Precambrian Research 172 (2009) 301–310 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Correlation of Sturtian diamictite successions in southern Australia and northwestern Tasmania by Re–Os black shale geochronology and the ambiguity of “Sturtian”-type diamictite–cap carbonate pairs as chronostratigraphic marker horizons Brian Kendall a,b,∗, Robert A. Creaser b, Clive R. Calver c,TimothyD.Raubd, David A.D. Evans e a School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada c Mineral Resources Tasmania, P.O. Box 56, Rosny Park, Tasmania 7018 Australia d Division of Geological and Planetary Sciences, California Institute of Technology 170-25, Pasadena, CA 91125, USA e Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA article info abstract Article history: Recent geochronological studies have raised concerns regarding the accuracy of regional and global Received 27 October 2008 correlations of Neoproterozoic sedimentary successions using relative dating methods such as carbon Received in revised form 14 May 2009 isotope chemostratigraphy and glaciogenic diamictite–cap carbonate lithological and geochemical char- Accepted 26 May 2009 acteristics. Precise and accurate radio-isotopic age constraints for Neoproterozoic sedimentary rocks thus remain a high priority for establishing a reliable Neoproterozoic chronostratigraphy. Here, we present a Keywords: new Re–Os age of 640.7 ± 4.7 Ma from black shales of the upper Black River Dolomite (Togari Group, Neoproterozoic northwestern Tasmania) that represents a minimum age for deposition of the immediately underlying, Sturtian Black shale diamictite-bearing Julius River Member. The upper Black River Dolomite Re–Os age is statistically identi- ± Re–Os cal to a Re–Os age of 643.0 2.4 Ma previously determined for the Sturtian post-glacial Tindelpina Shale Glaciation Member (Umberatana Group, Adelaide Rift Complex, southern Australia). Consistent with previous lithos- Australia tratigraphic and stromatolite biostratigraphic evidence, the Re–Os ages suggest broadly coeval deposition Tasmania of the Julius River Member and Sturtian diamictites and overlying organic-rich marine shales. Consid- eration of the Neoproterozoic geochronological database suggests that “Sturtian”-type diamictite–cap carbonate pairs cannot be used as chronostratigraphic marker horizons for global correlation of the Neoproterozoic rock record. © 2009 Elsevier B.V. All rights reserved. 1. Introduction zoic global correlation schemes based on these methods commonly assume global (“Snowball Earth”) or near-global (“Slushball Earth”) Global and regional correlation of the Neoproterozoic rock glaciations at ca. 700 Ma (“Sturtian”) and ca. 635 Ma (“Marinoan”) record is significantly hampered by a sparse paleontological record and a glaciation of uncertain extent at ca. 580 Ma (“Gaskiers”). and a general paucity of suitable igneous material for precise However, the number, timing, extent, and duration of Neoprotero- U–Pb zircon age determinations. Consequently, efforts at estab- zoic glaciation remain an ongoing topic of debate (Fairchild and lishing a Neoproterozoic chronostratigraphy are largely based on Kennedy, 2007), so the above tripartite division of Neoprotero- chemostratigraphy (primarily ␦13 C and 87Sr/86Sr from carbon- zoic glacial deposits may be erroneous. For example, Kendall et al. ates) and identification of distinctive lithological and geochemical (2006) show that the archetypal “Sturtian” glaciation of southern characteristics of carbonates capping glaciogenic diamictites (e.g., Australia is significantly younger than ca. 685–750 Ma glaciogenic Kaufman and Knoll, 1995; Kaufman et al., 1997; Kennedy et al., diamictites on other continents previously suggested as correla- 1998; Jacobsen and Kaufman, 1999; Walter et al., 2000; Halverson et tives. al., 2005, 2007; Halverson, 2006; Hoffman et al., 2007). Neoprotero- Carbon, sulfur, and strontium isotope chemostratigraphy are rel- ative dating tools that do not unambiguously provide chronological information and thus require numerical calibration by indepen- ∗ dent radio-isotopic and biostratigraphic data to be useful for global Corresponding author at: School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA. Fax: +1 480 965 8102. correlation schemes (Meert, 2007). Neoproterozoic carbon isotope E-mail address: [email protected] (B. Kendall). chemostratigraphic profiles for similar aged rocks can exhibit a sig- 0301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2009.05.001 302 B. Kendall et al. / Precambrian Research 172 (2009) 301–310 nificant degree of scatter both within and between cratonic blocks 2. Togari Group, northwestern Tasmania because of diagenetic alteration, incomplete stratigraphy, and surface-to-deep ocean ␦13 C gradients (e.g., Melezhik et al., 2001; The paleogeographic position of the Western Tasmania Terrane Knauth and Kennedy, 2005; Fairchild and Kennedy, 2007; Jiang et in the Late Neoproterozoic is not well constrained, but this crustal al., 2007; Giddings and Wallace, 2009). These problems compli- block may have been connected to the East Antarctic margin of the cate attempts at establishing useful composite regional and global Mawson continent before ca. 580 Ma (Berry et al., 2008); paleo- chemostratigraphic profiles. Strontium isotope chemostratigraphy magnetic evidence indicates western Tasmania was connected to shows some promise for correlation of Neoproterozoic sedimen- East Gondwanaland by Cambro-Ordovician boundary time (Li et al., tary successions because of a systematic rise in 87Sr/86Sr during 1997). In the Smithton Synclinorium of northwestern Tasmania, an the Neoproterozoic, but the 87Sr/86Sr curve requires significant unconformity separates unmetamorphosed and gently deformed improvements in sample resolution (Halverson et al., 2007) and will strata of the Togari Group from the underlying Rocky Cape Group generally be an insensitive chronometer at timescales much shorter (Fig. 1). Locally, the coarse-grained Forest Conglomerate defines than the seawater Sr residence time (∼ 2–3 M.y. for the modern the base of the Togari Group and is overlain by 600 m of stroma- ocean; Ravizza and Zachos, 2006). The resolution of the Neopro- tolitic dolostones, organic-rich chert and shale, and diamictite of terozoic sulfur isotope record remains too coarse for detailed global the Black River Dolomite (Calver, 1998; Calver and Walter, 2000). chemostratigraphy, although some broad temporal trends appear Vase-shaped microfossils (remains of testate amoebae; Porter and preserved in some basins (e.g., Fike et al., 2006). Knoll, 2000) are preserved in chert facies of the lower Black River Thus, precise and accurate U–Pb zircon and Re–Os black shale Dolomite (incorrectly termed Smithton Limestone by Saito et al., ages remain a high priority for reliable regional and global correla- 1988). tions of the Neoproterozoic rock record. Radio-isotopic calibration Two lines of evidence suggest a ≤200 m thick diamictite interval of the Neoproterozoic geological timescale is still limited to a rela- (Julius River Member) in the upper Black River Dolomite may be cor- tively small number of precise U–Pb zircon ages from volcanic rocks relative with the thick (up to 6 km) Sturtian glaciogenic diamictites (Evans, 2000; Trindade and Macouin, 2007) and Re–Os ages from of the lower Umberatana Group, Adelaide Rift Complex, southern black shales (Kendall et al., 2004, 2006). Geochronology represents Australia (Fig. 2): (1) iron formation is associated with both Stur- one critical test for distinguishing among hypotheses proposed to tian diamictites (Preiss, 1990, 1993; Lottermoser and Ashley, 2000) explain low-latitude Neoproterozoic glaciation, especially the hard and correlatives of the Julius River Member in western Tasma- “Snowball Earth” (Kirschvink, 1992; Hoffman and Schrag, 2002) nia (Calver, 1998), and (2) dolomite clasts within the Julius River and “Slushball Earth” (Hyde et al., 2000; Schrag and Hoffman, Member contain the stromatolite Baicalia burra (Griffin and Preiss, 2001) hypotheses predicting global and near-global synchronous 1976; Calver, 1998), that is also present within the pre-Sturtian glaciations, respectively, and the “Zipper-rift” hypothesis suggest- Skillogalee Dolomite, Burra Group (Belperio, 1990; Preiss, 1990). ing diachronous glaciation on uplifted rift margins associated with Lithological (stromatolitic dolostone and organic-rich chert) and ␦13 ∼ Rodinia breakup (Eyles and Januszczak, 2004). The “High Obliquity, carbon isotope ( Ccarb +2‰ to +6‰) similarities may suggest Low-latitude Ice, STrong seasonality” (HOLIST) hypothesis would correlation of the Skillogalee Dolomite and Black River Dolomite permit either synchronous or diachronous glaciation preferentially (Schidlowski et al., 1975; Veizer and Hoefs, 1976; Belperio, 1990; in low-latitudes (Williams, 2008), but this model has been chal- Calver, 1998). lenged by both theoretical and empirical evidence (Maloof et al., In the North Flinders Zone and Baratta Trough of the Adelaide 2002; Evans, 2006). Rift Complex, the Sturtian glacial succession was deposited by The likelihood of tectonic proximity between Western Tasma- sediment gravity flows contemporaneously with two glacial