Taken with a grain of salt: Resolving evaporite stratigraphy through accessory mineral geochronology

Rock salt caverns enable large-scale underground hydrogen storage, with potential significance for the green energy transition. However, in many salt-bearing basins, salt stratigraphy remains enigmatic due to inherent stratigraphic mobility and a lack of direct age constraints. Here, we present age and geochemistry of apatite and zircon grains from salt within the Canning Basin, Western Australia, which hosts Australia’s most extensive evaporite unit. The recovered apatite grains have shape, oxygen isotope, and geochemical affinity with magmatic volcanic crystals. The apatite grains yield a U–Pb age of 482 ± 13 Ma, coeval with volcanic ash beds elsewhere in the local stratigraphy. A syn-depositional interpretation of these apatite constrains the timing of salt deposition to ca. 482 Ma, ca. 40 Myr older than more well-constrained evaporites elsewhere in the Canning Basin, demonstrating multiple salt generations. This temporal constraint is supported by a maximum depositional age of 483 ± 12 Ma from detrital zircon grains. Previously unrecognised volcanic and clastic inputs of accessory minerals into salt provide a new means to constrain the stratigraphy and evolution of evaporitic basins, despite the mobility of salt. Such insights are critical for delineating the extent and geometry of salt bodies, whether bedded or diapiric, and thereby enhance site selection and risk assessment for hydrogen storage.