Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi, South China

Abstract

The formation of carbonate concretions is a cementation process which passively infills the pore spaces within sediments. They record the original environments of deposition and diagenetic conditions of the host rocks. Little is known about the precise mechanisms responsible for the precipitation of carbonate concretions. The most common host rocks are mudstones/shales, sandstones, and limestones. This study presents an example of large carbonate concretions from an unusual host rock, the black bedded cherts of the Gufeng Formation (Guadalupian) at Enshi on the northern Yangtze Platform, South China. Petrographic observations (X‐ray diffraction, optical microscopy, scanning electron microscopy) and multiple geochemical analyses (pyrite‐ and carbonate‐associated‐sulfate (CAS)‐sulfur isotopes, carbon isotopes) indicate that (a) the studied carbonate concretion are mainly composed of micritic calcite with subordinate dolomite; (b) the concretions may have been mainly formed in the bacterial sulfate reduction (BSR) zone during very early diagenesis near the sediment–water surface; (c) the paleo‐bottom water overlying the sediments during formation of the concretions was mainly euxinic; and (d) the growth of the studied concretions proceeded via a pervasive model, where later cementation phase initiated in the lower part of the concretions and progressed upward.

Provenance and Tectonic Setting of the Lower Cretaceous Huanhe Formation in the Northwestern Ordos Basin and Its Implications for Uranium Mineralization

“…In oxidized seawater, Mo exists in the form of stable and inactive molybdate ions (MoO 4 2– ). Given the very limited accumulation of authigenic Mo in an oxidized environment, the concentration of seabed sediments in the modern continental rim is as low as 1–5 ppm. − Under anoxic sulfur-rich conditions, a specific concentration of hydrogen sulfide (approximately 50–250 μM) is capable of activating Mo, thereby catalyzing the conversion of molybdate into thiomolybdate (MoO x S (4– x ) 2– , x = 0–3), ,,, and the latter is easily deposited with sulfurized organic matter or iron sulfides. − Under oxidized conditions, U primarily occurs in the form of soluble hexavalent uranyl carbonate complexes and shows chemical inertness. − The enrichment of authigenic U is relatively limited in an oxidizing environment, and the U concentrations are only 1–5 ppm in seabed sediments of the modern continental rim. ,, Under anoxic conditions, hexavalent U­(VI) is reduced into tetravalent U­(IV) in the possible form of insoluble uranyl ions UO 2+ or weakly soluble uranyl fluoride complexes. Therefore, the above chemical properties of Mo and U show that these elements can be used to evaluate the redox conditions of ancient waters.…”

“… 77 − 80 Under oxidized conditions, U primarily occurs in the form of soluble hexavalent uranyl carbonate complexes and shows chemical inertness. 81 − 85 The enrichment of authigenic U is relatively limited in an oxidizing environment, and the U concentrations are only 1–5 ppm in seabed sediments of the modern continental rim. 30 , 86 , 87 Under anoxic conditions, hexavalent U(VI) is reduced into tetravalent U(IV) in the possible form of insoluble uranyl ions UO 2+ or weakly soluble uranyl fluoride complexes.…”

Response of Paleogene Fine-Grained Clastic Rock Deposits in the South Qiangtang Basin to Environments and Thermal Events on the Qinghai-Tibet Plateau

“…In oxidized seawater, Mo exists in the form of stable and inactive molybdate ions (MoO 4 2– ). Given the very limited accumulation of authigenic Mo in an oxidized environment, the concentration of seabed sediments in the modern continental rim is as low as 1–5 ppm. − Under anoxic–sulfur-rich conditions, a specific concentration of hydrogen sulfide (approximately 50 to 250 μM) is capable of activating Mo, thereby catalyzing the conversion of molybdate into thiomolybdate (MoO x S (4– x ) 2– , x = 0–3), ,,, and the latter is easily deposited with sulfurized organic matter or iron sulfides. − Under oxidized conditions, U exists in the primary form of soluble hexavalent uranyl carbonate complexes and shows chemical inertness. − The enrichment of authigenic U is relatively limited in an oxidizing environment, and the U concentrations are only ∼1–5 ppm in seabed sediments of the modern continental rim. ,, Under anoxic conditions, hexavalent U­(VI) is reduced into tetravalent U­(IV) in the possible form of insoluble uranyl ions UO 2+ or weakly soluble uranyl fluoride complexes. Therefore, the above chemical properties of Mo and U show that they can be used to evaluate the redox conditions of ancient waters.…”

Biostratinomy and Diagenetic Impact on Exceptional Preservation of Coccospheres from Lower Oligocene Coccolith Limestones

“…The timing of cement crystallization which protected the coccospheres from destruction within the light-dark laminae couplets of the Tylawa Limestones and overlying deposits, can be inferred from compaction characteristics, using preserved texture [96,97]. The SEM images show that only a small number of complete coccospheres are partly crushed and/or cracked (Figure 5a).…”