ABSTRACT

Calcite cement is one of the principal destroyers of porosity in limestones. Studying the environment of calcite cementation, therefore, might make it possible to predict trends toward open porosity in carbonates.

Certain calcite cements contain traces of ferrous iron, detectable by artificial staining. In carbonate provinces ferrous iron is stable in solution only under reducing conditions; it will, therefore, not enter cements precipitated in the oxidizing zone above the water table. Once incorporated in a calcite lattice, however, ferrous iron can normally survive subsequent passages through oxidizing media. The ferroan character of a cement is thus good and lasting evidence for its precipitation under reducing conditions, principally found below the water table. In this latter environment, however, reducing conditions are only established in the presence of organic matter, the absence of which could result in local oxidizing conditions and thus in the formation of ferrous-iron-free cements. The latter are therefore not necessarily indicative of precipitation above the water table.

Certain carbonate cements exhibit variations in ferrous iron content. These variations give rise to a number of zones which are crystallographically disposed. They are thus considered to represent diagenetic time horizons, which can be correlated either laterally or vertically through rock sequences. Correlation and the use of other stratigraphic principles in the study of cements may expose trends toward the gradual disappearance of zones of cement and thus toward open porosity in limestones.