Stratigraphy of Outcropping Rocks, continued
Cretaceous System (?)--Lower Cretaceous Series (?)
Dakota (?) Formation
Gravel, of possible Cretaceous age, having a distinctive lithology was found at one site on the upland surface in central Johnson County. Because Cretaceous rocks have not been described previously in the county, a brief description of their present and probably former distribution in Kansas and nearby areas is given, and the lithology of the basal Cretaceous rocks is described.
Outcrops of Dakota Formation in central Kansas (Jewett, 1954) and in southwestern Iowa (Hershey and others, 1960, p. 5) are about 125 miles from Johnson County. The basal Cretaceous rocks in Kansas have been considered by some authors to have been derived from the north and cast and to formerly have been much more extensive in northeastern Kansas than they are today. In early Cretaceous time erosion beveled the Pennsylvanian and Permian rocks in eastern Kansas. The erosion surface was of moderate relief and is covered in central Kansas by the progressively overlapping Cheyenne Sandstone, Kiowa Formation, and Dakota Formation. Genetically related to this erosion surface, rather than to a stratigraphic position within the Cretaceous rocks, is a zone of rounded pebbles and cobbles consisting predominantly of chert, quartz, quartzite, and siliceous and ferruginous quartzose sandstone judged, in Kansas, to have been derived from the north and east (Frye and Leonard, 1952, p. 180-181).
In southwestern Iowa the Dakota was originally extensive but is now present only as scattered remnants partly covered by glacial deposits. In southwestern Iowa the Dakota consists of sandstone, clayey shale, and conglomerate; the conglomerate most often consists of siliceous pebbles in a ferruginous matrix (Hershey and others, 1960, p. 6).
In Johnson County the gravel of possible Cretaceous age consists of scattered rounded pebbles and cobbles of light and dark quartz, quartzite, and sub-rounded chert. These are lithologically similar to the pebbles and cobbles found in the basal Cretaceous of north-central Kansas, In addition, there are pebbles and cobbles of a subangular siliceous and ferruginous conglomeratic sandstone that weathers dark reddish brown. The siliceous and ferruginous conglomeratic sandstone pebbles are lithologically similar to cemented sandstone found in the Dakota of Saline County, Kans., and are distinctively different from any sandstone in the Pennsylvanian rocks of eastern Kansas.
The gravel, at an altitude of about 1,040 feet, was observed in a new road cut in the NW SE sec. 30, T. 14 S., R. 23 E., during construction of Interstate Highway 35. The scattered pebbles and cobbles of gravel occur in a red soil developed over the Weston Shale Member and below a thin mantle of loess.
The gravel cannot be of glacial origin because it lies south of the glacial border in the Marais des Cygnes River drainage basin, and lacks certain igneous and metamorphic rock types that are characteristic of weathered till or outwash in Kansas (Davis, 1951). The angularity (if the siliceous-cemented ferruginous sandstone pebbles precludes their having been transported by streams for any great distance. The high topographic position and the lack of a local source for rocks of this lithology make it unlikely that the gravel is a remnant of a Tertiary or early Pleistocene stream deposit. Therefore, the scattered pebbles and cobbles of lithologically distinctive gravel probably represent a formerly extensive deposit of Cretaceous rocks and a Cretaceous erosional surface. The rocks are not extensive enough to map.
Kansas Geological Survey, Johnson County Geohydrology
Web version April 2002. Original publication date Dec. 1971.
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