The Proterozoic Eon

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The Proterozoic Eon

• CHAPTER 7

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Proterozoic- Last chapter of Precambrian time

• The Proterozoic boundary is placed _at_ 2.5 By,
  mostly arbitrarily
• Essentially modern plate tectonic activity
• Archean cratons provide basis for continental
  development and attachment (accretion)
• Extensive plutonism/metamorphism and reworking of
  crustal rocks through orogeny

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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

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The Phanerozoic

• Yet the Phanerozoic, consisting of
• Paleozoic,
• Mesozoic,
• Cenozoic eras,
• lasted a comparatively brief 545 million years

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Proterozoic Eon can be 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, 540 my ago)

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North American continental developments

• Precambrian provinces of North America were
  welded together to form a large continent called
  Laurentia during the early Proterozoic.
• Welding occurred along mountain belts or orogens
  by 1.7 by.
• Laurentia continued to grow by accretion
  throughout the Proterozoic.
• Sedimentation on and around the craton consisted
  of clastic and carbonate sediments deposited in
  shallow water on broad continental shelves and
  epeiric seas.

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Precambrian provinces of North America
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Wilson Cycle

• A complete Wilson cycle involves
• Fragmentation of a continent
• Opening followed by closing of an ocean basin
• Finally, reassembly of the continent
• Many example (more abundant) in Phanerozoic

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Wopmay Orogen

• Some of the rocks in Wopmay orogen show
• An assemblage of rocks typical of passive
  continental margins that first become widespread
  during the Proterozoic

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Ocean formation/closure

1. See examples of tensional faulting- where ocean
   began after being ripped apart
2. Alluvial fans develop lava comes up through
   these faults (interspersed with sediments)
3. Ocean widened
4. Closure of ocean buckled sediments producing
   trough for further sedimentation crumpled rocks
   as buckled

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Figure 7-4 (p. 244)Location of the Labrador
Trough and Trans-Hudson Orogen.
Trans-Hudson Orogen
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Labrador Trough

• East of the Superior province are rocks
  indicating deposition on a continental shelf,
  slope, and rise of the craton.
• These rocks were folded, metamorphosed, and
  thrust-faulted during the Hudsonian orogeny,
  which separates the Paleoproterozoic from the
  Mesoproterozoic.

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Satellite image of Labrador Trough
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Paleoproterozoic Ice Age

• A Paleoproterozoic ice age is recorded in rocks
  north of Lake Huron
• Evidence for glaciation includes finely laminated
  mudstones and tillites (or poorly sorted
  conglomerates of glacial debris).
• fine laminations can indicate seasonal deposition

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Aulacogens

• Defined as the inactive rift (or failed rift arm)
  at a Y-shaped triple junction where three plates
  are separating.
• Two of the three rifts are dominant and active,
  and the third is inactive--later it fills with
  sediment.

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Y-shaped triple junction
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The Mesoproterozoic Era

• The Midcontinent Rift and the Keweenawan Sequence
• Extending southward from the Lake Superior
  region, and overlying Animikian rocks or
  crystalline basement, the Keweenawan consists of
  
• Clean quartz sandstones
• Conglomerates
• Basaltic lava flows
• Native copper in rocks
• An old rift zone 1.2 by - 1.0 by old.
• Extends as far south as Kansas.

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Precambrian provinces of North America. Note
Mid-Continent Rift and Grenville Province.
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The Grenville Province and Rodinia

• The Grenville Province in eastern North America
  consists of sedimentary rocks that were
  metamorphosed and intruded by igneous bodies.
• Deformation of these rocks occurred during the
  Grenville orogeny, 1.2 by to 1.0 by ago.
• This orogeny was associated with the formation of
  the supercontinent Rodinia (Proto-Pangea)
• Ocean around Rodinia was called Mirovia

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Figure 7-8 (p. 247) The Neoproterozoic
supercontinent Rodinia as it began to break
apart. (After Hoffman, P. F. 1991. Science 252
1409-1412.)
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The Neoproterozoic

• The Neoproterozoic ( "new" Proterozoic) ranges
  from about 1.0 by to 0.54 by (540 my).
• Neoproterozoic rocks were deposited in basins and
  shelf areas surrounding the North American
  craton. Most of these rocks were deformed during
  the Paleozoic orogenies.

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Glacial deposits in the Neoproterozoic -
Snowball Earth?

• Tillites or lithified, unsorted conglomerates and
  boulder beds are found nearly worldwide (on all
  land masses except India and Siberia-Mongolia) at
  this time, and are interpreted by many geologists
  as glacial deposits.
• Stratified cobbles and dropstones
• Glacial deposits are so widespread at this time
  that geologists refer to it as "snowball Earth
  (Varangian glaciation)
• HIGHLY debated

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Precambrian rocks of the Grand Canyon

• Vishnu Schist - the oldest unit. Complex unit of
  metasediments and gneisses, intruded by granites
  about 1.4 by to 1.3 by. Topped by an
  unconformity.
• Grand Canyon Supergroup - a Neoproterozoic unit
  consisting mainly of sandstones, siltstones, and
  shales. Top of this unit is an unconformity
  overlain by Cambrian sedimentary rocks.

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Vishnu Schist, Grand Canyon Supergroup, and other
rocks in the Grand Canyon of the Colorado River.
http//www.valdosta.edu/phy/hist_geo_lab/gp_AZ/gp_
column/gp_col.html
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Late Proterozoic Fossil Record

• Microfossils of the Gunflint Chert
• First Precambrian fossils to be discovered were
  Proterozoic fossils in the Gunflint Chert, NW of
  Lake Superior. Fossils resemble living
  photosynthetic organisms and include
  Cyanobacteria
• Probably photosynthetic - altered atmosphere.

• Rock contains organic compounds that are
  regarded as the breakdown products of chlorophyll

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Gunflint Microfossils
Photomicrograph of spheroidal and filamentous
microfossils from the Gunflint Chert of Ontario
Canada
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Proterozoic Fossil Record

• Eukaryotes (large cells with nuclei and
  organelles) appeared by Mesoproterozoic time.
  Appeared by 1.6 by to 1.4 by. Increased in
  abundance about 1.4 by ago.
• Potential for sexual reproduction and increasing
  variation (evolution)

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Acritarchs

• Acritarchs are single-celled spherical,
  organic-walled microfossils.
• Not known what sort of organism they were, but
  they may have been phytoplankton. First appeared
  1.6 by ago. Maximum diversity and abundance 850
  my ago. They declined steadily during the
  Neoproterozoic glaciation, and few remained by
  675 my ago. Useful for correlation in Proterozoic
  strata.

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Origin of Eukaryotes

• Eukaryotes are thought to have arisen from an
  originally endosymbiotic relationship between two
  or more prokaryotic cells.
• Symbiosis - two or more organisms living
  together, where each organism usually derives
  some benefit froom the relationship.
• Endosymbiosis - One organism lives inside the
  other, e. g. bacteria in stomachs of cattle.

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Origin of Eukaryotes

• Organelles (Mitochondria, plastids) were once
  free living bacteria, that entered or were
  engulfed by another prokaryote. Eventually,
  relationship became mutually beneficial.
• E. g. host cell provided proto-mitochondrion
  (primitive oxidizing bacteria) with plenty of
  food, mitochondrion performed oxidation and
  released energy for a bacterium that previously
  could only ferment. Eventually proto-mitochondrion
  gives up trying to reproduce.
• E.g. host cell waste products (carbon dioxide
  etc.) used by a cyanobacterium (proto-plastid) to
  photosynthesize. Plastid leaks energy to host
  cell. Eventually gives up reproducing on its own.

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Neoproterozoic Metazoan Life
Doushantuo Formation Metazoans--Eggs Embryos
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The Ediacaran Fauna

• In 1947, an Australian geologist, R.C. Sprigg,
  discovered impressions of soft-bodied animals in
  the Pound Quartzite in the Ediacara Hills of
  South Australia

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Ediacaran Fauna

• The Ediacaran fauna of Australia
• Tribrachidium heraldicum, a possible primitive
  echinoderm or cnidarian

Spriggina floundersi, a possible ancestor of
trilobites
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Ediacaran Fauna

• Pavancorina minchami

• Restoration of the Ediacaran Environment

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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

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Distinct 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,
• are collectively referred to as the Ediacaran
  fauna
• 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

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Latest Proterozoic Kimberella

• Kimberella, an advanced metazoan in the Ediacaran
  fauna, which is the first known organism to have
  a coelom or body cavity. Possibly mollusc-like.
  Specimens found in Russia in 1993. 550 my old.

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Figure 7-32 (p. 261)Reconstruction of
Kimberella. (After Fedonkin, M. A., and
Waggoner, B. M. 1997. The Late Precambrian fossil
Kimbrella is a mollusk-like bilateral organism.
Nature 388(28) 868-871).
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Soft 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

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Small Shelly Fauna The Origin of Hard Parts

• Small fossils with hard parts or shells (mostly a
  few mm long ) appeared in the Vendian
• Cloudina, an organism with a small (few cm long),
  tubular shell of calcium carbonate is interpreted
  as a tube-dwelling worm. It is the earliest known
  organism with a CaCO3 shell.

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Geologic time scale across the Precambrian-Cambria
n boundary, showing the Vendian, and indicating
the occurrence of the Ediacaran fauna, the small
shelly fauna (first known hard parts),
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Figure 7-37 (p. 263) Correlation of major
events in the history of the biosphere,
lithosphere, and atmosphere.