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1. Integrated Litho-, Chemo- and Sequence Stratigraphy of the Ediacaran Gametrail Formation Across a Shelf-Slope Transect in the Wernecke Mountains, Yukon, Canada

   https://doi.org/10.2475/001c.74874  

   Busch, James F; Boag, Thomas H; Sperling, Erik A; Rooney, Alan D; Feng, Xiahong; Moynihan, David P; Strauss, Justin V (January 2023, American Journal of Science)

   The Ediacaran Gametrail Formation of northwestern Canada chronicles the evolution of a complex carbonate ramp system in response to fluctuations in relative sea level and regional tectonic subsidence alongside exceptional global change associated with the Shuram carbon isotope excursion (CIE). Here, we use extensive outcrop exposures of the Gametrail Formation in the Wernecke Mountains of Yukon, Canada, to construct a shelf-slope transect across the Shuram CIE. Twelve stratigraphic sections of the Gametrail Formation are combined with geological mapping and a suite of geochemical analyses to develop an integrated litho-, chemo-, and sequence stratigraphic model for these strata. In the more proximal Corn/Goz Creek region, the Gametrail Formation represents a storm-dominated inner to outer ramp depositional setting, while slope depositional environments in the Nadaleen River region are dominated by hemipelagic sedimentation, turbidites, and debris flows. The magnitude of the Shuram CIE is largest in slope limestones which underwent sediment-buffered diagenesis, while the CIE is notably smaller in the inner-outer ramp dolostones which experienced fluid-buffered diagenesis. Our regional mapping identified a distinct structural panel within the shelf-slope transect that was transported ~30 km via strike-slip motion during the Mesozoic–Cenozoic Cordilleran orogeny. One location in this transported structural block contains a stromatolite reef complex with extremely negative carbon isotope values down to ~ -30‰, while the other location contains an overthickened ooid shoal complex that does not preserve the characteristic negative CIE associated with the Shuram event. These deviations from the usual expression of the Shuram CIE along the shelf-slope transect in the Wernecke Mountains, and elsewhere globally, provide useful examples for how local tectonic, stratigraphic, and/or geochemical complexities can result in unusually large or completely absent expressions of a globally recognized CIE. 

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2. Deep‐water first occurrences of Ediacara biota prior to the Shuram carbon isotope excursion in the Wernecke Mountains, Yukon, Canada

   https://doi.org/10.1111/gbi.12597  

   Boag, Thomas_H; Busch, James_F; Gooley, Jared_T; Strauss, Justin_V; Sperling, Erik_A (May 2024, Geobiology)

   Abstract Ediacara‐type macrofossils appear as early as ~575 Ma in deep‐water facies of the Drook Formation of the Avalon Peninsula, Newfoundland, and the Nadaleen Formation of Yukon and Northwest Territories, Canada. Our ability to assess whether a deep‐water origination of the Ediacara biota is a genuine reflection of evolutionary succession, an artifact of an incomplete stratigraphic record, or a bathymetrically controlled biotope is limited by a lack of geochronological constraints and detailed shelf‐to‐slope transects of Ediacaran continental margins. The Ediacaran Rackla Group of the Wernecke Mountains, NW Canada, represents an ideal shelf‐to‐slope depositional system to understand the spatiotemporal and environmental context of Ediacara‐type organisms' stratigraphic occurrence. New sedimentological and paleontological data presented herein from the Wernecke Mountains establish a stratigraphic framework relating shelfal strata in the Goz/Corn Creek area to lower slope deposits in the Nadaleen River area. We report new discoveries of numerousAspidellahold‐fast discs, indicative of frondose Ediacara organisms, from deep‐water slope deposits of the Nadaleen Formation stratigraphically below the Shuram carbon isotope excursion (CIE) in the Nadaleen River area. Such fossils are notably absent in coeval shallow‐water strata in the Goz/Corn Creek region despite appropriate facies for potential preservation. The presence of pre‐Shuram CIE Ediacara‐type fossils occurring only in deep‐water facies within a basin that has equivalent well‐preserved shallow‐water facies provides the first stratigraphic paleobiological support for a deep‐water origination of the Ediacara biota. In contrast, new occurrences of Ediacara‐type fossils (including juvenile fronds,Beltanelliformis,Aspidella, annulated tubes, and multiple ichnotaxa) are found above the Shuram CIE in both deep‐ and shallow‐water deposits of the Blueflower Formation. Given existing age constraints on the Shuram CIE, it appears that Ediacaran organisms may have originated in the deeper ocean and lived there for up to ~15 million years before migrating into shelfal environments in the terminal Ediacaran. This indicates unique ecophysiological constraints likely shaped the initial habitat preference and later environmental expansion of the Ediacara biota. 

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3. An algorithm-guided Ediacaran global composite δ13Ccarb–Bayesian age model

   https://doi.org/https://doi.org/10.1016/j.palaeo.2024.112321  

   Hagen, Cedric J; Creveling, Jessica R (September 2024, Palaeogeography palaeoclimatology palaeoecology)

   While it remains uncertain whether excursions in the stable carbon isotopic composition of Ediacaran marine carbonate (δ13Ccarb) represent globally synchronous events (or a direct measure of ocean carbon cycling), the absence of widely distributed and readily preservable fauna, and the presence of several iconic carbon isotope excursions (CIEs), has sustained δ13Ccarb correlation as the primary means to establish relative time relationships for Ediacaran successions. Here we present an Ediacaran global δ13Ccarb composite built with a dynamic time warping (DTW) time-normalization algorithm that generates libraries of least-squares alignments between chemostratigraphic records of unequal length and distinct sediment accumulation rates. When developing a δ13Ccarb composite for each of 16 globally distributed Ediacaran paleo-depositional regions, we selected high Pearson r alignments that conformed with published geological guidance about the correlation of constituent sections. When applying DTW to align these regional algorithmic composites into one global δ13Ccarb stack, we selected alignments that allied the excursions that field workers have established (or speculated) are the Marinoan cap carbonate excursion, the Shuram excursion, and/or the basal Cambrian excursion. There are strengths and weaknesses to making explicit the temporal relationships (point-to-point correspondences) often left implicit in visual correlation. One strength is to extrapolate depositional ages by means of isotopic correlation, and here we explored this with a Bayesian Markov chain Monte Carlo age model that predicts a median age, and uncertainty, for every carbonate stratum in the global Ediacaran δ13Ccarb composite. Yet, one must caution against a false accuracy that can arise from selecting one alignment among many possibilities––the likelihood that time-uncertain time series can be stretched and squeezed into one unequivocal alignment is low. Thus, while these alignments are grounded in the expert assessment of the field worker, this global Ediacaran δ13Ccarb–Bayesian age model should be viewed as a working hypothesis to enrich, but not arbitrate, discussions of the correlation, synchrony, and completeness of Ediacaran successions. 

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4. Upper Windermere Supergroup and the transition from rifting to continent-margin sedimentation, Nadaleen River area, northern Canadian Cordillera

   https://doi.org/10.1130/B32039.1  

   Moynihan, David P.; Strauss, Justin V.; Nelson, Lyle L.; Padget, Colin D. (March 2019, GSA Bulletin)

   Abstract Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin. 

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5. Calibrating the Russøya excursion in Svalbard, Norway, and implications for Neoproterozoic chronology

   https://doi.org/10.1130/G49593.1  

   Millikin, Alexie E.G.; Strauss, Justin V.; Halverson, Galen P.; Bergmann, Kristin D.; Tosca, Nicholas J.; Rooney, Alan D. (February 2022, Geology)

   Abstract The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway, represents one of the most complete and well-preserved Neoproterozoic sedimentary successions worldwide. With diverse fossil assemblages, an extensive carbonate δ13C record, and sedimentary evidence for two distinct Cryogenian glaciations, this succession will continue to yield insights into the Neoproterozoic Earth system; however, at present there are no direct radiometric age constraints for these strata. We present two new Re-Os ages and initial Os isotope data that constrain the timing of Neoproterozoic glaciation in Svalbard, providing further support for two globally synchronous Cryogenian glaciations and insight into pre- and post-snowball global weathering conditions. An age from the Russøya Member (Elbobreen Formation) facilitates correlation of the negative carbon isotope excursion recorded therein with the pre-glacial “Islay” excursion of the Callison Lake Formation of northwestern Canada and the Didikama and Matheos Formations of Ethiopia. We propose that this globally synchronous ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion with northeastern Svalbard as the type locality. This new age provides an opportunity to construct a time-calibrated geological framework in Svalbard to assess connections between biogeochemical cycling, evolutionary innovations within the eukaryotes, and the most extreme climatic changes in Earth history. 

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