Age and composition of oxalate-rich crusts in the Northern Territory, Australia

The nature, origin and age of some thick multi-layered oxalate-rich crusts from quartz-rich rocks in Australia are presented, with a view to alerting restorers and conservators to their value in palaeo-environmental and rock art dating studies. Mineralogical and geochemical data, together with field observations and evidence from cross-section analysis, suggest that these deposits are formed naturally by chemical reaction of organic acids in rainwater acting on calcium-rich dust particles which have accreted on stable siliceous rock ledges and other sheltered surfaces. Carbon-14 dating of the oxalate mineral, whewellite, found in the surface crusts ranges from modern to 8800 years BP: evidence that the natural processes which form oxalate-rich surface deposits have been continuous for many thousands of years. Such dating of oxalate layers provides a means for establishing time-frames in which different prehistoric painting styles can be fixed; the method does not give a direct age for individual artistic motifs.