A biostratigraphically well-constrained K-bentonite U-Pb zircon age of the lowermost Darriwilian stage (Middle Ordovician) from the Argentine Precordillera

Abstract

New concordant zircon U-Pb ages from lower Middle Ordovician K-bentonites in the Argentine Precordillera yield an emplacement age of 464 ± 2 Ma. These K-bentonites occur in the U. austrodentatus Zone, the lowermost graptolite zone of the Darriwilian Stage, which was recently ratified as a global stage by the ICS and IUGS. Because the estimated length of the U. austrodentatus Zone is no more than 1-2 Ma, the new isotopic age, which is one of the biostratigraphically best constrained from the Ordovician, can be considered representative of the base of the Darriwilian Stage.