Chapter 18. the Ediacaran Period

Home , Beothukis, Charnia, Charniodiscus, Corumbella, Dengying Formation, Doushantuo Formation

S.H. Xiao and G.M. Narbonne Chapter 18

600 Ma Ediacaran

1800

Ediacaran

CR Scotese, PALEOMAP Project

Chapter outline 18.1 Historical background 522 18.3.4 Chemical evolution of Ediacaran oceans 537 18.2 Cap dolostones and the base of the Ediacaran System 525 18.3.5 Radioisotopic dating 543 18.3 The biostratigraphic basis for the Ediacaran System 526 18.4 Toward an Ediacaran chronostratigraphy 544 18.3.1 Ediacaran megafossils and trace fossils 526 Acknowledgments 547 18.3.2 Ediacaran microfossils 535 References 547 18.3.3 Ediacaran glaciations 537

Abstract founded on a holistic approach integrating biostratigraphic, che- The Ediacaran Period (635À538 Ma) has the longest duration mostratigraphic, paleoclimatic and geochronometric data, particu- among all stratigraphically defined geological periods. The basal larly carbon and strontium isotopes, glacial diamictites, boundary of the Ediacaran System is defined by a horizon near acanthomorphic acritarchs, Ediacara-type megafossils, and certain the base of the Nuccaleena Formation overlying the Cryogenian tubular fossils. Our preferred scheme is to divide the Ediacaran diamictite of the Elatina Formation at the Enorama Creek section System into two series separated by the 580 Ma Gaskiers glaciation. Stage-level subdivisions at the bottom and top of the in South Australia. Most Ediacaran fossils represent soft-bodied Ediacaran System, including the definition of the second organisms and their preservation is affected by taphonomic biases. Ediacaran stage (SES) and the terminal Ediacaran stage (TES), are Thus the Phanerozoic approach of defining stratigraphic bound- feasible in the near future. Additional Ediacaran stages between aries using the first appearance datum of widely distributed, rap- the SES and TES can be envisioned, but formal definition of these idly evolving, easily recognizable, and readily preservable species stages are not possible until various stratigraphic markers are thor- would have limited success in the Ediacaran System. The subdivi- oughly tested and calibrated at both regional and global scales. sion and correlation of the Ediacaran System must therefore be

18.1 Historical background The designation of the Ediacaran Period reflected the gradual solution of a problem that had vexed even The basal boundary of the Ediacaran System was ratified Darwin (1859) in his writing of “The Origin of Species,” in 2004 (Knoll et al., 2004, 2006), with its GSSP (Global the apparently abrupt appearance of diverse groups of Boundary Stratotype Section and Point) defined by a hori- shelly fossils at the base of the Cambrian System without zon near the base of the Nuccaleena Formation overlying any obvious Precambrian ancestors. Darwin attributed this the Cryogenian diamictite of the Elatina Formation at the absence to massive record failure, a view formalized by Enorama Creek section in South Australia (Fig. 18.1). Walcott (1914) in his designation of the “Lipalian

Base of the Ediacaran System at Enorama Creek, Flinders Ranges, South Australia

N Parachilna Acraman impact ejecta Australia Blinman N GSSP Ediacara siltstone GSSP Mbr Wilpena trough Chace crossbed Hawker Mbr sandstone Rownsley Quartzite Rownsley

Bonny sandstone Port Augusta Sandstone diamictite 1000 m 1000

silty Wonoka limestone

cap Ediacaran dolostone

Bunyeroo AIE Top of cap carbonate ABC 6 5 Adelaide 4 3 Brachina 2 0 100km (A) 1 -3 -2 -1 stratigraphic height (m) 13 o Nuccaleena GSSP δ C /oo Elatina (B) Nuccaleena Cry. Formation

20 cm Teepee-structure, Nuccaleena Formation 0 cm GSSP Elatina Formation (C) (D)

FIGURE 18.1 GSSP for base of the Ediacaran at Enorama Creek section, central Flinders Ranges, Adelaide Rift Complex, South Australia. (A) Map showing location of the GSSP. (B) Generalized stratigraphic column showing the GSSP level, which is defined as the sharp base of the cap carbonate (Nuccaleena Formation) overlying the Marinoan glacial and glaciomarine diamictite deposits (Elatina Formation). Carbonate carbon isotopevalues in the cap carbonate are negative and decrease upsection in the cap dolostone. (C) Field photograph showing the exact location of the GSSP. (D) Enigmatic teepee-like structures, up to 1 m in amplitude, in the Nuccaleena cap dolostone. GSSP, Global Boundary Stratotype Section and Point. (A) Modified from Ogg et al. (2016); (B) Modified from Knoll et al. (2006) and Xiao et al. (2013). (C) Photograph by S. Xiao. (D) Photo courtesy Gabi Ogg. The Ediacaran Period Chapter | 18 523

Interval” of erosion beneath the base of the Cambrian. of an Ediacarian/Ediacaran System as the terminal The concept of a global period of erosion prior to the Neoproterozoic chronostratigraphic interval has evolved, Cambrian was soon contradicted by discovery of thick although Ediacara-type megafossils as a key symbol of successions of largely unmetamorphosed strata concor- this interval have remained the same. What has evolved is dantly beneath the base of the Cambrian in numerous the basal boundary age and the duration of the Ediacaran localities worldwide, most notably the “Sinian” in China System (Fig. 18.2). Jenkins (1981) formally proposed the (Grabau, 1922; Lee, 1924), the “Marinoan” in Australia Ediacaran System and placed its basal boundary at the (Mawson and Sprigg, 1950), and the “Vendian” in Russia base of the Wonoka Formation at Bunyeroo Gorge in (Sokolov, 1952). These names were originally proposed South Australia. The basal boundary of Jenkins’ (1981) as regional lithostratigraphic units that later assumed a Ediacaran System is thus stratigraphically higher than the chronostratigraphic significance, but scarcity of reliable GSSP of the Ediacaran System as currently defined radioisotopic dates and a lack of consistent criteria for (Fig. 18.1) and corresponds to the sequence boundary correlation frustrated early attempts to extend these divi- underlying the oldest unequivocal Ediacara-type fossils in sions globally. Australia. Lacking any robust geochronological data, Subsequent paleontological research confirmed Darwin’s Jenkins (1981) estimated that his Ediacaran System began view that there was abundant life before the beginning or at B640 Ma. Independent of Jenkins (1981), Cloud and the Cambrian but showed that most Precambrian organisms Glaessner (1982) formalized the Ediacarian System, with were soft-bodied and many were also microscopic. Perhaps most distinctive among these organisms were Ediacara-type megafossils, best seen in the Ediacara Hills of South

Australia, which gives its name to the Ediacaran Period. 2012 2004

These are centimeter- to meter-scale impressions of soft- 540 Ma Narbonne et al. Robb et al. bodied organisms that typically were preserved as impres- 1982

sions at the bases of event beds of sand or volcanic ash Cloud & Glaessner 550 Ma (Narbonne, 2005; Fedonkin et al., 2007). They were first described in the late 19th century (Billings, 1872)butfew paleontologists at that time were willing to accept the wide- 560 Ma spread occurrence of megascopic Precambrian life, and Jenkins, 1981 even the complex Ediacaran fossils reported from Namibia 570 Ma 1990 Harland et al., 1991 Plumb, and Australia in the 1930s and 1940s were tentatively regarded as “Cambrian” by their discoverers (Gu¨rich, 1930, 580 Ma 1933; Sprigg, 1947). Ford’s (1958) description of the frond Ediacaran Series Charnia from unequivocally Precambrian strata in central 590 Ma 1982 Harland et al., Ediacaran System England led Glaessner (1959) to propose a global “Ediacara Ediacaran System

Fauna” of large, soft-bodied animal-like fossils that immedi- System Vendian 600 Ma

ately preceded the Cambrian, a concept that exists to the Sinian Erathem present day and has proven instrumental in subsequent rec- Varangian Series Varangian ognition of a terminal Proterozoic System. The discovery of 610 Ma Ediacarian System Ediacaran System Ediacaran abundant and diverse microfossils, particularly acantho- III Neoproterozoic morphic acritarchs, has also significantly enhanced the pro- 620 Ma Ediacaran Series spects for Ediacaran biostratigraphy. This area of research Sinian Erathem was first pioneered by Russian and later Chinese micro- 630 Ma paleontologists (Timofeev, 1966; Yin and Li, 1978; Vendian System Vendian Jankauskas et al., 1989), and some of Ediacaran acantho- 640 Ma Sinian Erathem

morphs have been shown to have restricted time ranges Sturtian System

and a global distribution (Zang and Walter, 1992; Mortensnes Epoch 650 Ma Moczydłowska et al., 1993; Zhang et al., 1998; Grey, 2005; Vorob’eva et al., 2009b; Moczydłowska and

660 Ma Cryogenian System Cryogenian System Nagovitsin, 2012; Liu et al., 2014f; Xiao et al., 2014b). Series Varangian A year after Glaessner’s recognition of a global 670 Ma Epoch Smalfjord Ediacaran macrofossil assemblage below the Cambrian, Sturtian Termier and Termier (1960) proposed the “Ediacarien” as System Cryogenian System Precambrian chronostratigraphic interval characterized by FIGURE 18.2 Evolution of the concept of Ediacaran/Ediacarian this distinctive fossil assemblage. Since then, the concept System or Series. 524 PART | III Geologic Periods: Planetary and Precambrian

its lower boundary defined at the base of the Nuccaleena Working Group (later Subcommission) on the Terminal Formation in the Bunyeroo Gorge in South Australia, and Proterozoic Period at the IGC in Washington, DC. Over estimated that the Ediacarian System began at 670 Ma. the succeeding decade and a half, the Working Group/ Building upon the work of Jenkins (1981), Harland et al. Subcommission formally visited terminal Neoproterozoic (1982) introduced the Ediacaran Series, placed it in the sections on five continents and held a series of international Vendian System of the Sinian Erathem, and estimated its symposia to discuss the features that could be used to age to be 630À590 Ma, although they later revised define and correlate a terminal Neoproterozoic system. its age to be 590À570 Ma (Harland et al., 1990). After These scientific discussions corresponded with rapid dis- Ediacaran System was ratified (Knoll et al., 2004, 2006) coveries of new scientific techniques in geochronology, and precise radioisotopic ages were published to constrain isotopic chemostratigraphy, sequence stratigraphy and pale- the CryogenianÀEdiacaran boundary (Hoffmann et al., ontology that significantly elucidated the characteristics 2004; Condon et al., 2005), the base of the Ediacaran and history of the Neoproterozoic Earth. After 10 years of System was fixed at 635 Ma (Robb et al., 2004; Narbonne investigation a series of ballots increasingly focused the et al., 2012a). decision on the position of the GSSP and name for this Climatic indicators also provide critical information for new period (see Knoll et al., 2006 for details of the ballots Neoproterozoic subdivision. Late Precambrian glacial and opinions). The first ballot (December 2000) asked deposits were first recognized in the late 1800s (Thomson, whether the stratigraphic level of the GSSP should be 1871; Reusch, 1891) with a steadily increasing number of placed at (1) the base of the Varanger/Marinoan glacial reports throughout the 20th century (Hoffman et al., 2017). deposits (Harland et al., 1982; Sokolov and Fedonkin, In a landmark paper, Harland and Rudwick (1964) summa- 1982); (2) the cap carbonate atop these deposits (Cloud and rized Precambrian glacial deposits on every continent Glaessner, 1982); (3) a biostratigraphic level corresponding except Antarctica and proposed the “Infra-Cambrian” as a to the first appearance of Ediacaran macrofossils in a local period of continental glaciation before the Cambrian. section (Jenkins, 1981); or (4) some other level. Most of Evidence that these glacial deposits consistently predated the voting members believed that a boundary at the top Ediacara-type fossils in the same sections led Harland and rather than the base of the Varanger/Marinoan glacial colleagues to propose a Vendian System that consists of a deposits would be more significant in the Earth evolution lower Varangian Series characterized by glaciations and an since most Neoproterozoic glacial deposits would be upper Ediacaran Series characterized by Ediacara-type regarded as Cryogenian, whereas all known assemblages of megafossils (Fig. 18.2)(Harland and Herod, 1975; Harland diverse Ediacara-type fossils would fall into the succeeding et al., 1982, 1990). It is now known that a glaciation—the period, and that the end of a glacial interval marked by a 580 Ma Gaskiers glaciation—also occurred in the Ediacaran distinctive cap carbonate would probably be more easily Period (Eyles and Eyles, 1989; Pu et al., 2016), and this correlated than the base of glacial deposits, which would event may serve as an important tool for the subdivision likely be highly diachronous both regionally and globally. and correlation of the Ediacaran System. There was little support for placing the system-level bound- Ediacaran strata are characterized by major, apparently ary at the first appearance of megafossils but it was consid- synchronous chemostratigraphic excursions in carbon iso- ered that this level might prove useful in later subdivision topes (Knoll et al., 1986; Kaufman and Knoll, 1995; Zhou into series or stages. The second ballot (March 2003) and Xiao, 2007; Halverson et al., 2010; Macdonald et al., received a strong mandate (63% of votes cast) for placing a 2013), strontium isotopes (Kaufman et al., 1993; Halverson GSSP defined using these agreed characteristics at the et al., 2007, 2010) and sulfur isotopes (Halverson et al., “Enorama Creek Section, Flinders Ranges, South 2010), which provide another criterion for recognition and Australia.” The final ballot (September 2003) received correlation of a terminal Neoproterozoic system worldwide. overwhelming approval (89% of votes cast) to establish the Neoproterozoic isotope excursions are not unique in either GSSP for the Terminal Proterozoic Period “at the base of shape or magnitude, but in conjunction with biostratigraphy the Nuccaleena Formation cap carbonate, immediately (Narbonne et al., 1994; Knoll et al., 1995; Xiao et al., above the Elatina diamictite in the Enorama Creek section, 2012), climatic indicators (Kaufman et al., 1997) and pre- Flinders Ranges, South Australia” and a clear mandate cise UÀPb dates (Grotzinger et al., 1995; Condon et al., (79% of votes cast) that the new period and system be 2005; Noble et al., 2015), these can provide an exception- named the “Ediacaran.” These decisions were ratified by ally useful tool for global correlation of Neoproterozoic the ICS on February 20, 2004. A full-length description of strata. the GSSP and the selection process was subsequently pub- Internationally coordinated efforts to establish the lished in Lethaia (Knoll et al., 2006). Ediacaran GSSP using integrative biostratigraphic, chemo- Shortly after the ratification in 2004, the Subcommission stratigraphic and paleoclimatic data started in 1989 when on Ediacaran Stratigraphy (later renamed the Subcommission the International Commission on Stratigraphy launched a on Neoproterozoic Stratigraphy) was established to facilitate The Ediacaran Period Chapter | 18 525

the correlation and subdivision of the Cryogenian and China (2018), Spain (2019) and South America (planned Ediacaran periods. The Subcommission on Neoproterozoic in 2020). In 2018 the Subcommission launched a working Stratigraphy conducted a survey in 2009 to gauge the com- group to focus on the series-level subdivision of the munity’s opinion on how to proceed with the subdivision Ediacaran System. and correlation of the Ediacaran System. The survey results suggested that the Ediacaran System should be first divided into two series, with the lower series characterized by 18.2 Cap dolostones and the base of the acanthomorphic acritarchs and the upper series by Ediacara- Ediacaran System type macrofossils (Xiao et al., 2016). It further suggested that, ideally, the base of the upper Ediacaran series should be The GSSP for the Ediacaran System in Enorama Creek, placed within an outcrop section that has good radioisotopic Australia was fixed at the base of the Nuccaleena Dolomite, age constraints and has the potential for carbon isotope che- a 6-m-thick, distinctive dolostone that caps the Marinoan mostratigraphic and acanthomorphic acritarch biostrati- glaciomarine diamictites of the Elatina Formation graphic correlation. At that time, it was felt that (1) the (Fig. 18.1AÀC). The Nuccaleena cap dolostone is com- subdivision of the Ediacaran System should start with the posed of buff-weathering, creamy pink microcrystalline establishment of two series before the definition of stages; dolomite organized in centimeter-scale event beds. Meter- (2) the most useful correlation tools included chemostratigra- scale teepee-like structures (Fig. 18.1D) and horizontal phy such as δ13Cand87Sr/86Sr excursions, acanthomorphic sheet cracks filled with synsedimentary calcite cement are acritarchs and Ediacara-type macrofossils; and (3) good common. Carbonate carbon isotopes become increasingly GSSPs should be fossiliferous, geochronometrically dated, negative from about 22m to 23.5m upward through the and have mixed lithologies and the potential for chemostrati- Nuccaleena cap dolostone (Fig. 18.1B)(seeKnoll et al., graphic and biostratigraphic correlations. 2006 and references therein). In 2012 the Subcommission on Neoproterozoic Strikingly similar “cap dolostones” occur worldwide on Stratigraphy was split into the Subcommission on the top of terminal Cryogenian Marinoan-age glacial depos- Cryogenian Stratigraphy and the Subcommission on its (Fig. 18.3D) or an unconformity surface corresponding Ediacaran Stratigraphy. The stated goal of the to this glaciation (Hoffman, 2011) and serve as a superb Subcommission on Ediacaran Stratigraphy is to facilitate global lithostratigraphic and chemostratigraphic marker for international communication and scientific cooperation in thebaseoftheEdiacaran.Thesecapdolostonesaretypi- Ediacaran stratigraphy, with the ultimate goal to define, cally less than 10 m thick, and like the cap at the GSSP are by means of GSSPs, a hierarchy of chronostratigraphic buff-weathering dolostones with horizontal sheet cracks and units that provide the framework for global correlation. In meter-scale teepee-like structures (Fig. 18.1D). Other fea- the past decade the Subcommission on Ediacaran tures typifying the basal Ediacaran cap dolostone include Stratigraphy and its predecessor organized and sponsored macropeloids, unusual stromatolite facies, barite crystals, several field workshops to examine potential criteria for and cylindrical tubes of carbonate (Kennedy, 1996; James Ediacaran subdivision and correlation (Moczydłowska- et al., 2001; Xiao et al., 2004; Corsetti and Grotzinger, Vidal et al., 2008; Kumar and Sharma, 2010; Xiao et al., 2005; Jiang et al., 2006; Hoffman et al., 2007, 2017; 2011, 2014a; Gehling and Droser, 2012; Gehling et al., Hoffman, 2011). The cap dolostones are commonly over- 2012; Kumar and Sharma, 2012; Narbonne et al., 2012b; lain by limestones rich in aragonite crystal fans Kaufman et al., 2014; Pandey and Dimri, 2014; Singh and (Fig. 18.3C) and/or by deep-water fine-grained siliciclastics Ansari, 2014; Xiao and Sharma, 2014; Liu et al., 2018a). (Hoffman et al., 2007; Hoffman, 2011). This suite of fea- As part of the efforts toward the establishment of intra- tures permits most or all basal Ediacaran (post-Marinoan) Ediacaran GSSPs, the Subcommission has encouraged— cap dolostones to be readily distinguished from the dark, at various scientific meetings and workshops, notably the bituminous limestones that characterize older post-Sturtian 2014 Ediacaran Workshop in Wuhan, China, and the caps (Kennedy et al., 1998) and from the light gray micritic STRATI 2015 meeting in Graz, Austria—community- (recrystallized to sparry calcite) limestone atop the younger wide discussions about the subdivision and correlation of (mid Ediacaran) Gaskiers diamictite (Fig. 18.3G)(Myrow Ediacaran strata. In 2015 the Subcommission on and Kaufman, 1999). Interpretations of Neoproterozoic cap Ediacaran Stratigraphy established the Second Ediacaran dolostones vary considerably, but most interpretations Stage Working Group and the Terminal Ediacaran Stage involve a perturbation in the saturation state of the oceans Working Group to focus on these stages. This was fol- accompanying the rapid meltdown of continental ice sheets lowed by several focused field workshops to examine crit- (Hoffman et al., 2017). ical terminal Ediacaran Stage (TES) sections in Namibia Although the Ediacaran GSSP in South Australia has (2016) (Xiao et al., 2017b), Brazil (2017) (Xiao et al., not been dated, radioisotopic ages constrain the base of 2018), Oman (2018), western United States (2018), South the Ediacaran System to be around 635 Ma. The 526 PART | III Geologic Periods: Planetary and Precambrian

(A) (B) (C)

“C3”

1 m (E) (F) C2 Streptichnus & other complex traces 0 m Swartpuntia Pteridinium

cap dolostone C1 Doushantuo Fm Nantuo Fm (D) (G) (H) volcanic ash 540.61 ± 0.67 Ma

FIGURE 18.3 Field photographs of Ediacaran outcrops: (A) Overview of the Flinders Ranges succession at Wilpena Pound. The lip of the syncline marks the disconformable base of the Cambrian over shallow-water Ediacaran sandstones and shales. Ediacara-type fossil impressions occur in the banded sandstones below the massive quartzite ridge at the top of the bluff. (B) Overview of c. 1 km upper CryogenianÀbasal Ediacaran strata near Shale Lake, NW Canada. The prominent light ridge and scree marks the basal Ediacaran cap dolostone (Ravensthroat Formation) overlying the terminal Cryogenian Ice Brook Formation (after Fig. 4A of James et al., 2001). (C) Aragonite crystal fans in the Hayhook Formation (limestone) overlying the Ravensthroat Formation (cap dolostone). (D) Basal Ediacaran cap dolostone (Doushantuo Formation) overlying the terminal Cryogenian Nantuo Formation at Huajipo, Yangtze Gorges area, South China. Unit “C3” was previously considered as part of the cap dolostone (Jiang et al., 2006) but is now excluded from the cap dolostone (Wang et al., 2017b). (E) Ribbon rocks (interbedded limestone and dolostone) in the upper Doushantuo Formation at Guancaiyao, Yangtze Gorges area, South China. This lithofacies records EN3, which is considered as an equivalent of the Shuram negative δ13C excursion. Coin is B20 mm in diameter. (F) Ribbon rocks of the Shuram Formation at Wadi Bani Awf, Jabal Akhdar, northeastern Oman, which records the Shuram negative δ13C excursion. (G) Mid-Ediacaran (B580 Ma) Gaskiers Diamictite in Harbour Main, Newfoundland. Strata strongly dipping with stratigraphic top to the left of the photograph. Note abundant clasts and increasing red color upward through the Gaskiers Formation, white capping limestone (left of figure), and overlying thin turbidites of the Drook Formation. Inset shows a striated clast in the Gaskiers Formation. (H) Uppermost Ediacaran strata at Swartpunt, Namibia showing stratigraphic levels of key fossils and a dated volcanic ash bed (Grotzinger et al., 1995). The section is approximately 125 m thick. See Narbonne et al. (1997) for details. (A) Photo by J.G. Gehling.

CryogenianÀEdiacaran boundary is constrained to be suggest that the CryogenianÀEdiacaran boundary is at ,636.41 6 0.45 Ma in Kings Island of Tasmania, Australia 635 Ma. (Calver et al., 2013), ,635.21 6 0.59 Ma in northern Namibia (Prave et al., 2016), and .632.3 6 5.9 Ma in northwestern Canada (Rooney et al., 2015). Importantly, the 18.3 The biostratigraphic basis for the CryogenianÀEdiacaran boundary is bracketed between a Ediacaran System 634.57 6 0.88 Ma age from the uppermost Nantuo Formation of terminal Cryogenian age (Zhou et al., 2019)anda 18.3.1 Ediacaran megafossils and trace fossils 635.23 6 0.57 Ma age from just above the basal Ediacaran Ediacara-type megafossils represent the first abundant, cap dolostone (Condon et al., 2005). These two ages are large, and architecturally complex organisms in the Earth indistinguishable within analytical errors, and together they evolution. Their impressions on the bases of event beds The Ediacaran Period Chapter | 18 527

provide the most distinctive and readily recognizable et al., 2017; Dunn et al., 2018), an interpretation that has character of the Ediacaran System and appear to be a reli- been independently confirmed by biomarker data able indicator of the upper part of this system worldwide (Bobrovskiy et al., 2018). (Figs. 18.4 and 18.5). The affinities of the Ediacara biota Ediacara-type fossils have been reported from more are contentious—some groups such as the rangeomorphs, than 30 regions worldwide (Fedonkin et al., 2007; arboreomorphs and erniettomorphs may not be ancestral Laflamme et al., 2013). A few possible pre-Ediacaran pre- to any Phanerozoic or living lifeforms (Dececchi et al., cursors (Hofmann et al., 1990) and post-Ediacaran survi- 2017, 2018), whereas other forms such as Dickinsonia, vors are known (Jensen et al., 1998; Hagadorn et al., Yorgia, and Kimberella preserving evidence of locomo- 2000; Hoyal Cuthill and Han, 2018), but in general, tion and feeding arguably represent stem-group animals Ediacara-type fossils are strictly restricted to the upper (Gehling et al., 2005, 2014; Ivantsov, 2013). The iconic half of the Ediacaran System. Some occurrences (e.g., Ediacara fossil Dickinsonia has highly regulated develop- Finnmark in northern Europe) are low in diversity and/or mental patterns indicative of a eumetazoan or bilaterian contain only simple disks such as Aspidella, Nemiana affinity (Gold et al., 2015; Evans et al., 2017; Hoekzema and Hiemalora that are of limited use in biostratigraphy

FIGURE 18.4 Representative Ediacara-type megafossils. Parts (A and B), (C and D), and (E and F) are examples from the Avalon, White Sea, and Nama assemblages, respectively. (A) Rangeomorph Beothukis mistakensis (NFM F-758), Trepassey Formation, Spaniard’s Bay, Newfoundland (Narbonne et al., 2009). (B) Rangeomorph Fractofusus misrai, surface E, Mistaken Point, Newfoundland (Gehling and Narbonne, 2007). (C) Tribrachidium heraldicum (lower) and possibly Solza margarita (upper) from the Yorga Formation, Zimnie Gory, White Sea, Russia. (D) Eoandromeda octobrachiata from the upper Doushantuo Formation, northeastern Guizhou Province, South China. (E) Rangea schneiderhoehoni from the Nama Group, southern Namibia. Specimen reposited in Geological Survey of Namibia. (F) Swartpuntia germsi from the Spitskopf Member, Urusis Formation, Nama Group, Farm Swartpunt, southern Namibia. (A and C) Courtesy Marc Laflamme. 528 PART | III Geologic Periods: Planetary and Precambrian

Cambrian Erniettomorphs 540 Chapel Island Schwarzrand

545 Rangeomorphs Khatyspyt

Nama Kuibis Arboreomorph Discoids

Assemblage Shibantan

550 Dickinsoniomorphs Octoradialomorph Kimberellomorph Bilaterialomorphs Triradialomorphs Pentaradialomorph Tetraradialomorph Ediacara Ernietta

555 Zimmie Gory Cloudina Arkarua White Sea Assemblage Phyllozoon Yorgia Vendia Eoandromeda 560 Rangea

Charnwood Spriggina Kimberella Conomedusites Pteridinium Dickinsonia Ventogyrus Fermeuse Swartpuntia Parvancorina Trepassey Tribrachidium 565 Mistaken Point Ediacara-type megafossil assemblages Eoandromeda Briscal Ernietta Late Ediacaran Period 570 Drook Dickinsonia Avalon Assemblage Avalon Kimberella Hiemalora Cloudina Palaeopascichnus

Bradgatia Tribrachidium

575 Charnia Fractofusus Aspidella Doushantuo biota Charniodiscus / Arborea Fractofusus Arborea 580 Gaskiers Glaciation (582 Ma)

FIGURE 18.5 Ediacara-type megafossil zonation of the upper Ediacaran System. After Xiao and Laflamme (2009), with updated age constraints from Schmitz (2012), Pu et al. (2016), and Linnemann et al. (2019).

(Ho¨gstro¨m et al., 2013). Diverse assemblages suitable for calcification (Droser et al., 2017). Use of these assemblages biostratigraphy have been described from Australia for biostratigraphy is complicated by obvious evidence for (Flinders Ranges); Europe (White Sea, Urals, Ukraine, both environmental (Grazhdankin, 2004; Gehling and Droser, and central England); North America (Newfoundland, 2013; Boag et al., 2016) and taphonomic (Gehling et al., Mackenzie Mountains, Nevada, North Carolina); Africa 2005; Narbonne, 2005) influences on their composition, but (Namibia); and Asia (South China, Siberia). Possible this problem is not unique to Ediacaran megafossils and Ediacara-type fossils described from India and South affects all fossil groups of all ages to varying degrees. America require further study and substantiation. In gen- The Avalon assemblage (B571À560 Ma; Figs. 18.4A eral, Ediacara-type fossils and trace fossils are relatively and B, 18.5) is known only from deep-water deposits in common in both shallow- and deep-water siliciclastics Newfoundland (Misra, 1969; Narbonne and Gehling, (Narbonne et al., 2014) and have also been reported spo- 2003; Narbonne, 2004; Hofmann et al., 2008; Liu et al., radically from shallow- and deep-water carbonates 2014a,c; Mitchell et al., 2015), England (Ford, 1958; (Narbonne and Aitken, 1990; MacNaughton et al., 2000; Brasier and Antcliffe, 2009; Wilby et al., 2011), and the Grazhdankin et al., 2008; Chen et al., 2014; Bykova June beds in SE Mackenzie Mountains of NW Canada et al., 2017), enhancing global correlation based on (Narbonne and Aitken, 1990, 1995; Narbonne et al., Ediacara-type megafossils and trace fossils. Calcified 2014). Grazhdankin et al. (2008) showed that some long- megafossils such as Cloudina, Namacalathus and ranging and cosmopolitan taxa of the Avalon assemblage Sinotubulites are common in latest Ediacaran shallow- (such as Charnia and Hiemalora) persist in deep-water water carbonates worldwide, but Ediacara-type impres- deposits to the end of the Ediacaran, but these younger sions occur only rarely in these facies. deep-water assemblages typically also contain Ediacaran Available dates allow three broad assemblages to be rec- fossils typical of younger Ediacaran assemblages (e.g., ognized, including the Avalon, White Sea, and Nama assem- Hofmann and Mountjoy, 2010). The Avalon assemblage blages (Fig. 18.5)(Waggoner, 2003). Each assemblage (s.s.) consists largely of rangeomorphs (Fig. 18.4A and exhibits a major diversity change and evolutionary innovation B), fossils consisting of centimeter-scale elements exhibit- in complex multicellularity, segmentation, mobility, or ing self-similar branching that were used as modules to The Ediacaran Period Chapter | 18 529

build decimeter- to meter-scale constructions such as from the Doushantuo Formation of China (Xiao et al., 2002; Beothukis (Fig. 18.4A), Fractofusus (Fig. 18.4B), Charnia Ye et al., 2017) are also age-equivalent to the White Sea and Bradgatia (Narbonne, 2004). The oldest Ediacara- assemblage, a view supported by the presence of the Miaohe type fossils are specimens of the conical fossil Thectardis fossil Eoandromeda (Fig. 18.4D) and certain algal fossils in that occur approximately 150 m below an ash dated at theWhiteSeaassemblage(Xiao et al., 2013; Golubkova 570.94 6 0.38 Ma (Pu et al., 2016) in the Drook et al., 2018). The White Sea assemblage also witnessed the Formation of eastern Newfoundland (Narbonne and first appearance of abundant and reasonably unequivocal ani- Gehling, 2003), and a diverse assemblage of juvenile spe- mal burrows (Gehling and Droser, 2018). Simple, cimens of Trepassia wardae, Charnia masoni and other unbranched, subhorizontal burrows such as Planolites and Ediacaran fronds directly underlies this ash and is directly Helminthoidichnites dominate the assemblage, with some like dated by it (Liu et al., 2012). These fossils are ,9.5 Torrowangea showing beaded fills implying peristalsis and million years after the end of the 580 Ma Gaskiers glacia- active backfill (Narbonne and Aitken, 1990; Jensen et al., tion, which also corresponds with geochemical evidence 2006; Gehling and Droser, 2018). Most of these simple for a significant rise in deep-sea oxygen levels (Canfield Ediacaran trace fossil genera also range into the Phanerozoic, et al., 2007), implying a causal relationship between but the appearance of abundant Ediacaran burrows may mark Neoproterozoic glaciation, oxygenation, and the rise of a significant evolutionary and biostratigraphic event (Jensen complex eukaryotic life (Narbonne, 2010). Ediacaran fos- et al., 2006). sils have not been found in age-equivalent shallow-water The Nama assemblage (B550À538 Ma; Figs. 18.4E deposits, implying that these early experiments in com- and F, 18.5) may represent a declining phase of the plex megascopic life originated in deep-sea settings Ediacara biota (Shen et al., 2008; Boag et al., 2016; (Xiao, 2004b; Narbonne et al., 2014). Liu et al. (2010, Darroch et al., 2016, 2018a,b; Muscente et al., 2018), 2014a), Menon et al. (2013), and Liu and McIlroy (2015) although new taxa and new ecology did evolve in this described biological features interpreted as trace fossils assemblage (Schiffbauer et al., 2016; Droser et al., 2017). from the Avalon assemblage in Newfoundland, but the The Nama assemblage includes Ediacara-type fossils and origin of these structures as trace fossils is uncertain biomineralized tubular fossils that are best known from (Buatois and Ma´ngano, 2016) and trace fossils are not terminal Ediacaran siliciclastic successions in Namibia known from equivalent strata in NW Canada (Narbonne (Gu¨rich, 1930; Germs, 1972; Narbonne et al., 1997; et al., 2014). In general, evidences of mobility are either Grotzinger et al., 2000; Grazhdankin and Seilacher, 2002, absent from or exceedingly rare in the Avalon assemblage 2005; Vickers-Rich et al., 2013; Darroch et al., 2016; worldwide. Ivantsov et al., 2016); NW Canada (Carbone et al., The White Sea assemblage (B560À550 Ma; Figs. 18.4C 2015); Nevada (Smith et al., 2016, 2017); and Brazil and D, 18.5) has the highest taxonomic diversity among the (Babcock et al., 2005; Warren et al., 2012; Pacheco et al., three assemblages of Ediacara-type fossils (Shen et al., 2008; 2015; Adoˆrno et al., 2017; Becker-Kerber et al., 2017), as Droser and Gehling, 2015; Droser et al., 2017). It is best well as carbonate successions in China (Sun, 1986; Xiao known from shallow-water settings in the White Sea, Urals, et al., 2005; Chen et al., 2008, 2014; Cai et al., 2015, and Ukraine in Eastern Europe (Fedonkin, 1981, 1990, 1992) 2017) and northern Siberia (Grazhdankin et al., 2008; and in the Flinders Ranges of Australia (Sprigg, 1947; Bykova et al., 2017). These contain depauperate assem- Glaessner and Wade, 1966; Jenkins, 1992; Gehling et al., blages of Ediacara-type fossil impressions, mainly rangeo- 2005; Droser and Gehling, 2015; Droser et al., 2018). The morphs and erniettomorphs also known from the preceding White Sea assemblage contains a depauperate assemblage of assemblages (Xiao and Laflamme, 2009; Boag et al., rangeomorph taxa, many of them holdovers from the preced- 2016), but Swartpuntia (Narbonne et al., 1997; Hagadorn ing Avalon assemblage, along with other fronds such as and Waggoner, 2000)andErnietta (Ivantsov et al., 2016; Arborea and “Charniodiscus.” New developmental plans Smith et al., 2017) first appear in the Nama assemblage of include erniettomorphs (e.g., Pteridinium and Phyllozoon), Namibia and western North America. Abundant and mod- which show a modular construction of soda strawÀshaped erately complex trace fossils occur in the Nama assemblage elements. Segmented forms (e.g., Dickinsonia), some of (Jensen et al., 2000; Jensen and Runnegar, 2005; Chen which show polarity and possible cephalization (e.g., et al., 2013, 2018, 2019; Meyer et al., 2014a, 2017; Spriggina, Kimberella), provide iconic images of the Ediacara Buatois et al., 2018). UÀPb dates from Namibia biota. These segmented fossils are commonly regarded as (Grotzinger et al., 1995; Linnemann et al., 2019), Oman metazoans and likely stem-group bilaterians (Gehling, 1991; (Amthor et al., 2003; Bowring et al., 2007), South Fedonkin and Waggoner, 1997; Sperling and Vinther, 2010; China (Condon et al., 2005; Yang et al., 2017a,b)and Gold et al., 2015; Evans et al., 2017; Hoekzema et al., 2017; Brazil (Parry et al., 2017) constrain the Nama assemblage Bobrovskiy et al., 2018). The diverse macroscopic algae and between c. 550 and 538 Ma (Table 18.1). The decline and other carbonaceous compressions of the Miaohe assemblage eventual disappearance of the Nama assemblage at or near TABLE 18.1 Key radioisotopic dates constraining the Ediacaran System and its correlation.

Region Locality Period Unit Age Error Reported Reported Recalculated w/o λ w/λ Type of Reference plotted in plotted in age error age Schmitz error error age Fig. 18.10 Fig. 18.10 (2012) Australia Kings Island of Cryogenian Cottons Breccia 636.41 0.45 636.41 0.45 TIMS Calver et al. Tasmania, Australia (2013) Cryogenian

Namibia northern Namibia Cryogenian Ghaub Diamictite 635.21 0.59 635.21 0.59 TIMS Prave et al. (2016) Cryogenian Namibia northern Namibia Cryogenian Ghaub Diamictite 639.29 0.26 639.29 0.26 TIMS Prave et al. (2016) Cryogenian

South Yunnan, South Cryogenian Nantuo Formation 634.57 0.88 634.57 0.88 TIMS Zhou et al. (2019) China China Cryogenian Avalon Avalon Peninsula, Ediacaran Drook Formation, 150 m 570.94 0.38 570.94 0.38 TIMS Pu et al. (2016) Newfoundland, above first appearance of Canada Ediacara fossils

Avalon Avalon Peninsula, Ediacaran Mistaken Point Formation 566.25 0.35 566.25 0.35 TIMS Pu et al. (2016) Newfoundland, Canada Avalon Avalon Peninsula, Ediacaran Mall Bay, below Gaskiers 580.34 0.52 580.34 0.52 TIMS Pu et al. (2016) Newfoundland, Canada

Avalon Avalon Peninsula, Ediacaran Mall Bay, below Gaskiers 580.9 0.4 580.9 0.4 TIMS Pu et al. (2016) Newfoundland, Canada Avalon Avalon Peninsula, Ediacaran Drook Formation, above 579.88 0.44 579.88 0.44 TIMS Pu et al. (2016) Newfoundland, Gaskiers Canada

Avalon Bonavista Ediacaran Rocky Harbour Formation, 579.63 0.15 579.63 0.15 TIMS Pu et al. (2016) Peninsula, below Trinity Diamictite Newfoundland, Canada Avalon Bonavista Ediacaran Rocky Harbour Formation, 579.35 0.33 579.35 0.33 TIMS Pu et al. (2016) Peninsula, within Trinity Diamictite Newfoundland, Canada

Avalon Bonavista Ediacaran Rocky Harbour Formation, 579.24 0.17 579.24 0.17 TIMS Pu et al. (2016) Peninsula, above Trinity Diamictite Newfoundland, Canada Avalon England Ediacaran Park Breccia Member 559.3 7.3 559.3 2 559.3 7.3 7.3 SHRIMP Compston et al. (2002)

(Continued ) TABLE 18.1 (Continued)

Avalon England Ediacaran Stretton Shale Formation 566.6 2.9 566.6 2.9 SHRIMP Compston et al. (2002) Avalon England Ediacaran Lightspout Formation 555.9 3.5 555.9 3.5 SHRIMP Compston et al. (2002)

Avalon Wales Ediacaran Padarn Tuff Formation 604.7 1.6 604.7 1.6 SHRIMP Compston et al. (2002) Avalon Wales Ediacaran Padarn Tuff Formation 605.9 3.8 605.9 3.8 SHRIMP Compston et al. (2002)

Avalon Wales Ediacaran Fachwen Formation 574.3 2.7 574.3 2.7 SHRIMP Compston et al. (2002) Avalon Wales Ediacaran Fachwen Formation 572.5 1.2 572.5 1.2 SHRIMP Compston et al. (2002)

Avalon England Ediacaran Hanging Rock Formation 556.6 6.4 556.6 6.4 LA-ICP- Noble et al. (2015) MS

Avalon England Ediacaran Bradgate Formation 561.85 0.66 561.85 0.66 561.85 0.66 0.89 TIMS Noble et al. (2015) Avalon England Ediacaran Beacon Hill Formation 565.22 0.65 565.22 0.65 565.22 0.65 0.89 TIMS Noble et al. (2015)

Avalon England Ediacaran Benscliffe Breccia 569.08 0.73 569.08 0.73 569.08 0.73 0.94 TIMS Noble et al. (2015) Baltica Russia Ediacaran Uppermost Ust-Pinega 555.3 0.3 555.3 0.3 552.85 0.77 2.62 TIMS Martin et al. Formation (2000)

Brazil Corumba, Mato Ediacaran Upper Tamengo 541.85 0.77 541.85 0.75 541.85 0.77 0.97 TIMS Parry et al. (2017) Grosso do Sul Formation State, Brazil Brazil Corumba, Mato Ediacaran Upper Tamengo 542.37 0.32 542.37 0.28 542.37 0.32 0.68 TIMS Parry et al. (2017) Grosso do Sul Formation State, Brazil

Brazil Corumba, Mato Ediacaran Upper Bocaina Formation 555.18 0.34 555.18 0.3 555.18 0.34 0.7 TIMS Parry et al. (2017) Grosso do Sul State, Brazil Namibia Namibia Ediacaran Urusis Formation, upper 540.61 0.67 543.3 1 540.61 0.67 0.88 TIMS Grotzinger et al. Spitskopf Member (1995)

Namibia Namibia Ediacaran Urusis Formation, upper 542.68 1.25 545.1 1 542.68 1.25 2.8 TIMS Grotzinger et al. Spitskopf Member (1995)

(Continued ) TABLE 18.1 (Continued)

Namibia Namibia Ediacaran Urusis Formation, upper 538.99 0.21 538.99 0.21 TIMS Linnemann et al. Spitskopf Member (2019) Namibia Namibia Ediacaran Urusis Formation, 539.64 0.19 539.64 0.19 TIMS Linnemann et al. Spitskopf Member (2019)

Namibia Namibia Ediacaran Urusis Formation, 539.52 0.14 539.52 0.14 TIMS Linnemann et al. Spitskopf Member (2019) Namibia Namibia Ediacaran Urusis Formation, 539.58 0.34 539.58 0.34 TIMS Linnemann et al. Spitskopf Member (2019)

NW Mackenzie Ediacaran basal Sheepbed Formation 632.3 5.9 632.3 5.9 ReÀOs Rooney et al. Canada Mountains, (2015) northwestern Canada Oman Oman Cambrian Ediacaran Ara Group, 1 m above A4 541 0.29 541 0.21 541 0.29 0.63 TIMS Bowring et al. carbonate (2007)

Oman Oman Ediacaran Ara Group, 9 m below top 542.37 0.28 542.23 0.19 542.37 0.28 0.63 TIMS Bowring et al. of A3 carbonate (2007) Oman Oman Ediacaran Ara Group, 3 m above 542.9 0.29 542.9 0.2 542.9 0.29 0.63 TIMS Bowring et al. base of A3 carbonate (2007)

Oman Oman Ediacaran Ara Group, A0 carbonate 546.72 0.34 546.72 0.29 546.72 0.34 0.66 TIMS Bowring et al. (2007) Oman Oman Ediacaran Fara Formation 542.54 0.53 542.54 0.53 TIMS Bowring et al. (2007)

Oman Oman Ediacaran Fara Formation 547.23 0.36 547.23 0.36 TIMS Bowring et al. (2007)

South Huajipo, Yangtze Ediacaran Atop Doushantuo cap 635.26 0.84 635.23 0.57 635.26 0.84 1.07 TIMS Condon et al. China Gorges area, South dolostone (2005) China

South Jiuqunao, Yangtze Ediacaran Doushantuo member II 632.48 0.84 632.5 0.48 632.48 0.84 1.02 TIMS Condon et al. China Gorges area, South (2005) China

South Jiuqunao, Yangtze Ediacaran Doushantuo member IV 551.09 0.84 551.07 0.61 551.09 0.84 1.02 TIMS Condon et al. China Gorges area, South (2005) China

(Continued ) TABLE 18.1 (Continued)

Region Locality Period Unit Age Error Reported Reported Recalculated w/o λ w/λ Type of Reference plotted in plotted in age error age Schmitz error error age Fig. 18.10 Fig. 18.10 (2012) South Zhangcunping, Ediacaran Lower Doushantuo 609 5 609 5 SHRIMP Zhou et al. China South China Formation, just above (2017b) middle dolostone and middle phosphorite South Zhangcunping, Ediacaran Lower Doushantuo 614 9 614 7.6 614 9 9 SHRIMP Liu et al. (2009) China South China Formation, just above lower phosphorite

South Weng’an, South Ediacaran Doushantuo Formation, 599 4 599 4 PbÀPb Barfod et al. China China middle phosphorite (2002) South Fanglong, Guizhou, Ediacaran Lower Dengying 557 3 557 3 SHRIMP Zhou et al. (2018) China South China Formation

South Fanglong, Guizhou, Ediacaran Lower Liuchapo 550 3 550 3 SHRIMP Zhou et al. (2018) China South China Formation South Yunnan, South Ediacaran Basal Jiucheng Member, 553.6 3.8 553.6 2.7 553.6 3.8 SIMS Yang et al. (2017b) China China Dengying Formation

South Yunnan, South Ediacaran Middle Jiucheng Member, 546.3 3.8 546.3 2.7 546.3 3.8 SIMS Yang et al. (2017b) China China Dengying Formation

South Longbizui, Hunan, Ediacaran Lower Liuchapo 545.76 0.66 545.76 0.66 SIMS Yang et al. (2017a) China South China Formation, Longbizui

South Ganqiping, Hunan, Ediacaran Lower Liuchapo 542.1 5 542.1 5 SIMS Chen et al. (2015) China South China Formation, below Palaeopascichnus occurrence South Bahuang, Guizhou, Ediacaran Upper Liuchapo 542.6 3.7 542.6 3.7 SIMS Chen et al. (2015) China South China Formation

Namibia Namibia Cambrian Urusis Formation, 538.58 0.19 538.58 0.19 TIMS Linnemann et al. Spitskopf Member (2019)

Namibia Namibia Cambrian Cambrian Nomtsas Formation 538.18 1.11 539.4 1 538.18 1.11 1.24 TIMS Grotzinger et al. (1995) South Ganqiping, Hunan, Cambrian Upper Liuchapo 536.3 5.5 536.3 5.5 SIMS Chen et al. (2009) China South China Formation, above Palaeopascichnus occurrence

In "Type of age" column: LA-ICP-MS, laser ablation inductively coupled plasma mass spectrometer; SHRIMP, Sensitive High-Resolution Ion MicroProbe; SIMS, secondary ion mass spectrometery; TIMS, isotope dilution thermal ionization mass spectrometry. 534 PART | III Geologic Periods: Planetary and Precambrian

FIGURE 18.6 Representative tubular or ribbon-shaped fossils from the terminal Ediacaran stage. Parts (AÀC) are biomineralized skeletal fossils and Parts (DÀE) are nonbiomineralized fossils. (A and B) Cloudina riemkeae, (C) Sinotubulites baimatuoensis, and (D) Conotubus hemiannulatus, Dengying Formation, southern Shaanxi Province, South China. Part (B) is longitudinal cross section of (A), showing nested funnels (Hua et al., 2005). (E) Shaanxilithes ningqiangensis, Taozichong Formation, Guizhou Province, South China (Hua et al., 2004). Modified from Xiao et al. (2016). the EdiacaranÀCambrian boundary represents an area of Ediacaran strata in western Canada (Hofmann and current research (Laflamme et al., 2013; Smith et al., 2016; Mountjoy, 2001), Oman (Amthor et al., 2003), and Siberia Darroch et al., 2018b; Muscente et al., 2018; Tarhan et al., (Kontorovich et al., 2008; Grazhdankin et al., 2015). 2018; Linnemann et al., 2019). In addition, the Nama assemblage contains a number of The Nama assemblage witnessed the appearance of nonbiomineralized ribbon-shaped or tubular fossils such the earliest skeletal animal fossils, which represent an as Conotubus (Fig. 18.6D), Shaanxilithes (Fig. 18.6E), important evolutionary event of probable biostratigraphic Wutubus, Sekwitubulus, Saarina and Sabellidites (Selly significance. These include weakly biomineralized fossils et al., 2019). Shaanxilithes has been found in terminal such as Cloudina (Fig. 18.6A and B), Sinotubulites Ediacaran rocks in South China (Meyer et al., 2012), (Fig. 18.6C) and Namacalathus. Among these skeletal North China and Chaidam Blocks (Shen et al., 2007), fossils, Cloudina was the first described (Germs, 1972) India (Tarhan et al., 2014), Siberia (Zhuravlev et al., and is the most widespread, occurring in terminal 2009; Cai and Hua, 2011; Rogov et al., 2012), and possi- Ediacaran successions in Namibia (Grant, 1990), Oman bly Namibia (Darroch et al., 2016). Conotubus is known (Conway Morris et al., 1990), South China (Hua et al., from terminal Ediacaran successions in South China (Cai 2005; Cai et al., 2013; Cortijo et al., 2015b), Spain et al., 2011), and it has been regarded as an evolutionary (Cortijo et al., 2010, 2015a), Siberia (Kontorovich et al., precursor of Cloudina (Hua et al., 2007; Wood et al., 2008; Zhuravlev et al., 2012; Grazhdankin et al., 2015), 2017). Sabellidites cambriensis first appears in terminal Canada (Hofmann and Mountjoy, 2001), Mexico (Sour- Ediacaran rocks in the East European Platform Tovar et al., 2007), South America (Warren et al., 2017), (Moczydłowska et al., 2014), but it also extends into the and eastern California and Nevada where it was described basal Cambrian (Narbonne et al., 1987; Landing, 1994). as Nevadatubulus dunfeei by Signor et al. (1987) but con- These tubular fossils have the potential to improve the sidered as Cloudina by Grant (1990) and Zhuravlev et al. precision of biostratigraphic correlation of terminal (2012). Although the stratigraphic range of Cloudina may Ediacaran strata. extend into the basal Cambrian (Zhuravlev et al., 2012; One important but geochronologically poorly con- Yang et al., 2016; Han et al., 2017; Zhu et al., 2017), it is strained assemblage is the Lantian biota in South China. clear that the first appearance of Cloudina is in the termi- The Lantian biota is loosely constrained somewhere nal Ediacaran Period. Sinotubulites also has a wide geo- between 635 and 551 Ma on the basis of correlation with graphic distribution and has been recovered from terminal the Doushantuo Formation in the Yangtze Gorges area. It Ediacaran strata in South China (Cai et al., 2015), Mexico consists of centimeter-scale carbonaceous compressions (McMenamin, 1985), eastern California and Nevada interpreted as fleshy algae along with problematic fossils (Signor et al., 1987), and Spain (Cortijo et al., 2015a). informally compared with cnidarians and simple bilater- Namacalathus was first described from the terminal ians (Yuan et al., 2011; Van Iten et al., 2013; Wan et al., Ediacaran Nama Group in Namibia (Grotzinger et al., 2014, 2016). Several Lantian taxa also range into the 2000; Zhuravlev et al., 2015) and also occurs in terminal younger Miaohe biota of the Doushantuo Formation The Ediacaran Period Chapter | 18 535

(Xiao et al., 2002; Ye et al., 2017), which is capped by a processes of different morphologies (Fig. 18.7). Although a 551 Ma ash bed (Condon et al., 2005) and contains a number of MesoproterozoicÀTonian acritarch species can diverse flora of megascopic algae and problematica, develop processes (Tang et al., 2015) and a few can reach including Eoandromeda (Fig. 18.4D)(Tang et al., 2008; 400 μmindiameter(Agic´ et al., 2017), Ediacaran acantho- Zhu et al., 2008; Xiao et al., 2013). These exceptional morphs are morphologically diverse and taxonomically dis- fossil assemblages are important in showing the level of tinct. There are more than 200 described species of Ediacaran macroscopic complexity achieved in Ediacaran soft- acanthomorphs and the list keeps growing (Cohen and bodied algae and potentially even animals, but their lim- Macdonald, 2015). These taxonomically diverse and morpho- ited geographic occurrence and loose age constraints limit logically complex acanthomorphs are in sharp contrast to their value in biostratigraphy. basal Cambrian acanthomorphs that are extremely small (typically ,50 μmindiameter)(Moczydłowska, 1991, 1998; Yao et al., 2005; Dong et al., 2009; Ahn and Zhu, 2017). 18.3.2 Ediacaran microfossils Ediacaran acanthomorphs provide a useful biostrati- The lower part of the Ediacaran System is characterized by a graphic tool for Ediacaran subdivision and correlation. group of relatively large acanthomorphic (spiny) acritarchs These acanthomorphs have been described from Ediacaran that are variously known as DoushantuoÀPertatataka acri- shales, cherts, and phosphorites in South China (Yuan and tarchs (Zhou et al., 2001, 2007), Ediacaran Complex- Hofmann, 1998; Zhang et al., 1998; Zhou et al., 2001; Xiao, Acanthomorph-dominated Palynoflora (Grey et al., 2003), or 2004a; McFadden et al., 2009; Liu et al., 2014f; Xiao et al., large ornamented Ediacaran microfossils (Cohen et al., 2014b; Ouyang et al., 2017; Liu and Moczydłowska, 2019; 2009a). These acritarchs are 50À1000 μm in diameter Ouyang et al., 2019); Australia (Zang and Walter, 1992; (Vorob’eva and Sergeev, 2018) and are ornamented with Grey, 2005; Willman et al., 2006; Willman and

FIGURE 18.7 Representative acanthomorphic acritarchs from the Ediacaran System. (A) Tianzhushania spinosa from lower Member II of the Doushantuo Formation in the Yangtze Gorges area, South China. (B) Mengeosphaera reticulata from the Doushantuo Formation at Weng’an, South China. (CÀE) Hocosphaeridium anozos from Tanana Formation, Officer Basin, Australia (Willman and Moczydłowska, 2008); upper Doushantuo Formation (Member III) at Niuping, Yangtze Gorges area, South China (Liu et al., 2014f); and lower Doushantuo Formation (Member II) at Siduping, Hunan Province, South China (Hawkins et al., 2017). Parts (A), (D), and (E) are preserved in chert nodules, (B) in phosphorites, and (C) in shales. (C) Courtesy Sebastian Willman and (D) Courtesy Pengju Liu. 536 PART | III Geologic Periods: Planetary and Precambrian

Moczydłowska, 2008, 2011); Siberia (Moczydłowska et al., Ediacaran cap dolostone and range upward to and perhaps 1993; Nagovitsyn et al., 2004; Moczydłowska, 2005; above the upper Doushantuo negative δ13C excursion EN3 Vorob’eva et al., 2008; Golubkova et al., 2010; Sergeev that is believed to be equivalent to the Shuram negative δ13C et al., 2011; Moczydłowska and Nagovitsin, 2012); India excursion (Zhou et al., 2007; Ouyang et al., 2017). Viewed at (Tiwari and Azmi, 1992; Tiwari and Knoll, 1994; Shukla the broadest scale, there is seemingly a discrepancy between et al., 2008; Shukla and Tiwari, 2014; Joshi and Tiwari, the Australia and South China record; in Australia, Ediacaran 2016); Svalbard (Knoll, 1992); and Baltica (Vidal, 1990; acritarchs began with leiospheres and it is not until after the Veis et al., 2006; Vorob’eva et al., 2006, 2009a,b). Globally, Acraman Impact that the first acanthomorphs appear, whereas these acanthomorphs seem to be restricted in the lower in South China acanthomorphs diversified almost immediately Ediacaran System: in southern Norway, they occur below after the basal Ediacaran cap dolostone (Yin et al., 2007, the Gaskiers age Moelv diamictite (Bingen et al., 2005), and 2009; Zhou et al., 2007). This discrepancy indicates that there in South China, South Australia and Siberia, they predate are regional variations in Ediacaran acanthomorph diversifica- negative δ13C excursions that are interpreted as equivalent to tion and/or preservation, which can hinder inter-regional bio- the Shuram Excursion. However, recent studies have shown stratigraphic correlation using acanthomorph biozones. that some species of DoushantuoÀPertatataka-type acantho- In South China, Tianzhushania spinosa (Fig. 18.7A)is morphs may occur in terminal Ediacaran rocks (Golubkova one of the first acanthomorph taxa to appear after the et al., 2015; Anderson et al., 2017a, 2019)orpostdatethe basal Ediacaran cap dolostone (Yin et al., 2007; Zhou Shuram Excursion (Ouyang et al., 2017). et al., 2007; McFadden et al., 2009), in close proximity to In search for a biostratigraphic resolution, one is tempted an ash bed dated at 632.48 6 1.02 Ma age (Schmitz, to ask whether biozones of Ediacaran acanthomorphs can be 2012). Liu et al. (2013, 2014e,f) recognized two lower recognized and used in biostratigraphic subdivision and corre- Ediacaran acanthomorph biozones in the Yangtze lation. Indeed, five Ediacaran acritarch assemblage zones Gorges area of South China, namely the T. spinosa have been distinguished in Australia and these biozone in Member II of the Doushantuo Formation can be correlated in the Adelaide Rift Complex, Officer (below EN2) followed by the T. conoideumÀ sub-Basin, and Amadeus Basin (Grey, 2005). The biozones Hocosphaeridium scaberfaciumÀHocosphaeridium ano- are in ascending order: (1) the Leiosphaeridia zos biozone in lower Member III (below EN3) of the jacuticaÀLeiosphaeridia crassa Assemblage Zone; (2) the Doushantuo Formation. They propose that the T. spinosa Appendisphaera tabifica (5Appendisphaera barbata)À biozone only occurs in South China and northern India Alicesphaeridium medusoidumÀGyalosphaeridium pulchra (Tiwari and Knoll, 1994; Joshi and Tiwari, 2016), but it is Assemblage Zone; (3) the Tanarium conoideumÀSchizofusa absent in Australia, Siberia, and Baltica. The T. risoriaÀVariomargosphaeridium litoschum Assemblage conoideumÀH. scaberfaciumÀH. anozos biozone may be Zone; (4) Tanarium irregulareÀCeratosphaeridium correlated with Grey’s (2005) assemblage zones (2)À(5) glaberosumÀMultifronsphaeridium pelorium Assemblage and Vorob’eva et al.’s (2009a) upper Vychedga assem- Zone; and (5) Ceratosphaeridium mirabileÀDistosphaera blage zone. Several recent studies, however, call australicaÀApodastoides verobturatus Assemblage Zone. for a revision of the acanthomorph biozonation proposed Except for the first assemblage zone, which is characterized by Liu et al. (2014f). For example, H. anozos by smooth-walled leiospheres, the other four zones are all (Fig. 18.7CÀE) has been shown to occur in Member II of characterized by large acanthomorphs. Although it is unclear the Doushantuo Formation and its stratigraphic range may whether these assemblage zones can be recognized outside extend below the δ13C feature EN2 (Hawkins et al., 2017; Australia, there is some encouraging evidence that a moderate Liu and Moczydłowska, 2019), T. spinosa and H. anozos biostratigraphic resolution can be achieved in interregional cooccur in the middle Doushantuo Formation at Weng’an correlation based on Ediacaran acanthomorphs. For example, (Xiao et al., 2014b), some acanthomorph taxa range Vorob’eva et al. (2009a,b) recognized three acritarch assem- above the δ13C chemostratigraphic feature EN3 (Ouyang blages from the Vychedga Formation in the northeastern mar- et al., 2017), and the taxonomic distinction between gin of Baltica. These authors interpreted the lower the T. spinosa biozone and the T. conoideumÀH. Vychedga assemblage as Tonian in age, but the middle and scaberfaciumÀH. anozos biozone may have been more upper assemblages share some broad similarity with Grey’s subtle than previously thought (Ouyang et al., 2019). Liu assemblage zone (1) and zones (2)À(5), respectively; their and Moczydłowska (2019), on the other hand, proposed correlation predicts that the Cryogenian System, including the four acanthomorphic biozones based on early Ediacaran Sturtian and Marinoan glaciations, is represented by a cryptic material from the Yangtze Gorges area of South China: unconformity between the lower and middle Vychedga Appendisphaera grandisÀWeissiella grandistellaÀT. assemblages. In South China, Ediacaran acanthomorphs are spinosa, Tanarium tuberosumÀSchizofusa zangwenlongii, best known in the Doushantuo Formation of the Yangtze T. conoideumÀCavaspina basiconica, and Tanarium Gorges area. They first occur immediately after the basal pycnacanthumÀC. glaberosum assemblage zones. The Ediacaran Period Chapter | 18 537

Although there are some promising data suggesting the Gnilovskaya, 1990; Cohen et al., 2009b),andtheymayalso biostratigraphic importance of Ediacaran acanthomorphs, have biostratigraphic significance. there are several challenges limiting the full potential of these microfossils. First, Ediacaran acanthomorphic acritarchs are well known from continental shelf succes- 18.3.3 Ediacaran glaciations sions in Asia, Australia and Europe, but these successions In contrast with the Cryogenian, in which abundant evidence typically do not contain Ediacara-type megafossils. This of continental glaciation dominates the sedimentary record could be due to their differences in environmental, tapho- despite largely equatorial positions for most of the continents, nomic or biostratigraphic ranges, which make it difficult to Ediacaran glacial deposits are rare and typically isolated define the exact biostratigraphic relationships between despite their generally more polar positions. In their recent Ediacaran acanthomorphs and Ediacara-type megafossils. compilation, Hoffman and Li (2009) listed 13 probable Second, Ediacaran acritarchs can be preserved in cherts Ediacaran glacial deposits located on eight paleocontinents (Fig. 18.7A, D, and E), in phosphorites (Fig. 18.7B), or in (see also Li et al., 2013b). The best known Ediacaran glacial shales (Fig. 18.7C). Thus different methods (e.g., acid mac- deposit is the Gaskiers Formation from Avalonian eration and thin sectioning) are used in the extraction and Newfoundland (Fig. 18.3G)(Eyles and Eyles, 1989; Myrow observation of these microfossils, resulting in different taxo- and Kaufman, 1999), which is constrained between nomic practices (Xiao et al., 2014b). The situation has been 580.90 6 0.40 and 579.88 6 0.44 Ma in the Avalon exacerbated by taphonomic alteration that is common in Peninsula and between 579.63 6 0.15 and 579.24 6 0.17 Ma Ediacaran acanthomorphs (Grey and Willman, 2009). As in the Bonavista Peninsula (Pu et al., 2016)(Table 18.1). such, there have been some taxonomic inconsistencies The Gaskiers Formation is a 250-m thick, deep-water, glacio- between systematic treatments based on thin section and marine deposit that is similar to Cryogenian glacial deposits maceration materials. Also, environmental factors and pre- in showing significant iron enrichment upward and in locally servational bias can lead to variation in the geographic and exhibiting a cap carbonate (0.5 m thick composed of light stratigraphic distribution of Ediacaran acanthomorphs (Zhou gray sparry calcite) in an otherwise completely carbonate- et al., 2007); this is especially evident in the smooth leio- free succession. Sparse biostratigraphic, chemostratigraphic, spheres, which characterize terminal Ediacaran strata world- and geochronological data imply that the Squantum Tillite in wide but also occur as default taxa in some early Ediacaran the Boston Basin (Thompson and Bowring, 2000), the Croles microfossil assemblages where acanthomorphs may have Hill diamictite in Tasmania (Calver et al., 2004), the Moelv been excluded by environmental or taphonomic variables. It diamictite in southern Norway (Bingen et al., 2005), the should be noted, however, that these challenges are not Serra Azul diamictite in the Paraguay belt of Brazil unique to Ediacaran acanthomorphs; they are general pro- (de Alvarenga et al., 2007), possibly the Hankalchough dia- blems that all biostratigraphers face. Careful taphonomical mictite in Tarim (Xiao et al., 2004), and perhaps the and paleoenvironmental analyses will help us to maximize Mortensnes Formation of northern Norway (Rice et al., the biostratigraphic potential of Ediacaran acanthomorphs. 2011) may be correlative with the Gaskiers. If true, then Ediacaran successions also yield several other groups of DoushantuoÀPertatataka-type acanthomorphs from the microfossils, but these fossils have limited biostratigraphic Biskopa˚s Formation (Vidal, 1990), which is older than the significance, either because of their limited geographic distri- Moelv Formation in southern Norway, can provide insights bution or because of their rather long stratigraphic ranges. Of into the temporal relationship between Ediacaran acantho- the former category are animal embryo-like fossils, tubular morph biozonation and the Gaskiers glaciation: some if not microfossils, and multicellular algal fossils from the most DoushantuoÀPertatataka-type acanthomorphs appeared Doushantuo Formation at Weng’an of South China (Xiao before the Gaskiers glaciation. et al., 2014c). Of the latter category are various coccoidal and filamentous cyanobacteria (Venkatachala et al., 1990; Tiwari and Azmi, 1992; Zhang et al., 1998; Shukla et al., 18.3.4 Chemical evolution of Ediacaran oceans 2005). Of potential but unproven biostratigraphic significance are Salome hubeiensis—a multisheathed filamentous cyano- 18.3.4.1 Carbon isotopes bacterium that has been found in several lower Ediacaran Basal Ediacaran “cap dolostones” are characterized by 13 units, including the Doushantuo Formation in South China negative δ Ccarb values (Knoll et al., 2006), representing (Zhang et al., 1998), the lower Krol Group in northern India a negative excursion designated as EN1 in South China (Shukla et al., 2008) and the Shuurgat Formation in the (Zhou and Xiao, 2007)(Fig. 18.8). Although there are Zavkhan Terrane of southwestern Mongolia (Anderson et al., some facies-dependent regional variations in the magni- 2017b). Similarly, Vendotaenia antiqua and related fossils tude of this negative excursion (Jiang et al., 2007; Sato have been found in upper Ediacaran (,555 Ma) successions et al., 2016; Lang et al., 2017) and there are localized in Russia, South China and Namibia (Zhao et al., 1988; microfabric-dependent occurrence of extremely negative 538 PART | III Geologic Periods: Planetary and Precambrian

13 δ Ccarb values (Jiang et al., 2003; Wang et al., 2008), et al., 2011; Lu et al., 2013; Xiao et al., 2017a; Zhou 13 bulk-sample δ Ccarb values of the cap dolostone are about et al., 2017a); (3) its age and duration: whether it lasted a 25m (Hoffman et al., 2007), after which they return to few million years or up to 50 million years (Le Guerroue near 0m within about 3 million years (Condon et al., et al., 2006); and (4) its stratigraphic relationship 13 2005). A subsequent rise to highly positive δ Ccarb values with the c. 580 Ma Gaskiers glaciation, (16m to 10m) has been noted from many sections [e.g., DoushantuoÀPertatataka acanthomorphs, and Ediacara- NW Namibia (Cui et al., 2018), NE Svalbard (Halverson type fossils. et al., 2005), Brazil (Sial et al., 2016), and South China In some regions a negative δ13C excursion regarded as a (McFadden et al., 2008)]. In South China a regionally Shuram equivalent sits stratigraphically below a glacial dia- consistent negative excursion (EN2) in the middle mictite interpreted as a Gaskiers equivalent, suggesting that Doushantuo Formation punctuated the highly positive the Gaskiers glaciation may postdate the initiation or even 13 δ Ccarb values (Zhou and Xiao, 2007); additional nega- the entirety of the Shuram Excursion. For example, on the tive excursions are documented in some sections (Sawaki basis of a basin subsidence rate model Witkosky and et al., 2010; Zhu et al., 2013), but they are not regionally Wernicke (2018) estimated that the Shuram Excursion (as consistent and may represent diagenetic alterations. recorded in the Rainstorm Member) occurred 585À579 Ma, The most prominent feature in the Ediacaran δ13C thus older than the c. 580 Ma Gaskiers glaciation (as repre- 13 record is an unusually negative δ Ccarb excursion with sented by the incision of the Rainstorm Member shelf). values below 210m, commonly referred to as the Other possible examples of ShuramÀGaskiers relationships Shuram (or ShuramÀWonoka) Excursion or anomaly include the negative δ13C excursion in Member E in the (Burns and Matter, 1993). Possible equivalents of the uppermost Nyborg Formation that underlies the Mortensnes Shuram Excursion include the EN3 excursion in the diamictite in northern Norway (Halverson et al., 2005), the upper Doushantuo Formation in South China, the Zhuya negative δ13C excursion in the Shuiquan Formation that (5Nikol’skoe 1 Chencha) excursion in Siberia, the underlies the Hankalchough diamictite in the Tarim Block Wonoka excursion in South Australia, the Gametrail (Xiao et al., 2004), and the negative δ13C excursion in the excursion in northwestern Canada, and the Kanies/Mara Hongzaoshan Formation that underlies the Hongtiegou excursion in southern Namibia (Fig. 18.8 and references Diamictite in the Chaidam Basin of northwestern China cited in figure caption). Additional correlatives include (Shen et al., 2010). These relationships would suggest that the Rainstorm Member in Death Valley (Kaufman et al., the Gaskiers glaciation postdated the initiation of the 2007; Bergmann et al., 2011; Verdel et al., 2011; Shuram negative δ13C excursion. However, whether these Witkosky and Wernicke, 2018), the Member E of the examples represent Shuram and Gaskiers equivalents has Nyborg Formation in northern Norway (Halverson et al., not been tested thoroughly. Indeed, there are some prelimi- 2005; Rice et al., 2011), and possibly the Shuiquan nary data implying that there may have been more than Formation in the Tarim block (Xiao et al., 2004), but one glaciation in the Ediacaran Period. For example, the these successions are dominated by siliciclastics and, as a Hankalchough and Hongtiegou diamictites may represent result, their δ13C records are stratigraphically sporadic apostÀShuram Excursion near the EdiacaranÀCambrian and can be more disproportionately compromised by boundary rather than equivalents of the Gaskiers authigenic carbonates (Macdonald et al., 2013; Schrag glaciation (Shen et al., 2010), and several authors have et al., 2013; Cui et al., 2017). Current controversies sur- argued for the presence of glacial diamictites near the rounding the Shuram Excursion and its potential correla- EdiacaranÀCambrian boundary in Kazakhstan, Kyrgyzstan tives include (1) its origin: whether the Shuram and Siberia (Chumakov, 2009; Kaufman et al., 2009). Excursion represents a disturbance of the global ocean In other regions, there is evidence suggesting that the dissolved inorganic carbon (DIC) reservoir due to oxida- Gaskiers is older than the Shuram. For example, Prave tion of organic carbon or methane (Rothman et al., 2003; et al. (2009) argued that the InishowenÀLoch na Fike et al., 2006; Kaufman et al., 2007; McFadden et al., CilleÀMacDuff ice-rafted debris beds in the Dalradian 2008; Bjerrum and Canfield, 2011), reflects conditions Supergroup represent the Gaskiers glaciation in Scotland conducive to authigenic carbonate precipitation and Ireland, and the overlaying Girlsta Limestone records (Grotzinger et al., 2011; Macdonald et al., 2013; Schrag a negative δ13C excursion equivalent to the Shuram et al., 2013; Cui et al., 2017), or results from burial or Excursion; if correct, then the Gaskiers glaciation would meteoric diagenesis (Knauth and Kennedy, 2009; Derry, predate the Shuram Excursion. In Newfoundland a 2010); (2) its complexity: whether the Shuram Excursion Shuram-like excursion has been reported from the upper consists of a simple negative anomaly (Fike et al., 2006; Briscal and Mistaken Point Formations, and a possible Le Guerroue et al., 2006; An et al., 2015) or a complex equivalent of EN2 from the Mall Bay Formation, indicat- negative anomaly punctuated with one or more small- ing that the Gaskiers Formation—which immediately scale positive anomalies (Condon et al., 2005; Verdel overlies the Mall Bay Formation but predates the Briscal The Ediacaran Period Chapter | 18 539

Oman South Australia 1200 South China UT 550 1100 Ara 541.00±0.29 542.37±0.28 Siberia 1000 546.72±0.34 3000 500 YJH BACE 1000 TN 800 2800 Buah 450 900 2600 600 Chencha 2400 400 800 400 2200 NK 200 350 700 2000 0 600 1800 300 1600 -200 500 Shuram 250 1400 -400

1200 Bunyeroo Wonoka Pound 200 400 -600 1000 ABC 300 Khufai -800 150 551.09±0.84 800 Stratigraphic height (m) Stratigraphic EN3 200 600 -1000 100 400 -1200 EN2 Brachina 100 Barakun Valyukhta 50 Masirah 200 -1400 Nuccaleena; GSSP 632.48±0.84 Doushantuo Dengying 0 Bay 0 0 635.26±0.84 Ghadir DK -1600 EN1 Manqil -200 Elatina Nantuo -100 -50 -10 -5 0 5 10 -1800 -400 -10 -5 0 -10 -5 0 5 10 13 -600 13 13 C Ccarb Ccarb

carb Mariinsk -800 -15 -10 -5 0 5 10 S. Namibia 13C Newfoundland carb 538.18±1.11 538.58±0.19 538.99±0.21 540.61±0.67 539.64±0.19 SE Mackenzie Mountains 539.52±0.14 542.68±1.25 539.58±0.34 Signal Hill Gp

NW Canada Schwarzrand 1600 Ingta Head 1500 Urikos Wernecke Mountains Risky 400 Renews NW Canada 1400 900 1300 300

Hoogland 547.32±0.31 Ingta 1200 Fermeuse 800 200 Risky 1100 Blueflower

BF 1000 Omkyk 700 100 900 566.25±0.35 GT GT

600 800 Point JB 0 Kanies ? Mistaken 700 -10 -5 0 5 10 500 13C 600 carb 570.94±0.38 400 Sheepbed Carbonate 500 Outjo Basin, N. Namibia 579.88±0.44 Drook Briscal Trepassey 400 June beds 450

300 GpConception St. Johns Gp 300 400 350

300 Gaskiers 200 200 250 580.34±0.52 Bay 100 200 Mall Sheepbed Hayhook

150 Karibib 100 Sheepbed 632.3±5.9 (Re-Os)

Stratigraphic height (m) Stratigraphic 0 100 -10 -5 0 5 10 50 Harbor 0 13C 0 Main Gp Ice Brook Ravensthroat carb -50 Ghaub Keele -5 0 5 -100 13 -10 -5 0 5 10 Ccarb 13 Ccarb

Ediacara-type Ediacara-type Ediacara-type Ediacara-type Cloudina & related Acanthomorphs megafossils megafossils megafossils megafossils skeletal fossils Trace fossils (undifferentiated) (Avalon) (White Sea) (Nama)

Terminal Cryogenian EN1 13Cexcursion EN2 13Cexcursion Shuram/EN3 13Cexcursion Gaskiers glacial Cambrian strata glacial diamictite and correlatives and possible correlatives and potential correlatives diamictite FIGURE 18.8 Representative δ13C profiles with approximate horizons of fossil occurrences in Ediacaran successions, highlighting possible equivalents of the Shuram Excursion. Oman: δ13C data shown in blue from Fike et al. (2006); δ13C data shown in red from Amthor et al. (2003). Siberia: δ13C data shown in blue from Ura Uplift Section and δ13C data shown in red from Zhuya River section (Pokrovskii et al., 2006); biostratigraphic data from Sergeev et al. (2011) and Moczydłowska and Nagovitsin (2012). South Australia: δ13C data of the Wonoka Formation from canyon shoulder section 1 in the Flinders Ranges (Husson et al., 2015) and δ13C data of the Nuccaleena Formation from Calver (2000); biostratigraphic data from Jenkins (1995), Gehling (2000), and (Grey, 2005). Wernecke and Mackenzie Mountains, Northwestern Canada: δ13C data from Macdonald et al. (2013); radiometric date from Rooney et al. (2015); biostratigraphic data from Macdonald et al. (2013), Narbonne et al. (2014), and Carbone and Narbonne (2014). South China: δ13C data of the Doushantuo Formation from McFadden et al. (2008) and Li et al. (2010), δ13C data of Dengying Formation from Wang et al. (2014), δ13C data of Yanjiahe Formation from Ishikawa et al. (2008), Jiang et al. (2012), and Ishikawa et al. (2013); biostratigraphic data from Cai et al. (2010), Meyer et al. (2012), Chen et al. (2013), Chen et al. (2014), Cai et al. (2015), and Cortijo et al. (2015b). Southern Namibia: δ13C data shown in blue from Brak section and δ13C data shown in red from Zebra River section, both in Zaris Sub-basin (Wood et al., 2015); biostratigraphic data from Germs (1995), Grotzinger et al. (1995), Jensen et al. (2000), Jensen and Runnegar (2005), and Wood et al. (2015). Northern Namibia: δ13C data from Halverson et al. (2005). Newfoundland: biostratigraphic data from Narbonne and Gehling (2003), and Liu et al. (2010, 2014b); radiometric dates from Pu et al. (2016); the potential occurrence of EN2 and Shuram Excursion from Canfield et al. (2020). All radiometric dates are cited from Schmitz (2012) unless otherwise noted. ABC, ABC Range Quartzite; BF, Blueflower Formation; DK, Dzhemkukan Formation; GT, Gametrail Formation; JB, June beds; NK, Nikol’skoe Formation; TN, Tinnaya Formation; UT, Uratanna Formation; YJH, Yanjiahe Formation. Modified from Xiao et al. (2016). 540 PART | III Geologic Periods: Planetary and Precambrian

and Mistaken Point Formations—must be older than the assemblages clearly postdate the Shuram Excursion Shuram Excursion but slightly younger than EN2 (Xiao et al., 2016). However, in southeastern Mackenzie (Canfield et al., 2020). If true, then the Shuram Excursion Mountains of northwestern Canada, the Shuram would be constrained between c. 570 and 560 Ma given Excursion (which occurs in the Gametrail Formation; the high-precision radioisotopic dates from Ediacaran suc- Macdonald et al., 2013; Moynihan et al., 2019) is sand- cessions in Newfoundland (Pu et al., 2016; Canfield et al., wiched between the Nadaleen Formation containing a 2020; see also Rooney, 2019). In the Yangtze Gorges area low-diversity assemblage of rangeomorphs (Narbonne of South China, Sawaki et al. (2010) and Tahata et al. et al., 2014) regarded as equivalent to the early Avalon (2013) presented δ18O data to show a cooling event asso- assemblage and the Blueflower Formation containing ciated with EN2 in the middle Doushantuo Formation and Nama assemblage fossils (Carbone et al., 2015), imply- interpreted this event as equivalent to the Gaskiers glacia- ing that the negative excursion that separates them may tion, implying that the Gaskiers glaciation coincides with correspond to the late Avalon assemblage and/or the EN2 but is older than the Shuram Excursion, which is White Sea assemblage. thought to be equivalent to EN3 in the upper Doushantuo The above mentioned issues highlight the uncertainties Formation. Recently, Wang et al. (2017c) reported glen- with regard to the application of the Shuram Excursion donites in association with EP1 (which is below EN2) in in chemostratigraphic correlation. We desperately need the lower Doushantuo Formation and suggested that these high-precision radioisotopic ages to constrain the Shuram glendonites may record a spell of freezing conditions Excursion and its purported equivalents. Without such related to the Gaskiers; if true, the Gaskiers may even be geochronological constraints, we cannot unambiguously older than EN2 and significantly older than the Shuram. prove the correlation of the Shuram and its purported In addition, EN3 in the upper Doushantuo Formation equivalents. Similarly, the purported equivalents of the underlies a 551 Ma volcanic ash (Condon et al., 2005), Gaskiers glaciation also need to be constrained geochro- and this has been taken by some geologists as evidence nologically in order to prove their correlation with the that the Shuram Excursion (5EN3) is 551 Ma and thus B580 Ma Gaskiers. The uncertain stratigraphic relation- younger than the 580 Ma Gaskiers glaciation. However, ship between the Shuram and the Gaskiers is the main this interpretation is not secure because recent studies reason behind the two different versions of Ediacaran cor- have shown that EN3 is succeeded by additional δ13C var- relation and subdivision presented in Narbonne et al. iations before being capped by the 551 Ma ash (An et al., (2012a), Xiao et al. (2016), and this chapter. 2015; Xiao et al., 2017a; Zhou et al., 2017a), suggesting The Ediacaran Period is closed by another negative that the Shuram may be significantly older than 551 Ma. δ13C anomaly, dubbed BACE or Basal Cambrian Carbon In fact, a K-bentonite in the lower Dengying Formation in Isotope Excursion (Zhu et al., 2006). This excursion has Guizhou Province of South China yielded a UÀPb age of been used to approximate the PrecambrianÀCambrian 557 6 3Ma(Zhou et al., 2018); if correct, this places a boundary (Kaufman and Knoll, 1995; Amthor et al., minimum age constraint on the termination of EN3. 2003), but the precise relationship between BACE and the The stratigraphic relationship between the Shuram basal Cambrian boundary needs additional refinement. Excursion and DoushantuoÀPertatataka acanthomorphs at a global scale is another unresolved issue. Earlier studies have shown that DoushantuoÀPertatataka acanthomorphs in the 18.3.4.2 Strontium isotopes Doushantuo Formation are restricted to strata below EN3 Reconstruction of the seawater 87Sr/86Sr curve before the (McFadden et al., 2008; Liu et al., 2014f), but recent studies Ordovician Period suffers greatly from a lack of suitably have shown that some elements of DoushantuoÀPertatataka- well-preserved materials, which necessitates careful sam- type acanthomorphs may postdate EN3 in South China ple selection and preparation (Kaufman et al., 1993; (Ouyang et al., 2017) or may occur in terminal Ediacaran Bailey et al., 2000; Liu et al., 2014d)(Fig. 18.9). rocks in the East European Platform (Golubkova et al., 2015) Nevertheless, strontium isotope stratigraphy can poten- and northern Mongolia (Anderson et al., 2017a, 2019), open- tially resolve stratigraphic conflicts caused by the ambigu- ing the possibility that the stratigraphic ranges of the Shuram ity and circular reasoning inherent in matching otherwise Excursion, DoushantuoÀPertatataka-type acanthomorphs, identical δ13C excursions because of the magnitude of the and Ediacara-type megafossils may partially overlap. rise in seawater 87Sr/86Sr during the Ediacaran Period The stratigraphic relationship between the Shuram (from B0.7071 to B0.7087). Excursion and Ediacara-type megafossils at a global Basal Ediacaran cap dolostones tend to have highly scale is not fully resolved. In South China, South variable and sometimes highly elevated 87Sr/86Sr ratios Australia, and Namibia where both Ediacara-type mega- based on bulk samples (Yoshioka et al., 2003; Sawaki fossils and purported Shuram Excursion are recorded, et al., 2010), probably due to variable diagenetic/hydro- Ediacara-type megafossilsoftheWhiteSeaorNama thermal alterations (Huang et al., 2011; Zhao et al., 2018) The Ediacaran Period Chapter | 18 541

or mixing with glacial meltwaters carrying radiogenic sig- Cambrian 87Sr/86Sr to c. 0.70845 6 5. Least altered sam- nals derived from continental weathering (Liu et al., ples from Mongolia (Brasier et al., 1996), Siberia 2013a, 2014d, 2018b). Stepwise leaching technique can (Kaufman et al., 1996), and South China (Li et al., 2013a) help to minimize the contamination of clay minerals, indicate that seawater 87Sr/86Sr decreased through the revealing end-member 87Sr/86Sr values from the cap PrecambrianÀCambrian transition interval to reach a low dolostone in Australia and Mongolia, including high ratios of 0.70805 6 5 during Cambrian Stage 2. reflecting glacial meltwaters as well as low ratios (0.7072À0.7073) interpreted as primary seawater signatures (Liu et al., 2013a, 2014d, 2018b). These low 18.3.4.3 Sulfur isotopes 87Sr/86Sr ratios from the cap dolostone in Australia and Evaporite sulfate deposits are scarce during the Ediacaran Mongolia are similar to 87Sr/86Sr values of laminated Period and so with few exceptions, knowledge of seawa- 34 micrite and fibrous calcite seafloor cement in the Sete ter δ Ssulfate through this interval derives mainly from Lagoas cap dolostone in Brazil (Misi et al., 2007). After carbonate-associated sulfate (CAS), trace sulfate in phos- the deposition of the cap dolostone, limestone samples are phorite, and barite. A high-resolution analysis of the cap 87 86 34 readily available for Sr/ Sr analysis. For example, dolostone in South China reveals high δ SCAS values Sr-rich limestone of the Hayhook Formation (NW (25À40m, mean 30m) in intrashelf facies, low values Canada) yielded 87Sr/86Sr ratios of 0.70714 6 2(James (17À33m, mean 26m) in outer shelf facies, and still lower et al., 2001), which are consistent with data from postgla- values (17À33m, mean 22m) in slope facies (Huang 34 cial limestones of Namibia (Halverson et al., 2007). In et al., 2013). δ Sbarite data indicate that seawater 87 86 34 Namibia, least altered Sr/ Sr values subsequently rise δ Ssuphate had reached values of 20À45m by the end of to c. 0.7080 as δ13C values recover from 24.4m to 0m, the cap dolostone (Shields et al., 2007). CAS data show 34 and a rise of similar magnitude has been recorded in predominantly high and variable δ SCAS values lower Ediacaran carbonate in the Doushantuo Formation (20À45m) values in the Ediacaran Period (Fike et al., of South China (Sawaki et al., 2010; Lv et al., 2018). 2006; McFadden et al., 2008; Halverson et al., 2010). 34 High-Sr samples from NW Canada show an increase to There is some evidence suggesting a decline in δ SCAS 0.70753, while least altered 87Sr/86Sr data from South values during the Shuram Excursion (Fike et al., 2006; America trace a rise from c. 0.7074 to 0.70777 6 2 during McFadden et al., 2008; Li et al., 2017; Shi et al., 2018), δ13 34 the C recovery (Nogueira et al., 2007; de Alvarenga but δ SCAS rises again to high values (B40m) in the ter- et al., 2008). Nearly identical values (c. 0.7077À0.7078) minal Ediacaran (Fike et al., 2006; Cui et al., 2016a,b; 34 have been reported for basal Ediacaran barite samples of Tostevin et al., 2017). By the early Cambrian δ Ssulfate, NW Africa at a comparable point in the postglacial δ13C as measured in a variety of minerals, including evaporites, curve (Shields et al., 2007). Taken together, these data francolite, and carbonate, was also high (35mÀ40m)in indicate a rise in seawater 87Sr/86Sr from c. 0.7071 to c. most basins (Holser and Kaplan, 1966; Strauss, 1993; 0.7077 or higher during the postglacial δ13C recovery to Shields et al., 1999; Kampschulte and Strauss, 2004; positive values, which lasted ,3 million years (Condon Schro¨der et al., 2004). 34 et al., 2005). Pyrite sulfur isotope data (δ Spyrite) are available for The most complete 87Sr/86Sr record derives from many Ediacaran successions, including those in Oman South China (Sawaki et al., 2010; Cui et al., 2015; (Fike et al., 2006), Australia (Gorjan et al., 2000), Furuyama et al., 2016; Lv et al., 2018). This record indi- Namibia (Ries et al., 2009; Tostevin et al., 2017), South cates that, following the initial rise in 87Sr/86Sr ratios after China (McFadden et al., 2008; Cui et al., 2015; Wang the cap dolostone, 87Sr/86Sr ratios seem to stabilize at a et al., 2017a), Newfoundland (Canfield et al., 2007), plateau of 0.7080 that characterizes the lower Ediacaran Eastern European Platform (Johnston et al., 2012), and System, then rise to a peak of B0.7090 roughly concur- Siberia (Cui et al., 2016a). There are significant strati- 34 rent with the Shuram Excursion, and finally fall back to graphic and geographic variations in δ Spyrite, which can values around 0.7085 in terminal Ediacaran (Fig. 18.9). A range from 230m to 140m (Tostevin et al., 2017). B À 34 0.7085 0.7090 peak in association with purported Given the many factors that can affect δ Spyrite values, Shuram Excursion has also been reported from Siberia including primary production and availability of organic (Pokrovskii et al., 2006; Melezhik et al., 2009), Oman matter (Leavitt et al., 2013), marine sulfate concentration (Burns et al., 1994), and Australia (Calver, 2000). Several (Gomes and Hurtgen, 2015; Bradley et al., 2016), cell- 87Sr/86Sr studies span the PrecambrianÀCambrian specific sulfate reduction rate (Bradley et al., 2016), oxi- boundary, the most comprehensive being that of Brasier dative recycling of sulfide (Fike et al., 2015), and the et al. (1996). That study and other work (Derry et al., location of pyrite formation (Gomes and Hurtgen, 2015), 1994; Kaufman et al., 1996; Nicholas, 1996; Valladares as well as the complex redox structure of Ediacaran et al., 2006) constrain latest Ediacaran and earliest oceans with generally low sulfate concentrations laeg ta.(2008) al. et Alvarenga h prxmt g fteSua xuso smre yadul-ro-eddln.Aeetmtsaebsdo h w orltossonin shown correlations two the on based are estimates Age line. double-arrow-headed a by marked is Excursion Shuram the of age approximate The hs rmtebslEicrncpdlsoe High dolostone. cap Ediacaran basal the from those ilmlwtr ( meltwaters cial 542 i ta.(2018b) al. et Liu 18.10 Fig. 18.9 FIGURE

87 86 87 86

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, Sr/ Sr Sr/ Sr de , The Ediacaran Period Chapter | 18 543

34 (Li et al., 2010), it is not surprising that Ediacaran δ Spyrite events occurred at B632, B580, and B560À551 Ma, data exhibit significant stratigraphic and geographic varia- respectively (Fig. 18.10). Additional deep-water oxygen- tions. Thus, compared with carbon and strontium isotopes, ation events may have occurred during the Shuram 34 δ Spyrite is perhaps inherently less useful in global Excursion (Fike et al., 2006; McFadden et al., 2008; chemostratigraphic correlation of Ediacaran successions. Zhang et al., 2019) and following the Gaskiers glaciation 34 Nonetheless, the δ Spyrite record from Oman (Fike et al., (Canfield et al., 2007). The B560À551 Ma oxygenation 2006) and South China (McFadden et al., 2008; Cui et al., event is also supported by a global compilation of Mo 2015; Shi et al., 2018) shows a steady decline during the concentration data (Scott et al., 2008) as well as δ238U Shuram (and EN3) anomaly, but it remains to see whether and δ98Mo data (Kendall et al., 2015). After 551 Ma, the this decline represents a global signature that overwhelms deep ocean returned to pervasive anoxia as inferred from local and regional heterogeneities. exceptionally low δ238U data from terminal Ediacaran carbonates (Tostevin et al., 2018; Wei et al., 2018; Zhang et al., 2018). If these episodic oxygenation events had a 18.3.4.4 Redox proxies global impact, they can be used for global correlation of The strong stratigraphic and geographic variations in Ediacaran strata. In this respect, proxies for global redox 34 34 δ238 δ98 Ediacaran δ Spyrite and δ Ssulfate values are commonly conditions (such as U and Mo) may prove to be interpreted in terms of a low-sulfate ocean (Halverson and useful. Hurtgen, 2007). A low-sulfate ocean coupled with a low- Against the backdrop of ferruginous conditions in oxygen atmosphere is prone to develop ferruginous deep deep waters, Fe speciation data suggest that mid-depth waters. The prevalence of ferruginous deep waters in the and shallow waters experienced more complex and Ediacaran Period has been borne out by Fe speciation dynamic redox conditions. For instance, Li et al. (2010) data (Canfield et al., 2008). The combined use of FeHR/ have shown that euxinic or sulfidic conditions may have FeT (highly reactive iron vs total iron) and FeP/FeHR developed in mid-depth waters along distal continental (pyrite iron vs highly reactive iron) ratios can help to dis- margins of the Yangtze Platform. In shallow waters on tinguish sediments deposited under oxic, ferruginous, and continental shelves, local redox conditions may have been euxinic water columns (Raiswell et al., 1988; Poulton and modulated by oceanic upwelling, sea-level change, tem- Canfield, 2005; Lyons and Severmann, 2006). A compila- perature, and isolation from global ocean isolation—fac- tion of Fe speciation data shows that about half of tors contributing to the temporally dynamic and spatially Ediacaran samples from outer shelf and basinal environ- heterogeneous redox conditions that controlled the distri- ments are characterized by high FeHR/FeT ratios indicative bution of Ediacaran metazoan ecosystems (Wood et al., of deposition in anoxic conditions, and the majority 2015; Bowyer et al., 2017). As such, geochemical proxies ( . 90%) of these anoxic samples were deposited in ferru- for local redox conditions such as Fe speciation data are ginous deep waters (Sperling et al., 2015). Because ferru- less useful for the global correlation of Ediacaran strata ginous conditions can only arise when the molar flux of (Xiao et al., 2017a). FeHR to the deep ocean is greater than half the flux of sulfide, these Fe speciation data are consistent with a gener- 18.3.5 Radioisotopic dating ally low-sulfate ocean reservoir in the Ediacaran Period. Trace metal concentrations and isotopes, on the other Key UÀPb dates from the Ediacaran System and immedi- hand, indicate that deep waters were not completely and ately adjacent Cryogenian and Cambrian strata are listed continuously ferruginous throughout the entire Ediacaran in Table 18.1, where recalculated ages are presented to Period. Instead, there were probably several episodes of take into consideration of tracer and decay constant uncer- deep-water oxygenation to various extent. The concentra- tainties. The base of the Ediacaran is well constrained by tion of redox-sensitive trace metals (such as Mo, V, and UÀPb dates of: (1) 635.21 6 0.59 Ma from the terminal U) in euxinic black shales can be used to track the oce- Cryogenian Ghaub Formation in Oman (Hoffmann et al., anic reservoir size of these metals and to deduce the rela- 2004; Prave et al., 2016); (2) 634.57 6 0.88 Ma from the tive extent of deep-ocean oxygenation. Based on trace terminal Cryogenian Nantuo Formation in South China metal data from the Doushantuo Formation in South (Zhou et al., 2019); (3) 636.41 6 0.45 Ma from the termi- China, Sahoo et al. (2016) have identified three episodes nal Cryogenian Cottons Breccia in King Island, Tasmania, of deep-ocean oxygenation events that punctuated more Australia (Calver et al., 2013); (4) 635.2 6 0.6 Ma atop the or less ferruginous conditions. These three events are Doushantuo Formation in South China and 632.5 6 0.5 Ma recorded in black shales immediately above the basal a few meters above its basal Ediacaran cap dolostone Ediacaran cap dolostone, the middle Doushantuo (Condon et al., 2005); and (5) a ReÀOs date of Formation, and the uppermost Doushantuo Formation. 632.3 6 5.9 Ma from the basal Sheepbed Formation of the Sahoo et al. (2016) proposed that these three oxygenation lower Ediacaran System in northwest Canada (Rooney 544 PART | III Geologic Periods: Planetary and Precambrian

et al., 2015). The top of the Ediacaran in Namibia is con- from multiple regions to provide a robust Ediacaran sub- strained by dates of 540.61 6 0.67 Ma immediately below division. The lower third of the Ediacaran System and 538.18 6 1.11 Ma immediately above the disconform- (B635À600 Ma) contains acanthomorphic acritarchs but able contact with the overlying Lower Cambrian (recalcu- apparently lacks Ediacara-type megafossils. In addition, it lated from Grotzinger et al., 1995). A recent study further is characterized by relatively low 87Sr/86Sr ratios 13 constraint the EdiacaranÀCambrian boundary in Namibia (0.7071À0.7080) and a significant rise in δ Ccarbonate between 538.58 6 0.19 and 538.99 6 0.21 Ma, with addi- from c. 25m in cap dolostone to high values of c. 5m in tional dates of 539.64 6 0.19, 539.52 6 0.14 and the lower Ediacaran System. The uppermost part of the 539.58 6 0.34 Ma from the terminal Ediacaran Spitskopf Ediacaran System (B550À538 Ma), or the TES, typically Member (Linnemann et al., 2019). In Oman the carbon contains a leiosphere microfossil assemblage (but see isotope feature BACE and the last appearance of Golubkova et al., 2015; Anderson et al., 2017a, 2019) and Cloudina were constrained by dates of 542.37 6 0.28 Ma numerous Ediacaran megafossils. It is characterized by immediately below and 541.00 6 0.29 Ma immediately high 87Sr/86Sr ratios ( . 0.7080) and moderately positive 13 above the BACE (recalculated from Bowring et al., δ Ccarbonate values terminated by a negative excursion 2007). However, given that BACE may slightly predate (BACE). It is also constrained by many high-precision the EdiacaranÀCambrian boundary (Smith et al., 2016; radioisotopic dates and abundant fossils from the overly- Ahn and Zhu, 2017), and considering that the ing Cambrian Strata. EdiacaranÀCambrian boundary in Namibia is constrained Uncertainty in correlating siliciclastic and carbonate around 538 Ma (Linnemann et al., 2019), it is possible facies in the middle part of the Ediacaran that the EdiacaranÀCambrian boundary in Oman is above (B600À560 Ma) hinders global correlation and our the 541.00 6 0.29 Ma ash. understanding of the transition between the basal and Key UÀPb dates within the Ediacaran Period are uppermost Ediacaran. Phosphatic carbonates, chert available to constrain the beginning and end of the nodules, and fine siliciclastics of this age in central Asia Gaskiers glaciation in Avalon, all three Ediacara-type contain diverse acanthomorphic acritarchs that are amena- megafossil assemblages, the beginning of the Ediacaran ble to an array of chemostratigraphic techniques but acanthomorphic acritarchs in South China, and the end apparently lack either radioisotopic age constraints or of EN3—a purported equivalent of the Shuram Ediacara-type megafossils. In contrast, deep-water silici- Excursion in South China (Fig. 18.10; Table 18.1). clastics in Newfoundland and England contain abundant These dates provide a geochronological framework for Ediacara-type megafossils of the Avalon assemblage the subdivision and correlation of the Ediacaran System. beneath volcanic ash beds suitable for radioisotopic However, the Shuram Excursion has not been tightly dating, but carbonates suitable for chemostratigraphy are bracketed by radioisotopic dates, regional acanthomorph virtually absent and only nondiagnostic leiosphere micro- biozones have not been bracketed geochronologically, fossils are preserved in these thermally mature strata. The and there is a general scarcity of radioisotopic dates B580 Ma Gaskiers glaciation marks a significant divide from lower Ediacaran successions. Clearly, more high- that at least regionally separates lower Ediacaran strata precision radioisotopic dates are needed to constrain the lacking Ediacaran megafossils from upper Ediacaran age of Ediacaran carbon isotope features, to test whether strata with abundant Ediacaran megafossils (Pu et al., the purported equivalents of the Shuram Excursion can 2016). However, the Gaskiers glaciation is known only be correlated, and to resolve the stratigraphic relation- from Avalonia and a few other continents, and its global ship between the Shuram Excursion, the Gaskiers glacia- correlation remains uncertain. Although there is strong tion, and acanthomorph biozones. evidence to suggest that all Ediacara-type megafossils postdate the B580 Ma Gaskiers glaciation, the stratigraphic relationship among the Gaskiers glaciation, the 18.4 Toward an Ediacaran Shuram Excursion, and Ediacaran acanthomorphic acritarchs is not resolved with confidence. To reflect these chronostratigraphy uncertainties, two distinct correlations consistent with From the above, it is clear that the integration of acantho- all available radioisotopic dates and biostratigraphic and morphic acritarchs, Ediacara-type megafossils, carbon and chemostratigraphic data are presented in Fig. 18.10. strontium isotopes, precise UÀPb dates, and climatic These two correlations are built upon the assumption events has the potential to subdivide the Ediacaran into that the negative C-isotope excursion marked by the broad divisions. All of these indicators are strongly Gaskiers cap carbonate is a global event (Myrow and affected by facies and preservational factors (see the Kaufman, 1999) and thus the short-lived Gaskiers glacia- respective discussions previously), requiring the integration ought to be temporally associated with a negative tion of multiple biological and geochemical proxies δ13C excursion (but see Wang et al., 2017c and The Ediacaran Period Chapter | 18 545 assemblage

Radiometric ages: bubble size = uncertainty

and other Possible (Ma) TIMS Other (Ma) 13 assemblage 530 C Bilaterian trace fossils Divisions 530 Series Stage deepwater redox: green = oxic; black anoxic terminal Ediacaran terminal Leiosphaeridia assemblage Rangeomorphs Palaeopascichnids Dickinsoniomorphs Ernieomorphs Bilateralomorphs Kimberellomorphs Cloudina calciﬁed metazoans Cambrian 540 540

BACE Tanarium conoideum– Hocosphaeridium scaberfacium– Hocosphaeridium anozos TES (Nama) Miaohe biota Miaohe 550 EP3 550 4’ (White Sea) 560 Tianzhushania spinosa Upper 560 EN3 Avalon 570 3’ (Avalon) 570 EP2 Australia Balca 580 EN2 GASKIERS GLACIATION 580 Brazil Namibia 590 590 NW Canada Oman

600 Ediacaran 600 South China 610 EP1 Lower SES 610

620 620

630 630 EN1 FES 640 MARINOAN GLACIATION 640 (A) CORRELATION 1

Cryogenian (Condon et al., 2005; Sawaki et al., 2010; Macdonald et al., 2013; Canﬁeld et al., 2020) assemblage Radiometric ages: bubble size = uncertainty

(Ma) and other TIMS Other Possible (Ma) 13 530 C Bilaterian trace fossils Divisions 530

terminal Ediacaran terminal Leiosphaeridia assemblage Rangeomorphs Palaeopascichnids Dickinsoniomorphs Ernieomorphs Bilateralomorphs Kimberellomorphs Cloudina calciﬁed metazoans deepwater redox: green = oxic; black anoxic Series Stage Cambrian 540 BACE 540 Tanarium conoideum– Hocosphaeridium scaberfacium– Hocosphaeridium anozos

Miaohe biota Miaohe TES (Nama) 550 EP3 550 assemblage 5” (White Sea) 560 Upper 560 Avalon 570 4” (Avalon) 570 Australia Balca 580 GASKIERS GLACIATION 580 EN3 spinosa Tianzhushania Brazil Namibia 590 EP2 590 NW Canada Middle 3” Oman 600 Ediacaran 600 South China EN2 610 610 EP1 SES 620 Lower 620

630 630 EN1 FES 640 MARINOAN GLACIATION 640 (B) CORRELATION 2

Cryogenian (Xiao et al., 2004; Xiao, 2008; Fike et al., 2006; Zhou et al., 2007; Halverson et al., 2005, 2010; also see Witkosky and Wernicke, 2018)

FIGURE 18.10 Correlation and internal subdivision of the Ediacaran System: (A) Correlation 1 and (B) Correlation 2. The key difference between these two correlations relates to how the Gaskiers glaciation is correlated with δ13C excursions (EN2 vs EN3 5 Shuram). Both correlations assume that the Gaskiers glaciation coincides with a negative δ13C excursion, although this assumption remains to be tested (see Wang et al., 2017c; Canfield et al., 2020). Canfield et al. (2020) proposed a correlation similar to Correlation 1, but with EN2 slightly predating the Gaskiers glaciation. 546 PART | III Geologic Periods: Planetary and Precambrian

Canfield et al., 2020). Certainly, this assumption needs to distinct from a negative excursion associated with the be verified, and the two correlations presented next Gaskiers glaciation earlier in the Ediacaran Period. (Fig. 18.10A and B) are working hypotheses to be refined An alternative and partly overlapping scheme, or refuted in the future. Correlation 2 (Fig. 18.10B), regards the initiation of EN3 Correlation 1 (Fig. 18.10A) regards EN2 in the in the Doushantuo Formation of South China as correla- Doushantuo Formation of central China as correlative tive with the Gaskiers glaciation in Avalonia (Xiao et al., with the Gaskiers glaciation in Avalonia (Condon et al., 2004; Halverson et al., 2005, 2010; Fike et al., 2006; 2005; Sawaki et al., 2010; Macdonald et al., 2013). This Zhou et al., 2007; Xiao, 2008). This correlation results in correlation results in two series of approximately equal three series. The “lower series” would range from the length, the upper of which can be subdivided into three base of the Ediacaran (635 Ma) to EN2, which has not stages. The “lower series” (B635À580 Ma) may be fur- been dated but probably ,609 6 5Ma (Wang et al., ther divided into two stages (Xiao et al., 2016)—the “first 2017c; Zhou et al., 2017b). The “lower series” is charac- Ediacaran stage” (FES) and the “second Ediacaran stage” terized by local presence of an acanthomorph assemblage (SES), with the SES characterized by the rise of acantho- dominated by T. spinosa, the lack of Ediacara-type mega- morphic acritarchs. Overall, the lower series is characterized fossils anywhere in the world, and relatively low 87Sr/86Sr by Ediacaran acanthomorphs, the lack of Ediacara-type ratios in marine carbonates. The “lower series” can be megafossils, generally positive δ13C values except further divided into the FES and SES, with the latter char- in the cap dolostone, relatively low 87Sr/86Sr ratios in acterized by the rise of an acanthomorph assemblage marine carbonates, and widespread deep-water anoxia. dominated by T. spinosa. The “middle series” (approxi- The “upper series” (B580À538 Ma) begins at the top of mately 600À580 Ma based on this correlation) is charac- the Gaskiers glaciation or its equivalent level outside of terized by a more diverse acanthomorph assemblage; all Avalonia and is characterized by abundant Ediacara-type other biological and geochemical characters are broadly megafossils, generally positive δ13C values punctuated by similar to those of the underlying “lower series.” Given the Shuram Excursion, and high 87Sr/86Sr ratios in marine the problems associated with the T. spinosa biozone and carbonates. The “upper series” can be further divided into the T. conoideumÀH. scaberfaciumÀH. anozos biozone three stages: stages 3À5 would be roughly equivalent to (Hawkins et al., 2017; Liu and Moczydłowska, 2019; the Avalon, White Sea, and Nama assemblages. Stage 3 Ouyang et al., 2019), the distinction between the “lower (B580À560 Ma) is dominated by rangeomorphs; stage 4 series” and the “middle series” may be a challenge. The (B560À550 Ma) is characterized by more complex and “upper series” would be identical to that defined in corre- more diverse Ediacara-type megafossils, including the lation 1 and might similarly be divisible into three stages erniettomorphs and bilateralomorphs, as well as bilaterian based primarily on Ediacara-type megafossils. This corre- burrows; and stage 5 (TES; B550À538 Ma) is character- lation is consistent with the view that there may be a dis- ized by a less diverse assemblage of Ediacara-type mega- tinct stratigraphic separation between acanthomorphic fossils, plus abundant bilaterian trace fossils and calcified acritarchs in the “lowerÀmiddle” series (Liu et al., 2014f; metazoans such as Cloudina, Namacalathus and Xiao et al., 2014b) and Ediacara-type megafossils in the Sinotubulites. One testable implication of this correlation “upper series” (Knoll and Walter, 1992), but determina- is that the T. conoideumÀH. scaberfaciumÀH. anozos tion of the microfossil assemblage coeval with the Avalon biozone should stratigraphically overlap with the Avalon macrofossil assemblage is needed to test this model. assemblage and perhaps Ediacaran acanthomorphs may Correlation 2 implies a longer Shuram Excursion that also range into the White Sea and Nama assemblages lasted B20 million years and that extended continuously (Golubkova et al., 2015; Anderson et al., 2017a, 2019; through the Gaskiers glaciation and the Avalon megafos- Ouyang et al., 2017). Correlation 1 implies a relatively sil assemblage. short time frame for the Shuram Excursion, less than 10 The fundamental difference between the two correla- million years and largely corresponding to the White Sea tions is how to correlate the middle Ediacaran System, megafossil assemblage (Darroch et al., 2018b), that is specifically whether the Gaskiers glaciation is correlated L The termination of the Shuram Excursion may be between 557 6 3 and 566.25 6 0.35 Ma (Zhou et al., 2018; Canfield et al., 2020) and is herein shown at 560 Ma. Schematic δ13C curve (Zhou and Xiao, 2007; Zhou et al., 2017a), fossil ranges, deep-ocean redox conditions (black: anoxia; green: oxygenation; updated from Canfield et al., 2007; Sahoo et al., 2016; Zhang et al., 2019), and radiometric ages (circle diameter representing uncertainty) are presented. Radiometric ages are listed in Table 18.1. See Liu and Moczydłowska (2019) for more recent acanthomorphic acritarch biozonation based on material from the Yangtze Gorges area of South China. FES, first Ediacaran stage; SES, second Ediacaran stage; TES, terminal Ediacaran stage; EN, Ediacaran negative δ13C excursions; EP, Ediacaran positive δ13C excursions. Modified from Narbonne et al. (2012a) and Xiao et al. (2016). The Ediacaran Period Chapter | 18 547

with the δ13C chemostratigraphic feature EN2, EN3, or et al., 2000; Jensen and Runnegar, 2005; Chen et al., 2013, neither. They make different predictions about the age of 2018, 2019; Meyer et al., 2014b; Buatois and Ma´ngano, EN2 and EN3 (or Shuram Excursion), which have not 2016; Buatois et al., 2018). Chemostratigraphically, the TES been precisely bracketed by radioisotopic dates: correla- is characterized by slightly to moderately positive δ13C tion 1 predicts an EN2 at 580 Ma and a shorter EN3 at values (Wood et al., 2015; Cui et al., 2016b), sandwiched B570À560 Ma, whereas correlation 2 predicts an EN2 at between the Shuram Excursion beneath and the BACE at its B600 Ma and a longer EN3 at B580À560 Ma. These top (Fig. 18.10), significantly facilitating recognition of this predictions can be tested with precise age constraints on stage using multiple criteria. TES sections that exhibit many EN2 and EN3, as well as potential δ13C chemostrati- or most of these criteria occur in Namibia, South China, graphic data from well-dated siliciclastic sequences (e.g., Siberia, Spain, northern Norway, western North America Canfield et al., 2020). and eastern South America. Sections in Namibia and South Both correlations recognize a similar “upper series” China are geochronologically well constrained between rich in Ediacara-type megafossils and overlying the B550 and B538 Ma (Grotzinger et al., 1995; Condon Gaskiers glacial deposits or its equivalents. This strati- et al., 2005; Chen et al., 2015; Yang et al., 2017a,b; graphic interval is remarkably similar to Sokolov’s (1952) Zhou et al., 2018; Linnemann et al., 2019). original concept of “Vendian Series” of western Russia, Defining the base of the SES is more challenging although Sokolov’s elegant designation has regrettably because fewer stratigraphic markers are available in the been extended by others to include most of what is pres- lower Ediacaran. Xiao et al. (2016) listed several potential ently defined as the Ediacaran Period. This “upper series” criteria, including δ13C crossing over to positive values, is recognizable worldwide and could usefully be formal- 87Sr/86Sr ratios rising from B0.7073 to 0.7080, and the ized. The main problem at present is the reliable correla- first appearance of DoushantuoÀPertatataka-type acantho- tion of its lower boundary between the siliciclastic strata morphs, all of which seem to occur near 632 Ma based on of Europe and North America (rich in Ediacara-type currently available evidence. These biostratigraphic and megafossils) and the phosphatic carbonates of central chemostratigraphic criteria need to be used in conjunction Asia (rich in microfossils and amenable to a wide with geochronological data in order to test their potential array of chemostratigraphic techniques). This resolution is to define the SES. We are optimistic that an integrative likely to come from integrated macrofossil, microfossil, approach will eventually lead to a better stratigraphic and chemostratigraphic studies of mixed siliciclasticÀ framework to subdivide and correlate the Ediacaran carbonate successions and the discovery of additional Period, the most recently named period of the geologic datable horizons. time scale. Both correlations also agree on the recognition of the TES and the SES, where consensus is emerging with regard to stratigraphic markers of global significance (Xiao et al., Acknowledgments 2016). The most important criterion for recognizing the This chapter is an update of the Ediacaran chapter in the Geologic basal boundary of the TES is the rise of skeletal animals Time Scale 2012 (Narbonne et al., 2012a) and integrates discussion such as Cloudina, Sinotubulites and Namacalathus,which presented in Xiao et al. (2016). Thus we would like to thank all are abundant and globally distributed. A few Cambrian-style coauthors of Narbonne et al. (2012a) and (Xiao et al., 2016), includ- skeletal fossils, such as Cambrotubulus and Anabarites, ing Jim Gehling, Graham Shields-Zhou, Chuanming Zhou, Marc have also been reported from the terminal Ediacaran Laflamme, Dmitriy V. Grazhdankin, Małgorzata Moczydłowska- (Nagovitsin et al., 2015; Rogov et al., 2015; Zhu et al., Vidal and Huan Cui. We would also like to thank the entire 2017; Cai et al., 2019), but they are less abundant in the Ediacaran community for sharing their knowledge about this critical Ediacaran than in the Cambrian. Biostratigraphically, the geological time period. Xiao acknowledges financial support provided by National Science Foundation (EAR-1528553) and NASA TES is also characterized by numerous nonmineralized tubu- Exobiology and Evolutionary Biology Program (80NSSC18K1086); lar or ribbon-shaped fossils [e.g., Corumbella (Babcock Narbonne acknowledges financial support provided by a Natural et al., 2005; Pacheco et al., 2015; Walde et al., 2015; Sciences and Engineering Research Council of Canada (NSERC) Warren et al., 2017), Conotubus (Caietal.,2011), Wutubus Discovery Grant (05561-2014) and a Queen’s University Research (Chen et al., 2014), Sekwitubulus (Carbone et al., 2015)and Chair. Saarina (Selly et al., 2019)], and these provide alternative biological criteria for correlation. Ediacara-type fossil impressions are low in diversity and dominated by long- References ranging forms (Boag et al., 2016), with Ernietta being the Adoˆrno, R.R., Do Carmo, D.A., Germs, G., Walde, D.H.G., Denezine, only global taxon that is restricted to this stage (Ivantsov M., Boggiani, P.C., et al., 2017, Cloudina lucianoi (Beurlen & et al., 2016; Smith et al., 2017). Horizontal trace fossils of Sommer, 1957), Tamengo Formation, Ediacaran, Brazil: taxonomy, moderate complexity occur sporadically in this stage (Jensen analysis of stratigraphic distribution and biostratigraphy. 548 PART | III Geologic Periods: Planetary and Precambrian

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