Lower–Middle Permian Boundary Deposits of the Omolon Massif and Its Southeastern Framing (Northeast Russia): U–Th–Pb SIMS Dating of Zircons, Paleontological and Isotopic (δ13C) Characteristics

New results of U–Th–Pb SIMS dating of zircons from the Lower–Middle Permian boundary deposits of the Omolon massif and its southeastern framing are presented, a U–Pb age (275 ± 2 Ma) of the Lower–Middle Permian boundary of the Regional Stratigraphic Scale of the Kolyma-Omolon region is obtained for the first time. Stratigraphic sequences of the Lower–Middle Permian boundary deposits in the reference sections of the Omolon Massif and its southeastern framing are discussed. The characteristics of the ecosystems of the Kolyma-Omolon region in the interval of the end of the Early–the beginning of the Middle Permian are given. A large negative excursion of δ13Ccarb was established in the upper part of the Kungurian Stage of the Lower Permian, coinciding with the previously identified negative excursion of δ13Corg in the southern part of the Omulevka Block and a similar negative excursion in South China and the continental deposits of Northern Italy. The paleontological characteristics of the corresponding regional units are presented, illustrated by images of the main representatives of the biota of the lower zone of the middle series, including the characteristic Roadian ammonoid Sverdrupites harkeri (Ruzhencev, 1961). A new species of productid brachiopod Terrakea koragoi, sp. nov. is described.