HR: 1340h
AN: V13A-0520    [Abstracts]
TI: Calibration of the Permo-Triassic Magnetostratigraphic Time Scale: Constraints from the Dewey Lake Formation, West Texas
AU: * Chang, S
EM: su_chin@berkeley.edu
AF: Earth and Planetary Science Dept., University of California, Berkeley, Berkeley, CA 94720-4767 United States
AU: Knight, K B
EM: kbk@berkeley.edu
AF: Earth and Planetary Science Dept., University of California, Berkeley, Berkeley, CA 94720-4767 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Earth and Planetary Science Dept., University of California, Berkeley, Berkeley, CA 94720-4767 United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 United States
AB: Magnetostratigraphy is potentially a powerful tool for deciphering the high resolution chronostratigraphy of events across the Permo-Triassic boundary, but few well-dated polarity reversals exist to serve as calibration. Red beds of the Dewey Lake Formation (DLF) of West Texas span three reversed polarity intervals (Steiner, 2001) in a section of the DLF at Caprock Canyons State Park, where two tuffs occur. Sanidine separated from these tuffs was analyzed by ⁴⁰Ar/³⁹Ar methods. Single crystal laser fusion ⁴⁰Ar/³⁹Ar analyses of 40 grains from the upper tuff yield a weighted mean age of 249.9 ± 2.4 Ma (2σ errors here and throughout). The clustering of single crystal data provides some assurance against xenocrystic contamination. Two age spectra from multigrain sanidine separates from the lower tuff yielded integrated ages of 248.9 ± 2.8 Ma and 249.7 ± 2.8 Ma and consistent plateau ages of 249.2 ± 2.4 Ma and 249.6 ± 2.4 Ma. Two age spectra from multigrain upper tuff sanidines lack strict plateaus but with overall flat age spectra, with integrated ages of 249.7 ± 2.8 Ma and 250.3 ± 2.8 Ma and plateau-like segments (>70% of ³⁹Ar released) with ages of 249.9 ± 2.6 Ma and 249.9 ± 2.6 Ma, respectively. These results, compared with ⁴⁰Ar/³⁹Ar data (using the same FCs = 28.02 Ma standard calibration) from the GSSP section at Meishan, China, suggest that the Permo-Triassic boundary (249.8 Ma; recalculated from Renne et al., 1995) definitely occurs within the lower Dewey Lake Formation. The two tuffs, which bracket a normal to reverse geomagnetic polarity transition polarity (Steiner, 2001), have indistinguishable ages. The age of this Permo-Triassic polarity transition is thus best represented by the weighed average of their ages, ca. 249.7 Ma (based on accepted calibrations of the ⁴⁰Ar/³⁹Ar system). Further such constraints will facilitate high-resolution comparison of terrestrial and marine records across this critical time interval.
DE: 1100 GEOCHRONOLOGY
DE: 1115 Radioisotope geochronology
DE: 1520 Magnetostratigraphy
DE: 9612 Triassic
DE: 9615 Permian
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005