RE-OS DEPOSITIONAL AGES FOR NEOPROTEROZOIC POST-GLACIAL BLACK SHALES IN AUSTRALIA: EVIDENCE FOR DIACHRONOUS NEOPROTEROZOIC GLACIATIONS

Fine-grained sedimentary rocks of the Neoproterozoic Aralka Formation (Amadeus Basin, central Australia) and Tapley Hill Formation (Adelaide Rift Complex, southern Australia) overlie Areyonga and Sturtian diamictites, respectively, and have been correlated on the basis of lithostratigraphy and carbon isotope chemostratigraphy. Both glacial deposits show distinctive geological and geochemical characteristics (e.g., distinctive carbonates that cap glacial deposits and which exhibit unusual negative-to-positive carbon isotope excursions) also observed in other Neoproterozoic diamictites worldwide and these observations have been interpreted as evidence for a putative global “Sturtian” ice age at ca. 700 Ma. We have obtained new Re-Os depositional ages of 657.2 ± 5.4 Ma (n = 10, 2σ, Model 1, MSWD = 1.2) and 643.0 ± 2.4 Ma (n = 15, 2σ, Model 1, MSWD = 1.1) from immediately post-glacial black shales of the basal Aralka Formation and Tindelpina Shale Member (basal Tapley Hill Formation), respectively, that indicate these rock units were not deposited contemporaneously, thus indicating that the Areyonga and Sturtian diamictites do not represent a synchronous ice age. Additionally, these glacial intervals are much younger than other dated intervals (ca. 750-685 Ma from western North America, Oman, and Namibia) with which they have been previously correlated. Given the U-Pb ages of ca. 582-575 Ma for younger “Marinoan” Elatina correlatives in Tasmania, the Neoproterozoic Sturtian and Marinoan glaciations of southern Australia are significantly younger (ca. 643 Ma and 580 Ma) than the previously assigned ages of ca. 700 and 600 Ma. Our new Re-Os ages provide geochronological evidence for diachronous Neoproterozoic glaciations of regional extent and preclude the “snowball” or “slushball” Earth models as the only explanations for the origin of Neoproterozoic glacial rocks. When considered in conjunction with geological evidence for actively functioning hydrological cycles and oscillatory waxing and waning of ice sheets found in some Neoproterozoic glacial deposits, it is probable that ice ages more akin to those of the familiar Pleistocene Epoch may have existed during Neoproterozoic times.