Chapter 9—The Proterozoic: Dawn of a More Modern World

Chapter 9—The Proterozoic: Dawn of a More Modern World

CHAPTER OVERVIEW

The Proterozoic Eon which began 2.5 billion years ago and ended only 540 million years ago comprises 42 percent of the Earth’s history. During this Eon the Precambrian provinces that developed in the Archean become welded together along belts of crustal compression, mountain building, and metamorphism called an orogen to form Earth’s first large continent, Laurentia. Highlights of the Paleoproterozoic included development pf orogenic belts primarily around the provinces of Canada forming the Wopmay, Trans-Hudson, and Grenville Orogens. Wilson Cycles are defined and discussed and evidence of Earth’s first ice age is described including depositional sequences from the Animikie Group and the Labrador Trough. The discussion of deposition continues in the Mesoproterozoic with the Keweenawan rocks, lava flows which contained significant deposits of copper. There is also a discussion of the formation of the supercontinent Rodinia and the proto-Pacific Ocean called Panthalassa. Highlights of the Neoproterozoic include evidence of widespread glaciation. This is followed by a discussion of the Proterozoic rocks south of the Canadian Shield.

There is extensive discussion of the life forms throughout the Proterozoic including stromatolites, prokaryotes, eukaryotes, and metazoans. Microfossil significance is also discussed including the proposed addition of a new phylum, Vendoza, for Ediacaran creatures and animals that do not exist today. The chapter ends with a discussion of oxygen content in the atmosphere and the climate of the Proterozoic environment.

LEARNING OBJECTIVES

By reading and completing information within this chapter, you should gain an understanding of the following concepts:

  • Describe the orogenic events that formed Laurentia from the Precambrian provinces.
  • Discuss the relationship of orogen development to a Wilson Cycle.
  • Describe geologic evidence of the first ice age.
  • Discuss the Labrador Trough’s relationship to the Trans-Hudson orogen.
  • Describe the aborted ocean rift in the Lake Superior region, massive extrusions of basaltic lava flows, and the addition of copper ores to the Lake Superior region.
  • Describe the differences between eukaryotes and prokaryotes.
  • Explain the significance of the metazoans of the Proterozoic and the three groups of Ediacaran metazoans.
  • Describe the history of stromatolites and what other organisms may have contributed to their post-Proterozoic decline.

CHAPTER OUTLINE

  1. Highlights of the Paleoproterozoic (2.5 to 1.6 billion years ago)
  2. Early Plate Tectonics: Evidence from Canada’s Northwest Territory
  3. Evidence of Earth’s First Ice Age
  4. End of the Paleoproterozoic
  1. Highlights of the Mesoproterozoic (1.6 to 1.0 billion years ago)
  2. An Aborted Rift, Rich in Copper
  3. The Grenville Orogeny
  4. Building a New Supercontinent - Rodina
  1. Highlights of the Neoproterozoic (1.0 billion to 540 million years ago)
  2. A Big Chill
  3. Earth’s Glacial History
  1. Proterozoic Rocks South of the Canadian Shield

A. Grand Canyon Precambrian Rocks

  1. Proterozoic Life
  2. Microfossils of the Gunflint Chert
  3. The Rise of Eukaryotes
  4. Acritarchs
  5. Many-Celled Animals Arrive: The Metazoans
  6. Ediacaran Fauna
  7. The Vendoza Controversy
  8. What the Ediacaran Fossils Tell Us About the Proterozoic
  9. Oxygen and Climate Changes in the Proterozoic Environment

Key Terms (Pages 243–265)

acritarchs (256): Unicellular, spherical microfossils with resistant single-layered walls. Useful in correlating Proterozoic strata. First appear in rocks about 1.6 million years old. Reached maximum diversity and abundance 850 million years ago and then suffered a steady decline which coincided with the major episode of glaciation underway near the end of the Proterozoic.

Animikie Group (249): Paleoproterozoic rocks surrounding the western shores of Lake Superior are included in this group; famous for its bonanza iron ore content. Coarse sandstones and conglomerates deposited in shallow water lie near the base of the Animikie group. These rocks are overlain by cyclic successions of chert, cherty limestone, shales, and banded iron formations.

coelum (261): In Kimberella, a body cavity in which the digestive tract and other internal organs are suspended.

cyanobacteria (255?[TU1]): Formerly known as blue-green algae. These fossils suggest that oxygen-producing organisms had already evolved by an early age (Archean Eon) in Earth history. Microfossils were found in a chert bed that occurs within the Apex Basalt Formation of the Warrawoona group.

dropstone (251): Chunks of rock released from melting icebergs.

Ediacaran fauna (258): Impressions of large, soft-bodied animals. The best-known fossils of Neoproterozoic metazoans found in the Rawnsley Quartzite of the Pound subgroup. According to their shape, they fall into three groups, discoidal, frond-like, and ovate to elongate. Discoidalforms such as Cyclomedusa were initially thought to be jellyfish. Another circular form, Tribrachidium, appears to have no modern counterpart and may be a member of an extinct phylum. Frond-like fossils resemble the living soft corals informally called sea pens. They look rather like fronds of ferns, except that tiny coral polyps are aligned along the branchlets. Ovate to elongate were originally regarded as impressions made by flatworms and annelid worms. Typical of these fossils is Dickinsonia, which attained lengths of up to a meter, and Springgina, a more slender animal with a distinctive crescent-shaped structure at its anterior end.

epicontinental (245): A shallow, inland sea that flooded the continental interiors.

Grenville orogeny (251): Orogenic event that occurred 1.2 to 1.0 billion years ago in eastern Canada. Typical Grenville rocks consist of carbonates and sandstones that have been metamorphosed and intruded by igneous bodies. In eastern Canada, these rocks have been compressed into overturned folds that have northeasterly axes.

Hudsonian orogeny (249): The event that separates Paleoproterozoic from Mesoproterozoic geologic history. It was a period of intense folding, metamorphism, and westward thrust faulting.

Keweenawan (249): Clean quartz sandstones and conglomerates as well as basaltic volconics that rest on either crystalline basement or Animikian strata that extended from the Lake Superior region southward beneath cover of Phanerozoic rocks.

Labrador Trough (249): A curving, elongate structural depression found east of the Superior province where lie an extensive outcrop of rocks.

Laurentia (245): A large continent formed during the Proterozoic by the welding of Archeon elements. (The Superior, Slave, Hearne, Rae, Wyoming, and Nain provinces.)

metazoan (258): Multicellular animals that possess more than one kind of cell and have their cells organized into tissues and organs.

Mesoproterozoic Era (243): The middle era of the Proterozoic Era, approximately1.6 to 1.0 billion years ago.

molecular fossil (256): Biochemical remnants of microfossils, i.e. eukaryotes, that are chemical clues to early life forms.

Neoproterozoic Era (243): The “new” era of the Proterozoic Era, approximately1.0 billion years ago to the beginning of the Paleozoic Era, 540 million years ago.

orogen (245): Belt of crustal compression, mountain building, and metamorphism.

Paleoproterozoic Era (243): The early era of the Proterozoic Era, approximately2.5 to 1.6 billion years ago.

Precambrian province (245): The divisions of the Canadian Shield based on differences in the trends of faults and folds, the style of folding, and the ages of component rocks.

Rodinia (245): Supercontinent found by the gathering of continents during the Neoproterozoic. One of the most significant global events of the Neoproterozoic.

tillite (248): Unsorted glacial deformation (till) that has been converted into solid rock. Used as a Paleoclimatic indicator of glaciation.

Varangian glaciation (251): The episode of frigid conditions that occurred during the Neoproterozoic Era has been named the Varangian glaciation after Varangian Fiord in Norway, where Neoproterozoic tillites provided early evidence of this great ice age.

varves (248): A thin sedimentary layer or pair of layers that represent the depositional record of a single year.

Vendoza controversy (261): A separate taxonomic category proposed for Ediacaran creatures with dissimilarities to animals that exist today and possessing their own unique characteristics. One view suggests that they should not be placed in existing phyla.

Wilson Cycle (246): Sequence of events including the opening of an ocean basin, sedimentation along resulting new continental margins, and closure of the ocean basin through plate tectonic processes. Named after J. Tuzo Wilson, one of the pioneers of the plate tectonics theory.

Multiple-Choice Questions

1.The eon that began 2.5 billion years ago and ended 544 million years ago and contains 42 percent of the earth’s history is the

a.Neoproterozoic.c.Proterozoic.

b.Archean.d.Hadean.

2.The sequence of orogenic events in the Canadian Shield during the Proterozoic, from youngest to oldest, is

a.Wopmay, Hudsonian, and Grenvillian.c.Hudsonian, Grenvillian, and Wopmay.

b.Grenvillian, Hudsonian, and Wopmay.d.Grenvillian, Wopmay, and Hudsonian.

3.Opening of an ocean basin, sedimentation along continental margins, and closure of ocean through plate tectonic process is called

a.an orogeny.c.the Wilson cycle.

b.uniformitarianism.d.suturing.

4.Glaciation in the Early Proterozoic is recorded by conglomerates and laminated mudstone of the

a.Animikie group.c.Gunflint chert.

b.Gowganda formation.d.Belt supergroup.

5.Laminated mudstones that record yearly variations in sediment flux are called

a.greenstones.c.varves.

b.granulites.d.tillites.

6.The gathering of continents during the Neoproterozoic formed a supercontinent named

a.Pangea.c.Iapetus.

b.Rodinia.d.Mirovia.

7.The “Vishnu” represents the oldest rock unit at the Grand Canyon and is an example of a

a.sandstone.c.slate.

b.schist.d.granite.

8.The oldest rocks in the Grand Canyon region are the

a.Grand Canyon series.c.Belt supergroup.

b.Vishnu Schist.d.Animikie group.

9.The Trans-Hudsonian orogeny records the closure and suturing of the

a.North American plate to the Greenland plate.

b.Superior province to the Hearne and Wyoming province.

c.North American plate to the South American plate.

d.North American plate to the African plate.

10.The rocks of the Proterozoic that contain the oldest evidence of two continental glaciers are

a.Autcogen and Wopmay.c.Gowganda and Huronian.

b.Rhiphaen and Mazatal.d.Gowganda and Varangian.

11.Unicellular, spherical microfossils with resistant single-layered walls that are useful in correlating Proterozoic strata are termed

a.acritarchs.c.metazoans.

b.eukaryotes.d.heliotrophs.

12.One of the first eukaryote unicellular, spherical microfossils with resistant single-layered walls is

a.algae.c.cyanobacteria.

b.stromatolites.d.acritarchs.

13.The generic (genus) name given to the star-shaped fossils found by J. William Schopf in the 3.5 billion year old (Archean) Gunflint chert is

a.Gunflintia.c.Cyclomedusa.

b.Acritarchs.d.Eoastrion.

14.During this orogeny, the east coast of Laurentia (North America) lay adjacent to a block of western

South America termed Amazonia.

a.Greenville[TU2]c.Varanglan

b.Gowganded.Hudsonian

15.The Vishnuschist of the Grand Canyon of the southwestern U.S. was emplaced 1.4 – 1.3 billion years ago as part of what orogeny?

a.Mazatzal orogenyc.Trans-Hudson orogeny

b.Wopmay orogenyd.Grenville orogeny

16.The Varagean glaciation which is described as a period of extensive glaciation as evidenced by the tillites, dropstones, and varied clays occurred during

a.Neoprotozoic.c.Paleoproterozoic.

b.Grenville.d.Mesoproterozoic.

17.This Paleoproterozoic orogenic belt lies along the western margin of the Slove province in Canada’s Northwest Territory and is the oldest.

a.Wopmay orogen[TU3]c.Grenville orogen

b.Hudson orogend.Gowganda orogen

18.The Superior province that welded (joined) the Hearne and Wyoming provinces during

a.Grenville orogen.c.Trans-Hudson orogen.

b.Wopmay orogen.d.Keweenawan.

19.The Proterozoic Eon began about how many billion years ago?

a.3.5c.2.5

b.3.0d.2.0

20.The Proterozoic cyclic succession of the banded iron formations is

a.quartz sandstones, arkoses, conglomerates, basalts.

b.pillow lavas, basalts, mafic intrusives.

c.quartz, conglomerates, shales, banded iron.

d.cherts, cherty limestone, shales, banded iron.

Fill in the Blank

1.The large supercontinent formed during the early Proterozoic that was essentially what we now call North America was called .

2.The sequence of events given as evidence of the opening of an ocean basin, sedimentation along continental margins, and the subsequent closure of this basin is called
______.

3.The oldest rocks in the Grand Canyon region are called the .

4.A thin sedimentary layer or pair of layers that represent the depositional record of a single year is called a .

5.The orogeny[TU4] that separates the early Proterozoic from the Mesoproterozoic geologic history is called .

6.The formation where glaciation in the Early Proterozoic is recorded by conglomerates and laminated mudstones is called .

7.The oldest recorded exposures of prokaryotes occur in the now famous rock unit known as
.

8.The most significant event of the late Proterozoic that was the gathering of Precambrian continents to form the large supercontinent was called .

9.Episodes of Proterozoic glaciation are indicated by the presence of deposits.

10.The term that is given the unicellular, spherical microfossils with resistant, single-layered walls is
.

11.The orogen [TU5]that began with opening of an oceanic rift, progressed to the deposition of continental shelf and is located along the western margin of the slave provinces in Canada’s Northwest Territories is called Orogen.

12. Evidence for the oxidizing atmosphere is indicated in the geologic record of the Proterozoic by the
presence of .

13. East of the Superior province, along a curving, is an elongate structural depression called the

______.

14.Unsorted, lithified glacial debris are called .

15. are multicellular animals that possess more than one kind of cell and have their cells organized into tissues and organs.

True/False

1. At the beginning of the Proterozoic (2.5 billion years ago) many of the smaller elements of the Archean came together to form Laurentia.

2. The sequence of orogens [TU6] in the Canadian Shield in order of oldest to youngest is Wopmay, Trans-Hudson, and Grenville.

3. The Animikie group that surrounds the western shores of Lake Superior are world famous for their copper ores.

4. The Gowganda formation, a rock unit, is notable because of its surface magma that cooled and solidified to form ultramafic rocks.

5. Banded iron deposits are important because of their mineral resources that reflect the build-up of sufficient oxygen in the atmosphere to oxidize iron.

6. Life at the beginning of the Proterozoic resembled that of the late Archean since it consisted of prokaryotes such as cyanobacteria.

7. Strata of the Belt supergroup contain reef-like masses of stromatolites.

8. East of the Superior province along a curving is an elongate structural depression called the Keweenawan Trough are Paleoproterozoic rocks.

9. The Hudson Orogeny [TU7]marks the dose of the Paleoproterozoic.

10. The most significant biological event of the Neoproterozoic was the appearance of the multicellular animals called metazoans.

Chapter 9—The Proterozoic: Dawn of a More Modern World

Answer Key

Chapter 9—The Proterozoic: Dawn of a More Modern World

Multiple Choice

  1. c
  2. b
  3. c
  4. b
  5. c
  6. b
  7. b
  8. b
  9. b
  10. d
  11. a
  12. d
  13. d
  14. a
  15. a
  16. a
  17. a
  18. c
  19. c
  20. d

Fill Ins

  1. Laurestia
  2. Wilson’s cycles
  3. Vishua schist
  4. varve
  5. Hudsonian Orogen
  6. Gowganda
  7. Gunflint chert
  8. Rodira
  9. tillites
  10. acritarchs
  11. Wopmy
  12. red belts
  13. Labrador Trough
  14. tillites
  15. Metazoans

True/False

  1. T
  2. T
  3. F
  4. F
  5. T
  6. T
  7. T
  8. F
  9. T
  10. T

Chapter 9—The Proterozoic: Dawn of a More Modern World

RESPONSES TO QUESTIONS ACCOMPANYING SELECTED FIGURES

FIGURE 9–3 (p. 246) The faults beneath the turbidites are normal (tensional) faults. These faults are associated with the passive margins of tectonic plates.

FIGURE 9–7 (p. 249) Hematite serves as the red pigment in the banded iron formations.

FIGURE 9–15 (p. 254) A nonconformity separates the Vishnu Schist and Zoroaster Granite from the sedimentary sequence of the overlying Grand Canyon supergroup.

FIGURE 9–26 (p. 260) Branchlets in the Fronds fossils are fused together and do not have passages through which water currents might pass placing doubt on their identification as soft corals.

FIGURE 9–29 (p. 261) Kimberella shows evidence of a true coelum, or body cavity, in which a complete digestive tract and other internal organs can be suspended.

[TU1]Remember to check page number

[TU2]Grenville?

[TU3]Why are these “orogen” and below it is “orogeny”?

[TU4]Orogen?

[TU5]Orogeny?

[TU6]Orogenys?

[TU7]Orogen?