Abstract

Four lithostratigraphic alluvial formations and eight chronostratigraphic subdivisions occur along the middle Duck River Valley. Each late Quaternary flood-plain accumulation consists of about 2 m of sandy and gravelly bottom-stratum facies overlain by 2.5 to 4 m of clayey and silty top-stratum facies. An additional 1 to 2 m of clayey and silty “terrace veneer facies” blankets all but the youngest accumulation. On the basis of numerous excavations and 14 radiocarbon dates, a history of flood-plain sedimentation can be traced. (1) Yellowish-brown clay loam was deposited during the late Pleistocene, above a bedrock thalweg 5 m higher than the present level. (2) Severe bedrock and flood-plain erosion occurred near the end of the Pleistocene, and a major erosional unconformity was created. (3) During the early Holocene, lateral and vertical aggradation of dark yellowish-brown clay loam occurred over a bedrock valley floor already as deep as that at present. Scattered within this unit are early Archaic (ca. 9000 B.P.) chert artifacts. (4) After a brief interval of stability, brown silty clay loam accreted, but by 7,200 14C yr B.P., the flood-plain surface was again stable, and soil formation dominated over deposition. Mid-Archaic chert artifacts as young as 6,400 14C yr B.P. were left on this fossil flood-plain surface, and pollen analytical studies from the region document an effectively drier climate at this time. (5) By 6,200 14C yr B.P., renewed overbank accretion was under way, and pollen analyses indicate an increasingly humid climate. Aggradation continued until 7,200 14C yr B.P. and buried the older surface, artifacts, and soil with a veneer of dark brown silty clay loam, itself containing late Archaic artifacts. (6) Two periods of flood-plain stability and soil formation, separated by aggradation, occurred during the past 4,000 yr. The last period of stability ended in the early 1800s; the introduction of row crop agriculture into the basin at this time probably caused the historic episode of renewed accretion that is still under way.

These results suggest the importance of environmental controls over fluvial systems at time scales of 102 to 104 yr. The stratigraphic information also indicates that after periods of stability and soil development and in response to altered hydrologic regimes, the Duck River forms new flood plains by lateral and vertical suspended-load accretion on channel banks and in-channel bars. At the same time, the older flood-plain surface is covered by as much as several metres of the younger alluvium and becomes a terrace. In contrast, lateral accretion of point-bar sands and gravels is not an important flood-plain–forming process along this river.

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