Radiometric ages of the Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian, Duckmantian]: A comparison of U–Pb zircon single-crystal ages and Ar/Ar sanidine single-crystal plateau ages

The Fire Clay tonstein Pennsylvanian (Upper Carboniferous), Westphalian Series, Duckmantian Stage]–a kaolinized, volcanic-ash deposit occurring in Kentucky, West Virginia, Tennessee, and Virginia–is the most widespread bed in the Middle Pennsylvanian of the central Appalachian basin, USA. A concordant single-crystal U–Pb zircon datum for this tonstein gives a ²⁰⁶Pb/²³⁸U age of 314.6 ± 0.9 Ma (2σ). This age is in approximate agreement with a mean sanidine plateau age of 311.5 ± 1.3 Ma (1σ, n = 11) for the Fire Clay tonstein. The difference between the two ages may be due to bias between the ⁴⁰K and ²³⁸U decay constants and other factors. The age of the Fire Clay tonstein has important implications for Duckmantian Stage (Westphalian Series) sedimentation rates, correlations with the Westphalian Series of Europe, Middle Pennsylvanian volcanic events, and the late Paleozoic time scale.