1. THE EARTH THROUGH TIME H A R O L D L. L E V I N TENTH EDITION
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.

2. Proterozoic: Dawn of a More Modern World
Chapter 9
Proterozoic: Dawn of a More Modern World

3. Proterozoic Eon 2.5 billion years to 542 million years ago
Comprises 42% of Earth history
Divided into three eras:
Paleoproterozoic Era ( by ago)
Mesoproterozoic Era (1.6 to 1.0 by ago)
Neoproterozoic Era (1.0 by ago to the beginning of the Paleozoic, 542 my ago)

4. The Beginning of Proterozoic Marks the Beginning of:
More modern style of plate tectonics
More modern style of sedimentation
More modern global climate with glaciations
Establishment of the beginnings of an oxygen-rich atmosphere
Emergence of eukaryotes

5. Precambrian Provinces in North America
Precambrian provinces were welded (or sutured) together to form a large continent called Laurentia during Early Proterozoic.
Suturing occurred along mountain belts or orogens.
Provinces were assembled by about 1.7 b.y. ago.
Laurentia continued to grow by accretion throughout Proterozoic.

6. Proterozoic Sedimentation
Sedimentation on and around the craton consisted of shallow water clastic and carbonate sediments deposited on broad continental shelves and in epicontinental seas.

7. Proterozoic Climate and Tectonics
Proterozoic glaciations occurred during:
Paleoproterozoic, about 2.4–2.3 b.y. ago (Huronian glaciation)
Neoproterozoic, 850–600 m.y. ago (Varangian glaciation)
During Late Proterozoic, the continents became assembled into a supercontinent called Rodinia.

8. Overview of the precambrian
FIGURE 9-31 Correlation of major events in the biosphere, lithosphere, and atmosphere.

9. Overview of Proterozoic Events
FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events.

10. Paleoproterozoic Era The oldest part of Proterozoic
Ranges from about 2.5 b.y. to 1.6 b.y.
Covers 900 million years

11. Major Events of the Paleoproterozoic
Active plate tectonics
Major mountain building on all major continents
Earth's first glaciation
Widespread volcanism (continental flood basalts)
Rise in atmospheric oxygen (great oxidation event)

12. Major Events of the Paleoproterozoic
Accumulation of high concentrations of organic matter in sediments (Shunga event) 2000 m.y. ago, and generation of petroleum
Oldest known phosphorites and phosphate concretions

13. orogenic belts
Orogenic belts developed around margins of the Archean provinces.
Wopmay belt in NW Canada
Trans-Hudson belt, SW of Hudson Bay

14. Wopmay orogenic belt contains evidence of:
Rifting and opening of an ocean basin (with normal faults, continental sediments, and lava flows)
Sedimentation along new continental margins (with shallow marine quartz sandstones and carbonate deposition)
Closure of the ocean basin (with deep water clastics overlain by deltaic and fluvial sands), followed by folding and faulting.

15. Wilson Cycle
This sequence of events in the Wopmay orogenic belt is called a Wilson Cycle, and is a result of plate tectonics.
Rifting and opening of an ocean basin
Sedimentation along new continental margins
Closure of the ocean basin
The sequence of events in the Wopmay belt is similar to that in Paleozoic rocks of the Appalachians.

16. Trans-Hudson orogenic belt
Trans-Hudson belt contains the sedimentary record of a Wilson Cycle, with evidence of:
Rifting
Opening of an ocean basin
Deposition of sediment
Closure of the ocean basin along a subduction zone, associated with folding, metamorphism, and igneous intrusions.
This closure welded the Superior province to the Hearne and Wyoming provinces to the west.

17. Paleoproterozoic Glaciation—Earth's First Ice Age?
A Paleoproterozoic ice age is recorded in rocks north of Lake Huron in southern Canada (called the Huronian glaciation).
Gowganda Formation.
Age of Huronian glaciation = 2450–2220 m.y.
Apparent rapid onset of global glaciations from what had been relatively stable climatic conditions.

18. Evidence for glaciation includes:
Mudstones with laminations or varves: fine laminations indicating seasonal deposition in lakes adjacent to ice sheets.
Glacial dropstones (dropped from melting icebergs) in varved sedimentary rocks.
Tillites or glacial diamictites (poorly sorted conglomerates of glacial debris).
Scratched and faceted cobbles and boulders in tillite, due to abrasion as ice moved.

19. Widespread Glaciation
Age of global glaciations = 2.6–2.1 b.y. ago (2600–2100 m.y.).
Widespread glaciation at this time as indicated by glacial deposits found in:
Europe
Southern Africa
India

20. Banded iron formations and prokaryote fossils
Extensive banded iron formations (BIF's) on the western shores of Lake Superior, indicate that photosynthesis was occurring and oxygen was being produced.

21. Banded iron formations and prokaryote fossils
Animikie Group.
Some BIF deposits are >1000 m thick, and extend over 100 km.
The Gunflint Chert, within the BIF sequence, contains fossil remains of prokaryotic organisms, including cyanobacteria.
Age = 1.9 b.y.

22. Labrador Trough
East of the Superior province are rocks deposited on a continental shelf, slope, and rise.
Rocks are similar to those of the Wopmay orogenic belt.
These rocks were folded, metamorphosed, and thrust-faulted during the Hudsonian orogeny, which separates the Paleoproterozoic from the Mesoproterozoic.

23. Highlights of the Mesoproterozoic
The Mesoproterozoic (or middle Proterozoic) ranges from about 1.6 b.y.–1.0 b.y.
The Midcontinent rift, an abandoned oceanic rift in the Lake Superior region with massive basaltic lava flows
Copper mineralization in the Lake Superior region
Continental collisions producing the Grenville orogeny in eastern North America
The assembly of continents to form the supercontinent, Rodinia.

24. Midcontinent Rift and the Keweenawan Sequence
Midcontinent rift extends southward from Lake Superior region.
Overlies Archean crystalline basement rocks and Paleoproterozoic Animikian

25. Midcontinent Rift and the Keweenawan Sequence
Large volumes of basaltic rock indicate presence of an abandoned rift zone called the Midcontinent rift.
This was the first stage of a Wilson Cycle.
Rift developed 1.2 b.y.–1.0 b.y. ago.
Extended from Lake Superior to Kansas.
Rifting ceased before the rift reached the edge of the craton, or the eastern U.S. would have drifted away from the rest of North America.

26. Midcontinent Rift and the Keweenawan Sequence
The Keweenawan Sequence consists of:
Clean quartz sandstones
Arkoses
Conglomerates
Basaltic lava flows more than 25,000 ft thick (nearly 5 mi) with native copper
Basaltic rock beneath the surface crystallized as the Duluth Gabbro, 8 mi thick and 100 mi wide.
Native copper fills vesicles (gas bubbles) in the Keweenawan basalt, and joints and pore spaces in associated conglomerates.

27. Grenville Province and Grenville Orogeny
Grenville rocks were originally sandstones and carbonate rocks.
Grenville Province was the last Precambrian province to experience a major orogeny.
Grenville orogeny = 1.2 b.y.–1.0 b.y. ago

28. Grenville Province and Grenville Orogeny
Orogeny occurred when Eastern North America (Laurentia) collided with western South America (Amazonia).
Orogeny was associated with formation of the supercontinent, Rodinia.
Later, during Paleozoic, Grenville rocks were metamorphosed and intruded during the three orogenies involved in the building of the Appalachians.

29. The Supercontinent, Rodinia
The supercontinent, Rodinia, as it appeared about 1.1 b.y. ago. The reddish band down the center of the globe is the location of continental collisions and orogeny, including the Grenville orogeny.
FIGURE 9-10 The supercontinent Rodinia as it appeared about 1100 million years ago.

30. The Supercontinent, Rodinia
Rodinia formed as the continents collided during the Grenville Orogeny.
Rodinia persisted as a supercontinent for about 350 million years.
It was surrounded by an ocean called Mirovia.
Rodinia began to rift and break up about 750 million years ago, forming the proto-Pacific Ocean, Panthalassa, along the western side of North America.

31. Rifting in Rodinia
An early failed attempt at rifting began in eastern North America about 760 m.y. ago, with the deposition of sediments of the Mount Rogers Formation in a fault-bounded rift valley.
Felsic and mafic volcanic rocks are interlayered with the sedimentary rocks of the Mount Rogers Formation.

32. Neoproterozoic Era
The Neoproterozoic (or "new" Proterozoic) ranges from about 1.0 b.y. to b.y. (542 m.y.).

33. Highlights of the Neoproterozoic
Extensive continental glaciations
Sediments deposited in basins and shelf areas along the eastern edge of the North American craton.
Most of these rocks were deformed during Paleozoic orogenies.

34. Glacial deposits—Neoproterozoic
Glacial deposits formed roughly 635–850 m.y. ago.
Evidence for glaciation:
Glacial striations (scratched and grooved pebbles and boulders)
Tillites (lithified, unsorted conglomerates and boulder beds) found nearly worldwide
Glacial dropstones (chunks of rocks released from melting icebergs)
Varved clays from glacial lakes

35. Glacial deposits - Neoproterozoic
Varangian glaciation (named after an area in Norway).
Ice age lasted about 240 m.y.
Glacial related deposits are widespread (even in tropical latitudes).
Cryogenian period
It has been proposed that the entire planet was cover in ice "snowball Earth" or that ice did not extend beyond the mid latitudes “slushball Earth”
Glaciation was followed by tropical condition as evidence by the deposition of carbonates
FIGURE 9-11 Earth has seen several major episodes of widespread continental glaciation (orange).

36. Plate Tectonics and Glaciation
Plate tectonics may have had a role in cooling the planet.
Continents were located around the equator about 600 to 700 m.y. ago.
No tropical ocean.

37. Plate Tectonics and Glaciation
Heat lost by reflection from the rocks on the surface of the continents may have caused global cooling. (Land plants had not yet appeared.)
As continental glaciers and ice caps formed, reflectivity of snow and ice caused further temperature decrease.

38. Atmospheric Gases and Glaciation
Glaciation was associated with:
Decrease in CO2 and
Increase in O2.
CO2 causes the greenhouse effect and global warming. Decrease in CO2 may have caused cooling.
Decrease in CO2 was probably caused by increase in the number of photosynthetic organisms (cyanobacteria, stromatolites).

39. Limestones and Glaciations
Limestones are associated with glacial deposits, which is unusual, since limestones generally form in warm seas, not cold ones.
Association of limestones with glacial deposits suggests that times of photosynthesis and CO2 removal alternated with times of glaciation.
Limestones (made of CaCO3) are a storehouse of CO2, which was removed from the atmosphere.

40. Limestones and Glaciations
Glacial conditions may have inhibited photosynthesis by stromatolites.
As a result, CO2 may have accumulated periodically and triggered short episodes of global warming.
This produces the paradox of glaciers causing their own destruction.

41. Rifting in Rodinia
Around 570 million years ago, rifting began again, and South America began to separate from North America, forming the Iapetus Ocean (or proto-Atlantic Ocean).
The rift ran along what is now the Blue Ridge province. Basaltic lava flows formed the Catoctin Formation.
As the Iapetus Ocean opened, sands and silts were deposited in the shelf areas.

42. Proterozoic Rocks South of the Canadian Shield
Extensive outcrops of
Precambrian rocks are
present in the
Canadian Shield.
Precambrian rocks are also present in other areas,
including:
Rocky Mountains
Colorado Plateau (Grand Canyon)

43. Events Recorded in Proterozoic Rocks
Collision of an Archean terrane with volcanic island arc, 1.7 or 1.8 b.y.a. (Wyoming and western Colorado)
Extensive magma intrusion in Mesoproterozoic, 1.5–1.4 b.y.a. (California to Labrador)
Widespread rifting
Rifts with thick sequences of shallow water Neoproterozoic sedimentary rocks, 1.4–0.85 b.y.a. Belt Supergroup (Glacier National Park, Montana, Idaho, and British Columbia).

44. Precambrian rocks of the Grand Canyon
Vishnu Schist metasediments and gneisses, intruded by Zoroaster Granite about 1.4 b.y. to 1.3 b.y.a. during the Mazatzal orogeny.
Top of Vishnu Schist is an unconformity.
FIGURE 9-15 Vishnu Schist, Grand Canyon Supergroup, and other rocks in the Grand Canyon of the Colorado River.

45. Precambrian rocks of the Grand Canyon
Grand Canyon Supergroup unconformably overlies the Vishnu Schist while unconformably underling the Paleozoic rocks.
The Grand Canyon Supergroup consist of Neoproterozoic sandstones, siltstones, and shales. Correlates with Belt Supergroup.
FIGURE 9-15 Vishnu Schist, Grand Canyon Supergroup, and other rocks in the Grand Canyon of the Colorado River.

46. Life at the beginning of Proterozoic was similar to that of Archean
Archaea in deep sea hydrothermal vents
Planktonic prokaryotes floated in seas and lakes
Anaerobic prokaryotes in oxygen-deficient environments
Photosynthetic cyanobacteria (prokaryotes) constructed stromatolites (algal filaments)
Eukaryotes (as indicated by molecular fossils)

47. Other forms of life appeared during Proterozoic
More diverse eukaryotes including acritarchs
Metazoans or multicellular animals with soft bodies
Metazoans with tiny calcium carbonate tubes or shells
Metazoans that left burrows in the sediment

48. Microfossils of the Gunflint Chert
First definitive Precambrian fossils to be discovered (in 1953) were in the 1.9 b.y. old Gunflint Chert, NW of Lake Superior (Paleoproterozoic).

49. Microfossils of the Gunflint Chert
The fossils are well-preserved, abundant and diverse and include:
String-like filaments
Spherical cells
Filaments with cells separated by septae (Gunflintia)
Finely separate forms resembling living algae (Animikiea)
Star-like forms resembling living iron- and magnesium-reducing bacteria (Eoastrion)

50. Microfossils of the Gunflint Chert
A = Eoastrion ( = dawn star), probably iron- or magnesium-reducing bacteria
B = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameter
C = Animikiea (probably algae)
D = Kakabekia, an organism or uncertain affinity
FIGURE 9-20 (A) Eoastrion, (B) Eosphaera, (C) Animikiea, and (D) Kakabekia from the Gunflint Chert.

51. Microfossils of the Gunflint Chert
Gunflint fossil organisms resemble photosynthetic organisms.
The rock containing these organisms contains organic compounds that are regarded as the breakdown products of chlorophyll.
The Gunflint Chert organisms altered the composition of the atmosphere by producing oxygen.

52. The Rise of Eukaryotes
The appearance of eukaryotes is a major event during the history of life.
Eukaryotes have the potential for sexual reproduction, which increases variation through genetic recombination.
Genetic recombination provides greater possibilities for evolutionary change. Diversification of life probably did not occur until after the advent of sexual reproduction, or until oxygen levels reached a critical threshold.

53. The Rise of Eukaryotes
Eukaryotic cells can be differentiated from prokaryotic cells on the basis of size.
Eukaryotes tend to be much larger than prokaryotes (larger than 60 microns, as compared with less than 20 microns).
Harold Levin
FIGURE 8-37 (A) Comparison of a prokaryote cell (left) and a eukaryote cell.

54. The Rise of Eukaryotes
Eukaryotes appeared by Archean (as determined by molecular fossils or biochemical remains).
Larger cells begin to appear in the fossil record by 2.7 b.y. to 2.2 b.y.
Eukaryotes began to diversity about 1.2 to 1.0 b.y. ago.

55. Acritarchs Eukaryotes Single-celled, spherical microfossils
Thick organic covering
May have been phytoplankton
First appeared 1.6 b.y. ago (at Paleoproterozoic-Mesoproterozoic boundary)
Some resemble cysts or resting stages of modern marine algae called dinoflagellates.

56. Acritarchs Reached maximum diversity and abundance 850 m.y. ago
Declined during Neoproterozoic glaciation
Few acritarchs remained by 675 m.y. ago
Extinction during Ordovician
Useful for correlating Proterozoic strata

57. The First Metazoans (Multicellular Animals)
Metazoans are multicellular animals with various types of cells organized into tissues and organs.
Metazoans first appeared during Neoproterozoic, about 630 m.y. ago (0.63 b.y.). Preserved as impressions of soft-bodied organisms in sandstones.

58. Examples of metazoan fossils in Proterozoic rocks
Ediacara fauna: Imprints of soft-bodied organisms, first found in Australia during the 1940s
Metazoan eggs and embryos in uppermost Neoproterozoic Doushantuo Formation, south China
Trace fossils of burrowing metazoans in rocks younger than the Varangian glaciation.
Tiny shell-bearing fossils (small shelly fauna)

59. Ediacara fauna
Ediacara fauna is an important record of the first evolutionary radiation of multicellular animals.
Some were probably ancestral to Paleozoic invertebrates.
Oldest Ediacara-type fossils are from China. Youngest Edicara-type fossils are Cambrian (510 m.y., Ireland).

60. Types of Ediacara fossils
Discoidal
Frondlike
Elongate or ovate
FIGURE 9-28 Reconstruction of Kimberella.

61. Ediacara fauna
Because the Ediacara creatures are not really similar to animals that are living today, this has led to the suggestion that they be placed in a separate taxonomic category or new phylum.
The name proposed for this new category is Vendoza (named after the Vendian, or the latest part of the Neoproterozoic in Russia).

62. Small Shelly Fauna: The Origin of Hard Parts
Small fossils with hard parts or shells appeared during Neoproterozoic.
Cloudina, an organism with a small, tubular shell of calcium carbonate (CaCO3).
Resembles structures built by a tube-dwelling annelid worm.
Earliest known organism with a CaCO3 shell.
Found in Namibia, Africa.
FIGURE 9-29 Cloudina, the earliest known calcium carbonate shell-bearing fossils.

63. Small Shelly Fauna: The Origin of Hard Parts
Other latest Proterozoic and earliest Cambrian small fossils with shells include:
Possible primitive molluscs
Sponge spicules,
Tubular or cap-shaped shells, and
Tiny tusk-shaped fossils called hyoliths
Some early shelly material is made of calcium phosphate.

64. Precambrian Trace Fossils
Trails, burrows, and other trace fossils are found in Upper Neoproterozoic rocks.
In rocks deposited after Neoproterozoic Varangian glaciation.
Mostly simple, shallow burrows.
Trace fossils increase in diversity, complexity, and number in younger (Cambrian) rocks.
H. J. Hofman

65. What stimulated the appearance of metazoans?
May be related to the accumulation of sufficient oxygen in the atmosphere to support an oxygen-based metabolism.
Ancestral metazoans may have lived in "oxygen oases" of marine plants.
Ediacaran life may have evolved gradually from earlier forms that did not leave a fossil record.

66. Review of Proterozoic Events
FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events.

67. IMAGE CREDITS
FIGURE 9-31 Correlation of major events in the biosphere, lithosphere, and atmosphere.
FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events. Source: Harold Levin.
FIGURE 9-10 The supercontinent Rodinia as it appeared about 1100 million years ago. Source: Hoffman, 1991, Did the Breakout of Laurentia Turn Gondwanaland Inside-Out?, The American Association for the Advancement of Science.
FIGURE 9-11 Earth has seen several major episodes of widespread continental glaciation (orange). Source: Harold Levin.
FIGURE 9-15 Vishnu Schist, Grand Canyon Supergroup, and other rocks in the Grand Canyon of the Colorado River.
FIGURE 9-20 (A) Eoastrion, (B) Eosphaera, (C) Animikiea, and (D) Kakabekia from the Gunflint Chert. Source: Barghoorn, E., "The Oldest Fossils," Scientific American 224:30-42.
FIGURE 9-28 Reconstruction of Kimberella. Source: Harold Levin.
FIGURE 9-29 Cloudina, the earliest known calcium carbonate shell-bearing fossils. Source: Harold Levin.