Welcome to the Cryogenian webinar series, hosted by the Subcommission on Cryogenian Stratigraphy and organised by Ying Zhou. We aim to invite researchers to present their current research to prompt discussion around the icy middle part of the Neoproterozoic, the time leading to the Cryogenian Period and its aftermath. If you are interested in stratigraphy, geochemistry (organic, inorganic, isotope), palaeobiology of or related to the Cryogenian Period, feel free to join us (please email Ying) or forward to anyone you think may be interested.
19th February, 2021. 14:00 GMT
Time-calibrated records of the Tonian and the Tonian/Cryogenian transition from the Tambien Group of the Arabian-Nubian Shield
Abstract: The Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations — the Sturtian Snowball Earth. Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0–2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ13C and 87Sr/86Sr data and U-Pb ID-TIMS ages from the Tambien Group can be used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. The composite Tonian 87Sr/86Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater 87Sr/86Sr rose ca. 880–770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, support interpretations that the 87Sr/86Sr increase was influenced by increased subaerial weathering of radiogenic lithologies during initial rifting of Rodinia at low latitudes. The following 87Sr/86Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO2 and contributing to the initiation of the Sturtian glaciation. Arc accretion events create steep landscapes that include mafic lithologies. Our work has shown that when such collisions occur within the tropical rain belt they have been particularly effective at enhancing global weatherability and cooling Earth's climate over the past 520 million years. This process may have also played a role in initiating the glacial climate state of the Cryogenian.
18th December, 2020. 14:00 GMT
A More Complete Cryogenian Record of SE Idaho, U.S.A.
Carol DehlerMark Schmitz, Matt Ellison, Adolph Yonkee, Vince Isakson, F. Macdonald, Corey Wall,
The Cryogenian record of SE Idaho and correlatives in northern Utah have long been thought to record two stages of the ~717-660 Ma Sturtian glaciation, with the ~645-635 Ma Marinoan glaciation represented stratigraphically ~2000 m above by regional incision into the Inkom Fm of the Brigham Group. However, recent work on the Pocatello Fm of SE Idaho indicate that characteristic Marinoan cap carbonates are present above the diamictites, which suggested that some of the diamictites previously correlated with the Sturtian glaciation are instead Marinoan in age. To distinguish between these models, one of the thickest (650 m) and best preserved sections of the Pocatello Fm (Scout Mountain section) was sampled for igneous and detrital zircon U-Pb geochronology, which involved an in-tandem method of LA-ICPMS, In combination with facies analysis, new geochronology has allowed us to construct a new model for the chrono- and physical-stratigraphy of the Pocatello Fm. The base of the Pocatello Fm at Scout Mountain contains mafic and dacitic volcanics interbedded with a diamictite containing striated and faceted clasts. The dacite yielded two ages of 696 Ma and 696 Ma, suggesting that these dacites must have erupted under ice, in the middle of the Sturtian episode. Stratigraphically above the dacites in diamictite, detrital zircon grains yielded a maximum depositional age (MDA) of ~687 Ma. This diamictite is sharply overlain by an arkosic sandstone, which yielded an MDA of ~658 Ma. We interpret this relationship as an unconformity with 10s of Ma of missing strata, including the top of the Sturtian diamicite and the Sturtian cap carbonate, if they were deposited. The arkose is overlain by sandstone and clast-supported conglomerate, and an upper diamictite with faceted clasts, striations, and dropstones that grade into a sandy cap dolostone with negative isotope values of -3 to -6 per mil and an MDA on zircon of ~637 Ma. Above the sandy cap dolostone lies a succession dominated by dark gray argillite and subordinate blue limestone with replaced aragonite fans, indicating post glacial transgression. The new age model for the Pocatello Formation shows that both the Sturtian and Marinoan episodes of the Cryogenian are preserved, as well as the basal Ediacaran [Marinoan} cap dolostone. Correlation with the Kingston Peak Fm in Death Valley, California provides additional geochronologic tie points, and a keystone for correlation with other Cryogenian strata along the Western Cordillera of North America and beyond. These ages also implicate that the Ediacaran stratigraphy of the region is significantly thicker (3500 m vs 1500 m) and had higher sediment accumulation rates than previously estimated. The incised valleys stratigraphically higher in the Brigham Group may correlate with Ediacaran incised valleys of Nevada and California, however this incision episodes likely post-dates the Gaskiers glaciation at ca. 580 Ma, and may be due either to later Ediacaran sea-level fall or to continued tectonic activity along the margin. Lastly, although the base of the Pocatello Formation is not exposed, it may provide a good auxiliary reference section for the chosen GSSP for the basal Cryogenian or the existing Ediacaran GSSP, especially because the key sections of the Pocatello Fm are accessible along the Interstate 15 corridor and the Sturtian, interglacial, Marinoan, and basal Ediacaran strata can be easily viewed.