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1. Late Neoproterozoic – early Paleozoic basin evolution in the Coal Creek inlier of Yukon, Canada: implications for the tectonic evolution of northwestern Laurentia

   https://doi.org/10.1139/cjes-2020-0132  

   Busch, James F.; Rooney, Alan D.; Meyer, Edward E.; Town, Caleb F.; Moynihan, David P.; Strauss, Justin V. (April 2021, Canadian Journal of Earth Sciences)

   null (Ed.)

   The age and nature of the Neoproterozoic – early Paleozoic rift–drift transition has been interpreted differently along the length of the North American Cordillera. The Ediacaran “upper” group (herein elevated to the Rackla Group) of the Coal Creek inlier, Yukon, Canada, represents a key succession to reconstruct the sedimentation history of northwestern Laurentia across the Precambrian–Cambrian boundary and elucidate the timing of active tectonism during the protracted breakup of the supercontinent Rodinia. These previously undifferentiated late Neoproterozoic – early Paleozoic map units in the Coal Creek inlier are herein formally defined as the Lone, Cliff Creek, Mount Ina, Last Chance, Shade, and Shell Creek formations. New sedimentological and stratigraphic data from these units is used to reconstruct the depositional setting. In the Last Chance Formation, chemostratigraphic observations indicate a ca. 5‰ δ 13 C carb gradient coincident with the globally recognized ca. 574–567 Ma Shuram carbon isotope excursion. Map and stratigraphic relationships in the overlying Shell Creek Formation provide evidence for latest Ediacaran – middle Cambrian tilting and rift-related sedimentation. This provides evidence for active extension through the Cambrian Miaolingian Series in northwestern Canada, supporting arguments for a multiphase and protracted breakup of Rodinia. 

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2. New multicellular marine macroalgae from the early Tonian of northwestern Canada

   https://doi.org/10.1130/G48508.1  

   Maloney, Katie M.; Halverson, Galen P.; Schiffbauer, James D.; Xiao, Shuhai; Gibson, Timothy M.; Lechte, Maxwell A.; Cumming, Vivien M.; Millikin, Alexie E.G.; Murphy, Jack G.; Wallace, Malcolm W.; et al (March 2021, Geology)

   Abstract Molecular phylogenetic data suggest that photosynthetic eukaryotes first evolved in freshwater environments in the early Proterozoic and diversified into marine environments by the Tonian Period, but early algal evolution is poorly reflected in the fossil record. Here, we report newly discovered, millimeter- to centimeter-scale macrofossils from outer-shelf marine facies of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores Creek Formation in the Wernecke Mountains, northwestern Canada. These fossils, variably preserved by iron oxides and clay minerals, represent two size classes. The larger forms feature unbranching thalli with uniform cells, differentiated cell walls, longitudinal striations, and probable holdfasts, whereas the smaller specimens display branching but no other diagnostic features. While the smaller population remains unresolved phylogenetically and may represent cyanobacteria, we interpret the larger fossils as multicellular eukaryotic macroalgae with a plausible green algal affinity based on their large size and presence of rib-like wall ornamentation. Considered as such, the latter are among the few green algae and some of the largest macroscopic eukaryotes yet recognized in the early Neoproterozoic. Together with other Tonian fossils, the Dolores Creek fossils indicate that eukaryotic algae, including green algae, colonized marine environments by the early Neoproterozoic Era. 

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3. Newly discovered stromatolites and bimodal magmatism at ~1.23 Ga in the Burro Mountains of New Mexico: Mesoproterozoic rifting formed a shallow ocean basin in southwestern Laurentia

   https://doi.org/10.56577/SM-2024.2963  

   Millo, Amit; Amato, Jeff; Bell, Jordan (April 2024, New Mexico Geological Society Annual Spring Meeting)

   We interpret a newly discovered laminated limestone associated with Middle Proterozoic rocks in the Redrock and Brushy Mountain quadrangles of southwestern New Mexico as representing stromatolites, possibly Conophyton. This locality is geographically aligned with other outcrops of ~1.2 Ga stromatolites documented in the southwest U.S., such as those in the Bass Formation (Grand Canyon, AZ), the Mescal Limestone (Salt River Canyon, AZ), the Castner Marble (Franklin Mountains, TX), and the Allamoore Formation (Van Horn, TX). These stromatolites have similar morphologies to those found in Texas and represent the first instance of Proterozoic fossils identified within the state of New Mexico. The stromatolite-bearing unit, termed here the Ash Creek Limestone, is exposed along with a marble unit surrounded by the ~1225 Ma Redrock Granite (Williams, 2015). Mapping at 1:6,000 scale shows that the carbonates are also associated with a unit consisting of serpentinite interbedded with talc (forming a rock informally referred to as ricolite), meaning they share a similar depositional setting. The carbonates are only found surrounded by granite, whereas ricolite outcrops are typically in contact with a metabasalt. The metabasalt yielded a U-Pb zircon weighted mean 207Pb/206Pb age of 1229 ± 12 Ma (n= 35; MSWD= 0.8). This represents the first direct dating of ~1.2 Ga mafic volcanism in the area, and these racks are similar in age to anorthosite dated at 1223 ± 6 Ma and 1231 ± 4 Ma (Ramo et al., 2003). Together, the ~1.2 Ga bimodal magmatism and shallow marine limestone units indicate that the tectonic setting of southwest Laurentia at this time involved a NW-SE-trending Mesoproterozoic seaway that formed as a result of coeval regional extension during the early stages of the Grenville orogeny. 

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4. Pre-Mississippian Stratigraphic Architecture of the Porcupine Shear Zone, Yukon and Alaska, and Significance in the Evolution of Northern Laurentia

   https://doi.org/10.2113/2021/7866155  

   Faehnrich, Karol; McClelland, William C.; Colpron, Maurice; Nutt, Charlotte L.; Miller, Rebecca S.; Trembath, Matthew; Strauss, Justin V.; Abu-Alam, ed., Tamer S. (May 2021, Lithosphere)

   Abstract The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments. 

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5. 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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