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.

Precambrian Provinces in North America

Oldest (Archean) rocks are shown in orange.

Younger (Proterozoic) rocks are shown in green.

Proterozoic Plate Tectonics

During Late 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:

– 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

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

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 CycleThis 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 basin2. 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 beltTrans-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.

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

• 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 druing 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 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.

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 proto-Pacific 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.

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 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 - Neoproterozoic

• This time is referred to as "snowball Earth" because glacial deposits are so widespread.

• Varangian glaciation (named after an area in Norway).

• Late Proterozoic ice age lasted about 240 m.y.

• 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.

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.

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 bacteriaB = Eosphaera, an organism or uncertain affinity, about 30 micrometers in diameterC = 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 during 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.

Eukaryotes tend to be much larger than prokaryotes (larger than 60 microns, as compared with less than 20 microns).

Eukaryotic cells can be differentiated from prokaryotic cells on the basis of size.

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 glaciation9. Few acritarchs remained by 675 m.y. ago10.Extinction during Ordovician11.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

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.

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 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.

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