Chapter 9

1. Chapter 9 The Proterozoic: Dawn of a More Modern World

2. Proterozoic Eon • 2.5 billion years to 542 million years ago • Comprises 42% of Earth history • Divided into three eras: • Paleoproterozoic Era (2.5 - 1.6 by ago) • Mesoproterozoic Era (1.6 to 1.0 by ago) • NeoproterozoicEra (1.0 by ago to the beginning of the Paleozoic, 542 my ago)

3. The Beginning of the Proterozoic Marks the Beginning of: • More modern style of plate tectonics • More modern style of sedimentation • More modern global climate with glaciations • Establishment of the beginnings of an oxygen-rich atmosphere • Emergence of eukaryotes

4. Precambrian Provinces in North America Precambrian provinces were welded (or sutured) together to form a large continent called Laurentia during the early Proterozoic.

5. Precambrian Provinces in North America Oldest (Archean) rocks are shown in orange. Younger (Proterozoic) rocks are shown in green.

6. Precambrian Provinces in North America • Suturing occurred along mountain belts or orogens. • Provinces were assembled by about 1.7 b.y. ago. • Laurentia continued to grow by accretion throughout the Proterozoic.

7. Proterozoic Plate Tectonics Late in the Proterozoic, the continents became assembled into a supercontinent called Rodinia.

8. Proterozoic Sedimentation Sedimentation on and around the craton consisted of shallow water clastic and carbonate sedimentsdeposited on broad continental shelves and in epicontinental seas.

9. Proterozoic Climate • Proterozoic glaciations occurred during the: • Paleoproterozoic, about 2.4-2.3 b.y. ago (Huronian glaciation) • Neoproterozoic, 850-600 m.y. ago (Varangian glaciation)

10. Overview of the Precambrian

11. Overview of Proterozoic Events

12. Paleoproterozoic Era • The oldest part of the Proterozoic • Ranges from about 2.5 b.y. to 1.6 b.y. • Covers 900 million years

13. Major Events of the Paleoproterozoic • Active plate tectonics • Major mountain building on all major continents • Earth's first glaciation • Widespread volcanism (continental flood basalts) • Rise in atmospheric oxygen (great oxidation event)

14. Major Events of the Paleoproterozoic • Accumulation of high concentrations of organic matter in sediments (Shunga event) 2000 m.y. ago, and generation of petroleum • Oldest known phosphorites and phosphate concretions

15. Orogenic belts developed around margins of the Archean provinces. • Wopmay belt in NW Canada • Trans-Hudson belt, SW of Hudson Bay

16. Wopmay orogenic belt contains evidence of: • Rifting and opening of an ocean basin (with normal faults, continental sediments, and lava flows) • Sedimentation along new continental margins (with shallow marine quartz sandstones and carbonate deposition) • Closure of the ocean basin (with deep water clastics overlain by deltaic and fluvial sands), followed by folding and faulting.

17. Wilson Cycle This sequence of events in the Wopmay orogenic belt is called a Wilson Cycle, and is a result of plate tectonics. • Rifting and opening of an ocean basin • Sedimentation along new continental margins • Closure of the ocean basin The sequence of events in the Wopmay belt is similar to that in the Paleozoic of the Appalachians.

18. Trans-Hudson orogenic belt Trans-Hudson belt contains the sedimentary record of a Wilson Cycle, with evidence of: • Rifting • Opening of an ocean basin • Deposition of sediment • Closure of the ocean basin along a subduction zone, associated with folding, metamorphism, and igneous intrusions. This closure welded the Superior province to the Hearne and Wyoming provinces to the west.

19. Paleoproterozoic Glaciation - Earth's First Ice Age? • A Paleoproterozoic ice age is recorded in rocks north of Lake Huron in southern Canada (called the Huronian glaciation). • Gowganda Formation. • Age of Huronian glaciation = 2450-2220 m.y. • Apparent rapid onset of global glaciations from what had been relatively stable climatic conditions.

20. Evidence for glaciation includes: • Mudstones with laminations or varves - fine laminations indicating seasonal deposition in lakes adjacent to ice sheets. • Glacial dropstones (dropped from melting icebergs) in varved sedimentary rocks. • Tillites or glacial diamictites (poorly sorted conglomerates of glacial debris). • Scratched and faceted cobbles and boulders in tillite, due to abrasion as ice moved.

21. Widespread Glaciation • Age of global glaciations = 2.6 - 2.1 b.y. ago (2600-2100 m.y.). • Widespread glaciation at this time as indicated by glacial deposits found in: • Europe • southern Africa • India

22. Banded iron formations and prokaryote fossils Extensive banded iron formations (BIF's) on the western shores of Lake Superior, indicate that photosynthesis was occurring and oxygen was being produced.

23. Banded iron formations and prokaryote fossils • Some BIF deposits are >1000 m thick, and extend over 100 km. • Animikie Group. • Rich iron deposits were foundation of steel industry in Great Lakes region (Illinois, Indiana, Ohio, Pennsylvania). • Mining has declined because U.S. imports most of its iron ore and steel.

24. Banded iron formations and prokaryote fossils The Gunflint Chert, within the BIF sequence, contains fossil remains of prokaryotic organisms, including cyanobacteria.Age = 1.9 b.y.

25. Labrador Trough • East of the Superior province are rocks deposited on a continental shelf, slope, and rise. • Rocks are similar to those of the Wopmay orogenic belt. • These rocks were folded, metamorphosed, and thrust-faulted during the Hudsonian orogeny, which separates the Paleoproterozoic from the Mesoproterozoic.

26. Mesoproterozoic Era The Mesoproterozoic (or middle Proterozoic) ranges from about 1.6 b.y. - 1.0 b.y.

27. Highlights of the Mesoproterozoic • The Midcontinent rift, an abandoned oceanic rift in the Lake Superior region with massive basaltic lava flows • Copper mineralization in the Lake Superior region • Continental collisions producing the Grenville orogeny in eastern North America • The assembly of continents to form the supercontinent, Rodinia.

28. Midcontinent Rift and the Keweenawan Sequence • Midcontinent rift extends southward from Lake Superior region. • Overlies Archean crystalline basement rocks and Paleoproterozoic Animikian rocks (Animikie Group BIF).

29. Midcontinent Rift and the Keweenawan Sequence • Large volumes of basaltic rock indicate presence of an old abandoned rift zone called the Midcontinent rift. • This was the first stage of a Wilson Cycle. • Rift developed 1.2 b.y. - 1.0 b.y. ago. • Extended from Lake Superior to Kansas. • Rifting ceased before the rift reached the edge of the craton, or the eastern U.S. would have drifted away from the rest of North America.

30. Midcontinent Rift and the Keweenawan Sequence The Keweenawan Sequence consists of: • Clean quartz sandstones • Arkoses • Conglomerates • Basaltic lava flows more than 25,000 ft thick (nearly 5 mi) with native copper • Basaltic rock beneath the surface crystallized as the Duluth Gabbro, 8 mi thick and 100 mi wide.

31. Copper Mineralization • Native copper fills vesicles (gas bubbles) in the Keweenawan basalt, and joints and pore spaces in associated conglomerates. • Native Americans mined the copper as early as 3000 BC. • Copper was mined extensively from 1850 to 1950, but copper production ceased in the 1970's.

32. Grenville Province and Grenville Orogeny The Grenville province in eastern North America extends from northeastern Canada to Texas.

33. Grenville Province and Grenville Orogeny • Grenville rocks were originally sandstones and carbonate rocks. • Grenville Province was the last Precambrian province to experience a major orogeny. • Grenville orogeny = 1.2 b.y. to 1.0 b.y. ago

34. Grenville Province and Grenville Orogeny • Orogeny occurred when Eastern North America (Laurentia) collided with western South America (Amazonia). • Orogeny was associated with formation of the supercontinent, Rodinia. • Later, during the Paleozoic Era, Grenville rocks were metamorphosed and intruded during the three orogenies involved in the building of the Appalachians.

35. The Supercontinent, Rodinia The supercontinent, Rodinia, as it appeared about 1.1 b.y. ago.The reddish band down the center of the globe is the location of continental collisions and orogeny, including the Grenville orogeny.

36. The Supercontinent, Rodinia • Rodinia formed as the continents collided during the Grenville Orogeny. • Rodinia persisted as a supercontinent for about 350 million years. • It was surrounded by an ocean called Mirovia.

37. Rifting in Rodinia Rodinia began to rift and break up about 750 million years ago, forming the proto-Pacific Ocean, Panthalassa, along the western side of North America.

38. Rifting in Rodinia An early failed attempt at rifting began in eastern North America about 760 m.y. ago, with the deposition of sediments of the Mount Rogers Formation in a fault-bounded rift valley. Felsic and mafic volcanic rocks are interlayered with the sedimentary rocks of the Mount Rogers Formation.

39. Neoproterozoic Era The Neoproterozoic (or “new” Proterozoic) ranges from about 1.0 b.y. to 0.542 b.y. (542 m.y.).

40. Highlights of the Neoproterozoic • Extensive continental glaciations • Sediments deposited in basins and shelf areas along the eastern edge of the North American craton. • Most of these rocks were deformed during the Paleozoic orogenies.

41. Glacial deposits in the Neoproterozoic • Glacial deposits formed roughly 600 - 700 m.y. ago. • Evidence for glaciation: • Glacial striations (scratched and grooved pebbles and boulders) • Tillites (lithified, unsorted conglomerates and boulder beds) found nearly worldwide • Glacial dropstones(chunks of rocks released from melting icebergs) • Varved clays from glacial lakes

42. Rifting in Rodinia Around 570 million years ago, rifting began again, and South America began to separate from North America, forming the Iapetus Ocean (or proto-Atlantic Ocean). The rift ran along what is now the Blue Ridge province. Basaltic lava flows formed the Catoctin Formation. As the Iapetus Ocean opened, sands and silts were deposited in the shelf areas.

43. Glacial deposits in the Neoproterozoic • This time is referred to as "snowball Earth“ because glacial deposits are so widespread. • Varangian glaciation (named after an area in Norway). • The late Proterozoic ice age lasted about 240 m.y.

44. Plate Tectonics and Glaciation • Plate tectonics may have had a role in cooling the planet. • Continents were located around the equator about 600 to 700 m.y. ago. • No tropical ocean.

45. Plate Tectonics and Glaciation • Heat lost by reflection from the rocks on the surface of the continents may have caused global cooling. (Land plants had not yet appeared.) • As continental glaciers and ice caps formed, reflectivity of snow and ice caused further temperature decrease.

46. Atmospheric Gases and Glaciation • Glaciation was associated with: • Decrease in CO2 and • Increase in O2. • CO2 causes the greenhouse effect and global warming. Decrease in CO2 may have caused cooling. • Decrease in CO2 was probably caused by increase in the number of photosynthetic organisms (cyanobacteria, stromatolites).

47. Limestones and Glaciations • Limestones are associated with glacial deposits, which is unusual, since limestones generally form in warm seas, not cold ones. • Association of limestones with glacial deposits suggests that times of photosynthesis and CO2 removal alternated with times of glaciation. • Limestones (made of CaCO3) are a storehouse of CO2, which was removed from the atmosphere.

48. Limestones and Glaciations • Glacial conditions may have inhibited photosynthesis by stromatolites. • As a result, CO2 may have accumulated periodically and triggered short episodes of global warming. • This produces the paradox of glaciers causing their own destruction.

49. Proterozoic Rocks South of the Canadian Shield Extensive outcrops of Precambrian rocks are present in the Canadian Shield. Precambrian rocks are also present in other areas, including: • Rocky Mountains • Colorado Plateau (Grand Canyon)

50. Events Recorded in Proterozoic Rocks • Collision of an Archean terrane with volcanic island arc, 1.7 or 1.8 b.y.a. (Wyoming and western Colorado) • Extensive magma intrusion in Mesoproterozoic, 1.5-1.4 b.y.a. (California to Labrador) • Widespread rifting • Rifts with thick sequences of shallow water Neoproterozoic sedimentary rocks, 1.4 - 0.85 b.y.a. Belt Supergroup (Glacier National Park, Montana, Idaho, and British Columbia).