U-Pb zircon age constraints on the vertebrate assemblages and palaeomagnetic record of the Guadalupian Abrahamskraal Formation, Karoo Basin, South Africa

Abstract

The rich fossil record of the Permo-Triassic Beaufort Group of the main Karoo Basin serves as a reference sequence for tetrapod biochronology and provides a unique window into the evolution of terrestrial ecosystems between the Guadalupian and Middle Triassic. Absolute temporal constraints on the Beaufort Group have improved in the last decade with the application of U-Pb zircon geochronology but precise ages have not yet been reported for its basal Abrahamskraal Formation. Consequently, the age of the Eodicynodon Assemblage Zone and the lower boundary of the Tapinocephalus Assemblage Zone have remained less clear than younger Permian biozones. An improvement in temporal constraints also has implications for the correlation of palaeomagnetic logs reported from the Abrahamskraal Formation. In this paper we present new CA-ID-TIMS U-Pb geochronology from tuff beds that provide the first high-resolution chronostratigraphy for the upper two thirds of Abrahamskraal Formation. These ages constrain the base of the Tapinocephalus Assemblage Zone to be older than 264.4 Ma, and closely constrain the boundary between its two subzones to ca. 262 Ma. Combined with published ages from the overlying Teekloof Formation, our data show the Tapinocephalus Assemblage Zone is latest Wordian to late Capitanian in age. These complement previously published CA-ID-TIMS U-Pb ages for younger Beaufort rocks and suggest that the normal polarity interval observed in the middle Abrahamskraal Formation represents a normal (probably third) chron within the mixed-polarity Illawarra Superchron.