LECTURE NOTES: PROTEROZOIC EON (2.5by - 545my)

PRECAMBRIAN TIME-PROTEROZOIC EON
Transition to the Proterozoic

During the late Archean life developed as single-celled bacteria, plate tectonics constructed large Precambrian shields with sedimentary rocks along their margins.

In the Proterozoic, plate tectonics processes continue at a slower rate with cratons growing larger, new oceans basins & new mountains form.

We find the first evidence of radically different climates and glaciation. 

Life in the oceans begins to evolve.  First multi-cellular & soft-bodied organism to the appearance of animals with hard external skeletons.

Growth of Continents

Growth of landmass during this time by a process called continental accretion. Microcontinent are sutured to the craton as volcanic arcs collide during subduction.

Orogeny or mountain building thickens the crust and hardens unconsolidated materials. Regional and contact metamorphism results forming more greenstone belts with granitic and gneissic intrusions.

Assembly of North America

Between 2.0 and 1.85 bya, at least six small continents were accreted to the proto-North American continent and parts of Scandinavia and Baltica. This large continent was called Laurentia.  It is believed that a number of island arc terrains were suture together to form this large Proterozoic continent.

Growth of the Laurentia by Continental Accretion

Plate Tectonics 2.0 1.85 bya

Mid-Continental Rift of N. America

  • The Mid-Continent Rift began to form between 1.2 and 1 bya.

  • The rocks formed during this rift event include basalts, redbed (mudstones) and alluvial fan conglomerates & sandstones.

  • The rifting stopped sometime late in this time period therefore, it is referred to as a "failed rift zone"

Grenville Orogeny

The Grenville Orogeny was the last major accretion stage during the Proterozoic Rocks of the Grenville underlie much of the eastern US (including Ohio). They include sandstones, shales and carbonate rocks like those that form along a passive continental margin.

Assembly and breakup of Supercontinents

Between 1 bya and 550 mya there were several events of suturing and fragmentation of the continents.

Supercontinent Rodinia was formed as a result of Grenville orogeny and later attachment of Laurentia, southern Africa, Australia and Antarctica.

Global events between 2.5 and 1.0 bya

Early Proterozoic glaciation spreads over Canadian Shield about 2 bya and is represented by fine laminations or varved mudstone (Gowganda formation).

Rocks called tillite (lithified glacial sediments or till). Similar evidence of glaciation is found in Finland, southern Africa, and India.

Early Proterozoic life

Early Proterozoic life is represented by stromatolites (appearing about 2.5 by ago, gaining maximum diversity about 1.2 by ago). These deposits form when algal materials accumulate on the sea floor trapping calcite-rich sediments between the layers. Stromatolites are forming today off the western coast of Australia.

Atmospheric oxygen

Recall, atmospheric oxygen started to rise about 3.5 bya as photosynthetic prokaryotes began to release free oxygen.

Oxygen concentration in the ocean increased, allowing more banded iron formations (BIF's) to form.

By 1.9 ago the oceans began dissolving more atmospheric oxygen, oxygenating the deep ocean waters which ceased the formation of BIF's.
By the late Proterozoic, highly oxidized soils also form indicating an oxygen-rich atmosphere.

Life in the Proterozoic

Near the end of the proterozoic more modern forms of life appear.

Acritarch species (single-celled algae that later might have evolved into dinoflagellates). Stromatolites declined in abundance and diversity. Multicellular organism with soft body parts evolve.

However, a site in the Ediacara Hills, Australia provide evidence for a massive radiation of species occur at the end of the Proterozoic.

Ediacara Fauna and Evolution of animals

There are three major types of the fossils that bring our understanding of that era: trace fossils (there were not trace fossils in earlier times, the oldest trace fossils represent tubes made worm like species), soft body fossils (Cnidaran floating jellyfish and attached to the seafloor sea pens, preserved in sandy well-oxidized sea floor), and skeletal fossils (tube like microscopic Cloudina). The reason for the life evolution of this time is not quit clear.

 

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