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

Abstract

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 formed in lacustrine 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 2AE5 m in 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 d 13 C ranging between 0& and )15& (Vienna Peedee Belemnite, V-PDB), and highly variable and highly depleted d 18 O (down to 14& Vienna Standard Mean Ocean Water, V-SMOW). The late generation of cement is most depleted in both 13 C and 18 O. The geochemical and isotopic patterns are best explained by interaction between surface oxic waters, pore waters and underground, 18 Odepleted, reducing, ice-meltwaters accumulated in the underlying coal-bearing sediments during the Permian deglaciation. The invariant d 13 C 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.

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

“…O compositions of early authigenic calcite are typically more depleted in 18 O than co-existing host carbonates (e.g., Sass et al, 1991;Mozley and Burns, 1993;Melezhik et al, 2007). The detailed mechanism remains unclear, but may be associated with either potential mixing of more…”

Carnivore coprolites from the lower Carnian (Upper Triassic) Chañares Formation, northwestern Argentina

“…Although they display individual variation, modern amniote feces typically are quite consistent in shape and only vary in size well within a single order of magnitude (Chame, 2003). The structures also show a strong similarity in size and shape to early diagenetic carbonate concretions documented elsewhere in the geologic record (e.g., Melezhik et al, 2007;Francischini et al, 2016). Internally, at least some of the structures show faint concentric banding often found in sedimentary concretions.…”

Calcite‐cemented concretions in non‐marine sandstones: An integrated study of outcrop sedimentology, petrography and clumped isotopes

“…These examples mostly deal with marine sandstones. Knowledge remains relatively limited concerning the nature (shape, source of minerals and distribution patterns) of calcite concretions in freshwater sediments lacking internally‐redistributed detrital carbonate as a source of cement components (Tandoon & Narayan, 1981; Dutton et al ., 2002; Hall et al ., 2004; EI Khoriby, 2006; McBride & Milliken, 2006; Melezhik et al ., 2007; Zhu et al ., 2020).…”