MAGNETOSTRATIGRAPHY OF THE KAROO BASIN, REFINING THE GLOBAL POLARITY TIMESCALE FOR THE LATE PERMIAN AND EARLY TRIASSIC

We have conducted magnetostratigraphic studies from three sites in the Karoo Basin of South Africa with a total stratigraphic thickness of 1.4 km. The age of deposition is constrained by U-Pb SHRIMP dating of zircon from volcanic ashbeds observed in two of these sections, which range in age from ca. 267 Ma to 251 Ma. In all three sections, variable degrees of overprinting are observed with secondary magnetizations removed between 450C and 600C. This partial remagnetization overlaps the Jurassic reference directions for South Africa, and is inferred to be caused by regional re-heating caused by emplacement of mafics sills and dykes of the Karoo Large Igneous Province. U-Th-He ages of volcanic zircon are almost uniformly reset to 180 Ma, the age of the Karoo LIP, indicating regional heating to at least 180C at that time. The oldest section (700m thick) was deposited in the 268-262 Ma age, and displays a mostly reversed magnetostratigraphy punctuated by brief normal zones, demonstrating a post-Kiaman age. A second, 600 m thick section is unconstrained by geochronology, but demonstrates dominantly normal magnetozones similar to the late Permian Global Polarity Timescale (GPTS). The third section (130 m thick) is continuous through the Permo-Triassic boundary, as constrained by occurrences of fossil skulls from Dicynodon and Lystrosaurus species. New SHRIMP dating of volcanic ashes confirms the ca. 252 Ma age of this section, which also preserves magnetozones that show a close correspondence to marine records of the Permo-Triassic GPTS. Implications for late Permian paleogeography will be discussed.