Rocks of Permian Age
The Permian rocks of Kansas have been reclassified to conform with the recommendations in the report of the Permian Committee of the American Association of Petroleum Geologists (C. W. Tomlinsori et al., 1940), and divided by Moore (1940, p. 42) for the State Geological Survey of Kansas into the following series and groups, in descending order:
|Council Grove group|
Only the Wolfcampian series is represented in the outcrops in the Forest City basin.
Detailed descriptions of the outcrops of strata of the Wolfcampian series in Riley and Geary counties have been published by Jewett (1941a). The rocks included under this term were formerly known in Kansas as the Big Blue series, but since their correlation with the standard section established in Texas the term Big Blue has been abandoned by the State Geological Survey of Kansas. Under the new classification the Sumner group, formerly included in the Big Blue, has been excluded from the Wolfcampian and added to the overlying Leonardian series. The western counties of the Forest City basin are underlain by the Admire and Council Grove groups which constitute the lower part of the Wolfcampian series. These rocks, concealed in many places by glacial deposits, occupy broad belts trending east of north on both sides of the Nemaha anticline in northeastern Kansas. Older rocks of Wabaunsee age come to the surface on the crest of the Nemaha anticline in this part of Kansas. Only small areas east of the Nemaha anticline are underlain by the Chase group.
North of Kansas River, the Admire group crops out in two southwesterly trending belts separated by the Wabaunsee which is exposed between them on the crest of the Nemaha anticline. These belts merge south of Wabaunsee county. The Admire group consists chiefly of shale, much of which is red. The shale is interstratified with a number of limestones, named in ascending order, Aspinwall, Falls City, Five Points, and others which are thinner and less prominent than those in the overlying Council Grove group. The lowest rocks of the Admire group were deposited in channels, possibly 125 feet deep, eroded in the Pennsylvanian rocks. The channel deposits consist mainly of deposits known as the Indian Cave sandstone. The Admire group without the channel deposits has a thickness of only about 100 feet. It is overlain by the Council Grove group without obvious disconformity.
Council Grove group
This group of rocks consists principally of shale with a number of resistant but thin limestone beds, the Americus, Neva, Cottonwood, Funston, and others. Much of the shale is red or green. The lowest member is the Americus limestone. The highest member is the Speiser shale which underlies the conspicuous Wreford limestone at the base of the Chase group.
The Council Grove group north of Kansas river underlies the surface in the syncline east of the Nemaha anticline. It crops out again on the west side of the anticline. South of Kansas River these two belts of outcrops come together and the Council Grove takes it place in the parallel bands in which successive groups crop out across the state.
According to sections measured by Jewett (1941a, pl. 6) in outcrops in Riley County, the Council Grove group has a thickness of 300 feet. This group overlies the Admire group and underlies the Chase group with essential conformity.
The Chase group is composed of cherty limestones alternating with shale. The lowest member of the group is the Wreford limestone; the highest is the Herington limestone. Only the lowest Chase rocks occur in the Forest City basin north of Kansas river where they have been preserved locally in a synclinal area in northeastern Nemaha county. The lower part of the Chase group occurs in a narrow belt at the surface of the Flint hills on the western border of the Forest City basin.
Permian rocks younger than the Chase group are not now found in the Forest City basin, although eastern Kansas was undoubtedly covered at one time by many hundreds of feet of upper Permian rocks.
Glacial till--No rocks of Mesozoic age are known to occur now in or adjacent to the Forest City basin area of northeastern Kansas, but it is probable that Cretaceous rocks at one time extended far into the basin. Deposits of Tertiary age, consisting dominantly of chert gravels, have been reported (Todd, 1920; Frye, 1941, p. 246) at some localities overlying the bedrock and overlain by Pleistocene glacial till. The stratified rocks of Pennsylvanian and Permian age in the northern counties of the Forest City basin were covered by unconsolidated glacial debris in Pleistocene time. At least part of the area was covered by both the Nebraskan and Kansan glaciers (Frye, 1941, pp. 146, 147), so the original thickness of these deposits is not known. The Kansan ice sheet advanced approximately to the present valley of Kansas river, and glacial deposits formerly covered all of eastern Kansas north of this line (Schoewe, 1930). The glacial deposits have been eroded from much of the area since the retreat of the ice, especially in the major valleys and their tributaries. Deposits of Pleistocene age attain a maximum thickness of more than 100 feet and conceal the stratified rocks in large areas of the upland surface between the valleys.
Alluvium--The valley flats of the major valleys are underlain by deposits of alluvium. These deposits have in large part been deposited since the retreat of the ice, but the development of the major valleys may have been initiated before the advance of the Kansan ice sheet.
Kansas Geological Survey, Forest City Basin
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Web version July 2005. Original publication date Dec. 1943.