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Transcript
Chapter 9
The 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 the 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 the early
Proterozoic.
Precambrian Provinces in
North America
Oldest (Archean)
rocks are shown in
orange.
Younger (Proterozoic)
rocks are shown in
green.
Precambrian Provinces in
North America
• Suturing occurred
along mountain belts
or orogens.
• Provinces were
assembled by about
1.7 b.y. ago.
• Laurentia continued
to grow by accretion
throughout the
Proterozoic.
Proterozoic Plate Tectonics
Late in the Proterozoic, the continents
became assembled into a supercontinent
called Rodinia.
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
• Proterozoic glaciations occurred during
the:
– Paleoproterozoic, about 2.4-2.3 b.y. ago
(Huronian glaciation)
– Neoproterozoic, 850-600 m.y. ago (Varangian
glaciation)
Overview
of the
Precambrian
Overview of
Proterozoic
Events
Paleoproterozoic Era
• The oldest part of the
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
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 the Paleozoic 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.
• Animikie Group.
• Rich iron deposits were foundation of steel
industry in Great Lakes region (Illinois,
Indiana, Ohio, Pennsylvania).
• Mining has declined because U.S. imports
most of its iron ore and steel.
Banded iron formations and
prokaryote fossils
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.
Mesoproterozoic Era
The Mesoproterozoic
(or middle Proterozoic)
ranges from about 1.6
b.y. - 1.0 b.y.
Highlights of the Mesoproterozoic
• 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 rocks
(Animikie Group BIF).
Midcontinent Rift and the
Keweenawan Sequence
• Large volumes of basaltic rock indicate
presence of an old 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.
Copper Mineralization
• Native copper fills vesicles (gas bubbles)
in the Keweenawan basalt, and joints and
pore spaces in associated conglomerates.
• Native Americans mined the copper as
early as 3000 BC.
• Copper was mined extensively from 1850
to 1950, but copper production ceased in
the 1970's.
Grenville Province and
Grenville Orogeny
The Grenville
province in eastern
North America
extends from
northeastern
Canada to Texas.
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. to 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 the Paleozoic Era, 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.
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.
Rifting in Rodinia
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 faultbounded 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
the Paleozoic orogenies.
Glacial deposits in the
Neoproterozoic
• Glacial deposits formed roughly 600 - 700 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
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.
Glacial deposits in the
Neoproterozoic
• This time is referred to as
"snowball Earth“ because
glacial deposits are so
widespread.
• Varangian glaciation
(named after an area in
Norway).
• The late Proterozoic ice age
lasted about 240 m.y.
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.
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.
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.
Precambrian rocks of the
Grand Canyon
Grand Canyon Supergroup overlies unconformity.
Neoproterozoic sandstones, siltstones, and
shales. Correlates with Belt Supergroup.
Unconformably overlain by Cambrian rocks.
Proterozoic Life
Life at the beginning of the Proterozoic
was similar to that in the 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 the 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 magnesiumreducing bacteria
B = Eosphaera, an organism
or uncertain affinity, about 30
micrometers in diameter
C = Animikiea (probably
algae)
D = Kakabekia, an organism
or uncertain affinity
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 in the history of life.
Eukaryotes have the potential for sexual
reproduction, which increases variation
through genetic recombination.
The Rise of Eukaryotes
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
• Eukaryotes appeared by Archean time (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.
2.
3.
4.
5.
Eukaryotes
Single-celled, spherical microfossils
Thick organic covering
May have been phytoplankton
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.Extinct in Ordovician time
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 in the
Neoproterozoic, about 630 m.y. ago (0.63
b.y.). Preserved as impressions of softbodied organisms in sandstones.
Examples of metazoan fossils in
the Proterozoic
• Ediacara fauna - Imprints of soft-bodied
organisms, first found in Australia in the 1940's
• 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)
Geologic time scale
across the
PrecambrianCambrian boundary,
showing the
Ediacaran fauna and
other faunas.
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
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 in the Neoproterozoic.
Small Shelly Fauna:
The Origin of Hard Parts
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.
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 late Neoproterozoic rocks.
• In rocks deposited after the
Neoproterozoic Varangian glaciation.
• Mostly simple, shallow burrows.
• Trace fossils increase in diversity,
complexity, and number in younger
(Cambrian) rocks.
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
Review
of the
Precambrian