Precambrian Earth History

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Chapter 9
Precambrian Earth HistoryThe Proterozoic Eon
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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

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Archean-Proterozoic Boundary

• Geologists have rather arbitrarily placed the
  Archean-Proterozoic boundary at 2.5 billion years
  ago
• it marks the approximate time of changes in the
  style of crustal evolution

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Age of Continental Crust
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Archean vs. Proterozoic

• Proterozoic is characterized by
• differing crustal evolution
• less metamorphism
• plate tectonics similar to the present
• less heat!

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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

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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

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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

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Southern Margin Accretion

• Laurentia grew along its southern margin
• by accretion of the Central Plains, Yavapai, and
  Mazatzal orogens

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Grenville Orogeny

• A final episode of Proterozoic accretion occurred
  during the Grenville orogeny

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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

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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

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Jormua Complex, Finland

• Metamorphosed basaltic pillow lava

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Early Supercontinent

• Possible configuration of the Late Proterozoic
  supercontinent Rodinia
• before it began fragmenting about 750 million
  years ago

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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?

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Proterozoic Glacial Evidence

• Bagganjarga tillite in Norway
• overlies striated bedrock surface

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Ediacaran Fauna

• The Ediacaran fauna of Australia
• Tribrachidium heraldicum, a possible primitive
  echinoderm

Spriggina floundersi, a possible ancestor of
trilobites
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Ediacaran Fauna

• Pavancorina minchami

• Restoration of the Ediacaran Environment

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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

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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

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Distinct Evolutionary Group

• However, some scientists think these Ediacaran
  animals represent an early evolutionary group
  quite distinct from the ancestry of todays
  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)