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Kansas Geological Survey, Bulletin 41, Part 9, originally published in 1942

Environment of Sedimentation

The advance of the Cretaceous seas over the eroded surface of the pre-Cretaceous rocks of the Kansas region ended a long depositional hiatus. Where the base of the Cretaceous beds is exposed in Kansas, the system lies unconformably on an eroded surface of Permian rocks that has little relief. Triassic, Jurassic, and a considerable thickness of Permian rocks were either never deposited in this area or were removed by erosion prior to the beginning of Cretaceous deposition. In our opinion, the Cretaceous beds studied in the course of this investigation were deposited under varying conditions of a nearly continuously rising sea level or subsiding basement, the variable factor being the rate of deposition.

The lowermost Cretaceous formation in Kansas, the Cheyenne sandstone, is largely composed of sand that is inferred to have been deposited near or on the strand line of the advancing sea. The sediments are dominantly continental in character and locally they contain plant remains in abundance.

The Kiowa shale was deposited after the advancing sea had covered the land surface. A basal unit of the Kiowa at many outcrops is the Champion Shell Bed, which is composed mainly of the hard parts of marine clams and which thus furnishes obvious record of the presence of a sea in Kansas. The clayey and silty sediments of the Kiowa formation are also largely marine in origin. Occasional changes in the character of the sediments carried to the Cretaceous sea from the not-far-distant land resulted in the deposition of sand and silt, producing, on the whole, thin-bedded, fine-grained sandstone. The dark fissile shale of the Kiowa was probably deposited as a black mud containing some organic material. The very fine particle size of most of the clay minerals and the paper-thin lamination of the shale indicate deposition in relatively quiet water, which we infer to have been highly saline, inasmuch as gypsum occurs very abundantly in the form of selenite crystals.

Reducing conditions, rather than an oxidizing environment, seemingly prevailed during the time of deposition of shale, silt, and sand of the Kiowa formation. This is indicated by the prevalence of pyrite in both the fine and coarse sediments, and also by the presence of glauconite in the sands.

Illite is the dominant clay mineral in the dark-colored Kiowa shale, but the shale probably contains appreciable amounts of montmorillonite, also. It is reasonable to assume that the Kiowa shale was at no time subject to prolonged weathering and leaching near the surface. Such conditions have presumably been responsible for alteration of clay minerals to kaolinite in the Pennsylvanian underclays and in Dakota clays immediately overlying the Kiowa shale.

A large percentage of the iron is tied up as a replacement of the alumina in the molecules of the montmorillonite and illite clay minerals of the Kiowa shale. The most plentiful accessory minerals are pyrite or marcasite and selenite. The iron sulphide occurs in sizes ranging from microscopic crystals to fairly large concretionary nodules, and it forms pyritized marine fossils several centimeters in diameter. The selenite crystals range in size from tiny needles to euhedral crystals 10 centimeters in length.

In the ceramic tests, the Kiowa shales are characterized by a high percentage of water of plasticity, which is directly related to their ability to absorb water. The drying and firing shrinkage is high, and the fusion temperature comparatively low.

Although the sediments of the Dakota formation are non-marine in character, they were apparently deposited near sea level. Despite the fact that the analogy is not complete, it has seemed to us helpful to visualize the conditions of Dakota in Kansas sedimentation as similar to those of the lower Mississippi "delta country." The maximum elevation of land along the lower Mississippi is less than 5 feet above sea level. Areas totaling hundreds of square miles can be classified about equally well as land or water. After prolonged rains, most of the area is covered by a few inches of water. The streams, where such exist as more than a barely perceptible flow of water through swamp grass, lagoon, and bayou, are braided and of slight competency. The Mississippi river itself scarcely affects this picture, except that, in floods, it may leave its channel and break over the natural or artificial levees.

Possibly there was less vegetation in central Kansas during the time of Dakota deposition than in the Mississippi delta area at the present time, but most of the lower Mississippi country has little vegetation except grass and other low-growing plants. Well-preserved imprints of the leaves of dicotyledons are fairly plentiful throughout the Dakota formation; but, with the exception of the small root or stem cavities found in the siltstone at the top of the Janssen member, evidence of the presence of the trunks, roots, or stems of these plants is seldom found, and, in the rare cases when found, the trunks and stems were grouped as one would expect them to be if they had been deposited in stream drift. This is substantiated by the fact that such remains are found only in the coarser channel sandstones. It is possible that the leaves represent wind- and water-home debris carried a considerable distance; although the excellent state of preservation of the fossil imprints contradicts this suggestion.

Indications of vertebrate life have been found in the Dakota formation. Their rarity is indicated by the fact that the excellent opportunities given us to examine large quantities of freshly excavated Dakota clay and sand led to no discoveries of vertebrate remains. The meagerness of animal remains in the Dakota beds indicates that the environment during deposition was either not favorable to existence of vertebrates or that conditions of preservation of Dakota organisms have been poor. If it is true that vegetation was sparse during Dakota time, conditions would have been unfavorable for the existence of organisms that depended, either directly or indirectly, upon plants for food; and the low-lying, swampy land and shallow water which frequently shifted in pattern and distribution would have provided a poor environment for both the existence and the preservation of life. Tester's interpretation of the Dakota rocks (designated by him as the Dakota stage) of the type area is summarized as follows:

"…the Dakota stage represents the sediments laid down in an environment at and near the shore line of a broad, relatively shallow ocean. The material was derived from the north and northeast and was carried southward by streams of small competency which may have had wide flood plains and which when reaching the coast line built side and ramifying deltas. At times these deltas extended themselves seaward with a development of barlike fingers of relatively coarse material, with the minor distributaries of the streams throwing off the finer silt and clays which were deposited on the flank of the principal distributary" (1931, p. 280).

The coarser sandstone deposits of the Dakota formation are commonly typical channel fillings which interfinger with clay and shale at their margins. Presumably, the interfingering was produced by shifts of the stream that deposited the sand. From time to time many of the Dakota streams seem to have been shifted to entirely new locations, adding further complications to a far-from-simple depositional record. Relatively thin sandstone of considerable horizontal extent, which occurs in some places, may be interpreted as deposits formed by streams migrating laterally so as to deposit a more or less continuous sheet of relatively flat-lying sediments.

Commonly, an orderly sequence of clay beds can be correlated from one side of a channel sandstone to another, which implies that the conditions under which the sandstone was deposited did not interfere with the orderly deposition of clay.

Other sandstone deposits that are broadly arched at the top have been found. Beds of clay and evenly bedded thin sandstone adjoining or surrounding the arched-top sandstone can be correlated definitely. For example, in Washington County we observed a horizontally bedded, 4-inch sandstone which lies parallel to the clay beds and touches the top of an arch of massive sandstone. Such arched-top sandstone could be deposited by a distributary stream, dropping sand on clay. A later shift of the distributary would permit the orderly deposition of clay around and over the sand. The axis of the channel sandstone in Washington County is inferred to be parallel with the direction from which the sediments were brought in.

It is our opinion that the near sea-level condition of Dakota sedimentation existed over a wide area. Also, we conclude that this condition persisted throughout Dakota time owing to the fact that deposition kept pace with subsidence of the underlying rocks or the rising sea level (Lee, 1927, p. 25). The type of continental deposition prevalent during Dakota time ended, and the deposition of the marine Graneros shales began when the rate of subsidence became greater than the rate of deposition and the sea advanced over the watery land. It is evident that conditions were somewhat more stable near the close of Dakota time, when the sediments comprising the Janssen clay member were deposited. It seems probable that both deposition and subsistence proceeded at a slower rate during this time, and that better conditions for the growth of plants resulted. The increased amount of lignitized wood particles and a greater variety of plant remains, near the top of the formation, point in this direction. The bed of lignite or black lignitic clay that occurs almost invariably in the upper few feet of the Janssen clay member is interpreted to indicate the existence of swampy conditions for a considerable period of time. Plant debris accumulated, and extensive peat bogs resulted. The gray clay or clayey silt that underlie this bed strongly confirm this conclusion inasmuch as the clay minerals are even more characteristically kaolinitic than those found lower in the formation. They exhibit a marked conchoidal fracture (pl. 2B), and in many instances the limonite- or siderite-filled molds of roots are quite evident. It is probable that these clays are lithologically nearly identical to the fireclays which are mined from the upper Dakota west of Denver. They also closely resemble, both in the sequence of beds above them and in mineralogical and ceramic characteristics, the ball clays and plastic fireclays of the Ripley formation "of the Upper Cretaceous section in Western Tennessee (Whitlatch, 1940, p. 49-50). The conditions of sedimentation of these clays were probably analogous to those that produced the underclays of the Pennsylvania rocks. The alteration from other clay minerals to kaolinite takes place very slowly. In general, therefore, clays which are the most dominantly kaolinitic may be considered as having been subjected to leaching and weathering conditions favorable to the alteration to kaolinite over a much longer period of time than was the case with the illite or montmorillonite shales.

Immediately prior to the deposition of the Graneros sediments, the surface of the swamp and the vegetation growing on it now represented by the lignite beds, was covered with silt and silty clay. This was a relatively thin deposit of material and is represented by the upper siltstone which is characterized by molds of plant stems and roots or worm borings. The fact that worm-like imprints are found at many places on top of the siltstone substantiates the hypothesis that the borings were made by worms. Locally, the silt was not deposited, or it has been removed. At other places the lignite beds were eroded and mixed with clay and silt.

According to the interpretation of origin of the Dakota formation given by us, the swamp water that covered most of the surface in Kansas, near the close of Dakota time, may have contained enough organic acids to hold a considerable amount of iron in solution. As the sea, which eventually reached north-central Kansas in Graneros time, began to advance over the land, the water became brackish and increasingly saline, causing precipitation of much of the swamp water iron. This hypothesis explains the deposition of the thin bed of limonite which is commonly present at the base of the Graneros shale formation.

In many places the Dakota-Graneros contact cannot be exactly determined. The change from nonmarine to marine conditions was probably not abrupt. The gradual change was probably marked by oscillatory covering of the land by very shallow sea water. This is indicated by the common occurrence of alternating dark shale, silty clay, yellow sandstone, concretionary iron, and much gypsum in the form of selenite crystals in a zone sometimes as much as 5 feet thick near the contact. We commonly include this gradational bed with the Graneros shale because it occurs above the limonitic bed, which, according to our interpretation, was formed by the precipitation of iron caused by the brackish water of the advancing Graneros sea.

The Graneros shale closely resembles the Kiowa shale both in superficial appearance and in ceramic and chemical properties. The thin beds of bentonite, which are common in the Graneros, however, have not been observed in the Kiowa shale. The Graneros shale is essentially a marine deposit consisting of dark shale and some thick sandstone. Neither the Graneros shale nor the Kiowa shale was observed as carefully during our field work as the Dakota formation, inasmuch as we discovered early in the investigation that the Graneros and Kiowa shales were of slight value as ceramic raw materials. We continued, however, to take samples from both of the marine shales to aid in correlations. As previously stated in this paper, both the Graneros shale and the Kiowa shale samples always fired to a brownish color and showed excessive shrinkage both in drying and firing. Clays of the Dakota formation having an iron content equal to that of Graneros and Kiowa samples were found to shrink only moderately on drying, and very little on firing. Also, the fired colors of ferruginous Dakota clays are clear red, pink, or buff, not brown as are samples of the Graneros and Kiowa shale.

The mineralogical distinction between the Dakota clay and the marine shale, above and below, can be summarized by the generalization that continental Dakota clay is characteristically kaolinitic, whereas marine shale is composed dominantly of illite. According to Grim (1942, p. 261), illite is the dominant clay mineral in most of the shales he has studied, and there is some reason to believe that shaly structure is related to the presence of illite. The clays of all the mineral groups except that of kaolinite are not stable, owing partly to their greater capacity for base exchange; therefore, these clays respond readily to changing conditions. The kaolinite group of minerals, on the other hand, is stable. It is safe to assume that the kaolinite minerals of the Dakota formation were deposited in that form or became altered to kaolin shortly after deposition. It is not likely that the source of the materials deposited as Kiowa shale, Dakota clay, and Graneros shale changed as abruptly as the sediments change in character. If this is true, the conditions of sedimentation during deposition of the Dakota clay were necessarily favorable to the formation of kaolinite. In general, it is believed that prolonged leaching by slightly acidic soil water is favorable to the formation of kaolinite (Grim, 1939, p. 478-487). The process of leaching would have been promoted both by the reworking of the clays and by the continuous movement of water near the surface of the sediments. In general, the clays we have sampled tend to be slightly acidic. Although this fact does not necessarily mean that the acid condition prevailed at the time of deposition, it is favorable to the theory. Furthermore, the presence of organic material and of small amounts of sulphuric acid in the clays indicate the processes of decay of vegetable matter in the presence of abundant water in swamps, shallow lakes, or in a similar environment where organic acids are readily formed.

The hydrogen sulphide released by processes of decay would produce some sulphuric acid in the presence of water, and this acid could account for the occasional selenite crystals which are found in the Dakota clay into which ground water has penetrated. Calcium bi-carbonate in the ground water would be converted to calcium sulphate on contact with sulphuric acid.


Cragin, F. W., 1889, Contributions to the paleontology of the plains: Washburn Coll. Lab. Nat. Hist., vol. 2, no. 10, p. 65-68.

Cragin, F. W., 1890, On the Cheyenne sandstone and Neocomian shales of Kansas: Bull. Washburn Coll. Lab. Nat. Hist., vol. 2, no. 11, p. 69-81.

Cragin, F. W., 1891, Further notes on the Cheyenne sandstone and Neocomian shales of Kansas: Am. Geol., vol. 7, p. 179-181.

Cragin, F. W., 1895, The Mentor beds, a central Kansas terrane of the Comanche series: Am. Geol., vol. 16, p. 162-165.

Cragin, F. W., 1895, A study of the Belvidere beds: Am. Geol., vol. 16, p. 357-386.

Gould, C. N., 1900, The lower Cretaceous of Kansas: Am. Geol., vol. 25, p. 10-40.

Grim, R. W., 1939, Properties of clay: Illinois Geol. Survey Circular, no. 49, p. 466-490.

Grim, R. W., 1942, Modern concepts of clay materials: Jour. of Geol., vol. 50, no. 3, p. 225-275, figs. 1-12, tables 1-2.

Hill, R. T., 1895, On outlying areas of the Comanche series in Kansas, Oklahoma, and New Mexico: Am. Jour. Sci., 3rd ser., vol. 50, p. 205-234.

Latta, B. F., 1941, Geology and ground-water resources of Stanton County, Kansas: Kansas Geol. Survey, Bull., 37, p. 7-114, figs. 1-6, pls. 1-9.

Lee, W. T., 1927, Correlation of geologic formations between east-central Colorado, central Wyoming and southern Montana: U.S. Geol. Survey, Prof. Paper 149, p. 1-80, figs. 1-5, pls. 1-35.

Prosser, C. S., 1897, Comanche series of Kansas: Kansas Geol. Survey, vol. 2, p. 96-181; The Dakota sandstone, p. 182-194.

Rubey, W. W., and Bass, N. W., 1925, The geology of Russell County, Kansas, with special reference to oil and gas resources, Kansas Geol. Survey Bull. 10, p. 16, 57-65. [available online]

Tester, A. C., 1931, The Dakota stage of the type locality: Iowa Geol. Survey, vol. 35, p. 199-332, figs. 25-44, pls. 3-4.

Twenhofel, W. H., 1920, The Comanchean and Dakota strata of Kansas: Am. Jour. Sci., 4th ser., vol. 49, p. 281-297.

Twenhofel, W. H., 1924, Geology and invertebrate paleontology of the Comanchean and "Dakota" formations of Kansas: Kansas Geol. Survey Bull. 9, p. 1-135, pls. 1-23 (including maps).

Whitlatch, G. I., 1940, The clays of West Tennessee: Tennessee Div. of Geol. Bull. 49, p. 1-328, figs. 1-38, pls. 1-10, tables 1-38.

Wing, M. E., 1930, The geology of Cloud and Republic counties, Kansas: Kansas Geol. Survey Bull. 15, p. 1-49, figs. 1-2, pls. 1-18.


Kansas Geological Survey, Geology
Placed on web September 2005; originally published November 30, 1942.
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