Transcript Slide 1

The Proterozoic
Sub Divisions
► Neoproterozoic
 1.0-0.544 b.y
► Mesoproterozoic
 1.6- 1b.y.
► Paleoproterozoic
 2.5-1.6 b.y.
Laurentia: Proterozoic N. America
► Six
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Elements
Superior
Slave
Hearne
Rae
Wyoming
Nain
Laurentia: Proterozoic N. America
► During
Proterozoic the six elements (or
provinces) are welded together during
orogenic episodes
► This
period of continental welding ended by
1.7bya
► Extensive accretion of continental margin
(2.3-0.9bya)
Plate tectonics in the Proterozoic
► Rapid
lateral motion
 Vigorous subduction/orogeny
► Extensive
Rifting
► Rapid sea-floor spreading
Paleoproterozic Events
►Wopmay
Orogeny
 Western Margin of the Slave province
►Evidence
of opening of an ocean basin
►Sedimentation along the new
continental margin
►Closure of the ocean basin
Wilson Cycle
Depositional Zones
► Eastern
zone of ocean basin (next to Slave)
 Coastal Region
►Quartz
sandstone
►Shallow marine environment
►Over time Sandstone metamorphosed to Quartzite.
Also contained massive stromatolites (now
dolomitic)
 This is the Rocknest Formation
 Western Zone
►Continental Shelf deposits
 Deep water turbidites
Evidence of Wilson Cycle
► Numerous
tensional faults
► Alluvial Fan and Fluvial deposits
► As the ocean widened the western
edge of the province became passive
margin with two parallel zones of
deposition
Closure of the ocean basin
► As
westward edge of Slave subducted the
continental shelf buckled downward.
► As depth increased:
 Carbonate deposition ended
 Accumulation of deep water clastics began
► Subsequent
collisional forces caused
additional folding and faulting.
Aulacogens
►Aulacogens
are inactive rifts of a
radiating three-rift system that
develops over areas of crust that are
being uplifted
Forming an Aulacogen
 As the crust begins to bulge it breaks to form
the three rift zones
Forming an Aulacogen
► Two
arms fill with
water to form oceans
► The third, failed, arm
trends inland
Examples
Can you identify any modern Aulacogens on the world map?
Trans-Hudson Orogeny
►N
and W side of
Superior Province
► Records Wilson Cycle
► Joins Hearne and
Wyoming
 Severe folding and
metamorphism during
closure of wilson cycle
welded the Superior
plate to the Wyoming
and Hearne
Paleoproterozoic Ice Age
► Superior
Province 2.6-2.1 by
► Gowgonda Formation
 Notable for conglomerates and laminated
mudstones
►Laminations
represent repeated summer and winter
sediment layers (called varves)
►In Gowgonda Formation varves alternate with
tillites
► Formation
lies above 2.6 by crystalline rock
and is intruded by 2.1 by igneous rocks
Labrador Trough
► Curved
track on east
side of Superior
Province
 Records yet another
Wilson Cycle
 During closing phase
eastern zone underwent
intense folding, thrust
faulting, and
metamorphism- known
as Hudsonian Orogeny
Mesoproterozoic
► Hudsonian
Orogeny serves as the event that
separates paleoproterozoic from
mesoproterozoic
Keweenawan Sequence
► Rocks
rest on crystalline or animikian
strata
► Extend for hundreds of kilometers from Lake
Superior region
► Consist of Quartz sandstone, arkoses,
conglomerates, as well as basaltic volcanics.
 The lava flows are well known for their content
of native copper.
Keweenawan Sequence
In some areas lava flows accumulated to a thickness
of several kilometers.
► Although this seems like a lot of lava, most of the
magma remained underground forming the Duluth
Gabbro (12kmX160km)
 What is most basaltic magma/lava associated
with?
► Large amounts of mafic magma typically signals
the presence of a rift
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 Had the rift continued, the eastern US would have
drifted away
The Grenville Province
► Exposures
extend from Labrador to Lake Huron.
However, region actually extends down eastern US
and westward into Texas.
► Consist
of carbonates and sandstones that have
been deformed, metamorphosed, and intruded.
The Grenville Province
► Deformation
occurred 1.2-1.0 bya during
the Grenville Orogeny. (the beginnings of
the Appalachians)
 Only one part of a series of continental
collisions involved in the formation of the
supercontinent Rodinia
Rodinia
Neoproterozoic
► Rodinia
was assembled by 1.0 bya, however,
by 750mya it began to split apart
► During
this time the proto-pacific ocean
(Panthalassa) was created west of N.
America
Proterozoic Ice House
► Neoproterozoic
rocks on all
continents, except India and
Siberia, show glacial striations.
► Tillites
found
and varves can also be
 Evidence is so prevalent, geologist
refer to our planet during this time
as “Snowball Earth”
► Formally
this time is called the
Varangian Glaciation
Rodinia
What could have cause this?
► Plate
tectonics
 Most land masses located at low or mid
lattitudes
►Land
is much more reflective than water
►As glaciers formed land became even more reflective
► Atmospheric
 Loss of Carbon Dioxide
 During this time fungi, lichens and vascular
plants were present.
Sediment Deposition
► During
this period extensive layers of
limestone were deposited.
If Limestones are typically associated
with relatively warm seas how could
this be possible?
The Fossil Record
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Inherited Archean Life
 Photosynthetic cyanobacteria
(oxygenated)
 anaerobic prokaryotes
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Stromatolites
molecular fossils
Heliotropic Stromatolites
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Sine-wave growth
form=yearly cycles
Laminar structure=daily
growth phase
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Bitten Springs Fm.,
Australia (850 my)
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Anabaria juvensis
(Stromatolite)
435 laminae/cycle=435
days/year
Eukaryotes
Record of large cells: 1.6-1.4 b.y to present
Steranes: molecular fossils
Acritarchs (Planktonic Algae; Skiagia)
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Max age 1.6 b.y.
Fossil sites; Russia, California, Australia
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Eukaryotic Protozoans: Vase-shaped fossils from
Spitsbergen and Arizona
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Metazoans
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Australia: Ediacara Hills
China: Doushantuo Formation (~570 m.y.)
Russia: White Sea coast (~550 m.y.)
Some may not have modern equivalents
Body and trace fossil Metazoans
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Cloudina: 3cm, tube dwelling worm
Ediacaran trace fossils
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Cyclomedusa: discoidal (Jellyfish ?)
Tribrachidium: circular (unknown)
Charniodiscus: Frond shape (Sea pen)
Dicksonia: ovate-shape (Flatworm)
Spriggia: Elongate (Annelid)
Ediacaran
Fossil Expansions
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Mesoproterozoic (1.4 b.y. ago) Eukaryotes
Neoproterozoic (0.6 b.y. ago) Metazoans
Proterozoic Prokaryotes (Gunflint Fossils,
1.9b.y.)
Gunflintia: unbranched filament (algae)
► Animikiea: fine filament (algae)
► Eostrion: Dawnstar (bacterium)
► kakabekia: plumose form (unknown)
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Changing Environment
Free Oxygen begins to Accumulate in
atmosphere (2.0 b.y. ago)
Oxygen increases sporadically as CO2 level
decrease (1.0 b.y.)
Oxygen rises to 3-10% of current level (0.6 b.y.
ago)
Wide variety of climates
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Hot, dry (evaporates)
Tropical (algae reefs)
Ice Ages (glacial deposits)
Warm, tropical (Carbonates)