Geologic Map geodatabase of the valley corridor of the 59-mile reach of the Missouri National Recreational River, South Dakota, Nebraska, and Iowa:  Gavins Point Dam to North Sioux City

This geologic map area of 5,430 km2 spans a reach of the lower Missouri River valley and adjoining uplands for about 100 kilometers east of Gavins Point Dam, the easternmost mainstem dam on the Missouri River.  Understanding the surficial geologic history of the valley is relevant to natural resource management of the Missouri National Recreational River and is foundational to improved understanding of hydrology and ecology.  This geodatabase is a synthesis of recent FEDMAP, EDMAP, and STATEMAP work of the National Cooperative Geologic Mapping Program with previously published maps of the geologic surveys of South Dakota, Nebraska, Iowa, and the USGS.  Other data sources utilized for this map include NAIP ortho-imagery (especially for the modern river system), a photogrammetrically-produced DEM of the Missouri River Valley, and NRCS Soil Survey data.  Mapping herein is based on geomorphic and other surficial characteristics as well as sedimentary and stratigraphic characteristics from exposures and borehole data.  Quaternary glacial and nonglacial deposits up to 100 meters thick predominate the surface geology. Cretaceous sedimentary bedrock (largely from a former marine interior seaway) forms relatively limited exposures along valley margins of the uplands and underlies all Quaternary deposits of the map area.  Assemblages of Quaternary map units and associated landscapes vary markedly between three sectors in the map area:  1) the Missouri River valley proper, 2) late Pleistocene glacial deposits of South Dakota uplands, and 3) dissected uplands in northern Nebraska, western Iowa, and southeast South Dakota.  The Missouri River valley is predominantly covered by postglacial fluvial deposits overlying glaciofluvial sediments that dominate the lower part of the valley fill.  The western 70 percent of the South Dakota uplands in the map area are primarily late Wisconsinan glacial deposits  of the James lobe Lobe  of the Laurentide Ice sheet.  The remainder of the uplands, including all uplands south of the Missouri River Valley, are mantled with a discontinuous to locally thick and continuous late Quaternary loess over pre-Wisconsinan glacial and nonglacial deposits; these uplands are dissected by a surficial valley network pattern with a predominantly northwesterly orientation.   The northeast side of the Missouri River valley is dominated by backswamp mud, in contrast to the southwest side, which is dominated by point bar sand and other fluvial facies deposited in proximal association with past positions of the laterally migrating Missouri River channel. Postglacial aggradation of at least 7 meters has largely buried the earlier valley fill (about 20-25 meters thick) of Pleistocene outwash composed of gravelly sand.  The oldest known abandoned river meanders with surficial expression are late Holocene.  About 15% of the valley floor was reworked by lateral migration of the Missouri River between ca. 1892 and 1941 during a period of decreasing channel sinuosity.  After construction of the large Missouri River dams (mostly during the 1950s), a decrease in sediment load transformed the river to an incising regime that generally does not supply overbank sediment to the valley floor, in contrast to the paleo-environments indicated from the geologic record.

Tags: cretaceous, fluvial, geology, geomorphology, geoscientificinformation, glacial-geology, iowa, nebraska, quaternary, south-dakota, usgs-624cb901d34e21f82765072b
Organization: Department of the Interior