Constraints on the timing of ''Sturtian'' glaciation

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(ID-NTIMS) following the protocols outlined in Kendall et al. ... 1.2, 2 uncer- tainty, initial 187Os/188Os ratio [IOs] ... Tindelpina Shale Re-Os dates and IOs values.
Re-Os geochronology of postglacial black shales in Australia: Constraints on the timing of ‘‘Sturtian’’ glaciation Brian Kendall ⎤ Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, ⎥ Robert A. Creaser ⎦ Edmonton, Alberta T6G 2E3, Canada David Selby Department of Earth Sciences, University of Durham, Durham DH1 3LE, UK ABSTRACT New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ⴞ 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ⴞ 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. 685–750 Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the ‘‘Sturtian’’ ice age was markedly diachronous, and/or there was more than one ‘‘Sturtian’’-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than ‘‘snowball’’ or ‘‘slushball’’ Earth ice ages. Keywords: Neoproterozoic, age determination, Australia, black shale, rhenium, osmium. INTRODUCTION Contrasting interpretations of the geological, geochemical, and paleomagnetic records have resulted in a variety of hypotheses to explain the origin of Neoproterozoic glacial deposits. These include global (‘‘snowball Earth;’’ Hoffman and Schrag, 2002) and nearglobal (‘‘slushball Earth;’’ Hyde et al., 2000) ice ages, preferential low-latitude ice sheets resulting from an increased tilt (⬎54⬚) of the Neoproterozoic Earth’s spin axis (‘‘high orbital obliquity;’’ Williams and Schmidt, 2004), and widespread but regional and diachronous glaciation during protracted breakup of the supercontinent Rodinia (‘‘zipper-rift;’’ Eyles and Januszczak, 2004). Distinguishing among these hypotheses requires more rigorous geochronologic and paleomagnetic control on the Neoproterozoic rock record than currently exists. In the absence of a well-defined chronoand biostratigraphic Neoproterozoic framework, the Sturtian and Marinoan glacial deposits of southern Australia have served as marker horizons for global correlation schemes based largely upon litho- and chemostratigraphy (Kennedy et al., 1998; Walter et al., 2000; Halverson et al., 2005). Recently, direct U-Pb zircon age determinations for some glacial deposits have led to a new general consensus for three major Neoproterozoic glaciations at 580 Ma (‘‘Gaskiers;’’ Bowring et al., 2003), 635 Ma (‘‘Marinoan;’’ Hoffmann et al., 2004), and 712 Ma (‘‘Sturtian’’; Allen et al., 2002). Nevertheless, the number, timing, extent,

and duration of glaciation remain uncertain because of a general paucity of suitable material for radiometric dating of most Neoproterozoic glacial deposits. For example, direct radiometric age constraints for the southern Australia glacial intervals are particularly poor, comprising a U-Pb zircon age (777 ⫾ 7 Ma; Preiss, 2000) from volcanic rocks ⬎6 km below Sturtian glacial deposits, and imprecise Rb-Sr whole-rock shale dates from the Sturtian postglacial Tapley Hill Formation (750 ⫾ 53 Ma), interglacial Enorama Shale and Trezona Formation (690 ⫾ 21 Ma), and Marinoan postglacial Brachina Formation (601 ⫾ 68 Ma) (McKirdy et al., 2001). The Marinoan Elatina Formation of the Adelaide Rift Complex has been correlated with the Crole Hills Diamictite, Tasmania (maximum age of 582 ⫾ 4 Ma), and Cottons Breccia, King Island (minimum age of 575 ⫾ 3 Ma) (U-Pb sensitive high-resolution ion microprobe [SHRIMP] zircon ages on associated igneous rocks; Calver et al., 2004). A three-point ReOs date of 592 ⫾ 14 Ma obtained for organicrich siltstones (Aralka Formation) separating the Areyonga and Olympic glacial deposits in central Australia (Schaefer and Burgess, 2003) has been challenged because it is not consistent with global correlation schemes (Hoffmann et al., 2004). Precise age constraints for the Australian Neoproterozoic successions are thus required to test proposed global correlation schemes and the synchronicity of ice ages. Here we present new Re-Os depositional ages from two postglacial black shales in Australia that con-

strain the timing of glacial rocks used for global correlation of a putative global Sturtian ice age. NEOPROTEROZOIC GLACIATION IN CENTRAL AND SOUTHERN AUSTRALIA Within the Adelaide Rift Complex of southern Australia, the Sturt–Appila–Pualco–Bolla Bollana–Merirjina Tillites and WilyerpaLyndhurst Formations (Yudnamutana Subgroup) represent the waxing and waning stages of the Sturtian glaciation, respectively (Preiss et al., 1998). The glaciogenic Areyonga Formation (Amadeus Basin, central Australia; Walter et al., 1995) has been correlated with the Sturtian glacial deposits on the basis of litho- and chemostratigraphy (e.g., Walter et al., 2000) (Fig. 1A). Deposition of these glaciogenic sediments was accompanied by rifting in the Adelaide Rift Complex (Preiss, 2000) and tectonically induced faulting in the Amadeus Basin (Walter et al., 1995). Overlying the Sturtian and Areyonga glacial deposits are dolomitic siltstones and shales of the Tapley Hill and Aralka Formations, respectively, deposited during widespread postglacial transgression (Preiss, 2000). A hiatus of unknown duration separates the Aralka Formation from the overlying glaciogenic Olympic Formation, interpreted as a Marinoan equivalent (e.g., Walter et al., 2000). Following initiation of continental separation in the Adelaide Rift Complex during Sturtian postglacial times, sag-phase sedimentation continued (Brighton Limestone and Lower Upalinna Subgroup) followed by a marine transgression (Upper Upalinna Subgroup), and ultimately, Marinoan glaciation (Yerelina Subgroup) (Preiss, 2000). Distinctive geological and geochemical features (e.g., cap carbonates and their negative ␦13C excursions) associated with the glacial intervals in Australia and other continents have been interpreted as evidence for global or near-global ‘‘Sturtian’’ and ‘‘Marinoan’’ ice ages (Kennedy et al., 1998; Hoffman and Schrag, 2002). SAMPLES AND ANALYTICAL METHODS Dolomitic, organic-rich shales (total organic carbon [TOC] ⫽ 0.5%–1%; McKirdy et al., 2001; Schaefer and Burgess, 2003) were sam-

䉷 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; September 2006; v. 34; no. 9; p. 729–732; doi: 10.1130/G22775.1; 2 figures; Data Repository item 2006153.

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cursion (also seen for ␦13Corg in SCYW-1a) at the base of the Tindelpina Shale Member in Blinman-2 is immediately followed by increasingly more positive ␦13C values upsection (Walter et al., 2000; McKirdy et al., 2001), a trend considered typical of Sturtian postglacial ‘‘cap carbonate sequences’’ (Kennedy et al., 1998; Halverson et al., 2005). Our samples from both drill holes are located stratigraphically beneath the most negative ␦13C values. Our Aralka samples (drill hole Wallara-1, central Amadeus Basin, depth 1298.2–1300.2 m) occur ⬃7–9 m stratigraphically above the contact with the underlying Areyonga diamictites. This contact appears conformable in drill cores, whereas an abrupt contact exists between the Aralka Formation and the overlying Olympic Formation. Thus, the very thin stratigraphic thickness of the Aralka Formation preserved in Wallara-1 (⬃27 m) may not result from a condensed section, but rather reflect the presence of a lacuna unconformity between the Aralka and Olympic Formations at ⬃1281 m, such that only the lowermost Aralka strata are preserved in this drill hole (e.g., Gorjan et al., 2000). Based on kerogen atomic H/C ratios, thermal alteration of both stratigraphic successions sampled is minimal (Calver, 2000; McKirdy et al., 2001). Re and Os concentrations and isotopic compositions were measured through isotope dilution–negative thermal ionization mass spectrometry (ID-NTIMS) following the protocols outlined in Kendall et al. (2004), including the CrVIH2SO4 digestion method for organic-rich sedimentary rocks. Regressions were performed with the program Isoplot V.3.0 (Ludwig, 2003) using a 187Re decay constant of 1.666 ⫻ 10⫺11 yr⫺1 (Smoliar et al., 1996), 2␴ uncertainties for 187Re/188Os and 187Os/188Os isotope ratios, and the error correlation function (rho) (Kendall et al., 2004).

Figure 1. A: Location of sampled drill cores in the Amadeus Basin and Adelaide Rift Complex and accompanying stratigraphic columns showing proposed correlations of Neoproterozoic strata between these regions (from Walter et al., 2000). B: Stratigraphic columns and carbon isotope data for each drill hole (from Walter et al., 2000, and McKirdy et al., 2001). Re-Os ages and sampling intervals (shaded regions) in the chemostratigraphic profiles are from this study; Re-Os ages shown in the Wallara-1 stratigraphic column (representing sampled intervals of 1294–1304 m and 1297.3–1298.9 m) are from Schaefer and Burgess (2003).

pled from the basal Aralka Formation and Tindelpina Shale Member (basal Tapley Hill Formation) (Fig. 1B). Tindelpina Shale samples were obtained from two drill holes located ⬃160 km apart (Blinman-2, Central Flinders Zone, depth 1610.0–1610.9 m; and SCYW-1a, 730

Stuart Shelf, depth 1369.3–1370.1 m). The Blinman-2 and SCYW-1a samples are 7–8 m and 3–4 m above the contact with the underlying Sturtian glacial deposits, respectively, which appears conformable in both drill cores (Gorjan et al., 2000). A negative ␦13Cdol ex-

Re-Os RESULTS Re and Os abundances in the Aralka Formation range from 4.1 to 5.9 ppb, and 82 to 151 ppt, respectively, with 187Re/188Os and 187Os/188Os ratios between 252 and 447, and 3.59 and 5.75, respectively. The Tindelpina Shale Member contains similar abundances of Os (93–145 ppt) but displays a greater range in Re content (1.7–10.0 ppb) and 187Re/188Os (102–1010) and 187Os/188Os (2.06–11.83) ratios. Re-Os isotope data for the Aralka Formation yield a Model 1 Re-Os date of 657.2 ⫾ 5.4 Ma (⫾0.8%, n ⫽ 10, mean square of weighted deviates [MSWD] ⫽ 1.2, 2␴ uncertainty, initial 187Os/188Os ratio [IOs] ⫽ 0.82 ⫾ 0.03) (Fig. 2A). The Re-Os isotope data for the Tindelpina Shale Member from the Blinman-2 and SCYW-1a drill holes yield GEOLOGY, September 2006

Figure 2. Re-Os isochron diagrams for Australian postglacial shales. A: Aralka Formation, Amadeus Basin. B: Tindelpina Shale Member, Adelaide Rift Complex. Circle diameters are larger than calculated error ellipses and have been used for clarity. MSWD—mean square of weighted deviates.

Model 1 Re-Os dates of 645.1 ⫾ 4.8 Ma (⫾0.7%, n ⫽ 11, MSWD ⫽ 1.2, IOs ⫽ 0.95 ⫾ 0.01) and 647 ⫾ 10 Ma (⫾1.5%, n ⫽ 4, MSWD ⫽ 0.79, IOs ⫽ 0.89 ⫾ 0.12), respectively, that are equivalent within 2␴ uncertainties (Fig. 2B). Combining the two data sets for the Tindelpina Shale Member yields a Model 1 date of 643.0 ⫾ 2.4 Ma (⫾0.4%, n ⫽ 15, MSWD ⫽ 1.1, IOs ⫽ 0.95 ⫾ 0.01). TIMING OF STURTIAN AND AREYONGA POSTGLACIAL SEDIMENTATION Given the excellent linear fit of our Re-Os data for the Aralka Formation and Tindelpina Shale Member (see the GSA Data Repository1), and the demonstrated precision and accuracy of Re-Os geochronology of organicrich sedimentary rocks (including cross-calibration with the U-Pb zircon chronometer; Selby and Creaser, 2005), we interpret the Re-Os isochron dates to represent depositional ages corresponding to postglacial 1GSA Data Repository item 2006153, Re-Os data for the Aralka Formation and Tindelpina Shale Member, is available online at www.geosociety.org/ pubs/ft2006.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

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sedimentation. Our new Aralka Re-Os age of 657.2 ⫾ 5.4 Ma is significantly older than the previously reported three-point Re-Os date of 592 ⫾ 14 Ma (or the seven-point date of 623 ⫾ 18 Ma; Schaefer and Burgess, 2003) and indicates the previously determined ages may reflect sampling and/or analytical artifacts related to sample digestion using inverse aqua regia (concentrated HCl and HNO3) (Kendall et al., 2004). The similarity of the individual Tindelpina Shale Re-Os dates and IOs values for the Blinman-2 and SCYW-1a drill cores is consistent with the inferred synchronicity of the sampled shales suggested by their stratigraphic position beneath the postglacial ␦13C excursions. Thus, we consider the pooled ReOs age of 643.0 ⫾ 2.4 Ma to be the best estimate of the depositional age of the Tindelpina Shale Member. Because of the close stratigraphic proximity of the sampled shales to the conformably underlying diamictites, the Re-Os ages provide a tight minimum age constraint for the end of the Sturtian and Areyonga glaciations. Both the combined Tindelpina Shale Re-Os age of 643.0 ⫾ 2.4 Ma and the Blinman-2 ReOs age of 645.1 ⫾ 4.8 Ma are younger (given 2␴ uncertainties) than the Aralka Re-Os age of 657.2 ⫾ 5.4 Ma. These data suggest diachronous deglaciation and postglacial sedimentation, or synchronous deglaciation but different sediment accumulation rates for the Aralka Formation and Tindelpina Shale Member. The latter scenario would require very slow sedimentation rates for the Tindelpina Shale Member relative to the Aralka Formation to explain the observed Re-Os ages. However, this seems unlikely given the ⬎1600 m and ⬎100 m thickness of the Tapley Hill Formation and Tindelpina Shale Member, respectively, in Blinman-2, and the ⬃27 m thickness of the Aralka Formation in Wallara1. Alternatively, the end of the Sturtian-Areyonga glaciation was synchronous, but the onset of postglacial sedimentation was diachronous because of differences in depositional environment between the Tapley Hill Formation (developing passive margin) and Aralka Formation (intracratonic basin) (Preiss, 2000). However, this requires a significant hiatus between the Wilyerpa Formation and Tindelpina Shale Member to explain the younger Blinman-2 Re-Os age. No obvious hiatus is observed, although this could reflect the deepwater environment of deposition recorded in this core (McKirdy et al., 2001). If the end of the Sturtian and Areyonga glaciations was indeed diachronous, the timing of glaciation in southern and central Australia may have still overlapped, considering the great thickness of Sturtian glacial deposits (e.g., ⬎6 km in the North Flinders Zone; McKirdy et al., 2001).

IMPLICATIONS FOR GLOBAL CORRELATIONS Previous radiometric age constraints for the ‘‘Sturtian’’ ice age on other continents include U-Pb zircon ages of 684 ⫾ 4 and 685 ⫾ 7 Ma for the Edwardsburg Formation (central Idaho; Lund et al., 2003), 711.8 ⫾ 1.6 Ma for the Ghubrah Formation (Oman; Allen et al., 2002), between 667 ⫾ 5 and 709 ⫾ 5 Ma for the Scout Mountain Member, Pocatello Formation (southern Idaho; Fanning and Link, 2004), between 663 ⫾ 4 and 761 ⫾ 8 Ma for the Changan and Tiesiao Formations (southern China; Zhou et al., 2004), and between 741 ⫾ 6 and 771 ⫾ 6 Ma for the Kaigas Formation (southwestern Namibia; Frimmel et al., 1996, 2001). The minimum Re-Os age constraints of 643.0 ⫾ 2.4 and 657.2 ⫾ 5.4 Ma for the Sturtian and Areyonga glaciations suggest they are not likely to be correlative with the above ca. 685–750 Ma glacial intervals, although the duration of glaciation in Australia is unknown and some overlap in ages is possible. Nevertheless, the end of the ‘‘Sturtian’’ ice age was likely markedly diachronous worldwide, and/ or there was more than one ‘‘Sturtian’’-type glaciation. Global correlations of ‘‘Marinoan’’ glacial deposits are based in part on preglacial negative ␦13C excursions (Trezona anomaly) and cap carbonate litho- and chemostratigraphy (Kennedy et al., 1998; Hoffman and Schrag, 2002; Halverson et al., 2005). Precise age constraints for this ice age include U-Pb zircon ages of 635.51 ⫾ 0.54 and 635.23 ⫾ 0.57 Ma from ash beds within the upper Ghaub Formation in Namibia (Hoffmann et al., 2004) and the Nantuo cap carbonate (Doushantuo Formation) in southern China (Condon et al., 2005), respectively. These ages suggest the possibility of a worldwide ‘‘Marinoan’’ glaciation terminating synchronously at 635 Ma. Our 643 Ma Tindelpina Shale Member age does not necessarily refute the prevailing Marinoan correlation scheme, although accepting an age of 635 Ma for the undated Elatina glacial deposits requires high sedimentation rates to accommodate ⬎4 km of interglacial strata in the Central Flinders Zone (Preiss et al., 1998). Alternatively, Calver et al. (2004) suggest the Marinoan glaciation in southern Australia is more likely correlative with the 580 Ma Gaskiers glaciation (eastern Canada) based on correlation of the Elatina Formation with the Crole Hills Diamictite in Tasmania (maximum age of 582 ⫾ 4 Ma) and the Cottons Breccia in King Island (minimum age of 575 ⫾ 3 Ma). This scenario would be more compatible with the 643 Ma Tindelpina Shale age. However, evidence of a glacial influence on the Tasmania diamictites is not conclusive (e.g., Eyles and Januszczak, 2004), and as acknowledged 731

by Calver et al. (2004), stratigraphic considerations permit the possibility of a much older age than 575 Ma for the Cottons Breccia. Consequently, an age of either 635 or 580 Ma for the Elatina Formation remains compatible with present geochronologic data. Paleomagnetic and sedimentological studies suggest that Neoproterozoic glaciomarine deposition occurred at low latitudes in some instances (in contrast to the mid- and highlatitude Phanerozoic glaciations), thus implying unusually severe glaciations or preferential low-latitude ice cover due to a high orbital obliquity (Williams and Schmidt, 2004). Available radiometric age constraints for Neoproterozoic glaciation permit the possibility of a global or near-global ice age at 712 Ma (Ghubrah–Scout Mountain) and 635 Ma (Ghaub-Nantuo). However, our new ReOs age constraints for Australian glacial deposits also provide geochronological evidence for diachronous and/or multiple episodes of ‘‘Sturtian’’ glaciation between ca. 750 and 643 Ma, thus ruling out a simple tripartite division of all Neoproterozoic glacial deposits. The Sturtian and Edwardsburg diamictites of Australia and western Laurentia, respectively, may have been products of local ice sheet growth on uplifted rift margins during the protracted breakup of Rodinia (Preiss, 2000; Lund et al., 2003; Eyles and Januszczak, 2004). Geological evidence for actively functioning hydrological cycles and oscillatory waxing and waning of Neoproterozoic ice sheets (e.g., Leather et al., 2002) suggests some Neoproterozoic glaciations were likely similar to those of the Pleistocene epoch. ACKNOWLEDGMENTS Insightful reviews by Samuel Bowring, Graham Shields, and Clive Calver improved the manuscript. This research was supported by a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to Creaser and an NSERC Canada Graduate Scholarship, Alberta Ingenuity Fund Ph.D. Studentship, and Geological Society of America Student Research Grant to Kendall. The Radiogenic Isotope Facility at the University of Alberta is supported in part by an NSERC Major Facilities Access Grant. Sample collection was assisted by the staff of the Alice Springs (Northern Territory Geological Survey) and Glenside (Department of Primary Industries and Resources, South Australia) core libraries, Australia. REFERENCES CITED Allen, P.A., Bowring, S., Leather, J., Brasier, M., Cozzi, A., Grotzinger, J.P., McCarron, G., and Amthor, J.J., 2002, Chronology of Neoproterozoic glaciations: New insights from Oman: International Sedimentological Congress, 16th, Johannesburg, South Africa, July 8–12, 2002, Abstract Volume, p. 7–8. Bowring, S.A., Myrow, P., Landing, E., and Ramenzani, J., 2003, Geochronological constraints on terminal Neoproterozoic events and the rise of metazoans: Geophysical Research Abstracts, v. 5, p. 13,219.

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