New Insights on the Age and Stratigraphy of the Cisuralian Succession in the Cooper Basin, Australia, Based on U–Pb CA-TIMS Dating of Volcanic Air-Fall Tuffs

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Absolute age constraints for Cisuralian units in the Cooper Basin have been hampered by poorly calibrated palynostratigraphic schemes, with implications for pan-Australian and global correlations. New U–Pb CA-TIMS (uranium–lead chemical abrasion-thermal ionisation mass spectrometry) dates acquired from volcanic air-fall tuff beds from the Patchawarra Formation combined with legacy palynological data offer new insights on the age, stratigraphy and tectonic history of the basin. Six tuffs from different regions of the Cooper Basin give dates from 284.87 to 277.36 Ma, which suggest that the Patchawarra Formation was deposited between the Artinskian and Kungurian rather than from the Sakmarian to Kungurian, as previously thought. The new dates indicate a possible unconformity at the base of the Patchawarra Formation and the underlying Tirrawarra Sandstone. They also suggest that the overlying REM succession (Roseneath Shale, Epsilon Formation and Murteree Shale) is entirely Guadalupian age rather than ranging from the Cisuralian to the Guadalupian. Consequently, existing Cisuralian and Guadalupian high-resolution spore-pollen zonations should be used with care when applied in conjunction with U–Pb CA-TIMS dates. The tuff dates in the thin Patchawarra Formation over the prominent Gidgealpa–Merrimelia–Innamincka Ridge within the Cooper Basin support the interpretation that the ridge was an area of relatively slow subsidence rather than being a positive feature during deposition. Compacted sediment accumulation rates of between 25 and ∼40 m/My for this unit are faster than for most cratonic basins, which raise questions on the tectonic setting of the Cooper Basin. The use and further acquisition of tuff dates will enhance correlations and facilitate improved paleogeographic reconstructions and our understanding of the late Paleozoic ice age.