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The Precambrian Record. Key Events of Precambrian time. Acasta Gneiss is dated at 3.96 bya. It is near Yellowknife Lake , NWT Canada Zircons possibly a bit older in Australia. Precambrian 4.6 billion years to, say, 548 or 544 million years (depending on method).

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Key events of precambrian time
Key Events of Precambrian time

Acasta Gneiss is dated at

3.96 bya. It is near Yellowknife Lake , NWT Canada

Zircons possibly a bit older in Australia


  • Precambrian

  • 4.6 billion years to, say, 548 or 544 million years (depending on method).

  • Represents 88% of all of the history of the earth.

  • Referred to as the Cryptozoic Eon.

    • “hidden life”

(no more BIFs)

(prokaryotes)


Early hadean highlights 1
EarlyHadean Highlights 1

  • Earth formed about 4.6 billion years ago from coalescing interstellar dust.

  • Earth was bombarded by large planetesimals  adding to earth’s mass (adds heat)

  • Hot spinning pre-earth mass melted, caused differentiation of materials according to density.

  • Distinct earth layers begin to form

    • Dense iron and nickel migrate to center (core)

    • silicate material moves out to mantle


Early hadean highlights 2
EarlyHadean Highlights 2

  • Huge impact from a Mars-sized planetessimal created the moon.

    • Caused earth to spin faster.

    • Possible Tilt change

    • Moon controls earth’s spin and creates tidal forces.

    • Moon’s orbit at an angle to planets around Sun

    • Earth got most of the core – outer part molten. Earth rotates. We have magnetic field and, therefore, an atmosphere


Moon origin hypotheses 1
Moon Origin hypotheses -1

Speed and approach angle unlikely


Moon origin hypotheses 2
Moon Origin hypotheses - 2

Chemical composition of the Moon suggests

that it could not have co-formed with the earth.



  • PrecambrianEarly Atmosphere

  • First earth atmosphere H He lost to solar wind. No magnetic field

  • Early permanent earth atmosphere mostly Nitrogen (inert) and CO2

    Post-differentiationstart of liquid core dynamo

  • Liquid water is required to remove CO2 from atmosphere.

    • Mars is too cold to have liquid water.

    • Venus is too hot to have liquid water.

    • So both have CO2 atmospheres.

  • On Earth, most of the world’s CO2was converted to O2 by photosynthesis.

  • Enough by 2.0 bya to sustain life.

  • CO2 is locked up in life, limestones, dolomites.

Mars

Venus

Earth


Early permanent atmosphere
Early Permanent Atmosphere

  • Gasses from cooling magmas formed early atmosphere mostly N2, CO2, with CH4, H2O

  • Early earth not conducive to modern oxygen breathing organisms: too much solar UV gets through atmosphere.

  • Little oxygen occurred in the atmosphere until the evolution of photosynthetic organisms (Eubacteria) 3.5 billion years ago. Fully oxygenated about 1.9 billion years ago.


  • PrecambrianEarly Oceans from 4 bya

  • Much water vapor from volcanic degassing.

  • Salt in oceans is derived from weathering and

    carried to the oceans by rivers.

  • Blood of most animals has chemistry of seawater.

  • Part of the earth’s water probably came from comets.

    • Comets are literally large dirty snowballs.

    • Provide fresh water.

OCEANS


  • Archean To Proterozoic Sedimentary Rocks

  • Archean

    • 4 bya: mostly deep water clastic deposits such as mudstones and muddy sandstones.

    • high concentration of eroded volcanic minerals (Sandstones called Graywackes).

    • 3 bya: absence of shallow water shelf carbonates.

    • increasing chert.

    • low oxygen levels, free iron was much more common in the Archean.

    • Iron formed “chemical sinks” that consumed much of the early planetary oxygen.

    • Formed banded ironstones, commonly with interbedded chert.

  • Proterozoic– 2 bya Carbonates* become important *Franklin Marble Field Trip 1

    - Non-marine sediments turn red – iron is oxidized by the oxygen in AIR


  • Precambrian Hadean

  • Formation of Continents

  • Early earth surface was magma sea, gradually cooled to form the crust.

  • Continents did not always exist but grew from the chemical differentiation of early, mafic magmas in the young hot earth. Floating “Volcanic Islands” of less dense higher silica magmas.


  • Precambrian Hadean and Archean

  • Formation of Felsic Islands

  • Convection was fast due high temperatures – ultramafic magma

  • Partial Melting of base of Ultramafic Islands, OR

  • Fractional Crystallization of Mafic Magmas THEN

  • Once both mafic and felsic rocks (with different densities) exist, subduction under protocontinents possible.

  • Water squeezed from subducted ocean materials partially melts mantle

  • Volcanic arcs add land to protocontinents.

  • Increasing amounts of Felsic continental material


First continental crust
First continental crust

At high temperatures, only Olivine and Ca-Plagioclase crystallize “Komatiite”

Then:

First

Water out

Komatiite partially melts, Basalt gets

to surface, piles up. The stack sinks,

partially melts when pressure high

enough. Fractionation makes

increasingly silica-rich magmas

Density differencesallow subduction of mafic rocks. Further partial melting and fractionation makes higher silica melt that won’t subduct


Archean growth of the early continents
Archean: Growth of the early continents

Magmatism from Subduction Zones causes thickening


Growth of the early continents
Growth of the early continents

Island Arcs and other terranes accrete as

intervening ocean crust is subducted

Little Archean ocean crust survives: most subducted

But silica-rich continental crust too buoyant to subduct.


Growth of the early continents1
Growth of the early continents

Sediments extend continental materials seaward

Quartz sand becomes SS or quartzite, too buoyant to subduct


Growth of the early continents2
Growth of the early continents

  • Continent-Continent collisions result in larger continents

  • Again, not very big in Archean; convection cells too small



  • PrecambrianEarly Continents (Cratons) Archean

  • Archean cratons consist of regions of light-colored felsic rock (granulite gneisses)

  • surrounded by pods of dark-colored greenstone (chlorite-rich metamorphic rocks).

    • Pilbara Shield, Australia

    • Canadian Shield

    • South African Shield.

Chlorite is the Low T metamorphic product of clay muds.

Mafic Greenstone Belts

Felsic Islands

40km


Archean crustal provinces were once separated canadian shield assembled from small cratons
Archean Crustal Provinces were once separatedCanadian Shield assembled from small cratons

Discussion: Where to look for diamonds

Intensely folded rocks where cratons

were later sutured together in

Early Proterozoic

Longest: Trans-Hudson Orogen


Granulite gneiss and greenstone
Granulite gneiss and greenstone

Canadian Shield

Exposed by Pleistocene glaciers


Stratigraphic sequence of a greenstone belt
Stratigraphic Sequence of a Greenstone belt

Banded Iron Formations

Younger lavas richer in silica

Increasingly Silica-rich extrusives,

some rhyolites with granites

below them.

Komatiites form at very high temps. They are absent later as Earth cooled

DEMO: Banded Iron Sample


Archean formation of greenstone belts
Archean Formation of greenstone belts

  • Early continents formed by collision of felsic proto-continents.

  • Greenstone belts represent volcanic rocks and sediments that accumulated

  • along and above subduction zones and then were sutured to the protocontinents during collisions.

  • Protocontinents small, rapid convection breaks them up


Proterozoic tectonics the wilson cycle
Proterozoic Tectonics: The Wilson Cycle

  • Proterozoic – Convection Slows

  • Rift Phase

    • Coarse border, valley and lava rocks in normal faulted basins

  • Drift Phase

    • Passive margin sediments

  • Collision Phase

    • Subduction of ocean floor, island arcs form

    • Then collision


Crustal provinces proterozoic tectonics
Crustal provinces: Proterozoic Tectonics

Slave Craton Rift and Drift

Followed by Wopmay Orogen:

remnants of old collisional mountains

Intensely folded rocks where cratons

were sutured together in

Early Proterozoic


Wilson cycle 1 2 rift drift coronation supergroup
Wilson Cycle 1&2 Rift & DriftCoronation Supergroup

1. Rift 2. Passive Margin sediments

Much later stuff

1. Rift Valley

Proterozoic 2 bya as Slave craton pulled apart



3 end of wilson cycle in the wopmay orogeny
3. End of Wilson cycle in the Wopmay Orogeny Orogen

Coronation Supergroup thrust faulted eastward over Slave Craton

Note the vertical exaggeration



Proterozoic assembly of laurentia
Proterozoic Orogen Assembly of Laurentia

  • Trans-Hudson Orogen mostly 2.5 - 2 bya

    • Superior, Wyoming, Hearne plates sutured

    • Mountain range now eroded away

  • Greenland, N. Gr. Brit., Scandinavia by 1.8 bya

  • Continued accretion 1.8-1.6 bya of island arcs. Most of S. US “Mazatzal Province”

  • Last piece “Grenville Orogeny” 1.3-1 bya

  • Assembly of Rodinia by about 750 mya


Proterozoic oxygen rich atmosphere
Proterozoic Oxygen - Rich Atmosphere Orogen

  • Eubacteria are photosynthetic

    2 bya formed stromatolites along shores

  • Free oxygen in atmosphere

  • Band Iron Formations (common 3.8 – 2 bya) become rare, probably depended on disappearing conditions

  • 2 bya Redbeds begin forming when iron in freshwater sediment is exposed to abundant atmosphere oxygen

  • Oxygen in atmosphere irradiated - Ozone layer forms, protecting shallow water and land life forms from UV




Final assembly of rodinia grenville orogeny 1 3 1 0 bya
Final Assembly of Rodinia OrogenGrenville Orogeny 1.3 – 1.0 BYA

  • Eastern US Grenville collided with west coast of S.America (maybe)

  • We saw this Field Trip 1, Volcanic Arc at Park n Ride

  • Southwest US collided w/ Antarctica

    • Grenville Orogeny continues in Antarctica

  • South collided with Africa

  • Rifted apart by about 600 mya, after time of “Snowball Earth”


Growth of laurentia
Growth of Laurentia Orogen

Grenville: Shallow Water sandstones (lots of graywacke), mudstones and carbonates subjected to high-grade metamorphism and igneous intrusion



Proterozoic rifting
Proterozoic Rifting America

  • Grenville Time Rifting 1.3 – 1 bya

  • Kansas to Ontario to Ohio

  • Rift Valley sediments and lavas 15 km

    (9 miles) thick!

  • Rich in Copper, as are the rift valley sediments here.

  • Why?


Midcontinent rift
Midcontinent rift America

1500 km long, exposed near L. Superior



Plenty of highlands, America

equator to poles

What Plate Tectonic conditions favor glaciation?

Grenville

Orogen


Snowball earth
Snowball Earth America

  • Rodinia: abundant basalts with easily weathered Ca feldspars. Ocean gets Ca+ + . CO2 tied up in extensive limestones. Less greenhouse effect. Atmosphere can’t trap heat – Earth gets colder

  • Grenville Orogeny left extensive highlands

    • From high latitudes to equator

  • About 635 mya glacial deposits found in low latitudes and elevations

  • Huge Ice sheet reflects solar radiation “Albedo”

  • Some workers believe oceans froze


Stable isotopes of C and O America

d13C and d18O:3 - 4 Proterozoic Glaciations

Earth surface became cold enough to produce glaciations and ice ages

G - Glaciation

BIF - Banded Iron Formation

Cambrian

Snow-ball

Earth


Break up of rodinia
Break up of Rodinia America

  • Hypothesis: Ice an insulator, heat builds up

  • Heavy volcanic activity poured CO2 into atmosphere – greenhouse effect

  • Warming melted snowball earth


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