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The Moeraki Boulders; anatomy of some septarian concretions

J. R. Boles, C. A. Landis and P. Dale
The Moeraki Boulders; anatomy of some septarian concretions
Journal of Sedimentary Petrology (May 1985) 55 (3): 398-406


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.

ISSN: 0022-4472
Serial Title: Journal of Sedimentary Petrology
Serial Volume: 55
Serial Issue: 3
Title: The Moeraki Boulders; anatomy of some septarian concretions
Affiliation: Univ. Calif., Dep. Geol. Sci., Santa Barbara, CA, United States
Pages: 398-406
Published: 198505
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 25
Accession Number: 1986-040158
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch maps
S47°30'00" - S34°30'00", E166°30'00" - E178°30'00"
Secondary Affiliation: Univ. Otago, Geol. Dep., NZL, New ZealandDep. Lands and Surv., NZL, New Zealand
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by Institute of Geological and Nuclear Sciences Limited (GNS Science), Lower Hutt, New Zealand
Update Code: 1986
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