Ca-Mg-Distribution in Carbonates from the Lower Keuper in NW-Germany

Abstract

55 samples of carbonate sediments from a vertical stratigraphic section through the Lower Keuper Formation (Triassic) in the Weser area (North Western Germany) were evaluated as to their Mg/Ca ratio, taking their genetic aspects under consideration. Subsequently, the dolomite/calcite ratio, the Ca/Mg ratio of the dolomites, and the Mg-content of the calcite are discussed.

Results:

• 1. The Ca/Mg ratio of the early diagenetically formed solid solution members between calcite and dolomite depends on the Ca/Mg ratio of the depositional environment, i.e. on its salinity.

• 2. In the fossiliferous carbonates investigated, this relationship is only reflected by the dolomite, the CaCO₃-excess of wich decreases a) within a locally developed carbonate bank, b) within the entire section with increasing salinity until it disappears completely.

• 3. Accordingly, stoichiometric dolomites are formed during early diagenesis only in a hypersaline environment.

• 4. The unstable CaCO₃-excess in the dolomite appears to be unaffected by secondary processes; consequently, it can be considered as metastable for the relationship under consideration.

• 5. Because of its instability, the Mg-content of the early diagenetically formed calcite does not show a dependence on the stratigraphy and facies.

• 6. Much of the calcite is secondary. Its Mg-content is influenced by the Mg-content of the pore-solutions and is an indicator for the degree of dedolomitization.

• 7. Dolomite and calcite often belong to different phases of formation, so that their quantitative ratio cannot be used to determine facies. The Ca/Mg-ratio in the dolomites may be used instead.