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Clays from the Dakota

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The clays described in this report were sampled from the Dakota formation, which is the lowest stratigraphic unit of the Gulfian Series of the Cretaceous System in Kansas. The relation of this formation to the underlying Kiowa shale and the overlying Graneros shale is shown in the following stratigraphic sequence.

Cretaceous System
Gulfian Series
Graneros shale
Dakota formation
Janssen clay member
Terra Cotta clay member
Comanchean Series
Kiowa Shale

As shown above, the Dakota formation is subdivided into the Terra Cotta and Janssen clay members. The Terra Cotta member, which includes two-thirds to four-fifths of the total thickness of the Dakota formation, consists of massive clay, silt, and sandstone. The most conspicuous lithological feature of this member is wide-spread distribution of gray and red mottled clay. The Janssen member includes beds of lignite, gray to dark-gray clay, silt, and some shale and sandstone (Plummer and Romary, 1942; Moore and others,1951).

Location and Description of Deposits

The four clays described were chosen from a sample file of about 1,400 Dakota formation clays. These particular clays were selected for study because they represent a wide range of types of commercially valuable clays. Clay EL-60-6 and clay O-5-6 represent two types most commonly recognizable in the Dakota formation in Kansas. Although the locations sampled were miles apart, the beds are closely related stratigraphically. Both occur in the Janssen member of the Dakota formation, and generally the dark-colored, commonly fine-grained O-5-6 type occurs immediately above the coarser, silty, light-gray EL-60-6 type. Clays representing both of these types are used in the manufacture of face brick in brick plants at Concordia and Great Bend, Kansas.

Clay O-38-4 represents a less common but important type of clay found in deposits of irregular extent near the middle of the Terra Cotta member of the Dakota formation. The environment of deposition obviously was farther removed from marine than that of the other three clays studied. The Janssen clays EL-60-6 and O-5-6 are judged to have been deposited in a littoral environment and are immediately overlain by the marine Graneros shale. Clay EL-69-2 was deposited under similar conditions near sea level immediately after the final retreat of the sea in which the marine Kiowa shale was deposited.

Clay O-38-4 is a "clean" clay, lacking superficial impurities, and is the moderately fine-grained, smooth, plastic type commonly classified as ball clay. This clay was used in the manufacture of pottery at Miltonvale and Topeka.

Clay EL-69-2 represents a type common to the lower part of the Dakota formation, occurring in association with a persistent series of beds which probably should be given the same member status as the Terra Cotta and the Janssen. Some lignite or thin lignite beds and thin pure kaolin beds commonly are associated with the EL-69-2 type clay. It is slightly less smooth and plastic than clay O-38-4, and probably is not quite clean enough to be classified as a ball clay. Actually it represents a rather common type of fire clay. This clay was used at one time in the manufacture of insulating fire brick and glazed tile blocks by Ludowici-Celadon Company at Coffeyville, Kansas.

Clay EL-60-6

The clay identified in this report as EL-60-6 was sampled in the SE SW sec. 19, T. 15 S., R. 9 W., Ellsworth County. Where sampled the bed is 4.1 feet thick and consists of relatively gritty, light-gray silty clay that contains lignite particles. An irregular kaolin streak at the top has a maximum thickness of 0.2 foot. The bed is underlain by 0.8 foot of lignitic silt and immediately overlain by 10 feet of gray silty to plastic refractory clay which is in turn overlain by 3.5 feet of gray clayey silt. The top of the silt bed is only a few inches below the Dakota formation-Graneros shale contact.

Clay EL-60-6 is of a type commonly found in the Janssen member of the Dakota formation. The bed from which sample EL-60-6 was taken is recognizable locally along at least 1.5 miles of outcrop.

Clay EL-69-2

Clay EL-69-2 was sampled in the NW NW sec. 30, T. 15 S., R. 6 W., Ellsworth County. The bed sampled includes 10 feet of gray to light-gray clay with some yellow stain and contains lignite particles. The upper 0.5 foot of the bed is an impure kaolin. The bed is underlain by more than 10 feet of yellow sandstone and overlain by 6 feet of gray clay mottled with red, yellow, and lavender.

Although clays EL-60-6 and EL-69-2 are similar in appearance and both have a kaolin streak at the top, the EL-69-2 bed occurs in the lower part of the Terra Cotta member, a few feet above the Kiowa shale-Dakota formation contact, whereas the EL-60-6 bed occurs about 14 feet below the top of the formation.

Clay O-5-6

Clay sample O-5-6 was taken in the SW NE sec. 18, T. 11 S., R. 4 W., Ottawa County. This clay bed was exposed in three separate pits in which the thickness of the bed ranged from 2.8 to 5.7 feet. Channelings taken in all three pits were mixed for the test sample.

This clay is very dark gray to black, is unusually fine-grained and slick in appearance, and has a conchoidal fracture. In contrast to the majority of Dakota formation clays, which are moderately soft, clay O-5-6 is hard and rattles when handled. It contains numerous small slickensides, and in some places has the appearance of having been squeezed into irregular cavities in the overlying bed.

This clay represents one of the most persistent beds in the Dakota formation. The slick, highly plastic appearance is common, although this bed may be represented in some exposures by impure lignite or very silty lignitic clay.

Clay O-38-4

Where clay O-38-4 was sampled in the NW NE sec. 8, T. 9 S., R. 2 W., Ottawa County, the bed is 6.3 feet thick. The bed occurs as a long, narrow body extending at least 2 miles north-northwest from this location, and probably about the same distance to the south-southeast also.

The clay is light gray in color and is smooth and plastic. It contains some lignite particles and a small amount of limonite stain. The overlying clay is similar but is mottled with red, brown, and yellow. The underlying clay is definitely more silty and has a gritty texture.

This clay bed occurs about 100 feet below the top of the Dakota formation, probably about the middle of the Terra Cotta member.

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Kansas Geological Survey, Geology
Placed on web June 22, 2007; originally published Dec. 1954.
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