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Chapter 9

Chapter 9. Precambrian Earth and Life History—The Proterozoic Eon. 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. How long was the Proterozoic?.

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Chapter 9

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  1. Chapter 9 Precambrian Earth and Life History—The Proterozoic Eon

  2. 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

  3. How long was the Proterozoic? • 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

  4. How is the Archean-Proterozoic Boundary defined? • arbitrarily placed • the Archean-Proterozoic boundary • at 2.5 billion years ago • because it marks the approximate time • of changes in the style of crustal evolution

  5. Different Style of Crustal Evolution? • Archean crust-forming processes generated • granite-gneiss complexes • and greenstone belts • that were shaped into cratons • same rock associations • continued during the Proterozoic, • BUT at a considerably reduced rate

  6. Contrasting Metamorphism? • Unlike Archean rocks, vast exposures of Proterozoic rocks show • little or no effects of metamorphism, • and in many areas they are separated • from Archean rocks by a nonconformity

  7. Other Differences with Archean rocks? • the Proterozoic is characterized • by widespread rock assemblages • that are rare or absent in the Archean, • by a plate tectonic style essentially the same as that of the present • by important evolution of the atmosphere and biosphere • by the origin of some important mineral resources

  8. Proterozoic Evolution of Oxygen-Dependent Organisms! • During the Proterozoic • oxygen-dependent organisms • made their appearance • and the first cells evolved

  9. When did Continents evolve? • Proterozoic accretion at Archean island arcs and minicontinents margins thereby forming much larger landmasses

  10. Proterozoic Greenstone Belts • Most greenstone belts formed • during the Archean • between 2.7 and 2.5 billion years ago • not as common after the Archean, • and differed in one important detail • the near absence of ultramafic rocks • which resulted from Earth's decreasing amount of radiogenic heat

  11. What is 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

  12. When and how did Laurentia come into existence? • originated and underwent important growth • between 2.0 and 1.8 billion years ago • collisions among various plates formed several orogens • linear or arcuate deformation belts in which rocks have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths

  13. Proterozoic Evolution of Laurentia • Archean cratons were sutured • along deformation belts called orogens, • thereby forming a larger landmass • 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

  14. 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

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

  16. 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

  17. Early Proterozoic Rocks in Great Lakes Region • Early Proterozoic sandstone-carbonate-shale assemblages are widespread near the Great Lakes

  18. Outcrop of Sturgeon Quartzite • The sandstones have a variety of sedimentary structures • such as • ripple marks • and cross-beds • Northern Michigan

  19. Outcrop of Kona Dolomite • Some of the carbonate rocks, now mostly dolostone, • such as the Kona Dolomite, • contain abundant bulbous structures known as stromatolites • NorthernMichigan

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

  21. When did the southern portion of the continent accrete? • From 1.8 to 1.6 billion years ago, • as successively younger belts were sutured to Laurentia, • forming the Yavapai and Mazatzal-Pecos orogens

  22. 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

  23. What else happened during the Proterozoic? Deposition of most of Earth’s banded iron formations (BIF) • First deposition of continental red beds at ~ 1.8 billion years ago • sandstone and shale with oxidized iron • excellent evidence for widespread glaciation

  24. Other events? • Extensive igneous activity • from 1.8 to 1.1 billion years ago unrelated to orogenic activity • Although quite widespread, • this activity did not add to Laurentia’s size • because magma was either intruded into • or erupted onto already existing continental crust

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

  26. Cause of Igneous Activity? • According to one hypothesis • large-scale upwelling of magma • beneath a Proterozoic supercontinent • produced the rocks

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

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

  29. With what was the Grenville Orogeny associated? • 1) closure of an ocean basin • the final stage in a Wilson cycle • 2) intracontinental deformation or major shearing • Whatever the cause, • it was the final Proterozoic stage of Laurentia continental accretion

  30. How much of North American continent was in existence by the end of the Proterozoic? • about 75% of present-day North America existed • The remaining 25% • accreted during the Phanerozoic Eon

  31. What’s the Midcontinent Rift? • Grenville-age extension, volcanism and sedimentation in Laurentia 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

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

  33. Midcontinental Rift • Most of the rift is buried • except in the Lake Superior region • various igneous and sedimentary rocks are well exposed • The central part of the rift contains • numerous overlapping basalt lava flows • forming a volcanic pile several kilometers thick

  34. Midcontinental Rift • Along the rift's margins • coarse-grained sediments were deposited • in large alluvial fans • that grade into sandstone and shale • with increasing distance • from the sediment source • In the vertical section • Freda Sandstone overlies • Cooper Harbor conglomerate, • which overlies Portage Lake Volcanics

  35. Cooper Harbor Conglomerate Michigan

  36. Portage Lake Volcanics Michigan

  37. Middle and Late Proterozoic Sedimentation • sediment deposition in what is now • the eastern United States and Canada, • in the Death Valley region of California and Nevada, • and in three huge basins in the west

  38. Sedimentary Basins in the West • Map showing the locations of sedimentary Basins • in the western United States and Canada • Belt Basin • Uinta Basin • Apache Basin

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

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

  41. Proterozoic Sedimentary Rocks • Proterozoic rocks • of the Grand Canyon Super-group lie • unconformably upon Archean rocks The rocks consist mostly • of sandstone, shale, and dolostone, • deposited in shallow-water marine • and fluvial environments • The presence of stromatolites and carbonaceous • impression of algae in some of these rocks • also indicate probable marine deposition

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

  43. When did the current style of Style of Plate Tectonics come into play? • almost certainly by the Early Proterozoic • the oldest complete ophiolite is the Jormua mafic-ultramafic complex in Finland • It is about 1.96 billion years old, • but nevertheless compares closely in detail • with younger well-documented ophiolites

  44. Jormua Complex, Finland • Reconstruction • of the highly deformed • Jormua mafic-ultramafic complex • in Finland • This sequence of rock • is the oldest known complete ophiolite • at 1.96 billion years old

  45. Jormua Complex, Finland • Metamorphosed basaltic pillow lava 12 cm

  46. Jormua Complex, Finland • Metamorphosed gabbro between mafic dikes 65 cm

  47. Proterozoic Supercontinents? • A supercontinent consists of all • or at least much of the present-day continents, • The supercontinent Pangaea, • existed at the end of the Paleozoic Era,

  48. Pre-Pangean Supercontinents? • Supercontinents may have existed • as early as the Late Archean, • but if so we have little evidence of them • The first that geologists recognize • with some certainty, known as Rodinia • assembled between 1.3 and 1.0 billion years ago • and then began fragmenting 750 million years ago

  49. How did Rodinia look? • Possible configuration • of the Late Proterozoic supercontinent Rodinia • before it began fragmenting about 750 million years ago

  50. Pannotia: The next supercontinent • Rodinia's separate pieces reassembled • and formed Pannotia • about 650 million years ago • Fragmentated • by the latest Proterozoic, about 550 million years ago,

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