Frye and Leonard (1952) outlined the geologic history of the Pleistocene, the period during which the present landscape of Russell County was developed. No major changes in the reconstruction are apparent from this study, but some subtle details, such as the absolute ages of depositional and erosional events have emerged. Specifically, the late Quaternary stratigraphic record, including the late Wisconsinan and Holocene, is better resolved because younger deposits are better preserved and datable by the radiocarbon dating technique.
During the Miocene and Pliocene, the existing landscape of Russell County was partially mantled by Rocky Mountain alluvium. Due to conflicting evidence, it is unclear whether deposits remaining from this time are part of the Ogallala Formation, which has been identified to the west, north, and south of Russell County, or reworked Tertiary sediments. In spite of this uncertainty, remnants of alluvial sand and gravel, especially in the northwest part of the county, indicate that streams functioned as high-energy, braided systems during this time.
At some time during the early Pleistocene, after aggradation of stream channels with a Rocky Mountain sediment load, a period of major erosion, stream-system development, and entrenchment occurred in Russell County. No sediments from this time period have been positively identified owing to their antiquity, unconsolidated nature, and probable high topographic position. The present drainage pattern in Russell County was apparently established no later than late pre-Illinoian. This observation is supported by the stratigraphic association of pre-Illinoian and younger alluvial deposits with terraces and by the deposition of upland loess deposits. Late pre-Illinoian deposits, possibly correlating to the Grand Island and Sappa Formations, may be preserved on valley side walls of the Smoky Hill River, Saline River, and their major tributaries (e.g., Wolf Creek).
Illinoian time brought renewed incision in Russell County, removing much of the earlier deposits both laterally and vertically. Wolf Creek, for example, entrenched at least 17 m (56 ft) below the base of the pre-Illinoian fill. During one or more periods of valley aggradation during the Illinoian. deposition of sand and gravel (possibly correlative with the Crete Formation) occurred in the major stream valleys whereas the fine fraction, transported by the wind, accumulated on the uplands as the Loveland Loess. Several brief periods of landscape stability, resulting in a series of paleosols, probably occurred during Illinoian loess deposition. The most intensive period of landscape stability, resulting in the extremely well-developed Sangamon Soil, took place as deposition of Loveland Loess terminated. Gaps in the stratigraphic record are too extensive to precisely reconstruct the sequence of events in Russell County during the early and early middle Wisconsinan, although a period of regional, post-Sangamon erosion has been documented (Johnson, 1993).
During the late middle and late Wisconsinan (latest Altonian and Farmdalian), deposition of a thin loess mantle (Gilman Canyon Formation) probably occurred throughout Russell County on the uplands. valley side slopes, and bottomlands. Deposition of the loess was sufficiently slow that pedogenesis could incorporate new accumulations. Potentially. two or more distinct periods of landscape stability occurred, but individual A horizons have been obscured through bioturbation.
Late Wisconsinan (Woodfordian) time was a period of large-scale Peoria Loess deposition in Russell County when the Gilman Canyon Formation was slowly and conformably buried. Terminal ages on the Gilman Canyon and basal A horizon ages from the Brady Soil indicate that Peoria Loess accumulated between 20 ka and 10 ka, although reduction in the depositional rate may have occurred at 18-17 ka and 14-13 ka as suggested by regional botanical, pedologic, and geomorphic evidence. Streams in Russell County entrenched to a level beneath the present floodplain from about 13 ka to shortly before 10.5 ka, when a well-developed soil, chronologically equivalent to the upland Brady Soil, formed as it did elsewhere in the central Great Plains.
Except for deposition of a thin, discontinuous cover of Bignell Loess on the uplands, slumping in steep valley walls, and isolated sinkhole formation, the majority of Holocene landscape change in Russell County has been confined to stream systems. Reconstruction of Holocene cut-and-fill sequences elsewhere in the Kansas River basin indicates that Wolf Creek basin in northeastern Russell County can be confidently used as a proxy for events in large streams elsewhere in the county, although subtle differences in the timing or magnitude of events may exist.
During the early Holocene, from approximately 10.5 ka to 6 ka, aggradation of floodplains in Russell County was widespread. Deposition was episodic, with a brief period of soil formation recognized in Wolf Creek basin at 6.7 ka. Sometime during the middle Holocene (6-5 ka), a major erosional event occurred that flushed most early Holocene alluvium from Wolf Creek. Evidence elsewhere in the Kansas River basin suggests that erosion during the middle Holocene was probably not as intensive in the Saline and Smoky Hill River valleys. Aggradation of floodplains in Russell County was renewed at 5 ka and lasted episodically until approximately 1 ka. Evidence from Wolf Creek indicates that brief periods of floodplain stability occurred at 1.8 ka, 1.5 ka, and 1.2 ka in the county. Recent entrenchment to a depth of 5-9 m (16-29 ft) in the last 1 ka has elevated a prominent terrace throughout Russell County and resulted in a series of ill-defined landforms in the bottomlands adjacent to the stream channel.
We are grateful to the many landowners who provided access to their land and information concerning local geology, soils, landforms, and associated historical events. Funding for radiocarbon dating was provided by grants from The University of Kansas General Research Fund and Sigma Xi. D. W. May of the University of Northern Iowa and D. E. Hattin of Indiana University kindly served as manuscript reviewers and provided many valuable comments.
Kansas Geological Survey, Geology
Placed on web April 8, 2013; originally published 1996.
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