The Principle of Diagenetic Facies and Some of Its Implications

Abstract

Chemical reorganization occurs during diagenesis as a response of a rock to its physicochemical environment. The mineral assemblages produced will be dependent on the environment and the materials present. Physicochemical factors of importance in diagenesis appear to be environmental pH-cation concentration, environmental Eh, pressure, and temperature. Mineral assemblages which can be expected to appear in given rocks under given conditions can be inferred from data derived from inherent properties of minerals, such as abrasion pH, from laboratory studies on mineral stability fields and hydrothermal alteration, from theoretical studies on mineral stabilities, from hydrothermal deposits, and from diagenetic depth sequences. These data suggest the possibility of erecting a sequence of facies, analogous to metamorphic facies, starting at the surface and passing down into the low-grade metamorphic facies. Each diagenetic facies can be defined as including all rocks or sedimentary detritus which have, by a process of diagenesis, developed mineral assemblages that are the result of adjustment to a particular diagenetic environment. The concept of equilibrium cannot be universally applied because metastable minerals are characteristically present in some assemblages. Teodorovich (1954) has erected 13 "fades" (here termed "parfacies"), representative of halmyrolosis and early diagenesis, which constitute a low-rank facies of diagenesis. Satisfactory definition of higher-ranking facies is not yet possible, but facies characterized, with increasing rank, by heulandite-analcite, laumontite, prehnite-pumpellyite, and albite-epidote may be recognizable in rocks of suitable composition modified under alkaline conditions. Variations of assemblage with composition and pH are not satisfactorily known. However, kaolinite will become important as pH decreases.