A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations

The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history. INTRODUCTION After more than one billion years without robust evidence of glaciation, Cryogenian (ca. 850–635 Ma) strata record arguably the most extreme episodes of climate change in Earth’s history. The widespread occurrence of lowlatitude glacial deposits on every paleocontinent, coupled with the unique geochemistry and sedimentology of cap carbonates (Hoffman et al., 1998; Bao et al., 2008), inspired the snowball Earth hypothesis (Kirschvink, 1992). However, the general paucity of radiometric age constraints from multiple paleocontinents for the onset and demise of Cryogenian glaciations (Sturtian ca. 717–660 Ma, and Marinoan ending ca. 635 Ma), as well as reports of putative pre-Sturtian glaciations (Frimmel et al., 1996), has fostered doubts about the synchronicity and global extent of these events (e.g., Allen and Etienne, 2008; Kendall et al., 2006). In particular, an apparent disagreement between various geochronological constraints has fueled the idea Cryogenian glaciations were not particularly unique or extreme events; however, it remains unclear if these age differences represent true geological mismatches or the combination of analytical error and/ or poor cross-calibration between different geochronometers. Here we present four new Re-Os ages from strata that bound Cryogenian glacial deposits in northwest Canada, Zambia, and Mongolia. We then integrate these data with preexisting age constraints from multiple paleocontinents to produce an updated global Cryogenian chronology. GEOLOGICAL SETTING Black carbonaceous shales were sampled at four separate localities on three different Neoproterozoic paleocontinents (Fig. 1; Table DR1 in the GSA Data Repository1). The Callison Lake Formation of the Mount Harper Group is exposed in the Ogilvie Mountains of Yukon, Canada, and is composed of an ~400-m-thick succession of mixed carbonate and siliciclastic strata deposited in an episodically restricted marine basin (Strauss et al., 2014; Fig. 1). Current age constraints on the Mount Harper Group include an Re-Os age of 739.9 ± 6.1 Ma from the uppermost Callison Lake Formation and a U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry age on zircon of 717.4 ± 0.1 Ma from the overlying Mount Harper Volcanics (Macdonald et al., 2010a; Strauss et al., 2014). To further refine the geological history of this succession, a black shale horizon was sampled from the lower Callison Lake Formation for Re-Os geochronology (Fig. 1; Fig. DR1 in the Data Repository). The Katanga Supergroup of the Congo craton has been subdivided into the Roan, Nguba, and Kundelungu Groups and comprises a mixed carbonate and siliciclastic succession with two diamictite horizons (Fig. 1; Wendorff, 2003; Master et al., 2005). In the Chambishi area of Zambia, the Mwashya subgroup of the Nguba Group records subtidal deposition in a restricted marginal marine setting and comprises an ~120-m-thick succession of black carbonaceous shale with a gradational to locally disconformable contact with the overlying glaciogenic deposits of the Grand Conglomerate (Fig. 1; Selley et al., 2005). Age constraints on the Katanga Supergroup are limited to a maximum age for the onset of Roan Group sedimentation from a U-Pb sensitive high-resolution ion microprobe (SHRIMP) age of 883 ± 10 Ma (Armstrong et al., 2005) and ages of ca. 760 Ma from various volcanics within the Nguba Group (Key et al., 2001). Samples of carbonaceous black shale were collected from the Mwashya subgroup over a vertical interval of 6.49 m up to ~0.5 m below the Grand Conglomerate for Re-Os geochronology (MJCZ9 drill core; Bodiselitsch et al., 2005). The Tsagaan-Olom Group of the Zavkhan terrane in southwest Mongolia consists of as much as 2 km of carbonate-dominated strata that host two glacial deposits, the Maikhan-Uul and Khongor diamictites, which are considered to be the Sturtian and Marinoan equivalents, respectively (Fig. 1; Macdonald et al., 2009). Samples for Re-Os geochronology were sampled at the Taishir locality (Macdonald et al., 2009), 1.2 m above the contact with the Maikhan-Uul diamictite (Fig. 1) and within the Sturtian cap carbonate. The Cryogenian–Ediacaran-age Hay Creek and upper groups of the Windermere Supergroup are exposed in the Mackenzie Mountains, northwest Canada, and host Marinoan-age glacial deposits of the Stelfox Member of the Ice Brook Formation (Fig. 1; Aitken, 1991; James et al., 2001). A Marinoan age (ca. 635 Ma) for the Stelfox Member is supported by carbon isotope profiles and sedimentological characteristics of the Ravensthroat and Hayhook cap carbonate. Samples for Re-Os geochronology were collected from the Sheepbed Formation near Shale Lake (Aitken, 1991), 0.9 m above the contact with the underlying Hayhook limestone. The Sheepbed Formation consists of >700 m of siliciclastics deposited in a proximal to distal slope environment during a pronounced glacioeustatic transgression (Fig. 1; Dalrymple and Narbonne, 1996). GEOCHRONOLOGY Black shale from the lower part of the Callison Lake Formation of Yukon yields a Re-Os 1GSA Data Repository item 2015157, summary of sampling techniques, detailed analytical methods, and data tables containing all isotopic and/or geochronological data, is available online at www .geosociety.org/pubs/ft2015.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. GEOLOGY, May 2015; v. 43; no. 5; p. 459–462; Data Repository item 2015157 | doi:10.1130/G36511.1 | Published online 27 March 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. 460 www.gsapubs.org | Volume 43 | Number 5 | GEOLOGY depositional age of 752.7 ± 5.5 Ma (all age uncertainties also include the uncertainty in the 187Re decay constant, l, 2s, n = 5, mean square of weighted deviates, MSWD, of 0.30) with an initial 187Os/188Os (Osi) composition of 0.33 ± 0.03 (Fig. 2A). Regression of the Re-Os isotopic composition data from the Mwashya subgroup of Zambia yields a depositional age of 727.3 ± 4.9 Ma (2s, n = 7, MSWD = 0.50) with an unradiogenic Osi value of 0.35 ± 0.03 (Fig. 2B). The basal Taishir Formation of Mongolia yields a depositional Re-Os age of 659.0 ± 4.5 Ma (2s, n = 6, MSWD = 0.67), with a moderately radiogenic Osi value of 0.60 ± 0.01 (Fig. 2C). Regression of the isotopic composition data from samples of the Sheepbed Formation of northwest Canada yields a Re-Os age of 632.3 ± 5.9 Ma (2s, n = 5, MSWD = 0.58), with a highly radiogenic Osi value of 1.21 ± 0.04 (Fig. 2D). These new Re-Os ages coupled with existing Re-Os and magmatic U-Pb age constraints are combined to produce a refined geochronological framework for the Cryogenian as summarized in Figure 3 (Table DR2). DISCUSSION The existence of a pre-Sturtian, global Kaigas glaciation has been suggested from the apparent relationship between inferred glacial deposits and the following age constraints: a 741 ± 6 Ma Pb-Pb zircon evaporation age in the Gariep belt of the Kalahari craton (Frimmel et al., 1996); a 740 ± 7 Ma U-Pb SHRIMP age from near the base of the Bayisi diamictite on the Tarim craton (Xu et al., 2009); and a 735 ± 5 Ma U-Pb SHRIMP age from the Kundelungu Basin of the Congo craton (Key et al., 2001). However, previously published U-Pb and Re-Os ages from time-equivalent strata in Laurentia (Karlstrom et al., 2000; Macdonald et al., 2010a; Strauss et al., 2014) and the Re-Os age presented here from the Callison Lake Formation (Fig. 2A) document nonglacial sedimentation from ca. 753 to 717 Ma on the western margin of Laurentia, arguing against low-latitude glaciation during this interval. Macdonald et al. (2010b) observed that the 741 ± 6 Ma Pb-Pb evaporation age from the Gariep belt (Frimmel et al., 1996) was sampled from volcanic rocks that are not in direct contact with glacial deposits and that conglomerate of the Kaigas Formation was previously miscorrelated with glacigenic strata of the Numees Laurentia