Geochemical Facies

the combination of those physicochemical conditions of the environment that determine the nature of sedimentation and the diagenesis of sediments. The geochemical facies is characterized by narrow fluctuations in hydrogen ion concentration (pH), oxidation-reduction potential (Eh), temperature, mineralization and salt composition of waters, and the concentration of organic substances in sediments. It is accompanied by typical associations of authigenic minerals. The parameters characteristic of a given geochemical facies can be measured directly in contemporary marine and intracontinental bodies of waters and can be reconstructed only with a certain known approximation for ancient basins. The quantitative ratios of authigenic minerals of multivalent elements (Fe, Mn, U, S, and so forth) usually serve as a basis for this, these minerals having definite fields of stability within the Eh-pH system.

Two major groups of geochemical facies can be differentiated: continental and marine. The former are characterized by the primary development of oxidizing conditions (excess free oxygen). The latter are characterized by a wide range of conditions, from markedly reducing ones, usually developed in sediments enriched with organic substances (sulfide geochemical facies), through neutral (leptochlorite geochemical facies), to markedly oxidizing ones (iron oxides and hydroxides).

Geochemical facies change with time in the direction of a greater degree of reduction under conditions of the stable accumulation of sediment at the bottom of subsiding basins or else toward their greater degree of oxidation, when the sea bottom rises. The concept of geochemical facies was first introduced by the Soviet lithologist L. V. Pustovalov (1933).