NEW QUATERNARY GEOLOGIC MAPPING AND GEOCHRONOLOGY OF THE CORRIDOR AREA OF THE MISSOURI NATIONAL RECREATIONAL RIVER, NEBRASKA AND SOUTH DAKOTA

New Quaternary geologic mapping and geochronological research along the lower Missouri River below Gavins Point Dam provide new insights into the evolution of this critical river corridor. Our methods include uranium-series and optically-stimulated luminescence (OSL) dating. Geomorphology and soil characteristics were used to map the crosscutting fluvial meander belts that cover the 3-12 km wide valley floor and the glaciated uplands south of the Missouri River valley in Nebraska. In the latter area, contacts approximating the maximum first derivative of slope show a predominantly northwest-southeast orientation of tributary valleys and an anastamozed valley pattern in which multiple low passes commonly separate adjacent tributary watersheds. Erosion by strong periglacial winds has been suggested to form the orientation. However, the common occurrence of Canadian shield-derived glacial gravels and diamicts in both major and minor valleys indicates that glacial and fluvial erosion and deposition may have followed joint patterns in the Cretaceous and Tertiary sedimentary bedrock in the uplands. Involuted gravelly sand underlying a few meters of silt in lower Bow Creek valley in Nebraska yielded OSL dates of about 11-17 ka. The markedly oriented glaciated Nebraska uplands contrast with the younger glaciated terrain in South Dakota.

A few miles north of the Missouri River, laminated travertine exists on the marginal scarp of the James River Valley within the latest Wisconsin ground moraine of the James lobe of the Laurentide ice sheet. Robust 230Th/U dates on calcite laminae range from 10.5 ka to 12.5 ka (mean 2s error of 0.4 ka) and indicate ground-water discharge from glacial aquifers that closely followed the latest Wisconsin advance of the James lobe. Ages of secondary, vug-filling calcite as young as 8.1 ±0.1 ka help define a trend of decreasing initial 234U/238U with age. Modern spring discharge at one site has a 234U/238U activity ratio of 1.325, which plots near the zero-age end of the trend. The isotopic evolution of discharge, considered with quality of nearby ground and surface water, is being used to improve understanding of geologic and climatic controls on surface/ground water interactions, and their significance to adaptive management of riverine ecosystems.