Holocene Landscape Evolution in the Pawnee River Valley, Southwestern Kansas Rolfe D. Mandel Bulletin 236 1994 117 pages, 81 figures, soil profile descriptions, references, and an index
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Abstract

A basinwide study of terraces, Holocene alluvial fills, and soils was conducted in the valley of the Pawnee River, a tributary of the Arkansas River that drains nearly 4,000 km² in southwestern Kansas. By focusing on all levels of the drainage hierarchy, I was able to evaluate the similarities among drainage elements composing the basin and the differences in stratigraphy and chronology that occur in various parts of the basin. This information was used to infer causes of Holocene erosion, alluviation, and landscape stability and to define their temporal and spatial relationships in the Pawnee River basin.

Only one terrace (T-1) is present in the upper and middle reaches of small streams (less than fifth-order), but remnants of a slightly higher terrace (T-2) occur in lower reaches of fourth-order streams near their confluences with the Pawnee River. The bulk of the valley fill underlying the T-1 terrace aggraded between 2,800 and 1,000 yr B.P. Most Of the alluvium beneath the T-2 terrace accumulated between 10,000 and 5,000 yr B.P.

Three terraces, numbered consecutively upward from T-1, are present in the valley bottoms of large streams (greater than fourth-order). The modern floodplain (T-0) is the lowest surface: the T-2 and T-3 surfaces are Pleistocene terraces, and the T-1 terrace is the surface of Holocene valley fill. Radiocarbon assays suggest that the upper 8-9 m (26-30 ft) of the T-1 fill aggraded between ca. 10,500 and 1,600 yr B.P. Aggradation of the adjacent T-0 fill was underway by 1,000-500 yr B.P.

Radiocarbon ages determined on humates from multiple buried paleosols in valley fill of large streams suggest that the period 10,000-5,000 yr B.P. was punctuated by several episodes of floodplain stability and soil development. However, Holocene valley fills in the Pawnee River basin are devoid of any evidence of soil development between 7,000 and 5,000 yr B.P. Radiocarbon assays suggest that soil development was underway by at least 5,000 yr B.P. on floodplains in large valleys.

Two discrete periods of paleosol development were detected in large valleys: one at 2,750-2,600 yr B.P. and another at 2,000-1,600 yr B.P. The older of these two episodes partially coincides with the soil-forming period dated to 2,800-2,000 yr B.P. in small valleys. However, the most recent episode of paleosol development in large valleys (2,000-1,600 yr B.P.) precedes the beginning of the major soil-forming period dated to 1,350-1,000 yr B.P. in small valleys. Hence episodes of late Holocene deposition appear to have been time transgressive throughout the entire extent of the drainage network.

Radiocarbon assays indicate that Holocene erosion and alluviation as well as periods of net transport and storage of sediment were diachronous throughout the Pawnee River basin but were roughly synchronous in similar-sized streams of the drainage network. Early, middle, and late Holocene alluvium is stored in valley fill of large streams, but only late Holocene deposits are present in valley bottoms of small streams.

Although valley erosion and alluviation may have several causes, major bioclimatic changes explain the pattern of Holocene fluvial activity detected in the stratigraphic record in the Pawnee River basin. Reduced vegetative cover combined with infrequent but intense rainfalls during the warm Altithermal (8,000-5,000 yr B.P.) favored erosion and net transport of sediment in small valleys. As mean annual precipitation increased during the late Holocene, vegetation recovered and erosion rates decreased, promoting sediment storage in small valleys.

The Holocene record of entrenchment, alluviation, and soil formation in the Pawnee River valley generally agrees with alluvial chronologies for valleys elsewhere in the Great Plains and Midwest. Detailed correlation between river basins cannot be done at this time, however, because of a lack of sufficient detail from adjacent areas.

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