Chapter 9 Precambrian Earth History— The Proterozoic Eon

Chapter 9

Precambrian Earth History—The Proterozoic Eon

• ~2 billion years

– 42.5% of all geologic time

– yet we review this long episode of Earth and life history very briefly

The Length of the Proterozoic

• 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

Archean-Proterozoic Boundary

Age of Continental Crust

• Proterozoic is characterized by– differing crustal evolution– less metamorphism– plate tectonics similar to the

present– less heat!

Archean vs. Proterozoic

• 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

Evolution of Continents

• 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

Focus on Laurentia

Proterozoic Evolution of Laurentia

• Laurentia grew along its southern margin by accretion

• 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

Southern Margin Accretion• Laurentia grew along its southern margin

– by accretion of the Central Plains, Yavapai, and Mazatzal orogens

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

Grenville Orogeny

• 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

Building North America

• 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

Style of Plate Tectonics

Jormua Complex, Finland• Metamorphosed basaltic pillow lava

• Possible configuration of the Late Proterozoic supercontinent Rodinia – before it began

fragmenting about 750 million years ago

Early Supercontinent

• 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?

Ancient Glaciers

• Bagganjarga tillite in Norway– overlies striated bedrock surface

Proterozoic Glacial Evidence

• The Ediacaran fauna of AustraliaTribrachidium heraldicum, a possible primitive

echinoderm

Ediacaran Fauna

Spriggina floundersi, a possible ancestor of trilobites

Pavancorina minchami

Ediacaran Fauna

• Restoration of the Ediacaran Environment

• 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

Ediacaran Fauna

• 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

Represented Phyla

• 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)

Distinct Evolutionary Group