1. Precambrian Earth History—The Proterozoic Eon
Chapter 9
Precambrian Earth History—The Proterozoic Eon

2. 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

3. 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

4. Age of Continental Crust

5. Archean vs. Proterozoic
Proterozoic is characterized by
differing crustal evolution
less metamorphism
plate tectonics similar to the present
less heat!

6. 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

7. 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

8. 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

9. Southern Margin Accretion
Laurentia grew along its southern margin
by accretion of the Central Plains, Yavapai, and Mazatzal orogens

10. Grenville Orogeny
A final episode of Proterozoic accretion occurred during the Grenville orogeny

11. 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

12. 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

13. Jormua Complex, Finland
Metamorphosed basaltic pillow lava

14. Early Supercontinent
Possible configuration of the Late Proterozoic supercontinent Rodinia
before it began fragmenting about 750 million years ago

15. 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?

16. Proterozoic Glacial Evidence
Bagganjarga tillite in Norway
overlies striated bedrock surface

17. Ediacaran Fauna The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive echinoderm
Spriggina floundersi, a possible ancestor of trilobites

18. Ediacaran Fauna Pavancorina minchami
Restoration of the Ediacaran Environment

19. 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

20. 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

21. 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)