INTRODUCTION
EVOLUTION OF PROTEROZOIC
CONTINENTS
Paleoproterozoic History of Laurentia
¬Perspective The Sudbury Meteorite Impact and Its Aftermath
Mesoproterozoic Accretion and Igneous Activity
Mesoproterozoic Orogeny and Rifting
Meso- and Neoproterozoic Sedimentation
PROTEROZOIC SUPERCONTINENTS
ANCIENT GLACIERS AND THEIR DEPOSITS
Paleoproterozoic Glaciers
Glaciers of the Neoproterozoic
THE EVOLVING ATMOSPHERE
Banded iron Formations (BIFs)
Continental Red Beds
LIFE OF THE PROTEROZOIC
Eukaryotic Cells Evolve
Endosymbiosis and the Origin of Eukaryotic Cells
The Dawn of Multicelled Organisms
Neoproterozoic Animals
PROTEROZOIC MINERAL
RESOURCES
SUMMARY
The following content objectives are presented in Chapter 9:
¬ A large landmass called Laurentia, made up mostly of Greenland and North America, formed by the amalgamation of Archean cratons along deformation belts during the Paleoproterozoic.
¬ Following its initial stage of amalgamation, Laurentia grew by accretion along its southern and eastern margins.
¬ The Mesoproterozoic of Laurentia was a time of widespread igneous activity, orogenies, and rifting.
¬ Widespread Proterozoic assemblages of sandstone, shale, and carbonate rocks look much like the rocks deposited now on passive continental margins.
¬ Plate tectonics, essentially like that occurring now, was operating during the Proterozoic and one or possibly two supercontinents formed.
¬ The presence of banded iron formations and continental red beds indicate that at least some free oxygen was present in the atmosphere.
¬ Extensive glaciation took place during the Paleoproterozoic and the Neoproterozoic.
¬ The first eukaryotic cells, that is, cells with a nucleus and other internal structures, evolved from prokaryotic cells by 1.2 billion years ago.
¬ Impressions of multicelled animals are found on all continents except Antarctica.
¬ Banded iron formations, as sources of iron ore, are important Proterozoic resources, as are deposits of copper, platinum, and nickel.
To exhibit mastery of this chapter, students should be able to
demonstrate comprehension of the following:
¬ the important sequence of events in the evolution
of Laurentia, including major orogenies and the Midcontinent rift
¬ Archean and Proterozoic styles of plate tectonics
¬ the evidence for Proterozoic glaciation episodes
¬ the evolution of the atmosphere
¬ the distribution, age, and origin of banded iron
formations
¬ the evidence and significance of the first
eukaryotic cells
¬ characteristics of eukaryotes, and the possible
role of endosymbiosis in their
development
¬ characteristics of Neoproterozoic animals
¬ types and geologic associations of Proterozoic
ore deposits
1. The crust-forming processes that yielded Archean
granite-gneiss complexes and greenstone belts continued into the Proterozoic
but at a considerably reduced rate.
Figure 9.1 Proterozoic
Rocks
2.
Paleoproterozoic
collisions between Archean cratons formed larger cratons that served as nuclei,
around which crust accreted. One large landmass so formed was Laurentia,
consisting mostly of North America and Greenland. The collisions among plates formed
several orogens. Sedimentary rocks
in the Wopmay orogen record the opening and closing of an ocean basin, or
Wilson Cycle.
Some of the sedimentary rocks in the Wopmay organ
consist of a sedimentary rock suite called a sandstone-carbonate-shale
assemblage that forms on passive margins.
Figure 9.2 Proterozoic Evolution of Laurentia
Figure 9.3 The Wopmay
Orogen and the Wilson Cycle (Active figure)
Figure 9.4 Paleoproterozoic Sedimentary Rocks
3. Paleoproterozoic amalgamation of cratons, followed by Mesoproterozoic
igneous activity, the Grenville orogeny, and the Midcontinent rift, were
important events in the evolution of Laurentia.
Figure 9.5 Rocks of the Grenville Orogen
Figure 9.6 The Midcontinent Rift
Figure 9.7 Proterozoic Rocks in the Western United States and Canada
4. Sandstone-carbonate-shale assemblages deposited on passive continental
margins were very common by Proterozoic time.
5. Ophiolite sequences marking convergent plate
boundaries are first well documented from the Neoarchean and Paleoproterozoic,
indicating that a plate tectonic style similar to that operating now had become
established.
Figure
9.8 The
Jormua Mafic-Ultramafic Complex in Finland
6. The supercontinent Rodinia assembled between 1.3
and 1.0 billion years ago, fragmented, and then reassembled to form Pannotia
about 650 million years ago, which began fragmenting about 550 million years
ago.
Figure
9.9 Rodinia
7. Glaciers
were widespread during both the Paleoproterozoic and the Neoproterozoic.
Figure 9.10 Paleo- and Neoproterozoic Glaciers
Figure 9.11 Proterozoic Glaciation
Investigate the Snowball Earth hypothesis in more detail. Gabrielle Walker details the story of Paul Hoffman, who originally coined the phrase ÒSnowball Earth,Ó in Snowball Earth: the Story of a Maverick Scientist and His Theory of a Global Catastrophe That Spawned Life as We Know It (2004).
The hypothesis rests on evidence of glaciers in tropical regions, only 11 degrees from the equator, such as Panama today. It is hypothesized that because tropical glaciers existed, it is likely that glaciers covered the landmasses of the entire globe. ÒSnowball Earth,Ó was repeated at least once more 700 million years ago, when glaciers were even closer to the equator. The plummeting temperatures were likely caused by a lack of carbon dioxide, which may have been utilized and removed by abundant plants. A large event—such as a volcanic eruption, a meteorite impact, or the sudden release of frozen methane deposits—would be needed to release carbon dioxide into the atmosphere. Science News, March 29, 1997 v.151 n.13 p.196.
8. Photosynthesis
continued to release free oxygen into the atmosphere, which became increasingly
rich in oxygen through the Proterozoic.
9. Fully 92% of EarthÕs
iron ore deposits in the form of banded iron formations (BIFs) were deposited
between 2.5 and 2.0 billion years ago.
Figure
9.11 Paleoproterozoic
Banded Iron Formation (BIF)
Enrichment Topic 2. Elements and Evolution
The bulk of EarthÕs surface is covered by ocean
regions, where life is scarce. Although thinly-populated ecosystems do not lack
in energy (sunshine), water, or the basic building blocks (carbon, hydrogen,
oxygen, nitrogen), they are deficit in other elements needed to sustain
life.
Massive deposits of BIFs changed the
environmental chemistry of the oceans. Whereas the oceans were rich in iron
during the first half of Earth history, iron is scarce in oceans today. It is
often called a limiting nutrient. Sulfur was also affected. Other changes in
bioessential elements are more subtle, but the changing environment affected
manganese, cobalt, nickel, copper, zinc, and molybdenum. Changes in the chemical composition of
the ocean affected the biosphere as well. A. Anbar, ÒElements and Evolution.Ó Science,
December 5, 2008, v. 322, p. 1481-1483.
10. Continental red beds appeared about 1.8
billion years ago, and indicate that EarthÕs atmosphere had enough free oxygen
for oxidation of iron compounds.
11. Most of the known Proterozoic organisms are single-celled prokaryotes
(bacteria). When eukaryotic cells first appeared is uncertain, but they were
probably present by 2.1 billion years ago.
Endosymbiosis is a widely accepted theory for their origin.
Figure
9.12 Proterozoic Fossil
Bacteria and Stromatolites
Figure 9.13 Prokaryotic and Eukaryotic Cells
Table 9.1 The Six-Kingdom Classification of Organisms
Figure 9.14 The Three Domain Classification System
Figure 9.15 The Oldest Known Eukaryote and Megafossil
Figure 9.16 Proterozoic Fossils
Figure 9.17 Endosymbiosis and the Origin of Eukaryotic Cells
Enrichment Topic 3. Tracing Evolution to its Roots
Microbiologist
Mitchell Sogin traced human evolution back to the first organism that gave rise
to humans. When he used DNA
technology to trace the molecular evolution of todayÕs known species back to
their origins, he was surprised to find out that our oldest animal evolutionary
link is the sponge. Tracing DNA
beyond the animal lineage yielded an even bigger surprise: Sogin discovered that animals are more
closely related to fungi. McClintock,
ÒThis is Your Ancestor,Ó Discover, Nov.
2004 v.25 n.11 p.64-69.
12. Multicelled organisms were present
by the Neoproterozoic, but the fossil record does not tell us how the
transition took place.
Figure
9.18 Single Celled
and Multicelled Organisms
13. The first
controversy-free fossils of animals come from the Ediacaran fauna of Australia
and other locations. Animals were widespread at this time, but because all
lacked durable skeletons their fossils are not common.
Figure
9.19 The Ediacaran
Fauna of Australia
Figure
9.20 Ediacaran-type
Fossils from Mistaken Point Formation of
Newfoundland
Figure
9.21 Neoproterozoic
Fossils
Enrichment Topic 4. Ediacaran Fauna.
The Ediacaran fauna may provide the first glimpse
of life after ÒSnowball Earth.Ó One species, Charnia, that consists of slender fronds 2 meters in length, is the
longest Ediacaran fossil. Its
presence between glacial deposits provides the first glimpse of megafauna after
the melting of the glaciers. Narbonne & Gehling, Geology, January 2003, v.31 n.1 p.27-30.
The Ediacaran fauna
subdivided vertical space within a community, a characteristic known as
epifaunal tiering. In
Neoproterozoic Ediacaran communities, at least three tiers were present: a lower level 0-8 cm above the seafloor,
an intermediate level 8-22 cm above the seafloor, and an upper level from 22
cm to as high as 120 cm. This structure is consistent with
suspension feeders or organisms that absorb nutrients directly from the
water. Clapham and Narbonne, Geology, July 2002, v.30 no7 p627-630.
Enrichment Topic 5. The Cost of Diversity
The diversity exhibited in faunas such as the
Ediacaran is not without cost. Drew
Allen, an ecologist at the University of California at Santa Barabara,
calculated how much energy it costs to generate a new species. The answer is a staggering 1023
joules, more energy released by all the fossil fuels burned on the Earth in one
year. Allen investigated
foraminifera, one-celled plankton, and used both the body size of the organism
and its metabolismÕs dependence of temperature. Although the energy required for a new
species is constant, species form more quickly at the equator. Discover,
Sept. 2006, p. 14.
14. Most
of the worldÕs iron ore production is from Proterozoic banded iron formations.
Other important resources include nickel and platinum. Figure 9.22 Iron
Mining from the Lake Superior Region
Banded Iron
Formations
Help students to gain an understanding of the importance of
banded iron formations. Banded iron
formations represent a significant economic resource. Ninety percent of the
world's iron ore (1 billion tons yearly) comes from these types of deposits.
From the period of 2.0 to 1.8 billion years ago, it is estimated that 1014-15 tons of iron were precipitated. Have students calculate the rate of deposition of the iron deposits, as well as how long it will take at our current rate of consumption to deplete the iron ore.
Diversity of Life in
the Proterozoic Eon
Emphasize that the fossil record is very incomplete, and have students hypothesize how much of the Proterozoic EonÕs life forms are actually represented as fossils. Before the advent of eukaryotic cells, reproduction was accomplished asexually. Once eukaryotic cells evolved, evolution proceeded fairly rapidly to the diversity seen in the Neoproterozoic fauna of the Ediacaran. Review the connections between sexual reproduction and speed of speciation.
Ediacaran Fauna
Investigate the earliest animals of the Ediacaran fauna, and have students predict the ecosystems in which these organisms lived. Are the animals all herbivores? How do students predict the animals acquired food? Do these same organisms exist today? If not, why? Why arenÕt Edicaran fauna as well-preserved as some later animal specimens?
Relationship of the
Precambrian to Geologic Time
At the end of this chapter, you may want to remind students of the enormity of time that is contained in the Precambrian. Visual aids, such as adding machine tape or toilet paper timelines, are great tools to help students visualize just how much time has passed before the onset of our current eon, the Phanerozoic. Ask students to recall the great events of the Archean (development of tectonics, origin of prokaryotic life, photosynthesis), and the Proterozoic (modern tectonics, eukaryotic cell, multicelled life, oxygenated atmosphere). Then ask students to list some of the events that they believe will take place in the Phanerozoic Eon (origin of vertebrates, fish, amphibians, reptiles, mammals, insects, terrestrial plants, humans). How do these two lists compare—Precambrian versus Phanerozoic Eon—in time, as well as in events?
1. According to the plate tectonic theory, what
should be the orogenic belt-shield-craton structure of all of the continents?
2. The Belt Basin in northern Idaho and western
Montana was the site of the accumulation of a phenomenal amount of silt and
clay sized sediment. Have students calculate the amount of time it would take
to deposit 42,000 feet of sediment given deposition rates ranging from 1 to
about 5 mm/year.
3. Has the location of
BIF deposits affected the industrial revolution in the U.S.? Has this location affected the
development of the automobile industry in the U.S.?
|
banded iron formation (BIF) |
Laurentia |
sandstone-carbonate-shale |
|
continental red beds |
Midcontinent rift |
assemblage |
|
Ediacaran fauna |
multicelled organism |
supercontinent |
|
Endosymbiosis |
Orogen |
Wilson cycle |
|
eukaryotic cell |
Pannotia |
|
|
Grenville orogeny |
Rodinia |
|
|
1. a |
5. c |
9. c |
|
2. c |
6. c |
10. a |
|
3. e |
7. a |
|
|
4. b |
8. d |
|
11. The first fossils that all paleontologists agree
are animals are found in the Ediacaran fauna of Australia and other continents
(except Antarctica), and these are about 545- to 600-million-years old. Their
fossils are rare because the animals were soft bodied and so did not fossilize
easily.
12. The Midcontinent rift is a long narrow trough
with two branches. One branch extends southeast as far as Kansas, and one extends
southeasterly through Michigan into Ohio. The rifting began about 1.1 billion
years ago Although not all geologists agree, many think that the Midcontinent
rift is a failed rift where Laurentia began splitting apart.
The
central part of the rift contains numerous overlapping basaltic lava flows.
Along the riftÕs margins, conglomerate was deposited in large alluvial fans
that grade into sandstone and shale with increasing distance from the sediment
source.
13.
The endosymbiosis theory for the origin of
eukaryotic cells states that eukaryotes formed by a complex, sequential
incorporation of symbiotic prokaryotes into an original prokaryotic host. The
evidence comes from studies of living eukaryotic cells containing internal
structures called organelles. The
organelles have their own genetic material and synthesize proteins just as
prokaryotic cells do. These organelles, with their own genetic material and
protein-synthesizing capabilities, are thought to have been free-living
bacteria that entered into a symbiotic relationship, eventually giving rise to
eukaryotic cells.
14. Proterozoic banded iron formations and
continental red beds contain iron oxides, which indicate the presence of oxygen
in the atmosphere. The Archean had little or no free oxygen but abundant carbon
dioxide. Iron is a high reactive element, and in an oxidizing atmosphere it
combines with oxygen to form rustlike oxides that do not dissolve in water. If
oxygen is absent, iron is easily taken into solution and can accumulate in
large quantities in oceans. Therefore, the presence of the BIFs and red beds
indicate that oxygen was present in the Proterozoic, although it was not in the
Archean.
15.
The evolution of
Laurentia was long, complex, and is still not fully understood. However, it involves the amalgamation of cratons,
the accretion of volcanic arcs and oceanic terranes, and extensive plutonism,
metamorphism, and volcanism. Between 2.0 and 1.8 billion years ago, Archean
cratons collided and formed several orogens
16. A Wilson cycle, named for Canadian geologist J. Tuzo Wilson, is a full
cycle recording the opening and closing of an ocean basin. There is evidence of Wilson cycles in
Precambrian rocks. Some geologists think the Wopmay orogen represents a Wilson
cycle. Evidence for the opening of
an ocean basin include sandstone-carbonate-shale assemblages (a passive margin
sedimentary sequence), while ophiolites indicate subduction, or the closing of
an ocean basin.
17. Using the principle of uniformitarianism,
you should look for the known features of glacial movement in addition to the
tillite, such as varves, dropstones, and polished and striated bedrock.
18. The sedimentary assemblages of sandstone-carbonate-shale represent a
passive continental margin. Passive margin deposits are rare or absent in
Archean rocks, but they became common during the Proterozoic and thereafter.
19. The Archean was characterized by the origin of granite-gneiss terrains
and greenstone belts that were shaped into cratons. Although these rock associations formed
in the Proterozoic, they did so at a considerably reduced rate. Archean rocks
are metamorphic, while Proterozoic rocks are unaltered or nearly so.
Apply Your Knowledge
1. 4000 meters = 4,000,000 mm, 1.45 billion –
850 million = 600 million years. So the sedimentation rate =
4,000,000/600,000,000 = 0.0067 mm/year. This is unlikely to represent the
actual rate of sediment accumulation because it does not take erosion into
account.
2. If we were to look for
signs that oceans and continents formed through tectonic activity, such as a
Wilson cycle that records the opening and closing of an ocean basin, we should
look for evidence of passive continental sedimentation
(sandstone-carbonate-shale assemblages) that mark the opening of an ocean
basin, and we should look for evidence of subduction (ophiolites) that mark the
closing of an ocean basin.
3. On the upper and lower
surfaces of the Purcell sill are metamorphosed zones, or light-colored marble.
This indicates that the sill was emplaced into existing limestone (contact
metamorphism). Slower cooling in the middle of the sill would yield larger
crystals than the upper and lower boundaries, which would cool quicker in
contact with existing sedimentary rocks. In addition, we can use the principle
of inclusions. If the Purcell sill was a buried lava flow, we should see signs
of erosion on its upper surface, and the inclusion of some of the diorite in
the upper layers of the Siyeh Limestone. Therefore, the Purcell sill is YOUNGER
than the Siyeh Limestone, and if it is 1.45 billion years old, the Siyeh
Limestone must be older than 1.45 billion years.
4. a. The Vishnu Schist
appears older than the Zoraster Granite. The schist was metamorphosed before the
granite was emplaced. We can use the principle of cross-cutting relationships,
and our knowledge of metamorphism to determine this. (If the schist was younger
than the granite, the granite should also have been subjected to metamorphism.)
b. The
uncomformity present is a noncomformity, in which metamorphic or
igneous rocks underlie
younger sedimentary rocks.
c. The unconformity between the Grand
Canyon supergroup and the Tapeats sandstone is an angular unconformity because
the Grand Canyon supergroup is at an angle to the overlying Tapeats.
d. The sequence of the Tonto Group
(sandstone-shale-limestone) appears to
represent a
transgression, with the shoreline migrating toward the continent.
This
will yield deeper water and sedimentation patterns in the same area as the
shoreline migrates toward the craton.
All Pages Copyright © GeoClassroom. All Rights Reserved.