Comment and Reply Integrated stratigraphic, geochemical, and paleontological late Ediacaran to early Cambrian records from southwestern Mongolia: Comment

Ed Landing1,† and Peter D. Kruse2 1New York State Museum, 222 Madison Avenue, Albany, New York 12230, USA 2South Australian Museum, Adelaide, SA 5000, Australia

Global correlation of sedimentary rock suc- are strong advocates of the potential utility of Formation (ca. >200 m to 500 m thick), which cessions requires an integrated approach that carbon isotope excursions in correlation (e.g., unconformably overlies or is in thrust contact uses all fossil-based and alternative (lithostrati- Landing et al., 2013), avoidance of the fossil with units as low as the Bayan Gol Formation graphic, geochemical, paleomagnetic, geo- record­ is certainly not part of the “integrated … (Brasier et al., 1996b, p. 455; Smith et al., 2016, chronologic, sequence stratigraphic) data (e.g., geochemical and paleontological … record” of their Khairkhan and Bayangol Formations). Van Couvering and Berggren, 1977). Proposed their report’s title. Smith et al.’s (2016, p. 458) Smith et al.’s (2016) report focuses on two global correlations and their role in interpret- use of the fossil record is seemingly limited to units. One of these is the Salaany Gol Formation ing Earth and biotic history require a thorough “plac[ing] previously reported small shelly fos- (their Salaagol Formation), which commonly examination of the utility and temporal resolu- sil horizons ... into the new stratigraphic and underlies the Khayrkhan Formation. The Zav­ tion allowed by each correlation technique in d13C chemostratigraphic context.” khan terrane was apparently latitudinally sepa- comparison to the others. The article by Smith It is unfortunate that fossils from their “new rated from the Siberian Platform through much et al. (2016) is a welcome contribution to under­ small shelly fossil horizons” have “not yet been of the early Cambrian (e.g., Evans et al., 1996; standing the Upper Ediacaran(?)–Lower Cam- dissolved, identified, or imaged” (Smith et al., Wilhelm et al., 2012). However, the presence of brian succession of the Zavkhan terrane in the 2016, p. 458), as this means that all comment on many Siberian archaeocyath genera and species Khasagt-Khairkhan Range of southwestern Cambrian biostratigraphy of the Zavkhan ter- in the limestone-rich, upper Salaany Gol For- Mongolia. Their work complements earlier rane, including the present evaluation, must, in mation (Brasier, 1989, p. 61; Kruse et al., 1996) syntheses on the Zavkhan terrane completed by the interim, rely on older reports (e.g., ­Voronin indicates extensive faunal exchange between Soviet-Mongolian teams (e.g., Voronin et al., et al., 1982; Gibsher and Khomentovsky, 1990; the Siberian Platform and the Zavkhan terrane. 1982; Esakova and Zhegallo, 1996) and under Brasier et al., 1996b; Kruse et al., 1996; Esa- The lower Salaany Gol did not yield fossils in International Geoscience Programme (IGCP) kova and Zhegallo, 1996; Khomentovsky and earlier studies (e.g., Brasier et al., 1996b, and Project 303, “Late Precambrian–Cambrian Gibsher, 1996). Similarly, their report of unex- references therein) and locally features a thick Event Stratigraphy” (Brasier et al., 1996a; see pectedly low occurrences of archaeocyaths interval of conglomerates (~150 m) that em- associated papers in Geological Magazine, in the basal Salaany Gol Formation (Salaagol phasizes the likely unconformable base of the v. 133, no. 4). Formation of Smith et al., 2016) remains un- formation and a lengthy intra–early Cambrian Smith et al. (2016) provide a more compre- documented (Smith et al., 2016, p. 458, their hiatus (Missar­zhevsky, 1982; Astashkin et al., hensive understanding of onshore-offshore and fig. 12), as no earlier study has ever recovered 1995; Brasier et al., 1996b). lateral shelf lithofacies changes along the highly archaeocyaths at that level in the formation, and Earlier studies by archaeocyath specialists tectonized sections of the Khasagt-Khairkhan these fossils would provide an important lower concluded that the upper Salaany Gol Forma- Range (i.e., their Figs. 6B–6E). However, their age bracket on the Salaany Gol Formation. In tion bears two archaeocyathan faunas that can strong reliance on proposed global geochemi- this comment, we discuss Smith et al.’s (2016) be readily correlated with the upper Lower Cam- cal excursions (primarily d13C) and the incom- use of earlier documented fossil distributions brian (upper Atdabanian and Botomian Stages) pletely documented geochronology of the ter- in the Mongolian sections and interpretations of Siberia (e.g., Voronin et al., 1982; Esakova minal Ediacaran–Lower Cambrian (e.g., Maloof of the chronostratigraphic significance of these and Zhegallo, 1996; Brasier et al., 1996b). This et al., 2005, 2010a, 2010b) has led to a number fossil assemblages. correlation was followed by Kruse et al. (1996), of questionable correlations that significantly The Zavkhan terrane features a thick termi- yet Smith et al. (2016, p. 447, 462) repeatedly affect global correlation, as well as a “too old” nal Ediacaran–upper Lower Cambrian, mixed claim that Kruse et al. (1996) assigned a much earliest appearance of diverse biomineralized carbonate-siliciclastic succession that comple- older, Tommotian to Botomian age to that fauna. small metazoan fossils, and an anomalously ments the biostratigraphic and geochemical rec­ To add to the confusion, Smith et al. (2016, their early appearance of archaeocyaths. Smith et al.’s ord of the coeval, carbonate-dominated Siberia fig. 12) limit the Salaany Gol Formation to the (2016, p. 461) technique assumes that terminal and South China Platforms and the siliciclastic-­ Tommotian, while also recognizing the signifi- Ediacaran–Cambrian d13C excursions “mark dominated, global stratotype section of the Edi­ cantly older d13C 6p excursion of the Siberian synchronous, global markers” and are a way acaran-Cambrian boundary interval in Ava­ Nemakit-Daldynian Stage (e.g., Maloof et al., to avoid “poorly established, facies-dependent,­ lonian eastern Newfoundland (e.g., Brasier et al., 2010b) in the Salaany Gol Formation (Smith and highly regional biozones.” Although we 1996a). The forearc succession of the Zavkhan et al., 2016, their fig. 11, left column). terrane includes the siliciclastic-dominated, This Tommotian equivalency is justified by †ed.landing@​nysed​.gov upper Lower Cambrian, foredeep Khayrkhan Smith et al. (2016, p. 462) on the basis of an

GSA Bulletin; July/August 2017; v. 129; no. 7/8; p. 1012–1015; doi: 10.1130/B31640.1; published online 16 May 2017.

1012 GeologicalFor permission Society to of copy, America contact [email protected] Bulletin, v. 129, no. 7/8 © 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.

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absence of trilobites in the Salaany Gol Forma- gion noted here belies this claim. Even at the stone or conglomerate within the Salaany Gol tion, which in their view makes this interval genus level, the Salaany Gol Formation fauna Formation, and this alone does not suggest a older than the Siberian Atdabanian and, thus, can be confidently correlated, because Ajaci­ regional unconformity within the unit. How- renders it a Tommotian equivalent. However, cyathus, Robustocyathellus, Rotundocyathus, ever, a more plausible hiatus of such a length numerous other papers have long shown that Ur­cyathus, Leptosocyathus, Baikalocyathus, may be marked by the thick conglomerate and Atdabanian-Botomian–equivalent successions Bipalli­cyathus, Pretiosocyathus, Ladae­cyathus, sandstone interval at the base of the formation may lack trilobites and may have “late” appear- Plicocyathus, Agyrekocyathus (= Menneri­ (Brasier et al., 1996b, fig. 7), as also suggested ances of trilobites if they represent very shallow cyathus), Fransuasaecyathus, Alataucyathus, by Smith et al. (2016, p. 456, their fig. 9) for habitats, were paleogeographically isolated (as Archaeopharetra, and Tabulacyathellus are their section E1340 southeast of Khukh-Davaa. in the case of Avalonia), or have carbonate-poor globally limited to Atdabanian-equivalent and, Mixed carbonate and siliciclastic rocks of lithofacies that may have led to dissolution of in some cases, younger strata, while Ichnuso­ the Bayan Gol Formation underlie the Salaany early trilobite remains (as, for example, in Bal- cyathus, Fallocyathus, Chouberticyathus, and Gol Formation. Correlation of the Bayan Gol tica; e.g., Landing et al., 1989, 2013; Landing the non-archaeocyathan spongelike Acanthino­ Formation has long been problematic, with and Westrop, 2004). cyathus are restricted to Botomian equivalents correlations to the Nemakit-Daldynian and/or Smith et al. (2016, p. 462, 467) repeatedly de- (Debrenne­ et al., 2015). Tommotian Stages of Siberia long advocated scribe the Salaany Gol Formation archaeocyath­ Smith et al. (2016, p. 462, their fig. 11) state (see summary in Landing and Kouchinsky, fauna as comprising only “a few” species— that they have found the oldest known archaeo- 2016). However, Smith et al. (2016, p. 458), even though Voronin et al. (1982, their fig. 3) cyaths: They correlate the lower Salaany Gol on the basis of carbon and strontium isotope recorded 78, and Kruse et al. (1996) resampled Formation archaeocyaths with the Nemakit-­ excursions, claim to be the first to limit corre- 34 constituent nominal species (the latter identi- Daldynian Stage of Siberia. Elsewhere, lation of this unit to the Nemakit-Daldynian, fications by F. Debrenne and A.Yu. Zhuravlev). archaeocyaths­ have their global lowest known even though Landing et al. (2013) and Land- It is noteworthy that 43 of the 78 species (55%) appearance in the overlying Tommotian Stage ing and Kouchinsky (2016) had earlier used the listed by Voronin et al. (1982) are also known of Siberia and have never been recovered from Early Skeletalized Fossils of the unit to estab- from the Siberia–Far East region. Perhaps the Nemakit-Daldynian Stage and equivalents lish a Nemakit–­Daldynian equivalency of the the question is: How many archaeocyath spe- (Landing et al., 1989, 2013; Knoll et al., 1995; Bayan Gol Formation. The latter conclusion cies are needed before they can be regarded as Brasier et al., 1996a; Landing and Kouchinsky,­ was presaged by Brasier et al. (1996a, p. 367), more than just “a few”? In addition, Smith et al. 2016), and strong evidence is needed for a who noted that the abundant calcimicrobial (2016, p. 462, col. 2) conclude, “We suggest that claim of a Nemakit-Daldynian–equivalent ap- buildups of the Bayan Gol Formation lacked the few archaeocyath species from the upper pearance of archaeocyaths in Mongolia. Smith archaeocyaths­ (Kruse et al., 1996), and that one Salaagol Formation that are thought to be diag­ et al.’s (2016, their fig. 11) evidence for such explanation was that the formation was a pre- nostic of the Atdabanian and Botomian were old archaeocyaths lies only in their interpreta- Tommo­tian equivalent. either misidentified or have longer stratigraphic tion of the carbon isotope excursions of the A highly resolved interregional correlation of ranges than was previously thought.” They fur- ­Salaany Gol Formation. As noted already, Smith the Nemakit-Daldynian–equivalent Bayan Gol ther claim (Smith et al., 2016, p. 465, col. 2) et al. (2016, p. 447, 462) depict the entire for- Formation remains problematic, particularly as that these are “archaeocyath species with poorly mation variously as a Tommotian equivalent it seems unlikely that the Bayan Gol–Salaany constrained age ranges.” Smith et al. (2016) (Smith et al., 2016, their fig. 12) or as Nemakit-­ Gol contact is part of a stratigraphically continu- seemingly have not used decades of research Daldynian–Tommotian (their fig. 11). Alterna- ous section. This means that the d13C excursion on Siberian archaeocyaths that have established tively, they also equated a strong positive d13C record through the two formations cannot be a well-founded biostratigraphic scheme that excursion in the lower part of the formation directly compared with the supposed excursion is tied to other fossil groups, such as trilobites with excursion 6p of the Nemakit-Daldynian “standard” of Maloof et al. (2010b) without a and a variety of Early Skeletalized Fossils, and (Maloof et al., 2005, 2010a, 2010b; = excursion greater concern for stratigraphic discontinuities is applicable throughout the Siberia–Far East– I of Brasier and Sukhov, 1998). However, based and biostratigraphy. Smith et al. (2016, p. 462, Mongolia region (for a recent English-language on the biostratigraphic evidence, the prominent their fig. 12) are disposed to believe in a sim- compilation, see, for example, Varlamov et al., “6p” excursion in the basal Salaany Gol Forma- ple, not “multistage,” subsidence history for 2008). The Siberian zonation was and remains tion is more likely to be the local expression of the Zavkhan terrane, with apparently continu- the preeminent succession for global archaeo­ the IV excursion within the lower Atdabanian ous deposition and a continuous carbon isotope cyath correlation. of Siberia (e.g., Landing et al., 2013). excursion history from the Bayan Gol through Smith et al. (2016) acknowledge the apparent Altogether, these considerations warrant Salaany Gol Formations. However, no evidence conflict between theird 13C isotope-based corre- discounting Smith et al.’s (2016) correlation is provided by Smith et al. (2016, p. 462) for a lations and the Salaany Gol Formation archaeo­ of the Salaany Gol Formation, and indeed cast Zavkhan terrane that continuously subsides for cyath data, and they put forward a resolution doubt on their entire scheme. Smith et al. (2016, “~16–20 m.y.” of this supposed conflict. They propose that p. 462, their fig. 8) supported their argument Smith et al. (2016, fig. 12) assign the Zuun- archaeocyath biostratigraphy was developed against correlation of the upper Salaany Gol Arts Formation and most of the lower half of the largely in Siberia and may not be appropriate for with the upper Atdabanian–Botomian as they overlying Bayan Gol Formation to a “Precam- correlations elsewhere. It is true that archaeo­ surmised that this would require a 6–10 m.y. brian–Cambrian transition” interval. They place cyath species are provincial and lithofacies con- hiatus within the “middle–upper Salaagol For- much emphasis on the lowest (and only) occur- trolled (e.g., Kruse et al., 1996, and references mation” as marked by a “medium sandstone to rence of the ichnofossil Trichophycus pedum in therein). However, the strong faunal common- pebble conglomerate.” This argument is con- only one of their sections as suggesting the “P-C ality between Mongolian and late Atdabanian–­ fusing, as only one of their sections (E1340 in boundary, as defined at GSSP [global stratotype Botomian faunas of the Siberia–Far East re- Smith et al., 2016, their fig. 9) shows a sand- section and point]” in the middle Bayan Gol

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Formation (at about the base of their informal by Smith et al. (2016), as discussed above, a REFERENCES CITED unit BG4). This correlation may be regarded as confident correlation of the carbon isotope Astashkin, Y., Jacobsen, S.B., Knoll, A.H., Butterfield, N.J., forcing the numbered carbon isotope excursion peaks and nadirs in the Nemakit-Daldynian– and Swett, K., 1995, The Cambrian System of the fold- history presented in their figure 12 into incorrect equivalent Bayan Gol Formation is not pos- belts of Russia and Mongolia: International Union of Geological Sciences Publication 32, 132 p. correlations. sible. Excursion 6p of Maloof et al. (2010a, Bowring, S.A., Grotzinger, J.P., Isachsen, C.E., Knoll, A.H., The base of the Cambrian is defined at the 2010b) may actually equate to excursion “5p” Pelechaty, S.M., and Kolosov, P., 1993, Calibrating first appearance datum (FAD; i.e., lowest) of of Smith et al. 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It should be ­Kouchinsky, 2016). physics 22, p. 40–74. noted that T. pedum has a range of ~220 m in If this alternative correlation of Smith et al.’s Brasier, M.D., 1992, Background to the Cambrian explosion: Journal of the Geological Society [London], v. 149, the Fortune Head GSSP section (Landing et al., (2016) “5p” positive excursion (as representing no. 4, p. 585–587, doi:​10​.1144​/gsjgs​.149​.4​.0585​. 1988). Thus, the single known horizon with 6p) were accepted, their lowest positive excur- Brasier, M.D., and Sukhov, S.S., 1998, The falling amplitude T. pedum in Mongolia does not provide a basis sion termed “2p” in the Bayan Gol would be- of carbon isotope oscillations through the Lower to Middle Cambrian: Canadian Journal of Earth Sciences, for highly resolved correlation. Indeed, the trace come Maloof et al.’s (2010b, fig. 5) excursion v. 35, no. 4, p. 353–373. fossils Helminthoida cf. miocenica, Spatangop­ 3p, which lies just below the lowest range of Brasier, M.D., Anderson, M.M., and Corfield, R.M., 1992, Oxygen and carbon isotope stratigraphy of early Cam- sis, and Rusophycus that appear below T. pedum mollusks, cap-shaped shells, and hyoliths in brian carbonates in southeastern Newfoundland and in the Bayan Gol Formation appear above the Siberia but is not associated with any faunal­ England: Geological Magazine, v. 129, no. 3, p. 265– lowest T. pedum and, more importantly, in the change in Mongolia. Although Smith et al. 279, doi:​10​.1017​/S001675680001921X​. Brasier, M.D., Dorjnamjaa, D., and Lindsay, J., 1996a, The lowest Cambrian at the GSSP section. (2016, fig. 12) regard the Mongolian lowest Neoproterozoic to early Cambrian in southwest Mon- Given the stratigraphic range of T. pedum, Cambrian faunal succession as complement- golia: An introduction: Geological Magazine, v. 133, its local FAD need not necessarily represent ing that known from other paleocontinents, a no. 4, p. 365–369, doi:​10​.1017​/S0016756800007548​. Brasier, M.D., Shields, G., Kuleshov, V.N., and Zhegallo, the base of the Cambrian, and this is the case in diverse Early Skeletalized Fossil assemblage E.A., 1996b, Integrated chemo- and biostratigraphic the Zavkhan terrane sections. In the Smith et al. occurs at the level of their excursion “3p,” calibration of early animal evolution: Neoprotero- zoic–early Cambrian of southeast Mongolia: Geologi- (2016, fig. 12) report, the FAD of anabaritids is which would be equivalent to Maloof et al.’s cal Magazine, v. 133, no. 4, p. 445–489, doi:​10​.1017​ near the base of the Bayan Gol Formation, and (2010b, fig. 5) excursion 4p. If true, then the /S0016756800007603​. the lowest occurrence of this group of trilobed carbon isotope excursions of the lower Bayan Debrenne, F., Zhuravlev, A.Yu., and Kruse, P.D., 2015, Gen- eral features of the Archaeocyatha. Systematic descrip- tube-forming Early Skeletalized Fossils has been Gol Formation demonstrate a higher known ap- tions: Archaeocyatha, in Selden, P.A., ed., Treatise on regarded as providing the most likely approxi- pearance of diverse Early Skeletalized Fossils Invertebrate Paleontology, Part E, Porifera Revised, mation to the base of the Cambrian (e.g., Brasier, in the Zav­khan terrane than in Siberia, South Hypercalcified Porifera, Volume 5: Lawrence, Kan- sas, University of Kansas, Paleontological Institute, 1992; Brasier et al., 1992, 1996b; Bowring et al., China, and Avalonia (Landing and Kouchinsky, p. 845–1084. 1993). However, several caveats are associated 2016). An obvious caveat to both this and the Esakova, N.V., and Zhegallo, E.A., 1996, Biostratigrafiya i Fauna Nizhnego Kembriya Mongolii: Moscow, Sovmest- with definition of the base of the Cambrian near Smith et al. (2016, fig. 12) correlations is that, naya Rossiysko-Mongol’skaya Paleon­tologicheskaya the lowest occurrence of anabari­tids in the Zav­ as reported by Brasier et al. (1996b), this more Ekspeditsiya, Trudy 46, 214 p. [in Russian]. khan terrane. As noted herein, Smith et al. (2016, diverse Purella zone assemblage appears at a Evans, D.A., Zhuravlev, A.Yu., Budney, C.J., and Kirschvink, J.L., 1996, Palaeomagnetism of the Bayan Gol Forma- p. 458) did not acid-disaggregate any Early lithofacies and environmental change in the tion, western Mongolia: Geological Magazine, v. 133, Skeletalized Fossil–bearing horizons, so that lower Bayan Gol Formation, marked by the re- no. 4, p. 487–496, doi:​10​.1017​/S0016756800007615​. the lowest occurrence of anabaritids is presently placement of sandstones and shales by thick Gibsher, A.S., and Khomentovsky, V.V., 1990, The sections of the Tsagaan Oloom and Bayan Gol formations of the unknown in their sections, while Brasier et al. limestones, and it does not necessarily record Vendian–Lower Cambrian in the Zavkhan zone of Mon- (1996b) and Khomentovsky and Gibsher (1996) the oldest global appearance of diverse Early golia, in Pozdniy Dokembriy i Ranniy Paleozoy Sibiri: Voprosy Regional’noy Stratigrafii: Novosibirsk, Insti- recorded anabaritids in lower strata from what is Skeletalized Fossil faunas. Only relatively tuta Geologii i Geofiziki, p. 79–91 [in Russian]. now the underlying Zuun Arts Formation. The crude estimates exist on the ages of faunal as- Khomentovsky, V.V., and Gibsher, A.S., 1996, The Neo- lowest occurrence of anabaritids can only be semblages through the Nemakit-Daldynian and proterozoic–Lower Cambrian in northern Govi-Altay, western Mongolia: Regional setting, lithostratigraphy, said to fall in the Ediacaran-Cambrian boundary equivalents. Thus, a date of 530 Ma may be and biostratigraphy: Geological Magazine, v. 133, interval, with Zhuravlev et al. (2012) recording more appropriate as an estimate of the age of p. 371–390, doi:​10​.1017​/S001675680000755X​. members of the group notionally from the termi- the (herein) renumbered 4p peak in Mongolia Knoll, A.H., Kauffman, A.J., Semikhatov, M.A., Grotzinger, J.P., and Adams, W., 1995, Sizing up the sub-Tommo- nal Ediacaran. (Landing and Kouchinsky,­ 2016). tian unconformity in Siberia: Geology, v. 23, no. 12, Similarly, the list of body fossils reported by A highly resolved Ediacaran-Cambrian p. 1139–1143, doi:10​ .1130​ /0091​ -7613​ (1995)023​ ​ <1139:​SUTSTU>2​.3​.CO;2​. Smith et al. (2016, fig. 12; e.g., coelosclerito­ boundary level remains undetermined in the Kruse, P.D., Gandin, A., Debrenne, F., and Wood, R., 1996, phorans, orthothecimorphs, tommotiids, cap- Mongolian sections. Excursion 1p of Smith Early Cambrian bioconstructions in the Zavkhan Basin shaped fossils [possibly Purella]) to appear et al. (2016) in the lower Zuun Arts Formation of western Mongolia: Geological Magazine, v. 133, no. 4, p. 429–444, doi:​10​.1017​/S0016756800007597​. lower in the Bayan Gol Formation (their unit may correspond to the terminal Ediacaran 1p Landing, E., and Kouchinsky, A., 2016, Correlation of the BG2) are all taxa that indicate a lowest Cam- excursion of Maloof et al. (2005, 2010b). This Cambrian evolutionary radiation: Geochronology, evo- brian correlation. Voronova et al. (1982, fig. 12) is succeeded by a strongly negative peak higher lutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic and Brasier et al. (1996b, fig. 6) have provided in the Zuun Arts Formation, which likely corre- significance: Geological Magazine, v. 153, p. 750–756, more comprehensive lists of fossils through the sponds to excursion W of Brasier et al. (1996b) doi:​10​.1017​/S0016756815001089​. Landing, E., and Westrop, S.R., 2004, Environmental pat- Bayan Gol Formation. and possibly to the basal Cambrian carbon iso- terns in the origin and evolution and diversification With excursion 6p of Maloof et al. (2010a, tope excursion (BACE) in South China (e.g., loci of early Cambrian skeletalized Metazoa: Evidence 2010b) placed in the Salaany Gol Formation Peng and Babcock, 2008, fig. 4.4). from the Avalon microcontinent, in Lipps, J.H., and

1014 Geological Society of America Bulletin, v. 129, no. 7/8

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