More Modern World

choppedspleenMechanics

Feb 21, 2014 (3 years and 4 months ago)

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Chapter 9


The Proterozoic: Dawn of a
More Modern World

Highlights



A number of large, distinct crustal segments

called Precambrian provinces developed in North

America.



Called



Occurred along
, belts of crustal

compression, mountain building, and


metamorphism.



Laurentia


Areas where these continents are joined together

are called sutures.



Happened about 1.7


1.8 billion years ago.



Joshua Tree Pinto Gneiss is part of this
orogen
.


Continents had fairly low relief (not a lot of mountains).



Frequent

(broad, shallow) seas

flooded the continents.



Laurentia


Areas where these continents are joined together

are called sutures.



Happened about 1.7


1.8 billion years ago.



Joshua Tree Pinto Gneiss is part of this
orogen
.


Continents had fairly low relief (not a lot of mountains).



Frequent

(broad, shallow) seas

flooded the continents.



Rodina Begins to Form

.

Older single large continent called Rodina
begins.


First evidence of “Wilson Cycle”.


Opening of an ocean basin (divergence).



Normal faulting



Deep basin sedimentation with




interlayered basalt


Sedimentation along margins of separating


continents


Closing of ocean basin

Wilson Cycle

.


Subduction of oceanic crust beneath


continental crust



Trench sedimentation



Intense folding and reverse faulting.


An earlier version of formation of
Appalachian Mountains during formation of
Pangaea.

Evidence of First Ice Age

.

Tillites


unsorted, angular lithified glacial

debris.


Varved mudstones


laminated muds formed

by differences in summer and winter ice

melting.


Has Day
-
Length Changed?

.

Shorter Proterozoic Day


18.4 hour day!



Earth slowing at 2 seconds per



100,000 years


Lunar tides


Rhythmites



Alternating light/dark silty deposits


produced by daily tidal deposits.


900 million years ago



Banded Iron Formation

.

A lot already discussed.


O
2

in atmosphere.


Up to 1000 m thick and 100 km along



strike!


Up to 80% of iron ore bodies.


Only found in old shield areas


Cyanobacteria
-

prokaryotes





Aborted Mid
-
Continent Rift

.

Lake Superior region



Rich in copper and silver



“Black Smokers”


Developed 1.0
-

1.2 billion years ago



New Madrid EQ 1811
-
1812





Grenville Orogeny

.

1.0
-

1.2 billion years ago

Extends from Adirondack Mtns. to central
Mexico and California



Part of the building of Rodina



As Rodina breaks a new ocean forms



Proto
-
Pacific Ocean: Panthalassa



Ocean



Proto
-
Atlantic Ocean begins closing up



Iapetus

Snowball Earth

Photosynthesis is using up CO
2

and creating O
2

in earth’s
atmosphere.


What’s going to happen next?


Temperatures drop.


The Big Chill


Snowball Earth begins 716 million years before present.



Drop Stone



Snowball Earth



Snowball Earth

Earth is frozen over to the very low latitudes.


What color is earth?


What is earth’s reflectivity?


0.1 albedo snow and ice vs. 0.8 albedo for oceans


Will earth ever de
-
ice? Unlikely.


Except earth is currently in an unfrozen state.


What could have happened?



Snowball Earth Thaws Out

Are volcanoes still erupting? What do they erupt?


CO
2
? Why is CO
2

important?





Snowball Earth Cycles

Many freeze
-
thaw cycles where life is under extreme


stress



Grow
-

no grow





Snowball Earth Environmental Stress

First multi
-
cellular life evolves.




570 million years before present



China



Preservation by phosphate seeped into and



coated cells.



Burrowing organisms.



Change from fissile shales to siltstones that have


no layering.




What causes this?


Ediacaran Fauna

First important discovery of large metazoans



Australia


Large, snail
-
sized metazoa


Appear to contain coelum (body cavity)



Digestive tract and other organs


Snails?



630 to 580 million years ago



Ancestors of Paleozoic invertebrates



O
2

up to 1%!

End of Banded Iron Formation

.

Banded Iron Formation e