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Chapter 9

Chapter 9. Precambrian Earth and Life History—The Proterozoic Eon. Proterozoic Rocks, Glacier NP. Proterozoic rocks of the Belt Supergroup, Glacier National Park, Montana were deposited as sediments in marine and terrestrial environments

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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 rocks • of the Belt Supergroup, Glacier National Park, Montana • were deposited as sediments • in marine and terrestrial environments • and deformed during the Cretaceous to Early Cenozoic Laramide orogeny

  3. The Length of the Proterozoic • The Proterozoic Eon alone, • at 1.958 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 chapter

  4. The Phanerozoic • The Phanerozoic, • consisting of • Paleozoic, • Mesozoic, • Cenozoic eras, • lasted a comparatively brief 542 million years • but is the subject of 10 chapters!

  5. Disparity in Time • Precambrian rocks are deeply buried • in many areas. • Many have been metamorphosed and complexly deformed. • The Proterozoic Eon is subdivided • into three eras • with prefixes Paleo, Meso,and Neo • which are strictly terms denoting time

  6. Proterozoic Rocks • The Vishnu schist in the Grand Canyon • was originally lava flows and sedimentary rocks, but • was intruded by the Zoraster Granite • 1.7 billion years ago

  7. Proterozoic Rocks • The outcrop of sandstone and mudstone • 1.0 billion years old • has only been slightly altered by metamorphism

  8. Archean-Proterozoic Boundary • Geologists have rather 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 • However, we must emphasize "approximate," • because Archean-type crustal evolution • was not completed at the same time • in all areas

  9. Style of Crustal Evolution • Archean crust-forming processes generated • granite-gneiss complexes • and greenstone belts • that were shaped into cratons • Although these same rock associations • continued to form during the Proterozoic, • they did so at a considerably reduced rate

  10. Archean vs. Proterozoic • Many Archean rocks have been metamorphosed, • However, vast exposures of Proterozoic rocks • Are unaltered or nearly so, • In many areas Archean rocks are separated from Proterozoic rocks • by an unconformity • Widespread associations of sedimentary rocks of passive continental margins • were deposited during the Proterozoic • by a plate tectonic style essentially the same as it is now

  11. Other Differences • The Proterozoic was also a time in evolution of • the atmosphere and biosphere • as well as the origin of some important natural resources • Oxygen-dependent organisms • evolved during this time • The first multicelled organisms and animals • made their appearance. • The fossil record is still poor compared to the Phanerozoic

  12. Evolution of Proterozoic Continents • Archean cratons assembled during collisions • of island arcs and minicontinents, • providing the nuclei around which • Proterozoic crust accreted, • thereby forming much larger landmasses • Proterozoic accretion • probably took place more rapidly than today • because Earth possessed more radiogenic heat, • and the plates moved faster

  13. 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

  14. Proterozoic Greenstone Belts • Most greenstone belts formed • during the Archean • They also continued to form • during the Proterozoic and at least one is known • from Cambrian-aged rocks in Australia • They were not as common after the Archean, • and differed in one important detail • the near absence of ultramafic rocks, komatiites, • which no doubt resulted from • Earth's decreasing amount of radiogenic heat production

  15. Early Proterozoic History of Laurentia • Laurentia underwent important changes • between 2.0 and 1.8 billion years ago • During this time, collisions • among various plates formed several orogens, • which are linear or arcuate deformation belts • in which many of the rocks have been • metamorphosed • and intruded by magma • thus forming plutons, especially batholiths

  16. Proterozoic Evolution of Laurentia • Archean cratons were sutured • along these orogens, • thereby forming a larger landmass • which makes up much of • Greenland, central Canada, • and the north-central United States

  17. Craton-Forming Processes • Examples of these craton-forming processes • are recorded in rocks • in the Thelon orogen in northwestern Canada • where the Slave and Rae cratons collided,

  18. 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 Penokean orogen

  19. 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 • rifting of a continent, • opening and closing of an ocean basin, • and finally reassembly of the continent

  20. Wilson Cycle • Some geologists think that the Wopmay orogen • represents a complete Wilson cycle

  21. 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

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

  23. Outcrop of Mesnard Quartzite • The crests of the ripple marks • point toward the observer

  24. Outcrop of Kona Dolomite • The bulbous structures • are stromatolites • that resulted from the activities • of cyanobacteria.

  25. Penokean Orogen • These rocks of the Great Lakes region • of the United States and Canada • includes sandstone-carbonate shale assemblages

  26. Accretion along Laurentia’s Southern Margin • Following the initial episode • of amalgamation of Archean cratons • accretion took place along Laurentia's southern margin • as it collided with volcanic island arcs and oceanic terranes • From 1.65 to 1.76 billion years ago, • the Yavapai and Mazatzal orogens were added to the evolving continent • The rocks have been deformed, altered by metamorphism, intruded by granitic batholiths, and • incorporated into Laurentia.

  27. Southern Margin Accretion • Laurentia grew along its southern margin • by accretion of the Central Plains, Yavapai, and Mazatzal orogens

  28. 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 • A significant Paleoproterozoic event was a • huge meteorite impact that took place in • northern Ontario, Canada • In addition, some Early Proterozoic rocks • and associated features provide excellent evidence • for widespread glaciation

  29. Mesoproterozoic Accretion and Igneous Activity • During the interval • from 1.35 to 1.55 billion years ago, • extensive igneous activity took place • that seems to be unrelated to orogenic activity • and accounted for the addition of the Granite-Rhyolite province • Some of the igneous activity resulted in • plutons being emplaced in existing continental crust. • The resulting igneous rocks are exposed in eastern Canada • extend across Greenland, • and are also found in the Baltic Shield, Scandinavia

  30. Granite-Rhyolite province • The last episodes in the Proterozoic accretion of Laurentia • involved the origin of the Granite-Rhyolite province • and the Grenville-Llano provinces

  31. Igneous Activity • However, the igneous rocks are deeply buried • by younger rocks in most areas • The origin of these • granitic and anorthosite plutons, • Anorthosite is a plutonic rock composed • almost entirely of plagioclase feldspars • calderas and their fill, • and vast sheets of rhyolite and ash flows • are the subject of debate • According to one hypothesis • large-scale upwelling of magma • beneath a Proterozoic supercontinent • produced the rocks

  32. Mesoproterozoic Orogeny and Rifting • A Mesoproterozoic event in Laurentia • was the Grenville orogeny • in the eastern part of the evolving continent • 1.3 to 1.0 billion years ago • Grenville rocks are well exposed • in the present-day northern Appalachian Mountains • as well as in eastern Canada, Greenland, and Scandinavia • The Llano province in Texas is probably • A westward extension of the Grenville.

  33. Grenville Orogeny • Rocks of the Grenville Orogen • These metamorphic rocks are uncomformably overlain • by the Upper Cambrian Potsdam Formation.

  34. Grenville Orogeny • Many geologists think the Grenville orogen • resulted from closure of an ocean basin, • the final stage in a Wilson cycle • The Grenville may have been the final episode • in the assembly of the supercontinent Rodinia • Whatever the cause of the Grenville orogeny, • it was the final stage • in the Proterozoic continental accretion of Laurentia

  35. 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

  36. Midcontinent Rift • Beginning 1.1 billion years ago • tensional forces opened 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 Proterozoic rocks • and terminates against the Grenville orogen • in the east

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

  38. Midcontinental Rift • Most of the rift is buried beneath younger rocks • except in the Lake Superior region • where 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 • Although not all geologists agree, many think • That the Midcontinent rift is a failed rift • where Laurentia began splitting apart

  39. Midcontinental Rift • Along the rift's margins • conglomerate was 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

  40. Cooper Harbor Conglomerate

  41. Portage Lake Volcanics Michigan

  42. Meso- and Neoproterozoic Sedimentation • Remember the Grenville orogeny • took place 1.3 and 1.0 billion years ago, • the final episode of large-scale deformation • in Laurentia until the Ordovician Period • Nevertheless, important geologic events • were taking place, • such as 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

  43. Sedimentary Basins in the West • Meso- to Neoproterozoic basin • in the western United States and Canada • Belt Basin • Uinta Basin • Apache Basin

  44. Sedimentary Rocks • Meso- and Neoproterozoic sedimentary rocks • are exceptionally well exposed • in the northern Rocky Mountains • of Montana and Alberta, Canada • Indeed, their colors, deformation features, • and erosion by Pleistocene and recent glaciers • have yielded some fantastic scenery • Like the Paleoproterozoic rocks in the Great Lakes region • they are mostly sandstones, shales, • and stromatolite-bearing carbonates

  45. Belt Basin, Glacier National Park • Meso- and Neoproterozoic rocks in the Belt basin

  46. Rocks of the Uinta Mountain Group Utah

  47. Proterozoic Sandstone • Proterozoic rocks • of the Grand Canyon Super-group lie • unconformably upon Archean rocks • and in turn are overlain unconformably • by Phanerozoic-age rocks • The rocks, consisting mostly • of sandstone, shale, and dolostone, • were deposited in shallow-water marine • and fluvial environments • The presence of stromatolites and carbonaceous • impressions of algae in some of these rocks • indicate probable marine deposition

  48. Grand Canyon Super-group • Neoproterozoic sandstone in the Grand Canyon

  49. Proterozoic Supercontinents • A continent is a landmass • made up of granitic crust • with much of its surface above sea level • A supercontinent consists of • at least two continents merged into one, but usually includes • all or most of all Earth’s landmasses • The supercontinent Pangaea, • which existed at the end of the Paleozoic Era, • is familiar, • but few people are aware of earlier supercontinents

  50. Style of Plate Tectonics • The present style of plate tectonics • involving opening and then closing ocean basins • had almost certainly been established by the Paleoproterozoic • In fact, the oldest known ophiolites • providing evidence for an ancient convergent plate boundaries • are known from Neoarchean and Paleoproterozoic rocks of Russia and probably China • They compare closely with younger, well-documented ophiolites, • such as the Jormua mafic-ultramafic complex in Finland

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