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

The Earth Through Time
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
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
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)
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.
2.
3.
4.
5.
6.
7.
8.
VII.
Discovered in the 1940s
Older members found in China more recently
Ancestors to some Cambrian fauna
Three body types (impressions in sediment)
a. discoidal (flat and circular; Cyclomedusa)
b. frondlike (Tribrachidium; Tribrachidium)
c. ovate to elongate (Dicksonia; Spriggia; Kimberella)
Fossils with hard exoskeleton: Cloudina
Trace fossils (shallow burrows)
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
Appearance may have followed soon after the end of global glaciation
(the “snowball Earth” hypothesis)
Changing Environment
A. Oxygen Builds up in Atmosphere: rises to 10% of current level by
end of
Proterozoic
B. Wide range of climatic conditions
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 O2 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 membrane-bound 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.
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?
Chapter 9—The Proterozoic: Dawn of a More Modern World
CHAPTER OVERVIEW
The eon we call Proterozoic began 2.5 billion years ago and ended only 540
million years ago, and thus 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
with Mesoproterozoic 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 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 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
I.
Highlights of the Paleoproterozoic (2.5 to 1.6 billion years ago)
A. Early Plate Tectonics: Evidence from Canada’s Northwest Territory
B. Evidence of Earth’s First Ice Age
C. End of the Paleoproterozoic
II.
Highlights of the Mesoproterozoic (1.6 to 1.0 billion years ago)
A. An Aborted Rift, Rich in Copper
B. The Grenville Orogeny
C. Building a New Supercontinent - Rodina
III.
Highlights of the Neoproterozoic (1.0 billion to 540 million years ago)
A. A Big Chill
B. Earth’s Glacial History
IV.
Proterozoic Rocks South of the Canadian Shield
A. Grand Canyon Precambrian Rocks
V.
Proterozoic Life
A. Microfossils of the Gunflint Chert
B. The Rise of Eukaryotes
C. Acritarchs
D. Many-Celled Animals Arrive: The Metazoans
E. Ediacaran Fauna
F. The Vendoza Controversy
G. What the Ediacaran Fossils Tell Us About the Proterozoic
H. Oxygen and Climate Changes in the Proterozoic Environment
KEY TERMS (pages given in parentheses)
acritarchs (265): 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 Proterozoic.
Animikie Group (257): 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 (267): In Kimberella, a body cavity in which the digestive tract and other
internal organs are suspended.
cyanobacteria (257): 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 (259): Chunks of rock released from melting icebergs.
Ediacaran fauna (266): Impressions of large, soft-bodied animals. The bestknown 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.
Discoidal forms 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 (253): A shallow, inland sea that flooded the continental interiors.
Grenville orogeny (259): 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 (257): The event that separates Paleoproterozoic from
Mesoproterozoic geologic history. It was a period of intense folding,
metamorphism, and westward thrust faulting.
Keweenawan (258): 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 (257): A curving, elongate structural depression found east of
the Superior province where there is an extensive outcrop of rocks.
Laurentia (253): A large continent formed during Proterozoic by the welding of
Archeon elements. (The Superior, Slave, Hearne, Rae, Wyoming, and Nain
provinces.)
metazoan (266): Multicellular animals that possess more than one kind of cell
and have their cells organized into tissues and organs.
Mesoproterozoic (251): The middle part of Proterozoic, approximately 1.6 to 1.0
billion years ago.
molecular fossil (265): Biochemical remnants of microfossils, i.e. eukaryotes,
that are chemical clues to early life forms.
Neoproterozoic (251): The youngest part of Proterozoic, approximately 1.0
billion years ago to the beginning of the Paleozoic Era, 540 million years ago.
orogen (253):
metamorphism.
Belt
of
crustal
compression,
mountain
building,
and
Paleoproterozoic (251): The early part of Proterozoic, approximately 2.5 to 1.6
billion years ago.
Precambrian province (253): 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.
red beds (271):
Rodinia (251): Supercontinent found by the gathering of continents during
Neoproterozoic. One of the most significant global events of Neoproterozoic.
tillite (256): Unsorted glacial deformation (till) that has been converted into solid
rock. Used as a paleoclimatic indicator of glaciation.
varves (254): A thin sedimentary layer or pair of layers that represent the
depositional record of a single year.
Vendobionta (269): New name given to the Vendoza to avoid confusion with the
Russian time term Vendoza.
Vendoza (269): 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.
Also, a name used in Russia for the final period of
Neoproterozoic.
Wilson Cycle (254): 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.
RESPONSES TO QUESTIONS ACCOMPANYING SELECTED FIGURES
FIGURE 9–3 (p. 254) The faults beneath the turbidites are normal (tensional)
faults. These faults are associated with the passive margins of tectonic plates.
FIGURE 9–7 (p. 257) Hematite serves as the red pigment in the banded iron
formations.
FIGURE 9–15 (p. 262) A nonconformity separates the Vishnu Schist and
Zoroaster Granite from the sedimentary sequence of the overlying Grand Canyon
Supergroup.
FIGURE 9–28 (p. 268) Kimberella shows evidence of a true coelum, or body
cavity, in which a complete digestive tract and other internal organs can be
suspended.
CHAPTER 9
Proterozoic: Dawn of a More Modern World
PROTEROZOIC EON
2.5 billion years to 542 million years ago
Comprises 42% of Earth history
Divided into three eras:
Paleoproterozoic Era (2.5 - 1.6 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)
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
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.
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.
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.
OVERVIEW OF THE PRECAMBRIAN
FIGURE 9-31 Correlation of major events in the biosphere, lithosphere, and
atmosphere.
OVERVIEW OF PROTEROZOIC EVENTS
FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events.
PALEOPROTEROZOIC ERA
The oldest part of Proterozoic
Ranges from about 2.5 b.y. to 1.6 b.y.
Covers 900 million years
MAJOR EVENTS OF THE PALEOPROTEROZOIC
1.Active plate tectonics
2.Major mountain building on all major continents
3.Earth's first glaciation
4.Widespread volcanism (continental flood basalts)
5.Rise in atmospheric oxygen (great oxidation event)
MAJOR EVENTS OF THE PALEOPROTEROZOIC
6.Accumulation of high concentrations of organic matter in sediments (Shunga
event) 2000 m.y. ago, and generation of petroleum
7.Oldest known phosphorites and phosphate concretions
Orogenic belts developed around margins of the Archean provinces.
Wopmay belt in NW Canada
Trans-Hudson belt, SW of Hudson Bay
OROGENIC BELTS
WOPMAY OROGENIC BELT CONTAINS EVIDENCE OF:
1.Rifting and opening of an ocean basin (with normal faults, continental sediments,
and lava flows)
2.Sedimentation along new continental margins (with shallow marine quartz
sandstones and carbonate deposition)
3.Closure of the ocean basin (with deep water clastics overlain by deltaic and fluvial
sands), followed by folding and faulting.
WILSON CYCLE
This sequence of events in the Wopmay orogenic belt is called a Wilson Cycle, and is
a result of plate tectonics.
1.Rifting and opening of an ocean basin
2.Sedimentation along new continental margins
3.Closure of the ocean basin
The sequence of events in the Wopmay belt is similar to that in Paleozoic rocks of the
Appalachians.
TRANS-HUDSON OROGENIC BELT
Trans-Hudson belt contains the sedimentary record of a Wilson Cycle, with evidence
of:
1.Rifting
2.Opening of an ocean basin
3.Deposition of sediment
4.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.
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.
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.
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
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.
BANDED IRON FORMATIONS AND PROKARYOTE FOSSILS

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.
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.
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.
MIDCONTINENT RIFT AND THE KEWEENAWAN SEQUENCE
Midcontinent rift extends southward from Lake Superior region.
Overlies Archean crystalline basement rocks and Paleoproterozoic Animikian
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.
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.
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
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.
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.
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 protoPacific Ocean, Panthalassa, along the western side of North America.
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.
NEOPROTEROZOIC ERA
The Neoproterozoic (or "new" Proterozoic) ranges from about 1.0 b.y. to 0.542 b.y.
(542 m.y.).
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.
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
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).
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.
PLATE TECTONICS AND GLACIATION
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.
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).
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.
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.
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.
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)
EVENTS RECORDED IN PROTEROZOIC ROCKS
1.Collision of an Archean terrane with volcanic island arc, 1.7 or 1.8 b.y.a. (Wyoming
and western Colorado)
2.Extensive magma intrusion in Mesoproterozoic, 1.5–1.4 b.y.a. (California to
Labrador)
3.Widespread rifting
4.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).
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.
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.
LIFE AT THE BEGINNING OF PROTEROZOIC WAS SIMILAR TO THAT OF ARCHEAN
1.Archaea in deep sea hydrothermal vents
2.Planktonic prokaryotes floated in seas and lakes
3.Anaerobic prokaryotes in oxygen-deficient environments
4.Photosynthetic cyanobacteria (prokaryotes) constructed stromatolites (algal
filaments)
5.Eukaryotes (as indicated by molecular fossils)
OTHER FORMS OF LIFE APPEARED DURING PROTEROZOIC
1.More diverse eukaryotes including acritarchs
2.Metazoans or multicellular animals with soft bodies
3.Metazoans with tiny calcium carbonate tubes or shells
4.Metazoans that left burrows in the sediment
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).
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)
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.
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.
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.
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).
THE RISE OF EUKARYOTES FIGURE 8-37 (A) Comparison of a prokaryote cell (left)
and a eukaryote cell.
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.
ACRITARCHS
1.Eukaryotes
2.Single-celled, spherical microfossils
3.Thick organic covering
4.May have been phytoplankton
5.First appeared 1.6 b.y. ago (at Paleoproterozoic-Mesoproterozoic boundary)
6.Some resemble cysts or resting stages of modern marine algae called
dinoflagellates.
ACRITARCHS
7.Reached maximum diversity and abundance 850 m.y. ago
8.Declined during Neoproterozoic glaciation
9.Few acritarchs remained by 675 m.y. ago
10.Extinction during Ordovician
11.Useful for correlating Proterozoic strata
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.
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)
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).
TYPES OF EDIACARA FOSSILS
Discoidal
Frondlike
Elongate or ovate
FIGURE 9-28 Reconstruction of Kimberella.
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).
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.
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.
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.
I. J. Hofman
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.
REVIEW OF PROTEROZOIC EVENTS
FIGURE 9-1 Pathway through the Proterozoic, depicting major geologic events.
• 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: .
• 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: .
• 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., 1971. "The Oldest Fossils,"
Scientific American 224:30-42.
• FIGURE 9-28 Reconstruction of Kimberella. Source: .
• FIGURE 9-29 Cloudina, the earliest known calcium carbonate shell-bearing
fossils. Source: .