A BIOSTRATIGRAPHICALLY WELL-CONSTRAINED K-BENTONITE U-PB ZIRCON AGE OF THE LOWERMOST DARRIWILIAN STAGE (MIDDLE ORDOVICIAN) FROM THE ARGENTINE PRECORDILLERA

New concordant zircon U-Pb ages from lower Middle Ordovician K-bentoni tes in the Argentine Precordillera yield an emplacement age of 464 +/- 2 Ma. These K-bentonites occur in the U. austrodentatus Zone, the low er-most graptolite zone of the Darriwilian Stage, which was recently r atified as a global stage by the ICS and IUGS, Because the estimated l ength of the U. austrodentatus Zone is MO more than 1-2 Ma, the new is otopic age, which is one of the biostratigraphically best constrained from the Ordovician, can be considered representative of the base of t he Darriwilian Stage.