ABSTRACT

This study concerns the origin and composition of clay minerals in the lower part of the Atoka Formation (Pennsylvanian) of Crawford County, Arkansas. Samples studied are from a 237' core which penetrated a complex interval of interbedded sandstones and shales.

Chemical and mineralogical variations in these rocks are primarily related to the post-depositional formation of iron-rich chlorite and destruction of kaolinite. Such effects increase with sandiness of the rocks and are thought to be related to the preferential circulation of formation waters through these relatively porous and permeable sandy sediments.

Associated diagenetic phenomena include the formation of authigenic siderite rhombohedra and silica cementation; the latter being related to proximity to shale. The overall pattern of diagenesis is compatible with reducing alkaline conditions.

Constancy of inter-element ratios suggests that chemical and mineralogical changes have occurred without gain or loss of a number of elements (Al, B, Ti, Ga, K, Rb) on a whole-rock basis. The most important chemical variation is a reciprocal relationship between iron and aluminum.

Detrital interpretations of clay minerals should he made with caution; possible diagenetic effects must also he considered. Clay minerals in sandstone are particularly likely to be of diagenic origin.