Late Permian–Early Triassic (Vyatkian (Changhsingian)–Induan) Warming of the Russian Plain according to Mössbauer Spectroscopic Data on Terrigenous Deposits

For the first time, Mössbauer spectroscopy of Permian–Triassic boundary rocks in the European part of Russia (Nizhny Novgorod region, Vetluga River) was performed. Sands and clays of the Vyatka Formation and the Astashikha member of the Vokhma Formation (upper Vyatkian substage, Changhsingian stage) and the overlying part of the Vokhma Formation (Induan) have been studied. During the transition from the Vyatka Formation to the Vokhma Formation, the chlorite content increases (main part of Fe(II), max 21% of the total Fe), which correlates with the active erosion of rocks in the Ural orogen. In sands, the Uralian epidote (up to 9%) is present as an important carrier of paramagnetic silicate Fe(III). The magnetically ordered phase (Fe(III)magn) is represented by highly dispersed magnetite-unrelated particles likely represented mainly by maghemite. The content of Fe(III)magn in clays is 12–24% in the Vyatka Formation, 13–40% in the Astashikha member, and 19–43% in the Induan stage. Synthesis of the compounds of Fe(III)magn is reconstructed immediately after sedimentation during pedogenesis, after the infiltration of clay suspensions into alluvium due to Fe oxidogenesis in a matrix of waterproof clays during the alternation of hydrologically contrasting seasons or longer time episodes. High content of Fe(III)magn in the Induan correlates with the increase of magnetic susceptibility and intensity of reddening. Increase of these characteristics coincides with the trend of global warming at the Permian–Triassic boundary in marine sections and confirms the opinion that the reddening of Permian–Triassic rocks in the Russian Plain was due to climatic fluctuations synchronous with sedimentation, rather than deep diagenesis.