Wordian (Middle Permian) U-Pb CA-IDTIMS isotopic ages from the Lightjack Formation, Canning Basin, Western Australia

Abstract

Coal exploration coreholes in the Permian of the Fitzroy Trough (Canning Basin, Western Australia) have intersected thin (<15 cm) ash-fall tuffs in seams from the 30-300-m thick Lightjack Formation. Four of these tuffs within the 'Didecitriletes ericianus' spore-pollen zone have yielded high-precision U-Pb zircon CA-IDTIMS ages of 268.63-270.14 Ma, consistent with a Wordian age. The formation is dominated by siltstone and sandstone, with minor coal and fossiliferous beds near the base, and extends along the major depocentre of the basin (Fitzroy Trough - Gregory Sub-basin), and its margins. The unit spans the Roadian-Wordian-Capitanian based on ammonoids ('Daubichites goochi' and 'Bamyaniceras australe'), brachiopods ('Neochonites ('Sommeriella') afanasyevae' zone), and palynomorphs ('Dulhuntyispora granulata' to 'D. parvithola' zones). Rare conodont genera from the formation include 'Vjalovognathus', 'Mesogondolella' and 'Hindeodus'.