Microfauna and Stratigraphy of the Ogarkovo Jurassic–Cretaceous Reference Section on the Unzha River (Russia, Kostroma Region)

The results of a comprehensive microfaunistic study of the Ogarkovo Upper Jurassic–Lower Cretaceous reference section on the Unzha River (Kostroma oblast) are presented. The Lower Kimmeridgian, the Upper Volgian and the Ryazanian stages contained variably mineralized three-dimensional remains of radiolarians. The radiolarian genera Protovallupus and Vallupus are recorded for the first time from the Russian Platform. Representative foraminiferal and nannoplankton assemblages and isolated ostracod shells Schuleridea triebeli were identified in the Lower Kimmeridgian. The foraminiferal-based Lenticulina kuznetsovae–Epistomina praetatariensis Zone and were established in the Lower Kimmeridgian. The occurrences of the Mniovniki Formation are for the first time recorded in the Kostroma Zavolzhye region. The age of the Ogarkovo Unit (Ryazanian Tzikwinianus Zone—Lower Valanginian Undulatoplicatilis Zone) is updated.