Moscovian–Kasimovian boundary conodont assemblages from the Kalinovo section, Donets Basin, Ukraine.

The conodonts of the Moscovian–Kasimovian boundary beds were restudied in the Kalinovo section—one of the most continuous successions in the Donets Basin, eastern Ukraine. Species of Idiognathodus and Swadelina are most common in the studied interval. Swadelina subexcelsa and Idiognathodus sagittalis, two of four species proposed as hypothetical index species for the Moscovian–Kasimovian boundary are registered in the Kalinovo section. I. turbatus and I. heckeli were not identified in the Donets Basin so far. Four conodont zones were distinguished in the studied Moscovian–Kasimovian boundary interval: the Swadelina gurkovaensis, Sw. subexcelsa, Sw. makhlinae and Idiognathodus sagittalis–I. neverovensis zones. Two potential biostratigraphic events were investigated for selection of the lower boundary of the Kasimovian Stage. One of them is defined by the FAD of Swadelina subexcelsa. It coincides with the base of the Krevyakinian Regional Substage, which is the traditional Moscovian–Kasimovian boundary in the type Kasimovian. But in the Donets Basin the Sw. subexcelsa and Sw. makhlinae zones belong to the upper part of the Lomovatkian Regional Stage, which is late Moscovian in age. This contradicts to the correlation by conodont data. The second event is defined by the FOD of Idiognathodus sagittalis and a number of new conodont species recovered close to the base of the Toretzian Regional Stage of the Donets Basin, and described herein shortly in open nomenclature. This event is more prominent in conodont evolution due to occurrence and then dominance of new species of Idiognathodus and extinction of the Swadelina species characteristic of a whole Upper Moscovian Substage. This event corresponds to the Khamovnikian Regional Substage of the Kasimovian type area, one substage higher than the traditional Moscovian–Kasimovian boundary. This event with outburst of new species documents the new step in conodont evolution. If the position of the Moscovian–Kasimovian boundary will be selected by the FAD of one of the three species—I. sagittalis, I. turbatus or I. heckeli, the boundary will be placed in the lower or middle part of the Khamovnikian Regional Substage of the type Kasimovian and at the base of the Toretzian Regiostage in the Donets Basin. At present according to the Ukrainian Carboniferous Stratigraphical Scheme the Moscovian–Kasimovian boundary in the Donets Basin takes place already at the base of the Toretzian Regiostage. Its position is based on the cyclo-stratigraphic analysis. It does not correspond to that in the Kasimovian stratotype. The correlation of the Donets Basin with the Moscow Syneclise, Ural Mountains, North Spain and partly with South China (Tethyan Province) is mostly reliable but correlation to the U.S. Midcontinent is complicated due to presence of mostly endemic species in both Eurasia and North America. The Moscovian–Kasimovian boundary interval is characterized by a sea-level lowstand that isolated marine basins. [ABSTRACT FROM AUTHOR]