2 research outputs found
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