Biostratigraphy underpins the Phanerozoic time scale, but its application to pre-Ediacaran strata has remained limited because Proterozoic taxa commonly have long or unknown stratigraphic ranges, poorly understood taphonomic constraints, and/or inadequate geochronological context. Here we report the discovery of abundant vase-shaped microfossils from the Callison Lake dolostone of the Coal Creek inlier (Yukon, Canada) that highlight the potentialfor biostratigraphic correlation of Neoproterozoic successions using species-level assemblage zones of limited duration. The fossiliferous horizon, dated here by Re-Os geochronology at 739.9 ± 6.1 Ma, shares multiple species-level taxa with a well-characterized assemblage from the Chuar Group of the Grand Canyon (Arizona, USA), dated by U-Pb on zircon from an interbedded tuff at 742 ± 6 Ma. The overlapping age and species assemblages from these two deposits suggest biostratigraphic utility, at least within Neoproterozoic basins of Laurentia, and perhaps globally. The new Re-Os age also confi rms the timing of the Islay δ13Ccarbonate anomaly in northwestern Canada, which predates the onset of the Sturtian glaciation by >15 m.y. Together these data provide global calibration of sedimentary, paleontological, and geochemical records on the eve of profound environmental and evolutionary change.
The Taconic and Salinic orogenies in the northern AppalachianMountains record the closure of the Iapetus Ocean, which separatedperi-Laurentian and peri-Gondwanan terranes in the early Paleozoic.The Taconic orogeny in New England is commonly depicted asan Ordovician collision between the peri-Laurentian Shelburne Fallsarc and the Laurentian margin, followed by Silurian accretion of peri-Gondwanan terranes during the Salinic orogeny. New U-Pb zircongeochronology demonstrates that the Shelburne Falls arc was insteadconstructed on a Gondwanan-derived terrane preserved in the MoretownFormation, which we refer to here as the Moretown terrane.Metasedimentary rocks of the Moretown Formation were depositedafter 514 Ma and contain abundant ca. 535–650 Ma detrital zircon thatsuggest a Gondwanan source. The Moretown Formation is bound to thewest by the peri-Laurentian Rowe belt, which contains detrital zirconin early Paleozoic metasedimentary rocks that is indistinguishable inage from zircon in Laurentian margin rift-drift successions. These datareveal that the principal Iapetan suture in New England is between theRowe belt and Moretown terrane, more than 50 km farther west thanpreviously suspected. The Moretown terrane is structurally below andwest of volcanic and metasedimentary rocks of the Hawley Formation,which contains Laurentian-derived detrital zircon, providing a linkbetween peri-Laurentian and peri-Gondwanan terranes. The Moretownterrane and Hawley Formation were intruded by 475 Ma plutonsduring peak activity in the Shelburne Falls arc. We propose that theperi-Laurentian Rowe belt was subducted under the Moretown terranejust prior to 475 Ma, when the trench gap was narrow enoughto deliver Laurentian detritus to the Hawley Formation. Interactionbetween peri-Laurentian and peri-Gondwanan terranes by 475 Ma is20 m.y. earlier than documented elsewhere and accounts for structuralrelationships, Early Ordovician metamorphism and deformation, andthe subsequent closure of the peri-Laurentian Taconic seaway. In thisscenario, a rifted-arc system on the Gondwanan margin resulted in theformation of multiple terranes, including the Moretown, that independentlycrossed and closed the Iapetus Ocean in piecemeal fashion.
. Earth and Planetary Sciences Letters, 404, pp.22-32. Abstract
Neoproterozoic cap carbonates host distinctive geochemical and sedimentological features that reflect prevailing conditions in the aftermath of Snowball Earth. Interpretation of these features has remained contentious, with hypotheses hinging upon timescale and synchronicity of deposition, and whether or not geochemical signatures of cap carbonates represent those of a well-mixed ocean. Here we present new high-resolution Sr and Mg isotope results from basal Ediacaran cap dolostones in South Australia and Mongolia. Least-altered Sr and Mg isotope compositions of carbonates are identified through a novel incremental leaching technique that monitors the purity of a carbonate sample and the effects of diagenesis. These data can be explained by the formation of these cap dolostones involving two chemically distinct solutions, a glacial meltwater plume enriched in radiogenic Sr, and a saline ocean residue with relatively lower 87Sr/86Sr ratios. Model simulations suggest that these water bodies remained dynamically stratified during part of cap-dolostone deposition, most likely lasting for ∼8thousand years. Our results can potentially reconcile previous conflicts between timescales estimated from physical mixing models and paleomagnetic constraints. Geochemical data from cap carbonates used to interpret the nature of Snowball Earth and its aftermath should be recast in terms of a chemically distinct meltwater plume.
After nearly a billion years with no evidence for glaciation, iceadvanced to equatorial latitudes at least twice between 717 and635 Mya. Although the initiation mechanism of these NeoproterozoicSnowball Earth events has remained a mystery, the broadsynchronicity of rifting of the supercontinent Rodinia, the emplacementof large igneous provinces at low latitude, and the onset ofthe Sturtian glaciation has suggested a tectonic forcing. We presentunique Re-Os geochronology and high-resolution Os and Sr isotopepro!les bracketing Sturtian-age glacial deposits of the RapitanGroup in northwest Canada. Coupled with existing U-Pb dates, thepostglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronologicalconstraints for both the onset and demise of a Cryogenianglaciation from the same continental margin and suggests a 55-Myduration of the Sturtian glacial epoch. The Os and Sr isotope dataallow us to assess the relative weathering input of old radiogeniccrust and more juvenile, mantle-derived substrate. The preglacialisotopic signals are consistent with an enhanced contribution ofjuvenile material to the oceans and glacial initiation throughenhanced global weatherability. In contrast, postglacial stratafeature radiogenic Os and Sr isotope compositions indicative ofextensive glacial scouring of the continents and intense silicateweathering in a post–Snowball Earth hothouse.
. Journal of Paleontology, 88(2), pp.299-308. Abstract
Here we describe large, complex trace fossils in the late Ediacaran Omkyk Member of the Zaris Formation,Nama Group, southern Namibia. The horizontal trace fossils are preserved on a number of talus blocks from a beddingplane of a cm-thick sandstone lens from a single stratigraphic horizon less than 100 m below an ash bed dated at 547.3 60.7 Ma. The forms consist of overlapping U-shaped spreiten elements with parallel limbs surrounded by an outer tube.Individual U-shaped elements are 0.2 to 1 cm in diameter, the outer tube is less than 3 mm in diameter, and the forms as awhole range from 5 to 30 cm long and 3 to 10 cm wide. The specimens commonly show a change in direction and changein diameter. The morphology of these trace fossils is comparable to backfill structures, particularly specimens of PaleozoicZoophycos from shallow water environments. Here we interpret these horizontal spreiten-burrows to record the grazing ofthe trace-maker on or below a textured organic surface. The identification of large late Ediacaran trace fossils is consistentwith recent reports of backfilled horizontal burrows below the Precambrian–Cambrian boundary and is suggestive of theappearance of complex feeding habits prior to the Cambrian trace fossil explosion.