Vol 27, No 2 (2019)

Clay subfractions (SFs) with particles size of 2–5, 0.6–2.0, 0.3–0.6, 0.2–0.3, and 0.1–0.2 μm of two shale samples (Upper Riphean Inzer Formation, South Urals) are studied using transmission electron microscopy, X-ray diffractometry (XRD), and U–Pb, Sm–Nd, Rb–Sr, and K–Ar methods. The SFs are composed of low-temperature 1M_d illite; quartz, chlorite, and 2M₁ illite are observed only in some coarse SFs. Irrespective of the size, the clay particles are equant. The standardized illite crystallinity indices (CIS) of all SFs are typical of the dia(cata)genesis zone. The CIS value increases, the I₀₀₂/I₀₀₁ ratio of XRD patterns decreases, and the K content and K/Rb ratio increase with decreasing particle size of the SFs from 2–5 to 0.1–0.2 μm. Leaching with 1N HCl and 1N NH₄OAc and U–Pb, Sm–Nd, and Rb–Sr analysis of untreated SFs, leachate, and residue allowed us to study the isotope systematic of mixing in mobile and silicate material of shales. The ²⁰⁸U/²⁰⁴Pb and ⁸⁷Rb/⁸⁶Sr ratios of leachates are lower and the ¹⁴⁷Sm/¹⁴⁴Nd ratio is higher than those of residues. The leachates are also characterized by less radiogenic Pb and Sr and more radiogenic Nd relative to residues. With decreasing size of SF particles, the U, Pb, Sm, Nd, and Sr contents of leachates gently decrease and the Rb content increases. The ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratios of leachates of fine SFs are significantly higher, whereas their ²³⁸U/²⁰⁴Pb ratio is lower in comparison with coarse SFs. What is more, the data points of residues of various SFs occur along the mixing lines in the ⁸⁷Rb/⁸⁶Sr‒⁸⁷Sr/⁸⁶Sr and 1/⁸⁶Sr‒⁸⁷Sr/⁸⁶Sr plots. The data points of corresponding leachates also form linear trends in ²³⁸U/²⁰⁴Pb‒²⁰⁶Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb‒²⁰⁷Pb/²⁰⁴Pb, ¹⁴⁷Sm/¹⁴⁴Nd‒¹⁴³Nd/¹⁴⁴Nd, and ⁸⁷Rb/⁸⁶Sr‒⁸⁷Sr/⁸⁶Sr coordinates. Apparent Rb–Sr ages calculated from slopes of “inner isochrons” (“leachochrons”) as well as K-Ar ages gradually decrease from 835–836 and 721–773 Ma, respectively, for SFs of 2–5 μm and to 572‒580 and 555‒580 Ma, respectively, for SFs of 0.1–0.2 μm. Thus, the XRD and isotopic data indicate that both clay and mobile shale constituents are mixtures at least of two components, and the silicate phase contains authigenic illites of various ages. First-generation illite abundant in the coarse SFs of 2–5 μm and 0.6–2.0 μm was formed immediately after the deposition of the Inzer sediments and its age of 803–836 Ma is consistent with stratigraphic age of the Inzer Formation. The formation of this illite was facilitated either by lithostatic burial or intense horizontal fluid flow caused by tectonic inversion in the eastern regions of the Uralian paleobasin. The age of the second-generation illite is 572–580 Ma; it was formed as a result of vertical movements or renovation of the composition of the pore fluids during deformations and metamorphism of the South Urals related to the evolution of the Beloretsk metamorphic complex.

The Cambrian malacofauna of Australia is taxonomically among the most diverse of synchronous faunas. In the number of described mollusk species, it rivals the Siberian and Chinese faunas. To date, 80 valid species and 12 species in open nomenclature, apparently representing new undescribed taxa, have been recorded from the Lower–Middle Cambrian successions of Australia. In addition, six species names can be considered as junior synonyms. The ranges of distribution of mollusk species plotted over the modern stratigraphic scheme reveal four major molluskan evolutionary assemblages in the Tommotian–Undillan interval. In a paleogeographical context, the Cambrian malacofauna of Australia has 29 species in common with the Siberian Platform, Kazakhstan, Altai-Sayan, Transbaikalia, Mongolia, North and South China, Morocco, Antarctica, Europe (Denmark, Germany), Greenland, North America, and New Zealand, providing important correlation links between these regional stratigraphic schemes.

Ammonoids of the genus Yakutosirenites (Sirenitidae) from the Carnian deposits of Northeast Asia have been revised. On the basis of the study of morphogenesis of the most important shell structures, the genus Yakutosirenites is subdivided into two subgenera: Yakutosirenites with the type species Sirenites pentastichus Vozin, 1964 and Vozinites with the type species Sirenites armiger Vozin, 1965. A description of the genus and its subgenera and species is given. The significance of the species of these subgenera for biostratigraphic subdivision and correlation of the lower/upper Carnian boundary interval is substantiated. The Boreal–Tethyan correlation of the Yakutosirenites pentastichus Zone has been clarified. For the first time, taking into account the Yakutosirenites revision data, the upper part of this zone is compared only to the Arctosirenites canadensis beds of Arctic Canada and to the lower subzone of the Tropites welleri Zone of British Columbia, which are equivalent to the lower part of the Tropites subbullatus Zone of the Alpine standard.

High-resolution quantitative analyses of the planktonic foraminifera and calcareous nannofossil content have been carried out on IODP Leg 342, Hole U1406A (Northwest Atlantic Ocean) in the core interval 10H2–2H4, plotting the abundance distributions of the biostratigraphically and quantitatively most significant species. Qualitative analyses have been used to identify the precise depth of marker bioevents. For almost the whole succession, relevant magnetostratigraphic data were provided by previous studies. All chrons and subchrons in the interval C6Cn–C6AAr were recognised in core interval 10H2–4H1 and calibrated to the GPTS 2012, but no magnetostratigraphic data are available for the highest portion (above 35 m). The investigated succession falls between Sphenolithus delphix FO (First Occurrence)—uppermost Chattian bioevent—in core 10H2 and the Sphenolithus belemnos LO (Last Occurrence)–Burdigalian bioevent – in core 2H5, embracing the Zone O7 (uppermost part)–M3 foraminiferal zone interval and the Zone CNO6–CNM5 calcareous nannofossil zone interval. A calcareous plankton biochronological framework has been produced integrating calcareous plankton and magnetostratigraphic data. Calculated ages for the most significant recognised bioevents are S. delphix FO 23.35 Ma, S. delphix LO 23.02 Ma, Paragloborotalia kugleri FO 23.00 Ma, Globoquadrina dehiscens FO 22.59 Ma, Sphenolitus disbelemnos FO 22.56 Ma, Helicosphaera carteri FCO (First Common Occurrence) 22.18 Ma, P. kugleri LO 21.23 Ma. Two hiatuses have been recognised within core 4H, whereas only one was recognised in previous studies. The first one in core 4H5 spans the interval 21.17–19.01 Ma and coincides with a phosphate-rich layer; the second in core 4H2 spans the interval from 18.87 to 18.21 Ma and corresponds to a glauconite-rich layer. The results obtained from Hole U1406A allow some considerations about potential reference bioevents concerning the still open issue of Burdigalian GSSP (Global Stratotype Section and Point).