Middle and Upper Ordovician conodonts of the Moscow Syneclise: New data on stratigraphy of the Borehole Gavrilov Yam-1 section

On the basis of the first data on the taxonomic composition and stratigraphic distribution of conodont assemblages in the Borehole Gavrilov Yam-1 section (central part of the Moscow Syneclise), its biostratigraphic subdivision is proposed with specification of some formations previously substantiated by finds of acritarchs and benthic faunal assemblages. The Nefedovo Formation, which was previously attributed to the Volkhov Horizon (Dapingian Stage), is correlated by conodonts with the Kunda Horizon (basal part of the Darriwilian Stage). The middle part of the overlying Veksa Formation, which was formerly considered to be of Kunda age, yielded conodonts that mark the lower boundary of the Azeri Horizon. Conodonts found in the Mitino Formation with highly ambiguous age allow it to be correlated with the Johvi and Keila horizons of the Sandbian Stage (Upper Ordovician). The lower part of the Varlyga Formation, which is attributed to the Lower Silurian in the regional stratigraphic scale, is correlated by conodonts with the Oandu and Rakvere regional stages (lower part of the Katian Stage, Upper Ordovician). In total, 44 conodont species were found in 79 samples; most of them are illustrated. A brief description is given for representatives of the genera Pectinognathus, Phragmodus, and Semiacontiodus. The conodont assemblage includes species distributed in the northwestern part of the East European Platform (Baltoscandian paleobasin) and taxa characteristic of the Timan-Pechora region.