POST-LARAMIDE TECTONICS AND TECTOGENIC SEDIMENTATION IN THE BLACK HILLS-PINE RIDGE REGION

Preliminary mapping on the Pine Ridge near Chadron, Nebraska, indicates extensive normal faulting in Miocene-age rocks. Structural features also include small south-vergent thrusts. Previously unmapped probable upper Miocene-age cross-bedded feldspathic conglomerate and sandstones (maximum clast size 12 mm) are interpreted as tectogenic products of penecontemporaneous uplift. Crossbed dips indicate transport from the north, and angular coarse alkali feldspar clasts suggest a Black Hills origin. Rounded, finer-grained plagioclase (maximum clast size 0.5 mm) might have a different source, and are likely recycled. These lines of evidence suggest a tectonically active highland coincident with the conglomerate's proximal source area. A proposed chronology is as follows: during the Oligocene northwestern Nebraska had been blanketed in thick volcaniclastic deposits of the White River Group. More tuffaceous and epiclastic sediments followed as mostly channel cut and fill deposits during the Miocene. The Whiteclay Gravel Beds (Ogallala Group), previously interpreted as fault-rupture fills of early Miocene age, indicate that uplift to the north began during the early Miocene. The conglomerate and faulted upper-Miocene rocks indicate that uplift continued into the late Miocene. Nearly all evidence of the contiguous Black Hills-Pine Ridge highland has been eroded by the structurally controlled White and Cheyenne rivers. Additional study of this area is needed to discover the evolving role of the Pine Ridge in the tectonics and paleogeography of the Rockies and Great Plains.