Precambrian Facts

Makes up over 88% of all geologic time. No rocks are known for the first 640 million years of earth. Subdivided into Hadean, Archean Eon, and Proterozoic Eon.

Hadean

Oldest mineral Zircon from Australia dated at 4.4 billion years old, may confirm oldest continental crust existed at that time - shortly after the earth-moon system was formed.

Shields

Large areas of exposed precambrian rock (canadian shield)

Platforms

Precambrian rocks covered with younger rocks

Cratons

shield and platform together, forming a stable continental core

Why was early earth very hot?

Heat was generated by colliding particles as earth accreted, gravitational compression, and decay of short lived radioactive isotopes.

Hadean compared to later Precambrian

Relatively high frequency of meteorite and comet impacts. Lethal doses of ultraviolet radiation as no ozone layer. Moon much closer to earth. Day only 10 hours.

Archean early continents

Sediments extend continental materials seaward. Continent-content collisions result in larger continents. Not very big in Archean, Plate tectonics too fast. Magmatism from subduction zones causes thickening.

Archean Plate Tectonics

Radiogenic heat production was very high in the Archean, thus new crust was generated at spreading ridges more quickly than today. More volcanism and more rapid growth of continental crust at convergent boundaries.

Proterozoic Crust

2 billion years ago the first major episode of Proterozoic crustal evolution began. Orogens built large cratons by collisions of Archean crust. Laurentia encompassing most of North America, Greenland and parts of Scotland was formed by 1.8 bya.

Supercontinents (Proterozoic)

Several continents existed during the Proterozoic. They moved around and united at times to form supercontinent. First supercontinent Columbia formed durign 2.1-1.8 bya and it began to fragment about 1.5-1.35 bya.

Rodinia

Laurentia, Australia, and Antarctica collided during the Late Proterozoic to form another supercontinent, Rodinia, which fragmented 750 mya.

Proterozoic Deposits

Thick sedimentary packages of sand, shale, and carbonates were deposited on passive margins

Proterozoic Rocks

Ophiolite sequences are first observed from the Proterozoic suggesting a differentiation of the Earth crust - oceanic and continental. Passive margin sediments, banded iron formations and red beds (O2)

Snowball Earth

A major glaciation occurred during the Proterozoic period. Two episodes - An early proterozoic ice sheet centered southwest of the Hudson Bay. A late proterozoic glaciation that occured on all continents. Glacial deposition (till) can be used to correlate rocks from different continents (dropstone, varves, diamictite)

Huronian Glaciation

The oldest and longest ice age, and led to a mass extinction on earth. Extended from 2.4 to 2.1 billion years ago. Happened because increased atmospheric oxygen decreased atmospheric methane, the oxygen combined with the methan to form carbon dioxide and water, which do not retain heat as well as methane does.

The greatest ice age known to have occured.

Cryogenian Glaciation - Slushball/snowball earth, between 720 to 635 million years ago.

Five stages of Snowball earth

1. Cold spells, oceans start freezing. 2. Lowered reflectivity causes further cooling. 3. CO2 cycle in the ocean stops. 4. Strong greenhouse effect melts it. 5. CO2 cycle restarts, pulling CO2 back into oceans.

Early atmosphere made of?

Gasses from cooling magmas formed the early atmosphere, mostly N2, CO2, with CH4 and H2O. It was not breathable due to low oxygen.

Atmosphere change

The change to an oxidizing atmosphere caused the formation of banded iron formations about 2.5 billion years ago. Banded inro formations (silica and iron oxide), source of most of the worlds iron ore deposits, and red beds of sandstone, siltstone, and shale about 1.8 billion years ago.

Great Oxidation Event (GOE)

Also known as the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Great Oxygenation Event, or Oxygen Revolution. A period when the earth's atmosphere and the shallow ocean experienced a rise in oxygen, approximately 2.5/2.4 bya to 2.1/2.0 bya during the Paleoproterozoic era.

Origin of Life

Life was present on Earth at least 3.5 billion years ago (fossils). Only fossils of bacteria are known from the Archean, whereas today millions of species are recognized.

Archean Early Life

No oxygen, no ozone layer (harmful UV radiation bombards the earth), acidic oceans. Life originated when earth's primitive atmosphere had little or no free oxygen. (Precambrian cyanobacteria)

Definition of Life

A living organism must reproduce and practice some kind of metabolism (ensures long-term and short-term survival). Two requirements are necessary for the origin of life: 1. a source of the appropriate elements from which organic molecules could have been synthesized. 2. an energy source to promote chemical reactions that synthesized organic molecules

Origin of life

All organisms are composed mostly of C, H, N, and O, which were present in the earth's early atmosphere contributed as gasses from volcanic eruptions. Energy is needed for these elements to combine which was likely lightning and UV radiation.

Where do we think life originated? (location)

Many scientists hypothesize that the earliest organisms originated in the oceans or the ocean atmosphere interface.

Minimum requirements for life to form (3)

1. a membrane-enclosed capsule to contain the bioactive chemicals 2. energy-capturing chemical reactions capable of promoting other reactions. 3. some chemical system for replication (RNA-DNA)

RNA

can replicate and act as a catalyst that drives other nucleic acid reactions

DNA (deoxyribonucleic acid)

easier once RNA was formed in early oceans

Earliest organisms

Likely Anaerobic (require no oxygen), dependent on external source of nutrients, lacked a cell nucleolus and other internal structures, and reproduced asexually.

Fossil evidence

Supports the origins of life on earth earlier than 3.5 billion years ago. The Apex Chert microfossils from Australia are complex enough that more primitive cells must have existed earlier.

Archean organisms

Stromatolites are cyanobacteria that originate when sediment is trapped on sticky mats of algae. They are at least 3.5 byo and contributed to the increasing oxygen levels of the Archean.

Proterozoic organisms

like the archean, the proterozoic was dominated by a biota of single celled bacteria without a cell nucleus. In the middle proterozoic sexually reproducing cells appeared. This caused an increase in the rate of evolution and many new organisms started to develop.

Gunflint Microfossils

2 billion years ago. The gunflint is a succession of peculiarly banded silica and iron rich rocks - iron oxide alternating with layers of red, yellow, and grey chert.

Eukaryotic cells

A major event in life history was the appearance of these cells during the early to middle proterozoic. They are large, have a membrane bound nucleus, nucleus contains genetic materials, and they reproduce sexually.

Oldest Eukaryotes

A fossil from 2.1 billion years ago, Negaunee Iron Formation at Marquette, Michigan, is probably some type of multicelled algae. (Grypania)

Proterozoic organisms

This suggests multicellular organism existed. These organisms consisting of cells specialized to perform special functions like reproduction and respiration appeared in the Late proterozoic. No fossil evidence of the transition from their unicellular ancestors.

Ediacaran fauna

Is the impressions of multicellular, soft-bodied animals in rocks. Preserved as molds and casts, likely similar to present day jellyfish, sea pens, and segmented worms. Likely lived in a nearshore, shallow marine environment. Widespread on most continents. Not common as fossils as they lacked skeletons. Present in 670 to 570 myo sandstone.

Late Proterozoic worms

Very rare and even absent before 700 million years ago. Includes trails and burrows made by metazoans, simple/ shallow burrows, and an apparently backfilled burrow.

Late Proterozoic Shelly Fossils

Small shells or scraps of shell-like material and spicules occur in late proterozoic rocks (evidence that the earliest stages of skeletonization occured during this period). By the lastest Proterozoic several skeletonized animals probably existed.

Resources in Precambrian Rocks

Gold (Yellowknife, Snow Lake), Nickel (Thompson, Sudbury), Uranium (Cluff Lake, Key Lake), Iron (Wawa, Schefferville) and Copper-Zinc (Flin Flon, Snow Lake).

Manitoban Precambrian Rocks

About 60% of the province is underlain by Precambrian rocks recording the earliest geological history of Manitoba. Churchill and Superior Provinces are subdivided into a number of smalled sub-provinces. World's largest nickel deposit is best known in Thompson.