The Longford Member is a mappable unit and a distinctive facies of the late Albian Kiowa Formation in north-central Kansas. The top of the member is marked by a conspicuous siltstone, whereas the lower parts consist of varied assemblages of kaolinitic and smectitic clay rocks, siltstone, sandstone, and lignite. The unit is named for exposures near Longford, southwestern Clay County. Brightly colored, red-mottled clay rocks in the lower parts of the member and the nearly white siltstone that caps the member contrast with olive-gray shale and brown sandstone in facies-equivalent and overlying parts of the Kiowa Formation. Longford rocks rest with transgressive disconformity on Lower Permian rocks and are overlain conformably by higher parts of the Kiowa Formation. As the Kiowa Formation thins and pinches out northward between Permian rocks below and the Dakota Formation above, the rocks of the Longford Member also are truncated by the Dakota Formation. The northward thinning and disappearance of the Kiowa Formation and its Longford Member coincide with the northward increase in breadth of the gently dipping western limb of the Nemaha anticline. The siltstone marking the top of the member is the northernmost kind of Kiowa rock that can be identified in any practical way as the formation pinches out beneath red-mottled clay rocks and sandstone of the Dakota Formation near the Clay County-Washington County boundary, about 30 mi (48 km) south of the Kansas-Nebraska border (Figs. 1, 2; Pl. 1).
Longford rocks have been confused with rocks belonging to the Dakota Formation in central Kansas. Both units contain red-mottled clay rocks, gray to brownish-gray clay rocks, lignite, light-colored siltstone showing reed or trace fossils, and sandstone. The stratigraphic position of the Longford Member at the base of the Kiowa Formation, in combination with the characteristic sequence of rock types, allows easy recognition of the member in most places. From top to bottom, the characteristic sequence of strata is: Kiowa shale or sandstone overlies silty or shaly beds that grade sharply downward into the nearly white siltstone that marks the top of the member; the siltstone in turn grades downward into varied assemblages of clay rocks, siltstone, sandstone, and lignite; these assemblages in turn rest unconformably on Permian rocks (Pl. 2; Fig. 15). Most, but not all, lignite in the Dakota Formation is near the top of the unit. Consequently, stratigraphic position beneath the capping siltstone is the distinguishing feature of Longford lignite. The same is true for Longford sandstone, much of which resembles other sandstone in the Kiowa and Dakota formations. Longford clay rocks commonly contain appreciable smectite. As a result, their weathering characteristics differ from those shown by most Dakota clay rocks. Slopes developed on exposed Longford clay rocks tend to be puffy and to have abundant shrinkage cracks (Fig. 7), in contrast to the relatively smooth, resistant surfaces formed on kaolinitic Dakota clay rocks. The siltstone that marks the top of the Longford Member could be confused with nearly white Dakota siltstone that also contains reed or trace fossils, except that such Dakota siltstone is near the top of that formation. Where the capping Longford siltstone lacks reed or trace fossils, however, it can be mistaken for siltstone near the base of the Dakota Formation. Along the pinchout of the Kiowa Formation, therefore, where the Dakota Formation rests on and truncates the siltstone marking the top of the Longford Member, attempts to distinguish between Longford and Dakota rocks commonly become impractical. Consequently, some of the rocks mapped as part of the Dakota Formation north of the Kiowa pinchout shown in Plate 1 probably are coextensive with Longford rocks.
The distribution of Longford rocks in the subsurface west of the outcrop belt is incompletely known. Longford rocks are present in the subsurface of Ottawa County, and probably also in the subsurface of northwestern Ellsworth County (Fig. 1) (O. S. Fent, 1977, written communication). Despite the northward pinchout of the Kiowa Formation along the Nemaha anticline in Kansas (Fig. 1, Pl. 1), the depositional strike of the Kiowa Formation (about N 25° W) implies that equivalents of Longford rocks may be present in the subsurface of southern and central Nebraska. Longford rocks, accordingly, may be important to the unravelling of Cretaceous stratigraphy there. Longford rocks, especially the red-mottled and brownish-gray to gray clay rocks, should not be equated with similar rocks in the Albian Cheyenne Sandstone of southern Kansas, or with similar rocks in the Albian (?) to Cenomanian Dakota Formation of central Kansas and southern Nebraska. Longford rocks are facies equivalents of Kiowa shale and sandstone in Scott's (1970a, 1970b) Inoceramus bellvuensis and I. comancheanus concurrent-range zones (Fig. 2). The stratigraphic relations, if any, between the Longford Member and widespread sandstone and sequences of nearly white to greenish-gray siltstone, shale, and sandstone at the base of the Cretaceous System in the subsurface of western Kansas need to be examined.
The thickness of the Longford Member ranges from 0 to about 100 ft (0 to 30 m). The thickness, especially of the lower parts, is controlled mainly by topography developed on underlying Permian rocks prior to deposition of the Cretaceous beds. In places, the siltstone that marks the top of the member rests directly on Permian rocks, but elsewhere the lower parts of the member are as much as 80 ft (24 m) thick. Topography developed on Permian rocks before transgression of the Kiowa sea not only influenced the kinds of Longford sediment that accumulated from place to place, but they also exerted profound control over Longford environments of deposition.
Longford sediments were deposited in fluvial, estuarine, and inner shore realms behind barrier systems that formed linear clastic Kiowa shorelines. Light-gray, commonly silty Kiowa shale overlying the siltstone that marks the top of the Longford Member is inferred to be the product of deposition of muds in bays or lagoons, whereas the nearly white, thin-bedded to thin-laminated, well-sorted siltstone is judged to represent deposits that accumulated along the landward, inner shores of the lagoons or bays (Fig. 15). Evidence that tides affected those inner shores mostly is indirect and depends on indications that tides influenced deposition of the thick, lenticular deposits of sand that formed the barrier systems. The siltstone does not show the kinds of upward fining, progradatiodal sequences that characterize many tidal-flat deposits (Klein, 1971, 1972). The siltstone formed under conditions of general transgression, and opportunities for deposition of diagnostic, progradational sequences were limited. Current ripple marks, flaser bedding, reed fossils, plant rootlets, and probable insect trails preserved in the siltstone, however, are consistent with intertidal to supratidal deposition of much of the silt. Locally detected upward changes from siltstone containing reed fossils or plant rootlets into siltstone showing low-angle cross-strata, oscillation and current ripple marks, and rod-shaped burrows indicate submergence of the inner shore deposits as Kiowa lagoons or bays expanded or shifted landward.
Relatively scarce sandstone and abundant red-mottled clay rocks in the lower parts of the member represent fluvial deposits that accumulated in the broad valleys that were eroded into Permian bedrock before the Kiowa sea transgressed north-central Kansas. Log-probability plots of grain-size distributions in the sandstone have shapes that compare well with those of fluvial sands studied by Visher (1969). The orientation of cross-strata (vector resultant of S 44° W) in one lens in Clay County (sections 16 and 21, T. 9 S., R. 2 E., Pl. 1) also accords with fluvial transportation of the sand down the Kiowa depositional slope. The scour-fill contact at the base of the lens and the incorporation of fragments of penecontemporaneously reworked clay rocks in the sandstone also are consistent with fluvial processes. Intercalation of other sandstone with red-mottled clay rocks also suggests fluvial sedimentation of the sand.
Red-mottled clay rocks and less common red-mottled siltstone in the lower parts of the member stem from overbank deposition of clay and silt by Longford streams. Slickensided fractures, microscopic evidence of clay skins, and other evidence of soil-matrix fabrics (Brewer, 1964) support the idea. The red, hematitic mottles are attributed to early diagenetic reduction, redistribution, re-oxidation, and dehydration of iron oxides in the floodplain sediments, which may have been stained red or brown initially. The mottled rocks are products of gleying and formation of so-called soft plinthite in low-lying alluvial soils in which fluctuating groundwater tables lay close to the floodplain surfaces. The red-mottled rocks resemble groundwater laterites, but they have more in common with immature alluvial soils that have not undergone extensive weathering. Like immature alluvial soils along the Amazon River (Sombroek, 1966), the red-mottled Longford rocks do not show evidence of well-developed soil profiles, even though they are extensively mottled; they lack layers of concretionary hard plinthite near the tops of the mottled lenses; although they contain abundant kaolinite, the red-mottled rocks also contain appreciable 2:1 clay minerals, chiefly smectite and lesser amounts of illite; and they do not contain gibbsite. The red-mottled rocks seem to be products of initial soil-forming processes or "soil ripening" on floodplains (Pons and Zonneveld, 1965).
Other Longford sediments may have accumulated in fluvial or estuarine settings. Lenses and seams of lignite are composed of detrital plant debris that may have been deposited in more than one environmental realm. Those low in the member probably formed as floodbasin deposits, whereas those near the base of the siltstone that caps the member could have formed either as floodbasin deposits or on the plains of estuary-head deltas. The same is true for lenses of nearly black, highly plastic, smectitic claystone. The claystone shows slickensided fractures, churned fabrics, and downward projections into underlying clay rocks. It likely formed as a tropical vertisol (Soil Survey Staff, 1960), a common soil in floodbasins or estuarine settings where there is repeated wetting and drying of the soils. The abundant gray and brownish-gray mudstone and claystone, much of it containing appreciable carbonaceous matter, likewise may have formed from deposits that accumulated in fluvial or estuarine settings. Because the Kiowa sea flooded the gentle topography and broad valleys developed on Permian bedrock, products of estuarine sedimentation are to be expected among the rocks underlying, or not far below, the siltstone marking the top of the Longford Member (Fig. 15).
Much of the siltstone below that marking the top of the member probably stems from estuarine sedimentation, as is indicated by its position between fluvial deposits below and inner shore deposits above. Many of the siltstone beds show oscillatory and current-ripple structures and contain abundant carbonaceous plant debris or imprints of plant debris, features that accord with estuarine deposition of silts, perhaps near or on bayhead deltas.
The climate that prevailed during deposition of Longford sediments was equable, humid, and tropical. The lack of marked seasonal banding in the piece of silicified gymnosperm wood that was found near the base of the member in Marion County indicates the equable nature of the climate. The similarity of the red-mottled clay rocks and the dark, highly plastic, smectitic claystone to modern, alluvial tropical soils is suggestive of the humidity and warmth of the climate. These inferences reinforce interpretations about Kiowa climates by Franks (1966, 1975) and Scott (1970a, 1970b).
Heavy minerals noted in thin sections of Longford sandstone also are found in other Kiowa sandstone (Franks, 1966, 1975). The tourmaline and zircon in the heavy-mineral suites indicate derivation of the sands by reworking of Paleozoic sedimentary materials in the continental interior to the east of Kansas. Staurolite in the sandstone, however, suggests that some of the sand was derived from the crystalline terrains of the central Appalachian Mountains, either directly, or indirectly by reworking of Upper Permian or younger rocks now eroded from the continental interior (Franks, 1966, 1975).
The abundant kaolinite and smectite in Longford clay rocks probably were derived by reworking of fossil soils similar to those preserved locally along the Permian-Cretaceous unconformity in central Kansas. Similar soils probably were widespread on Paleozoic rocks in the continental interior during Early Cretaceous time. Those in Kansas likely developed under conditions of tropical weathering and poor drainage of the illitic and chloritic parent rocks. The small amounts of illite and mixed-layer chlorite or vermiculite in Longford rocks are consistent with derivation of those minerals by erosion of weathered Paleozoic source rocks in the continental interior.
Irregular upward increases in the proportions of kaolinite to smectite in Longford clay-mineral assemblages may depend on upward increases in the abundance of siltstone in the member. Post-depositional alteration of smectite or illite to kaolinite in the more permeable silty beds may be the chief factor (Glass, 1958; Glass and others, 1956; Potter and Glass, 1958; Wilson and Pittman, 1977), but the data are inconclusive. Alternatively, the estuarine environments in which much of the silt was deposited may have favored the formation of relatively coarse kaolinite settling aggregates, thus leading to differential transport of smectite and illite to more saline realms (Edzwald and O'Melia, 1975).
Lateral variations in the relative amounts of illite, smectite, and kaolinite in Longford and facies-equivalent Kiowa rocks generally correspond to the observations summarized by Parham (1966). Illite tends to be most abundant in sediments deposited in offshore realms, whereas kaolinite is most abundant in sediments deposited in nonmarine realms. Smectite (montmorillonite) may show distributions similar to those of illite, or it may be most abundant in nearshore or nonmarine sediments. The lateral variations in clay-mineral assemblages in Longford and Kiowa rocks can be interpreted as reflecting different source areas for much of the illite and smectite in Kiowa shale and much of the kaolinite and smectite in Longford rocks (Weaver, 1968a, 1968b), or as a response to differential transport and sedimentation of the clay minerals in fluvial, estuarine, lagoon, and marine environments (Edzwald and O'Melia, 1975). Definitive data are not at hand, but the small amount of illite in most Longford fluvial overbank deposits implies that differences in source areas may best account for the lateral variations. Floodplains and floodbasins would not be places where differential settling tendencies of differently sized clay aggregates could bring about effective sorting and separation of clay-mineral species.
The apparently scattered distribution of Kiowa shale and thick, lenticular deposits of Kiowa sandstone above the Longford Member affords insight into the mechanisms by which the Kiowa sea flooded onto the gentle topography developed on the western flank of the Nemaha anticline in north-central Kansas. Transgression must have been accompanied by inplace growth and eventual submergence of Kiowa barrier systems. Submergence resulted in step-wise, landward shifting of surf zones (Sanders and Kumar, 1975) without effective ravinement (shore-face and tidal-inlet erosion) of barrier-island, lagoon or bay, estuarine, and fluvial deposits (Fig. 16-C). The landward shift of surf zones resulted in construction of new barriers close to the former inner shores of lagoons or bays, and in renewal of the process. Submergence of Kiowa barrier systems and landward shifting of surf zones readily accounts for the sequences of open-sea Kiowa shale that overlie lagoonal shales and the capping Longford siltstone in many places, and for the presence of thick, lenticular deposits of Kiowa barrier sandstone above lagoon or bay and inner shore deposits in other places. It also accounts for the excellent preservation of thick sections of Longford alluvial and estuarine deposits along valleys eroded into Permian bedrock.
This study of Longford rocks indicates a number of projects that could add to the understanding of Cheyenne, Kiowa, and Dakota stratigraphy and sedimentation. The subsurface distribution of Longford rocks north and west of the outcrop belt is of especial interest, as would be more accurate determination of the facies relationships between Kiowa shale and sandstone and Longford rocks. Similarly, the stratigraphic relations between Longford rocks and strata that have been called Cheyenne Sandstone in the subsurface of western Kansas warrant careful analysis. Detailed petrologic and stratigraphic studies of thick, lenticular deposits of Kiowa sandstone should further understanding of the factors that controlled the development of Kiowa barrier systems. Conceivably, increased knowledge of the sandstone could lead to exploitation of similar sandstone in the subsurface of western Kansas and southern Nebraska as traps for oil and gas.
Detailed study of clay-mineral assemblages in Kiowa and Longford rocks might permit assessment of the extent to which soil-forming processes controlled Longford clay-mineral assemblages, and the extent to which differences in Longford and Kiowa assemblages reflect differential transport and sedimentation or differences in source materials. Detailed study of plant fossils in Longford and facies-equivalent Kiowa rocks might aid interpretations of Longford and Kiowa depositional environments, as well as contribute to the growing body of knowledge on angiosperm evolution.
Field and laboratory work for this report was sponsored mainly by the Kansas Geological Survey. Additional field and laboratory work was supported by The University of Akron (Research Grants 295 and 443). D. I. Good and Pei-lin Tien helped with much of the field and laboratory work. Valuable discussions in the field and in the office with Norman Plummer, O. S. Fent, R. W. Scott, and Ada SWineford contributed to the report. R. J. Bain, Jane Denne, O. S. Fent, R. W. Scott, and C. T. Siemers reviewed all or parts of the manuscript, and Siemers kindly examined and commented on some of the trace fossils. Gloria J. Price helped by typing some of the tabular information and descriptions. Especial credit is due my wife, Rhea, who patiently endured the aggravations and clatter of typewriter entailed in preparation of the manuscript.
Kansas Geological Survey, Geology
Placed on web March 10, 2009; originally published November, 1979.
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