Vol 26, No 6 (2018)

The aggregated section of Middle Cambrian at the Khorbusuonka River, northeastern Siberian Platform, is described. The aggregated sequence is compiled on the basis of investigations at four bank outcrops. The findings of trilobites at different stratigraphic levels allowed us to identify the intervals comparable to regional zones of the Amgan and Mayan stages (Middle Cambrian). New findings and analysis of stratigraphic distribution of trilobites helped to specify the position of the Middle Cambrian lower boundary. The lower boundary of the Mayan Stage is drawn by the first appearance of Corynexochus perforatus in the section, 9 m from the base of the Yunkyulyabit–Yuryakh Formation. Trilobites of Cyclolorenzella sp. and Kuraspis sp. found in the upper Tyussala Formation are characteristic of the boundary layers of the Middle and Upper Cambrian. It was supposed earlier that the Upper Cambrian part of the sequence began in the overlying Lapar Formation; however, the aforementioned findings suggest that the Middle Cambrian lower boundary can be located within the limits of the upper Tyussala Formation.

The Ludfordian deposits of the boundary interval of the Gerd’yu and Greben’ superhorizons in the north of the western slope of the Urals and the Pre-Urals Foredeep corresponding to the mid-Ludfordian interval of the Lau Event are studied. The results were obtained using lithological, facies, paleontological, geochemical, spectroscopic, electron-microscopic, and isotopic chemostratigraphic methods. The Lau Event in the examined section is recognized by a distinct change in facies, by a change in zonal species in assemblages of brachiopods, vertebrates, and conodonts, and by climatic changes. The Subpolar Urals section includes a member of clayey-carbonate rocks with interbeds of carbonaceous shales, argillites, and clayey dolomites, reflecting a prominent Greben’ transgression that followed the variously pronounced erosion of a carbonate plate at the end of the Gerd’yu time and differentiation of back-reef relief of this platform. A positive excursion of the isotopic chemostratigraphic curves within the Lau Event interval is most pronounced in the sections of the Chernyshev and Chernov swells, where rocks are less strongly altered by secondary processes and δ¹³C_carb is 4.6 and 6.2–7.4‰, respectively. A decline in the Homerian–early Ludfordian reefs right until the late Lochkovian was an important result of this event as it marked a cessation of the Late Ordovician–Silurian marginal shelf reef formation in the Urals.

The isotopic composition and lithology of the hypostratotype of the Bashkirian Stage (Pennsylvanian) in the Askyn River section (Bashkortostan, western slope of the South Urals) are studied. The isotopic study (δ¹³C, δ¹⁸O) was conducted taking the carbonate lithology into account. The succession studied is composed of Upper Serpukhovian, Bashkirian, and Lower Moscovian carbonates of various lithological types, changeable throughout the section. The Serpukhovian carbonates are characterized by a relatively light carbon isotopic composition (negative values of δ¹³C), whereas the Bashkirian and Moscovian sediments have a heavy composition (mainly positive values of δ¹³C). The carbon isotope anomaly of the first order, fixed at the Mississippian–Pennsylvanian boundary and having a global distribution, is recorded in the studied hypostratotype section slightly higher than the uniformly accepted boundary level. This may indicate that the section contains “transitional beds” at the base of the Bashkirian. In the studied carbonates, a dependence (of the second order) of the isotopic composition of carbon and oxygen on the lithological type of rocks is sometimes recorded. The variation in the values of δ¹³C and δ¹⁸O in carbonates reflects changes in the conditions in which they were formed (temporary and local changes in paleotemperature, bioproductivity, desalination) and epigenetic changes.