| Four new zircon U-Pb ages for volcanic horizons from the Great Valley Group require adjustments to parts of the Lower Cretaceous Time Scale: a Lower Aptian concordia age of 124.07 +/- 0.13 Ma is in good agreement with the Global Time Scale 2008 Barremian-Aptian boundary age of 125.0 Ma, but Upper Berriasian (138.46 +/- 0.15 Ma) and two Lower Valanginian ages (137.63 +/- 0.14 Ma, 133.66 +/- 0.14 Ma) shift the Berriasian-Valanginian and Valanginian-Hauterivian boundaries to 138 Ma and 133 Ma, respectively. These new boundary ages, relative to previous estimations, indicate longer Berriasian and shorter Valanginian stage durations, which (1) adjust the timing of the Valanginian delta13C positive excursion so that it is synchronous with the main pulse of the Parana-Etendeka flood basalt volcanism, (2) require a faster seafloor spreading rate during the Valanginian than previously assumed, and (3) suggest a sharper calcareous nannofossil species diversity peak during the Hauterivian. |
