Precambrian Earth History—The Proterozoic Eon 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 Archean-Proterozoic 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)