Summary of StratigraphyThe rocks that crop out in Sedgwick County are of sedimentary origin and range in age from Permian to Recent. Rock exposures in the County are poor owing to the nature of the rocks and the lack of topographic relief. Much of the interpretation presented in this report is based on subsurface data and knowledge of geology in nearby areas.
The oldest rocks cropping out in the County are a part of the Wellington Formation of the Cimmaronian Stage of the Permian System. The Wellington forms the bedrock surface in about the eastern two-thirds of the County and is exposed at the surface along streams east of the Arkansas River valley and along the southern edge of the County where it is not covered by younger rocks. The Ninnescah Shale, also of the Cimmaronian Stage, forms the bedrock surface in about the western one-third of the County. It conformably overlies the Wellington Formation in this area. Topographic relief in this part of the County is more pronounced, and the Ninnescah Shale is exposed at many places in the valley walls of the Ninnescah River and its tributaries. However, the Ninnescah Shale is covered by younger rocks in the uplands.
The unconsolidated deposits of south-central Kansas, particularly north of Sedgwick County, in McPherson, Harvey, and Reno counties, described by Haworth and Beede (1897) and by Beede (1898) were named the "McPherson Equus beds." On the old edition of the geologic map of Kansas (Moore and Landes, 1937) the unconsolidated rocks are shown as alluvium, Quaternary deposits, and, in the upland west of the Arkansas Valley, as Ogallala (Tertiary). Williams and Lohman (1949) and Hibbard (1952) recognized the presence of Tertiary deposits, which they called the Delmore Formation, in a part of McPherson County. All deposits of Pleistocene age there except the alluvium and dune sand were included in the McPherson Formation. Frye, Leonard, and Swineford (1956, p. 57) recognized that the Delmore Formation had a local source, but they included these beds within the Ash Hollow Member of the Ogallala Formation on the basis of fossil evidence. In subsequent geologic work in conjunction with ground-water investigations in Kansas members of the Federal and State Geological Surveys have established a sequence of Pleistocene events that can be correlated with the classic glacial section of the Mississippi Valley. The stratigraphic sequence thus established will be used in this report insofar as possible, although the term "Equus beds" is well established by local usage to include all unconsolidated water-bearing deposits in Sedgwick County and adjacent areas irrespective of age.
Unconsolidated deposits of clay, silt, sand, and gravel believed to be equivalent to a part of the Pliocene Ogallala Formation unconformably overlie Permian rocks in the basal part of the valley fill of the Arkansas River. Similar deposits are present as isolated remnants underlying loess on the uplands west of the Arkansas Valley. These rocks are known to be exposed at only one locality in the County--in a creek channel in the uplands north of the city of Goddard but they have been penetrated by many water wells in the valley.
Deposits representing at least four major depositional cycles of the Pleistocene Series are present in Sedgwick County. These deposits range in age from the Early Pleistocene Holdrege Formation of the Nebraskan Stage to Recent alluvium and consist of unconsolidated clay, silt, sand, and gravel. The sediments of Pleistocene age unconformably overlie Neogene(?) and Permian rocks in the Arkansas Valley and Permian rocks in other stream valleys and the upland areas of the County.
A generalized section of the rocks significant to the occurrence of ground water in Sedgwick County is given in Table 2.
The areal geology of the County and the geologic cross sections of it are shown on Plates 1 and 2 respectively.
Kansas Geological Survey, Geohydrology of Sedgwick County|
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Web version April 1998. Original publication date Dec. 1965.