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Spherical and columnar, septarian,18 O-depleted, calcite concretions from Middle-Upper Permian lacustrine siltstones in northern Mozambique : evidence for very early diagenesis and multiple fluids

By Victor A Melezhik, Anthony E. Fallick, Richard A. Smith and Dante M. Rosse

Abstract

Calcite septarian concretions from the Permian Beaufort Group in the Maniamba\ud Graben (NW Mozambique) allow controls on the composition and nature of\ud diagenetic fluids to be investigated.The concretions formedinlacustrine siltstones,\ud where they occur in spherical (1 to 70 cm in diameter) and columnar (up to 50 cm\ud long) forms within three closely spaced, discrete beds totalling 2Æ5 min thickness.\ud Cementation began at an early stage of diagenesis and entrapped non-compacted\ud burrows and calcified plant roots. The cylindrical concretions overgrew calcified\ud vertical plant roots, which experienced shrinkage cracking after entrapment. Two\ud generations of concretionary body cement and two generations of septarian crack\ud infill are distinguished. The early generation in both cases is a low-Mn, Mg-rich\ud calcite, whereas the later generation is a low-Mg, Mn-rich calcite. The change in\ud chemistry is broadly consistent with a time (burial)-related transition from oxic\ud to sub-oxic/anoxic conditions close to the sediment–water interface. Geochemical\ud features of all types of cement were controlled by the sulphate-poor environment\ud and by the absence of bacterial sulphate reduction. All types of cement present\ud have d13C ranging between 0&and )15&(Vienna Peedee Belemnite, V-PDB), and\ud highly variable and highly depleted d18O (down to 14& Vienna Standard Mean\ud Ocean Water, V-SMOW). The late generation of cement is most depleted in both\ud 13C and 18O. The geochemical and isotopic patterns are best explained by\ud interaction between surface oxic waters, pore waters and underground, 18Odepleted,\ud reducing, ice-meltwaters accumulated in the underlying coal-bearing\ud sediments during the Permian deglaciation. The invariant d13C distribution across\ud core-to-rim transects for each individual concretion is consistent with rapid\ud lithification and involvement of a limited range of carbon sources derived via\ud oxidation of buried plant material and from dissolved clastic carbonates. Syneresis\ud of the cement during an advanced stage of lithification at early diagenesis\ud is considered to be the cause of development of the septarian cracks. After\ud cracking, the concretions retained a small volume of porosity, allowing infiltration\ud of anoxic, Ba-bearing fluids, resulting in the formation of barite. The results\ud obtained contribute to a better understanding of diagenetic processes at the\ud shallow burial depths occurring in rift-bound, lacustrine depositional systems

Topics: Earth Sciences
Publisher: Wiley
Year: 2007
DOI identifier: 10.1111/j.1365-3091.2007.00886.x
OAI identifier: oai:nora.nerc.ac.uk:4209
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