Vrije Universiteit Brussel
Cyclic cold climate during the Nantuo Glaciation: Evidence from the Cryogenian Nantuo Formation in the Yangtze Block, South China Lang, Xianguo; Chen, Jitao; Cui, Huan; Man, Ling; Huang, Kang Jun; Fu, Yong; Zhou, Chuanming; Shen, Bing Published in: Precambrian Research
DOI: 10.1016/j.precamres.2018.03.004
Publication date: 2018
Document Version: Final published version
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Citation for published version (APA): Lang, X., Chen, J., Cui, H., Man, L., Huang, K. J., Fu, Y., ... Shen, B. (2018). Cyclic cold climate during the Nantuo Glaciation: Evidence from the Cryogenian Nantuo Formation in the Yangtze Block, South China. Precambrian Research, 310, 243-255. https://doi.org/10.1016/j.precamres.2018.03.004
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Precambrian Research
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Cyclic cold climate during the Nantuo Glaciation: Evidence from the T Cryogenian Nantuo Formation in the Yangtze Block, South China
Xianguo Langa,b,c, Jitao Chena,b, Huan Cuid,e, Ling Manf,g, Kang-Jun Huangh, Yong Fui, ⁎ Chuanming Zhoua,b, Bing Shenc, a CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China b Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, Nanjing 210008, China c Key Laboratory of Orogenic Belts and Crustal Evolution, MOE; School of Earth and Space Sciences, Peking University, Beijing 100871, China d NASA Astrobiology Institute, University of Wisconsin–Madison, Madison, WI 53706, USA e Department of Geoscience, University of Wisconsin–Madison, Madison, WI 53706, USA f Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China g Exploration and Development Research Institute of Southwest oil & gasfield Company, PetroChina, Chengdu 610051, China h Department of Geology, Northwest University, Xi'an 710069, China i College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China
ARTICLE INFO ABSTRACT
Keywords: Geological and paleomagnetic data indicate that the ice sheets might have extended to low-latitude regions in Ice sheets the Cryogenian (720–635 Ma). The ‘snowball Earth’ hypothesis proposed that the Earth was completely ice- Glacial cycle covered during global glaciation. However, the glacial sedimentary record seems to contradict with the snowball Snowball Earth Earth hypothesis. Detailed sedimentological investigations of the glacial deposits would provide the first-order Cryogenian constraint on the nature of global glaciation. The Nantuo Formation (∼654–635 Ma) in the Yangtze Block of Neoproterozoic South China has been correlated with the Marinoan snowball Earth. In this study, we conducted systematic sedimentological analyses of six sections/cores of the Nantuo Formation. Three facies associations were re- cognized: the proximal glaciomarine, distal glaciomarine, and non-glacial marine facies associations. The ver- tical stacking pattern of facies associations can be correlated among the five slope and basin sections, while their correlation with the shelf section remains obscure. Our data indicate two episodes of glaciation that are sepa- rated by an interglacial interval during the Nantuo Glaciation. The first glacial episode is recorded by successions of coarse-grained facies (e.g., massive diamictite) in the lower part of the Nantuo Formation. The re-appearance of massive diamictite in the middle to upper part of the Nantuo Formation indicates onset of the second glacial episode. These two glacial episodes were separated by a siltstone/shale sequence of several 10 s m thick, sug- gesting an interglacial period with limited influence from glaciation. The top of Nantuo Formation consists of alternative distal-glaciomarine and non-glacial marine facies associations, representing the deglacial sequence of the Nantuo Glaciation. The sedimentary facies analysis indicates that the cold climate during the Nantuo Glaciation may be cyclic. Finally, our sedimentary analysis confirms a lag between the deglaciation and pre- cipitation of cap carbonate.
1. Introduction these two global glaciations (Hoffman et al., 1998; Hoffman and Schrag, 2002). Global freezing resulted in the stagnation in hydro- Geological and paleomagnetic evidence indicates that ice sheets logical cycle, leading to a weak continental weathering and negligible might have extended to the low latitude sea level during the Sturtian marine primary productivity. As a result, atmospheric pCO2 level could (ca. 717–660 Ma) and Marinoan glaciations (ca.650–635 Ma) (Hoffman continuously rise due to volcanic degassing during the global glaciation et al., 1998; Hoffman and Li, 2009; Hoffman and Schrag, 2002; (Hoffman and Schrag, 2002). The Earth remained completely frozen for
Kirschvink, 1992; Macdonald et al., 2010). The snowball Earth hy- tens of million years until atmospheric pCO2 level reached a threshold pothesis proposed that the Earth’s surface was completely frozen during that caused a catastrophic meltdown of the global glaciation (Bao et al.,
⁎ Corresponding author. E-mail address: [email protected] (B. Shen). https://doi.org/10.1016/j.precamres.2018.03.004 Received 8 December 2017; Received in revised form 25 February 2018; Accepted 4 March 2018 Available online 13 March 2018 0301-9268/ © 2018 Elsevier B.V. All rights reserved. X. Lang et al. Precambrian Research 310 (2018) 243–255
2008; Fabre and Berger, 2012; Higgins and Schrag, 2003; Hoffman activities (Jiang et al., 2011; Jiang et al., 2003; Wang and Li, 2003). et al., 1998; Hoffman and Schrag, 2002; Le Hir et al., 2007; Lewis et al., This stage is represented by the Liantuo/Chengjiang Formation in 2007). northwest and the Banxi/Xiajiang/Danzhou Group in southeast (Jiang The snowball Earth hypothesis provided a reasonable interpretation et al., 2011)(Table 1). The Liantuo/Chengjiang Formation is composed for the global precipitation of 12C-enriched cap carbonate immediately of sandstones of < 300 m thick, whereas the sandstone-siltstone suc- above glacial deposits (Hoffman et al., 1998). However, this scenario cessions of the Banxi/Xiajiang/Danzhou Group have a thickness of cannot be completely supported by the glacial sedimentary record. several kilometers (Huang et al., 2014; Wang and Li, 2003; Zhang et al., Some sedimentary structures observed in glacial marine deposits, such 2011), suggesting the rift basin dipping toward southeast, i.e. dee- as thin-bedded mudstone intercalated with diamictite (Allen and pening toward southeast. The overlying Cryogenian (720–635 Ma) Etienne, 2008; Allen et al., 2004; Busfield and Le Heron, 2014; Hu strata thickens from < 100 m in northwest to several kilometers in et al., 2011), wave ripples, and hummocky cross stratification (Busfield southeast (Jiang et al., 2011; Wang and Li, 2003). Significant reduction and Le Heron, 2016; Le Heron, 2015; Le Heron et al., 2016), indicate of volcanic and magmatic activities in Cryogenian suggests the late the presence of water current and sediment transportation during the rifting stage of the Yangtze Block (Table 1)(Wang and Li, 2003) global glaciation. Although wave-induced current would have become (Fig. 1B). negligible when the ocean was completely covered by ice (Allen and The overlying Ediacaran succession, consisting of, in ascending Etienne, 2008; Allen et al., 2004), recent study proposed that water order, the Doushantuo and Dengying/Liuchapo/Laobao formations, motion could also be induced by tidal force and hydrothermal activ- attenuates from northwest (< 1000 m) to southeast (< 250 m) (Jiang ities, and thus may still exist even in the ice-covered ocean (Hoffman et al., 2011). It was proposed that the Ediacaran succession represents et al., 2017a). Therefore, the presence of certain sedimentary structure the thermal subsidence stage of the Yangtze Block (Jiang et al., 2011, alone hardly supports the presence of an open ocean (i.e. not covered 2003; Wang and Li, 2003). Alternatively, recent studies indicate by ice) during the global glaciation. Instead, analysis of multiple gla- widespread hydrothermal activities occurred in the Liuchapo Forma- ciomarine depositional sequences in a basin-to-platform transect might tion along the platform margin of the Yangtze Block, indicating an in- provide more valuable information about the nature of global glacia- tensified tectonism rather than a passive continental margin in late tions (Hoffman et al., 2017a). Ediacaran (Wang et al., 2012). This scenario is also supported by In this study, we conducted systematic sedimentary analyses of 6 multiple hydrothermal dolomitization in the Dengying Formation in the sections of the Nantuo Formation (654–635 Ma) in the Yangtze Block of Upper Yangtze Block area (Jiang et al., 2016). Therefore, the late South China. Our study demonstrates that the Nantuo Formation may Ediacaran successions may represent a gentle extensional to a thermos- deposit under a cyclic cold-warm climatic condition rather than a long- subsidence stage of the Yangtze Block (Zhao et al., 2017). lasting global glaciation with the Earth’s surface being entirely frozen. 2.2. The Cryogenian stratigraphy in the Yangtze Block 2. Geological settings The Cryogenian deposits in the Yangtze Block can be divided into 2.1. Neoproterozoic depositional history of the Yangtze Block three units: in chronological order, the Jiangkou/Chang’an glacial de- posits, the Datangpo/Fulu interglacial deposits, and the Nantuo glacial The South China Block consists of the Yangtze Block to the north- deposits (Zhang et al., 2011). The Jiangkou/Chang’an glacial deposits west (present orientation) and the Cathaysia Block to the southeast are represented by the Gucheng/Tiesiao/Dongshanfeng (GTD) forma- (Fig. 1A) (Wang and Li, 2003; Zhao and Cawood, 2012; Zheng et al., tions in the slope and the Chang’an and Fulu (only the lower portion) 2013). The Yangtze and Cathaysia blocks were considered as having formations in the basin environments (Table 1)(Zhang et al., 2011). been amalgamated along the Jiangnan Orogenic Belt during the as- The GTD formations unconformably overlie the Banxi Group, and are sembly of the supercontinent Rodinia at ~830 Ma, followed by a rifting mainly composed of massive diamictite with thickness varying be- and thermal-subsidence cycle in late Neoproterozoic (Wang and Li, tween ∼ 2 m and several tens of meters. In the basin environment, the 2003). Chang’an Formation is defined by a thick sequence of massive dia- The early rifting stage that was initiated at ca. 820 Ma was domi- mictite intercalated with sandstone/siltstone, and conformably overlies nated by siliciclastic deposition along with widespread magmatic the Tonian Gongdong Formation (Lan et al., 2014). The overlying Fulu
Fig. 1. (A) Paleogeographic map of the Nantuo Formation in the Yangtze Block. (Modified after Zhang et al. (2011)). Red dots showing the localities of investigated sections in this study. 1: Jiulongwan section; 2: Wuluo drill core; 3: Daotuo drill core; 4: Nangao section; 5: Shennongjia section; 6: Yazhai section. (B) Depositional model of the Tonian and Cryogenian successions in the Yangtze Block. (Modified after Jiang et al. (2011)).
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Table 1 The Neoproterozoic stratigraphic framework of the Yangtze Block. (Ages cited from Jiang et al. (2011) and references therein)
Formation can be divided into five lithological members (Member I–V), Marinoan glaciation (Jiang et al., 2006; Lang et al., 2016). In this study, and consists of, in stratigraphic order, banded iron formation (BIF) or 4 outcrop sections and 2 drill cores of the Nantuo Formation were in- ironstones (Member I), pebbly sandstone (Member II), stratified sand- vestigated. Basic information of the studied sections is presented in stone (Member III), massive diamictite (Member IV), and siltstone and Table 2. mudstone (Member V). It is proposed that the member IV of the Fulu Formation can be correlated with the GTD formations in the slope en- 3. Lithofacies vironment, and represents the second episode of the Sturtian glaciation (Lan et al., 2015). Three U–Pb zircon ages of 725 ± 10 Ma, Ten lithofacies were identified from the Nantuo Formation. The 715.9 ± 2.8 and 714 ± 5.2 Ma from tuff beds in the top of the Tonian descriptions and interpretations are summarized in Table 3 and dis- Banxi/Danzhou Group (Lan et al., 2014; Song et al., 2017; Zhang et al., cussed below. 2008a), suggest the lower glacial interval in the Yangtze Block can be correlated with the Sturtian glaciation, which is dated at 3.1. Massive diamictite (Dmm) 713.7 ± 0.5 Ma from Oman and 715.9 ± 2.8 Ma from Canada (Bowring et al., 2007; Macdonald et al., 2010; Rooney et al., 2015). 3.1.1. Description The interglacial deposits are represented by the Datangpo This lithofacies (grayish-green or less commonly purple-red in – Formation (663 654 Ma) in the slope and the Member V of the Fulu color) accounts for 30–50% of the Nantuo Formation in stratigraphic Formation in the basin environments (Zhang et al., 2008b; Zhou et al., successions (a few meter to > 50 m thick) (Fig. 2). The diamictite ∼ – 2004). The Datangpo Formation ( 10 500 m thick) is dominated by comprises dominantly of silty and clayey matrix and < 30% shale/mudstone or siltstone (Li et al., 2012), and conformably overlies (mostly < 5%) of polymictic gravels that are dominated by granite, the GTD formations. Manganese carbonate at the base of the Datangpo sandstone and mafic igneous rocks, with rare occurrences of carbonate Formation has been interpreted as the cap carbonate of the Sturtian and mudstone (Fig. 4A). Gravels are angular to rounded in outline, and ∼ – glaciation (Yu et al., 2017). The Member V ( 30 80 m thick) of the poorly sorted (∼0.2–10 cm in size) occasionally with giant boulders up Fulu Formation is mainly composed of siltstone with occurrences of to 1 m in size (Fig. 4B). Glacial striations are commonly observed on several carbonate layers in some sections. pebbles (Fig. 4C). Diamictite is commonly massive but sometimes The upper glacial interval is called the Nantuo Formation. The normal-graded, showing a fining upward in gravel size (Fig. 2). Inter- Nantuo Formation was radiometrically dated between ~654 and calated siltstone layers are rarely observed in massive diamictite. Li- 635 Ma (Condon et al., 2005; Zhang et al., 2011, 2008b), and thus can quefied fine-sand veins are sporadically present in diamictite of the be correlated with the Marinoan global glaciation (Zhang et al., 2011). Daotuo drill core (Fig. 4D and E). The bounding surfaces of massive The thickness of the Nantuo Formation ranges from several meters to diamictites are commonly planar, displaying a steady distribution in several tens of meters in the terrestrial and inner shelf settings to > transverse. 2000 m in the basinal settings (Jiang et al., 2011; Zhang et al., 2011). The Nantuo Formation unconformably overlies the Chengjiang/Liantuo 3.1.2. Interpretation formations in terrestrial and inner shelf environments (Figs. 1B and 3A, This lithofacies is deposited from a series of glaciogenic subaqueous Tables 1 and 2)(Zhang et al., 2011), and has a conformable contact debris flows (Allen et al., 2004; Boulton and Deynoux, 1981). In the ice- with the underlying Datangpo and Fulu formations in the slope and grounding line zone, debris flows generated during ice sheet melting basinal environments (Fig. 3C and D) (Zhang et al., 2008b). The Nantuo would result in a rapid deposition of massive diamictite, forming ice Formation underlies a 3–6 m thick dolostone in the base of Doushantuo margin fans and/or ice-grounding line fans (Boulton, 1990; Boulton and Formation (Fig. 3B and E), which represents the cap carbonate of the Deynoux, 1981). Massive sediments loaded in the debris flows can be
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Table 2 Locations and descriptions of 6 logged sections/cores.
Section name GPS Location Depositional Main lithologies The bottom boundary of the The top boundary of the Thickness of the environment Nantuo Formation Nantuo Formation Nantuo Formation (m)
Jiulongwan N30°48′14.49′′ Glacial littoral Massive diamictite, crudely Unconformable contact Conformable contact 83 E111°03′18.21′′ stratified diamictite, pebbly sandstone, shale Wuluo N28°04′14.30′′ Slope Massive diamictite, pebbly Conformable contact Conformable contact 172 E108°49′55.31′′ sandstone, massive sandstone, siltstone/shale Daotuo N28°07′11.83′′ Slope Massive diamictite, massive Conformable contact Conformable contact 220 E108°53′6.29′′ sandstone, siltstone/shale Nangao N26°24′11.59′′ Slope Massive diamictite, crudely Conformable contact Covered ca.210 E107°53′15.57′′ stratified diamictite, shale Shennongjia N31°27′19.37′′ Slope Massive diamictite, pebbly Erosive contact Covered ca.260 E110°11′28.61′′ sandstone, massive sandstone Yazhai N25°50′46.50′′ Basin Massive diamictite, massive Conformable contact Conformable contact 350 E109°44′36.69′′ sandstone, pebbly sandstone transported downslope through a canyon or submarine channel and detached from sediment surface and form a turbulent jet due to water deposited as massive diamictite in distal glaciomarine environment. buoyancy (Boulton, 1990; Boulton and Deynoux, 1981). Outsized Normal-graded diamictite and intercalated siltstone layers are likely gravels floated in the massive diamictite are supported by cohesive deposited from turbidity currents that are generated during retreating strength of clayey matrix and transported in debris flows. Purple-red and advancing of ice sheets. When the glacial efflux charge is low, color of massive diamictites likely resulted from the enrichment of iron subglacial or basal streams that previously acted on seabed would be oxides that were either transported into the ocean by glaciers or
Inner shelf Slope Basin
Jiulongwan Wuluo Daotuo Nangao Shennongjia Yazhai 100 m 240 360 DST m DST DST
1350 DST 635 Ma m m Dmm Dmm Fl 90 Sp 225 Fl Sp Dms 270 345 Fl 1365 m Dmm Sm Sp Fl 80 210 Sp Pl Sp 255 330 1380 Sp Sm Sm 210 Dms m Dms 70 Fl Fl 195 Dms Sp Dmm Pl Sm 240 315 Dms 1395 Fdl Pl 195 Sp Sp Sp Dmm Dmm Dms 60 180 Fdl Sp 225 300 Dmm Dms 1410 Sp Dms 180 Fdl Dmm Sp Dmm 50 Dmm 165 Sp 210 Sm 285 Dmm 1425 Dmm Dmm 165 Sm Fdl
Nantuo Formation Fl 150 40 Sp 195 ~50m 1440 Dmm Dmm covered Dmm 150 Dms Fl 135 Fl Nantuo Formation Pl 30 Fl 180 225
1455 Nantuo Formation Fr Dmm Sm 135
Fl 120 Dmm Sp 165 Dmm 20 Gg 210 1470 Sm Dmm 120 Sp Dmm Dmm Fl 105 Fl Dmm Fl 150 195 10 1485 Sm Nantuo Formation Dmm Fl Dmm Fl 105 Sm
LT Dmm 90 Sm ? Fl 135 180 Nantuo Formation 0 1500 Nantuo Formation Dmm 90 Fl Cl S F M C G Dmm Sp 75 Fl 120 Dmm 165 Sm 1515 75 Sp 60 105 654 Ma Sp 150 1530 Sm Dmm Fl Dms 60 Sm Sp 4545 Dmm 90 135
DTP Dmm 1545 45 Fl Dmm Fl Dmm 30 Sp 75 Pl 120 1560 Sp Sm 30 Sp Dms Cl S F M C G Sm 15 Sm Sp Fl 60 105 Fl Pl Facies codes 15 Sm Sm DTP 0 Fl Dmm 4545 Diamictite Cl S F M C G 90 0 DTP Dmm - Massive diamictite Sp Sand Fine graines Cl S F M C G Dmm 30 75 Dms - Crudely stratified diamictite Sm - Massive and normal-graded sandstone Fr - Ripple cross-laminated siltstone Sp Gravel Sp - Stratified to massive pebbly sandstone Fl - Laminated siltstone/mudstone Dmm Sm 15 60 Gg - Normal-graded conglomerate Sh - Parallel-laminated sandstone Fdl - Dropstone-bearing laminated siltstone Sm
Pl - Laminated dolostone DTP 0 4545 Lithologies and structures Cl S F M C G Sp
Crudely stratified diamictite Shale Proximal glaciomarine facies association Ripple marks 30 Fl Sm Massive diamictite Silty mudstone Distal glaciomarine facies association Soft deformation structure
Pebbly sandstone Siltstone Non-glacial facies association Water-escaping structure 15 Sp Sm Drop-stones structure Lamination Massive/graded sandstone Covered Fl 0 DTP Dolostone Faults Parallel bedding Cl S F M C G
Fig. 2. Six Stratigraphic logs of the Nantuo Formation from the Yangtze Block. The Nantuo Formation can be divided into four parts according to the stacking pattern of facies association. Lithofacies codes are corresponding to those in Table 3. LT: Liantuo Formation; DTP: Datangpo Formation; DST: Doushantuo Formation. Grain sizes from left to right: Cl = clay, S = silt, F=fine sand, M = medium sand, C = coarse sand, G = gravels.
246 X. Lang et al. Precambrian Research 310 (2018) 243–255
Fig. 3. Outcrop photographs. (A) At the Jiulongwan section, the Nantuo Formation has an erosional contact with the underlying Liantuo Formation. (B) The upper boundary of the Nantuo Formation at the Jiulongwan section, showing the conformably contact between the Nantuo Formation and the Doushantuo cap carbonate. (C) Black shale of the Datangpo Formation at the Shennongjia section. (D) The boundary between the Datangpo Formation and the Nantuo Formation at the Shennongjia section. The Nantuo massive diamictite conformably overlies the black shale/siltstone of the Datangpo Formation. (E) The Nantuo Formation is conformably underlain by the Ediacaran Doushantuo Formation at the Yazhai section. originated from oxidation of ferrous iron minerals in subaerial condi- 3.2.2. Interpretation tions. Liquefied sand veins are caused by dewatering of rapid deposition This lithofacies is commonly deposited in the ice margin or the ice of the massive diamictite. grounding line areas where hydrodynamic energy is high (Eyles et al., 1985). In the ice grounding line areas, massive sediments loaded in glaciers would be quickly dumped into sea water and deposited as 3.2. Crudely stratified diamictite (Dms) massive diamictite (Allen et al., 2004; Eyles et al., 1985). These poorly sorted, massive diamictites would be slightly reworked by tidal currents 3.2.1. Description or waves, generating crude stratification. Irregular stratifications in this Crudely stratified diamictite is widespread in the Nantuo Formation, lithofacies indicate a temporal reworking of previous sediments. but its abundance is subordinate to massive diamictite (Figs. 2 and 4F). Changes in the velocity of subglacial streams can also produce a weakly The total thickness of this lithofacies is commonly less than several stratified diamictite. Near ice margins, subglacial streams often gen- meters, except in the Jiulongwan and Nangao sections where it is > erate an underflow at the tunnel jet when massive sediments are loaded 10 m thick. This lithofacies is clay/mud matrix supported and contains (Eyles et al., 1985). At a constant velocity, the underflow would remain 1–5% of polymictic, sub-rounded to rounded gravels (0.2–10 cm in in contact with sediments along a runout distance after dumping into size). This lithofacies is weakly stratified and commonly overlies mas- the ocean. Once the velocity of the underflow increases, massive sedi- sive diamictite. Irregular stratifications are reflected mainly by the ments would be modified, producing weak stratifications. variation in clasts content and can only be traced in a limited distance laterally.
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Table 3 Summary of the lithofacies of the Nantuo Formation.
Lithofacies code Lithofacies Description Interpretation
Dmm Massive diamictite Several meters to 10 s meters in thickness (normally > 10 m), silty and clayey Deposits of a series of glacigenic subaqueous matrix supported, < 10% of polymictic, sub-angular to sub-rounded gravels debris flows (1–10 cm in size) Dms Crudely stratified Less than several meters in thickness, irregularly stratified, commonly overlain by Deposits after moderate modification of diamictite massive diamictite, silty and clayey matrix supported, 1–5% of polymictic, sub- massive diamictite by active tide currents or rounded to well-rounded gravels (0.2–10 cm in size) waves Fdl Dropstone-bearing Laminated siltstones with < 1% of polymictic, angular to rounded pebbles Deposits of dilute turbidity currents with fall- laminated siltstone (1–10 cm in size), dropstone downwarping siltstone laminae out of heterolithic gravels from icebergs Sp Stratified to massive Several centimeters to several meters in thickness, massive to less commonly Deposits of debris flows pebbly sandstone stratified, fine to medium sand matrix supported, < 5% dispersed, poorly sorted, sub-angular to sub-rounded pebbles
Gg Normal-graded ∼10 cm in thickness, normal graded, clast supported, > 50% of polymictic, poor The basal deposits (TA) of the Bouma sequece conglomerate sorted, rounded pebbles Sm Massive to normal graded 1–15 m in thickness, massive to normal-graded, laterally persistent, fine to The deposits from low density turbidity sandstone medium sand in size currents Sh Parallel-laminated ∼1 m to several meters in thickness, laterally persisitent > 10 m, overlain by Deposits of currents under the upper flow sandstone massive or normal graded sandstone, rare soft deformation structures regime
Fr Ripple cross-laminated < 10 cm in thickness, asymmetric ripples, rare soft deformation structures. TC deposits of dilute turbidity currents siltstone Pl Laminated dolostone 10 cm to 3 m in thickness, laminated, laterally persistent, intercalated with Deposits in low-energy, clean seawater laminated siltstone/mudstone conditions
Fl Laminated siltstone/ Several centimeters to 10 s meters in thickness, rhythmic laminae, rare pyrite TD deposits of dilute turbidity currents and mudstone laminations fall out of fine-grained terrestrial materials
3.3. Dropstone-bearing laminated siltstone (Fdl) grounding line fans could produce debris flow currents, depositing massive or graded pebbly sandstone. Overturn of icebergs in the open 3.3.1. Description ocean is another mechanism for pebbly sandstone deposition. Notably, Laminated siltstone contains rounded, heterolithic, and poor-sorted stratified to massive pebbly sandstone formed in the distal glaciomarine pebbles that range from 1 to 10 cm in size. This lithofacies makes up a environments is commonly thin bedded. Such a thin-bed pebbly sand- relatively small proportion of the Nantuo Formation (Fig. 2). The stone is associated with dropstone-bearing laminated siltstone. continuity of laminations is commonly interrupted by pebbles, showing a typical ‘drop-stone’ structure (Fig. 5A). These siltstones are commonly wavy laminated (Fig. 5B). 3.5. Normal-graded conglomerate (Gg)
3.3.2. Interpretation 3.5.1. Description Dropstone-bearing laminated siltstones are commonly deposited This lithofacies only appears in the Daotuo drill core (Fig. 6A), and ∼ with transient fall-out of heterolithic clasts from icebergs (Allen et al., the conglomerate bed is only 10 cm in thickness. The conglomerate is 2004; Condon et al., 2002). Ice-rafted deposition may take place in clasts-supported and normal graded and contains > 50% of hetero- either proximal or distal glaciomarine environments during ice sheet lithic, poorly sorted, but well-rounded pebbles. melting. In proximal glaciomarine environments, dropstone-bearing laminated siltstones are often intercalated with coarse-grained litho- 3.5.2. Interpretation facies, whereas distal glaciomarine settings are dominated by fine- The graded clasts-supported conglomerate represents the deposits of grained lithofacies, such as laminated siltstone/mudstone. Presence of high density turbidity currents (i.e., the basal units of the Bouma se- thin layers of siltstone suggests this lithofacies was likely deposited quence) (Allen et al., 2004; Le Heron et al., 2014). Rounded pebbles from a series of dilute turbidity currents in distal glaciomarine en- indicate that the gravels are transported over a long distance before vironments. deposition, or are sourced from pre-existing sediments.
3.4. Stratified to massive pebbly sandstone (Sp) 3.6. Massive/normal-graded sandstone (Sm) 3.4.1. Description fi This lithofacies consists of ne to medium sandy matrix with dis- 3.6.1. Description persed, poorly sorted, sub-angular to sub-rounded pebbles (< 5%) This lithofacies occupies a significant fraction (∼10–40%) of the (Fig. 5C and D). These pebbly sandstones are mostly massive and less Nantuo Formation and can be observed in all deep water sections fi commonly strati ed. The sandstone beds show various thickness ran- (Figs. 2 and 6B). The sandstone is mostly massive and sometimes ging from several centimeters to several meters (Fig. 2). Thin beds of normal-graded. The massive sandstone bed is > 1 m thick and can be fi strati ed pebbly sandstone are locally intercalated with thin-layer laterally traced for long distances. This lithofacies can upward change siltstone, whereas thick pebbly sandstone beds are normally massive to thin-bedded siltstone locally. and are commonly interbedded with conglomerate or massive fine sandstone (Fig. 2). Stratified to massive pebbly sandstone beds com- monly have planar bounding surfaces. 3.6.2. Interpretation Massive/normal-graded sandstones can be deposited by relatively
3.4.2. Interpretation low-density turbidity currents, as the basal unit (i.e., TA) of the Bouma Stratified to massive pebbly sandstones are transitional deposits sequence (Sumner et al., 2009; Talling et al., 2012a,b). The overlying between conglomerate and sandstone, representing deposits of debris siltstone represents the tail deposits of the turbidity currents (Talling flows (Allen et al., 2004; Eyles et al., 1985). Over-steepening of ice et al., 2012b).
248 X. Lang et al. Precambrian Research 310 (2018) 243–255
Fig. 4. Photographs showing lithofacies of the Nantuo Formation. (A) Massive diamictites occurred at the base of the Nantuo Formation at the Shennongjia section. Detrital clasts showing sub-angular to rounded outlines. (B) Large boulders from the middle part of the Nantuo Formation at the Shennongjia section. (C) Occurrence of glacial striations on a pebble surface at the Jiulongwan section. (D) and (E) Occurrence of liquefied sand veins in massive diamictite at the Daotuo drill core. (F) Purple-red crudely stratified diamictites at the Nangao section. Weak stratifications are reflected by the changes in clast content.
3.7. Parallel-laminated sandstone (Sh) 3.8. Ripple cross-laminated siltstone (Fr)
3.7.1. Description 3.8.1. Description This lithofacies is characterized by the parallel lamination, and This lithofacies is often underlain by massive sandstone or inter- consists of fine sands (Fig. 6C). The sandstone beds range from ∼1mto calated with laminated siltstone/mudstone (Fig. 6D). The siltstone beds several meters in thickness and can be laterally traced for > 10 m. The are usually < 10 cm in thickness. Ripples are asymmetric with a wa- parallel-laminated sandstone beds commonly overlie massive or velength of centimeter scale. Soft-sediment deformation structures are normal-graded sandstone conformably. commonly observed (Fig. 6E and F).
3.7.2. Interpretation 3.8.2. Interpretation
Parallel lamination is usually formed by currents under the upper Ripple cross-laminated siltstone represents the TC deposits of dilute flow regime (Southern et al., 2017). Association with massive or turbidity currents (Stow and Piper, 1984; Talling et al., 2012b). Soft- normal-graded sandstone in the Nantuo Formation indicates that this sediment deformation structures are formed by earthquakes or sedi- lithofacies is deposited under turbidity current (Talling et al., 2012b). ment over-steepening triggered liquefaction and fluidization. Water- escaping structures suggest this lithofacies may have a relatively high sedimentary rate.
249 X. Lang et al. Precambrian Research 310 (2018) 243–255
Fig. 5. Photographs and photomicrographs showing the texture of the Nantuo Formation. (A) Photograph showing dropstone structures preserved in the dropstone-bearing siltstone lithofacies from the Daotuo drill core. (B) Thin section photograph of background deposition of dropstone-bearing siltstone. (C) Photograph showing massive pebbly sandstone from the Daotuo drill core.
3.9. Laminated dolostone (Pl) 3.10.2. Interpretation Laminated mudstones represent suspension fall out of fine-grained 3.9.1. Description terrestrial materials from water column (Sumner et al., 2009), whereas
Laminated dolostone can be observed in all logged deep-water laminated siltstones may represent the TD deposits from a dilute tur- sections. This lithofacies is usually intercalated with siltstone or mud- bidity current (Talling et al., 2012b). This lithofacies was commonly stone. The dolostone beds can be traced laterally at outcrop scale and deposited in the slope environments. varies between 10 cm and 3 m in thickness (Fig. 6G). 4. Facies associations
3.9.2. Interpretation Based on the stacking pattern and the lateral relationship between This lithofacies is deposited in relatively low-energy, clean seawater lithofacies, three facies associations are recognized: the proximal gla- Ł conditions when siliciclastic input was shut down ( abaj and Pratt, ciomarine, distal glaciomarine and non-glacial marine depositional fa- 2016), representing deposits in non-glacial condition. cies (Eyles et al., 1985).
3.10. Laminated siltstone/mudstone (Fl) 4.1. Proximal glaciomarine facies association
3.10.1. Description In all logged section/cores, proximal glaciomarine facies association fi – Laminated siltstone/mudstone can be observed in all logged sec- constitutes a signi cant fraction (10 90%) of the Nantuo Formation. tions/cores and constitute a significant fraction (5–40%) of the Nantuo Massive diamictite (Dmm) is the dominant lithofacies in this facies Formation. This lithofacies comprises rhythmic lamination of siltstone association (Allen et al., 2004; Eyles et al., 1985). The coarse-grained fl and mudstone (Fig. 6H). The siltstone/mudstone beds vary in thickness sediments are deposited from a series of subglacial debris ows with between several centimeters and several tens of meters (Fig. 2). Locally, transient rainout of ice rafts by gravity. If hydrodynamic condition was fi this lithofacies may contain pyrite laminations (e.g. in the Wuluo drill high, massive diamictite would be modi ed, generating a weakly or fi fi core; Fig. 6I). crudely strati ed diamictite (Dms). Other than diamictite, ne-grained facies could also be deposited in the ice grounding line areas, such as
250 X. Lang et al. Precambrian Research 310 (2018) 243–255
Fig. 6. (A) Normal graded conglomerate from the Daotuo drill core. Normal grade conglomerate is clasts-supported, and gravels are polymictic, poorly sorted but well rounded. (B) Massive sandstone from the Daotuo drill core. (C) Parallel-laminated sandstone from the Shennongjia section. (D) Ripple cross-laminated siltstone from the Yazhai section. Yellow arrows indicate asymmetric ripples with a wave- length of less than several centimeters. (E and F) Soft-de- formation structures occurred in the ripple cross laminated siltstones from the Daotuo drill core. (G) Laminated dolos- tones alternated with laminated siltstone/mudstone from the Shennongjia section. Arrows indicate thin carbonate beds. These dolostones are Mn-enriched and are weathered in outcrop. (H and I) Laminated siltstones/mudstones from the Daotuo and Wuluo drill cores. Yellow arrows show pyrite laminations.
pebbly sandstone (Sp), massive sandstone (Sm) or even laminated calving line or ice grounding line zone, sediments in distal glaciomarine siltstone (Fl). Sandstones deposited in this environment are commonly environments were mainly deposited from low-density gravity flows enriched in sedimentary structures, such as soft-deformation and water- (Allen et al., 2004). Transient hyper-concentrated debris flows could be escaping structures. These structures were caused by rapid sedimenta- generated by overturn of icebergs, leading to the deposition of thin-bed tion or ice-sheet push. It is notable that these sandstones usually have a pebbly sandstone or conglomerate. A specific feature of the facies as- limited thickness and are always associated with coarse-grained facies. sociation in distal glaciomarine environments is frequent variation in lithofacies, which may attribute to pulsed transformations between 4.2. Distal glaciomarine facies association debris flows and dilute turbidity currents.
Distal glaciomarine facies association is commonly dominated by 4.3. Non-glacial marine facies association fine-grained facies, such as laminated siltstone/mudstone (Fl), and drop-stone bearing siltstone (Fdl). Sometimes coarse-grained facies, Non-glacial marine facies association represents deposits formed in including massive diamictite (Dmm) or pebbly sandstone (Sp), may also normal marine environments without the influence of glaciation. In present. In contrary to proximal glaciomarine facies association, mas- such environments, the most common facies is laminated siltstone/ sive diamictite (Dmm) and pebbly sandstone (Sp) in this facies asso- mudstone (Fl) and ripple cross laminated siltstone (Fr). Additionally, ciation are thinner, normally < 1 m thick. As far away from the ice massive sandstone (Sm), laminated carbonate (Pl) and parallel
251 X. Lang et al. Precambrian Research 310 (2018) 243–255 laminated sandstone (Sh) can also deposit in non-glacial marine en- deposition (Fig. 2). At the Nangao section, the middle part of the vironments. Some stratified fine-grained lithofacies might be the re- Nantuo Formation is dominated by massive diamictite of 50 m in worked deposits from massive diamictite (Dmm) by subaqueous debris thickness, representing an overwhelming proximal glaciomarine de- flow or liquefied/turbidity current along the submarine canyon. position. Upward, massive diamictite gradually grades into crudely However, this scenario only seems to explain vertical lithofacies var- stratified diamictite and thin-bed diamictite with total thickness iation within a short distance (kilometer-scale). Well-correlated fine- of ∼60 m (Fig. 2), indicating a transition from proximal to distal gla- grained/stratified lithofacies across the whole Yangtze Block (hundreds ciomarine deposition. At the Shennongjia and Yazhai sections, massive of kilometer-scale) cannot be simply interpreted as deposits after Dmm diamictite and massive sandstone facies directly overlie the non-glacial modification. Instead, these fine-grained lithofacies may reflect the deposit. Both sections show frequent changes in lithology, representing non-glacial or interglacial intervals intercalated within glaciations. the oscillation between the proximal and the distal glaciomarine en- Laminated carbonate (Pl) is most likely precipitated in a high sea-level vironments (Fig. 2). condition. This facies association is present in all logged successions Deposition of thin laminated siltstone above the glaciomarine de- with a thickness ranging from several to several tens of meters. High posits in the upper part of the Nantuo Formation may indicate a glacial sea-level conditions may result from substantial melting of ice sheets, to non-glacial transition. At the Daotuo and Wuluo drill cores, a sand- indicating an interglacial or deglacial period of the Nantuo Glaciation. stone-siltstone succession that ranges from 5 to > 20 m thick re- presents the deposition in non-glacial conditions (Fig. 2). This succes- 5. Lateral and vertical distributions of facies associations sion can be correlated with ∼20 m thick laminated siltstone succession at the Yazhai section. This suggests the ice sheet might have been In the shallow water environments (inner shelf), the Nantuo melted in the open ocean. The top of the Nantuo Formation in the slope Formation is mainly composed of coarse-grained glacial deposits. At the environment is characterized by the alternating deposition of massive Jiulongwan section (inner shelf), a thick sequence of massive diamictite sandstone-stratified sandstone-laminated siltstone (e.g., from the and pebbly sandstone (∼15 m) that represents the proximal glacio- Daotuo and Wuluo drill cores). The local occurrences of thin-layered marine deposit unconformably overlies the sandstones of the Tonian diamictite may indicate the distal glaciomarine deposition near the end Liantuo Formation. These proximal glaciomarine deposits are directly of the Nantuo Glaciation, although a debris flow deposition by re- succeeded by a 2-m thick shale layer in the lower part of the Nantuo working of glaciogenic diamictite cannot be ruled out either. At the Formation, indicating a possible non-glacial deposition. The middle Yazhai section, the top of the Nantuo Formation is dominated by cru- part of the Nantuo Formation is dominated by proximal glaciomarine dely stratified diamictites that are intercalated with thin-layer siltstone, facies association that consists of massive diamictite and crudely stra- representing distal glaciomarine deposits. tified diamictite, although there is a 3-m thick pebbly sandstone layer In summary, the Nantuo Formation displays pronounced variations indicating the possible distal glacial marine deposition (Fig. 2). The in facies association both laterally and vertically. However, the strati- upper part of the Nantuo Formation is mainly composed of crudely graphic correlation between the shallow water (i.e., inner shelf) and stratified diamictite lithofacies, representing proximal-glaciomarine deep water (i.e., slope and basin) sections are not straightforward, deposition. The top of the Nantuo Formation at the Jiulongwan section probably due to incomplete record of the glacial deposits in the shallow is represented by a massive diamictite unit of > 20 m thick, and this water sections. In addition, faults occurred in the lower part of the diamictite unit is separated from the basal Doushantuo cap dolostone Nantuo Formation in the Jiulongwan section may also obscure its by a 0.5-m thick gravelly siltstone layer (Fig. 2). correlation with other sections. In the slope and basin environments, the five investigated Nantuo sections have similar vertical stacking patterns of facies associations. In 6. Discussion these sections, the Nantuo Formation conformably overlies the inter- glacial deposits of the Datangpo and Fulu formations, as indicated by a 6.1. Dynamic evolution of the Nantuo Glaciation gradual transition from laminated siltstones of the upper Datangpo/ Fulu Formation to massive diamictites (Fig. 2). These observations in- It is proposed that excessive topography relief would result in dicate that the lower Nantuo Formation may represent a glacial deposit complex facies associations during the Neoproterozoic global glaciation with ice sheets probably extended to the deep-water environment of the (Hoffman et al., 2017b). Particularly, deposition of diamictite during Yangtze Block. ice sheet melting might create topographic variations even within short The lower Nantuo Formation is succeeded by a sequence of fine- distance. Oversteepening of diamictite could generate reworked sedi- grained deposits dominated by fine sandstone, siltstone or shale, re- ments, such as crudely stratified diamictite, pebbly sandstone and presenting the non-glacial marine facies association (Fig. 2). This non- stratified sandstone (Hoffman et al., 2017b). This scenario seems to be a glacial succession has been found throughout the Yangtze Block, but good explanation for variable thickness and vertical facies change of shows significant variations in thickness. This non-glacial interval is glacial deposits within a short distance. However, well-correlated la- represented by a ∼30 m and ∼60 m thick laminated siltstone/mud- minated siltstone/mudstones and carbonate beds in the middle of the stone layer in the Daotuo and Wuluo drill cores, whereas this non- Nantuo Formation cannot be simply interpreted as deposits during ice glacial layer is ∼20 m and ∼10 m thick in the Nangao and Shennongjia sheet melting. Rather, these fine-grained facies may reflect the non- section, respectively (Fig. 2). At the Yazhai section from the basin en- glacial or interglacial interval intercalated within glaciations, sug- vironment, this non-glacial deposit is dominated by laminated siltstone gesting multiple ice-sheet advancing-retreating cycles during the of ∼15 m thick (Fig. 2). Nantuo Glaciation (Fig. 7). The non-glacial deposit is directly overlain by glaciomarine de- Accordingly, four depositional stages of the Nantuo Formation are posits, suggesting the renewal of glaciation. At the Wuluo drill core, a 3- reconstructed in this study and are summarized below: m thick massive diamictite layer underlies a thick sequence of pebbly sandstone and massive sandstone, representing a dominant distal gla- 6.1.1. Stage I ciomarine deposition, whereas at the nearby Daotuo drill core, after a In the earliest stage of Nantuo Glaciation, ice sheets have experi- 35-m thick alternating deposition of massive diamictite and massive enced an entire advancing-retreating cycle (Fig. 7A). Deposition of sandstone, a 40-m thick sequence of massive diamictite deposition re- glaciomarine facies association at the base of the Nantuo Formation presents the proximal glaciomarine deposition (Fig. 2). In the following suggests that ice sheets might have extended to the open ocean of the 15 m interval, reoccurrence of pebbly sandstone-sandstone alternations Yangtze Block. Ice-sheet advance could have led to a significant sea- may indicate a transition from proximal to distal glaciomarine level fall, resulting in a coarsening-upward sequence (Fig. 2). Local
252 X. Lang et al. Precambrian Research 310 (2018) 243–255
massive diamictites at the Jiulongwan and Shennongjia sections and the Wuluo and Daotuo drill cores may indicate the ice grounding line may have maintained a balance between advancing and retreating during coarse debris release ice bergs ice sheets A this period. At the Nangao and Yazhai sections, the absence of massive diamictite at the top of this interval may be attributed to relatively far coarsening upwards distance away from the ice margin area. debris rich ice-grounding line depositional lobes layers overflows 6.1.2. Stage II intererflows debris downslope underflows This stage represents an interglacial period during the Nantuo LT Glaciation (Fig. 7B), as evidenced by a package of fine-grained facies distal glacimarine pervasively occurred in the middle part of the Nantuo Formation. No- Sturtian table thickness variations in the interglacial deposits may be attributed DTP to different accommodation space between the shallow and deep water glacier environments. Deposition of laminated carbonate implies a relatively low terrestrial input and warm climate (Tucker et al., 1990). Therefore, B delta alternative fine sandstone fi and siltstone these ne-grained siliciclastic lithofacies and laminated carbonates (dolostones) were most likely deposited in a high sea-level condition, rivers with no influence of ice sheet. Since the Yangtze Block was located delta submarinecanyon at ∼30° N in latitude in late Neoproterozoic (Li et al., 2013), ice sheets during the Stage II may have retreated to mid to high latitude regions. submarine fans low density turbidity currents LT 6.1.3. Stage III This stage was the second glacial episode of the Nantuo Glaciation (Fig. 7C). In the upper part of the Nantuo Formation, reappearance of massive diamictite represents re-advance of ice sheets after the inter-
abundant ice bergs glacial period. Above these massive diamictites, oscillations between and coarse debris rain out distal and proximal glaciomarine facies associations, such as in the interval between 105 m and 120 m at the Daotuo drill core, may in- ice grounding line fans C dicate a dynamic ice grounding line during this stage. Although massive
ice sheets calving sediments sandstone or siltstone commonly intercalate with massive diamictite, oversteepening facies analysis suggests that all sediments were deposited in glacio- marine environments. Stacked glacial-related facies assemblages strongly demonstrate an overall tendency of ice sheet advancing. The thicker interval of coarse-grained glacial deposits compared to the proximal glacimarine glacitectonic deformation lower interval (Stage I) suggests the second glacial episode may have been more severe than the first episode of the Nantuo Glaciation. meltwater channels 6.1.4. Stage IV During this period, ice sheets were gradually retreating and the Crudely stratified diamictite Nantuo Glaciation was finally terminated (Fig. 7D). This stage was re- rippled sands longshore currents corded in the top of the Nantuo Formation. Packages of non-glacial and distal glaciomarine facies associations can be correlated throughout the D Yangtze Block (Fig. 2), suggesting the ice sheet retreat may have oc- curred at least regionally. In addition, occurrence of fine-grained li- thofacies, such as ripple-mark siltstone, laminated siltstone or carbo- tides/waves action turbiditic sands nate, suggest some part of the oceans may have remained ice free before ultimate meltdown of the Marinoan glaciation. Taken together, the four depositional stages (I–IV) of the Nantuo sediments reworking Formation suggest two cycles of ice sheet advance and retreat before slumpling and soft deformation the deposition of the Marinoan cap carbonates at 635 Ma.
Fig. 7. Schematic diagram showing the four stages of the Nantuo Glaciation. LT: Liantuo 6.2. Was the Nantuo Glaciation a snowball Earth event? Formation, DTP: Datangpo Formation. (A) The lower part of the Nantuo Formation re- cords an ice-sheet advancing-retreating cycle, representing the first episode of the Nantuo Glaciation. (B) The interglacial period after the first glacial interval. Ice sheets may have Multiple cycles of ice sheet advance and retreat as well as non- retreated back to mid- to high-latitude regions. Laminated siltstones/mudstones are the glacial deposits in the middle of the Nantuo Formation may suggest that characteristic deposition in the interglacial interval. (C) Reappearance of massive dia- the Nantuo Glaciation as a whole cannot be a hard snowball Earth. mictites in the upper part of the Nantuo Formation indicates the onset of the second However, existing geochemical evidence strongly suggests the presence episode of the Nantuo Glaciation. (D) The melting of the Nantuo Glaciation might be a of snowball Earth interval (Bao et al., 2008; Huang et al., 2016), though gradual progress with an alternative deposition of distal glaciomarine and non-glacial its duration is poorly constrained. Then, how can the snowball Earth marine facies associations. hypothesis reconcile the sedimentology evidence? One possible solution that can explain such discrepancy is that, the erosive surface or unconformity observed at the base of the Nantuo onset of the Marinoan glaciation was a diachronous process in a global Formation in the shelf environment may result from ice-sheet trunca- scale. Although the Snowball Earth hypothesis asserts that its onset was tion. For example, at the Jiulongwan section, truncation of the siltstone synchronous (Hoffman et al., 2017a), regional glaciations might have at the top of the Liantuo Formation suggests that ice sheets may be occurred before the Marinoan snowball Earth. This argument is con- pushed to the inner shelf environment of the Yangtze Block. Stacked sistent with radiometric ages, although diachronous ages could also be
253 X. Lang et al. Precambrian Research 310 (2018) 243–255 attributed to low stratigraphic resolution of glacial deposits (Hoffman Sciences (grant number XDB18000000) and the Natural Science et al., 2017a). Therefore, the Stage I may represent a regional glaciation Foundation of China (41602343). We are grateful to two anonymous before the onset of Marinoan snowball Earth. If the Marinoan snowball reviewers for their constructive comments and suggestions. Earth indeed existed, we suggest that the snowball Earth might be re- presented by the Stage III deposition of the Nantuo Glaciation, re- References cording a more severe glaciation than the earlier regional glaciation (Stage I). It should be noted that the sedimentological analyses alone Allen, P.A., Etienne, J.L., 2008. Sedimentary challenge to Snowball Earth. Nature cannot differentiate a regional glaciation from a snowball Earth gla- geocience 1, 817–825. Allen, P.A., Leather, J., Brasier, M.D., 2004. The Neoproterozoic Fiq glaciation and its ciation. The assignment of the Stage III as the Marinoan snowball Earth aftermath, Huqf supergroup of Oman. Basin Res. 16, 507–534. deposit is mainly based on the assumption of the presence of a snowball Bao, H., Lyons, J.R., Zhou, C., 2008. Triple oxygen isotope evidence for elevated CO2 Earth interval during the Marinoan glaciation. The stage IV deposition levels after a Neoproterozoic glaciation. Nature 453, 504–506. Boulton, G., 1990. Sedimentary and sea level changes during glacial cycles and their that consists of a mixture of distal glaciomarine and non-glacial de- control on glacimarine facies architecture. Geol. Soc. Lond. Spec. Publ. 53, 15–52. posits indicates the melting stage of possible snowball Earth glaciation Boulton, G.S., Deynoux, M., 1981. Sedimentation in glacial environments and the iden- (Huang et al., 2016). tification of tills and tillites in ancient sedimentary sequences. Precambr. Res. 15, 397–422. Bowring, S.A., Grotzinger, J.P., Condon, D.J., Ramezani, J., Newall, M.J., Allen, P.A., 6.3. A Lag between deglaciation and cap carbonate precipitation 2007. Geochronologic constraints on the chronostratigraphic framework of the Neoproterozoic Huqf Supergroup, Sultanate of Oman. Am. J. Sci. 307, 1097–1145. fi Cap carbonate is commonly interpreted to be precipitated im- Bus eld, M.E., Le Heron, D.P., 2014. Sequencing the Sturtian icehouse: dynamic ice be- haviour in South Australia. J. Geol. Soc. 171, 443–456. mediately after termination of the glaciation or concurrent with de- Busfield, M.E., Le Heron, D.P., 2016. A Neoproterozoic ice advance sequence, Sperry glaciation (Hoffman et al., 2007; Shields, 2005; Zhou et al., 2010). Wash, California. Sedimentology 63, 307–330. However, facies analysis demonstrates that the upper part of the Condon, D., Zhu, M., Bowring, S., Wang, W., Yang, A., Jin, Y., 2005. U-Pb ages from the Neoproterozoic Doushantuo Formation, China. Science 308, 95–98. Nantuo Formation is dominated by distal glaciomarine facies associa- Condon, D.J., Prave, A.R., Benn, D.I., 2002. Neoproterozoic glacial-rainout intervals: tion and locally intercalated with non-glacial facies association (Fig. 2), observations and implications. Geology 30, 35–38. suggesting that deglaciation was initiated during the stage IV of the Eyles, C., Eyles, N., Miall, A., 1985. Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences. Palaeogeogr. Nantuo Glaciation. In addition, a thin siltstone or gravelly siltstone bed Palaeoclimatol. Palaeoecol. 51, 15–84. commonly underlies the Doushantuo cap carbonate in the Yangtze Fabre, S., Berger, G., 2012. How tillite weathering during the snowball Earth aftermath Block (Fig. 2)(Zhang et al., 2008b). These observations are consistent induced cap carbonate deposition. Geology 40, 1027–1030. Higgins, J.A., Schrag, D.P., 2003. Aftermath of a snowball Earth. Geochem. Geophys. with recent Mg isotopes study shows that an extreme chemical Geosyst. 4, 1–20. weathering event may have occurred in the upper part of the Nantuo Hoffman, P.F., Abbot, D.S., Ashkenazy, Y., Benn, D.I., Brocks, J.J., Cohen, P.A., Cox, G.M., Formation (Huang et al., 2016), considerably predating precipitation of Creveling, J.R., Donnadieu, Y., Erwin, D.H., Fairchild, I.J., Ferreira, D., Goodman, the Doushantuo cap carbonate. The moderate facies transitions from J.C., Halverson, G.P., Jansen, M.F., Le Hir, G., Love, G.D., Macdonald, F.A., Maloof, A.C., Partin, C.A., Ramstein, G., Rose, B.E.J., Rose, C.V., Sadler, P.M., Tziperman, E., the Nantuo Formation to the Doushantuo Formation indicate that a lag Voigt, A., Warren, S.G., 2017a. Snowball Earth climate dynamics and Cryogenian may have existed between the onset of deglaciation and the Doush- geology-geobiology. Sci. Adv. 3, e1600983. ff antuo cap carbonate precipitation. Ho man, P.F., Halverson, G.P., Domack, E.W., Husson, J.M., Higgins, J.A., Schrag, D.P., 2007. Are basal Ediacaran (635 Ma) post-glacial “cap dolostones” diachronous? Earth Planet. Sci. Lett. 258, 114–131. 7. Conclusions Hoffman, P.F., Kaufman, A.J., Halverson, G.P., Schrag, D.P., 1998. A Neoproterozoic snowball earth. Science 281, 1342–1346. Hoffman, P.F., Lamothe, K.G., LoBianco, S.J.C., Hodgskiss, M.S.W., Bellefroid, E.J., Detailed facies analysis of six successions of the Nantuo Formation Johnson, B.W., Hodgin, E.B., Halverson, G.P., 2017. Sedimentary depocenters on in the Yangtze Block was conducted in this study. The main conclusions Snowball Earth: Case studies from the Sturtian Chuos Formation in northern are listed as below: Namibia. Geosphere 13, 811–837. Hoffman, P.F., Li, Z.-X., 2009. A palaeogeographic context for Neoproterozoic glaciation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 277, 158–172. (1) The Nantuo Formation includes ten lithofacies and three facies as- Hoffman, P.F., Schrag, D.P., 2002. The snowball Earth hypothesis: testing the limits of sociations. These facies and facies associations display pronounced global change. Terra Nova 14, 129–155. ff Hu, J., Wang, J., Chen, H., Wang, Z., Xie, L., Lin, Q., 2011. Multiple cycles of glacier lateral and vertical changes among di erent sections. The change of advance and retreat during the Nantuo (Marinoan) glacial termination in the Three well-correlated facies associations may suggest dynamic ice sheets Gorges area. Front. Earth Sci. 6, 101–108. in certain period of the Nantuo Glaciation. Huang, J., Feng, L., Lu, D., Zhang, Q., Sun, T., Chu, X., 2014. Multiple climate cooling (2) Four depositional stages of the Nantuo Glaciation are recognized: prior to Sturtian glaciations: evidence from chemical index of alteration of sediments in South China. Scientific Rep. 4, 6868. Stage I includes an entire cycle of ice sheet advance-retreat, re- Huang, K.-J., Teng, F.-Z., Shen, B., Xiao, S., Lang, X., Ma, H.-R., Fu, Y., Peng, Y., 2016. presenting the first episode of the Nantuo Glaciation. Stage II re- Episode of intense chemical weathering during the termination of the 635 Ma – presents ice sheets retreat to high latitude regions, implying an Marinoan glaciation. Proc. Natl. Acad. Sci. 113, 14904 14909. Jiang, G., Kennedy, M.J., Christie-Blick, N., Wu, H., Zhang, S., 2006. Stratigraphy, interglacial period during the Nantuo Glaciation. Re-advance of ice Sedimentary Structures, and Textures of the Late Neoproterozoic Doushantuo Cap sheets in the second episode of Nantuo Glaciation (Stage III) is Carbonate in South China. J. Sediment. Res. 76, 978–995. succeeded by the deglacial interval of Stage IV. Jiang, G., Shi, X., Zhang, S., Wang, Y., Xiao, S., 2011. Stratigraphy and paleogeography of the Ediacaran Doushantuo Formation (ca. 635–551Ma) in South China. Gondwana (3) Sedimentary facies analysis of the Nantuo Formation indicates that Res. 19, 831–849. the Nantuo Glaciation includes at least two major episodes of gla- Jiang, G., Sohl, L.E., Christie-Blick, N., 2003. Neoproterozoic stratigraphic comparison of ciation that were separated by an interglacial interval. The dis- the Lesser Himalaya (India) and Yangtze block (south China): paleogeographic im- plications. Geology 31, 917–920. covery of non-glacial facies association in the top of the Nantuo Jiang, Y., Tao, Y., Gu, Y., Wang, J., Qiang, Z., Jiang, N., Lin, G., Jiang, C., 2016. Formation suggests that the melting of the Nantuo Glaciation was Hydrothermal dolomitization in Dengying Formation, Gaoshiti-Moxi area, Sichuan earlier than the deposition of the Doushantuo cap carbonate. Basin, SW China. Pet. Explor. Dev. 43, 54–64. Kirschvink, J.L., 1992. Late Proterozoic low-latitude global glaciation: the snowball Earth. The Proterozoic Biosphere: a multidisciplinary study. 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