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 itís 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

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