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
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
►
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
►
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
►
Bitten Springs Fm.,
Australia (850 my)
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
►
Eukaryotic Protozoans: Vase-shaped fossils from
Spitsbergen and Arizona
►
Metazoans
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
►
►
Cloudina: 3cm, tube dwelling worm
Ediacaran trace fossils
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)
►
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)