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

Graham A. Shields-Zhou; Susannah Porter; Galen P. Halverson

doi:10.18814/epiiugs/2016/v39i1/89231

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Episodes 2016; 39(1): 3-8

Published online March 1, 2016

https://doi.org/10.18814/epiiugs/2016/v39i1/89231

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

Graham A. Shields-Zhou^1,2, Susannah Porter³, Galen P. Halverson⁴

¹Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK. E-mail: g.shields@ucl.ac.uk
²State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, No.39 East Beijing Road, Nanjing 210008, China. E-mail: g.shields@nigpas.ac.cn
³Department of Earth Science, University of California at Santa Barbara, CA 93106, USA. E-mail: porter@geol.ucsb.edu
⁴Department of Earth & Planetary Sciences/Geotop, McGill University, 3450 University Street, Montreal, Quebec, H3A 0E8 Canada. E-mail:galen.halverson@mcgill.ca

Received: March 20, 2015; Accepted: August 18, 2015

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The Cryogenian Period was first established in 1988 along with other Precambrian eon, era and period-level subdivisions that were defined numerically by Global Standard Stratigraphic Ages (GSSAs). As absolute age constraints have improved, some of these time intervals no longer bracket adequately the geological event(s), for which they were named. For example, the age discrepancy between the basal Cryogenian GSSA at 850 Ma and the onset of widespread glaciation ca. 717 Ma has rendered the 850 Ma boundary obsolete. The International Commission on Stratigraphy has now formally approved the removal of the Cryogenian GSSA from its International Chronostratigraphic Chart and supports its replacement with a rock-based Global Stratotype Section and Point (GSSP). The new Cryogenian GSSP will be placed at a globally correlative level that lies stratigraphically beneath the first appearance of widespread glaciation and is assigned in the interim a ‘calibrated age’ of circa 720 Ma. This new definition for the Tonian/Cryogenian boundary should be used in future publications until a formal Cryogenian GSSP can be ratified. The change marks progress towards establishment of a ‘natural’ (rock-based) scale for Precambrian time.