Stages of Diagenesis in the Development of Sandstones
On the basis of mineral replacements and intergrowths, four oxide series are recognized among sandstones: (1) alumina, lime-magnesia, iron oxide, (2) silica, lime-magnesia, iron oxide, (3) silica, alumina, iron oxide, (4) silica, alumina, lime-magnesia. Typical reactions are: a, for series (1), hematite + calcite + Mg � siderite + ferro-dolomite + dolomite; b, for series (2), hematite + clay minerals + silica � chlorite + greenalite; c, for series (3), diaspore + silica � clay minerals, and diaspore + silica + K � glauconite + clay minerals; also, during late burial illite + Na + K + Ca → muscovite + biotite + feldspar; d, for series (4), illite or glauconite → chlorite + muscovite; and calcite + Mg + illite → micas + feldspar + dolomite; also, with deep and late burial, clay minerals → chlorite or sericite, K feldspar → sericite, plagioclase → chlorite + chert. From the type of reaction and the principal sequence of occurrence three stages of diagenesis are established: Stage (1), redoxomorphic, which dominates the episodes of deposition and early burial. Reactions principally are reduction and oxidation which form the basis for the name of the stage. Stage (2), identified as locomorphic, involves replacement of one mineral by another, the two minerals not entering into mutual reaction. Such reactions are typical of the early burial stage and are an important part of the process of lithification. Stage (3), identified as phyllomorphic, follows the principal locomorphic replacements and involves development of micas, principally from clays. Among sandstones, the progress of diagenesis may be arrested at either the locomorphic or phyllomorphic stages, in more or less parallel fashion to metamorphic grades. Also, the phyllomorphic stage overlaps the zeolite and chlorite grades of metamorphism.