NEOPROTEROZOIC PALEOGEOGRAPHY, RODINIA BREAKUP, AND STURTIAN GLACIATION: CONSTRAINTS FROM RE-OS BLACK SHALE AGES FROM SOUTHERN AUSTRALIA AND NORTHWESTERN TASMANIA

Black shales overlying diamictites of the Julius River Member (Black River Dolomite, Togari Group, northwestern Tasmania) yield a Re-Os date of 640.7 ± 4.7 Ma (2σ, n = 19, MSWD = 0.91, Model 1) that is identical (given 2σ uncertainties) to the Re-Os age of 643.0 ± 2.4 Ma¹ for the Sturtian post-glacial Tindelpina Shale Member (Umberatana Group, Adelaide Rift Complex). Consistent with previous lithostratigraphic (e.g., iron formation-diamictite association), chemostratigraphic (low ⁸⁷Sr/⁸⁶Sr values in the Black River Dolomite, and moderately positive δ¹³C_carb in stratigraphically underlying carbonates), and biostratigraphic (Baicalia burra stromatolites in the pre-Sturtian Skillogalee Dolomite and Julius River Member diamictite clasts) evidence, the new Re-Os ages suggest correlation of the Julius River Member with Sturtian diamictites. Remarkable litho- and chemo-stratigraphic similarities have been previously observed for cap carbonates and post-glacial shales (e.g., shale-hosted fossil benthic microbial mats) above the Elatina Formation and Cottons Breccia (Grassy Group, southeastern King Island). Collectively, these observations suggest a tectonic affinity between southeastern Australia, King Island, and western Tasmania during the Sturtian and Elatina glaciations.

Recent U-Pb zircon age constraints suggest the waxing phase of Sturtian glaciation in southern Australia is younger than ca. 658 Ma². Paleomagnetic data indicate the AUSMEX (Australia-Mexico) Rodinia connection between eastern Antarctica – Australia and southernmost Laurentia, if valid at all, was terminated prior to 755 Ma. Sturtian glacial and post-glacial deposition (ca. 658-643 Ma) significantly post-dates ca. 827-777 Ma magmatic activity in the Adelaide Rift Complex associated with Rodinia supercontinent breakup. Thus, marine inundation associated with the onset of continental separation and passive margin formation likely pre-dates Sturtian glaciation, and may be represented by sandstone and fine-grained siliciclastic and carbonate strata in the middle Burra Group. Sturtian glaciation itself may have been associated with an episode of crustal extension and uplift that did not result in a major phase of continental breakup.

¹Kendall et al. (2006) Geology 34, 729-732. ²Fanning and Link (2006) GSA abstract.