New U-Pb CA-IDTIMS isotopic age tie points for the Lightjack Formation, Canning Basin, Western Australia

Abstract

Ash-fall tuffs are extremely rare in the Permian of Western Australia, but recent coal exploration boreholes in the Fitzroy Trough of the Canning Basin have intersected several in the ~30-300 m thick Lightjack Formation. We here report four high-precision U-Pb zircon CA-IDTIMS ages of these tuffs. The formation contains marine to non-marine facies, and extends along the major depocentre of the Canning Basin (Fitzroy Trough - Gregory Sub-basin) and its margins, where it is sporadically exposed at localities including Lightjack Hill (type section), the Noonkanbah area, Liveringa Ridge and in Shore Range. The formation is dominated by siltstone and calcareous to ferruginous sandstone, with minor coal and fossiliferous beds near its base. It has previously been assigned a Roadian-Wordian age based on ammonoids ('Daubichites goochi' and 'Bamyaniceras australe') and brachiopods ('Neochonites (Sommeriella) afanasyevae' Zone), whereas palynomorphs ('Dulhuntyispora granulata' to 'D. parvithola' Zones) indicate a slightly younger age for the upper part of the formation.