Geochronology and chronostratigraphy of the Eocene - Oligocene transition
thesisposted on 26.06.2014, 15:46 by Claudia Diana Sahy
This thesis integrates high-precision (<0.2%, 2σ) [superscript 206]Pb/[superscript 238]U dating of zircons from volcanic tuffs intercalated in key Late Eocene-Oligocene marine and terrestrial sedimentary successions, with high-resolution biostratigraphic and magnetostratigraphic data sets in order to critically examine the accuracy and precision of the numerical age calibration of the Eocene – Oligocene transition (EOT). Weighted mean [superscript 206]Pb/[superscript 238]U ages from the Italian Umbria-Marche and North American White River Group (WRG) sedimentary successions are 0.4-1.0 Myr younger than legacy [superscript 40]Ar/[superscript 39]Ar biotite and sanidine data from the same tuffs (calibrated relative to Fish Canyon sanidine at 28.201 Ma). [superscript 206]Pb/[superscript 238]U calibrated age-depth models were used to constrain the age of magnetic reversals between 26.5-36 Ma (C8r-C16n.2n). Interpolated magnetic reversal ages are consistent with relatively constant seafloor spreading rates, and provide a fully integrated and robust chronostratigraphic framework for the EOT, as shown by mutual consistency of chron boundary ages from the Umbria-Marche basin and the WRG between 31-36 Ma. These data effectively eliminate the discrepancies between astronomically tuned and radio-isotopically calibrated time scales of the EOT. An evaluation of the fidelity of planktonic foraminifer bioevent based chronostratigraphy across the EOT indicates that the last occurrence of hantkeninids and the last common occurrence of Chiloguembelina cubensis which mark the Eocene-Oligocene (34.090 ± 0.074 Ma) and Rupelian – Chattian (28.126 ± 0.175 Ma) boundaries are not timetransgressive across oceanic basins. However, other Oligocene planktonic foraminifer bioevents occur 0.4-0.8 Myr later in the western Tethys than in tropical and subtropical open ocean settings. In the WRG sedimentary succession, the first and last appearance datums of key Late Eocene mammal taxa show diachroneity of ca. 1 Myr over a distance of 400 km. Long-term aridification recorded by the WRG appears to be time-transgressive, and progressed gradually from west to east, while abrupt Early Oligocene cooling reported from WRG outcrops in NE Nebraska was synchronous with Early Oligocene glaciation of Antarctica.