Argillaceous rocks show major chemical and mineralogic trends as a function of geologic age. If the trends are compared by eras, they are of global significance. The ratios of the various metal oxides to Al₂O₃ plotted against geologic age form 3 distinct behavioral groups and oxide-Al₂O₃ ratios, except those of K₂O and FeO, diminish with increasing rock age.

The first group, water (-H₂O), CaO, and CO₂, decreases with rock age from high to very low values. The covariance of H₂O with CaO and CO₂ is consistent with a gentle water leach of shale and loss of original calcium carbonate through time. MgO, Na₂O, and SiO₂ form a second group, but change with geologic age much less than the first group. The chemical trends of these second group oxides, along with K₂O, reflect the differences in shale mineralogy as a function of age. The increase in illite percentage in older rocks results in a slight "relative" enrichment in K₂O whereas the abundance of expanded clays in younger shales gives rise to Mesozoic-Cenozoic shales of higher Na₂O, MgO, and Si ₂ content. This interpretation requires that reactions of the following type obtain within shales in the first few hundred million years after burial: (low silica) kaolinite + (high silica) expanded clay + potassium = (intermediate silica) illite + MgO, Na₂O, and SiO₂ (lost from shale).

The third group includes FeO and Fe₂O₃. There is a reciprocal relation between these oxides; young rocks are high in oxidized iron, old rocks low, but the total iron oxide concentration in shale is almost constant with geologic age. This trend partly reflects the postdepositional oxidation of organic matter and attendant reduction of iron in shales as they progressively age.

These time-dependent chemical and mineralogic trends are in harmony with the general concepts of differential sedimentary cycling and chemical uniformitarianism; concepts that predict approximately the kinds of long-term, postdepositional, selective changes expected in argillaceous rocks as a function of geologic age, and that help to discriminate between primary and secondary chemical and mineralogic features.

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