ABSTRACT

Isotopic analysis of dolomite concretions from the Miocene Monterey Formation of California yields a covariance of ¹⁸O and ¹⁸C within every concretion analyzed. The total range for all concretions in ¹⁸O (SMOW) is +27.3 to +33.6. The total range in ¹³C (PDB) is - 10.2 to + 10.8. Variations of up to 3.6 in oxygen and 12.2 in carbon are encountered within a single concretion. Decreasing ¹⁸O is interpreted in terms of increasing temperatures of formation as the concretion grows during burial. The Claypool and Kaplan (1974) model can be successfully used to interpret carbon isotope variations as the result of progressive concretion growth in the zones of microbial sulfate reduction, microbial methane production, and thermocatalytic decarboxylation.

Monterey dolomite concretions are too rich in carbonate to have originated by simple cementation of initially porous sediment. Much of the originally diatomaceous sediment appears to have been replaced by dolomite. Isotopic zonation within one concretion is found to be complex, suggesting nucleation at several sites which then grew together. Complex growth patterns may result from local variations in lithology and porosity during precipitation.

Multiple sampling of several concretions at a single outcrop yields a more complete early diagenetic history than by sampling only one concretion. Isotopic diagenetic profiles inferred from the dolomite concretions are similar to those seen in Deep Sea Drilling Project cores where deep-sea dolomitization is occurring today. Single samples from a concretion cannot reveal the range or the trend of the diagenetic reactions that have occurred. Multiple sampling of diagenetic carbonate concretions is clearly essential in determining the early diagenetic history from isotopic measurements.