Proterozoic
Chapter 9.
Proterozoic
includes about 42% of the geologic time.
The
boundary between Archean and Proterozoic is not defined to a narrow layer (changes occurred during a wide range of time between
2.95-2.45.
Major
differences between Archean and Proterozoic
1.
Style of crustal evolution
Reduced rate (Less radioactive decay, colder crust and slower mantle convection)
plate tectonics as it is today (there is proof of an existing felsic
crust and mountain formation)
2.
Widespread sedimentary rocks (some of them representing shallow marine, river
environment)
3.
Occurrence of mature sedimentary rocks
4. Free
oxygen in the atmosphere (ozone layer)
Early
Proterozoic
Crustal
evolution
Evolution
of the early Proterozoic super continent Laurentia
between 2.0 and 1.8 billion years ago, was marked by major orogenesis. These
orogenesis already produced mountain ranges as a result of collision between two
continents. In these ranges highly deformed rocks were forming. Lots of granite
plutons also evidence the subduction before collision.
Thelon
orogen
Northwestern
Canadian shield where Slave and Rae cratons collided.
Geol.
record indicates subduction, granite plutons, volcanism, intense deformation,
and regional metamorphism.
Trans-Hudson
orogen
In
northern Saskatchewan there is a record of initial rifting, extrusive volcanism,
sedimentation, and the formation of oceanic crust, followed by closure of the
oceanic basin. This is followed by island arc subduction, granite plutons,
regional metamorphism. This orogenesis resulted the suture of Superior, Wyoming,
andearne cratons.
Wopmay
orogen
Adjacent
to Slave craton in northwestern Canada. This orogen gives us the best preserved
Wilson cycle of the Proterozoic.
This
orogen is studied by geologists in big detail.
1.The
evidence of rifting here are the normal faults found in the area representing
extension,. the extension created rift basins in which thick accumulation of
alluvial fan and fluvial deposits occurred. Through fault systems from
down-upward lavas represent the basaltic rift volcanism.
2.
Continued opening produced an oceanic crust, which slowly widened.
3. The
western edge of Slave craton became a passive margin with characteristic
sediments.
a. Coastal deposits close to the continent
b. Shallow marine quartz sandstone seaward (clean mature quartz sandstone
standing as a good evidence of existing chemical erosion;; well developed rain,
fluvial system on the continent).
c. Stromatolitic dolomite (stromatolite photosythetising cyanobacteria
evidencing that atmosphere starts to have free Oxygen)
d. Deeper water turbidites and shale.
4. The
closure of the ocean basin started. and finally collision, and mountain
formation. The collision produced outcrop of ophiolite sequences. Ophiolite are
remnants of oceanic crust. This is the first occurrence of them.
Banded
Iron Formation
The
latest Archean and the earlier part of Proterozoikum (especially between 2.5-2.o
billion years) is characterized by a special group sedimentary rocks consisting
alternating thin layers of silica (chert) and iron minerals (hematite and
magnetite). These rocks were deposited in shallow marine water.
They have a great
importance, bec.
1.they
prove the presence of shallow marine environment;
2.Since
the iron is oxidative here Fe3+ (hematite, magnetite), its presence prove the
presence of free oxygen in the atmosphere.
upwelling
deep water brings Fe2+.
By 1.8
billion years ago Greenland, Central Canada, north-central US were assemblage
together into a big huge cont. Laurentia.
Between
1.8-1.6 billion years ago this continent accretion continued. and Laurentia
became more than 1000km2.
There
are some evidence in Canada foe an early Proterozoic glaciation.
Red
beds
During
glacial time the presence of red beds are characteristic indicating arid
climate.
Red
beds are sandstones, where the red coloration is coming from hematite cement
Middle
Proterozoic
Igneous
activity
Between
1.6-1.3 billion years no further continent accretion happened, however there are
signs of tremendous igneous activity, which was apparently unrelated to
orogenesis.
They
are present on almost every shield on the earth.
They mostly granite batholites, rhyolite lava flows
gabbro are common also.
To
these igneous activity tremendous ore deposit relates. Mostly copper, gold,
Grenville
orogeny
1.3-1.0
billion years ago
Southeastern
Canada, and underneath this belt goes down to Kansas. And it’s rocks are found
in Scandinavia and Greenland.
The
rocks here are represent also a whole Wilson cycle from rifting to collision.
The
rifting broke up Laurentia. That was a mid-continent rift, It cut Archean and
early Proterozoic rocks.
Supercontinents
of the Middle Proterozoic
-Laurasia on the north as we talked about including Greenland,
Central Canada, north-central US and
-GONDWANA on the south.
Australia,
Antarctica, India, Africa, South America
Some
people think that probably at the end of the Middle Proterozoic Gondwana and
Laurasia were assembled into a supercontinent, but for this there is no
evidence.
Late
Proterozoic
The
late Proterozoic is characterized by large scale rifting, which resulted in the
break up of the supercontinent.
Because
plates were drifting apart, large part of Gondwana got to the south pole which
started a big scale continental glaciation.
Glaciation
How do
we know if something is glacial?
-Unsorted,
unstratified sediments called TILLITE
-tillite
is associated by striated bedrock
pavement
-finally
laminated argillites (varved sediments),
that contain large clasts dropped from
floating ice
Widespreaded
glaciation occurred
between 900-600 million years ago. Tillites and other glacial deposits were
recognized in all the continents but Antarctica. Of course glaciation was not
continuous, it had four periods.
Most
likely this glaciation was the largest in the earth history, evidenced by the
presence of tillites even near the equator.
Evolving
atmosphere
The
photosyntetizing algae produced more and more oxygen into the atmosphere, and by
2.5-1.8 billion years ago enough free oxygen existed in the atmosphere to start
the formation of ozone layer Ultraviolet radiation converted some of the oxygen
into O and O3, both of which are oxidize surface materials more effectively than
O2. Once an ozone layer became established,
no more ultraviolet radiation reached the earth, and O2 become the
primary source of surface oxidation.
Ozone
formed by 2000 million years ago.
Proterozoic
life
Stromatolites
Stromatolites
are the cyanobacteria (sometimes called blue green algae), and they are
Prokaryote, autotrophic, so photosynthesis is characteristic to them.
They
became abundant 2.3-2.2 billions of years ago, because at that time the amount
of shallow continental shelf increased.. (Algae mats like to live in the
intertidal, shallow subtidal zone)
They
remained abundant and did not decline until the Paleozoic. They are
living today also, especially in harsh environment where other animals
could not harm them.
They
had many different shapes, some of them built reeflike structure. They are not
too useful in relative dating, because they did not evolve too much they are the
same all the time. (They have asexual division, which does not allow wide
mutation to occur).
Appearance
of EUKARYOTIC cell
Their
appearance marks the most important step in the history of life.
They
have:
1.
Membrane-bounded nucleus, that contains the genetic material
2. They
have mitochondria
Their
origin
Most
Eukaryote reproduces sexually allowing more genetic mutation. They are mostly
aerobic, which mean that they could not form without oxygen in the atmosphere.
The
oldest fossil eukarypt was found in 1.2 1.4 billion years old Back spring
dolomite California.
They
are usually spherical or plate like structures.
Acritarch
These
are hollow fossils probably parts of planktonic algae, which was abundant during
late Proterozoic.
Multicellular
organisms
Some of
the cells are specialized to certain functions, such as reproduction. We know,
that they appeared in Late Proterozoic, but we do not have any transitional
form.
The
first for sure fossils of multicellular algae are known from rocks of 0.8
billion years old.
Ediacaran
fauna
In
south Australia Sprig discovered impressions of soft bodied animals in rocks of
the Ediacara hills. (570-670 million years ago).
The
rock is quartzite here.
Unique
assemblage of soft-bodied animals are preserved here as molds and casts on the
underside of the sandstone layers.
These
animals lived in a near shore shallow marine environment.
At
least three present day invertebrate phyla represented:
1.Yellyfish,
sea pens (ph.Cnidaria)
2.
Segmented worms (Ph. Annelid)
3.Primitive
members of Ph.Arthropoda.
People
believe, that a worm-like Ediacaran fossil named Spriggina is the possible
ancestor of Trilobites.
Tribaschidium
may be a primitive echinoderm.
Some
geologists however believe, that the Ediacaran fauna is an evolutionary dead
end.
Mineral
deposits in Proterozoic.
Banded
iron
Lake
Superior
Brazil,
Australia
China
Ukraine
Sweden
South
Africa
Nickel-Platinum
Sudbury
mineral district Canada
Busweld
South Africa
Oil,gas
China,
Russia
Pegmatites,
for gemstones and industrial minerals
south
Dakota
Main