The upper Visean-Serpukhovian in the type area for the Serpukhovian Stage (Moscow Basin, Russia): Part 1. Sequences, disconformities, and biostratigraphic summary

The upper Visean-Serpukhovian strata in the type region for the Serpukhovian Stage is an epeiric-sea succession ca. 90m in thickness. The predominantly Visean Oka Group (comprising the Aleksin, Mikhailov, and Venev formations) is dominated by photozoan packstones with fluvial siliciclastic wedges developed from the west. The Lower Serpukhovian Zaborie Group is composed of the Tarusa and Gurovo formations. The latter is a new name for the shale-dominated unit of Steshevian Substage age in the studied area. The Zaborie Group is composed of limestones and marls in its lower (Tarusa and basal Gurovo) part and black smectitic to grey palygorskitic shales in the main part of the Gurovo Formation. The Gurovo Formation is capped by a thin limestone with oncoids and a palygorskitic-calcretic palaeosol. The Upper Serpukhovian is composed of a thin (3-12m) Protva Limestone heavily karstified during a mid-Carboniferous lowstand. The succession shows a number of unusual sedimentary features, such as a lack of high-energy facies, shallow-subtidal marine sediments penetrated by Stigmaria, the inferred atidal to microtidal regime, and palustrine beds composed of saponitic marls. The succession contains many subaerial disconformities characterized by profiles ranging from undercoal solution horizons to palaeokarsts. Incised fluvial channels are reported at two stratigraphic levels to the west of the study area. The deepest incisions developed from the Kholm Disconformity (top of the Mikhailov Formation). This disconformity also exhibits the deepest palaeokarst profile and represents the major hiatus in the Oka-Zaborie succession. The new sea-level curve presented herein shows two major cycles separated by the Kholm Unconformity at the Mikhailov/Venev boundary. The Lower Serpukhovian transgression moved the base-level away from falling below the seafloor so that the section becomes conformable above the Forino Disconformity (lower Tarusa). The maximum deepening is interpreted to occur in the lower dark-shale part of the Gurovo Formation. The base of the Serpukhovian Stage is defined by FADs of the conodont Lochriea ziegleri and the foraminifer Janischewskina delicata in the middle of the sequence VN2. The Aleksinian-Mikhailovian interval is provisionally correlated with the Asbian (Lower-Middle Warnantian) in Western Europe. Based on FODs of Janischewskina typica and first representatives of Climacammina, the Venevian is correlated with the Brigantian in Western Europe. The Tarusian-Protvian interval contains diverse fusulinid and conodont assemblages, but few forms suitable for international correlation. FADs of the zonal conodont species Adetognathus unicornis and Gnathodus bollandensis at several metres above the Protvian base suggest correlation of the entire Zaborie Group and may be the basal Protvian to the Pendleian. Copyright (c) 2014 John Wiley & Sons, Ltd.