Transcript chapter9_Proterozoic..

Chapter 9
Precambrian Earth and Life History
Proterozoic
Introduction
 The Archean-Proterozoic boundary at 2.5 billion
years ago marks the approximate time of changes
in the style of crustal evolution,

 First, the Archean was characterized by the origin
of granite-gneiss terrains 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.
 The change in style of crustal evolution, the Proterozoic
was also an important time in the evolution of the
atmosphere and biosphere, as well as the origin of some
important natural resources. Oxygen dependant
organisms and the types of cells that make up most of
today's organisms evolved during this time.
Fig. 19.1, p. 494
Proterozoic Continents
Acretion continue during the Proterozoic
 Laurentia a North land mass known North America
Greenstone belts continue in Australia but without
Komatiites
Proterozoic Earth History
The Proterozoic 2.5 bya – 545 mya
Paleoproterozoic History of Laurentia
 By 1.8 bya, plate collisions had
formed several orogens – linear or
arcuate belts of deformed
metamorphic rocks intruded by
magma forming huge batholiths.
 The old Archean cratons were
sutured together by continental
accretion along their margins
creating a large landmass known
as Laurentia.
 Laurentia included part of North
America and Greenland.
Laurentia
 Excellent examples of these craton-forming processes are
recorded in rocks of the
 The Thelon orogen (1.92-1.96 BYA) in northwestern Canada
where the Slave and Rae cratons collided,
 The Trans-Hudson orogen (1.82-1.84BYA), in the United
States and Canada where the Superior, Hearne, and Wyoming
cratons were sutured (Figure 9.2a).
 the Penokean orogen formed along the southern margin of
Laurentia over tens of millions of years, although the most
intense episode of this event was about 1.85 BYA.
Proterozoic Earth History
The Proterozoic
2.5 bya – 545 mya
Paleoproterozoic
History of Laurentia
 Wilson cycle evidence –
The Wopmay Orogen
 Wilson cycles record
the opening and closing
of an ocean basin
Development of a
passive continental
margin
Uplift
Divergence
(spreading)
Passive continental
margin
Divergence
(spreading)
Convergence
(subduction)
Convergence
(collision)
and uplift
Convergence
and uplift
Stepped Art
Fig9.3
Wopway Ororgen
Northwestern Canada
 The Wopmay orogen consist of a suite of sedimentary rocks called a
sandstone-carbonate-shale assemblage that forms on passive
continental margins.
 Passive margin deposits are rare or absent in Archean rocks, but
they become common during the Proterozoic and thereafter.
 This assemblage of rocks is also well represented in the Penokean
orogen of the Great Lakes region of the United States and Canada
Proterozoic Earth History
The Proterozoic 2.5 bya – 545 mya
Paleo- and Mesoproterozoic Igneous Activity
Between 1.8 and 1.1 bya
extensive igneous activity
unrelated to orogenic
processes occurred,
thickening the continental
mass with:
Granite and anorthosite plutons
Volcanic calderas
Vast sheets of ash
Hypothesis: Mantle plume
Proterozoic Earth History
The Proterozoic 2.5 bya – 545 mya
Mesoproterozoic Orogeny and Rifting
 Between 1.3 and 1.1 bya the Grenville orogeny
occurred as the final stage of Proterozoic continental
accretion.
Beginning 1.1 bya, the
Midcontinental Rift
formed, thought to be a
failed rift that did not
break Laurentia apart.
Detrital sedimentary
rocks and basaltic lava
flows are found in the
central portion of the rift.
Proterozoic Earth History
The Proterozoic 2.5 bya – 545 mya
Meso- and Neoproterozoic Sedimentation
 In the western United States and
Canada, Proterozoic sedimentary
rocks are well-exposed.
 These are mostly sandstones and
shales, with some dolostones and
stromatolite-bearing carbonates.
 Indicate shallow marine and fluvial
environments of deposition.
Exposed Rocks
Proterozoic Earth History
The Proterozoic 2.5 bya – 545 mya
Proterozoic Supercontinents
 A continent is a landmass, consisting
of a granitic crust and most of its
surface above sea level.
 A supercontinent is larger and
composed of several continents that
have come together.
 The first supercontinent that
geologists recognize is known as
Rodinia.
 assembled 1.3 to 1.0 bya
 began fragmenting 750 mya
Proterozoic Earth History
Proterozoic Rocks
Sandstone-carbonate-shale assemblages
Deposited on passive continental margins and in
intracratonic basins
The most common Proterozoic-aged rocks.
Proterozoic Earth History
Proterozoic Earth History
Proterozoic Glacial Deposits
Widespread glaciers were present during the
Early and Late Proterozoic.
Fig. 19.15b, p. 510
The Evolving Atmosphere
 the Archean atmosphere contained little or no free oxygen (see Chapter 8),
so the atmosphere was not strongly oxidizing as it is now.
 Photochemical dissociation and photosynthesis were adding free oxygen
to the atmosphere,
 but the amount present at the beginning of the Proterozoic was 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.
 Remember that cyanobacteria (blue-green algae), and Stromatolites
These photosynthesizing organisms and, to a lesser degree, photochemical
dissociation both added free oxygen to the evolving atmosphere
Proterozoic Earth History
Proterozoic Earth History
Banded Iron Formations
Deposition of
widespread banded
iron formations
between 2.5 and 2.0
billion years ago
indicate that free
oxygen was absent in
the early atmosphere.
Fig. 19.16a, p. 511
Banded Iron Formations (BIF)
 BIFs, fully 92%, were deposited in shallow-water shelf
environments during the interval from 2.5 to 2.0 billion
years These deposits are much more extensive than
those of the Archean and they have important
implications for the evolving atmosphere.
Proterozoic Earth History
Proterozoic Earth History
Continental Red Beds
Deposition of
continental red beds
about 1.8 billion years
ago, however, indicate
that there now was
some free oxygen was
present in the
atmosphere.
Life-Its Origin and Early History
Life of the Proterozoic
A New Type of Cell Appears
 Eukaryotic Cells reproduce
sexually, and most are multi-celled
nearly all are aerobic, that is,
they depend on free oxygen to
carry out their metabolic
processes
Therefore, they could not have
evolved before at least some
free oxygen was present in the
atmosphere
Fig. 19.21, p. 515
Life on Proterozoic
 6 Kindoms of organisms
Life of Proterozoic
3 Domains
Life-Its Origin and Early History
Life of the Proterozoic
Endosymbiosis and the Origin of Eukaryotic Cells
Endosymbiosis – a process whereby one
cell lives within another, a process
practiced by prokaryotic cells was probably
responsible for the first eukaryotic cells.
Life-Its Origin and Early History
Life-Its Origin and Early History
Life of the Proterozoic
 The Dawn of Multi-celled Organisms
It is not known how multi-celled
organisms arose from singlecelled organisms.
Carbonaceous impressions of
Proterozoic multi-celled algae are
known from areas, some in rocks
more than 2 billion years old.
Fig. 19.22, p. 516
Multicelled vs Single Celled
Life-Its Origin and Early History
Life of the Proterozoic
Ediacaran Fauna
The Neoproterozoic
Ediacaran faunas
include the oldest welldocumented animal
fossils. None had
durable skeletons, so
their fossils are not
common.
Fig. 19.25b, p. 517
Why Edicaran Fauna ?
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
Ediacaran Fauna
Resources in Precambrian Rocks
Archean Resources
More than 50% of the world’s gold has come from
Archean rocks in South Africa.
Massive sulfide deposits containing zinc, copper,
and nickel are known from Australia, Zimbabwe
(Africa) and Canada. These are similar to the black
smokers on the seafloor.
About 25% of the world’s chromium reserves are
also in Archean rocks, along with platinum deposits.
Archean pegmatites in the African Shield and the
Canadian Shield have yielded valuable gem-quality
minerals.
Resources in Precambrian Rocks
Proterozoic Resources
 92% of all BIF’s (Banded iron formations), the world’s
main source of iron ore comes Proterozoic rocks
 Nickel, platinum, and chrome are found on the shields
of North America and Africa.
End of
Chapter 9