The magnetic polarity time scale across the Permian–Triassic boundary
Published:January 01, 2006
Maureen B. Steiner, 2006. "The magnetic polarity time scale across the Permian–Triassic boundary", Non-Marine Permian Biostratigraphy and Biochronology, Spencer G. Lucas, Giuseppe Cassinis, Joerg W. Schneider
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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 Gries 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 summary 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.
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Non-Marine Permian Biostratigraphy and Biochronology
During the Permian, the single supercontinent Pangaea stretched from pole to pole. Early Permian glacial deposits are found in southern Gondwana. Along the sutures of Pangaea, mountain ranges towered over vast tropical lowlands. Interior areas included dry deserts where dune sands accumulated. Gypsum and halite beds document the evaporation of hot, shallow seas that formed the most extensive salt deposits in the geological record. The Permian period (251 to 299 Ma) encompasses nine ages (stages) arranged into three epochs (series). Most of the Permian marine timescale has been defined by global stratotype sections and points for the stage boundaries. This volume presents new data regarding the biostratigraphy and biochronology of the non-marine Permian and provides a basis for temporally ordering Permian geological and biotic history on land, and correlating that history to events in the marine realm.