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740 Ma Vase-Shaped Microfossils from Yukon, Canada: Implications for Neoproterozoic Chronology and Biostratigraphy

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740 Ma Vase-Shaped Microfossils from Yukon, Canada: Implications for Neoproterozoic Chronology and Biostratigraphy 740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy Justin V. Strauss1, Alan D. Rooney1, Francis A. Macdonald1, Alan D. Brandon2, and Andrew H. Knoll1 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA 2Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, 77204, USA ABSTRACT global stratigraphic correlation that goes beyond Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran previous broad morphoclass-based biostrati- strata has remained limited because Proterozoic taxa commonly have long or unknown strati- graphic comparisons. graphic ranges, poorly understood taphonomic constraints, and/or inadequate geochrono- logical context. Here we report the discovery of abundant vase-shaped microfossils from the STRATIGRAPHY Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potential The Coal Creek inlier in the Ogilvie Moun- for biostratigraphic correlation of Neoproterozoic successions using species-level assemblage tains (Yukon, Canada) hosts an ~3-km-thick se- zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at quence of ca. 780–540 Ma Windermere Super- 739.9 ± 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from group strata (Fig. 1; Mustard and Roots, 1997). the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an The Mount Harper Group consists of three interbedded tuff at 742 ± 6 Ma. The overlapping age and species assemblages from these two informal units; in stratigraphically ascending deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, order , these are (1) the Callison Lake dolostone, 13 and perhaps globally. The new Re-Os age also confi rms the timing of the Islay δ Ccarbonate an ~400-m-thick mixed siliciclastic and carbon- anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by ate deposit; (2) the Mount Harper conglomer- >15 m.y. Together these data provide global calibration of sedimentary, paleontological, and ate, an ~1100-m-thick rift-related clastic suc- geochemical records on the eve of profound environmental and evolutionary change. cession; and (3) the Mount Harper volcanics, an ~1200-m-thick intermediate to mafi c volcanic INTRODUCTION (Porter and Knoll, 2000; Porter et al., 2003). complex (Mustard and Roots, 1997; Macdon- Neoproterozoic sedimentary deposits of west- Given their abundance, diversity, preservation, ald et al., 2010). Age constraints on the Mount ern North America record large fl uctuations in and time-calibrated record, VSMs could rep- Harper Group are provided by U-Pb chemical global biogeochemical cycles (e.g., Narbonne resent the fi rst temporally well-resolved bio- abrasion–thermal ionization mass spectrometry et al., 1994; Karlstrom et al., 2000; Halver- stratigraphic assemblage zone for pre-Ediacaran (CA-TIMS) ages on zircon of 811.51 ± 0.25 Ma son et al., 2005), the diversifi cation of multiple strata, opening a new window for regional and from a tuff in the underlying Fifteenmile Group, eukary otic clades (e.g., Porter and Knoll, 2000; Samuelsson and Butterfi eld, 2001; Cohen and Greenland Knoll, 2012), the fragmentation of Rodinia ac- Coal Creek Inlier, Ogilvie Mountains, Yukon companied by localized mafi c volcanism (e.g., Alaska Jefferson and Parrish, 1989; Prave, 1999; Mac- Upper Group Mount Gibben δ13 Corg(‰) donald et al., 2010), and multiple global glacia- Hay Creek Group 500 m Section J1204 –35 –30 –25 Yukon 33.1 tions (e.g., Aitken, 1991; Hoffman et al., 1998). Te r r i t o r y Rapitan Group Understanding the causal relationships among 716.47±0.2 Ma these events requires accurate stratigraphic cor- Mount Harper 717.43±0.1 Ma CANADA volcanics relation in the context of geochronologically constrained age models. However, the geograph- U.S.A. Mount Harper conglomerate 20 ically disparate Neoproterozoic sedimentary rec- 141°W Hyland Group 739.9±6.1 Ma ords along the length of western North America 68°N Windermere SG Callison Lake have yet to be clearly linked in time and space Mackenzie Mtns SG dolostone due a paucity of radiometric age constraints, Pinguicula Group 10 Wernecke SG non-unique chemostratigraphic ties, abundant Craggy Coal Creek synsedimentary tectonism and associated lat- M Dolostone Inlier acken 811.51±0.1 Ma eral facies change, and a lack of biostratigraphi- zie 0 m M –8 –4 0 4 cally useful microfossils. Correlations have been o 13 u Conglomerate VSM δ C (‰) n Reefal carb proposed for pre-glacial Neoproterozoic strata t Sandstone Cover Mount a Assemblage i in the southwestern United States (e.g., Dehler n Shale Nodular U-Pb Age Gibben Ogilvie s Evaporite Chert Re-Os Age et al., 2001, 2010), but these schemes have not Mtns GroupFifteenmile Harper Gp. Mount Diamictite Chandindu Intraclasts Exposure been extended to the rich sedimentary archives 400 km Dolostone Fine lamination surface of northwest Canada, due primarily to a lack of Whitehorse Mackenzie Mountains SG Windermere Supergroup Gibben Fm. Basalt/Rhyolite Stromatolite/Microbial age control. Here we document new vase-shaped microfossil (VSM) assemblages from Yukon, Figure 1. Simplifi ed map locations and schematic lithostratigraphy of the Coal Creek inlier, Canada, that are indistinguishable in taxonomic Yukon, Canada. Vase-shaped microfossils (VSMs) described herein are from Callison Lake dolostone. Measured section J1204 highlights location of fossil and Re-Os age hori zon, as composition and age from those described from δ13 δ13 well as bounding Ccarb and Corg (blue data points) data from the Islay anomaly. Geologic the Chuar Group of the Grand Canyon (Arizona, map of Yukon is adapted from Wheeler and McFeely (1991). Abbreviations: SG—Supergroup; USA) and successions of similar age worldwide Gp.—Group; Fm.—Formation; Mtns—mountains. GEOLOGY, August 2014; v. 42; no. 8; p. 1–8; Data Repository item 2014244 doi:10.1130/G35736.1 | Published online XX Month 2014 GEOLOGY© 2014 Geological | June Society 2014 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 data-point error ellipses are 2 717.43 ± 0.14 Ma from rhyolite in the upper 3.4 member of the Mount Harper volcanics, and G 716.47 ± 0.24 Ma from a tuff interbedded with 3.0 E H diamictite correlated with the glacially infl u- F 2.6 Os C enced Rapitan Group in the Mackenzie Moun- B 188 tains (Fig. 1; Macdonald et al., 2010). / 2.2 H D The lower Mount Harper Group records Os I mixed marine and terrestrial deposition inti- 187 1.8 mately associated with an east-west–trending, A Age = 739.9 ± 6.1[6.5] Ma 1.4 187 188 syndepositional north-side-down fault scarp that J Initial Os/ Os = 0.609 ± 0.01 outlines the remnants of a Proterozoic half-gra- MSWD = 0.62 1.0 ben (Mustard and Roots, 1997). Basal deposits Figure 2. Neoproterozoic vase-shaped micro- 20 60 100 140 180 220 of the Callison Lake dolostone unconformably 187 188 fossils (VSMs) from Callison Lake dolo- Re / Os overlie brecciated and silicifi ed strata of the stone, Yukon, Canada. (Slide number and Fifteenmile Group, and consist of an ~4–30-m- England Finder Coordinates are given for Figure 3. Re-Os isochron for upper Callison each image.) A: Low-magnifi cation image thick interval of sandstone, siltstone, and dis- Lake dolostone (Yukon, Canada) with an age showing abundance of VSM tests, with uncertainty of 6.5 m.y. (in brackets) when un- continuous beds of quartz and chert pebble centrally located specimen of Melano- certainty of 187Re decay constant is included. conglomerate that transition into ~5–30 m of cyrillium hexodiadema (J1204.16.8; K23/0). MSWD—mean square of weighted deviates. black to varicolored and mud-cracked shale Scale = 100 µm. B: Palaeoarcella athanata Isotope composition and abundance data interbedded with laterally discontinuous stro- (J1204.16.8; H31/1). Scale = 50 µm. C: Cross are presented in the Data Repository (see section of Melanocyrillium hexodiadema footnote 1). matolitic bioherms that host poorly preserved aperture (F930.15.5; O33/4). Scale = 50 µm. VSMs. Basal Callison Lake siliciclastic depos- D: Bombycion micron (J1204.18.1; H32/2). its are sharply overlain by an ~15–100-m-thick Scale = 40 µm. E: Bonniea dacruchares medium gray dolostone characterized by (J1204.18.1; W31/1). Scale = 50 µm. F: Cyclio- GEOCHRONOLOGY cyrillium torquata (J1204.18.1; L43/1). Scale = piso litic grainstone, microbial laminae, mor- 50 µm. Silicifi ed black shale of the Callison Lake phologically diverse stromatolites, evaporite dolostone was collected from a VSM-bearing pseudomorphs, intraclast conglomerate, and outcrop near Mount Gibben in the Coal Creek mechanically bedded dolomicrite and/or dolo- inlier (Fig. 1; section J1204, 16.8–18.2 m). This siltite, which also contains intercalated black pole opposite a tapered oral end with aperture; 2.4-m-thick exposure was sampled at high reso- shale composed predominantly of authigenic (3) broad size ranges, from ~20 to 200 µm in lution for Re-Os geochronology, and bounding talc [Mg3Si4O10(OH2)] (Tosca et al., 2011). length and ~15 to 120 µm in width; (4) morpho- stromatolitic dolostone was collected at ~1 m δ13 δ13 These strata are overlain by 200–300 m of logically diverse apertures ~10–40 µm wide; resolution for Ccarb and Corg chemo stratig- dolo stone characterized by abundant microbial and (5) ~1–3-µm-thick test walls. Most sections raphy (Fig. 1; details of the sampling proce- lamination, domal stromatolitic bioherms, and through Callison Lake specimens do not yield dure and analytical methods are provided in cross-bedded oolitic grainstone, with abun- systematically diagnostic characters; however, the GSA Data Repository1). A Re-Os age of dant early diagenetic chert.
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