Paralic to Fluvial Record of an Early Cretaceous Marine Transgression--Longford Member, Kiowa Formation, North-Central Kansas
By Paul C. Franks
Originally published in 1979 as Kansas Geological Survey Bulletin 219. This is, in general, the original text as published. The information has not been updated.
This report describes, names, and treats the origins of the Longford Member of the Kiowa Formation. Longford rocks form a distinctive part of the Kiowa Formation and include siltstone, relatively scarce sandstone, minor lignite, and abundant clay rocks. They mark the base of the Kiowa Formation in parts of north-central Kansas and are named for the excellent exposures near Longford, southwestern Clay County. The member also crops out in parts of Washington, Ottawa, Dickinson, Saline, and Marion counties. Longford sediments were deposited more than 100 million years ago during the Mesozoic Era, specifically during late Early Cretaceous time when much of what we now call Kansas was covered by the Kiowa sea.
Clay rocks of Cretaceous age have been an important economic resource in north-central Kansas, especially since World War II. Shale in the Kiowa Formation is used to make light-weight, bloated aggregate. Clay rocks in the Dakota Formation, which overlies the Kiowa Formation, are used to make bricks. Knowledge of Kiowa and Dakota stratigraphy has aided in the utilization of those ceramic resources. The essentials of Kiowa and Dakota stratigraphy were outlined by Norman Plummer and John F. Romary in 1942 (Kansas Geological Survey Bulletin 41, Part 9), and this report refines and extends some of the groundwork laid by them. Although this report does not deal specifically with the ceramic properties of Longford clay rocks, it does note that Longford rocks resemble and have been confused with those in the Dakota Formation. Longford clay rocks, like those near the base of the Dakota Formation, are demonstrably lenticular, and they contain variable proportions of the ceramically important clay minerals kaolinite and smectite.
Longford sediments were deposited in broad stream valleys, in estuaries, and in lagoons or bays along sandy barrier coasts as the Kiowa sea flooded into what is now north-central Kansas. The top of the Longford Member is marked by nearly white to pale yellowish-brown siltstone that formed from well-washed sediments that were deposited behind Kiowa barrier bars and islands along the landward, tide-influenced shores of lagoons or bays. The distribution of Kiowa shale and sandstone above the Longford Member offers insight into the mechanism by which Kiowa shorelines shifted landward, and that mechanism seems to differ significantly from the major mode of marine transgression that is inferred for many geologically young barrier shorelines, like those along the modern Atlantic coast of North America. Clay rocks in the Longford Member range from those that are light gray and blotched by striking, red, hematitic mottles to those that are nearly black and highly plastic. At least some of the clay rocks originated as floodplain or other overbank deposits of streams, but they were modified into immature tropical alluvial soils by the warm, humid climate that prevailed during Kiowa time.
A distinctive facies of the Albian Kiowa Formation is exposed along the eastern fringes of the Kiowa outcrop belt in north-central Kansas. The name "Longford Member of the Kiowa Formation" is proposed for the distinctive unit, which is as much as 100 ft (30 m) thick. Conspicuous, nearly white siltstone as much as 15 ft (4.6 m) thick, marks the top of the member. The lower parts of the member consist of clay rocks, siltstone, sandstone, and lignite. Red-mottled clay rocks, red-mottled siltstone, and the nearly white siltstone at the top of the member form a striking contrast to drab shale and yellowish-gray to brown sandstone in other parts of the Kiowa Formation. The Longford Member rests with transgressive disconformity on Lower Permian rocks. Along most of the outcrop belt, it is overlain conformably by higher parts of the Kiowa Formation. About 30 mi (48 km) south of the Kansas-Nebraska border, near the northward pinchout of the Kiowa Formation, the member is overlain by the Dakota Formation (Albian? and Cenomanian). The siltstone at the top of the Longford Member is the northernmost Kiowa rock that can be identified with certainty as the formation pinches out.
Longford sediments were deposited as the Kiowa sea transgressed onto the eroded, gently dipping, western flank of the Nemaha anticline. Kiowa barrier-bar systems and the topography developed on Permian bedrock influenced both the thickness and the depositional environments of the Longford Member. The siltstone that marks the top of the unit stems from sedimentation along the landward, inner shores of lagoons or bays that formed behind Kiowa barrier bars. Sedimentary structures and fossil reeds and rootlets suggest that the inner shores were affected by tides.
The lower parts of the Longford Member formed from sediments that accumulated in fluvial and estuarine realms that developed in broad valleys eroded into Permian bedrock. Log-probability plots of grain-size distributions support a fluvial origin for most Longford sandstone. Red-mottled, kaolinitic and smectitic clay rocks and scarce red-mottled siltstone represent floodplain deposits. The red mottling accords with gleying in immature, tropical soils. A pedogenic origin for the red-mottled clay rocks also is indicated by plexoidal and domain clay fabrics similar to those common in soils, and by "clay skins" around detrital quartz grains. Lenses of dark, smectitic claystone may be fossil vertisols that developed in floodbasins or on estuarine bayhead deltas. Lignite seams and lenses represent floodbasin deposits, but some of them may have formed from detrital accumulations of plant debris on estuarine delta plains. Sequences of light-gray siltstone may also represent estuarine sediments.
Light-gray, smectitic, commonly silty Kiowa shale overlying the Longford Member probably formed from lagoon or bay sediments. Kiowa rocks above the lagoon or bay deposits indicate that transgression was accompanied by in-place growth and eventual submergence of Kiowa barrier systems. Submergence resulted in landward shifting of surf zones and in construction of new barriers near the former inner shores of lagoons or bays. Open-sea, illitic, Kiowa shale overlies the lagoon or bay deposits in many places. Shoreface erosion, or other processes of ravinement, did not lead to the development of either marked or extensive disconformities during transgression. Instead, remarkably complete transgressive records of fluvial, estuarine, lagoon, and bay sedimentation were preserved.
Fossil soils along the Permian-Cretaceous unconformity in central Kansas indicate that the kaolinite and smectite in Longford clay rocks were reworked from similar soils developed on Paleozoic source rocks in the continental interior. Clay-mineral assemblages in facies-equivalent Longford and Kiowa rocks accord with, but do not prove, derivation of the abundant illite in Kiowa shale from sources that were different from those that supplied kaolinite and smectite to Longford depositional systems. Longford sandstone was derived mainly from Paleozoic terrains in the continental interior, but staurolite grains in the sandstone suggest that some of the sand ultimately came from the crystalline terrains of the central Appalachian Mountains.
Kansas Geological Survey, Geology
Placed on web March 10, 2009; originally published November, 1979.
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