. Bulletin of the British Museum (Natural History), Geology. The Ordovician-Silurian boundary beds of the north-east USSR T. N. Koren1, M. M. Oradovskaya2 and R. F. Sobolevskaya3 'VSEGEI, Srednii prospekt 74, 199026 Leningrad, USSR 2PGO 'Sevvostokgeologia', 44 Proletarskaya, 685000 Magadan, USSR 3VNIIOkeangeologia, 120 Moika, 190121 Leningrad, USSR Synopsis Graptolites of the supernus, extraordinarius, persculptus, acuminatus and ascensus Zones are present in sections in the north-east USSR, with the best section at Mirny Creek. Brachiopod and coral faunas also occur with the Tcherskidium and
. Bulletin of the British Museum (Natural History), Geology. The Ordovician-Silurian boundary beds of the north-east USSR T. N. Koren1, M. M. Oradovskaya2 and R. F. Sobolevskaya3 'VSEGEI, Srednii prospekt 74, 199026 Leningrad, USSR 2PGO 'Sevvostokgeologia', 44 Proletarskaya, 685000 Magadan, USSR 3VNIIOkeangeologia, 120 Moika, 190121 Leningrad, USSR Synopsis Graptolites of the supernus, extraordinarius, persculptus, acuminatus and ascensus Zones are present in sections in the north-east USSR, with the best section at Mirny Creek. Brachiopod and coral faunas also occur with the Tcherskidium and Holorhynchus beds in the supernus Zone and the Hirnantia? beds present in the persculptus Zone, both within the Tirekhtyakh Horizon. The succeeding acuminatus and ascensus Zone graptolites are developed in the Chalmak Horizon, which also bears a sparse shelly fauna. Introduction The late Ordovician and early Silurian boundary beds in the north-east USSR crop out on the Omulev Uplift in the upper Kolyma Basin. They are built up by terrigenous-carbonate and terrigenous deposits which are variable in composition and contain a mixed shelly-graptolite fauna. The rocks are exposed on limbs of extensive anticlines and show either a monoclinal succession, such as at Mirny Creek, Neznakomka River and Drevnyaya River, or represent large fragments of sections among complex faulted sequences, such as at the Ina River. The Upper Ashgill and Lower Llandovery deposits include the supernus, extraordinarius, per- sculptus, acuminatus and ascensus graptolite Zones and have a total thickness of about 300 m (Fig. 1). This part of the section is designated the Tirekhtyakh and Chalmak horizons. The lower part of the Tirekhtyakh horizon (the supernus Zone) (Fig. 2) shows a diversity of facies from deep water shales yielding graptolites, for example at Khekandya River and Lukavy Creek, to biohermal and biogenic-detrital carbonates with mixed brachiopod-coral-graptolite faunas as at Mirny Creek and the
Size: 1606px × 1556px
Photo credit: © Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorbritishmuseumnaturalhistory, bookcentury1900, bookcoll
