THE RELATIVE IMPACT OF LAURENTIDE DISCHARGE AND FOREBULGE MIGRATION ON THE UPS AND DOWNS OF THE MISSOURI RIVER PROFILE

Abstract

This Missouri River follows the Pleistocene glacial front and assuredly felt the effects of both glacial discharge and glacial tectonics. The position of the Missouri River, and flanking loess deposits, demands this common glacial interpretation. Yet, direct evidence of this glacial heritage from the valley fill itself remains unreported. This study maps and dates the valley fill of the Missouri River for the first time, and chronicles this presumed glacial record. The Missouri River records two cycles of aggradation and incision that proceeded since the Last Glacial Maximum (LGM). Timing of these cycles are here compared with both known glaciation and deglaciation events and models for glacial isostatic adjustment (GIA). GIA rates and locations do not coincide with magnitude and timing of incision and aggradation cycles, and thus do not appear to be the cause. Instead, incision and aggradation events are better explained by more coincident glacial outwash cycles of the Laurentide ice sheet. This study also indicates that the aggradation and incision events in the Missouri Valley at the end of the LGM are common to all glacial North American Rivers, including the Ohio and Mississippi Rivers and their tributaries. While uplift and subsidence from GIA does not appear to drive incisions and aggradation cycles, it does appear to influence river migration. The Missouri River laterally migrated away from the glacial front over the span of the Holocene. This lateral migration coincides with tilting of the Missouri River Valley associated with relaxation of the glacial forebulge since collapse of the glacial front.