Download Chapter 9 Proterozoic

The Length of the Proterozoic
• the
Proterozoic
Eon alone,
– at 1.955
billion years
long,
– accounts for
42.5% of all
geologic time
– yet we review
this long
episode of
Earth and life
history in a
single section
The Phanerozoic
• Yet the
Phanerozoic,
– consisting of
• Paleozoic,
• Mesozoic,
• Cenozoic
eras,
– lasted a
comparatively
brief 545
million years
– is the subject
of the rest of
the course
The Proterozoic Eon:
Top Ten Significant Events
•
•
•
•
Plate tectonics occurred similar to modern rates
Accretion at continental boundaries
Assembly of Laurentia; and two super continents
Widespread sandstone, carbonate, shale deposits
(continental shelf deposits)
•
•
•
•
Extensive continental glaciation
Mid-continent rift formed in North America
Widespread occurrence of stromatolites
Formation of banded iron and other mineral
resources (gold, copper, platinum, nickel)
• Free oxygen in atmosphere
• Evolution of eukaryotic cells
Style of Crustal Evolution
• Archean crust-forming processes
generated
– granite-gneiss complexes
– and greenstone belts
– that were shaped into cratons
• During the Proterozoic, these formed at a
considerably reduced rate and cooler
temperatures
Contrasting Metamorphism
• Many Archean rocks have been
metamorphosed,
• However, vast exposures of Proterozoic
rocks
– show little or no effects of metamorphism,
– and in many areas they are separated
– from Archean rocks by a profound
unconformity
Evolution of
Proterozoic Continents
• Archean cratons assembled during
collisions of island arcs and minicontinents,
– providing the center of today’s continents.
– Proterozoic crust accreted, at edges forming
much larger landmasses
• Proterozoic accretion at craton margins
– probably took place more rapidly than today
• Earth possessed more radiogenic heat,
– but the process continues even now
Proterozoic Greenstone
Belts
• They were not as common after the
Archean,
– near absence of ultramafic rocks
– WHY would this happen?
Focus on Laurentia
• Our focus here is on the geologic evolution
of Laurentia,
– a large landmass that consisted of what is now
• North America,
• Greenland,
• parts of northwestern Scotland,
• and perhaps some of the Baltic shield of
Scandinavia
Early Proterozoic History of
Laurentia
• Laurentia originated ~ 2.0 billion years ago
• collisions called orogens
formed linear or arcuate deformation belts
– in which many of the rocks have been
• metamorphosed
• and intruded by magma
• thus forming plutons, especially
batholiths
Proterozoic Evolution of Laurentia
• Archean cratons were sutured
– along deformation belts called orogens,
• By 1.8 billion years ago,
– much of what is now Greenland, central
Canada, and the north-central United
States existed
• Laurentia grew along its southern margin
– by accretion
What is the evidence?
Craton-Forming Processes
– Recorded in
rocks
– In northwestern
Canada
• where the Slave
and Rae cratons
collided
Craton-Forming Processes
• the Trans Hudson orogen
• in Canada and the
United States,
– where the Superior,
Hearne, and Wyoming
cratons
– were sutured
• The southern margin of
Laurentia
– is the site of the
Penokian orogen
Wilson Cycle
• Rocks of the Wopmay orogen
– in northwestern Canada are important
– because they record the Wilson cycle
• opening and closing of an ocean basin
• A complete Wilson cycle,
• named for the Canadian geologist J. Tuzo Wilson,
– involves
•
•
•
•
fragmentation of a continent (rifting)
opening of an ocean basin
followed by closing of an ocean basin,
and finally reassembly of the continent
Wopmay Orogen
• Some of the
rocks in
Wopmay orogen
– are sandstonecarbonateshale
assemblages,
– a suite of rocks
typical of passive
continental
margins
– that first become
widespread during
the Proterozoic
Early Proterozoic Rocks in Great Lakes
Region: Evidence of continental shelf
• Early Proterozoic sandstone-carbonate-shale
assemblages are widespread near the Great
Lakes
Where? N. Michigan
Outcrop of Sturgeon Quartzite
• The sandstones have a variety of
sedimentary structures
– such as
– ripple
marks
– and
crossbeds
– Northern
Michigan
Outcrop of Kona Dolomite
“warm shallow marine”
• Some of the carbonate rocks, now mostly
dolostone,
– such as the Kona Dolomite,
– contain
abundant
bulbous
structures
known as
stromatolites
– Northern
Michigan
Penokean Orogen
• These rocks of northern Michigan
– have been only moderately deformed
– and are now part
of the Penokean
orogen
Southern Margin Accretion
• Laurentia grew along its southern margin
– by accretion of the Central Plains, Yavapai,
and Mazatzal orogens
• Also notice that the
Midcontinental Rift
– had formed in the
Great Lakes region
by this time
BIF, Red Beds, Glaciers
• This was also the time during which
– most of Earth’s banded iron formations (BIF)
– were deposited
• The first continental red beds
– sandstone and shale with oxidized iron
– were deposited about 1.8 billion years ago
• We will have more to say about BIF
– and red beds in the section on “The Evolving
Atmosphere”
• In addition, some Early Proterozoic rocks
– provide excellent evidence for widespread
glaciation
Proterozoic Igneous Activity
• These igneous rocks are exposed
– in eastern Canada, extend across Greenland,
– and are also found
in the Baltic shield
of Scandinavia
Igneous Activity
Why? How do we know?
• However, the igneous rocks are deeply buried
– by younger rocks in most areas
• The origin of these
– are the subject of debate
• According to one hypothesis
– large-scale upwelling of magma
– beneath a Proterozoic supercontinent
– produced the rocks
Middle Proterozoic
Orogeny and Rifting
• The only Middle Proterozoic event in
Laurentia
– was the Grenville orogeny
– in the eastern part of the continent
– 1.3 to 1.0 billion years old
• Grenville rocks are well exposed
– in the present-day northern Appalachian
Mountains
– as well as in eastern Canada, Greenland, and
Scandinavia
Grenville Orogeny
• A final episode of Proterozoic accretion
– occurred during the Grenville orogeny
75% of North America
• By this final stage, about 75%
– of present-day North America existed
• The remaining 25%
– accreted along its margins,
– particularly its eastern and western margins,
– during the Phanerozoic Eon
Midcontinent Rift
• Grenville deformation in Laurentia
– was accompanied by the origin
– of the Midcontinent rift,
• a long narrow continental trough bounded by faults,
• extending from the Lake Superior basin southwest
into Kansas,
• and a southeasterly branch extends through
Michigan into Ohio
• It cuts through Archean and Early
Proterozoic rocks
– and terminates in the east against rocks
– of the Grenville orogen
Location of the Midcontinent Rift
• Rocks
filling the
rift
– are
exposed
around
Lake
Superior
– but are
deeply
buried
elsewher
e
Midcontinental Rift
• Most of the rift is buried beneath younger
rocks
– except in the Lake Superior region
– with various igneous and sedimentary rocks
exposed
• The Evidence:
• numerous overlapping basalt lava flows
– forming a volcanic pile several kilometers
thick
Portage Lake Volcanics
Michigan
Sedimentary Rocks
• Middle to Late Proterozoic sedimentary rocks
– are exceptionally well exposed
– in the northern Rocky Mountains
– of Montana and Alberta, Canada
• Glacier National Park
Proterozoic Rocks, Glacier NP
• Proterozoic sedimentary rocks
– in Glacier National Park, Montana
• The angular peaks, ridges and broad
valleys
– were carved by Pleistocene and Recent
Proterozoic Mudrock
• Outcrop of red mudrock in Glacier
National Park, Montana
Proterozoic Limestone
• Outcrop of limestone with stromatolites in
Glacier National Park, Montana
Grand Canyon Super-group
• Proterozoic Sandstone of the Grand
Canyon Super-group in the Grand Canyon
Arizona
Proterozoic Supercontinents
• A supercontinent consists of all
– Or much of the present-day continents,
– so other than size it is the same as a continent
• The supercontinent Pangaea,
– existed MUCH LATER but few people are
aware of earlier supercontinents
Early Supercontinents
• Rodinia
– assembled between 1.3 and 1.0 billion years
ago
– and then began fragmenting (rifting apart) 750
million years ago (THE Proterozoic ends at
545my ago)
Early Supercontinent
• Possible
configuration
– of the Late
Proterozoic
supercontinent
Rodinia
– before it began
fragmenting
about 750
million years
ago
• Rodinia's separate pieces reassembled
– and formed another supercontinent
– Pannotia
– about 650 million years ago
• Fragmentation was underway again,
– about 550 million years ago,
– giving rise to the continental configuration
– that existed at the onset of the Phanerozoic Eon – the Cambrian
Recognizing Glaciation
• How can we be sure that there were
Proterozoic glaciers?
• the extensive geographic distribution
– of other conglomerates and tillites
– and their associated glacial features
– is distinctive,
– such as striated and polished bedrock
Proterozoic Glacial Evidence
• Bagganjarga Tillite in Norway
Over bedrock
Geologists Convinced
• The occurrence of tillites
– in Michigan, Wyoming, and Quebec
– indicates that North America may have had
– an Early Proterozoic ice sheet centered
southwest of Hudson Bay
Early Proterozoic Glaciers
• Deposits in
North
America
– indicate that
Laurentia
– had an
extensive ice
sheet
– centered
southwest of
Hudson Bay
Late Proterozoic Glaciers
• The approximate distribution of Late
Proterozoic glaciers
• Late Proterozoic glaciers
– seem to have been present even
– in near-equatorial areas!!
– Geologists have recently named this
phenomenon
– “SNOWBALL EARTH”
The Evolving Atmosphere
– Archean: little or no free oxygen
– the amount present
– at the beginning of the Proterozoic was
probably no more than 1% of that present now
– Stromatolites—not common until:
– 2.3 billion years ago,
• that is, during the Early Proterozoic
• There is evidence of increasing oxygen….
Early Proterozoic
Banded Iron Formation
•
•
•
•
At this outcrop in Ishpeming, Michigan
the rocks are alternating layers of
red chert
and
silvercolored
iron
minerals
Banded Iron Formations (BIF)
• Banded iron formations (BIFs),
– consist of alternating layers of
• iron-rich minerals
• and chert
– about 92% of all BIFs
• formed during the interval
• from 2.5 to 2.0 billion years ago
BIFs and the Atmosphere
• How are these rocks related to the
atmosphere?
• Their iron is in iron oxides, especially
– hematite (Fe2O3)
– and magnetite (Fe3O4)
• Iron combines with oxygen in an oxidizing
atmosphere
– to from rustlike oxides
– that are not readily soluble in water
• If oxygen is absent in the atmosphere,
though,
– iron easily dissolves
– so that large quantities accumulate in the
world's oceans,
Formation of BIFs
• The Archean atmosphere was deficient in
free oxygen
• so that little oxygen was dissolved in
seawater
• However, as photosynthesizing organisms
– increased in abundance,
• as indicated by stromatolites,
– free oxygen,
• released as a metabolic waste product into the
oceans,
– caused the precipitation of iron oxides along
with silica
Formation of BIFs
• Depositional model for the origin of banded
iron formation
Source of Iron and Silica
• A likely source of the iron and silica
– was submarine volcanism,
– similar to that now talking place
– at or near spreading ridges
• Huge quantities of dissolved minerals are
– also discharged at submarine hydrothermal
vents
• In any case, the iron and silica
– combined with oxygen
– thus resulting in the precipitation
– of huge amounts of banded iron formation
• Precipitation continued until
– the iron in seawater was largely used up
Continental Red Beds
• Obviously continental red beds refers
– to red rocks on the continents,
– but more specifically it means red sandstone
or shale
– colored by
iron oxides,
– especially
hematite
(Fe2O3)
Red mudrock in
Glacier
National Park,
Red Beds
• Red beds first appear
– in the geologic records about 1.8 billion years
ago,
– increase in abundance throughout the rest of
the Proterozoic,
– and are quite common in rocks of Phanerozoic
age
• The onset of red bed deposition
– coincides with the introduction of free oxygen
– into the Proterozoic atmosphere
• However, the atmosphere at that time
– may have had only 1%
Red Beds
• Is this percentage sufficient to account
– for oxidized iron in sediment?
• Probably not,
– but no ozone (O3) layer existed in the upper
atmosphere
– before free oxygen (O2) was present
• As photosynthesizing organisms released
– free oxygen into the atmosphere,
– ultraviolet radiation converted some of it
– to elemental oxygen (O) and ozone (O3),
– both of which oxidize minerals more effectively
than O2
Red Beds
• Once an ozone layer became established,
– most ultraviolet radiation failed
– to penetrate to the surface,
– and O2 became the primary agent
– for oxidizing minerals
Important Events in Life History
• Archean fossils are not very common,
– and all of those known are varieties
– of bacteria and cyanobacteria (blue-green
algae),
– although they undoubtedly existed in profusion
• Likewise, the Early Proterozoic fossil
record
– has mostly bacteria and cyanobacteria
• Apparently little diversification
– had taken place;
– all organisms were single-celled prokaryotes,
– until about 2.1 billion years ago
Gunflint Microfossils
• Even in well-known Early Proterozoic
fossils assemblages, only fossils of
bacteria are recognized
Photomicrograph
of spheroidal
and filamentous
microfossils
from the Gunflint
Chert of Ontario
Canada
Prokaryote and Eukaryotes
• An organism made up of prokaryotic cells
is called a prokaryote
– whereas those composed of eukaryotic cells
are eukaryotes
• In fact, the distinction between prokaryotes
and eukaryotes
– is the basis for the most profound distinction
between all living things
Lack of Organic Diversity
• Actually, the lack of organic diversity
– during this early time in life history
– is not too surprising
– because prokaryotic cells reproduce asexually
• Most variation in
– sexually reproducing populations comes from
– the shuffling of genes,
– and their alleles,
– from generation to generation
• Mutations introduce new variation into a
population,
– but their effects are limited in prokaryotes
Genetic Variation in Bacteria
• A beneficial mutation would spread rapidly
– in sexually reproducing organism,
– but have a limited impact in bacteria
– because they do not share their genes with
other bacteria
• Bacteria usually reproduce by binary
fission
– and give rise to two cells
– having the same genetic makeup
• Under some conditions,
– they engage in conjugation during
– which some genetic material is transferred
Sexual Reproduction Increased the Pace of
Evolution
• Prior to the appearance of cells capable of
sexual reproduction,
– evolution was a comparatively slow
process,
– thus accounting for the low organic diversity
• This situation did not persist
• Sexually reproducing cells probably
– evolved by Early Proterozoic time,
– and the tempo of evolution increased
Eukaryotic Cells Evolve
• The appearance of eukaryotic cells
– marks a milestone in evolution
– comparable to the development
• of complex metabolic mechanisms
• such as photosynthesis during the Archean
• Where did these cells come from?
• How do they differ from their predecessors,
– the prokaryotic cells?
• All prokaryotes are single-celled,
– but most eukaryotes are multicelled,
– the notable exception being the protistans
Eukaryotes
• Most eukaryotes reproduce sexually,
– in marked contrast to prokaryotes,
• and nearly all are aerobic,
– that is, they depend on free oxygen
– to carry out their metabolic processes
• Accordingly, they could not have evolved
– before at least some free oxygen was present
in the atmosphere
Prokaryotic Cell
• Prokaryotic cells
– do not have a cell nucleus
– do not have organelles
– are smaller and not nearly as complex as
eukaryotic cells
Eukaryotic Cell
• Eukaryotic cells
have
– a cell nucleus
containing
– the genetic material
– and organelles
– such as mitochondria
– and plastids,
– as well as
chloroplasts in plant
cells
Eukaryotic Fossil Cells
• The Negaunee Iron Formation in Michigan
– which is 2.1 billion years old
– has yielded fossils now generally accepted
– as the oldest known eukaryotic cells
• Even though the Bitter Springs Formation
– of Australia is much younger --1 billion yrs old
– it has some remarkable fossils of single-celled
eukaryotes
– that show evidence of meiosis and mitosis,
– processes carried out only by eukaryotic cells
Evidence for Eukaryotes
• Prokaryotic cells are mostly rather simple
– spherical or platelike structures
• Eukaryotic cells
– are larger
– much more complex
– have a well-defined, membrane-bounded
cell nucleus, which is lacking in prokaryotes
– have several internal structures
– called organelles such as plastids and
mitochondria
– their organizational complexity
– is much greater than it is for prokaryotes
Acritarchs
• Other organisms that were
– almost certainly eukaryotes are the acritarchs
– that first appeared about 1.4 billion years ago
– they were very common by Late Proterozoic
time
– and were probably cysts of planktonic
(floating) algae
Acritarchs
• These common Late Proterozoic
microfossils
– are probably from eukaryotic organisms
• Acritarchs are very likely the cysts of algae
Late Proterozoic Microfossil
• Numerous
microfossils of
organisms
– with vase-shaped skeletons
– have been found
– in Late Proterozoic rocks
– in the Grand Canyon
• These too have tentatively been
identified as
– cysts of some kind of algae
Endosymbiosis and the
Origin of Eukaryotic Cells
• Eukaryotic cells probably formed
– from several prokaryotic cells
– that entered into a symbiotic relationship
– Symbiosis,
• involving a prolonged association of two or more
dissimilar organisms,
– is quite common today
• In many cases both symbionts benefit from
the association
– as occurs in lichens,
• once thought to be plants
• but actually symbiotic fungi and algae
Endosymbiosis
• In a symbiotic relationship,
– each symbiont must be capable
– of metabolism and reproduction,
– but in some cases one symbiont
– cannot live independently
• This may have been the case
– with Proterozoic symbiotic prokaryotes
– that became increasingly interdependent
– until the unit could exist only as a whole
• In this relationship
– one symbiont lived within the other,
– which is a special type of symbiosis
– called endosymbiosis
Evidence for Endosymbiosis
• Supporting evidence for endosymbiosis
– comes from studies of living eukaryotic
cells
– containing internal structures called
organelles,
• such as mitochondria and plastics,
– which contain their own genetic material
•
•
In addition, prokaryotic cells
– synthesize proteins as a single system,
whereas eukaryotic cells
– are a combination of protein-synthesizing systems
Organelles Capable of
Protein Synthesis
• That is, some of the organelles
– within eukaryotic cells are capable of protein
synthesis
• 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
Multicelled Organisms
• Obviously multicelled organisms
– are made up of many cells,
– perhaps billions,
– as opposed to a single cell as in prokaryotes
• In addition, multicelled organisms
– have cells specialized to perform specific
functions
– such as respiration,
– food gathering,
– and reproduction
Dawn of Multicelled
Organisms
• We know from the fossil record
– that multicelled organisms were present during the Proterozoic,
– but we do not know exactly when they appeared
• What seem to be some kind of multicelled
algae appear
– in the 2.1-billion-year-old fossils
• from the Negaunee Iron Formation in Michigan
– as carbonaceous filaments
• from 1.8 billion-year-old rocks in China
– as somewhat younger carbonaceous impressions
– of filaments and spherical forms
Multicelled Algae?
• Carbonaceous impressions
– in Proterozoic rocks, Montana
• These may be impressions of
multicelled algae
– Skip next slide
The Multicelled Advantage?
• Is there any particular advantage to
being multicelled?
• For something on the order of 1.5
billion years
– all organisms were single-celled
– and life seems to have thrived
• In fact, single-celled organisms
– are quite good at what they do
– but what they do is very limited
The Multicelled Advantage?
• For example, single celled organisms
– can not grow very large, because as size
increases proportionately less of a cell is
exposed to the external environment in relation
to its volume
– and the proportion of surface area decreases
• Transferring materials from the exterior
– to the interior becomes less efficient
The Multicelled Advantage?
• Also, multicelled organisms live longer,
– since cells can be replaced and more offspring
can be produced
• Cells have increased functional efficiency
– when they are specialized into organs with
specific capabilities
Late Proterozoic Animals
• Biologists set forth criteria such as
– method of reproduction
– and type of metabolism
– to allow us to easily distinguish
– between animals and plants
• Or so it would seem,
– but some present-day organisms
– blur this distinction and the same is true
– for some Proterozoic fossils
• Nevertheless, the first
– relatively controversy-free fossils of animals
– come from the Ediacaran fauna of Australia
– and similar faunas of similar age elsewhere
The Ediacaran Fauna
• In 1947, an Australian geologist, R.C.
Sprigg,
•
•
– in the Pound Quartzite in the Ediacara Hills of South Australia
Additional discoveries by others turned up what appeared to be
– discovered impressions of soft-bodied animals
– impressions of algae and several animals
– many bearing no resemblance to any existing now
Before these discoveries, geologists
– were perplexed by the apparent absence
– of fossil-bearing rocks predating the Phanerozoic
Ediacaran Fauna
• The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive
echinoderm
Spriggina floundersi, a
possible ancestor of
trilobites
Ediacaran Fauna
Pavancorina
minchami
• Restoration of the
Ediacaran Environment
Ediacaran Fauna
• Geologists had assumed that
•
– the fossils so common in Cambrian rocks
– must have had a long previous history
– but had little evidence to support this conclusion
The discovery of Ediacaran fossils and subsequent discoveries
– have not answered all questions about pre-Phanerozoic animals,
– but they have certainly increased our knowledge
– about this chapter in the history of life
Represented Phyla
• Three present-day phyla may be
represented
– in the Ediacaran fauna:
• jellyfish and sea pens (phylum Cnidaria),
• segmented worms (phylum Annelida),
• and primitive members of the phylum Arthropoda
(the phylum with insects, spiders crabs, and others)
• One Ediacaran fossil, Spriggina,
– has been cited as a possible ancestor of
trilobites
• Another might be a primitive member
– of the phylum Echinodermata
Distinct Evolutionary Group
• However, some scientists think
– these Ediacaran animals represent
– an early evolutionary group quite distinct from
– the ancestry of today’s invertebrate animals
• Ediacara-type faunas are known
– from all continents except Antarctica,
--were widespread between 545 and 670 million
years ago
– but their fossils are rare
• Their scarcity should not be surprising,
though,
– because all lacked durable skeletons
Other Proterozoic Animal Fossils
• Although scarce, a few animal fossils
– older than those of the Ediacaran fauna are
known
• A jellyfish-like impression is present
– in rocks 2000 m below the Ediacara Hills
Pound Quartzite,
• Burrows, in many areas,
– presumably made by worms,
– occur in rocks at least 700 million years old
• Wormlike and algae fossils come
– from 700 to 900 million-year-old rocks in China
– but the identity and age of these "fossils" has
Wormlike Fossils from China
• Wormlike
fossils from
Late
Proterozoic
rocks in China
Soft Bodies
• All known Proterozoic animals were softbodied,
– but there is some evidence that the earliest
stages in the origin of skeletons was underway
• Even some Ediacaran animals
– may have had a chitinous carapace
– and others appear to have had areas of
calcium carbonate
• The odd creature known as Kimberella
– from the latest Proterozoic of Russia
– had a tough outer covering similar to
– that of some present-day marine invertebrates
Latest Proterozoic Kimberella
• Kimberella, an animal from latest
Proterozoic rocks in Russia
– Exactly what
Kimberella was
remains
uncertain
– Some think it
was a sluglike
creature
– whereas others
think it was
more like a
Durable Skeletons
• Latest Proterozoic fossils
– of minute scraps of shell-like material
– and small tooth like denticles and spicules,
• presumably from sponges
• indicate that several animals with skeletons
– or at least partial skeletons existed
• However, more durable skeletons of
• silica,
• calcium carbonate,
• and chitin (a complex organic substance)
– did not appear in abundance until the
beginning
– of the Phanerozoic Eon 545 million years ago
Proterozoic Mineral Resources
• Most of the world's iron ore comes
from
– Proterozoic banded iron formations
• Canada and the United States have
large deposits of these rocks
– in the Lake Superior region
– and in eastern Canada
• Thus, both countries rank among
– the ten leading nations in iron ore
production
Iron Mine
• The Empire Mine at Palmer, Michigan
– where iron ore from the Early Proterozoic
Negaunee Iron Formation is mined
Nickel
• In the Sudbury mining district in Ontario,
Canada,
– nickel and platinum are extracted from
Proterozoic rocks
• Nickel is essential for the production of
nickel alloys such as
• stainless steel
• and Monel metal (nickel plus copper),
– which are valued for their strength and
resistance to corrosion and heat
• The United States must import
– more than 50% of all nickel used
– mostly from the Sudbury mining district
Sudbury Basin
• Besides its economic importance, the
Sudbury Basin,
– an elliptical area measuring more than 59
by 27 km,
– is interesting from the geological
perspective
• One hypothesis for the concentration
of ores
– is that they were mobilized from metalrich rocks
– beneath the basin
– following a high-velocity meteorite impact
Platinum and Chromium
• Some platinum
– for jewelry, surgical instruments,
– and chemical and electrical equipment
– is exported to the United States from Canada,
– but the major exporter is South Africa
• The Bushveld Complex of South Africa
– is a layered igneous complex containing both
• platinum
• and chromite
– the only ore of chromium,
– United States imports much of the chromium
– from South Africa
– It is used mostly in stainless steel
Oil and Gas
•
•
•
Economically recoverable oil and gas
– have been discovered in Proterozoic rocks in China and Siberia,
– arousing some interest in the Midcontinent rift as a potential source
of hydrocarbons
So far, land has been leased for exploration,
– and numerous geophysical studies have been done
However, even though some rocks
– within the rift are know to contain petroleum,
– no producing oil or gas wells are operating
Proterozoic Pegmatites
• A number of Proterozoic pegmatites
– are important economically
• The Dunton pegmatite in Maine,
– whose age is generally considered
– to be Late Proterozoic,
– has yielded magnificent gem-quality specimens
– of tourmaline and other minerals
• Other pegmatites are mined for gemstones as well as for
– tin, industrial minerals, such as feldspars, micas, and quartz
– and minerals containing such elements
– as cesium, rubidium, lithium, and beryllium
Proterozoic Pegmatites
• Geologists have identified more than 20,000 pegmatites
– in the country rocks adjacent
– to the Harney Peak Granite
– in the Black Hills of South Dakota
• These pegmatites formed ~ 1.7 billion years ago
– when the granite was emplaced as a complex of dikes
and sills
• A few have been mined for gemstones, tin, lithium, micas,
– and some of the world's largest known
– mineral crystals were discovered in these pegmatites
Summary
• The crust-forming processes
– that yielded Archean granite-gneiss
complexes
– and greenstone belts
– continued into the Proterozoic
– but at a considerably reduced rate
• Archean and Proterozoic greenstone
belts
– differed in detail
• Early Proterozoic collisions
– between Archean cratons formed larger
cratons
Summary
• One such landmass was Laurentia
– consisting mostly of North America and
Greenland
• Important events
– in the evolution of Laurentia were
• Early Proterozoic amalgamation of cratons
• followed by Middle Proterozoic igneous activity,
• the Grenville orogeny, and the Midcontinent rift
• Ophiolite sequences
– marking convergent plate boundaries
– are first well documented from the Early
Proterozoic,
– indicating that a plate tectonic style similar
Summary
• Sandstone-carbonate-shale
assemblages
– deposited on passive continental margins
– are known from the Archean
– but they are very common by Proterozoic
time
• 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
• Glaciers were widespread
– during both the Early and Late Proterozoic
Summary
• Photosynthesis continued
– to release free oxygen into the atmosphere
– which became increasingly oxygen rich
through the Proterozoic
• Fully 92% of Earth's iron ore deposits
– in banded iron formations were deposited
– between 2.5 and 2.0 billion years ago
• Widespread continental red beds
– dating from 1.8 billion years ago indicate
– that Earth's atmosphere had enough free
oxygen
– for oxidation of iron compounds
Summary
• Most of the known Proterozoic organisms
– are single-celled prokaryotes (bacteria)
• When eukaryotic cells first appeared is
uncertain,
– but they may have been present by 2.1
billion years ago
• Endosymbiosis is a widely accepted
theory for their origin
• The oldest known multicelled organisms
– are probably algae,
– some of which may date back to the Early
Proterozoic
Summary
• Well-documented multicelled animals
– are found in several Late Proterozoic
localities
• Animals were widespread at this time,
– but because all lacked durable skeletons
– their fossils are not common
• Most of the world's iron ore produced
– is from Proterozoic banded iron formations
• Other important resources
– include nickel and platinum