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

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Integrated Stratigraphic, Geochemical, and Paleontological Late Ediacaran to Early Cambrian Records from Southwestern Mongolia Integrated late Ediacaran to early Cambrian records from southwestern Mongolia Integrated stratigraphic, geochemical, and paleontological late Ediacaran to early Cambrian records from southwestern Mongolia Emily F. Smith†, Francis A. Macdonald, Tanya A. Petach, Uyanga Bold, and Daniel P. Schrag Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA ABSTRACT minths, cap-shaped fossils, protoconodonts, limited to a small number of localities with large and Salanacus to just below the positive peak uncertainties in both local and global strati- The Zavkhan terrane in western Mongolia 3p. This interpretation differs from previ- graphic correlations and the lack of absolute preserves thick, fossiliferous, carbonate-rich ous chronostratigraphic placements of first ages directly linked to biostratigraphic and che- strata that span the Ediacaran-Cambrian appearance data of genera between positive mostratigraphic data, particularly with respect transition. Measured stratigraphic sections excursions 1p and 2p. Using this level as the to critical data from Asia. Because the global and geological mapping of these strata, first appearance datum in Mongolia for these data set is biased by just a few localities, refin- which include the Zuun-Arts, Bayangol, genera, we replot global fossil first appear- ing local correlations and grounding them in Salaagol, and Khairkhan Formations, reveal ances. With this new compilation, there are regional geology are necessary before an accu- large lateral facies changes over short dis- still three distinct pulses of fossil first appear- rate global synthesis of fossil occurrence data tances that necessitate revisions to previous ances, as was suggested in previous compila- can be constructed and interpreted. lithostratigraphic correlations and biostrati- tions; however, we suggest that this pattern is The Ediacaran-Cambrian boundary is defined graphic range charts. Here, we integrate new controlled largely by regional sedimentation at the global bondary stratotype section and geological mapping and measured strati- and taphonomy rather than the rate of taxo- point (GSSP) as the first appearance datum graphic sections across the Zavkhan terrane nomic origination. (FAD) of the bilaterian trace fossil Treptich- with high-resolution carbon isotope (δ13C) nus pedum (Landing, 1994), but the boundary chemostratigraphy, sequence stratigraphy, INTRODUCTION has also been associated with the FAD of small and biostratigraphy. Using these data, we shelly fossils and a large negative carbon iso- revise correlations within the Zavkhan ter- Since Charles Darwin’s observation of the tope excursion (Narbonne et al., 1994; Corsetti rane and place small shelly fossil and ichno- apparently rapid appearance of fossils in what is and Hagadorn, 2000). These occurrences in the fossil horizons in a refined temporal and spa- now known as Cambrian strata (Darwin, 1859), rock record are generally assumed to be broadly tial framework. Finally, we integrate these the Precambrian-Cambrian boundary has been coincident, but both the precise and relative tim- data from Mongolia with absolute ages and widely regarded as one of the evolutionary pivot ing of these evolutionary milestones and per- chemo stratig raphy from sections in Morocco points in the history of life. Despite the persist- turbations to the carbon cycle are not directly and Oman. Our revised age model suggests ing interest on this topic, the causes, triggers, constrained. The subsequent earliest Cambrian that deposition of the Bayangol Formation and tempo of change remain controversial. Fol- period is marked by FADs of small shelly fos- and the lower part of the Salaagol Formation lowing the extinction of Ediacaran biota and the sils in many clades and high-frequency oscilla- was limited to the Nemakit-Daldynian and calcifying metazoan Cloudina, the early Cam- tions in the carbon cycle (Maloof et al., 2010a), Tommotian Stages. brian (or Terreneuvian Series) encompasses an but again, there are few absolute ages tied to With the new correlations and age model, evolutionary interval characterized by increases these FADs or excursions. Because there is no we place the small shelly fossil first appear- in diversity and disparity (Marshall, 2006) that one section globally in which it is possible to ance datum in the basal Bayangol Formation coincide with multiple carbon isotope excur- integrate the ichnofossil record, body fossil instead of the basal Zuun-Arts Formation, sions with amplitudes of <8‰ (e.g., Zhu et al., record, carbon isotope chemostratigraphy, and which moves this horizon hundreds of meters 2006; Porter, 2007; Maloof et al., 2010a; Erwin absolute ages, the calibration of this evolution- higher in the stratigraphy, above the large et al., 2011) over a geologically brief inter- arily important transition remains piecemeal, negative excursion in the Zuun-Arts Forma- val in Earth history (Valentine, 2002). Most resulting in much uncertainty in determining tion. The first appearance datum of Treptich- recently, Maloof et al. (2010a) suggested that rates of origination and geochemical change. nus pedum is ~275 m above the large negative the Cambrian fossil first appearances occurred With a dearth of absolute ages tied to the fossil excursion in the Zuun-Arts Formation and in three discrete pulses associated with rapid record, carbon isotope (d13C) chemostratigra- ~250 m above the first appearance datum reorganizations in the carbon cycle. To test this phy has proven to be a powerful tool for global of small shelly fossils, highlighting the rarity hypothesis and others about mechanistic links correlation of Ediacaran-Cambrian successions and facies dependence of its preservation. between environmental change and evolution- (Brasier et al., 1990, 1993; Kirschvink et al., We shift the first appearance datums of ary milestones across the Ediacaran-Cambrian 1991; Knoll et al., 1995a; Zhu et al., 2006, 2007; tommotiids, orthothecimorphs, hyolithel- transition, it is necessary to integrate records Maloof et al., 2010b). around the globe. However, previous global The Zavkhan (Dzabkhan) terrane in western †E-mail: [email protected] syntheses (e.g., Maloof et al., 2010a) have been Mongolia hosts thick, well-exposed, low-grade, GSA Bulletin; Month/Month 2015; v. 1xx; no. X/X; p. 1–27; doi: 10.1130/B31248.1; 13 figures; 1 table; Data Repository item 2015265.; published online XX Month 2015. For permission to copy, contact [email protected] Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 © 2015 Geological Society of America Smith et al. carbonate-rich, fossiliferous successions of Tarim (Lehmann et al., 2010; Edel et al., 2014). Cryogenian to Early Ediacaran late Ediacaran through early Cambrian strata, Locally, on the Zavkhan terrane, this late Paleo- Stratigraphy providing an opportunity to more precisely zoic collision is manifested in dextral strike-slip calibrate evolutionary changes in a chemo- and wrench structures (Figs. 1A and 1C) and wide- Along the Zavkhan River, the oldest exposed sequence-stratigraphic context. This succession spread Permian plutonism (Jahn et al., 2009). rocks are felsic volcanic rocks of the Zavkhan hosts dozens of well-exposed sections with sev- The Neoproterozoic through Cambrian stra- Formation (Figs. 1B and 1C; Bold et al., 2013), eral horizons of small shelly fossils, archaeo- tigraphy of the Zavkhan terrane was first which have been dated using laser ablation cyathid reefs, and ichnofossils (Goldring and described by Bezzubtsev (1963) in the Bayan inductively coupled plasma mass spectrometry Jensen, 1996; Khomentovsky and Gibsher, Gorge. Before the early 1990s, research in (LA-ICP-MS) U/Pb dating on zircon to between 1996; Kruse et al., 1996), many of which are Mongolia was largely restricted to Mongolian 773.5 ± 3.6 and 803.4 ± 8.0 Ma (Levashova being reported and documented here for the and Russian researchers who worked primarily et al., 2010). Granites that intrude the Zavkhan first time. However, because Ediacaran to early on the Ediacaran through early Cambrian strata Formation have been dated using thermal ion- Cambrian strata in the Zavkhan terrane were in the Zavkhan terrane (Zhegallo and Zhuravlev, ization mass spectrometry (TIMS) bulk multi- deposited in a foreland basin (Macdonald et al., 1991). Shortly after Mongolia opened back up grain U/Pb on zircon at 755 ± 3 Ma (Yarmolyuk 2009), topography generated during tectonism to Western researchers, an international research et al., 2008). The Zavkhan Formation is overlain resulted in large lateral facies changes, which team participated in International Geoscience by the synrift siliciclastic deposits informally confounded previous lithostratigraphic correla- Programme (IGCP) Project 303, “Precambrian- referred to as the Khasagt Suite (Ruzhentsev tions and biostratigraphic compilations (Brasier Cambrian Event Stratigraphy,” in 1993, repre- and Burashnikov, 1996) and passive-margin et al., 1996b; Khomentovsky and Gibsher, senting the first effort to bring together specialists deposits of the Tsagaan-Olom Group (Macdon- 1996). In this study, we present new geological from different subdisciplines of earth sciences ald et al., 2009; Bold et al., 2013). mapping and small shelly fossil biostratigraphy for an integrated approach to addressing the Pre- The Tsagaan-Olom Group (note recent for the Nemakit-Daldynian through Tommotian cambrian-Cambrian transition in western Mon- changes in spelling and formalization of stra-
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