The Length of the Proterozoic

1. The Length of the Proterozoic • the Proterozoic Eon alone, • at 1.955 billion years long, • accounts for 42.5% of all geologic time • yet we review this long episode of Earth and life history in a single section

2. The Phanerozoic • Yet the Phanerozoic, • consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, • lasted a comparatively brief 545 million years • is the subject of the rest of the course

3. The Proterozoic Eon:Top Ten Significant Events • Plate tectonics occurred similar to modern rates • Accretion at continental boundaries • Assembly of Laurentia; and two super continents • Widespread sandstone, carbonate, shale deposits (continental shelf deposits) • Extensive continental glaciation • Mid-continent rift formed in North America • Widespread occurrence of stromatolites • Formation of banded iron and other mineral resources (gold, copper, platinum, nickel) • Free oxygen in atmosphere • Evolution of eukaryotic cells

4. Style of Crustal Evolution • Archean crust-forming processes generated • granite-gneiss complexes • and greenstone belts • that were shaped into cratons • During the Proterozoic, these formed at a considerably reduced rate and cooler temperatures

5. Contrasting Metamorphism • Many Archean rocks have been metamorphosed, • However, vast exposures of Proterozoic rocks • show little or no effects of metamorphism, • and in many areas they are separated • from Archean rocks by a profound unconformity

6. Evolution of Proterozoic Continents • Archeancratons assembled during collisions of island arcs and minicontinents, • providing the center of today’s continents. • Proterozoic crust accreted, at edges forming much larger landmasses • Proterozoic accretion at craton margins • probably took place more rapidly than today • Earth possessed more radiogenic heat, • but the process continues even now

7. ProterozoicGreenstone Belts • They were not as common after the Archean, • near absence of ultramafic rocks • WHY would this happen?

8. Focus on Laurentia • Our focus here is on the geologic evolution of Laurentia, • a large landmass that consisted of what is now • North America, • Greenland, • parts of northwestern Scotland, • and perhaps some of the Baltic shield of Scandinavia

9. Early Proterozoic History of Laurentia • Laurentia originated ~ 2.0 billion years ago • collisions called orogens formed linear or arcuate deformation belts • in which many of the rocks have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths

10. Proterozoic Evolution of Laurentia • Archean cratons were sutured • along deformation belts called orogens, • 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

11. What is the evidence?Craton-Forming Processes • Recorded in rocks • In northwestern Canada • where the Slave and Rae cratons collided

12. Craton-Forming Processes • the Trans Hudson orogen • in Canada and the United States, • where the Superior, Hearne, and Wyoming cratons • were sutured • The southern margin of Laurentia • is the site of the Penokian orogen

13. Wilson Cycle • Rocks of the Wopmay orogen • in northwestern Canada are important • because they record the Wilson cycle • opening and closing of an ocean basin • A complete Wilson cycle, • named for the Canadian geologist J. Tuzo Wilson, • involves • fragmentation of a continent (rifting) • opening of an ocean basin • followed by closing of an ocean basin, • and finally reassembly of the continent

14. Wopmay Orogen • Some of the rocks in Wopmay orogen • are sandstone-carbonate-shale assemblages, • a suite of rocks typical of passive continental margins • that first become widespread during the Proterozoic

15. Early Proterozoic Rocks in Great Lakes Region: Evidence of continental shelf • Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

16. Where? N. MichiganOutcrop of Sturgeon Quartzite • The sandstones have a variety of sedimentary structures • such as • ripple marks • and cross-beds • Northern Michigan

17. Outcrop of Kona Dolomite“warm shallow marine” • Some of the carbonate rocks, now mostly dolostone, • such as the Kona Dolomite, • contain abundant bulbous structures known as stromatolites • NorthernMichigan

18. Penokean Orogen • These rocks of northern Michigan • have been only moderately deformed • and are now part of the Penokean orogen

19. Southern Margin Accretion • Laurentia grew along its southern margin • by accretion of the Central Plains, Yavapai, and Mazatzal orogens • Also notice that the Midcontinental Rift • had formed in the Great Lakes region by this time

20. BIF, Red Beds, Glaciers • This was also the time during which • most of Earth’s banded iron formations (BIF) • were deposited • The first continental red beds • sandstone and shale with oxidized iron • were deposited about 1.8 billion years ago • We will have more to say about BIF • and red beds in the section on “The Evolving Atmosphere” • In addition, some Early Proterozoic rocks • provide excellent evidence for widespread glaciation

21. Proterozoic Igneous Activity • These igneous rocks are exposed • in eastern Canada, extend across Greenland, • and are also found in the Baltic shield of Scandinavia

22. Igneous ActivityWhy? How do we know? • However, the igneous rocks are deeply buried • by younger rocks in most areas • The origin of these • are the subject of debate • According to one hypothesis • large-scale upwelling of magma • beneath a Proterozoic supercontinent • produced the rocks

23. Middle Proterozoic Orogeny and Rifting • The only Middle Proterozoic event in Laurentia • was the Grenville orogeny • in the eastern part of the continent • 1.3 to 1.0 billion years old • Grenville rocks are well exposed • in the present-day northern Appalachian Mountains • as well as in eastern Canada, Greenland, and Scandinavia

24. Grenville Orogeny • A final episode of Proterozoic accretion • occurred during the Grenville orogeny

25. 75% of 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

26. Midcontinent Rift • Grenville deformation in Laurentia • was accompanied by the origin • of the Midcontinent rift, • a long narrow continental trough bounded by faults, • extending from the Lake Superior basin southwest into Kansas, • and a southeasterly branch extends through Michigan into Ohio • It cuts through Archean and Early Proterozoic rocks • and terminates in the east against rocks • of the Grenville orogen

27. Location of the Midcontinent Rift • Rocks filling the rift • are exposed around Lake Superior • but are deeply buried elsewhere

28. Midcontinental Rift • Most of the rift is buried beneath younger rocks • except in the Lake Superior region • with various igneous and sedimentary rocks exposed • The Evidence: • numerous overlapping basalt lava flows • forming a volcanic pile several kilometers thick

29. Portage Lake Volcanics Michigan

30. Sedimentary Rocks • Middle to Late Proterozoic sedimentary rocks • are exceptionally well exposed • in the northern Rocky Mountains • of Montana and Alberta, Canada • Glacier National Park

31. Proterozoic Rocks, Glacier NP • Proterozoic sedimentary rocks • in Glacier National Park, Montana • The angular peaks, ridges and broad valleys • were carved by Pleistocene and Recent glaciers

32. Proterozoic Mudrock • Outcrop of red mudrock in Glacier National Park, Montana

33. Proterozoic Limestone • Outcrop of limestone with stromatolites in Glacier National Park, Montana

34. Grand Canyon Super-group • Proterozoic Sandstone of the Grand Canyon Super-group in the Grand Canyon Arizona

35. Proterozoic Supercontinents • A supercontinent consists of all • Or much of the present-day continents, • so other than size it is the same as a continent • The supercontinent Pangaea, • existed MUCH LATER but few people are aware of earlier supercontinents

36. Early Supercontinents • Rodinia • assembled between 1.3 and 1.0 billion years ago • and then began fragmenting (rifting apart) 750 million years ago (THE Proterozoic ends at 545my ago)

37. Early Supercontinent • Possible configuration • of the Late Proterozoic supercontinent Rodinia • before it began fragmenting about 750 million years ago

38. Rodinia's separate pieces reassembled • and formed another supercontinent • Pannotia • about 650 million years ago • Fragmentation was underway again, • about 550 million years ago, • giving rise to the continental configuration • that existed at the onset of the Phanerozoic Eon – the Cambrian

39. Recognizing Glaciation • How can we be sure that there were Proterozoic glaciers? • the extensive geographic distribution • of other conglomerates and tillites • and their associated glacial features • is distinctive, • such as striated and polished bedrock

40. Proterozoic Glacial Evidence • Bagganjarga Tillite in Norway Over bedrock

41. Geologists Convinced • The occurrence of tillites • in Michigan, Wyoming, and Quebec • indicates that North America may have had • an Early Proterozoic ice sheet centered southwest of Hudson Bay

42. Early Proterozoic Glaciers • Deposits in North America • indicate that Laurentia • had an extensive ice sheet • centered southwest of Hudson Bay

43. Late Proterozoic Glaciers • The approximate distribution of Late Proterozoic glaciers

44. Late Proterozoic glaciers • seem to have been present even • in near-equatorial areas!! • Geologists have recently named this phenomenon • “SNOWBALL EARTH”

45. The Evolving Atmosphere • Archean: little or no free oxygen • the amount present • at the beginning of the Proterozoic was probably no more than 1% of that present now • Stromatolites—not common until: • 2.3 billion years ago, • that is, during the Early Proterozoic • There is evidence of increasing oxygen….

46. Early Proterozoic Banded Iron Formation • At this outcrop in Ishpeming, Michigan • the rocks are alternating layers of • red chert • and silver-colorediron minerals

47. Banded Iron Formations (BIF) • Banded iron formations (BIFs), • consist of alternating layers of • iron-rich minerals • and chert • about 92% of all BIFs • formed during the interval • from 2.5 to 2.0 billion years ago

48. BIFs and the Atmosphere • How are these rocks related to the atmosphere? • Their iron is in iron oxides, especially • hematite (Fe2O3) • and magnetite (Fe3O4) • Iron combines with oxygen in an oxidizing atmosphere • to from rustlike oxides • that are not readily soluble in water • If oxygen is absent in the atmosphere, though, • iron easily dissolves • so that large quantities accumulate in the world's oceans, • which it undoubtedly did during the Archean

49. Formation of BIFs • The Archean atmosphere was deficient in free oxygen • so that little oxygen was dissolved in seawater • However, as photosynthesizing organisms • increased in abundance, • as indicated by stromatolites, • free oxygen, • released as a metabolic waste product into the oceans, • caused the precipitation of iron oxides along with silica • and thus created BIFs

50. Formation of BIFs • Depositional model for the origin of banded iron formation