ORIGIN OF THE MALTA TERRACE AND OTHER LANDFORMS WITHIN THE MISSOURI RIVER VALLEY

The Pleistocene and Holocene floodplain deposits of the Missouri River Valley alluvial fill were mapped. The processes responsible for the landforms present within the valley were determined and a pioneer allostratigraphic framework for the valley deposits was developed. This study involved a detailed surface and subsurface mapping of the alluvial fill at Saline and Carroll counties, Missouri. Subsurface data were obtained from cores and borings using a truck-mounted Giddings hydraulic soil probe, a set of Dutch augers and water well logs. Samples were analyzed and logged using standard field and laboratory procedures. Organic materials from the terrace and unexposed point bar sand samples were dated using the Accelerated Mass Spectrometry (AMS) and Optically Stimulated Luminescence (OSL) techniques respectively.

The Pleistocene deposits within the valley comprise of three units: Peoria loess, Malta alloformation, and Pleistocene braid plain. The Peoria loess consists of Aeolian sediments of well-oxidized, massive, unconsolidated light brown to yellowish silt-loam, grading downwards into silt. The Malta alloformation consists of 8m of thin to thick interbedded laminated fine-medium sand (20%), loam (approximately 50%), silt loam (<10), sandy-loam (10%), and lenses of silt, silty clay, and clay lithologies (10%). AMS dates of sediments within this unit put the Malta alloformation at late Wisconsin. The Malta alloformation is underlain by the braid plain outwash which consists of a basal upward fining unit of boulders, gravel, coarse sand to fine sand, with a thickness ranging from 9.5 to about 15m.

Fluvial patterns observed within the study area suggest that the processes of crevassing and avulsion are responsible for the origin of the Malta Terrace. These processes were in response to rapid valley aggradation due to a possible channel blockage at the Dewitt Narrows due to increased sediment discharge with lower river competence and capacity.

Holocene allounits within the study area consist of four sets of river channel morphologies with varying geometries. These possibly represent different climatic episodes within the valley. The channel fill allounits consist of primary and secondary abandoned channels. OSL dates record a date of 3000 years for the dated point bar deposits within the study area.