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 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 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 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 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 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 Proterozoic Evolution of Oxygen Dependent Organisms During the Proterozoic oxygen dependent organisms made their appearance and the first cells evolved When did Continents evolve Proterozoic accretion at Archean island arcs and minicontinents margins thereby forming much larger landmasses 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 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 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 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 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 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 Wopmay Orogen Some of the rocks in Wopmay orogen are sandstonecarbonate shale assemblages a suite of rocks typical of passive continental margins that first become widespread during the Proterozoic Early Proterozoic Rocks in Great Lakes Region Early Proterozoic sandstone carbonate shale assemblages are widespread near the Great Lakes Outcrop of Sturgeon Quartzite The sandstones have a variety of sedimentary structures such as ripple marks and crossbeds Northern Michigan Outcrop of Kona Dolomite Some of the carbonate rocks now mostly dolostone such as the Kona Dolomite contain abundant bulbous structures known as stromatolites Northern Michigan Penkean Orogen These rocks of northern Michigan have been only moderately deformed and are now part of the Penokean orogen 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 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 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 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 Igneous Activity These igneous rocks are exposed in eastern Canada extend across Greenland and are also found in the Baltic shield of Scandinavia Cause of Igneous Activity According to one hypothesis large scale upwelling of magma beneath a Proterozoic supercontinent produced the rocks 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 Grenville Orogeny A final episode of Proterozoic accretion occurred during the Grenville orogeny 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 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 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
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