ABSTRACT

Closely interlaminated sandstones and shales from Desmoinesian deltaic strata in Oklahoma were investigated to determine the relative importance of diagenetic and detrital origin of constituent clay minerals. Close association of the two lithologic types eliminates many parameters that could have varying effects upon mineral content. To eliminate effects of modern weathering, subsurface samples were studied. Investigative techniques included x-ray diffraction, x-ray fluorescence, and thin-section observation.

Quantitatively, authigenic mineral formation was not significant, and bulk chemical analyses showed little difference between sands and shales except those related to expected compositional differences which are a function of size fractionation. Permeability was the basic control of the degree of diagenesis.

Consistent differences in relative amounts of chlorite, illite, and kaolinite were noted between sands and shales. Illite and chlorite are of detrital origin, whereas a significant portion of the kaolinite was formed diagenetically. Illite, the dominant clay mineral in all shales and many sands investigated, is relatively more abundant in shales and in finer sand fractions. Chlorite distribution varies because it is present both as clay and in metamorphic rock fragments and thus is not directly related to grain size. Kaolinite is an important constituent in sands, an occurrence attributed to authigenic formation in the more permeable sands.

Authigenic kaolinite formation was possible because four factors coexisted: (1) organic material helped maintain a low pH; (2) potassium feldspars provided a source of aluminum and silicon; (3) partly degraded illite was an efficient acceptor for potassium released during feldspar decomposition; and (4) water-filled pores provided both a medium and space for kaolinite growth. Authigenic kaolinite formation did not require long-distance transport of necessary ions; all constituents were present in situ.

During diagenesis additional minerals were formed, especially in the sands. The paragenesis was quartz overgrowths, kaolinite, calcite, dolomite and finally, pyrite and siderite.