranging the drainages. I demonstrate there are two knickzone fronts associated with mapped terraces. Preliminary field evidence of soil development shows that these terraces are time transgressive in nature. Digital elevation models from the Midwestern United States were used to test the validity of the climate model proposed by several authors. The results of the DEM study showed that streams change their long profiles in response to storm intensity rather than mean precipitation. Even though climate may play a role in the formation of terraces, the data clearly shows that migrating knickzones are responsible for the formation of terraces in the Black Hills and adjacent northern Great Plains.