Luminescence dating of spring mound deposits in the southwestern Great Artesian Basin, northern South Australia

Abstract

Artesian spring mound deposits in the southwestern part of the Great Artesian Basin arise from groundwater discharge from flowing springs in the southern and western margins which reaches the surface through faults and weaknesses in thin confining beds overlying the Mesozoic artesian aquifers. Carbonate in solution in the artesian groundwater is deposited by many springs as tufa, building ‘spring mounds.’ Active flowing and dry inactive spring mounds occur in a variety of sizes and shapes. The ages of a representative selection of spring mound deposits have been found by luminescence dating of quartz sand grains that have been incorporated in the mound deposits. The spring deposits of the active flowing springs, Big Bubbler, Blanche Cup and Beresford Spring have ages of 15.1 ± 2.2, 10.9 ± 1.5 and 13.9 ± 1.0 ka, respectively. Spring complexes with both active flowing and dry extinct mounds, Strangways and Beresford Springs, have ages of 60 ± 8 and 219 ± 35 ka, respectively. Spring deposits of springs which ceased flowing and are overlying pedestals of Cretaceous Bulldog Shale and of substantial height (up to >45 m above the surrounding plain) of Beresford Hill and Kewson Hill gave ages of 128 ± 33 ka and 400 ± 100 ka, respectively. Elizabeth Springs is a large spring mound, and parts contain active flowing springs; a sample from a dry inactive part gave an age of 740 ± 120 ka. The analyses for radioactive content, necessary for the estimation of dose rates for luminescence dating, were unusual because the uranium radioactive decay chain is in disequilibrium, with the parent uranium patently unable to support the lower part of the chain. We interpret this as being due to radium being carried up with the spring water from the confined Mesozoic aquifer, which overlies Proterozoic basement rocks. Calculations of dose rates take account of detailed measurements on individual nuclides and differ from site to site.