The Ludfordian Lau Event (Upper Silurian) in the Northeastern Regions of European Russia

— 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 δ13Ccarb 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.