A new rock-based definition for the Cryogenian Period (circa 720 – 635 Ma)

Shields, Graham A.; Porter, Susannah M.; Halverson, Galen P.

doi:10.18814/epiiugs/2016/v39i1/89231

Cited by 86 publications

(46 citation statements)

References 44 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…717–659 Ma “Sturtian” and ca. 650–635 Ma “Marinoan”, defining Cryogenian time (Bao et al., ; Macdonald et al., ; Rooney, Strauss, Brandon, & Macdonald, ; Shields‐Zhou, Porter, & Halverson, ). The glacial‐periglacial origin of Cryogenian diamictites is widely accepted and the snowball Earth hypothesis has gained favour since globally synchronous glacial terminations are increasingly supported radiometrically (Cox et al., ; Rooney et al., ).…”

Section: Introductionmentioning

confidence: 99%

Hit or miss: Glacial incisions of snowball Earth

et al. 2019

View full text Add to dashboard Cite

Estimated at ~58 Ma in duration, the Sturtian snowball Earth (ca. 717–659 Ma) is one of the longest‐known glaciations in Earth history. Surprisingly few uncontroversial lines of evidence for glacial incisions associated with such a protracted event exist. We report here multiple lines of geological field evidence for deep but variable glacial erosion during the Sturtian glaciation. One incision, on the scale of several kilometres, represents the deepest incision documented for snowball Earth; another much more modest glacial valley, however, suggests an erosion rate similar to sluggish Quaternary glaciers. The heterogeneity in snowball glacial incisions reported here and elsewhere was likely influenced by actively extending horst‐and‐graben topography associated with the breakup of supercontinent Rodinia.

show abstract

Section: Introductionmentioning

confidence: 99%

Hit or miss: Glacial incisions of snowball Earth

et al. 2019

View full text Add to dashboard Cite

show abstract

“…Attention has now turned to defining a basal Cryogenian boundary and the Cryogenian stratigraphic sub-commission seeks to do this beneath the oldest evidence of widespread glaciation, in the meantime adopting a numerical age of 720 Ma as an interim position (Shields-Zhou et al, 2016). Preservation of glacigenic diamictite in a region of low palaeolatitude would support the concept that the associated glaciation was widespread.…”

Section: Introduction and Previous Researchmentioning

confidence: 99%

Tonian-Cryogenian boundary sections of Argyll, Scotland

Fairchild

Spencer²,

Ali

et al. 2018

Precambrian Research

View full text Add to dashboard Cite

The Tonian-Cryogenian System boundary is to be defined at a GSSP (Global Boundary Stratigraphic Section and Point) beneath the first evidence of widespread glaciation. A candidate lies within the Dalradian Supergroup of Scotland and Ireland, which is least deformed and metamorphosed in Argyll, western Scotland. We present new stratigraphic profiles and interpretations from the Isle of Islay and the Garvellach Islands, update the chemostratigraphy of the Appin Group Tonian carbonates underlying the thick (ca. 1 km) glacigenic Port Askaig Formation (PAF) and demonstrate an environmental transition at the contact.The Appin Group forms a regionally extensive, >4 km-thick, succession of limestones, shales and sandstones deposited on a marine shelf. On Islay, the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure, oolite, stromatolitic dolomite and intraclastic microbial mounds. Significantly deeper erosion at the unconformity at the base of the overlying PAF is demonstrated in the southern section. Carbonate facies show a gradual decline in δ 13 C VPDB from +5 to +2‰ upwards.In NE Garbh Eileach (Garvellach Islands), a continuously exposed section of Appin Group carbonates, 70 m thick, here designated the Garbh Eileach Formation (GEF), lies conformably beneath the PAF. The GEF and the GEF-PAF boundary relationships are re-described with new sedimentological logs, petrological and stable isotope data. Interstratified limestone and dolomicrosparite with δ 13 C of -4 to -7 ‰ (a feature named the Garvellach anomaly, replacing the term Islay anomaly) are overlain by dolomite in which the isotope signature becomes weakly positive (up to +1 ‰) upwards. Shallow subtidal conditions become peritidal upwards, with evidence of wave and storm activity. Gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the GEF. The basal diamictite (D1) of the PAF is rich in carbonate clasts similar to slightly deeper-water parts of the underlying succession. D1 is typically several metres thick with interstratified sandstone and conglomerate, but dies out laterally. Scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft-sediment deformation is interbedded with carbonate below and above D1. Dolomite beds with derived intraclasts and gypsum pseudomorphs are found above D1 (or equivalent position, where D1 is absent).Published and new Sr isotope studies, including successive leach data, demonstrate primary Tonian 87 Sr/ 86 Sr values of 0.7066-0.7069 on Islay, decreasing to 0.7064-0.7066 in the younger GEF limestones on the Garvellachs, with 1700-2700 ppm Sr. Other typically Tonian characteristics of the carbonates are the Sr-rich nature of limestones, molar tooth structure, and dolomitized peritidal facies with evidence of aridity. Seabed surveys suggesting uniformly-dipping strata and shallow borehole core material illustrate the potential for extending the...

show abstract

“…The Neoproterozoic Era is divided into three geological periods. The Tonian (which means ‘stretching’) was originally defined chronologically as 1000 to 850 Ma, but since the supercontinent Rodinia is now known to have broken up largely after this time, it has now been redefined to end at 720 Ma (Shields‐Zhou et al ., ), pending designation of a ‘golden spike’ in a suitable geological succession below the first evidence of glaciation. The Tonian record is notable for the high δ 13 C values of platform carbonates (Halverson, ), as well as crown‐group eukaryotes becoming ecologically significant (Butterfield, ).…”

Section: Introductionmentioning

confidence: 99%

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

2016

View full text Add to dashboard Cite

Sedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post-glacial facies, including their age constraints; (iii) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep-sea oxidation through the Ediacaran Period. Techniques such as Fe-speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea-level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma, and plausibly indicate long durations for each (55 Myr for Sturtian and 5 to 15 Myr for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr-isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and the age and synchroneity of individual negative d 13 C anomalies. Sturtian deposits are typically thick, rift-related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice-free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO 2 was high, as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold-climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea-levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative (2016) 63, 253-306 doi: 10.1111/sed.12261 STATE OF THE SCIENCE model o...

show abstract

A new rock-based definition for the Cryogenian Period (circa 720 – 635 Ma)

Abstract: A new rock-based definition for the Cryogenian Period (circa 720 -635 Ma)

Cited by 86 publications

References 44 publications

Hit or miss: Glacial incisions of snowball Earth

Hit or miss: Glacial incisions of snowball Earth

Tonian-Cryogenian boundary sections of Argyll, Scotland

Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

Contact Info

Product

Resources

About