Archean EonFriday 21 March 2008

The Eoarchean Earth?

Archean rocksGreenstone belts:Typically found in belts which have a

synclinal form. • Show a sequential transition from ultramafic volcanics

at the bottom to felsic volcanics and capped by sediments.

• Mafic Volcanics - pillow lavas are common (environment?)

• Sedimentary Rocks - composed dominantly of graywackes ("dirty sandstones"), conglomerates and sandstones. - deposited in shallow water deltas, tidal flats, shallow marine shelf environments.

Archean

• Making and keeping crustal fragments

Arcs collide due to subduction to form larger land mass

Land masses continue to grow

Precambrian shields and cratons• Shield: area of exposed Precambrian rocks

• Craton: buried and exposed Precambrian rocks

Superior Craton of Canada• Small fragments before 3.0 billion• Explosion of crustal growth 3.0-2.5 billion (end of

Archean)

Model for assembly of cratons• Continental accretion: plates collide with

volcanic arcs and other plates.• Continents grow along margins

Archean greenstone pillows

Archean greenstone pillows

Archean conglomerate: more evidence for water

Archean sediment layers: more evidence for water

Canadian Shield Gneiss

• By end of Archean– Earth cooler– Easier to keep crust – Easier to make and

keep granite

Proterozoic to the Cambrian explosion

Rocks, life and climate

• ~1.8-2.6 Ga--first red Banded Iron Formations (BIF's) - red and gray zones of oxidized iron layers of silica.

• Responsible for world's most important iron deposits. As soluble iron was used up, O2 began to increase in atmosphere, and CO2 decreased

• continued until ~2 Ga=first appearance of red beds (and appearance of ozone!)

• Red beds = shallow-water, river, or soil deposits in which the iron has combined with O2 to form red iron oxide

Proterozoic rocks

Banded iron formation

• 92% of BIF are Proterozoic (2.5-2.0 Ga)

BIF’s

Banded Iron Formation, Alternating bands of red jasper and black hematite,about 2250 million years old (2.55 billion years old)

Jasper Knob, Ishpeming, Michigan

Model for origin of BIF• Oxygen-rich upper

ocean• Oxygen-poor deep

ocean• Upwelling brings Fe

and Si-rich water up• Iron oxide formation

occurs (rust-like minerals form)

•red beds=O2 increase in atmosphere

•BIFs decline, an indication that reducing compounds are disappearing from the oceans (

•weathering as we know it was probably set up at this time

Less CO2

more CO2

Proterozoic rocks

Life in Proterozoic• Until about 1.9 Ga we only have evidence of the

simplest kinds of life - the PROKARYOTES – bacteria and blue green algae

• Requirements to be Life – self-replicating (DNA) – metabolism (chemical processes that convert food into

energy)

• But at about 1.9 billion years we start to see fossils of much larger cells. These cells belong to the Eukaryotes - of which we are members

• First sexual reproduction!

Life in Proterozoic• ~650 mya (0.65 billion)

the first multicellular forms are present

• called the Ediacara assemblages

• seem to be elaborations of forms with – large surface areas– living in shallow relatively

high energy environments – often in red beds.

Life in Proterozoic• Originally discovered in Pound Qtzt, Ediacara Hills, S.

Australia; later found worldwide (including Piedmont area of NC) at low paleolatitudes.0.59 - 0.7 by (590 - 700 my)

• impressions and molds of animals (associated with trace fossils)

Mawsonites, similar to jellyfish, AustraliaSmithsonian Institution, Museum of Natural History

Dickensonia costata,segmented worm, from AustraliaSmithsonian Institution,Museum of Natural History

Unnamed "spindle-shaped organism"

from NewfoundlandSmithsonian Institution, Museum of Natural History

Tribrachidium heraldicum, Echinoderm?,from AustraliaSmithsonian Institution, Museum of Natural History

EDIACARA FAUNA

Cambrian explosion—545mya

What happened at the P-Tr boundary? What about all of the other “dips” in the diversity?

Paleogeography and climate

• Animals with hard-shells appeared in great numbers for the first time during the Cambrian.  The continents were flooded by shallow seas.  The supercontinent of Gondwana had just formed and was located near the South Pole.

NA=Laurentia

N.Europe=Baltica

Proterozoic climate

• Cold multiple times!!• Evidence:

– Till

– Polished, striated bedrock

N.A. Orogenies

• 8 major Archean-aged crustal blocks: Hearne, Nain, Penokean, Rae, Slave, Superior, Wopmay, and Wyoming.

• Collisions between these blocks resulted in orogenic (mountain building) events. - continent-continent collisions