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Integrated Stratigraphic, Geochemical, and Paleontological Late Ediacaran to Early Cambrian Records from Southwestern Mongolia: Comment

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Integrated Stratigraphic, Geochemical, and Paleontological Late Ediacaran to Early Cambrian Records from Southwestern Mongolia: Comment 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- rec ord 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 (Missar zhevsky, 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 silici clastic- 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. Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/129/7-8/1012/1002230/1012.pdf by guest on 01 November 2019 Comment and Reply 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 Ur cyathus, Leptosocyathus, Baikalocyathus, may be marked by the thick conglomerate and Atdabanian-Botomian–equivalent successions Bipalli cyathus, Pretiosocyathus, Ladae cyathus, 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
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