U–Pb calcite age of the Late Permian Castile Formation, Delaware Basin: a constraint on the age of the Permian–Triassic boundary (?)

Abstract

Organic-rich calcite laminated with gypsum in the evaporitic, non-fossiliferous Castile Formation of the Delaware Basin, southwest USA, yields a Total Pb/U isochron age of 251.5±2.8 Ma (MSWD=1.3). The Castile Formation is almost certainly Late Permian based on its 87Sr/ 86Sr value of 0.706923, identical to the distinct minimum in the late Permian Sr curve. In this paper we explore the potential for using U–Pb calcite ages to date traditionally undatable sections and show how this southern-midcontinent USA (far west Texas and southeast New Mexico) deposit can be correlated to the type section in China. We accept that diagenetic alteration can bias U–Pb results, but the data set we present shows no evidence for such alteration. Clearly with alteration the age we present would represent a minimum age for the Castile Formation. If the age actually dates the Castile Formation then it requires the Permian–Triassic boundary to be younger than 251.5±2.8 Ma.