Abstract
The geological timescale 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 timescale, 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.
Highlights
The term ‘Precambrian’, or more traditionally ‘pre-Cambrian’ (Glaessner 1962), is an informal geological term that refers to the time before the beginning of the Cambrian Period at c. 0.54 Ga (Peng et al 2020)
In summary: (1) we agree with previous workers (e.g. Van Kranendonk et al 2012; Griffin et al 2014) that the current chronometric subdivision of the Archean should be modified from four to three rock-based eras by discontinuing use of the Eoarchean as an era-level subdivision, (2) we suggest that the three remaining eras could be of approximately equal duration, comprising the Paleoarchean (c. 4.0–3.5 Ga), the Mesoarchean (c. 3.5–3.0 Ga) and the Neoarchean (c. 3.0–2.45 Ga) and (3) we concur that the Siderian should be moved to the terminal Neoarchean (Van Kranendonk et al 2012) and propose that it ends at c. 2.45 Ga (Fig. 1c)
4) We suggest that current period-level Global Standard Stratigraphic Ages (GSSAs) be replaced by improved rock-based concepts and interim chronostratigraphic units as soon as practicable, continuing recent progress towards that goal, illustrated, for example, by the establishment of an Ediacaran GSSP in 2004 and chronostratigraphic definition of the base of the Cryogenian at c. 720 Ma in 2016
Summary
A template for an improved rock-based subdivision of the pre-Cryogenian timescale. GAS, 0000-0002-7828-3966; SMP, 0000-0002-4707-9428; CJdA, 0000-0002-9676-5555; DMB, 0000-0003-4951-3360; ASC, 0000-0002-3408-5474; KC, 0000-0002-7743-9535; KD, 0000-0002-2372-2095; DADE, 0000-0001-8952-5273; AEF, 0000-0002-7649-6167; BSK, 0000-0002-8720-0608; SWP, 0000-0001-7621-189X; CS, 0000-0002-4945-7381; ES, 0000-
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