Geomorphology, Quaternary Stratigraphy, and Geoarcheology of Fox Creek Valley, Tallgrass Prairie National Preserve, Northeast Kansas

A geomorphological investigation was conducted in Fox Creek valley within the Tallgrass Prairie National Preserve, Chase County, Kansas. During the first stage of the study, Quaternary landforms were defined and mapped in the valley. This was followed by inspection of cutbank exposures along the creek and deep coring across the valley floor. Subsurface information gleaned from the exposures and cores was used to determine the lithostratigraphy and soil-stratigraphy of the landform sediment assemblages. The numerical ages of alluvial deposits and associated buried soils were determined by radiocarbon dating charcoal, plant macrofossils, and soil carbon. The radiocarbon chronology, combined with soil-stratigraphic data, was used to define temporal and spatial patterns of landscape evolut on (eros on, depos t on, and landscape stab l ty), and to assess the geolog c potent al for bur ed prehistoric cultural resources in Fox Creek valley. Also, late-Quaternary vegetative change was inferred from δ 3C analysis of soil and sediment organic carbon. The valley floor of Fox Creek consists of four Quaternary landforms: a low floodplain (T-0b), a slightly higher floodplain (T-0a), an alluvial terrace (T-1), and alluvial/colluvial fans. Members of the DeForest Formation, a lithostratigraphic unit that occurs throughout the eastern Plains, compose all of the Holocene landform sediment assemblages (T-0a, T-0b, and T-1) in the valley. Specifically, the T-0b surface is underlain by stratified, silty and loamy alluvium typical of the Camp Creek Member of the DeForest Formation. Although the numerical age of the T-0b fill is unknown, it probably is less than 400 years old and may be less than 200 years old in most of the valley. Two units of the DeForest Formation were identified beneath the T-0a surface: the Honey Creek Member and the Roberts Creek Member. The Honey Creek Member consists of brown (10YR 5/3-4/3, dry), silty alluvium with a moderately expressed surface soil (A-Bw-Bk horizonation). The numerical age of the Honey Creek Member in Fox Creek valley is unknown, but it aggraded sometime after ca. 2000 4C yr B.P. Aggradation of the Honey Creek Member was interrupted by at least one episode of landscape stability, indicated by a weakly developed soil about 1.5 m below the T-0a surface. The Roberts Creek Member consists of fine-grained, organic-rich alluvium that fills paleochannels cut into the Honey Creek Member. A subtle 1 m-high scarp separates the T-0a and T-1 surfaces in Fox Creek valley. The T-1 terrace dominates the valley floor and gently rises towards the valley wall where it either merges with alluvial/ colluvial fans, or is bounded by a steep bedrock wall. The Gunder Member of the DeForest Formation forms most of the valley fill beneath the T-1 surface. It consists of moderately oxidized, brown (10YR 5/3, dry) to yellowish brown (10YR 5/4, dry) silty clay loam that has been strongly modified by surface-soil development (A-Bt-Btk horizonation). Aggradation of the T-1 fill was underway at ca. , 00 4C yr B.P. and may have continued into the early Holocene. However, there is a gap in the alluvial record between ca. 11,200 and 4500 4C yr B.P. The early-through-middle Holocene appears to have been a period of net sediment removal on the valley floor of Fox Creek. Sediment storage resumed during the late Holocene and was characterized by rapid floodplain sedimentation from ca. 4500 4C yr B.P. until sometime between ca. 3500 and 2100 4C yr B.P. By ca. 2100 4C yr B.P., sedimentation slowed, and soil development was underway on the late Holocene floodplain soon after 00 4C yr B.P. There was another episode of floodplain sedimentation sometime after 2100 4C yr B.P., resulting in burial of the soil only on the lowest portion of the T-1 terrace near the stream channel. Large, low-angle alluvial/colluvial fans on the margins of the valley floor are composed of fineand coarse-grained sediment that accumulated during the late Pleistocene. The top of a strongly expressed, brown (7.5YR 4/3, dry) to reddish brown (5YR 4/3, dry) buried soil (paleosol) with At-Bt horizonation is about 1.25-1.50 m below the surface of the fans. Decalcified organic carbon from the upper 10 cm of this paleosol yielded radiocarbon ages of 24,560+350 and 22,620+340 yr B.P. Based on the lithology of the alluvial/colluvial deposits, the fans are composed of the Severance formation, an informal lithostratigraphic unit recognized in eastern Kansas and Nebraska. The δ 3C values determined on organic carbon in soil and sediment samples from Core 3 (T-1 fill) suggest that between ca. 4500 and 2100 4C yr B.P., the valley floor of Fox Creek was characterized by a mixed C3/C4 plant community. However, there is a distinct shift to less negative δ 3C values after ca. 00 4C yr B.P., suggesting that C4 vegetation (i.e., warm-season grasses) increased in abundance