MASS WASTING AND QUATERNARY LANDSCAPE DEVELOPMENT, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The goal of this study was to determine the timing and the mechanism for a landslide located near Norbeck Ridge in Badlands National Park, South Dakota. The Norbeck Ridge landslide is a large landslide adjacent to State Route 240, the main road into the park. Two landslides east of Norbeck Ridge, the Cedar Pass and Cliff Shelf slides, were stabilized in a $14 million US DOT project after landslide movement threatened to destroy the road (Monley 2000). Technical reports from investigation of those slides suggest that the Cliff Shelf was a block-glide-style landslide, that is, a coherent mass moving along a planar zone of weakness.

Landslides occurring along the wall in the Badlands are complex, and may be the result of more than one mass movement process. However, based on our field observation in 2010 and 2011, Norbeck Ridge landslide is a rotational slump, and the fault mapped near the landslide is a slump scarp. In support of this conclusion, we observed numerous zones of rotational slumping in the outcrop of the Norbeck Ridge. We observed this same mass movement style in adjacent areas, such as the landslide at Saddle Pass trail. Thus, while the larger Cliff Shelf slide may be a block glide, the dominant slide mechanism at Norbeck Ridge appears to be rotational slumping. Detachment in one or more clay-rich, poorly consolidated strata in the Upper Scenic Formation or the Lower Poleslide, perhaps above and below the informally named disappointment limestone interval as suggested by Evanoff (2010), appears to be the mechanism for slide motion.

After the 2010 expedition, the field team concluded tentatively that the Norbeck Ridge slide is inactive and too thin a deposit to threaten the park road. However, after the 2011 field season we now believe that the Norbeck Ridge slide is inactive, or only intermittently active, but has the potential to reactivate with destructive consequences for the park road. Ephemeral sag ponds at the head of the Norbeck Ridge slide could create conditions conducive to further slumping. Other geological characteristics that may have influenced landslide development in the field area include gentle southward dipping and vertical jointing of lithologies.