QUATERNARY SCOUR AND FILL OF THE MISSOURI RIVER VALLEY FROM FIELD MAPPING AND OSL DATES: A BUFFER VS. DENUDATION VALLEY ORIGIN FOR A CONTINENTAL-SCALE RIVER
Authors: John Patten, Jena Newman, Vince Egyed, Justin Anderson, Rachel Albertson, Diana Flores, Chris Hendrix, Shane Patterson, David Grasman, Adam Trimble, and John Holbrook
The Quaternary Geology of the Missouri River Valley has only recently seen extensive mapping and remains poorly understood. NSF/REU and USGS EDMAP students working through University of Texas at Arlington and University of Nebraska at Lincoln mapped the Missouri River Valley floodplain from Yankton, SD to Missouri City, IA (237km) between 2008-2010, and have identified well defined terraces and dated these using optically stimulated luminescence (OSL). The Holocene meander surface reached its current height of 100’ above the valley bottom approximately 7,000y.b.p. It rests 60’ above a buried 15,000y.b.p. Pleistocene braidplain and records rapid early Holocene aggradation. Terraces flank the valley formed from alluvial fans dispersing from the bedrock. These rise steeply 78’ above the valley below and record a stage of valley aggradation to meet a river surface later incised by the 15,00y.b.p. fluvial surface. The date of this higher surface is pending, but it is similar to a 24,000y.b.p. terrace found farther downdip. This provides evidence supporting rapid alluvial aggradation and incision at largely varied equilibrium profiles owing to shifting climate. The interval of incision and aggradation because of climatic fluctuation in equilibrium profile is the ‘buffer’ zone, and a valley resulting from this origin is a buffer valley. As much as 170 feet of the site’s valley scour is attributable to development of a buffer valley. Only the remaining valley depth of 60ft are the result of river incision because of seeking of ultimate base level and long-term denudation of the North American Continent.