Tertiary landslides, northwestern South Dakota and southeastern Montana

Landslide blocks of latest Oligocene or earliest Miocene age are preserved at several localities in northwestern South Dakota and southeastern Montan These tilted blocks contain Late Cretaceous to late Oligocene rocks and are unconformably overlain by nearly horizontal strata of the Arikaree Formation of Miocene age. Undisturbed rocks of late Oligocene age were completely stripped from the area by pre-Arikaree erosion. More than 100 landslide blocks are preserved in the Slim Buttes, Harding County, South Dakota. Some of the blocks are several miles long, 600 ft. wide, and contain a thickness of 300 ft. or more of relatively unbroken or unjumbled strata which have been displaced downward as much as 250 ft. The strike of the tilted blocks is about N. 60 degrees W., and the dip of the bedding in the blocks at most places is to the SW. and ranges from a few to more than 60 degrees . Most of the rock in the blocks consists of coarse arkosic sandstone, tuffaceous siltstone, claystone, and thick beds of bentonite of the Chadron and Brule formations of Oligocene age. In a few places, small sections of the underlying Paleocene strata are tilted, The blocks formed along NW.-trending pre-Arikaree valley walls and escarpments. The major joint system of underlying rocks apparently controlled the orientation of the slides, and the dip of these older rocks facilitated the downward and outward movement of the landslide blocks. The Short Pine Hills, Harding County, South Dakota, and the Long Pine Hills, Carter County, Montana, contain numerous landslide blocks that are similar in orientation and geologic setting to those of the Slim Buttes. In contrast the Finger Buttes of Carter County, Montana, contain landslide blocks that strike about N.70 degrees E. and dip to the SE. This lack of parallelism with blocks of nearby areas is apparently due to the lack of a well-developed joint system in underlying rocks of this area. The presence of the complete sequence of Oligocene rocks in the downdropped blocks indicates that before pre-Arikaree erosion the area was covered by 500 ft. or more of beds of the Chadron and Brule formations. These landslide blocks show that the time interval represented by the unconformity at the base of the Arikaree Formation was of relatively short duration.