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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 240, part 3
Chert Gravel and Neogene Drainage in East-central Kansas--page 8 of 15
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REGIONAL DISTRIBUTION OF UPLAND CHERT GRAVEL

Regional distribution of high-terrace and hilltop gravel deposits is depicted according to actual elevation on the raster database (fig. 8). This database overestimates the actual area of upland gravels, as individual gravel cells may represent full coverage to as little as 10 acres (4 hectares) of Olpe soil. Nonetheless, the database provides for a regional overview of gravel distribution and elevation. The highest gravel deposits are located in headwaters of the Walnut, Verdigris, Fall, and Cottonwood drainage basins at elevations greater than 400 m (1,310 ft). These gravels are preserved near the crest of the Flint Hills escarpment. The lowest gravels are found near the eastern and southeastern margins of the study region at elevations less than 300 m (980 ft).

Fig. 8. Kansas chert gravel (Olpe Soil) map. Raster grid of upland chert gravel deposits (Olpe soil) in eastern Kansas, according to township-and-range grid. Each pixel represents one quarter section in area (0.25 mile2, 0.65 km2); pixels are color coded according to elevation classes. Numbers indicate man-made reservoirs: 2 = Pomona, 3 = Melvern, 5 = Marion, 6 = John Redmond, 7 = El Dorado, 8 = Toronto, and 9 = Fall. See also Fig. 2.

Walnut Drainage Basin

Chert gravel deposits are abundant on high terraces and hilltops in southern Butler and northernmost Cowley counties (fig. 2). Gravel deposits are associated with the Little Walnut River, Hickory Creek, and Rock Creek valleys, as well as the lower Walnut River valley. These streams have entrenched their headwaters into or below the Florence Limestone Member, which is the primary source of residual chert. Streams in western and northern Butler County either have not yet entrenched or are just beginning to erode into the Florence Limestone Member. This explains the general lack of older chert alluvium in northern and western portions of the Walnut drainage basin.

Upland chert gravel deposits are formally named the Leon Gravel after the city of Leon (Aber, 1992). A stratotype is designated in NE sec. 35, T. 27 S., R. 5 E. As defined, the Leon Gravel is equivalent to the Olpe or Olpe-Norge soil map units within the Walnut drainage basin of Butler and Cowley counties. The Leon Gravel is usually 1-2 m (3.3-6.6 ft) thick and rests on Permian bedrock. It can be informally separated in some places into upper and lower members on the basis of topographic position in relation to adjacent modern valleys. The upper member occupies hilltop or drainage-divide positions, about 18-30 m (60-100 ft) above modern floodplains. The Olpe soil B2 unit color is typically moderate to dark reddish brown (10 R 4/6 to 3/4). The lower member occupies high-terrace positions, roughly 6-12 m (20-40 ft) above modern floodplains. The Olpe soil B2 unit color is typically moderate brown (5 YR 4/4).

Upland chert gravels in the Walnut drainage basin are preserved almost exclusively on the northern sides of eastern tributaries--Little Walnut, Hickory, and Rock valleys--and on the eastern side of the lower Walnut valley. The same pattern holds true for lower (that is, younger) terraces within these valleys, and rivers are cutting bedrock bluffs in many places on valley sides opposite the terraces. Exotic pebbles are found within chert gravel deposits in all portions of the Walnut basin and are especially abundant along the lower Walnut valley.

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Kansas Geological Survey
Web version March 18, 1998
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