Transcript A chronostratigraphic division of the Precambrian: possibilities and challenges Martin J. Van Kranendonk

A chronostratigraphic
division of the Precambrian:
possibilities and challenges
Martin J. Van Kranendonk
Geological Survey of Western Australia
Chair, ICS Precambrian Subcommission
Current ICS stratigraphic chart
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
Proterozoic timescale based
on Supercontinent assembly
Bleeker, 2003: Lithos 71, 99-134
Current ICS stratigraphic chart
Problem 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.
Problem 5: “Global” geodynamic events
are highly diachronous
e.g.
Archean features”
e.g.:“Classic
“Archean-Proterozoic
boundary”
=
granite-greenstone
crust=and
Pilbara
Craton (Australia)
2.78komatiites;
Ga,
typically
in 2.7 Ga
Superior Craton
(N.terranes,
America) = c. 2.5 Ga.
but also 2.1 Ga Birimian granitegreenstone 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.: “global rifting” at end of Archean
2750
2700
Black Range dyke
2650
2600
2550
2500
2450
Great dyke
Matachewan dykes
375 Million years!!
2400
2350
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 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
“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
Age dates of oldest rocks
3.825 Ga
3.890 Ga
4.03 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
Hamersley Gp.
2450 Ma
2460 Ma
2463 Ma
2490 Ma
2501 Ma
2562 Ma
Proterozoic
Hamersley Basin
Fortescue Gp.
2630 Ma
2719 Ma
2741 Ma
Archean
2597 Ma
2764 Ma
2775 Ma
Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.
Stable 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
Johnson et al., 2008: Ann. Rev. Earth Planet. Sci. 36, 457-493
Great Oxidizing Event
Holland, 1994
33
4
S( o/ oo)
8
0
-4
4.0
3.0
Time (Ga)
2.0
1.0
Glacials
BIFs
GIF
Melezhik, 2005: GSA Today 15, 4-11
~2.0-1.8 Ga: Granular iron formation
Earaheedy Gp., Australia
Animikie Gp., N. America
Mesoproterozoic environmental stability
Proterozoic glacial gap
environmental
stability
Sulphidic shales
Onset of Snowball events
Ca-sulphates
Climate modelling
Proterozoic
Phanerozoic
100
10
%PAL
1
0.1
2.0
1.0
Time (Ga)
Hamersley Basin
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
Pyrophyllite Al2Si4O10(OH)2
Quartz crystal ‘beds’
In contrast, under higher pO2, weathering is achieved through mechanical
breakdown of material:
This results in the transport and deposition of clastic sedimentary rocks.
Hamersley Gp.
2450 Ma
2460 Ma
2463 Ma
2490 Ma
2501 Ma
2562 Ma
Proterozoic
Hamersley Basin
Fortescue Gp.
2630 Ma
2719 Ma
2741 Ma
Archean
2597 Ma
2764 Ma
2775 Ma
Trendall et al., 2004: Australian Journal of Earth Sciences 51, 621-644.
Iron formation-related shales
Frere Fm., Earaheedy Gp.,
Australia
2 cm
~2.4 Ga glaciations
2220
2220
2220
2316
2450
2432
2450
Transition from BIF to glacials ~2.4 Ga
Bedded Mn-carbonate
Dropstone in 2.4 Ga Turee Creek Gp.
Summary of contingent events through
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 through
time
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
2800
2700
2600
2500
2400
Time (Ma)
2300
2200
2100
A revised Precambrian timescale: possibilities
CHRONOMETRIC BOUNDARIES
• 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
A revised Precambrian timescale: possibilities
CHRONOSTRATIGRAPHIC BOUNDARIES
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
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
Main BIFs and anoxic oceans
2780 Ma
2900 Ma
2630 Ma
2400 Ma
2450 Ma
1840 Ma
end of BIFs
Glaciations
Major crust fm. + CO2
outgassing
Oxidized atmosphere
Glacials and oxygenic photosynthesizers
~2.06-1.8 Ga: Granular iron formation
Frere Fm., Earaheedy Gp.,
Australia
2 cm
Great Oxidizing Event
Holland, 1994
3176 Ma
3190 Ma
3240 Ma
3325 Ma
3350 Ma
East Pilbara Terrane
• Three unconformities
• upward-younging U-Pb ages
• Distinct geochemical trends
upsection
• Discrete history from
neighbouring terranes
3458-3427 Ma
3470 Ma
3481 Ma
3498 Ma
3508 Ma
3515 Ma
3.48 Ga stromatolites