Origin and Classification of Chemical Sediments in Terms of pH and Oxidation-Reduction Potentials

Chemical sediments of marine origin are divided into three main classes, representing deposition in normal marine open-circulation environments, restricted humid (euxinic) environments, and restricted arid (evaporite) environments. The characteristics of these environments are briefly reviewed, and the position is taken that the hydrogen-ion concentration, pH, and the oxidation-reduction potential, Eh, afford two basic controls which largely determine the kinds of chemical end-members produced by both inorganic and biochemical reactions. Within the framework of these controls, the depositions of calcium carbonate, iron minerals, manganese minerals, phosphates, evaporites, and organic matter are shown in their relation to variations in pH and Eh of the environment. Certain of the end-members depend mainly upon one or the other of the two controls, and some depend upon both. A classification of chemical sediments is proposed which shows their relation to pH and Eh and serves to indicate genetic relations among the chemical end-members. Occurrences of chemical end-members are summarized on a pH-Eh graph, which shows the range of these values which normally occurs in the environments discussed. Observed mineral associations in typical chemical sediments are listed in support of the theoretical treatment. The influence of postdepositional changes on the original mineral associations is also pointed out. The writers conclude that the environment of deposition of many ancient chemical sediments can be reconstructed in terms of its essential physicochemical characteristics from study of the mineral assemblages among the chemical end-members present in the sediment.