Siderite microconcretions in the glauconite-bearing clayey-silty rocks of the Khaipakh Formation (Middle Riphean, Olenek Uplift)

Siderite microconcretions in the glauconite-bearing clayey-silty rock member of the lower sub-formation of the Khaipakh Formation (Middle Riphean, Olenek Uplift) are scrutinized for the first time. In two Khorbusuonka River sections located with a spacing of 12 km, the microconcretions occur as lenses and interlayers. Together with glauconitites, they serve as a distinct marker horizon of this stratigraphic interval. Their structures, morphologies, diffraction characteristics, chemical compositions, and isotope data are considered. They were examined comprehensively with modern investigation methods (X-ray diffraction, scanning electron microscopy with the application of local microprobe analysis, and others). Structural-morphological types of microconcretions are identified. Calculation of the chemical composition of carbonates based on numerous microprobe analyses made it possible to reveal different degrees of mineral heterogeneity in each type in terms of the distribution of macro- and microlevel isomorphous trace-elements and to refine the character of their secondary alterations at different stages of lithogenesis. The results of oxygen and hydrogen isotopic studies are presented for algal dolomites and limestones from the Middle and Lower Riphean sections in the Olenek Uplift (Debengda, Arymass, and Kyutingda formations). They demonstrated that siderites are similar to limestones and dolomites in terms of the oxygen isotopic composition (δ 18 O = 17.6–24.8, δ 18 O av = 20.0 ± 2.4‰), but are marked by low δ 13 C values (from −6.3 to −12.0‰ (δ 13 C av = −8.6 ± 2.1‰), suggesting the formation of microconcretions during early diagenesis. Siderite microconcretions were formed in the clayey-silty sediment slightly after glauconite, whose grains could serve as crystallization centers and (or) be entrapped during the growth of separate microcrystals. The role of catalyzers during the formation of both glauconite and siderite was played by bacterial communities, whose poorly preserved remnants have been detected not only in the studied member of the lower Khaipakh subformation, but also above and below the section. Separate types of microconcretion could be formed during the replacement of oncolites by siderite.