A chronostratigraphic A chronostratigraphic division of the Precambrian: division of the Precambrian: possibilities and challengespossibilities and challenges

Martin J. Van KranendonkMartin J. Van KranendonkGeological Survey of Western AustraliaGeological Survey of Western Australia

Chair, ICS Precambrian SubcommissionChair, ICS Precambrian Subcommission

Current ICS stratigraphic chart

Frequency distribution of juvenile continental crust

Condie, 2004

Hamersley Basin

Problem 1: Based on round numbers, from 80’s comp., not tied to rock record

Bleeker, 2003: Lithos 71, 99-134

Proterozoic timescale based on Supercontinent assembly

Current ICS stratigraphic chartProblem 2: Proterozoic system/period scheme is impractical

Problem 3: No lower limit

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.

e.g.: e.g.: “Archean-Proterozoic boundary”“Archean-Proterozoic boundary” Pilbara Craton (Australia) = 2.78 Ga,Pilbara Craton (Australia) = 2.78 Ga,Superior Craton (N. America) = c. 2.5 Ga.Superior Craton (N. America) = c. 2.5 Ga.

Problem 5: “Global” geodynamic events are highly diachronous

>2.83 Ga basement

<2.78 Ga volcano-sedimentary rocks

3.3 Ga Olivine-bladed spinifex3.3 Ga Olivine-bladed spinifex

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

e.g.: e.g.: “global rifting” at end of Archean“global rifting” at end of Archean

Problem 5: “Global” geodynamic events are highly diachronous

2750 2700 2650 2600 2550 2500 2450 2400 2350

Great dykeBlack Range dyke

Matachewan dykes

375 Million years!!

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.

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

Precambrian timescale revision cont’d

Propose:

Use the wealth of new geoscientific data to erect Use the wealth of new geoscientific data to erect a Precambrian timescale based on the extant a Precambrian timescale based on the extant rock record rock record - using golden spikes where possible – to reflect - using golden spikes where possible – to reflect the major, irreversible processes in Earth the major, irreversible processes in Earth evolutionevolution

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

“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.

Precambrian timescale: pertinent new data

3.96 Ga

3.73 Ga

3.65 Ga

4.03 Ga

3.825 Ga 3.890 Ga

3.64 Ga

Age dates of oldest rocks

3.55 Ga 3.81 Ga

3.55 Ga

3.4 Ga

Hamersley BasinFort

escu

e G

p.

Ham

ers

ley G

p.

2450 Ma

2460 Ma

2562 Ma2501 Ma

2597 Ma

2630 Ma

2463 Ma2490 Ma

Arc

hean

Pro

tero

zoic

2719 Ma2741 Ma

2764 Ma

2775 Ma

Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.

Johnson et al., 2008: Ann. Rev. Earth Planet. Sci. 36, 457-493

Stable isotope dataStable isotope data

Major perturbation from~2.8-2.4 Ga

• Coincides with unique episode of crustal growth, deposition of BIF and rise in atmospheric O2

Great Oxidizing EventGreat Oxidizing Event

3.04.0 1.02.0Time (Ga)

S(

/

)o

oo

33

8

4

0

-4

Holland, 1994

Melezhik, 2005: GSA Today 15, 4-11

BIFs

Glacials

GIF

~2.0-1.8 Ga: Granular iron ~2.0-1.8 Ga: Granular iron formationformation

Earaheedy Gp., Australia

Animikie Gp., N. America

Mesoproterozoic environmental stability

Proterozoic glacial gap

environmentalstability

Onset of Snowball events

Sulphidic shales Ca-sulphates

Climate modellingClimate modelling

2.0 1.0Time (Ga)

%PAL

0.1

1

10

100

Proterozoic Phanerozoic

Hamersley BasinHamersley Basin

Under high pCOUnder high pCO22, weathering is by chemical processes, as a result of: , weathering is by chemical processes, as a result of: HH22O + COO + CO22 = H = H22COCO33 (carbonic acid) (carbonic acid)

This results in formation of acidic waters and intense chemical weatheringThis results in formation of acidic waters and intense chemical weatheringA predictive consequence of the geochemical data and this model is that residues A predictive consequence of the geochemical data and this model is that residues of weathering should have Alof weathering should have Al22OO33 and SiO and SiO22 rich horizons, and that indeed is exactly rich horizons, and that indeed is exactly what occurs in Fortescue Group basaltswhat occurs in Fortescue Group basalts

Pyrophyllite Al2Si4O10(OH)2 Quartz crystal ‘beds’

In contrast, under higher pOIn contrast, under higher pO22, weathering is achieved through , weathering is achieved through mechanical mechanical breakdownbreakdown of material: of material:This results in the transport and deposition of clastic sedimentary rocks.This results in the transport and deposition of clastic sedimentary rocks.

Hamersley BasinFort

escu

e G

p.

Ham

ers

ley G

p.

2450 Ma

2460 Ma

2562 Ma2501 Ma

2597 Ma

2630 Ma

2463 Ma2490 Ma

Arc

hean

Pro

tero

zoic

2719 Ma2741 Ma

2764 Ma

2775 Ma

Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.

Iron formation-related shalesIron formation-related shales

Frere Fm., Earaheedy Gp.,Australia

2 cm

~2.4 Ga glaciations

2450 2432 2450

222022202220

2316

Transition from BIF to glacials ~2.4 Ga

Dropstone in 2.4 Ga Turee Creek Gp.

Bedded Mn-carbonate

Summary of contingent events Summary of contingent events through timethrough time

1. First crustal remnants: 4.404 Ga

2. First preserved rock: 4.03 Ga

3. First preservation of macroscopic life: 3.49 Ga

4. Unique and rapid growth of continental crust: 2.78-2.63 Ga

5. Global deposition of BIF: 2.63-2.43 Ga

6. Irreversible oxidation of oceanic Fe2+→ rise of oxygen in atmosphere → global glacial

deposits and rise in seawater sulphate: 2.43-2.25 Ga

7. Lomagundi-Jatuli carbon isotopic excursion: 2.25-2.06 Ga

8. Deposition of Superior-type BIFs and stilpnomelane shales = return to reducing conditions:

2.06-1.8 Ga

9. Sulphidic shales and environmental stability: 1.8-1.25 Ga

10. Onset of Neoproterozoic glaciations and snowball Earth: ~750-630 Ma

Summary of contingent events Summary of contingent events through timethrough time

2800 2700 25002600 2400 2300 2200 2100

Time (Ma)

3. Unique and rapid growth of continental crust

4. Highly reduced atmosphere: chemical weathering anddeposition 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

A revised Precambrian timescale: possibilities

• Formal definition of a Hadean Eon, from T0 = 4567 Ma to age of Earth’s oldest rock = 4030 Ma: base of the stratigraphic column on Earth

CHRONOMETRIC BOUNDARIES

A revised Precambrian timescale: possibilities

CHRONOSTRATIGRAPHIC BOUNDARIES

Neoarchean: widespread crust generation and onset of voluminous BIF deposition; GSSP = base of first stable flood basaltsMesoarchean: first stable crust, with macroscopic evidence of life; GSSP = base of first stromatolitic horizon

Mesoproterozoic: environmental stability; GSSP = top of GIF

Neoproterozoic: onset of environmental crisis, snowball Earth, and the rise of animals; GSSP = first widespread sulphates?

Archean-Proterozoic boundary at rise in atmospheric oxygen: GSSP at change from BIF to glacials

Moving forwardMoving 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

2450

Ma

2630

Ma

2900

Ma

Major crust fm. + CO2

outgassing

2780

Ma

Oxidized atmosphere

2400

Ma

Glaciations

Glacials and oxygenic photosynthesizers

1840

Ma

end

of B

IFs

Main BIFs and anoxic oceans

~2.06-1.8 Ga: Granular iron formation~2.06-1.8 Ga: Granular iron formation

Frere Fm., Earaheedy Gp.,Australia

2 cm

Great Oxidizing EventGreat Oxidizing Event

Holland, 1994

East Pilbara TerraneEast Pilbara Terrane

• Three unconformities• upward-younging U-Pb ages• Distinct geochemical trends upsection• Discrete history from neighbouring terranes

3176 Ma3190 Ma3240 Ma

3325 Ma

3350 Ma

3458-3427 Ma

3470 Ma

3481 Ma

3498 Ma

3508 Ma

3515 Ma

3.48 Ga stromatolites3.48 Ga stromatolites