Correlation of the Type Bashkirian Stage (Middle Carboniferous, South Urals) with the Morrowan and Atokan series of the midcontinental and western United States

Abstract

The graphic correlation technique has been used to directly relate the stratigraphic appearances of key species in the Bashkirian Stage stratotype to those in a North American composite section. The type Bashkirian is separated from the underlying Serpukhovian Stage by an erosional unconformity and associated lacuna of undetermined, but probably minor duration. Accordingly, the base of the type Bashkirian (base of Bogdanovkian Horizon) is only slightly younger than the international mid-Carboniferous boundary. A level within the upper part of the Tashastian Horizon (Upper Bashkirian Substage) most likely correlates with the Morrowan-Atokan boundary. This level roughly coincides with a sequence boundary at the Bashkirian stratotype and with a regional unconformity in the North American midcontinent. The top of the Bashkirian Stage (top of Asatauian Horizon) is lower Atokan in North American terms. On the basis of recent 40Ar/39Ar and SHRIMP zircon geochronology studies, the age of the mid-Carboniferous boundary is estimated at 314 Ma and a horizon of early Atokan age is dated at 310.8 Ma. Accepting the present biostratigraphic correlations, these values suggest a duration for the Bashkirian Stage of slightly more than 3.2 m.y. and a duration of the Morrowan Series of slightly less than 3.2 m.y.