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Phillips County Geology

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Recent geologic history

As Ager (1973, p. 34) states, "the sedimentary pile at any one place on the Earth's surface is nothing more than a tiny and fragmentary record of vast periods of Earth's history." This is certainly true of Phillips County. Nonetheless, Frye and A. B. Leonard (1949) and A. R. Leonard (1952) were able to reconstruct a general outline of the geologic history for the Pleistocene, the period during which the present landscape of the region was developed. No major changes have been made in the reconstruction, but details, including absolute ages of events, have come to light in this study. There is an improvement in resolution toward the Holocene because of the preservation of increasingly younger deposits and the readily available radiocarbon dating technique.

During the Miocene and earliest Pliocene, the existing landscape was progressively inundated by Rocky Mountain alluvium (Ogallala Formation), first aggrading river valley bottoms (Valentine member), then filling them to where deposits coalesced on divides (Ash Hollow member), and finally mantling the aggradational landscape (Kimball member). The capping algal limestone probably represents the trace of a calcareous surface soil developed during an indeterminate period of landscape stability.

The Pliocene and earliest Pleistocene was a time of major erosion of Ogallala deposits and stream-system development and entrenchment. Because of the antiquity, unconsolidated nature, and high topographic position of many alluvial deposits of this time period (early pre-Illinoian), no sediments are known to have been preserved. The present drainage patterns were established no later than late pre-Illinoian. This is apparent from the stratigraphic association of pre-Illinoian and younger alluvial deposits with associated terraces and from the accretionary nature of upland loess deposits. Pre-Illinoian (Grand Island and Sappa formations) deposits have been preserved as terrace deposits within the Prairie Dog Creek and North Fork Solomon River valleys.

Illinoian time brought renewed incision, which removed much of the earlier deposits both laterally and vertically. The North Fork Solomon River entrenched approximately 50 ft (15 m) below the base of the pre-Illinoian deposits to within 30 ft (9 m) of the present bedrock valley floors (Frye and A. R. Leonard, 1949, p. 55). Sand and gravel (Crete Formation) were deposited during one or more periods of valley aggradation during the Illinoian, and the fine fraction, primarily silt, was transported by wind and deposited as a mantle of loess on the uplands and valley side slopes, resulting in the Loveland Loess. The several brief periods of landscape stability occurring during deposition of the loess are manifested as a series of paleosols. The Sangamon soil, an extremely well-developed soil capping the Loveland, represents an extended period of landscape stability. Opal phytolith data from the Sangamon soil exposed in the Eustis ash pit, located in south-central Nebraska, indicate that the soil was developing under warm, perhaps dry, grassland conditions. The post-Sangamon period was one of erosion; typically the Sangamon soil is severely or completely eroded and overlain, at least locally, by sandy deposits. By early Wisconsin time major erosion and stream entrenchment were once again occurring. There may have been more than one cycle of cutting and filling during the middle Wisconsin (Altonian Substage), but knowledge of the stratigraphic record is too limited to provide that level of detail. Pre-35-ka fill exists beneath at least some parts of the Kirwin terrace.

The middle and late Wisconsin (latest Altonian and entire Farmdalian) were characterized by deposition of a thin loess mantle (Gilman Canyon Formation) on the uplands, valley side slopes, and bottomlands. The source of the silt-sized material is unknown but was likely from a single large area because of the regional extent of the event. Deposition of the loess was sufficiently slow that pedogenic processes (e.g., melanization and bioturbation) were able to operate nearly continuously. There may have been two or more distinct periods of soil formation, but the thin nature of the loess has welded or bioturbated any individual A horizons. Phytolith data from Gilman Canyon sediments at the Eustis ash pit suggest that the soils formed under a mesic tall-grass cover.

Late Wisconsin time was one of large-scale loess deposition, which began slowly and conformably on the Gilman Canyon Formation. The loess fall began at about 20 ka and ended at about 10 ka, based on terminal radiocarbon ages from the Gilman Canyon and on basal A horizon ages from the Brady soil. Although deposition of the Peoria loess was relatively continuous, there is botanical, pedologic, and geomorphic evidence to suggest that at least a reduction in the rate of deposition occurred, perhaps at 18-17 ka and 14-13 ka. The last date was also approximately the time of valley entrenchment, to a level beneath that of the Kirwin terrace and present floodplain. The cutting was not so laterally extensive that all of the Kirwin terrace was removed, and the cutting probably occurred shortly before 10.5 ka, a time of major soil development in fill elsewhere within the Kansas River basin.

Aggradation of stream valleys during the Holocene constructed a terrace level equivalent to the preexisting Kirwin terrace. This aggradation continued in episodic fashion, creating several poorly developed soils within the alluvial fill. Because of the areal synchroneity in periods of soil formation, the impact of climatic variation is directly or indirectly responsible. Recent entrenchment to a depth of 4- 11 m (l3-36ft) below the Kirwin terrace occurred after 2,500 years ago and probably as recently as 1,000 years ago. The event must have been recent because of the relatively narrow nature of the floodplain and dissected pattern of the Kirwin terrace. During this same period, the Brady soil formed in the Peoria loess, beginning at about 10.5 ka. Where favorable conditions existed, the Bignell Loess began to bury the Peoria loess, beginning at 9-8.5 ka. With loess deposits on the uplands and entrenched stream channels, the local relief of Phillips County is perhaps greater now than at any other time during the Illinoian and Wisconsin time intervals.

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Kansas Geological Survey, Geology
Placed on web Feb. 9, 2013; originally published 1993.
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