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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)
