Chapter 9

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