Abstract:

The Mississippian shales from Wardie (Scotland) were deposited in a large normally brackish-water basin. They host siderite concretions with δ13C values ranging from −3.5 to 12.1‰ which show that sulfate reduction and methanogenesis contributed to the production of bicarbonate. Fossils are found within each of the concretions, suggesting that the decay of these organic remains provided a catalyst for concretionary growth. The pore water was freshened, which is indicated by the very negative δ18O values (from −12.8 to −4.7‰). Many features indicate that the concretions are very early-diagenetic, precompactional precipitates.

The concretions are mostly homogeneous; only a few exhibit complex concentric zoning. The microstructure of the homogeneous concretions indicates a pervasive growth. However, their isotopic composition reveals a clear trend towards lighter δ13C and δ18O values from the center to the edge. It is proposed that the concretions grew due to methanogenesis in microenvironments around large organic remains, which acted as nuclei. We suggest that this process may be one of the reasons for the very common occurrence of cements as mineral concretions, not as disseminated precipitates along beds. These microenvironments developed entirely within the sulfate reduction zone, which resulted in the establishment of isotopic gradients in bicarbonate dissolved in the pore water across these microenvironments. These gradients were caused by fast consumption of sulfate by the intensive oxidation of organic remains. This resulted in the isotopic trends preserved in the homogeneous concretions. The formation of the zoned concretions proceeded generally concentrically, but detailed observations suggest that pervasive growth also contributed to their formation. Afterwards, all the concretions were deeply buried, probably within the oil window, where late-diagenetic phases crystallized (mainly kaolinite, quartz, and sulfides).

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