Cryptozoic Rocks
Archean rocks
• Greenstones:
– Meta-volcanic rocks, including
• Basalt (with pillows)
• Komatiites
• Andesite/rhyolite (less common, toward the tops of
sequences)
– Meta-greywackes
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Volcanic rock fragments
Feldspars
Poorly sorted and rounded
Graded bedding
Greenstones
http://www.newscientist.com/article/dn14818-discovery-ofworlds-oldest-rocks-challenged-.html
http://scienceblogs.com/highlyallochthonous/2007/07/wh
at_is_a_greenstone_belt.php
Archean rocks
• Gneiss belts
– Granite gneisses
– Granite
– quartzites
Gneiss belt
http://picasaweb.google.com/lh/p
hoto/Sifz5y7ygBA1hpZjsZ3xow
http://www.geosci.ipfw.ed
u/Geopics/Framesrc/Faults
/quartzitefolds.html
Interpretation
• Greenstones = oceanic & subduction rock
• Gneisses = teeny unstable continents
• Many small, fast-moving thin plates with many
subduction zones and many collisions
• Thin plates allowed intraplate activity – minirifts and plate over-rides
• Everything was much hotter, so faster rates
and more metamorphism
Proterozoic rocks
• Lower Proterozoic: 2 common rock suites in
North America
– Type 1:
• Well-sorted quartz sandstones
• Quartz-rich greywackes
• Limestones with stromatolites
– Type 2:
• Banded iron formations (BIFs)
• Slates and dark greywackes
BIFs
BIFs are puzzling
• Age: from Archean through Middle Proterozoic,
with a bit at end of Proterozoic; most date from
about 3.0-1.5 GY.
• 90% of iron in rock is in the BIFs; they hold 20X
more oxygen than currently in the atmosphere –
yet deposited in an Fe-poor atmosphere
• Very thin banding that goes for hundreds of
kilometers
BIFs are weird
• Fine layering:
– Iron-rich minerals (oxides, carbonates, sulfides,
clays, amphiboles, micas)
– Chert
• But no redbeds as we know them from
Phanerozoic rocks – no red shales or
sandstones. So there could not be much free
oxygen in the atmosphere.
Possible explanations for BIF’s
1. Why so much iron?
– Iron from volcanic eruptions
– Iron coming from hydrothermal vents
– Early weathering conditions were highly acidic –
that would weather out and transport the iron.
– So ocean was full of iron ions, and no oxygen
ions.
Possible explanations for BIF’s
2. Why alternating iron-rich & iron poor?
– Evolution of photosynthetic organisms: they
produce oxygen which immediately bonded with
iron.
– In warm water silica stays in solution but bacteria
would produce more O2 and iron precipitation.
Summer = red iron bands
– In cold water silica is deposited, and bacteria
become inactive. Winter = silica bands
Why did BIF production stop?
• Eventually enough O2 was produced to oxidize
available iron, and so it started to build in
atmosphere.
• Development of ozone layer allowed organisms
to invade surface waters: more efficient
photosynthesis, much more rapid production of
O2
• Free O2 set stage for evolution of more
heterotrophs – organisms that use more O2 to
find food, rather than more CO2 to make food
Late Proterozoic
• Mid-Continent:
– Keweenawan suite: basalt, gabbro, red sandstones
and shales
– What’s the tectonic suite?
• Yes, rift valley – a very long failed rift.
• We will look at other regions in more depth