Timescales of speleogenesis in an evolving syngenetic karst: The Tamala Limestone,Western Australia

Abstract

The Tamala Limestone, a vast Quaternary carbonate sequence exposed on the coast of Western Australia, forms one of the most geographically-extensive deposits of carbonate aeolianite in the world. It is also considered an excellent example of so-called ‘syngenetic karstification’, where the processes of lithification and karst development occur in close temporal association. Speleothems, hosted in caves throughout the region, provide an opportunity to explore the timescales of speleogenesis in such a young karst environment. A compilation of existing limestone ages suggests that the unit has been forming since at least 800 ka, mediated by glacial-interglacial cyclicity and with older units identified in the extreme south. We present 225 new U-Th and U-Pb age determinations on speleothem ‘rubble’ — providing minimum ages for cave formation — which reveal that speleogenesis has also occurred throughout the region for at least the last 200 kyr. Speleothems from Jewel Cave, the oldest yet documented from the region, extend far beyond the range of the U-Th chronometer, with U-Pb dating revealing a distinct early phase of cave formation extending from around 2 Ma. These data confirm previous suggestions that extensive karstification can occur rapidly, within a few tens to hundreds of thousands of years after limestone formation, but also reveal that initial deposition of the Tamala Limestone units began much earlier than previously suspected.