About This Item
Share This Item
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.
End_of_Article - Last_Page 858------------