Chemical and mineralogic evidence indicates significant clay-mineral diagenesis in a shallow (237 ft) core of Atoka (Pennsylvania) sandstone from Crawford County, Arkansas. This core spanned a complex sequence of interbedded sandstone and shale that is at least partly marine.

Clay shales are dominated by illite with significant amounts of kaolinite and minor amounts of chlorite. Increasingly sandy shales contain progressively more chlorite and less kaolinite; illite remains the dominant clay-mineral component. The clay fractions (< 2µ) of sandstones are dominated by chlorite with lesser amounts of illite and no kaolinite.

The strong contrast between the clay-mineral contents of these closely interbedded sandstones and shales suggests a diagenetic change that occurred primarily in the sandstones because of their greater permeability. The chlorite of the sandstones most likely has been formed authigenically with concurrent destruction of kaolinite.

Elimination of kaolinite from sandstones by differential sedimentation seems unlikely because (1) it is doubtful that natural mechanical processes could produce the perfect separation of kaolinite found in these rocks, and (2) numerous studies of Recent and ancient sediments indicate that kaolinite commonly is concentrated in sandstones rather than eliminated from them.

Clay-mineralogical changes in the sandstones may be part of an overall pattern of diagenesis in an alkaline, reducing environment that also included the formation of siderite rhombohedra and significant solution of silica (as indicated by straight or embayed contacts of quartz grains).

Much of the discussion on the origin of clayey sediments in modern deposits has been devoted to the relative importance of provenance versus environmental

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alteration or segregation. With respect to ancient deposits, however, approaches confined to these factors are inadequate where diagenesis of clay minerals has occurred, as it apparently has in the Atoka sandstone and sandy shale.

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