chapter 12 the proterozoic eon of precambrian time
TRANSCRIPT
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Chapter 12
The Proterozoic Eon
of Precambrian Time
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Guiding Questions• How did mountains form in Proterozoic time?• What global climatic and biological changes took
place in the Proterozoic Eon?• What evidence is there that oxygen began to build
up in the atmosphere about 2.3 million years ago?• Why do some scientists use the label “snowball
Earth” to describe out planet during part of the Neoproterozoic time?
• What did continental suturing accomplish on a global scale during Neoprotozoic time?
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Proterozoic Eon• Precambrian
– Proterozoic• 2.5 Billion to ~ 0.6
Billion years ago– Archean
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Proterozoic Orogeny• Formation of large
cratons
• By ~1 Billion years ago, modern style orogeny
• Wopmay
• 2 Billion years
• Slave Province– Ancient fold and
thrust belt
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Proterozoic Orogeny
• Thick deposit of quartz sandstone
• Carbonate platform
• Transitional mudstones
• Flysch deposits
• Turbidites then mudcracks and stromatolites
• Molasse deposition
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Proterozoic Orogeny• Carbonate platform
– Cyclic deposits indicated progradation of tidal flats across lagoon
– Laminated dolomite forms base
– Oolites or stromatolites are at top
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Proterozoic Events• Widespread glaciation
– Gowganda deposits– 2.3 Ga
• Stromatolites– Proliferate– Diverse shapes 1.2
Billion years ago
• Early Eukaryotes
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Evolution of Eukaryotes• Union of 2 prokaryotic cells
– Mitochondrian• Allow cells to derive energy
from their food by respiration• Evolved from 1 prokaryotic cell
– Chloroplast• Site of photosynthesis• Protozoan consumed, retained
cyanobacterial cell
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Algae
• Multicellular protists
• Algal ribbons wound into loose coils– 2.1 Billion years
ago
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Algae• Prokaryotic forms
– Gunflint flora 2 Billion years ago– Lake Superior
• Acritarchs– Multicellular forms abundant after 2 Billion years
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Proterozoic Life• Complex organisms
can be identified from trace fossils– None present until
about 570 Million years ago
• Belt Supergroup, Montana
• 1.3 Billion years ago
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Proterozoic Life• Trace fossils
provide evidence for past life in Neoproterozoic
• Increasingly complex and varied
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Proterozoic Life• Non-skeletal fossils
– Similar to Cnidaria– Imprints of soft-
bodied organisms– < 570 M years ago
• Ediacarian fauna– Lived before
predators– Some similar to
modern forms
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Proterozoic Life• Possible arthropod fossils
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Proterozoic Life• Skeletal fossils
– Vase-shaped and tubular
– Small size
• Timing may be related to glaciations
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Proterozoic Continents
• Continental accretion– Microplate accreted to
continent– Orogenic stabilization
• Welds sediment to crust
• Thickens crust, hardens unconsolidated sediments
• Remobilization– Alter character of pre-existing
rocks– Reset radiometric clocks
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Proterozoic Continents
• Canadian Shield– Includes Northern
U.S.
• Laurentia grew by accretion during Proterozoic
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Proterozoic Continents• 1.95–1.85 Billion years ago
– Superior Province is largest
• 1.9–1.8 Billion years old rock unit– Separates Superior Province
from Wyoming and Hearne provinces
• 1.8–1.6 Billion years ago– Exposed in Southern
Wyoming to Northern Mexico
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Proterozoic Continents• Wyoming Province
– Archean terrane exposed in Wyoming and Black Hills
• Laurentia was probably attached to Australia, Antarctica and Siberia
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Proterozoic Continents• Failed rift 1.2–1.0 Billion
years ago– Volcanic belt through
midwest– Keweenawan basalts
• Grenville Orogeny 1.1 Billion years ago– Accretionary event
• Added eastern belt– Exposed in Canada,
Adirondacks, Blue Ridge, Llano (TX) uplift
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Proterozoic Continents• Keweenawan
basalts– Copper ore– Form mid-
continent gravity high
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Proterozoic Continents• Rodinia
– Supercontinent• United Laurentia to
other land masses• Full assembled 1
Billion years ago
– Broke up 0.8–0.7 Billion years ago
• Created Pacific Ocean• Created failed rifts in
Western Laurentia
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Proterozoic Continents• Belt Supergroup
– 0.9–1.5 Billion years ago– Thick sequence of sediments formed in failed rift – Accumulated in shallow water during rapid subsidence
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Banded Iron Formations• Stopped forming 1.9
Billion years ago– Chert contaminated by
iron• Red or brown color
– Alternate with iron- rich layers (magnetite)
• Oxygen-poor ocean waters
• Iron was not oxidized
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Red Beds
• Never found in terranes older than 2 Billion years
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Snowball Earth• Neoproterozoic glacial deposits
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Snowball Earth
• Light carbon isotopes
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Snowball Earth
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Snowball Earth
• Growth of stromatolites over columnar crystals of aragonite
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Snowball Earth• Evidence for Marinoan ice age in Morocco
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