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

Abstract

Several samples of unaltered volcanic biotite were recovered from five biostratigraphically and magnetostratigaphically controlled bentonite horizons in the continuous upper Eocene–Oligocene pelagic carbonate sequence of Gubbio, northern Italy. K/Ar and Rb/Sr dates on these samples offer a unique opportunity to calibrate with closely spaced points the late Paleogene biostratigraphic and magnetostratigraphic time scales. Until now, time scales have been constructed by interpolating between less certain and more widely spaced data points. Our combined K/Ar and Rb/Sr dates are here used to calibrate the following five 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 nannoplankton zone, bottom of magnetic anomaly 9N: 28.0 ± 0.3 Ma. 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 ± 0.8 Ma. Level 3— Pseudohastigerina micra zone, upper part of NP21 and CP16b zones, top of anomaly 13N: 35.4 ± 0.4 Ma. Level 2—upper part of Globigerinatheka semiinvoluta zone, upper part of NP18 and CP15a zones, top of magnetic anomaly 16N: 36.0 ± 0.4 Ma. 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 ± 0.3 Ma. From these data, the interpolated age for the much-debated Eocene-Oligocene boundary is 35.7 Ma with a 2-standard-deviation uncertainty of 0.4 Ma.