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The Proterozoic

The Proterozoic. Sub Divisions . Neoproterozoic 1.0-0.544 b.y Mesoproterozoic 1.6- 1b.y. Paleoproterozoic 2.5-1.6 b.y. Laurentia: Proterozoic N. America. Six Elements Superior Slave Hearne Rae Wyoming Nain. Laurentia: Proterozoic N. America.

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The Proterozoic

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  1. The Proterozoic

  2. Sub Divisions • Neoproterozoic • 1.0-0.544 b.y • Mesoproterozoic • 1.6- 1b.y. • Paleoproterozoic • 2.5-1.6 b.y.

  3. Laurentia: Proterozoic N. America • Six Elements • Superior • Slave • Hearne • Rae • Wyoming • Nain

  4. Laurentia: Proterozoic N. America • During Proterozoic the six elements (or provinces) are welded together during orogenic episodes • This period of continental welding ended by 1.7bya • Extensive accretion of continental margin (2.3-0.9bya)

  5. Plate tectonics in the Proterozoic • Rapid lateral motion • Vigorous subduction/orogeny • Extensive Rifting • Rapid sea-floor spreading

  6. Paleoproterozic Events • Wopmay Orogeny • Western Margin of the Slave province • Evidence of opening of an ocean basin • Sedimentation along the new continental margin • Closure of the ocean basin Wilson Cycle

  7. Depositional Zones • Eastern zone of ocean basin (next to Slave) • Coastal Region • Quartz sandstone • Shallow marine environment • Over time Sandstone metamorphosed to Quartzite. Also contained massive stromatolites(now dolomitic) • This is the Rocknest Formation • Western Zone • Continental Shelf deposits • Deep water turbidites

  8. Evidence of Wilson Cycle • Numerous tensional faults • Alluvial Fan and Fluvial deposits • As the ocean widened the western edge of the province became passive margin with two parallel zones of deposition

  9. Closure of the ocean basin • As westward edge of Slave subducted the continental shelf buckled downward. • As depth increased: • Carbonate deposition ended • Accumulation of deep water clastics began • Subsequent collisional forces caused additional folding and faulting.

  10. Aulacogens • Aulacogens are inactive rifts of a radiating three-rift system that develops over areas of crust that are being uplifted

  11. Forming an Aulacogen • As the crust begins to bulge it breaks to form the three rift zones

  12. Forming an Aulacogen • Two arms fill with water to form oceans • The third, failed, arm trends inland

  13. Examples Can you identify any modern Aulacogens on the world map?

  14. Trans-Hudson Orogeny • N and W side of Superior Province • Records Wilson Cycle • Joins Hearne and Wyoming • Severe folding and metamorphism during closure of wilson cycle welded the Superior plate to the Wyoming and Hearne

  15. Paleoproterozoic Ice Age • Superior Province 2.6-2.1 by • Gowgonda Formation • Notable for conglomerates and laminated mudstones • Laminations represent repeated summer and winter sediment layers (called varves) • In Gowgonda Formation varves alternate with tillites • Formation lies above 2.6 by crystalline rock and is intruded by 2.1 by igneous rocks

  16. Labrador Trough • Curved track on east side of Superior Province • Records yet another Wilson Cycle • During closing phase eastern zone underwent intense folding, thrust faulting, and metamorphism- known as Hudsonian Orogeny

  17. Mesoproterozoic • Hudsonian Orogeny serves as the event that separates paleoproterozoic from mesoproterozoic

  18. Keweenawan Sequence • Rocks rest on crystalline or animikian strata • Extend for hundreds of kilometers from Lake Superior region • Consist of Quartz sandstone, arkoses, conglomerates, as well as basaltic volcanics. • The lava flows are well known for their content of native copper.

  19. Keweenawan Sequence • In some areas lava flows accumulated to a thickness of several kilometers. • Although this seems like a lot of lava, most of the magma remained underground forming the Duluth Gabbro(12kmX160km) • What is most basaltic magma/lava associated with? • Large amounts of mafic magma typically signals the presence of a rift • Had the rift continued, the eastern US would have drifted away

  20. The Grenville Province • Exposures extend from Labrador to Lake Huron. However, region actually extends down eastern US and westward into Texas. • Consist of carbonates and sandstones that have been deformed, metamorphosed, and intruded.

  21. The Grenville Province • Deformation occurred 1.2-1.0 bya during the Grenville Orogeny.(the beginnings of the Appalachians) • Only one part of a series of continental collisions involved in the formation ofthe supercontinentRodinia

  22. Rodinia

  23. Neoproterozoic • Rodinia was assembled by 1.0 bya, however, by 750mya it began to split apart • During this time the proto-pacific ocean (Panthalassa) was created west of N. America

  24. Proterozoic Ice House • Neoproterozoic rocks on all continents, except India and Siberia, show glacial striations. • Tillites and varves can also be found • Evidence is so prevalent, geologist refer to our planet during this time as “Snowball Earth” • Formally this time is called the Varangian Glaciation

  25. Rodinia

  26. What could have cause this? • Plate tectonics • Most land masses located at low or mid lattitudes • Land is much more reflective than water • As glaciers formed land became even more reflective • Atmospheric • Loss of Carbon Dioxide • During this time fungi, lichens and vascular plants were present.

  27. Sediment Deposition • During this period extensive layers of limestone were deposited. If Limestones are typically associated with relatively warm seas how could this be possible?

  28. The Fossil Record • Inherited Archean Life • Photosynthetic cyanobacteria (oxygenated) • anaerobic prokaryotes • Stromatolites • molecular fossils

  29. Heliotropic Stromatolites • Sine-wave growth form=yearly cycles • Laminar structure=daily growth phase • Bitten Springs Fm., Australia (850 my) • Anabaria juvensis (Stromatolite) • 435 laminae/cycle=435 days/year

  30. Eukaryotes • Record of large cells: 1.6-1.4 b.y to present • Steranes: molecular fossils • Acritarchs (Planktonic Algae; Skiagia) • Max age 1.6 b.y. • Fossil sites; Russia, California, Australia • Eukaryotic Protozoans: Vase-shaped fossils from Spitsbergen and Arizona • Metazoans • Australia: Ediacara Hills • China: Doushantuo Formation (~570 m.y.) • Russia: White Sea coast (~550 m.y.) • Some may not have modern equivalents

  31. Body and trace fossil Metazoans • Cloudina: 3cm, tube dwelling worm • Ediacaran trace fossils • Cyclomedusa: discoidal (Jellyfish ?) • Tribrachidium: circular (unknown) • Charniodiscus: Frond shape (Sea pen) • Dicksonia: ovate-shape (Flatworm) • Spriggia: Elongate (Annelid)

  32. Ediacaran

  33. Fossil Expansions • Mesoproterozoic (1.4 b.y. ago) Eukaryotes • Neoproterozoic (0.6 b.y. ago) Metazoans • Proterozoic Prokaryotes (Gunflint Fossils, 1.9b.y.) • Gunflintia: unbranched filament (algae) • Animikiea: fine filament (algae) • Eostrion: Dawnstar (bacterium) • kakabekia: plumose form (unknown)

  34. Changing Environment • Free Oxygen begins to Accumulate in atmosphere (2.0 b.y. ago) • Oxygen increases sporadically as CO2 level decrease (1.0 b.y.) • Oxygen rises to 3-10% of current level (0.6 b.y. ago) • Wide variety of climates • Hot, dry (evaporates) • Tropical (algae reefs) • Ice Ages (glacial deposits) • Warm, tropical (Carbonates)

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