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Earth and Environmental Sciences pres.

Special Lecture: What Really Happened in the Continental Realm During the First of Three Great Global Extinctions?: The Chronostratigraphy of Beaufort Group Strata Deposited Across the Permian/Triassic Boundary, Karoo Basin, South Africa?

John Geissman, University of Texas at Dallas

The commonly held, decades old model for the terrestrial response to the end-Permian extinction crisis is based on a turnover in the vertebrate-fossil record first documented in the Karoo Basin, South Africa, and since extrapolated globally. This model requires that the systematic loss exhibited by an abrupt turnover from the Daptocephalus Assemblage Zone (DAZ) to the Lystrosaurus Assemblage Zone (LAZ) is coincident with the timing of mass extinctions in the oceanic realm. Understanding the timing of these inferred environmental changes in the Karoo Basin, from Late Permian to possibly Early Triassic (?) time, as recorded in Beaufort Group strata, requires robust chronostratigraphic information, including high quality unequivocal magnetic polarity stratigraphy for sections previously interpreted to encompass end-Permian extinction events. The preservation of an early-acquired remanence in Beaufort strata is required for a valid magnetostratigraphy, yet is difficult to prove due to thermochemical effects related to the Early Jurassic (ca. 183 Ma) Karoo Large Igneous Province (LIP) and the NE to SW increase in burial diagenesis attending Cape Fold Belt tectonism. My very fond wonderings in parts of the Karoo Basin, along with several tremendous colleagues, have allowed me to collect well over 2500 independently oriented samples from several key inferred PTB sections, involving at least 240 distinct sites. At the well-studied Bethulie section, Free State Province, over 120 sites have been established in both Beaufort strata and several <2 m wide Karoo LIP dikes. Strata well-removed from dikes yield both normal and reverse polarity ChRM. The first-removed RM in sedimentary rocks is always a NNW seeking, moderate to steep negative inclination ChRM (normal polarity); NRM intensities are typically ~1 to 5 mA/m. A stratigraphic interval involving over ten sites in discrete beds, the top of which is located some 4 m below the often-cited “event bed” Permian/Triassic boundary interval is dominated by a well-defined reverse RM with a normal overprint RM unblocked below 400oC, implying elevated temperatures (i.e., ~ 100 to 250oC+) for ca. 1 Ma (+/-). The lower part of the section, extending close to the Caledon River, is exclusively of normal polarity. Contact tests are positive but complicated. Documentation of a primary RM in these strata, which appears in some areas to be preserved, requires careful laboratory- and field-based assessment. At Farm Nooitgedacht (“Neverland”), where previous workers have identified the position of the DAZ to LAZ boundary, we have, for the first time in upper Beaufort Group strata, a thin, pristine ash fall deposit. The high precision U-Pb zircon age data from this ash, in combination with magnetostratigraphic, palynostratigraphic, and geochemical observations are in a manuscript that has passed the first of two hurdles in Nature Communications. The new results have profound implications for previous interpretations of any turnover in vertebrates that may have occurred in relation to the end-Permian extinction event.

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