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Refractory-clay deposits are widespread in rocks of the lower part of the Pennsylvanian System in much of the Appalachian and midcontinent regions. Characteristically, they contain kaolin minerals having, in flint clays, a well-ordered form and, in the more plastic clays, the baxis disordered form. Generally the deposits are irregular in thickness, ranging from a film to more than 100 feet. Where they are more than a few feet thick, the deposits generally fill sinkholes or other topographic depressions in pre-Pennsylvanian carbonate rocks. The refractory clays are multiple-cycle sedimentary deposits. A long period of subaerial exposure and erosion in Late Mississippian and Early Pennsylvanian time produced a considerable insoluble residue from pre-Pennsylvanian carbonate rocks. Clays from this residue accumulated and were reworked in freshwater or brackish-water swampy areas under humid to tropical conditions. The chelating effects of organic acids during diagenesis resulted in desilication of the reworked clay minerals with accompanying concentration of Al+3 ; leaching depleted the clays of the fluxing ions Na+1 , Ca+2 , Mg+2 , Fe +2 , and K +1 . On the Ozark dome in Missouri the clay deposits are zoned. Highalumina clays occur near the crest of the dome, and flint, semiflint, and semiplastic varieties are at successively lower structural positions on the northwest side of the dome.

u.s. geological survey professional papers/u.s. geological survey professional paper

Paleotectonic investigations of the Pennsylvanian system in the United States, Part II: Interpretive summary and special features of the Pennsylvanian system