MOSAIC FAULTING, DRAINAGE DIVERSION, AND REJUVENATION RESULTING FROM RECENT DOMING IN CENTRAL NEBRASKA

After more than ten years of data collection and analysis, the concept of recent broad-scale tectonism affecting most of central Nebraska appears inescapable. The terrain of this area is characterized by numerous widely scattered "loess mesas", remnants of a nearly flat surface bearing irregular ridges and intervening blowouts of the final depositional event, accumulation of the Peoria Loess. Subsequently, regional deformation created a broad, low dome elongated east-west. This uplift resulted in the formation of an intricate mosaic of horsts, grabens, and tilted blocks that are readily apparent constituents of the modern topography. Regional drainage, which was from the west and northwest, was broadly obstructed, forming lakes and wetlands that dot the Sandhills. Master streams that managed to continue across the rising dome adjusted their courses by migration and piracy to develop the present structurally controlled angulate drainage pattern. On the southern flank of the dome the Platte River was progressively shifted downslope, creating the great arcuate bend toward the south and eating into adjacent uplands. Tributary streams were rejuvenated and eroded rapidly into the thick loess substrate, creating a network of "loess canyons" that, together with a myriad of fingertip cuts, are now destroying the newly raised upland. Remaining pseudo-mesas that still preserve parts of the domed surface rise westward from l800 feet (l00 feet above adjacent valley floor) at 98 degrees longitude to 2950 feet (250 feet above nearby valley floor) at l00 degrees longitude. Local exposures of consolidated bedrock and, especially, truncations of stacked paleosols attest to the mobility of modern erosional scarps. Some parts of the Plains have not been as stable as we thought.