FIRST RECORD OF LATE PLEISTOCENE DEPOSITS FROM THE MISSOURI RIVER VALLEY, NORBORNE QUADRANGLE, MO

We report the first surficial record of Late Pleistocene channel-belt deposits recovered from the floodplain of the lower Missouri River Valley. These strata were mapped utilizing aerial photographs, GPS receivers and hand auger samples (10 cm interval) taken in the Norborne Quadrangle. We developed a cross section of these strata from these core data. Pleistocene deposits occur beneath 4 m of backswamp muds, that post date Pleistocene channel deposition and likely derive from the adjacent Holocene meander plain. The Pleistocene bar deposits recovered include poorly sorted gravel and coarse sands and are adjacent to a > 4 m Pleistocene outwash channel consisting of loamy silts and muds. The channel belt sits beneath a thin (< 5 cm), slightly oxidized Pleistocene soil horizon.The gravel was optically (OSL) age dated at about 15,000 yr., thus placing these deposits at the beginning of the last glacial retreat during the Wisconsian glaciation of the Late Pleistocene. These strata are interpreted as Pleistocene fluvial braidplain deposits. Previous workers in the Missouri River Valley have assumed that Pleistocene braidplain deposits existed under the Holocene channels in the valley based upon the presence of adjacent Pleistocene loess deposits and the assumption that the Missouri is an ice front river. The newly recovered section from the Norborne Quadrangle represents the first non-reworked Pleistocene channel deposits and confirms the assumed existence of Pleistocene braidplains in the Missouri River Valley.