A LATE HOLOCENE TERRACE-FAN COMPLEX ON THE WESTERN MARGIN OF THE MIDDLE MISSOURI RIVER VALLEY, NEBRASKA, USA

Previous investigations at the Engineer Cantonment (Stephen Long Expedition 1819-1820) archeological site relied upon shallow backhoe trenching to identify a late Holocene terrace-fan complex on the western margin of the Missouri River valley margin (Dillon, 2004.) Trenching demonstrated that the site consisted of a small alluvial fan which overlies and interfingers with late Holocene Missouri River overbank deposits. Recently I have returned to the site and completed 14 soil borings to depths ranging from 3 to 7.2 meters b.g.s. Ternary plots of particle size data (N=84 samples) delineate the two basic depositional units: an alluvial fan composed of silt and silt loam, and Missouri River alluvium composed of silty clay and clay (USDA texture classes), but also an intermediate group composed of silty clay loam. The intermediate group includes a thin bed at the fan/alluvium contact which I interpret as a sediment mixing zone, and samples from well-developed A horizons within the fan.

The alluvial fan is composed of several lobes of sediment which together comprise at least 5 distinct beds with varying degrees of soil development. The late Holocene Missouri River alluvium is subdivided into 6 facies based upon texture, color, mottling, FeO accumulations, and degree of bioturbation/argilliturbation. The depth and thickness of these units in cross section indicate 2 younger fills within the late Holocene terrace fill. This interpretation is supported by 3 radiocarbon ages.

The site is only about 35 m wide, less than 1% of the valley bottom, and yet it contains a complex record of late Holocene channel cutting, abandonment, filling, and burial. Most of the Missouri valley bottom is underlain by a complex assemblage of Historic-aged channel and overbank deposits. However, terrace remnants and alluvial fans occur along the valley margins which preserve much of the late Wisconsin through Holocene record. Many of these fans and terrace remnants are small and rather subtle. Thus reconstruction of the Holocene history of the middle Missouri River valley will depend upon detailed investigations along its margins, rather than traditional cross-valley transects with widespread borings and trenches.