Detrital zircon U–Pb geochronology of the Mallowa Salt in the Canning Basin, Western Australia

Understanding the stratigraphy of buried rock salt formations is important for evaluating their potential to store hydrogen in underground salt caverns. However, efforts to resolve salt stratigraphy are frequently hindered by the lack of robust age constraints on salt deposition. Recently, the dating of accessory mineral phases has been shown to improve constraints on depositional ages and enhance stratigraphic interpretations of large Australian salt units. Here we present new zircon U–Pb data from Australia’s largest known bedded salt unit, the Mallowa Salt in the Canning Basin, Western Australia. Zircon grains extracted from two salt samples are predominantly subhedral in morphology and yield youngest age clusters of 485±5Ma and 488±8Ma. These results suggest an Early Ordovician maximum depositional age (MDA) of ca 485Ma, which is approximately 40Ma older than its previously proposed Late Ordovician to earliest Silurian depositional age (DA) of ca 440Ma inferred from palynology. Independent DA constraints from fossils and volcanic ash beds in the underlying strata corroborate a DA of ca 440Ma. The Mallowa Salt is younger than, and stratigraphically distinct from, the Frome Rocks salt recently dated through accessory minerals to ca 482Ma. More broadly, these results demonstrate that in the Canning Basin, discrepancies between zircon U–Pb MDAs and true DAs are controlled by the presence or absence of local coeval volcanic activity. Only where volcanism is demonstrably contemporaneous with sedimentation do MDAs meaningfully approximate depositional ages; such synchrony is expected to arise only in particular geodynamic and basin settings that sustain coeval magma flux and sediment accumulation. These findings have important implications for refining salt stratigraphy in the basin and assessing the limitations of relying solely on MDAs for constraining depositional ages more broadly.