NORTHWEST NEBRASKA’S CHADRON DOME IS NOT A DOME

Our recent geologic mapping and GIS analysis give new insights into the structure, stratigraphy, and evolution of the Chadron Arch in northwest Nebraska. A Cretaceous outcrop along this arch, 15 miles northeast of Chadron, Nebraska, long known as the Chadron Dome, is not a dome with 150 meters of closure as previously mapped, but is the plunging, broken, and eroded rump of the northwest striking Chadron Arch where it intersects the northeast striking White River Fault. The Chadron Arch, which reaches its apex 38 km southeast of this intersection near the town of Hay Springs, has also been referred to as either the Chadron Dome or the Hay Springs Dome by previous workers. To acknowledge historical precedence and avoid confusion with the misconstrued dome, we suggest that the names Chadron Dome and Hay Springs Dome be discarded and Chadron Arch be retained. Our analysis is based upon a structure contour map of the Carlile Fm/Niobrara Fm contact created with the ArcGIS Spatial Analyst spline tool. Our regional structure contour map of the Greenhorn Fm corroborates these results. Additionally, mapping of 189 bedding planes revealed no dome-like structure.

Outcrops of the overlying late Eocene fluvial Chamberlain Pass Fm were mapped and described. The coarse channel fill/overbank mudstone lithosome described in the literature was not encountered. Chert cobbles, remnants of the carapace of the Black Hills and a component of the channel fill, were found isolated and wholly supported by a sandy clay matrix in outcrop. Cobbles were oriented with long axes up to 90 degrees from horizontal. This suggests these fluvial sediments were subsequently transported colluvially, likely indicating local tectonism. The sandy clay matrix contains obsidian clasts, mostly smaller than 0.5 mm. These grains are subangular, adamantine, translucent black, and conchoidally fractured. The nearest known occurrence of volcanic glass is the Tomahawk Volcanic Area, located 150 km north in the Black Hills where a small outcrop of pitchstone has been dated at 56 ma, at least 16 million years prior to published Chamberlain Pass Fm deposition. If this is indeed the source area for these clasts, transport, preservation, and timing of deposition are all problematic.