Further Paleomagnetic and Rock Magnetic Data Bearing on the Magnetostratigraphy of Upper Permian to Lower Triassic (?) Beaufort Group Strata at Bethulie, Karoo Basin, Free State, South Africa

Understanding the timing of inferred environmental changes in the Karoo Basin, from Late Permian to possibly Early Triassic (?) time, as recorded in Beaufort Group strata, includes efforts to establish robust magnetic polarity records for sections previously interpreted to encompass end-Permian extinction events. The preservation of an early-acquired remanence (RM) in Karoo 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. We report data from over 75 sites collected at the well-studied Bethulie section, Free State Province, which also includes several <2 m wide Karoo LIP dikes. We obtain 7-10+ independent samples per individual horizon to assess ChRM consistency. 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. Site BT15, which is located in strata lying some 4 m below the often-cited "event bed" interval inferred to be the Permian/Triassic boundary is dominated by a well-defined reverse RM with a normal overprint RM unblocked below 400^oC, implying elevated temperatures (i.e., 100 to 250^oC+) 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. Host strata collected less than 1-2 dike widths from dikes have been thermally remagnetized and have high NRM intensities (> 50 mA/m). Collectively we interpret these data to indicate that any ChRM, with the exception of those from host strata in baked contacts, that persists above T_lub of 425^oC is pre-Karoo LIP in origin, and likely primary at the Bethulie locality. Documentation of a primary RM in these strata, which appears in some areas to be preserved, requires careful laboratory- and field-based assessment.