HistGeolChapt9 - Chapter 9 Precambrian Earth and Life...

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Chapter 9 Precambrian Earth and Life History – The Proterozoic The Proterozoic – 1.955 billion years, 42.5% of geologic time. Proterozoic beginning placed at 2.5 b.y., the end of the Archean-style of crustal tectonics.
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Changes: Proterozoic rocks are generally less meta- morphosed than Archean rocks. In some areas there is a long-term unconformity between Proterozoic and Archean rocks. Proterozoic plate tectonics similar to today. Proterozoic rock assemblages are different from Archean. Proterozoic atmosphere and biosphere more developed than Archean. Different mineral resources than Archean. Multi-celled organisms developed, as did Eucaryotes, i.e., cells with nuclei. 2
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Cratons “assembled” during Archean, accretion continued during Proterozoic = larger continents. Greenstone belts less common in Proterozoic, ultramafic lavas almost unknown due to lower temperatures. Focus on Laurentia , the Proterozoic landmass that portions of which include North America, Greenland, NW Scotland, possible portions of Baltic Shield of Scandinavia. Important Laurentian growth 2.0 to 1.8 b.y., collisions of smaller plates with craton leaving linear or arcuate deformation belts. 3
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During Proterozoic, Laurentia grew to southeast by accretion of other cratons. Collision zones = orogenic belts. Brown masses – Archean age. 4
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Rocks of the Wopmay orogenic belt in NW Canada are important because they record the opening and closing of an ocean basin, i.e., 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, Final reassembly of the continent Some of the Wopmay rocks include are sandstone- carbonate-shale assemblages, a suite of rocks typical of passive continental margins that first become widespread during the Proterozoic. 5
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Early Pro- terozoic rocks in the Great Lakes region. Sandstones with ripple marks and cross-beds suggest shallow water deposition. Stromatolites in dolostones suggest tidal flat conditions. Banded Iron Formations suggest fluctuations. 6
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Interlayered marble & hornfels (green/gray) = sea Breccia dike gives hints of what might lie below the Castner Marble. 7
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Middle Proterozoic Castner Marble, Franklin Mts., El Paso, TX Also see p. 156 – Kona
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HistGeolChapt9 - Chapter 9 Precambrian Earth and Life...

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