Radiometric time scale for the upper Eocene and Oligocene based on K/Ar and Rb/Sr dating of volcanic biotites from the pelagic sequence of Gubbio, Italy.

Several samples of unaltered volcanic biotite were recovered from 5 biostratigraphically and magnetostratigraphically controlled bentonite horizons in the continuous upper Eocene-Oligocene pelagic carbonate sequence of Gubbio. The combined K-Ar and Rb-Sr dates are here used to calibrate the following 5 stratigraphic levels, from top to bottom: Level 5-upper part of Globorotalia opima opima planktonic foraminiferal zone, upper part of indistinct NP23-NP24 nannoplankton zone and CP18 nonnoplankton zone, bottom of magnetic anomaly 9N: 28.0+ or -0.3Ma. Level 4-Globigerina ampliapertura and Cassigerinella chipolensis zone, lower part of NP23 zone and bottom of CP18 zone, upper part of anomaly 12R:32.0+ or -0.8Ma. Level 3-Pseudohastigerina micra zone, upper part of NP21 and CP16b zones, top of anomaly 13N:35.4+ or -0.4Ma. Level 2-upper part of Globigerinatheka semiinvoluta zone, upper part of NP18 and CP15a zones, top of magnetic anomaly 16N:36.0+ or -0.4Ma. Level 1-lower part of G. semiinvoluta zone, between zones NP18 and NP19, middle part of zone CP15a, upper part of anomaly 17N:36.4+ or -0.3Ma. From these data, the interpolated age for the much-debated Eocene-Oligocene boundary is 35.7Ma with a 2 standard-deviation uncertainty of 0.4Ma.-from Authors