Isotope stratigraphy of the Neoproterozoic Togari Group, Tasmania

A carbon and strontium isotope study of the Togari Group, northwest Tasmani a, provides significant new age constraints based on chemostratigraphic cor relation with key Neoproterozoic sections on other continents. The Black Ri ver Dolomite is late Riphean (middle Cryogenian: ca 750-650 Ma) on delta(13 )C, Sr-87/Sr-86 and stromatollte-biostratigraphic evidence. In the upper pa rt of the Black River Dolomite, a diamictite unit (the Julius River Member) is associated with a negative delta(13)C 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 Neoproterozoi c III in age (ca 650-580 Ma). The Smithton Dolomite is middle to late Neopr oterozoic III (i.e. Ediacarian/Vendian: ca 580-545 Ma) on delta(13)C and Sr -87/Sr-86 evidence. The rise in Sr-87/Sr-86 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 Block River Dolomite and lower Smithton Dolomite are syndepositional or early diagenetic and little altered in delta(13)C. In contrast, crystalline. isotopically altered massi ve dolostones comprise most of the upper two-thirds of the Smithton Dolomit e, but minor interbedded limestones retain little-altered delta(13)C and Sr -87/Sr-86.