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Pawnee River valley

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Summary and conclusions

The foregoing discussion described the nature of valley landscapes in the Pawnee River basin of southwestern Kansas and focused on the stratigraphy and chronology of alluvial deposits composing landform-sediment assemblages in different drainage elements of the basin. Here, I summarize the findings and make recommendations for future research in the Central Plains.

Landforms and valley fill

Three terraces were identified in the large valley (greater than fourth-order) of the Pawnee River. These terraces are designated, from lowest to highest, T-1, T-2, and T-3. Radiocarbon assays indicate that most of the Holocene valley fill underlies the T-1 terrace. Alluvium beneath the T-2 and T-3 terraces is thought to be Wisconsinan and Illinoian in age, respectively, though no absolute ages are available for these landform-sediment assemblages.

A modern floodplain (T-0) borders the Pawnee River throughout most of its course. The presence of weakly developed floodplain soils and buried Late Prehistoric cultural materials suggests that deposits underlying the T-0 surface are less than 1,000 years old.

The valley floors of second- and third-order streams in the Pawnee River basin are composed of two landforms: a narrow floodplain (T-0) and a low terrace (T-1). This sequence of landforms also is documented in the middle and upper reaches of fourth-order streams. Radiocarbon assays indicate that most of the alluvium composing the T-1 fill accumulated between ca. 2,800 and 2,000 yr B.P.; no early or middle Holocene valley fill is documented beneath these terraces.

A second terrace (T-2) was observed on valley floors of fourth-order streams near their confluence with the Pawnee River. Radiocarbon assays indicate that most of the alluvium composing the T-2 fill accumulated between ca. 9,000 and 5,000 yr B.P.

Holocene history

Temporal data, both absolute and relative, were used to reconstruct the Holocene history of the Pawnee River basin. During the early Holocene, alluviation in large valleys was punctuated by floodplain stability, soil formation, and erosion. However, the early Holocene record of small streams is unknown because no fill dating to that period has been found. During the middle Holocene, aggradation continued in large valleys, but there is no evidence of soil formation between ca. 7,000 and 5,000 yr B.P. Small streams (second- and third-order) and the upper and middle reaches of fourth-order streams were characterized by net transport of alluvium during the middle Holocene. However, a major shift in the focus of sediment storage took place during the late Holocene. Small streams, formerly zones of net sediment transport, became zones of net sediment storage. This period of alluviation in small valleys was punctuated by episodes of landscape stability and soil formation. Two major periods of floodplain stability and soil formation were documented: one at ca. 2,800-2,000 yr B.P. and another at ca. 1,350-1,000 yr B.P.

Alluviation slowed in the lower, middle, and upper Pawnee River valley by ca. 2,800, 2,000, and 1,500 yr B.P., respectively, allowing soils to develop on the late Holocene floodplain. The Pawnee River downcut sometime between 2,000 and 500 yr B.P., leaving the late Holocene floodplain as a terrace (T-1). This wave of late Holocene incision spread into small valleys at about 1,000 yr B.P., also creating a T-1 terrace.

The entire network of the Pawnee River received large volumes of sediment during the past 1,000 years, especially during the Historic Period. This sediment is stored beneath modern floodplains in large and small valleys. Also, Historic alluvium mantles portions of T-1 terraces in the main valley of the Pawnee River.

The Holocene alluvial chronology of the Pawnee River basin corresponds to the record of climatic change for the Central Plains. In general, early and middle Holocene erosion in small valleys accompanied by alluviation in large valleys corresponds to regional warming and drying (Altithermal). Increased sediment storage in small valleys during the late Holocene was in phase with a shift to wetter conditions. Deep entrenchment in large and small valleys sometime around 1,000 yr B.P. may be related to a short episode of warm, dry conditions documented elsewhere in the Great Plains [see Hall (1990)].

The Holocene stratigraphic record of the Pawnee River is similar to records of other streams in the Central Plains. This similarity is attributed to broadly similar activity of streams throughout the Plains during the Holocene. Major climatic changes appear to be the cause of the synchronous patterns of fluvial activity detected in the stratigraphic records of streams in the regions. However, extrinsic and intrinsic factors in the fluvial system interact to produce periods of stability and instability that may be detected in the stratigraphic record of one stream but not in the record of another stream in the same basin.

Directions for future research

Archeological research

The preceding discussion noted that alluvial deposits of certain ages are differentially but systematically preserved in various drainage elements of the Pawnee River basin. This information combined with geomorphic and soil-stratigraphic data can be used to direct future archeological research in the region. For example, the results of this study make it possible to predict where buried materials for each cultural period are likely to occur in the drainage network (table 6). The potential is based on the presence or absence of Holocene deposits and buried paleosols in the different drainage elements of the basins [after Bettis and Benn (1984, p. 22)].

The geologic potential for buried prehistoric deposits older than Plains Village in T-0 deposits of large and small valleys in the study area is considered low to nonexistent (table 6). Archeological evidence suggests that most of the fill beneath modern floodplains in the region is less than 1,000 years old, and no paleosols were found at depths of more than 50 cm (1.6 ft) below the T-0 surface. Also, preservation of sedimentary features throughout most of the T-0 fill indicates rapid deposition. Altogether these lines of evidence suggest that buried prehistoric sites probably are rare in T-0 deposits, and sites that do occur are likely to be Plains Village and Proto-Historic occupations. There is high potential, however, for Historic sites and artifacts below the surfaces of modern floodplains.

Table 6--Geologic potentials for buried archeological deposits in the study areaa.

Cultural period Large valleys Small valleys
T-0 T-1 T-2b T-0 T-1
Paleo-Indian (>10,000 yr B.P.)   +++ +++    
Early Archaic (10,000-6,000 yr B.P.)   +++ +++    
Middle Archaic (6,000--4,000 yr B.P.)   +++ +++    
Late Archaic (4,000-2,000 yr B.P.) + +++ ++   +++
Plains Woodland (2,000-1,000 yr B.P.) + +++ + + +++
Plains Village (1,000-500 yr B.P.) ++ +++   ++ ++
Proto-Historic (500-150 yr B.P.) +++ +++   +++ +
Historic (<150 yr B.P.) +++ +++   +++ +
a. +, low potential; ++, moderate potential; +++, high potential; -, impossible.
b. Applies only to T-2 terraces in lower reaches of fourth-order streams near their confluence with the Pawnee River.

There is high potential for buried archeological deposits dating to all cultural periods in T-1 fill of the Pawnee River valley. Paleo-Indian and Early Archaic sites are likely to be associated with buried early Holocene soils documented 3.5-7.0 m (11-23 ft) beneath the T-1 surface. Also, Middle Archaic materials may occur at the top of these buried soils. Late Archaic and younger cultural deposits are likely to be associated with buried late Holocene soils documented at many localities in the Pawnee River valley. Most of these late Holocene soils are beneath Historic alluvium that mantles large areas of the T-1 terrace.

Alluvial deposits and paleosols beneath T-1 terraces in small valleys are likely to contain Late Archaic and Plains Woodland sites, but no Middle Archaic or older sites are associated with T-1 fills. Only late Holocene deposits are stored in T-1 fill of second- and third-order streams, precluding an early archeological record. Deep entrenchment of small streams in the Pawnee River basin isolated T-1 surfaces from Historic floods and concomitant sedimentation. Hence Historic and Late Prehistoric cultural deposits should be present on these surfaces in small valleys. This has been confirmed by a recent archeological survey in small valleys of the Pawnee River basin (Timberlake, 1988).

The geologic potential for buried archeological deposits in valley fill underlying T-2 terraces of the Pawnee River is unknown because the age of the fill has not been determined. However, T-2 fill in the lower reaches of fourth-order tributaries to the Pawnee River contains early and middle Holocene deposits and paleosols; therefore it may contain materials dating from the Paleo-Indian through the Late Archaic period.

If the time-space distribution of alluvial deposits in the study areas holds true for other drainage basins of the Great Plains, it may explain the paucity of Archaic and Paleo-Indian sites in the region. In large valleys most of the archeological record may be deeply buried in early through late Holocene terrace fills. In small valleys erosion during the early and middle Holocene probably removed most of the early sites, and aggradation during the late Holocene and Historic Period favored deep burial of Late Archaic and younger sites. Also, surfaces of landforms that dominate valley bottoms throughout the drainage systems are geologically young, often postdating 2,000 yr B.P. Hence apparent gaps in the archeological record may be a result of (1) deep burial of sites, (2) removal of deposits that contain sites, and (3) young surfaces dominating valley landscapes.

Pedologic research

Soil data gleaned from the present study provide a basis for future pedologic research in the region. For example, radiocarbon ages can be used to determine rates of soil genesis in dry-subhumid and semi-arid regions of the Central Plains. Also, morphologic properties of buried Holocene soils raise interesting questions about soil-forming factors. For example, why do early and middle Holocene soils in the Pawnee River valley generally have strongly developed Bt and/or Btk horizons, whereas most late Holocene buried soils are characterized by weakly developed A-C, A-Bw, or ABk profiles? Preliminary results suggest that such variability in Holocene soils may be related to preconditioning of parent materials (Mandel, 1990b). However, additional study is needed to isolate the effects of parent materials from those of time and climate on soil morphology.

Paleoenvironmental research

Most of what has been stated here about Holocene paleoenvironments of the study region was inferred from data gathered elsewhere in the Central Plains. However, there is great potential for collecting paleoenvironmental data in the Pawnee River basin. For example, stratified archeological deposits in late Holocene valley fill of Buckner and Hackberry creeks contain large- and small-mammal bones. Different faunal assemblages detected in different cultural horizons may reflect environmental changes during the late Holocene.

Gastropods and pelecypods also are common in Holocene valley fill of the Pawnee River and its tributaries. Preliminary examination of these invertebrate fauna revealed that there is great potential for paleoenvironmental reconstruction based on changes in taxa from one stratigraphic unit to the next (Raymond Neck, personal communication, 1988).

In addition to examining faunal assemblages in the Holocene fills, an effort should be made to recover pollen and opal phytoliths from Holocene buried soils in the Pawnee River basin. Paleobotanic information combined with the available temporal data would be invaluable to a reconstruction of the region's late Quaternary environments.

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Kansas Geological Survey, Geology
Placed on web July 8, 2016; originally published 1994.
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