| dc.description.abstract | The Kiowa Foundation (formerly Kiowa Shale) of Early Cretaceous age and the Dakota Foundation of Early(?) and Late Cretaceous age have been studied and mapped on a detailed reconnaissance basis in an area encompassing some 3500 square miles (9100 krn2) in north-central Kansas. Mapping was initiated as a means of unravelling the stratigraphy, an understanding of which is essential to sedimentary petrographic studies. Neither formation had previously been mapped extensively despite long standing as rock-stratigraphic units. The Kiowa Formation is not present in the northern part of the area mapped. Both formations have an involved nomenclatural history. The Kiowa Foundation is a heterogeneous assemblage composed largely of olive-weathering gray illitic shale and abundant sandstone. The Foundation rests on the eroded surface of Permian rocks. Along the eastern fringes of its outcrop belt, the Kiowa Formation contains a diagnostic sequence of siltstone underlain by a heterogeneous assemblage of red-mottled and carbonaceous gray to black mudstone and siltstone. The mudstone contains variable amounts of montmorillonite and kaolinite. The siltstone and the underlying assemblage are designated the Longford Member (new name) of the Kiowa Foundation. Above the Longford Member, the Kiowa Formation contains numerous fossils of marine or brackish-water invertebrates. As in the Dakota Formation above, fossil deciduous leaves locally are abundant. The Dakota Formation is subdivided into two members, the Terra Cotta Clay Member below and the Janssen Clay Member above. The Terra Cotta is made up largely of light-gray kaolinitic mudstone and claystone showing abundant red mottles, but it also contains appreciable sandstone. The Janssen is composed chiefly of gray and dark-gray kaolinitic mudstone and claystone, locally abundant sandstone, and scattered beds of lignite. It contains marine and brackish-water fossils near its contact with the overlying Graneros Shale, into which its upper part grades laterally. The Kiowa Formation was deposited in, and near the margins of, the Early Cretaceous sea that transgressed from southwest· to northeast across Kansas. Part of the Longford Member of the Kiowa Formation is thought to have been deposited on the landward side of the shifting shoreline. The upper part of the formation is thought to include regressive deposits, sedimentation of which heralded deposition of the Dakota Formation. The Dakota Formation is mainly an alluvial plain deposit that developed and extended itself southwestward upon relatively rapid withdrawal of the Kiowa sea. Upward changes in the Janssen Clay Member largely reflect the influence of the transgressing sea in which the Graneros Shale was deposited. Sandstone in the Kiowa and Dakota formations is mature and contains as much as 95 percent detrital quartz, quartzite, and chert, and locally contains molds and casts of pelecypods. Conglomeratic sandstone in the Dakota Formation is coarser grained than conglomeratic sandstone in the Kiowa and contains numerous pebbles of penecontemporaneously reworked mudstone and claystone. Heavy mineral assemblages in both formations are similar and contain zircon, tourmaline, and staurolite as major components. The proportion of staurolite decreases upward in passing from the Kiowa Formation to the Janssen Clay Member of the Dakota Formation.Sandstone in both formations is abundantly cross-stratified. Computation of vector-resultant dip bearings of cross-strata in both formations indicates that the regional slope was inclined to the west-southwest. The heavy mineral suite therefore is attributed in part to source areas lying to the east and northeast, mainly Paleozoic or older rocks. The staurolite probably was derived from rocks in the central Appalachian Mountains. Variations in clay mineralogy are attributed to differential transport and sedimentation. Source areas for the clay minerals are thought to have been generally the same for both formations. The contact selected for mapping the Kiowa and Dakota formations is a sharp, consistent boundary that can be used in most of north-central Kansas and in the type area of the Kiowa Foundation in southwestern Kansas. In many places the contact is disconformable. | en_US |
