NEOGENE STRUCTURAL AND SEDIMENTOLOGIC CONNECTION BETWEEN THE PINE RIDGE AND BLACK HILLS, NEBRASKA-SOUTH DAKOTA, USA

The Pine Ridge escarpment, located in northwestern Nebraska, is the southernmost questa of the Black Hills uplift. While a Pine Ridge-Black Hills connection was postulated in the past, our study, linking remotely sensed lineaments and a series of approximately east-west oriented normal faults south of Chadron, NE, offers evidence of the genetic connection. The Pine Ridge escarpment is a physiographic boundary between the High Plains and the southern Black Hills, as well as a boundary for the High Plains Aquifer in northwestern Nebraska. The region between the Pine Ridge escarpment and the southern Black Hills is a broad basin drained by the structurally-controlled Cheyenne and White rivers. In the the area bounded by the Black Hills and the Niobrara River to the south regional dip is to the south. Mapping in the Spotted Tail range, a north-south oriented salient of the Pine Ridge near Chadron, NE, identified five major normal faults, all down to the south, with throws ranging from 2-50 m. A sixth, northernmost fault on the Spotted Tail range is a small-scale thrust fault. Reversal of dips occurs north of the thrust fault with beds dipping slightly to the north. Faults expressed in outcrop have not been traced beyond the extent of the Spotted Tail range. However, lineaments identified on remotely sensed imagery prior to field mapping extend well beyond the range. Statistical analysis of lineament azimuths and nearest mapped faults indicates correlation. We conclude that most lineaments in the study area represent the subtle expression of faults.