THE MAGNETIC POLARITY TIME SCALE ACROSS THE PERMIAN-TRIASSIC BOUNDARY

Abstract

Early Triassic and Late to Middle Permian magnetostratigraphic investigations are numerous and span the globe. More than 20 magnetostratigraphic sequences have documented all or part of the Early Triassic geomagnetic field polarity, and > 27 have examined the Late and Middle Permian; 13 span the Permian-Triassic boundary. In order to assess the exact polarity sequence in the time period surrounding the Permian-Triassic boundary, the sequences have been compared diagrammatically. Four distinctive intervals of geomagnetic polarity characterize the Early Triassic, and have been named for discussion purposes: Gries N, Diener R-N, Smith N, and Spath N. A polarity pattern for the Mid- and Late Permian is also recognizable. The Mid- and Late Permian are characterized by two normal polarity intervals (Chang N and Capitan N) of greater apparent duration than those of the Early Triassic. Below the Permo-Triassic Giles N, a distinctive short duration reversed-normal-reversed polarity pattern characterizes the uppermost Changhsingian. The oldest normal polarity in the Middle Permian occurred during the Wordian Stage, established by results from three global sequences. Therefore, the geomagnetic field resumed reversing behaviour after the ~50 Ma-long constant polarity of the Kiaman Reversed Polarity Superchron ('Illawarra reversals') during the Mid- to Late Wordian, or ~267 Ma. Very significantly, the magnetostratigraphic sum mary from this work indicates that the Siberian Traps were active in the Late Permian and spanned the Permian-Triassic boundary. This new geomagnetic polarity dating of the massive Siberian flood basalt activity suggests long-term eruption and environmental degradation, therefore making this igneous activity the most likely cause of the end-Permian mass extinctions. Magnetostratigraphy suggests that eruptions probably commenced in the Late Guadalupian; therefore, the eruptions of two large igneous provinces, Emishan and Siberian, were probably partly simultaneous during part of the Mid- to Late Permian. Environmental havoc throughout the late Mid- and Late Permian is easy to imagine, stressing the environment prior to probably more voluminous eruptions at the end of the Guadalupian and Permian. Siberian eruptions continued through the early Early Triassic, and probably contributed to the slow biotic recovery. © The Geological Society of London.