Isotope stratigraphy of the Neoproterozoic Togari Group, Tasmania

Abstract

A carbon and strontium isotope study of the Togari Group, northwest Tasmania, provides significant new age constraints based on chemostratigraphic correlation with key Neoproterozoic sections on other continents. The Black River Dolomite is late Riphean (middle Cryogenian: ca 750–650 Ma) on δ13C, 87Sr/86Sr and stromatolite‐biostratigraphic evidence. In the upper part of the Black River Dolomite, a diamictite unit (the Julius River Member) is associated with a negative δ13C excursion and may correlate with the Sturtian glaciation. The succeeding rift volcanics and clastics of the Kanunnah Subgroup, and the probably correlative Crimson Creek Formation of western Tasmania, are inferred to be late Cryogenian to early Neoproterozoic III in age (ca 650–580 Ma). The Smithton Dolomite is middle to late Neoproterozoic III (i.e. Ediacarian/Vendian: ca 580–545 Ma) on δ13C and 87Sr/86Sr evidence. The rise in 87Sr/86Sr through the Smithton Dolomite (0.7079–0.7085) is consistent with previous work showing a monotonic rise in the ratio through the Vendian. Dolostones in the Black River Dolomite and lower Smithton Dolomite are syndepositional or early diagenetic and little altered in δ13C. In contrast, crystalline, isotopically altered massive dolostones comprise most of the upper two‐thirds of the Smithton Dolomite, but minor interbedded limestones retain little‐altered δ13C and 87Sr/86Sr.