The Earth Through Time, 10th Edition
by Harold L. Levin
CHAPTER 9—THE PROTEROZOIC: DAWN OF A MORE
MODERN WORLD
CHAPTER OUTLINE FOR TEACHING
I.
Proterozoic subdivisions (numbers are in billions of years)
A. Neoproterozoic: 1.0 to 0.542
B. Mesoproterozoic: 1.6 to 1.0
C. Paleoproterozoic: 2.5 to 1.6
II. Laurentia: Proterozoic North America
A. Orogenic Suturing: joining elements
B. Period of Continental Suturing: ended by 1.7 billion years ago
C. Epicontinental Seas: sands and carbonates
D. Glaciation Episodes: 2.4-2.3 billion years ago and 850-600 million years ago
III. Paleoproterozoic Events
A. Formation of Wopmay Orogen
B. Development of Wilson Cycle (opening and closing of an ocean)
1. Opening of an ocean basin
2. Sedimentation along the margin of separating continents
3. Closing of the ocean basin through plate tectonics
C. Formation of Trans-Hudson Orogen
1. Northern and western sides of Superior province
2. Records Wilson cycle
3. Joins Hearne and Wyoming terrains
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
1
2
D. First Ice Age (Gowganda Formation, 2.6-2.1 billion years old)
1. Conglomerates (tillites)
2. Laminated (varved) mudstones (glacial lakes)
E. Deposition of the Animikie Group
1. Iron ores (banded-iron formations)
2. Gunflint Chert (fossil cyanobacteria and prokaryotes)
F. Formation of the Labrador Trough
1. Curved tract on east side of Superior Province
2. Records a Wilson cycle
3. Stratigraphy: east versus west
a. east: pillow lavas, mafic intrusions, and graywackes
b. west: sandstones, dolostones, and iron formations
4. Structure
a. east: folding and metamorphism
b. west: thrust faulting
5. Hudsonian Orogeny: close of trough and end of Paleoproterozoic
IV. Mesoproterozoic Events
A. Deposition of Keweenawan Rocks
1. Clean sandstone and vesicular basalts
2. Lavas several 1000 m thick with abundant copper deposits (in vesicles)
B. Rifting and Intrusion of Duluth Gabbro
1. Rifting: 1.2-1.0 billion years ago
2. Tensional faulting, then intrusion
3. Funnel-shaped Duluth Complex intrusion
a. gabbro in graded layers
b. dimensions: 20 km x 250 km
C. Development of Grenville Province: eastern North America
1. Age: 1.2-1.0 billion years
2. Metamorphosed carbonates and sandstones, intruded by igneous bodies
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
2
3
V. Neoproterozoic Events
A. Cryogenian Period: 215 million years of the big chill (850 to 635 million years ago)
1. Cryogenian glacial sediments found on all continents, except Antarctica
2. Glacial striations
3. Sediments: tillites, dropstones, varved clays
B. Cap carbonates: dolostones and limestones that abruptly overlie Cryogenian
glacial sediments; indicating a rapid return to warmer conditions
C. Proterozoic Rocks of the Rocky Mountains and Colorado Plateau
1.
2.
3.
4.
5.
Strongly deformed and metamorphosed granitic rocks
Volcanic rocks and greenstones
Island arc collisions about 1.7 to 1.8 billion years ago
Magma intrusion events in North America (1.5 to 1.4 billion years)
Widespread rifting and sedimentation (Belt Supergroup)
D. Proterozoic of the Grand Canyon
1. Grand Canyon Supergroup
2. Vishnu Schist
VI. Fossil Record
A. Inherited Archean Life
1.
2.
3.
3.
4.
Photosynthetic cyanobacteria
Anerobic prokaryotes
Thermophiles
Stromatolites
Molecular fossils of eukaryotes
B. Heliotropic Stromatolites
1. Laminar structure = daily growth phase
2. Bitter Springs Fm., Australia (850 million years old)
3. Results: 435 laminae/cycle = 435 days/year
C. Proterozoic Prokaryotes (microfossils of the Gunflint chert)
1.
2.
3.
4.
5.
Gunflintia: string-like filaments and ball-shaped cells (algae)
Animikiea: fine filament (algae)
Eoastrion: “dawn star” (Fe- and Mg-reducing bacteria)
Kakabekia: plumose form (unknown affinity)
Eosphaera: “dawn sphere” (unknown affinity)
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
3
4
D. Eukaryotes (including molecular fossils)
1. First appear: 2.7 to 2.2 billion years ago
2. Began to diversify: 1.2 to 1.0 billion years ago
E. Acritarchs (unicellular, spherical microfossils with single-layer walls)
1. First appear: 1.6 billion years ago
2. Maximum diversity: 850 million years
3. Fossil sites: Russia, California, Australia
F. Protozoan Eukaryotes
G. Ediacaran Biota
1. Discovered in the 1940s
2. Older members found in China more recently
3. Ancestors to some Cambrian fauna
4. Three body types (impressions in sediment)
a. discoidal (flat and circular; Cyclomedusa)
b. frondlike (Tribrachidium; Tribrachidium)
c. ovate to elongate (Dicksonia; Spriggia; Kimberella)
5. Fossils with hard exoskeleton: Cloudina
6. Trace fossils (shallow burrows)
7. Vendosa (Vendobionta) – possible affinities
a. fossils look like some modern jellyfish and corals, so they are probably the
ancestors for those groups
b. fossils may look like modern forms, but are really not related; some may not have
modern equivalents
8. Appearance may have followed soon after the end of global glaciation (the “snowball
Earth” hypothesis)
VII. Changing Environment
A. Oxygen Builds up in Atmosphere: rises to 10% of current level by end of
Proterozoic
B. Wide range of climatic conditions
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
4
5
Answers to Discussion Questions
1. In the Wopmay orogen of Canada, there is evidence of the opening of an ocean,
sedimentation along the resulting new continental margins, and closure of the ocean basin
through plate-tectonic processes.
2. Banded-iron formations indicate that a small amount of oxygen was present in the
atmosphere over 3 billion years ago. Red beds, which first occur about 2 billion years ago,
indicate the first real oxygenated atmosphere had developed, even though the percent of O 2
was much less than one percent.
3. The supercontinent of Rodinia was assembled during the Neoproterozoic. The Grenville
orogeny of the North American eastern coast is a result of the Rodinian assembly.
4. Crustal rifting and basaltic intrusion of the Keweenawan region developed during the
Mesoproterozoic (specifically 1.2 to 1.0 billion years ago) as a result of failed crustal rifting.
This rifting was likely induced by a mantle plume.
5. Prokaryotes have a cell wall and are capable of locomotion. Eukaryotes have well-defined
chromosomes, a nuclear wall, and are capable of sexual reproduction. Eukaryotes have
specialized organelles including chloroplasts and mitochondria. Living prokaryotes rarely
exceed 20 microns in diameter, whereas eukaryotes are usually larger than 60 microns.
Eukaryotes began to appear in the fossil record during Mesoproterozoic (specifically 1.6 to
1.4 billion years ago).
6. Cyannobacteria formed mats near shorelines and contributed to stromatolite development.
The photosynthetic activity of cyannobacteria contributed oxygen to Earth’s atmosphere and
food for more complex organisms that were evolving.
7. Acritarchs are unicellular, spherical microfossils with resistant single-layer walls. Acritarchs
are thought to be resting-stage cells of planktonic algae. Acritarchs had apparent membranebound nuclei and chloroplasts, hence they were eukaryotes. Acritarchs first appeared about
1.6 billion years ago, and reached their maximum diversity and abundance about 850 million
years ago. After that time, they experienced a steady decline until few remained at 675
million years ago.
Their decline coincided with the major glacial episode of the
Neoproterozoic.
8. Metazoans are multicellular animals that possess more than one kind of cell and have their
cells organized into tissues and organs. The earliest known occurrence of metazoans is in
the Ediacaran hills of Australia and is late Neoproterozoic. The three Neoproterozoic
metazoan groups know as fossils are: discoids, fronds, and elongates.
9. In the Inner Gorge of the Grand Canyon, Mesoproterozoic Vishnu Schist (dated at 1.4 to 1.3
billion years) is exposed. The Vishnu is overlain by Neoproterozoic Grand Canyon
Supergroup.
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
5
6
10. Evidence of glacial episodes includes tillite conglomerates, glacial lake deposits, and glacial
striations on bedrock. The two Proterozoic glaciations occurred at about 2.0 and 0.6 billion
years ago, respectively. Archean glaciation is unlikely due to elevated surface temperatures,
and (early on) a lack of viable atmosphere and oceanic realm.
11. The Ediacaran “jellyfish,” fronds, and sea pens do not exactly match modern forms and,
furthermore and more importantly, modern forms are too soft to have preserved as
impressions like those found in the rocks.
12. Stromatolites likely provided food for newly evolved small, shelled organisms and other
primitive invertebrates. Overgrazing on stromatolites seems to have lead to their downfall as
fossils in the rock record.
13. Belt Supergroup crops out in Montana, Idaho, and British Columbia (especially in Glacier
National Park). Shallow-water carbonates and clean sands suggest shallows conditions
during deposition.
14. a
15. b
16. c
17. a
Chapter Activities
Student activities for in-depth learning.
1. Using the resources on Wilson Cycles that is posted on the web at
http://csmres.jmu.edu/Geollab/Fichter/Wilson/Wilson.html, review and brief describe the nine
stages discussed. Relate as many of these stages as possible to one of the Wilson Cycles
discussed in the chapter text, for example, the Wopmay orogen.
2. Use the web resources on various pages by using your search engine to look for information
on acritarchs. The web page on dinoflagellates constructed by the University of California,
Berkeley, Museum of Paleontology is a good place to start. What is the nature of an acritarch?
What is the geological range? How are they fossilized and in what kind of rock? Why are there
no more acritarchs?
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
6