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A chronostratigraphic division of the Precambrian: possibilities and challenges

A chronostratigraphic division of the Precambrian: possibilities and challenges. Martin J. Van Kranendonk Geological Survey of Western Australia Chair, ICS Precambrian Subcommission. Problem 1: Based on round numbers, from 80’s comp., not tied to rock record. Hamersley Basin. Condie, 2004.

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A chronostratigraphic division of the Precambrian: possibilities and challenges

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  1. A chronostratigraphic division of the Precambrian: possibilities and challenges Martin J. Van Kranendonk Geological Survey of Western Australia Chair, ICS Precambrian Subcommission

  2. Problem 1: Based on round numbers, from 80’s comp., not tied to rock record Hamersley Basin Condie, 2004 Frequency distribution of juvenile continental crust Current ICS stratigraphic chart

  3. Proterozoic timescale based on Supercontinent assembly Bleeker, 2003: Lithos 71, 99-134

  4. Problem 2: Proterozoic system/period scheme is impractical Problem 3: No lower limit Current ICS stratigraphic chart

  5. Problem 4: Many significant geodynamic events are not reflected in current timescale e.g. appearance of first ophiolites at 2.0 Ga, reflecting what many believe is the onset of truly modern plate tectonics.

  6. e.g. “Classic Archean features” = granite-greenstone crust and komatiites; typically in 2.7 Ga terranes, but also 2.1 Ga Birimian granite-greenstone crust and 2056 Ma Lapland komatiites <2.78 Ga volcano-sedimentary rocks >2.83 Ga basement 3.3 Ga Olivine-bladed spinifex Problem 5: “Global” geodynamic events are highly diachronous e.g.: “Archean-Proterozoic boundary” Pilbara Craton (Australia) = 2.78 Ga, Superior Craton (N. America) = c. 2.5 Ga.

  7. Problem 5: “Global” geodynamic events are highly diachronous 2750 2700 2650 2600 2550 2500 2450 2400 2350 Black Range dyke Great dyke e.g.: “global rifting” at end of Archean Matachewan dykes 375 Million years!!

  8. Precambrian timescale revision Rationale and aims: “…we seek trend-related events that have affected the entire Earth over relatively short intervals of time and left recognizable signatures in the rock sequences of the globe. Such attributes are more likely to result from events in atmospheric, climatic, or biologic evolution than plutonic evolution..” i.e. crust-forming events operate at 100’s million year scale, vs. biological events at <1 million year scale Cloud, P., 1972. A working model of the primitive earth. American Journal of Science 272, 537-548.

  9. Precambrian timescale revision cont’d • A major criticism of this approach in the 1980’s compilation was that there was not enough geobiological change through the Precambrian to use this criterion for timescale purposes. • However, since that time there has been a veritable explosion of new information pertaining to Precambrian geobiology in the form of: • Detailed stratigraphic sections • High precision geochronology (U-Pb and Re-Os) • Stable isotope geochemical data (S, C, O) • Atmospheric/climatic modelling

  10. Precambrian timescale revision cont’d • Propose: • Use the wealth of new geoscientific data to erect a Precambrian timescale based on the extant rock record • - using golden spikes where possible – to reflect the major, irreversible processes in Earth evolution • The importance of this work is to: • document major events in Earth history • facilitate and promote communication amongst Earth Scientists • convey the history of events in Earth evolution to the general public

  11. “The organising principles of history are directionality and contingency. Directionality is the quest to explain (not merely document) the primary character of any true history as a complex, but causally connected series of unique events, giving an arrow to time by their unrepeatability and sensible sequence. Contingency is the recognition that such sequences do not unfold as predictable arrays under timeless laws of nature, but that each step is dependent (contingent) upon those that came before, and that explanation therefore requires a detailed knowledge of antecedent particulars.” Gould, S.J., 1994. Introduction: The coherence of history. In: Bengston, S. (ed.), Early Life on earth. Nobel Symposium 84, 1-8.

  12. Precambrian timescale: pertinent new data 4.03 Ga Age dates of oldest rocks 3.825 Ga 3.890 Ga 3.4 Ga 3.55 Ga 3.81 Ga 3.55 Ga 3.65 Ga 3.64 Ga 3.73 Ga 3.96 Ga

  13. 2450 Ma Proterozoic 2460 Ma 2463 Ma 2490 Ma 2501 Ma 2562 Ma 2597 Ma 2630 Ma Archean 2719 Ma 2741 Ma 2764 Ma 2775 Ma Hamersley Basin Hamersley Gp. Fortescue Gp. Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.

  14. Coincides with unique episode • of crustal growth, deposition of • BIF and rise in atmospheric O2 Stable isotope data Major perturbation from ~2.8-2.4 Ga Johnson et al., 2008: Ann. Rev. Earth Planet. Sci. 36, 457-493

  15. 8 4 33 S( / ) o oo 0 -4 4.0 3.0 2.0 1.0 Time (Ga) Great Oxidizing Event Holland, 1994

  16. BIFs GIF Glacials Melezhik, 2005: GSA Today 15, 4-11

  17. ~2.0-1.8 Ga: Granular iron formation Animikie Gp., N. America Earaheedy Gp., Australia

  18. Mesoproterozoic environmental stability Proterozoic glacial gap environmental stability Onset of Snowball events Ca-sulphates Sulphidic shales

  19. Climate modelling Proterozoic Phanerozoic 100 10 %PAL 1 0.1 2.0 1.0 Time (Ga)

  20. Hamersley Basin

  21. Quartz crystal ‘beds’ Pyrophyllite Al2Si4O10(OH)2 Under high pCO2, weathering is by chemical processes, as a result of: H2O + CO2 = H2CO3 (carbonic acid) This results in formation of acidic waters and intense chemical weathering A predictive consequence of the geochemical data and this model is that residues of weathering should have Al2O3 and SiO2 rich horizons, and that indeed is exactly what occurs in Fortescue Group basalts In contrast, under higher pO2, weathering is achieved through mechanical breakdown of material: This results in the transport and deposition of clastic sedimentary rocks.

  22. 2450 Ma Proterozoic 2460 Ma 2463 Ma 2490 Ma 2501 Ma 2562 Ma 2597 Ma 2630 Ma Archean 2719 Ma 2741 Ma 2764 Ma 2775 Ma Hamersley Basin Hamersley Gp. Fortescue Gp. Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.

  23. Iron formation-related shales Frere Fm., Earaheedy Gp., Australia 2 cm

  24. 2220 2220 2220 2450 2432 2450 ~2.4 Ga glaciations 2316

  25. Bedded Mn-carbonate Dropstone in 2.4 Ga Turee Creek Gp. Transition from BIF to glacials ~2.4 Ga

  26. Summary of contingent events through time • First crustal remnants: 4.404 Ga • First preserved rock: 4.03 Ga • First preservation of macroscopic life: 3.49 Ga • Unique and rapid growth of continental crust: 2.78-2.63 Ga • Global deposition of BIF: 2.63-2.43 Ga • Irreversible oxidation of oceanic Fe2+→ rise of oxygen in atmosphere → global glacial deposits and rise in seawater sulphate: 2.43-2.25 Ga • Lomagundi-Jatuli carbon isotopic excursion: 2.25-2.06 Ga • Deposition of Superior-type BIFs and stilpnomelane shales = return to reducing conditions: 2.06-1.8 Ga • Sulphidic shales and environmental stability: 1.8-1.25 Ga 10. Onset of Neoproterozoic glaciations and snowball Earth: ~750-630 Ma

  27. 3. Unique and rapid growth of continental crust 4. Highly reduced atmosphere: chemical weathering and deposition of BIF 5. Irreversible oxidation of crust and oceanic sinks (Fe2+) → rise of atmospheric oxygen → global glaciation and rise in seawater sulphate 6. Lomagundi-Jatuli carbon isotopic excursion Summary of contingent events through time 2800 2600 2500 2400 2300 2200 2100 2700 Time (Ma)

  28. A revised Precambrian timescale: possibilities CHRONOMETRIC BOUNDARIES • Formal definition ofa Hadean Eon, from T0 = 4567 Ma to age of Earth’s oldest rock = 4030 Ma: base of the stratigraphic column on Earth

  29. A revised Precambrian timescale: possibilities Neoproterozoic: onset of environmental crisis, snowball Earth, and the rise of animals; GSSP = first widespread sulphates? Mesoproterozoic: environmental stability;GSSP = top of GIF Archean-Proterozoic boundary at rise in atmospheric oxygen: GSSP at change from BIF to glacials Neoarchean: widespread crust generation and onset of voluminous BIF deposition; GSSP = base of first stable flood basalts Mesoarchean: first stable crust, with macroscopic evidence of life; GSSP = base of first stromatolitic horizon CHRONOSTRATIGRAPHIC BOUNDARIES

  30. Moving forward • Instigate working groups for Precambrian timescale boundaries • Solicit proposals for potential GSSPs in different countries • Assess proposals and develop research plan to constrain potential boundaries • Write formal proposals for voting by ICS members

  31. Glaciations end of BIFs 2450 Ma 2780 Ma 2630 Ma 2400 Ma 2900 Ma 1840 Ma Major crust fm. + CO2 outgassing Main BIFs and anoxic oceans Oxidized atmosphere Glacials and oxygenic photosynthesizers

  32. ~2.06-1.8 Ga: Granular iron formation Frere Fm., Earaheedy Gp., Australia 2 cm

  33. Great Oxidizing Event Holland, 1994

  34. 3176 Ma 3190 Ma 3240 Ma 3325 Ma 3350 Ma 3458-3427 Ma 3470 Ma 3481 Ma 3498 Ma 3508 Ma 3515 Ma East Pilbara Terrane • Three unconformities • upward-younging U-Pb ages • Distinct geochemical trends upsection • Discrete history from neighbouring terranes 3.48 Ga stromatolites

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