Isotope and elemental geochemistry of black shale-hosted fossiliferous concretions from the Cretaceous Santana Formation fossil Lagerstätte (Brazil)

© 2016 The Authors. Sedimentology © 2016 International Association of SedimentologistsCarbonate concretions hosted within organic carbon-rich shale sequences represent unique archives of often exceptionally preserved fossil biota. Besides providing high-fidelity preservation, their geochemical signatures can provide insight into the physical and chemical processes during early and later-stage concretion growth. Here, two fossiliferous carbonate concretions of the late Early Cretaceous Santana Formation (Araripe Basin, north-east Brazil) are analysed with an integrative geochemical approach including µ-XRF scanning, d13C, d18O, 87Sr/86Sr and ?47 (clumped isotope thermometry). Individual concretions show a concentric internal zonation with the outermost layer being composed of millimetre thick cone-in-cone calcite. A strong covariance of d13C and d18O values of the fine-crystalline concretion body indicates mixing of two different carbonate phases and supports a scenario of temporally separated pervasive growth stages. Microbially-mediated formation of an early porous calcite framework was controlled by the combined processes of fermentation and methanogenesis around the decaying carcass, forming localized environments within a zone of sulphate reduction. Microbial sulphate reduction is indicated by the concentric enrichment of pyrite in the outer part of the concretion body and by high pyrite abundance in the surrounding shale. Information on the later-stage diagenetic processes affecting the Santana concretions can be derived from the outermost fringing cone-in-cone calcite. The carbonate precipitating fluid was characterized by a more or less marine d18O composition (calculated d18Oporewater = -1·0 to -1·8‰) and by radiogenic Sr-isotope ratios (up to 0·713331 ± 7·0*10-6), the latter probably reflecting modification due to interaction with the surrounding shale or, alternatively, with underlying evaporitic sulphate deposits influenced by strong continental inflow or with crystalline basement rocks. The ?47-derived temperature estimates range between 37°C and 42°C ± 5, indicating precipitation of the cone-in-cone calcite at a depth of 650 to 850 m, which is only half as much as the maximum burial depth derived from existing fission-track data. Overall, the study of fossiliferous carbonate concretions in organic carbon-rich sedimentary sequences can reveal a complex growth history spanning incipient microbially-influenced precipitates as well as later-stage burial diagenetic phases

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

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