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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,