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Illite dates from the World𩀙s oldest caves

K-Ar dating of illitic clays at Jenolan Caves has given the first direct evidence for the great antiquity of eastern Australian caves. It shows that sections of the caves formed in the Visean, before 340 Ma, making them the World’s oldest dated accessible caves by more than 240 Ma. Early Devonian (394 Ma) volcaniclastic palaeokarst was also identified. Permian and Triassic dates for some <0.4 μm fractions indicate growth of acicular illite during burial below the Sydney Basin. As well as raising problems about landform persistence, these dates set a new minimum age for folding in the eastern Lachlan Fold Belt. The most recent “Kanimblan” folding must have occurred prior to 340 Ma.