Title: Proterozoic sedimentary rocks
1Proterozoic Rocks, Glacier NP
- Proterozoic sedimentary rocks
- in Glacier National Park, Montana
- The angular peaks, ridges and broad valleys
- were carved by Pleistocene and Recent glaciers
2The 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
3The 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
4Disparity in Time
- Perhaps this disparity
- between the coverage of the Proterozoic and the
Phanerozoic - seems disproportionate,
- but we know far more
- about Phanerozoic events
- than we do for either of the Precambrian eons
5Archean-Proterozoic Boundary
- Geologist have rather arbitrarily placed
- the Archean-Proterozoic boundary
- at 2.5 billion years ago
- because it marks the approximate time
- of changes in the style of crustal evolution
- However, we must emphasize "approximate,"
- because Archean-type crustal evolution
- was largely completed in South Africa
- nearly 3.0 billion years ago,
- whereas in North America the change took place
- from 2.95 to 2.45 billion years ago
6Style of Crustal Evolution
- Archean crust-forming processes generated
- granite-gneiss complexes
- and greenstone belts
- that were shaped into cratons
- Although these same rock associations
- continued to form during the Proterozoic,
- they did so at a considerably reduced rate
7Contrasting Metamorphism
- In addition, Archean and Proterozoic rocks
- contrast in metamorphism
- Many Archean rocks have been metamorphosed,
- although their degree of metamorphism
- varies and some are completely unaltered
- 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
8Other Differences
- In addition to changes in the style of crustal
evolution, - the Proterozoic is characterized
- by widespread rock assemblages
- that are rare or absent in the Archean,
- by a plate tectonic style essentially the same as
that of the present - by important evolution of the atmosphere and
biosphere - by the origin of some important mineral resources
9Proterozoic Evolution of Oxygen-Dependent
Organisms
- It was during the Proterozoic
- that oxygen-dependent organisms
- made their appearance
- and the first cells evolved
- that make up most organisms today
10Evolution of Proterozoic Continents
- Archean cratons assembled during collisions
- of island arcs and minicontinents,
- providing the nuclei around which
- Proterozoic crust accreted,
- thereby forming much larger landmasses
- Proterozoic accretion at craton margins
- probably took place more rapidly than today
- because Earth possessed more radiogenic heat,
- but the process continues even now
11Proterozoic Greenstone Belts
- Most greenstone belts formed
- during the Archean
- between 2.7 and 2.5 billion years ago
- They also continued to form
- during the Proterozoic and at least one is known
- from Cambrian-aged rocks in Australia
- They were not as common after the Archean,
- and differed in one important detail
- the near absence of ultramafic rocks
- which no doubt resulted from
- Earth's decreasing amount of radiogenic heat
12Focus 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
13Early Proterozoic History of Laurentia
- Laurentia originated and underwent important
growth - between 2.0 and 1.8 billion years ago
- During this time, collisions
- among various plates formed several orogens,
- which are linear or arcuate deformation belts
- in which many of the rocks have been
- metamorphosed
- and intruded by magma
- thus forming plutons, especially batholiths
14Proterozoic Evolution of Laurentia
- Archean cratons were sutured
- along deformation belts called orogens,
- thereby forming a larger landmass
- 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
15Craton-Forming Processes
- Examples of these craton-forming processes
- are recorded in rocks
- in the Thelon orogen in northwestern Canada
- where the Slave and Rae cratons collided,
16Craton-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
17Wilson Cycle
- Rocks of the Wopmay orogen
- in northwestern Canada are important
- because they record the opening and closing
- of an ocean basin
- or what is called a Wilson cycle
- A complete Wilson cycle,
- named for the Canadian geologist J. Tuzo Wilson,
- involves
- fragmentation of a continent,
- opening followed by closing
- of an ocean basin,
- and finally reassembly of the continent
18Wopmay Orogen
- Some of the rocks in Wopmay orogen
- are sandstone-carbonate-shale assemblages,
- a suite of rocks typical of passive continental
margins - that first become widespread during the
Proterozoic
19Early Proterozoic Rocks in Great Lakes Region
- Early Proterozoic sandstone-carbonate-shale
assemblages are widespread near the Great Lakes
20Outcrop of Sturgeon Quartzite
- The sandstones have a variety of sedimentary
structures - such as
- ripple marks
- and cross-beds
- Northern Michigan
21Outcrop of Kona Dolomite
- Some of the carbonate rocks, now mostly
dolostone, - such as the Kona Dolomite,
- contain abundant bulbous structures known as
stromatolites - NorthernMichigan
22Penkean Orogen
- These rocks of northern Michigan
- have been only moderately deformed
- and are now part of the Penokean orogen
23Accretion along Laurentias Southern Margin
- Following the initial episode
- of amalgamation of Archean cratons
- 2.0 to 1.8 billion years ago
- accretion took place along Laurentia's southern
margin - From 1.8 to 1.6 billion years ago,
- continental accretion continued
- in what is now the southwestern and central
United States - as successively younger belts were sutured to
Laurentia, - forming the Yavapai and Mazatzal-Pecos orogens
24Southern 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
25BIF, Red Beds, Glaciers
- This was also the time during which
- most of Earths 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
- and associated features provide excellent
evidence - for widespread glaciation
26Early and Middle Proterozoic Igneous Activity
- During the interval
- from 1.8 to 1.1 billion years ago,
- extensive igneous activity took place
- that seems to be unrelated to orogenic activity
- Although quite widespread,
- this activity did not add to Laurentias size
- because magma was either intruded into
- or erupted onto already existing continental
crust
27Igneous Activity
- These igneous rocks are exposed
- in eastern Canada, extend across Greenland,
- and are also found in the Baltic shield of
Scandinavia
28Igneous Activity
- However, the igneous rocks are deeply buried
- by younger rocks in most areas
- The origin of these
- granitic and anorthosite plutons,
- Anorthosite is a plutonic rock composed
- almost entirely of plagioclase feldspars
- calderas and their fill,
- and vast sheets of rhyolite and ash flows
- are the subject of debate
- According to one hypothesis
- large-scale upwelling of magma
- beneath a Proterozoic supercontinent
- produced the rocks
29Middle 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
30Grenville Orogeny
- A final episode of Proterozoic accretion
- occurred during the Grenville orogeny
31Grenville Orogeny
- Many geologists think the Grenville orogen
- resulted from closure of an ocean basin,
- the final stage in a Wilson cycle
- Others disagree and think
- intracontinental deformation or major shearing
- was responsible for deformation
- Whatever the cause of the Grenville orogeny,
- it was the final stage
- in the Proterozoic continental accretion of
Laurentia
3275 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
33Midcontinent 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
34Location of the Midcontinent Rift
- Rocks filling the rift
- are exposed around Lake Superior
- but are deeply buried elsewhere
35Midcontinental Rift
- Most of the rift is buried beneath younger rocks
- except in the Lake Superior region
- where various igneous and sedimentary rocks
- are well exposed
- The central part of the rift contains
- numerous overlapping basalt lava flows
- forming a volcanic pile several kilometers thick
- In fact, the volume of volcanic rocks,
- between 300,000 and 1,000,000 km3,
- is comparable in volume although not areal extent
- to the great outpourings of lava during the
Cenozoic
36Midcontinental Rift
- Along the rift's margins
- coarse-grained sediments were deposited
- in large alluvial fans
- that grade into sandstone and shale
- with increasing distance
- from the sediment source
- In the vertical section
- Freda Sandstone overlies
- Cooper Harbor conglomerate,
- which overlies Portage Lake Volcanics
37Cooper Harbor Conglomerate
38Portage Lake Volcanics
39Middle and Late Proterozoic Sedimentation
- Remember the Grenville orogeny
- took place 1.2 billion 900 million years ago,
- the final episode of continental accretion
- in Laurentia until the Ordovician Period
- Nevertheless, important geologic events
- were taking place,
- such as sediment deposition in what is now
- the eastern United States and Canada,
- in the Death Valley region of California and
Nevada, - and in three huge basins in the west
40Sedimentary Basins in the West
- Map showing the locations of sedimentary Basins
- in the western United States and Canada
- Belt Basin
- Uinta Basin
- Apache Basin
41Sedimentary Rocks
- Middle to Late Proterozoic sedimentary rocks
- are exceptionally well exposed
- in the northern Rocky Mountains
- of Montana and Alberta, Canada
- Indeed, their colors, deformation features,
- and erosion by Pleistocene and recent glaciers
- have yielded some fantastic scenery
- Like the rocks in the Great Lakes region
- and the Grand Canyon,
- they are mostly sandstones, shales,
- and stromatolite-bearing carbonates
42Proterozoic Mudrock
- Outcrop of red mudrock in Glacier National Park,
Montana
43Proterozoic Limestone
- Outcrop of limestone with stromatolites in
Glacier National Park, Montana
44Proterozoic Sandstone
- Proterozoic rocks
- of the Grand Canyon Super-group lie
- unconformably upon Archean rocks
- and in turn are overlain unconformably
- by Phanerozoic-age rocks
- The rocks, consisting mostly
- of sandstone, shale, and dolostone,
- were deposited in shallow-water marine
- and fluvial environments
- The presence of stromatolites and carbonaceous
- impression of algae in some of these rocks
- indicate probable marine deposition
45Grand Canyon Super-group
- Proterozoic Sandstone of the Grand Canyon
Super-group in the Grand Canyon Arizona
46Style of Plate Tectonics
- The present style of plate tectonics
- involving opening and then closing ocean basins
- had almost certainly been established by the
Early Proterozoic - In fact, the oldest known complete ophiolite
- providing evidence for an ancient convergent
plate boundary - is the Jormua mafic-ultramafic complex in Finland
- It is about 1.96 billion years old,
- but nevertheless compares closely in detail
- with younger well-documented ophiolites
47Jormua Complex, Finland
- Reconstruction
- of the highly deformed
- Jormua mafic-ultramafic complex
- in Finland
- This sequence of rock
- is the oldest known complete ophiolite
- at 1.96 billion years old
48Jormua Complex, Finland
- Metamorphosed basaltic pillow lava
12 cm
49Jormua Complex, Finland
- Metamorphosed gabbro between mafic dikes
65 cm
50Proterozoic Supercontinents
- You already know that a continent
- is one of Earth's landmasses
- consisting of granitic crust
- with most of its surface above sea level
- A supercontinent consists of all
- or at least much of the present-day continents,
- so other than size it is the same as a continent
- The supercontinent Pangaea,
- which existed at the end of the Paleozoic Era,
- is familiar,
- but few people are aware of earlier
supercontinents
51Early Supercontinents
- Supercontinents may have existed
- as early as the Late Archean,
- but if so we have little evidence of them
- The first that geologists recognize
- with some certainty, known as Rodinia
- assembled between 1.3 and 1.0 billion years ago
- and then began fragmenting 750 million years ago
52Early Supercontinent
- Possible configuration
- of the Late Proterozoic supercontinent Rodinia
- before it began fragmenting about 750 million
years ago
53Pannotia
- Rodinia's separate pieces reassembled
- and formed another supercontinent
- this one known as Pannotia
- about 650 million years ago
- judging by the Pan-African orogeny
- the large-scale deformation that took place
- in what are now the Southern Hemisphere
continents - Fragmentation was underway again,
- by the latest Proterozoic, about 550 million
years ago, - giving rise to the continental configuration
- that existed at the onset of the Phanerozoic Eon
54Ancient Glaciers
- Very few times of widespread glacial activity
- have occurred during Earth history
- The most recent one during the Pleistocene
- 1.6 million to 10,000 years ago
- is certainly the best known,
- but we also have evidence for Pennsylvanian
glaciers - and two major episodes of Proterozoic glaciation
55Recognizing Glaciation
- How can we be sure that there were Proterozoic
glaciers? - After all, their most common deposit
- called tillite is simply a type of conglomerate
- that may look much like conglomerate
- that originated by other processes
- Tillite or tillite-like deposits are known
- from at least 300 Precambrian localities,
- and some of these are undoubtedly not glacial
deposits
56Glacial Evidence
- But the extensive geographic distribution
- of other conglomerates
- and their associated glacial features
- is distinctive,
- such as striated and polished bedrock
57Proterozoic Glacial Evidence
- Bagganjarga tillite in Norway
- overlies striated bedrock surface
- on sandstone of the Veidnesbotn Formation
58Geologists Convinced
- Geologists are now convinced
- based on this kind of evidence
- that widespread glaciation
- took place during the Early Proterozoic
- The occurrence of tillites of about the same age
- in Michigan, Wyoming, and Quebec
- indicates that North America may have had
- an Early Proterozoic ice sheet centered southwest
of Hudson Bay
59Early Proterozoic Glaciers
- Deposits in North America
- indicate that Laurentia
- had an extensive ice sheet
- centered southwest of Hudson Bay
60One or More Glaciations?
- Tillites of about this age are also found
- in Australia and South Africa,
- but dating is not precise enough to determine
- if there was a single widespread glacial episode
- or a number of glacial events at different times
in different areas - One tillite in the Bruce Formation in Ontario,
Canada - may date from 2.7 billion years ago,
- thus making it Late Archean
61Glaciers of the Late Proterozoic
- Tillites and other glacial features
- dating from between 900 and 600 million years ago
- are found on all continents except Antarctica
- Glaciation was not continuous during this entire
time - but was episodic with four major glacial episodes
so far recognized
62Late Proterozoic Glaciers
- The approximate distribution of Late Proterozoic
glaciers
63Most Extensive Glaciation in Earth History
- The map shows only approximate distribution
- of Late Proterozoic glaciers
- The actual extent of glaciers is unknown
- Not all the glaciers were present at the same
time - Despite these uncertainties,
- this Late Proterozoic glaciation
- was the most extensive in Earth history
- In fact, Late Proterozoic glaciers
- seem to have been present even
- in near-equatorial areas
64The Evolving Atmosphere
- Geologists agree that the Archean atmosphere
- contained little or no free oxygen so the
atmosphere - was not strongly oxidizing as it is now
- Even though processes were underway
- that added free oxygen to the atmosphere,
- the amount present
- at the beginning of the Proterozoic
- was probably no more than 1 of that present now
- In fact, it might not have exceeded
- 10 of present levels even
- at the end of the Proterozoic
65Cyanobacteria and Stromatolites
- Remember from our previous discussions
- that cyanobacteria,
- also known as blue-green algae,
- were present during the Archean,
- but stromatolites
- the structures they formed,
- did not become common until about 2.3 billion
years ago, - that is, during the Early Proterozoic
- These photosynthesizing organisms
- and to a lesser degree photochemical dissociation
- added free oxygen to the evolving atmosphere
66Oxygen Versus Carbon Dioxide
- Earth's early atmosphere
- had abundant carbon dioxide
- More oxygen became available
- whereas the amount of carbon dioxide decreased
- Only a small amount of CO2
- still exists in the atmosphere today
- It is one of the greenhouse gases
- partly responsible for global warming
- What evidence indicates
- that the atmosphere became oxidizing?
- Where is all that additional the carbon dioxide
now?
67Evidence from Rocks
- Much carbon dioxide is now tied up
- in various minerals and rocks
- especially the carbonate rocks
- limestone and dolostone,
- and in the biosphere
- For evidence that the Proterozoic atmosphere was
evolving - from a chemically reducing one
- to an oxidizing one
- we must discuss types
- of Proterozoic sedimentary rocks, in particular
- banded iron formations
- and red beds
68Banded Iron Formations (BIF)
- Banded iron formations (BIFs),
- consist of alternating layers of
- iron-rich minerals
- and chert
- Some are found in Archean rocks,
- but about 92 of all BIFs
- formed during the interval
- from 2.5 to 2.0 billion years ago
69Early Proterozoic Banded Iron Formation
- At this outcrop in Ishpeming, Michigan
- the rocks are alternating layers of
- red chert
- and silver-colorediron minerals
70Typical BIF
- A more typical outcrop of BIF near Nagaunee,
Michigan
71BIFs 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, - which it undoubtedly did during the Archean
72Formation 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 - and thus created BIFs
73Formation of BIFs
- One model accounting for the details
- of BIF precipitation involves
- a Precambrian ocean with an upper oxygenated
layer - overlying a large volume of oxygen-deficient
water - that contained reduced iron and silica
- Upwelling,
- that is transfer of water from depth to the
surface, - brought iron- and silica-rich waters
- onto the shallow continental shelves
- and resulting in widespread precipitation of BIFs
74Formation of BIFs
- Depositional model for the origin of banded iron
formation
75Source 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
76Continental 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, Montana
77Red 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
- or perhaps 2 of present levels
78Red 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
79Red 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
80Important 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
- when more complex eukaryotic cells evolved
81Gunflint 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
82Prokaryote 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
83Lack 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
84Genetic 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
85Sexual 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
86Eukaryotic 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
87Eukaryotes
- 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
88Prokaryotic Cell
- Prokaryotic cells
- do not have a cell nucleus
- do not have organelles
- are smaller and not nearly as complex as
eukaryotic cells
89Eukaryotic 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
90Eukaryotic 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
91Evidence 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
92Acritarchs
- 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
93Acritarchs
- These common Late Proterozoic microfossils
- are probably from eukaryotic organisms
- Acritarchs are very likely the cysts of algae
94Late 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
95Endosymbiosis 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
96Endosymbiosis
- 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
97Evidence 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
98Organelles 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
99Multicelled 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
100Dawn 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
101Multicelled Algae?
- Carbonaceous impressions
- in Proterozoic rocks, Montana
- These may be impressions of multicelled algae
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102The 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
103The 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
104The 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
105Late 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
106The 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
107Ediacaran Fauna
- The Ediacaran fauna of Australia
- Tribrachidium heraldicum, a possible primitive
echinoderm
Spriggina floundersi, a possible ancestor of
trilobites
108Ediacaran Fauna
- Restoration of the Ediacaran Environment
109Ediacaran 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
110Represented 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
111Distinct Evolutionary Group
- However, some scientists think
- these Ediacaran animals represent
- an early evolutionary group quite distinct from
- the ancestry of todays 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
112Other 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
been questioned
113Wormlike Fossils from China
- Wormlike fossils from Late Proterozoic rocks in
China
114Soft Bodies
- All known Proterozoic animals were soft-bodied,
- 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
115Latest 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 mollusk
116Durable 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
117Proterozoic 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
118Iron Mine
- The Empire Mine at Palmer, Michigan
- where iron ore from the Early Proterozoic
Negaunee Iron Formation is mined
119Nickel
- 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
120Sudbury 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 metal-rich rocks
- beneath the basin
- following a high-velocity meteorite impact
121Platinum 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
122Oil 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
123Proterozoic 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
124Proterozoic 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
125Summary
- 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
- that served as nuclei
- around which Proterozoic crust accreted
126Summary
- 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
- to that operating now had been established
127Summary
- 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
128Summary
- 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
129Summary
- 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
130Summary
- 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