The Taconic and Salinic orogenies in the northern Appalachian
Mountains record the closure of the Iapetus Ocean, which separated
peri-Laurentian and peri-Gondwanan terranes in the early Paleozoic.
The Taconic orogeny in New England is commonly depicted as
an Ordovician collision between the peri-Laurentian Shelburne Falls
arc and the Laurentian margin, followed by Silurian accretion of peri-
Gondwanan terranes during the Salinic orogeny. New U-Pb zircon
geochronology demonstrates that the Shelburne Falls arc was instead
constructed on a Gondwanan-derived terrane preserved in the Moretown
Formation, which we refer to here as the Moretown terrane.
Metasedimentary rocks of the Moretown Formation were deposited
after 514 Ma and contain abundant ca. 535–650 Ma detrital zircon that
suggest a Gondwanan source. The Moretown Formation is bound to the
west by the peri-Laurentian Rowe belt, which contains detrital zircon
in early Paleozoic metasedimentary rocks that is indistinguishable in
age from zircon in Laurentian margin rift-drift successions. These data
reveal that the principal Iapetan suture in New England is between the
Rowe belt and Moretown terrane, more than 50 km farther west than
previously suspected. The Moretown terrane is structurally below and
west of volcanic and metasedimentary rocks of the Hawley Formation,
which contains Laurentian-derived detrital zircon, providing a link
between peri-Laurentian and peri-Gondwanan terranes. The Moretown
terrane and Hawley Formation were intruded by 475 Ma plutons
during peak activity in the Shelburne Falls arc. We propose that the
peri-Laurentian Rowe belt was subducted under the Moretown terrane
just prior to 475 Ma, when the trench gap was narrow enough
to deliver Laurentian detritus to the Hawley Formation. Interaction
between peri-Laurentian and peri-Gondwanan terranes by 475 Ma is
20 m.y. earlier than documented elsewhere and accounts for structural
relationships, Early Ordovician metamorphism and deformation, and
the subsequent closure of the peri-Laurentian Taconic seaway. In this
scenario, a rifted-arc system on the Gondwanan margin resulted in the
formation of multiple terranes, including the Moretown, that independently
crossed and closed the Iapetus Ocean in piecemeal fashion.

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 potential
for 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.

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, ice
advanced to equatorial latitudes at least twice between 717 and
635 Mya. Although the initiation mechanism of these Neoproterozoic
Snowball Earth events has remained a mystery, the broad
synchronicity of rifting of the supercontinent Rodinia, the emplacement
of large igneous provinces at low latitude, and the onset of
the Sturtian glaciation has suggested a tectonic forcing. We present
unique Re-Os geochronology and high-resolution Os and Sr isotope
pro!les bracketing Sturtian-age glacial deposits of the Rapitan
Group in northwest Canada. Coupled with existing U-Pb dates, the
postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological
constraints for both the onset and demise of a Cryogenian
glaciation from the same continental margin and suggests a 55-My
duration of the Sturtian glacial epoch. The Os and Sr isotope data
allow us to assess the relative weathering input of old radiogenic
crust and more juvenile, mantle-derived substrate. The preglacial
isotopic signals are consistent with an enhanced contribution of
juvenile material to the oceans and glacial initiation through
enhanced global weatherability. In contrast, postglacial strata
feature radiogenic Os and Sr isotope compositions indicative of
extensive glacial scouring of the continents and intense silicate
weathering in a post–Snowball Earth hothouse.

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 bedding
plane of a cm-thick sandstone lens from a single stratigraphic horizon less than 100 m below an ash bed dated at 547.3 6
0.7 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 a
whole range from 5 to 30 cm long and 3 to 10 cm wide. The specimens commonly show a change in direction and change
in diameter. The morphology of these trace fossils is comparable to backfill structures, particularly specimens of Paleozoic
Zoophycos from shallow water environments. Here we interpret these horizontal spreiten-burrows to record the grazing of
the trace-maker on or below a textured organic surface. The identification of large late Ediacaran trace fossils is consistent
with recent reports of backfilled horizontal burrows below the Precambrian–Cambrian boundary and is suggestive of the
appearance of complex feeding habits prior to the Cambrian trace fossil explosion.