3 Pander Society Symposium (Posters)

Monday, 24 March 2003
8:00 AM - 12:00 PM

INTEGRATED BIOSTRATIGRAPHY, MAGNETOSTRATIGRAPHY, AND GEOCHRONOLOGY OF THE OLENEKIAN-ANISIAN BOUNDARY IN MARINE STRATA OF GUANDAO SECTION, NANPANJIANG BASIN, SOUTH CHINA: IMPLICATIONS FOR TIMING OF BIOTIC RECOVERY FROM THE END-PERMIAN EXTINCTION

Paper number: 3-15
Program: SYMPOSIA SESSIONS
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The end-Permian extinction and the Early Triassic biotic recovery are exceptionally well recorded in marine-strata of the Nanpanjiang Basin, south China. Guandao section, in the basin center, is the key section for chronostratigraphy as it is continuous from the Permian-Triassic boundary to the Upper Triassic, and it contains abundant conodonts, preserved primary magnetic signature, and several volcanic ash horizons. The paleoenvironmental setting of the Guandao section is advantageous as tracing of strata from deep-water section at Guandao to an adjacent platform allows the timing of both shallow- and deep-marine events.

Abundant conodonts enabled delineation of 14 bio-zones for the Lower Triassic and basal Anisian. The Olenekian-Anisian boundary is characterized by the last occurrence of Neospathodus homeri and Ns. symmetricus and the first occurrence of Ns. gondelloides, Chiosella timorensis and Neogondolella regalis.

A magnetostratigraphy was constructed from 467 oriented plugs. Careful thermal demagnetization of samples yielded stable primary signals that pass fold and congolomerate tests and yield a detailed reversal stratigraphy. The Olenekian-Anisian boundary is marked by a shift from reversing polarity in the upper Olenekian to a long normal zone in the lower Anisian (Aegean). The overall pattern is consistent with that of Gradstein et al. (1995) and Muttoni et al.(1995,1996).

Several volcanic tuff horizons bracket the P-T boundary and Olenekian-Anisian boundary in Guandao section. Zircon U-Pb analysis from a volcanic horizon at the base of the Anisian in Guandao section (basal timorensis zone) indicates an age of ca. 247 Ma. Previous age estimates of the Olenekian-Anisian boundary ~240-242 Ma were based on extrapolation. A greater age for the Olenekian-Anisian boundary is consistent with a recently reported date of 241 Ma for the Anisian-Ladinian boundary (Mundil, 1996). Our data indicates a timeframe of approximately 4 Ma, thus shortening the timeframe for biotic recovery from the end-Permian extinction.