A template for an improved rock-based subdivision of the pre-Cryogenian timescale

Abstract

The geological time scale before 720 Ma uses rounded absolute ages rather than specific events recorded in rocks to subdivide time. This has led increasingly to mismatches between subdivisions and the features for which they were named. Here we review the formal processes that led to the current time scale, outline rock-based concepts that could be used to subdivide pre-Cryogenian time and propose revisions. An appraisal of the Precambrian rock record confirms that purely chronostratigraphic subdivision would require only modest deviation from current chronometric boundaries, removal of which could be expedited by establishing event-based concepts and provisional, approximate ages for eon-, era- and period-level subdivisions. Our review leads to the following conclusions: 1) the current informal four-fold Archean subdivision should be simplified to a tripartite scheme, pending more detailed analysis, and 2) an improved rock-based Proterozoic Eon might comprise a Paleoproterozoic Era with three periods (early Paleoproterozoic or Skourian, Rhyacian, Orosirian), Mesoproterozoic Era with four periods (Statherian, Calymmian, Ectasian, Stenian) and a Neoproterozoic Era with four periods (pre-Tonian or Kleisian, Tonian, Cryogenian and Ediacaran). These proposals stem from a wide community and could be used to guide future development of the pre-Cryogenian timescale by international bodies.

U–Pb zircon geochronology of a pyroclastic rock from the Parsoi Formation, Mahakoshal Group: Implications towards age and tectonics of the Basin in Central Indian Tectonic Zone

“…From the strongest and youngest peak in the probability density diagram of 207 Pb/ 206 Pb age values of the near‐concordant, (<5% discordance) spots of zircon grains, a 1894.3 ± 9.4 Ma age of crystallization and deposition for the pyroclastic unit and in turn, an Orosirian age is inferred. The studied acidic magmatic event in the Mahakoshal Basin correlates with the records of reports of similar age events (1,890–1,850 Ma) from different continents of the ‘Columbia’ supercontinent revealing the protracted accretion history of the supercontinent (Shields et al, 2021). We believe that the precise age of the Orosirian Mahakoshal Basin will provide new research avenues to explore whether this evolved as an oxygenic Basin in the aftermath of GOE, and LJE from the occurrence of iron formation rocks with dominant oxide mineralogy at different stratigraphic levels within the basin succession.…”

“…In the Indian Craton, Orosirian age acidic magmatism suggesting Columbian crustal consolidation is documented from the lesser Himalayan sequence (Kohn, Paul, & Corrie, 2010;Mandal et al, 2016;Sen, Sen, Srivastava, Singhal, & Phukon, 2019), Aravalli orogenic belt (Bhowmik, Bernhardt, & Dasgupta, 2010), Andean (A)-type granitoids from the Khetri region (Kaur, Chaudhri, Raczek, Kröner, & Hofmann, 2009), felsic volcanics in Hindoli Group (Deb, Thorpe, & Krstic, 2002) and Mangalwar complex (Dey et al, 2019). Bora et al (2013) with the records of reports of similar age events (1,890-1,850 Ma) from different continents of the 'Columbia' supercontinent revealing the protracted accretion history of the supercontinent (Shields et al, 2021). We believe that the precise age of the Orosirian Mahakoshal Basin will provide new research avenues to explore whether this evolved as an oxygenic Basin in the aftermath of GOE, and LJE from the occurrence of iron formation rocks with dominant oxide mineralogy at different stratigraphic levels within the basin succession.…”

The Anthropocene is a prospective epoch/series, not a geological event

“…The other such example quoted by Gibbard et al (2021), the invasion of the continents by land plants, has to our knowledge not been regarded as a geological 'event' (although individual events within this protracted invasion are recognised; Gibling and Davies, 2012), and in any case this process extended broadly from the Ordovician to the Carboniferous (Wellman, 2010). As reinterpreted by Shields et al (2021) for the GOE, these extended phenomena more closely fit the definition of an 'episode' of the North American Stratigraphic Code (NASC, 2005), as the highest in their series of diachronic units that intersect with, rather than replace, chronostratigraphic units (NASC, 2005, p. 1584 and their fig. 10).…”

“…The deeper-time examples quoted by Gibbard et al (2021), the Great Oxidation Event (GOE) and the Great Ordovician Biodiversification Event (GOBE), lie at the extreme end of this range -indeed, to the extent that the term 'event' has been contested for both of them. Shields et al (2021) consider the GOE to be a complex 'episode' that spans ~200 Ma and includes named globally correlatable events such as the Lomagundi Event, a carbon isotope excursion. Servais and Harper (2018) and Servais et al (2021) question whether the GOBE should be considered an event, as its >30 Ma duration includes a complex succession of more conventionally understood and named bio-and chemo-events.…”

The proposed Anthropocene Epoch/Series is underpinned by an extensive array of mid‐20th century stratigraphic event signals

“…Each such episode typically includes numerous more conventionally understood events – some of which may define or support chronostratigraphic boundaries, as in isotopic events associated with oxygenation now being considered for use in developing a GSSP‐based pre‐Cryogenian chronostratigraphy (Shields et al , 2022). More widely, event stratigraphy is now being increasingly employed in defining chronostratigraphic boundaries, as with the carbon isotope anomaly used as the primary marker for the Paleocene–Eocene boundary (Aubry et al , 2007), the palaeomagnetic reversal events that are the primary markers for the Neogene–Quaternary (Head, 2019) and Early–Middle Pleistocene boundaries (Suganuma et al , 2021), and most famously the bolide event that effectively defines the Cretaceous–Paleogene boundary (Molina et al , 2006).…”