Radiometric dating of the eocene-oligocene boundary at gubbio, italy

Unaltered volcanic biotite has been extracted from five bentonite levels in the upper Eocene-Oligocene pelagic carbonate sequence of the Contessa quarry, near Gubbio. The K-Ar and Rb-Sr age determination of the biotite separates are used here to calibrate, with closely spaced points, the late Paleogene biostratigraphic and magnetostratigraphic time scales. Two samples from the upper Eocene have yielded unexpectedly young ages making the duration of the Priabonian age 0.8 ± 0.4 m.y., instead of 3.5 m.y. as predicted by other time scales. The interpolated age of the Eocene-Oligocene boundary is 35.7 m.y. with a 2-standard-deviation uncertainty of 0.4 m.y.