Lower Moscovian Conodonts and Fusulinids: The Position of the Lower Boundary of the Moscovian Stage (Pennsylvanian)

Abstract

The definition of the base of the Moscovian Stage is one of the highest priorities of the Subcommission on Carboniferous Stratigraphy. This stage was established in 1890 in the Moscow Basin (Russia) by S. N. Nikitin. According to the unified Carboniferous chart of the Russian Platform, the Moscovian consists of four substages: the Vereian, Kashirian, Podolskian, and Myachkovian. In the stratotype region, the lower Moscovian includes the Vereian and Kashirian. Most of the Vereian is a siliciclastic succession, and carbonate marine units occur only at its base and top. The overlying Kashirian Substage is represented mainly by shallow-water marine carbonates with an abundant and diverse marine fauna. Several subordinate siliciclastic units also occur. The exact position of the base of the Moscovian cannot be defined properly in its type area, because the basal Vereian unconformably overlies Mississippian limestones or alluvial and lagoonal strata of the Aza Formation (uppermost Bashkirian). The Moscovian Stage constitutes the Middle Pennsylvanian Series of the Carboniferous System, but a biostratigraphic marker and GSSP for it have not yet been designated. The task group assigned to establish a GSSP close to the existing Bashkirian–Moscovian boundary has suggested several potential markers among foraminifers and conodonts, but the search for the base of the global Moscovian Stage marker near its traditional base has now stalled, and the most recent proposals have attempted to shift this boundary to a level within the middle or upper Bashkirian. In contrast, we suggest that it may be more productive to search for first appearance datums (FADs) of conodont species within the lower part of the traditional Moscovian that may have greater potential for boundary designation and international correlation. We have restudied the distribution of the lower Moscovian (Vereian and Kashirian substages) conodonts and fusulinids from the southwest Moscow Basin and the Oka–Tsna Swell. The greatest change in conodont assemblages does not occur at the base, but higher up in the Moscovian. The platform conodonts of the overlying Kashirian Substage differ significantly in their morphology and taxonomy from Vereian conodonts. The Bashkirian genera Declinognathodus and Idiognathoides became extinct close to end of the Vereian. The base of the Kashirian Substage, coinciding with the FAD of Neognathodus bothrops, would be an easier level for global correlation than would the base of the Vereian Substage, the current level of the base of the Moscovian in the Moscow Basin. Analysis of the fusulinids shows that the Kashirian assemblages also differ greatly from those of the Vereian. The FADs of Taitzehoella and Hemifusulina coincide with the lower boundary of the Kashirian in the type region of the Moscovian, and the species appeared almost simultaneously with Neognathodus botrops. For the purposes of correlation, the appearance of these genera is very useful and could probably be used as a marker or additional taxa for the definition of the lower boundary of the Moscovian Stage.