Petrology of the Nodaway Underclay (Pennsylvanian), Kansas

Abstract

The underclay of the Nodaway coal of the Wabaunsee group (Pennsylvanian), in Kansas has been studied with the object of determining its origin. Samples were collected from the underclay and some of the shales next below at 39 localities at approximately 5-mile intervals across Kansas from the Oklahoma border to the Nebraska border. In northeastern Kansas in the position of the Forest City basin the underclay is ash gray and plastic, but in southern Kansas on the Bourbon arch it is yellowish gray, not as plastic, and generally thinner.

Data obtained from mechanical, chemical, and x-ray analyses, and microscopic study of the very fine sand show that the underclay has a different mineralogical assemblage on the Bourbon arch than in the Forest City basin. Furthermore, in most places, the shale is markedly different from the overlying clay.

In the Forest City basin the Nodaway underclay contains illite, kaolinite, and mixed-layer illite-montmorillonite whereas the underlying shale lacks mixed-layer illite-montmorillonite, but has chlorite or chlorite-like clay. The poorly developed underclay on the Bourbon arch more closely resembles the underlying shale. Mechanical analyses show that the underclay in the Forest City basin is finer grained than the shale, and the reverse is true on the Bourbon arch.

Potassium is most concentrated in the topmost portion of the underclay, at the seven localities from which samples were chemically analyzed. Furthermore, where the coal is thick the amount of potassium is greater than under thin coal.

A comparison was made between Nodaway underclay and modern gley (bleached layer under peat), which develops beneath allochthonous peat or autochthonous peat. It is postulated that underclay is "fossil" gley. The characteristics of gley and underclay are due to action of organic compounds that have moved downward from the peat by diffusion. Biotite has been altered to chlorite in the poorly developed underclay, but the "gleying" process has altered chlorite to mixed-layer illite-montmorillonite in the well-developed underclay.