Early-Middle Triassic boundary interval: Integrated chemo-bio-magneto-stratigraphy of potential GSSPs for the base of the Anisian Stage in South China

Abstract

The Wantou and Youping sections of Guangxi, South China provide a detailed high-resolution integrated calibration of the Early-Middle Triassic boundary succession for lithostratigraphy, volcanic episodes, conodont first occurrences (FOs), ammonoid biostratigraphy, geomagnetic polarity, inorganic carbon isotopes, sea-surface temperatures derived from conodont-apatite oxygen-isotopes, and ID-TIMS U–Pb radiometric dating. The upper Spathian (late Early Triassic) magnetostratigraphy is characterized by normal polarity (magnetozone LT9n) that encompasses the FOs of the typical Spathian conodonts Triassospathodus homeri and Gladigondolella carinata, the late Spathian Neopopanoceras haugi ammonoid zone and the beginning of a progressive positive shift in inorganic carbon isotopes. The overlying reversed polarity interval (LT9r) contains two brief normal-polarity subzones (MT1n and MT2n) that can be recognized in several other marine and terrestrial sections. The FO of conodont Chiosella timorensis sensu stricto, a proposed base-Anisian global marker, is near MT1n and near the end of the positive δ13Ccarb excursion. Sea-surface temperatures were reported to have cooled by 4°C during this rise in δ13Ccarb, suggesting a sequestration of carbon dioxide. The lowermost Anisian at Wantou and Youping is dominated by normal polarity (MT3n, with the presence of one major reversed-polarity subzone MT3n.1r), contains the FO of typical Anisian conodonts (Gladigondolella tethydis/Magnigondolella alexanderi), and has the onset of a plateau in inorganic carbon isotopes values (stabilizing around +4‰). The combination of the FO of conodont Chiosella timorensis s.s., the brief normal polarity zone (MT1n) and the last portion of the rising carbon-isotope trend are suitable for primary proxies for global correlation of the Early-Middle Triassic boundary (base of Anisian) to other marine and non-marine settings. Radiometric dates at the Wantou and at the Guandao sections, coupled with a composite cyclostratigraphy for Early Triassic through Anisian, indicate that the FO of the conodont Chiosella timorensis s.s. is at approximately 246.7 Ma.