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Transcript
Chapter 9
Precambrian Earth History—
The Proterozoic Eon
The Length of the Proterozoic
• ~2 billion years
– 42.5% of all
geologic time
– yet we review this
long episode of
Earth and life
history very briefly
Archean-Proterozoic Boundary
• Geologists have rather arbitrarily placed the ArcheanProterozoic boundary at 2.5 billion years ago
– it marks the approximate time of changes in the style of
crustal evolution
Age of Continental Crust
Archean vs. Proterozoic
• Proterozoic is characterized by
–
–
–
–
differing crustal evolution
less metamorphism
plate tectonics similar to the present
less heat!
Evolution of Continents
• Archean cratons assembled during collisions of island
arcs and minicontinents
– nuclei around which Proterozoic crust accreted
– much larger landmasses formed
• Proterozoic accretion at craton margins probably took
place more rapidly than today
– because Earth still possessed more internal heat
– but the process continues even now
Focus on Laurentia
• Large landmass that consisted of what is now:
•
•
•
•
North America
Greenland
parts of northwestern Scotland
some of the Baltic shield of Scandinavia
• Laurentia originated and underwent important growth between
2.0 and 1.8 billion years ago
• During this time, collisions among various plates formed several
orogens
– linear or arcuate deformation belts in which many of the rocks have been
metamorphosed and intruded by magma
Proterozoic Evolution of Laurentia
• Archean cratons
were sutured
– along deformation belts called orogens,
– thereby forming a larger landmass
• By 1.8 billion years ago, much of what is now
Greenland, central Canada, and the north-central
United States existed
• Laurentia grew along its southern margin by accretion
Southern Margin Accretion
• Laurentia grew along its southern margin
– by accretion of the Central Plains, Yavapai, and
Mazatzal orogens
Grenville Orogeny
• A final episode of Proterozoic accretion occurred
during the Grenville orogeny
Building North America
• By this final stage, about 75% of present-day
North America existed
• The remaining 25% accreted along its margins,
particularly its eastern and western margins,
during the Phanerozoic Eon
Style of Plate Tectonics
• The present style of plate tectonics
– involving opening and then closing ocean basins
– had almost certainly been established by the Early
Proterozoic
• In fact, the oldest known complete ophiolite
– providing evidence for an ancient convergent plate boundary
– is the Jormua complex in Finland
• It is about 1.96 billion years old
– similar to younger well-documented ophiolites
Jormua Complex, Finland
• Metamorphosed basaltic pillow lava
Early Supercontinent
• Possible
configuration of
the Late
Proterozoic
supercontinent
Rodinia
– before it began
fragmenting about
750 million years
ago
Ancient Glaciers
• Very few times of widespread glacial activity have
occurred during Earth history
• Most recent one during the Pleistocene (1.6 Ma - 10
Ka): The Ice Age
– we also have evidence for Pennsylvanian glaciers
– two major episodes of Proterozoic glaciation
• How do we recognize past glacial periods?
Proterozoic Glacial Evidence
• Bagganjarga tillite in Norway
– overlies striated bedrock surface
Ediacaran Fauna
• The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive echinoderm
Spriggina floundersi, a possible
ancestor of trilobites
Ediacaran Fauna
Pavancorina minchami
• Restoration of the
Ediacaran Environment
Ediacaran Fauna
• Geologists had assumed that the fossils so common in
Cambrian rocks must have had a long previous history
– little evidence to support this conclusion
• The discovery of Ediacaran fossils dramatically
increased our knowledge about this chapter in the
history of life
Represented Phyla
• Three present-day phyla may be represented in the Ediacaran
fauna:
– jellyfish and sea pens (phylum Cnidaria)
– segmented worms (phylum Annelida)
– primitive members of the phylum Arthropoda (the phylum with insects,
spiders crabs, and others)
• One Ediacaran fossil, Spriggina, has been cited as a possible
ancestor of trilobites
• Another might be a primitive member of the phylum
Echinodermata
Distinct Evolutionary Group
• However, some scientists think these Ediacaran animals
represent an early evolutionary group quite distinct from the
ancestry of today’s invertebrate animals
• Ediacara-type faunas are known from all continents except
Antarctica
– collectively referred to as the Ediacaran fauna
– widespread between 545 and 670 million years ago
– fossils are rare (lacked durable skeletons)