The Moeraki Boulders; anatomy of some septarian concretions

Univ. Calif., Dep. Geol. Sci., Santa Barbara, CA, United States
Univ. Otago, Geol. Dep., New Zealand
Dep. Lands and Surv., New Zealand

The Moeraki boulders are large (to 2 m) calcite concretions with septarian veins of calcite and rare late-stage quartz and ferrous dolomite. The concretions are enclosed by Paleocene marine mudstones with vitrinite reflectance values of 0.29% R suggesting maximum burial temperatures of 25-30 degrees C. Microprobe analyses of these concretions indicate a Ca enrichment-Mg depletion trend with growth. Calcite in the core contains about 88 mole % Ca and about 93 mole % Ca in the rim. Both rim and cores contain less than 1 mole % Fe and Mn. Septarian veins also show a systematic calcium enrichment with growth of the vein. The initial vein fills are similar in composition to the outer rim of concretionary bodies, suggesting septarian cracks form contemporaneously with attainment of final concretion size. The later-stage calcite vein-fills have less than 1.5 mole % Mg and up to 11 mole % Fe+Mn. These elements systematically decrease both into and outward along the vein, with increasing calcium substitution. The above compositional trends in veins suggest continued crystallization during propagation of the fractures. The carbonate composition trends reflect interaction between the growing concretion and the enclosing mudstone pore-fluid system. The observed Fe, Mn, Mg depletion trends probably reflect depletion of elements released by short-time-scale diagenetic events of finite size. The growth time of the larger concretions is estimated at about 4 million years based on published diffusion growth models. Extrapolation of compositional trends versus growth time from these concretion bodies suggests that septarian veins form on a time scale of several million years.

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