A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations Alan D. Rooney1, Justin V. Strauss1, Alan D. Brandon2, and Francis A. Macdonald1 1Department of Earth and Planetary Sciences, 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 snowball Earth hypothesis predicts globally synchronous glaciations that persisted on Zambia, the Mwashya subgroup of the Nguba a multimillion year time scale. Geochronological tests of this hypothesis have been limited by Group records subtidal deposition in a restricted a dearth of reliable age constraints bracketing these events on multiple cratons. Here we pres- marginal marine setting and comprises an ent four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan ~120-m-thick succession 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 Laurentia 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 craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas 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 duration for the 717–660 Ma Sturtian glacial epoch 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 Marinoan glaciation 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 history. 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 years 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 proterozoic 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–Ediacaran-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 present 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 Month 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 Siberia 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).