Chapter 9 – The Proterozoic: Dawn of a More Modern World

1. Chapter 9 – The Proterozoic: Dawn of a More Modern World Highlights A number of large, distinct crustal segments called Precambrian provinces developed in North America. Called Laurentia Occurred along orogens, belts of crustal compression, mountain building, and metamorphism.

2. Laurentia Areas where these continents are joined together are called sutures. Happened about 1.7 – 1.8 billion years ago. Joshua Tree Pinto Gneiss is part of this orogen. Continents had fairly low relief (not a lot of mountains). Frequent epicontinental (broad, shallow) seas flooded the continents.

3. Laurentia Areas where these continents are joined together are called sutures. Happened about 1.7 – 1.8 billion years ago. Joshua Tree Pinto Gneiss is part of this orogen. Continents had fairly low relief (not a lot of mountains). Frequent epicontinental (broad, shallow) seas flooded the continents.

4. Rodina Begins to Form Older single large continent called Rodina begins. First evidence of “Wilson Cycle”. Opening of an ocean basin (divergence). Normal faulting Deep basin sedimentation with interlayered basalt Sedimentation along margins of separating continents Closing of ocean basin .

5. Wilson Cycle • Subduction of oceanic crust beneath continental crust • Trench sedimentation • Intense folding and reverse faulting. • An earlier version of formation of Appalachian Mountains during formation of Pangaea. .

6. Evidence of First Ice Age • Tillites – unsorted, angular lithified glacial debris. • Varved mudstones – laminated muds formed by differences in summer and winter ice melting. .

7. Has Day-Length Changed? • Shorter Proterozoic Day • 18.4 hour day! • Earth slowing at 2 seconds per 100,000 years • Lunar tides • Rhythmites • Alternating light/dark silty deposits produced by daily tidal deposits. • 900 million years ago .

8. Banded Iron Formation • A lot already discussed. • O2 in atmosphere. • Up to 1000 m thick and 100 km along strike! • Up to 80% of iron ore bodies. • Only found in old shield areas • Cyanobacteria - prokaryotes .

9. Aborted Mid-Continent Rift • Lake Superior region • Rich in copper and silver • “Black Smokers” • Developed 1.0 - 1.2 billion years ago • New Madrid EQ 1811-1812 .

10. Grenville Orogeny • 1.0 - 1.2 billion years ago • Extends from Adirondack Mtns. to central Mexico and California • Part of the building of Rodina • As Rodina breaks a new ocean forms • Proto-Pacific Ocean: Panthalassa Ocean • Proto-Atlantic Ocean begins closing up • Iapetus .

11. Snowball Earth Photosynthesis is using up CO2 and creating O2 in earth’s atmosphere. What’s going to happen next? Temperatures drop. The Big Chill Snowball Earth begins 716 million years before present.

12. Drop Stone

13. Snowball Earth

14. Snowball Earth Earth is frozen over to the very low latitudes. What color is earth? What is earth’s reflectivity? 0.1 albedo snow and ice vs. 0.8 albedo for oceans Will earth ever de-ice? Unlikely. Except earth is currently in an unfrozen state. What could have happened?

15. Snowball Earth Thaws Out Are volcanoes still erupting? What do they erupt? CO2? Why is CO2 important?

16. Snowball Earth Cycles Many freeze-thaw cycles where life is under extreme stress Grow - no grow

17. Snowball Earth Environmental Stress First multi-cellular life evolves. 570 million years before present China Preservation by phosphate seeped into and coated cells. Burrowing organisms. Change from fissile shales to siltstones that have no layering. What causes this?

18. Ediacaran Fauna First important discovery of large metazoans Australia Large, snail-sized metazoa Appear to contain coelum (body cavity) Digestive tract and other organs Snails? 630 to 580 million years ago Ancestors of Paleozoic invertebrates O2 up to 1%!

19. End of Banded Iron Formation • Banded Iron Formation e .