S.H. Xiao and G.M. Narbonne Chapter 18 The Ediacaran Period 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 Geologic Time Scale 2020. DOI: https://doi.org/10.1016/B978-0-12-824360-2.00018-8 © 2020 Elsevier B.V. All rights reserved. 521 522 PART | III Geologic Periods: Planetary and Precambrian 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 car- bonate (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,