A chronology: Two long-lasting synchronous glaciations

Alan D. Rooney1, Justin V. Strauss1, Alan D. Brandon2, and Francis A. Macdonald1 1Department of and Planetary , Harvard University, Cambridge, Massachusetts 02138, USA 2Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA

ABSTRACT Master et al., 2005). In the Chambishi area of The hypothesis predicts globally synchronous glaciations that persisted on Zambia, the Mwashya subgroup of the Nguba a multimillion scale. Geochronological tests of this hypothesis have been limited by Group records subtidal deposition in a restricted a dearth of reliable constraints bracketing these events on multiple . Here we pres- marginal marine setting and comprises an ent four new Re-Os age constraints on Sturtian (717–660 Ma) and Marinoan ~120-m-thick of black carbonaceous (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma shale with a gradational to locally disconformable age from the base of the Callison Lake Formation in Yukon, Canada, confi rms nonglacial contact with the overlying glaciogenic deposits sedimentation on the western margin of between ca. 753 and 717 Ma. Coupled of the Grand Conglomerate (Fig. 1; Selley et al., with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomer- 2005). Age constraints on the Katanga Super- ate on the Congo (Africa), these data refute the notion of a global ca. 740 Ma group are limited to a maximum age for the onset glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia con- of Roan Group sedimentation from a U-Pb sensi- fi rms previous constraints on a long for the 717–660 Ma Sturtian glacial and tive high-resolution ion microprobe (SHRIMP) a relatively short nonglacial interlude. In addition, we provide the fi rst direct radiometric age age of 883 ± 10 Ma (Armstrong et al., 2005) and constraint for the termination of the in Laurentia with an age of 632.3 ages of ca. 760 Ma from various volcanics within ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within the Nguba Group (Key et al., 2001). Samples of uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data carbonaceous black shale were collected from unite Re-Os and U-Pb geochronological constraints and provide a refi ned temporal frame- the Mwashya subgroup over a vertical interval of work for Cryogenian Earth . 6.49 m up to ~0.5 m below the Grand Conglom- erate for Re-Os geochronology (MJCZ9 drill INTRODUCTION GEOLOGICAL SETTING core; Bodiselitsch et al., 2005). After more than one billion without Black carbonaceous shales were sampled at The Tsagaan-Olom Group of the Zavkhan robust evidence of glaciation, Cryogenian (ca. four separate localities on three different Neo- terrane in southwest Mongolia consists of as 850–635 Ma) strata record arguably the most paleocontinents (Fig. 1; Table DR1 much as 2 km of carbonate-dominated strata that extreme episodes of climate change in Earth’s in the GSA Data Repository1). The Callison host two glacial deposits, the Maikhan-Uul and history. The widespread occurrence of low- Lake Formation of the Mount Harper Group Khongor diamictites, which are considered to be latitude glacial deposits on every paleoconti- is exposed in the Ogilvie Mountains of Yukon, the Sturtian and Marinoan equivalents, respec- nent, coupled with the unique geochemistry Canada, and is composed of an ~400-m-thick tively (Fig. 1; Macdonald et al., 2009). Samples and sedimentology of cap carbonates (Hoff- succession of mixed carbonate and siliciclastic for Re-Os geochronology were sampled at the man et al., 1998; Bao et al., 2008), inspired the strata deposited in an episodically restricted Taishir locality (Macdonald et al., 2009), 1.2 m snowball Earth hypothesis (Kirschvink, 1992). marine basin (Strauss et al., 2014; Fig. 1). Cur- above the contact with the Maikhan-Uul diamic- However, the general paucity of radiometric rent age constraints on the Mount Harper Group tite (Fig. 1) and within the Sturtian cap carbonate. age constraints from multiple paleocontinents include an Re-Os age of 739.9 ± 6.1 Ma from The Cryogenian–-age Hay Creek for the onset and demise of Cryogenian gla- the uppermost Callison Lake Formation and a and upper groups of the Windermere Super- ciations (Sturtian ca. 717–660 Ma, and Mari- U-Pb chemical abrasion–isotope dilution–ther- group are exposed in the Mackenzie Mountains, noan ending ca. 635 Ma), as well as reports of mal ionization mass spectrometry age on zircon northwest Canada, and host Marinoan-age gla- putative pre-Sturtian glaciations (Frimmel et of 717.4 ± 0.1 Ma from the overlying Mount cial deposits of the Stelfox Member of the Ice al., 1996), has fostered doubts about the syn- Harper Volcanics (Macdonald et al., 2010a; Brook Formation (Fig. 1; Aitken, 1991; James et chronicity and global extent of these events Strauss et al., 2014). To further refi ne the geo- al., 2001). A Marinoan age (ca. 635 Ma) for the (e.g., Allen and Etienne, 2008; Kendall et al., logical history of this succession, a black shale Stelfox Member is supported by carbon isotope 2006). In particular, an apparent disagreement horizon was sampled from the lower Callison profi les and sedimentological characteristics of between various geochronological constraints Lake Formation for Re-Os geochronology (Fig. the Ravensthroat and Hayhook cap carbonate. has fueled the idea Cryogenian glaciations 1; Fig. DR1 in the Data Repository). Samples for Re-Os geochronology were col- were not particularly unique or extreme events; The Katanga Supergroup of the Congo cra- lected from the Sheepbed Formation near Shale however, it remains unclear if these age dif- ton has been subdivided into the Roan, Nguba, Lake (Aitken, 1991), 0.9 m above the contact ferences represent true geological mismatches and Kundelungu Groups and comprises a mixed with the underlying Hayhook limestone. The or the combination of analytical error and/ carbonate and siliciclastic succession with two Sheepbed Formation consists of >700 m of or poor cross-calibration between different diamictite horizons (Fig. 1; Wendorff, 2003; siliciclastics deposited in a proximal to distal geochronometers. Here we four new slope environment during a pronounced gla- Re-Os ages from strata that bound Cryogenian 1GSA Data Repository item 2015157, summary cioeustatic transgression (Fig. 1; Dalrymple and glacial deposits in northwest Canada, Zambia, of sampling techniques, detailed analytical meth- Narbonne, 1996). and Mongolia. We then integrate these data ods, and data tables containing all isotopic and/or geochronological data, is available online at www with preexisting age constraints from multiple .geosociety.org/pubs/ft2015.htm, or on request from GEOCHRONOLOGY paleocontinents to produce an updated global [email protected] or Documents Secretary, Black shale from the lower part of the Cal- Cryogenian chronology. GSA, P.O. Box 9140, Boulder, CO 80301, USA. lison Lake Formation of Yukon yields a Re-Os

GEOLOGY, May 2015; v. 43; no. 5; p. 459–462; Data Repository item 2015157 | doi:10.1130/G36511.1 | Published online XX 2015 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 5 For| www.gsapubs.org permission to copy, contact [email protected] 459 A 1. Kipushi Basin, Zambia 2. Mackenzie Mtns., Canada 3. Ogilvie Mtns., Canada 4. Zavkhan Basin, Mongolia depositional age of 752.7 ± 5.5 Ma (all age Calcaire SHPB

632.3±5.9 Ma Ol

Ed. uncertainties also include the uncertainty in the Rose # Marinoan Hay Creek Petit Keele Gp. 187 Kunde- lungu Gp. Conglom. Undiff. Re decay constant, λ, 2σ, n = 5, mean square Twitya Hay Creek Gp. of weighted deviates, MSWD, of 0.30) with an 662.4±4.3 Ma

Gp. 187 188

Shezal Rapitan Gp. 200 m initial Os/ Os (Os ) composition of 0.33 ± Kakontwe 716.5±0.2 Ma i Fm. Sayeuni Sturtian 0.03 (Fig. 2A). Regression of the Re-Os isotopic

200 m Mt. Harper 717.4±0.1 Ma Gp. Rapitan Mt. Berg Volcanics Gp. Cu-cap 732.2±4.7 Ma Seela Taishir Fm. composition data from the Mwashya subgroup

Cryogenian 739.9±6.5 Ma Redstone Pass Tsagaan Olom of Zambia yields a depositional age of 727.3

Mount 659.0±4.5 Ma Nguba Gp. River Callison Harper Lake σ Gp. Thund. Grand Coates Lk. ± 4.9 Ma (2 , n = 7, MSWD = 0.50) with an Conglom. Fm. 752.7±5.5 Ma 727.3 ± 4.9 Ma Ram 777.7±2.5 Ma unradiogenic Os value of 0.35 ± 0.03 (Fig. 2B). Mwashya Head Craggy i Fm. Dolostone The basal Taishir Formation of Mongolia yields

T.S. 200 m Gp. U.R. Bancroft 200 m Gp. Little Dal S.S. a depositional Re-Os age of 659.0 ± 4.5 Ma Ton. Gp. Fm. Maikhan-Uul Fm. Gayna Fifteenmile Reefal A. 811.5±0.1 Ma σ B (2 , n = 6, MSWD = 0.67), with a moderately 635 Ma cap carbonate radiogenic Osi value of 0.60 ± 0.01 (Fig. 2C). Iron Formation Regression of the isotopic composition data Basalt/Rhyolite/Diabase Evaporite from samples of the Sheepbed Formation of Stratified diamictite northwest Canada yields a Re-Os age of 632.3 Massive diamictite ± 5.9 Ma (2σ, n = 5, MSWD = 0.58), with a 4 3 Shale 2 highly radiogenic Osi value of 1.21 ± 0.04 (Fig. Calcisiltite Laurentia Limestone/Dolostone 2D). These new Re-Os ages coupled with exist- Siltstone ing Re-Os and magmatic U-Pb age constraints Sandstone are combined to produce a refi ned geochrono- 1 Conglomerate logical framework for the Cryogenian as sum- Congo Unconformity Vase-shaped microfossil marized in Figure 3 (Table DR2). U/Pb zircon age Re/Os ORR age DISCUSSION Re/Os ORR age this paper at # Sample location The existence of a pre-Sturtian, global Kaigas ca. 715 Ma glaciation has been suggested from the apparent relationship between inferred glacial deposits Figure 1. A: Schematic stratigraphy of sample locations 1–4, adapted from Bodiselitsch et al. (2005), Macdonald et al. (2009, 2010a), and Strauss et al. (2014). Geochronological con- and the following age constraints: a 741 ± 6 Ma straints are from Jefferson and Parrish (1989), Macdonald et al. (2010a), Rooney et al. (2014), Pb-Pb zircon evaporation age in the Gariep belt and Strauss et al. (2014). Ton.—; Ed.—Ediacaran; Gp.—Group; Fm.—Formation; of the Kalahari craton (Frimmel et al., 1996); a Conglom.—Conglomerate; U.R.—Upper Roan; SHPB—Sheepbed Formation; Lk—lake; Cu- 740 ± 7 Ma U-Pb SHRIMP age from near the cap.—Coppercap Formation; Thund.—Thundercloud Formation; T.S.—Ten Stone Formation; S.S.—Snail Spring Formation; A.—assemblage; Mt.—Mount; Mtns.—mountains; #—includes base of the Bayisi diamictite on the Tarim cra- the Ravensthroat and Hayhook formations. B: Paleogeographic reconstruction of Rodinia at ton (Xu et al., 2009); and a 735 ± 5 Ma U-Pb ca. 715 Ma, modifi ed from Li et al. (2013). ORR—organic-rich rock. SHRIMP age from the Kundelungu Basin of the Congo craton (Key et al., 2001). However, previously published U-Pb and Re-Os ages from 187Re/188Os 187Re/188Os time-equivalent strata in Laurentia (Karlstrom et 100 200 300 400 500 600 0200 400 600 al., 2000; Macdonald et al., 2010a; Strauss et al., 7.5 2014) and the Re-Os age presented here from the A B 8.0 6.5 J1301-62.5-7 Callison Lake Formation (Fig. 2A) document MJ-31 nonglacial sedimentation from ca. 753 to 717 6.0 5.5 J1301-62.5-4 MJ-34 Ma on the western margin of Laurentia, argu- Os Os J1301-62.5-6 MJ-33 ing against low-latitude glaciation during this 188 188 4.5 J1301-62.5-2 MJ-36 4.0 interval. Macdonald et al. (2010b) observed that Os/ Os/ 187 187 MJ-32 3.5 the 741 ± 6 Ma Pb-Pb evaporation age from the MJ-35 Gariep belt (Frimmel et al., 1996) was sampled Age = 752.7 ± 5.5 Ma Age = 727.3 ± 4.9 Ma 2.0 MJ-37 2.5 Os = 0.33 ± 0.03 Os = 0.35 ± 0.03 from volcanic rocks that are not in direct contact J1301-62.5-1 i i MSWD = 0.50 MSWD = 0.50 with glacial deposits and that conglomerate of 1.5 0 4.0 the Kaigas Formation was previously miscor- C D 7.0 related with glacigenic strata of the Numees 3.6 SpB-6-8 A1309-I SpB-6C 3.2 Os

188 5.0 2.8 SpB-6A Figure 2. Re-Os isochron diagrams. A: Os Os/ A1309-D SpB-6-9 Lower Callison Lake Formation. B: Mwashya 188 187 2.4 subgroup (MJ-31–MJ-37, 172.61–179.10 m). Os/

A1309-E 187 C: Taishir Formation. D: Sheepbed Forma- 2.0 A1309-C 3.0 Age = 659.0 ± 4.5 Ma Age = 632.3 ± 5.9 Ma tion. All data point error ellipses are 2σ and A1309-B Os = 0.60 ± 0.01 Os = 1.21 ± 0.04 their diameters are larger than calculated 1.6 A1309-A i SpB-6B i MSWD = 0.67 MSWD = 0.58 error ellipses. All isotopic composition and 1.2 1.0 elemental abundance data are presented in 60 100 140 180 220 260 300 0 200 400 600 Table DR1 (see footnote 1). MSWD—mean 187Re/188Os 187Re/188Os square of weight deviates.

460 www.gsapubs.org | Volume 43 | Number 5 | GEOLOGY Formation. Therefore, this age only provides Rooney et al., 2014). The Osi value of 0.60 is Age (Ma) Reference a maximum age constraint on the Sturtian-age moderately unradiogenic, in contrast to modern- 632.5±1.0* † 1,2 Numees Formation. Similarly, no defi nitive evi- seawater, and is similar to values reported 632.3±5.9 3 635.5±1.1* 4,2 dence for glacial sedimentation has been dem- near the base of the post-glacial Twitya Forma- 635.3±1.1* 1,2 onstrated in the Bayisi diamictite of the Tarim tion in northwest Canada (0.54 versus 0.60). 636.3±4.9 Marinoan Glaciation 5 636.4±0.5 6 block, and the 740 ± 7 Ma U-Pb age of Xu et An existing Re-Os of 607.8 ± 4.7 Ma 640.7±5.7† 7 al. (2009) is potentially undermined by the fact from shale of the Windermere Supergroup, U-Pb magmatic that the analysis includes many zircon grains Canada, was suggested to represent a termina- zircon age that are possibly inherited from underlying ca. tion age for the Marinoan glaciation (Kendall et Re-Os ORR age 750 Ma volcanic rocks. Consequently, the 735 ± al., 2004). However, this Re-Os age is from an 654.5±3.8 5 5 Ma U-Pb SHRIMP age on the Luatama brec- isolated outcrop that is not in direct contact with

† cia of the Congo craton (Key et al., 2001) has the underlying Marinoan-age glacial deposits or 657.2±6.9 8 659.0±4.5† remained a fi nal holdout for the putative ca. 740 cap carbonate (Kendall et al., 2004). Our new † 3 659.6±10.2 9 Ma Kaigas glaciation. However, this U-Pb age is Re-Os age of 632.3 ± 5.9 Ma from 0.9 m above 662.4±4.6† 10 662.9±4.3 11 compromised not only by the inclusion of grains the distinctive Ravensthroat-Hayhook cap car- that were possibly inherited from the underly- bonate is within uncertainty of multiple ca. 635 ing ca. 765 Ma mafi c volcanic rocks of the low- Ma U-Pb zircon ages on the deglaciation of ermost Mwashya subgroup (Key et al., 2001), the Marinoan snowball Earth worldwide but also by the poorly constrained of the (Figs. 2D and 3; Hoffmann et al., 2004; Condon contact between the Luatama breccia and the et al., 2005; Calver et al., 2013. Sturtian Glacial Epoch glacial deposits (Selley et al., 2005). Our new 727.3 ± 4.9 Ma Re-Os age from directly below CONCLUSION the Grand Conglomerate is consistent with a The four Re-Os ages presented herein help 684.0±4.0 12 685.0±7.0 12 Sturtian age for the oldest Cryogenian glacial refi ne our current Neoproterozoic chronology. 685.5±0.4 13 deposits on the Congo craton (Figs. 2B and 3). These data refute the evidence for an earlier 687.4±1.3 14 688.9±9.5 15 The Re-Os age of 752.7 ± 5.5 Ma for the basal global Kaigas glaciation and suggest instead Callison Lake Formation provides a new deposi- that the initiation of the Sturtian glacial epoch tional age constraint for the lower portion of this ca. 717 Ma marked the fi rst unambiguous gla- unit and expands the range of vase-shaped micro- cial event in more than a billion years. Together fossil occurrences in northwest Canada (Strauss with existing geochronological data, the new

et al., 2014). It is interesting that the Osi value Re-Os ages constrain the onset and demise of from this basin at the time of deposition was the long-lasting (717–660 Ma) Sturtian glacial markedly unradiogenic (Os = 0.33), in contrast epoch and further bolster correlations for the 709.0±5.0 16 i to the Osi value presented for the upper Callison end-Cryogenian Marinoan glaciation ca. 635 711.5±0.3 17 Lake Formation at 739.9 ± 6.1 Ma (Osi = 0.60; Ma (Fig. 3). The long duration (>55 m.y.) of the 715.8±2.5 18 Strauss et al., 2014) and modern-day seawater Sturtian glacial epoch implies a relatively short 716.5±0.2 19 (Os = 1.06; Peucker-Ehrenbrink and Ravizza, Cryogenian nonglacial interlude (<25 m.y.), 717.4±0.1 19 i 719.5±0.3 20 2012). This Osi value suggests that the domi- consistent with a repeated trigger for glaciation nant source of Os entering this basin was either related to the tectonic background conditions 726.0±1.0 17 derived from the localized weathering of juve- that drive weathering and the consumption of 727.3±4.9† 3 nile crustal material (e.g., basalt) or sourced from CO2 on 1–10 m.y. time scales (Mills et al., 2011). hydrothermal inputs. Similarly, the unradiogenic This updated Neoproterozoic chronology pro- 732.2±4.7† 10 Os value of 0.35 for the Mwashya subgroup is vides new constraints to test and refi ne climate 735.0±5.0 21 i 736.0±23 22 suggestive of a signifi cant infl ux of unradiogenic models of a long-duration glacial epoch and the Os into this basin. These pre-Sturtian Os values, nature of a relatively short nonglacial interlude. † i 739.9±6.5 23 together with earlier work from northwest Can- The Osi data presented here confi rm enhanced ada (Rooney et al., 2014), lend further support weathering of juvenile crustal material prior to 741.0±6.0 24 742.0±6.0 25 to the so-called fi re and ice mechanism of snow- the onset of the Sturtian glacial epoch, consis- ball Earth initiation, whereby the weathering of tent with a basalt weathering trigger for initia- † voluminous unradiogenic basalts at low latitudes tion of the Sturtian glacial epoch (e.g., Goddéris 752.7±5.5 3 would have enhanced CO2 drawdown, resulting et al., 2003; Rooney et al., 2014). These ages Figure 3. Compilation of geochronological in a climate sensitive enough to enter a global confi rm the central prediction of the snowball constraints for Neoproterozoic strata for the glaciation (Goddéris et al., 2003). Earth hypothesis of long-lived (~10 m.y.) gla- interval 753–630 Ma. No detrital U-Pb zircon The Re-Os age of 659.0 ± 4.5 Ma for the ciation with globally synchronous deglaciation. data are included, because they cannot con- Taishir Formation constrains the termination strain the onset or cessation of glaciation. of the Sturtian-age Maikhan-Uul glaciation in ORR—organic-rich rock. Asterisks—U-Pb ACKNOWLEDGMENTS magmatic zircon ages including uncertain- Mongolia. This age is identical, within uncer- We thank Sharad Master for access to the Zambian ties recalculated by Schmitz (2012); dag- tainty, to Re-Os and U-Pb zircon age constraints drill core. Supported by the Massachusetts Institute gers—Re-Os ages including uncertainty for post-Sturtian horizons from northwest of Technology NASA Astrobiology Institute node, in 187Re decay constant (λλ)). Details of geo- Canada, China, and Australia, and confi rms a the National Foundation (NSF) Sedimentary chronological data and accompanying refer- Geology and Paleobiology Program (grant EAR- ences (numbered) are provided in the Data long duration (>55 m.y.) for the Sturtian glacial 1148058), an NSF Graduate Research Fellowship (to Repository (see footnote 1). epoch (Fig. 3; Zhou et al., 2004; Kendall et al., Strauss), the Yukon Geological Survey, and Fireweed 2006; Macdonald et al., 2010a; Lan et al., 2014; Helicopters and Canadian Helicopters (support and

GEOLOGY | Volume 43 | Number 5 | www.gsapubs.org 461 transportation). We also thank Lyle Nelson, Uyanga James, N.P., Narbonne, G.M., and Kyser, T.K., 2001, the Port Nolloth Group of Namibia and South Bold, and Sarah Moon for assistance in the fi eld; Late Neoproterozoic cap carbonates: Macken- Africa and implications for the age of Neopro- Charlie Roots, Erik Sperling, Galen Halverson, Dave zie Mountains, northwestern Canada: Precipi- terozoic iron formations: American Journal of Selby, and Paul Hoffman for fruitful conversations; tation and global glacial meltdown: Canadian Science, v. 310, p. 862–888, doi:10.2475 /09 and four reviewers for constructive comments. Journal of Earth Sciences, v. 38, p. 1229–1262, .2010.05. doi: 10.1139 /e01-046. 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