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The Precambrian

 

 

Introduction

 

The Precambrian Eon lasted from 4.6 bya to ~ 600 mya.

 

It comprises almost 80% of geologic time!

 

The Precambrian is not very well understood because:

 

1) Precambrian rocks are deep-seated (they are called basement rocks) and tend to be poorly exposed at Earth’s surface.

2) The rocks are severely metamorphosed.

3) The rocks contain few fossils (correlation is difficult).

 

The Precambrian is divided into three Eras:

 

1) Hadean Era- 4.5-4.0 bya.

2) Archean Era- 4.0-2.5 bya.

3) Proterozoic Era- 2.50-0.6 bya.

 

 

 

The Hadean (4.5 – 4 bya)

 

There is no rock record preserved for the Hadean.  The major events of this time include:

1) Solar nebular processes.

2) The formation of Earth by accretion.

3) Differentiation of Earth into the crust, mantle, and core.

 

  

Precambrian (Archean and Proterozoic) Rocks

 

What kinds of Precambrian rocks exist?

 

1) Igneous

2) Metamorphic

3) Some sedimentary

 

This implies some similarity to today’s Earth.

 

Where do Precambrian rocks occur?  They are best exposed in cratons, the interiors of continents that were not deformed by tectonic events.  These regions are also called Precambrian shields.  In North America, Precambrian shields occur at:

 

1) Canada

2) The Great Lakes region

3) Greenland

 

Precambrian rocks are the “basement” rocks over which younger sedimentary rocks were deposited.

 

 

 

The Accretion of North America

 

Different regions of the North American craton exhibit different isotopic dates (they vary within a few hundred million years).  Each regions is known as an isotopic date province.

 

The oldest provinces tend to be located in the center of the craton, and younger provinces tend to be on the edges.

 

Image:World geologic provinces.jpg

 

The provinces appear to represent “microcontinents” that collided and accreted to form the North American continent.

 

The collision and subduction of microcontinents resulted in mountain-building events called orogenies.

 

 

 

Archean Orogenic Belts (4.0 – 2.5 bya)

 

The oldest Precambrian rocks are preserved in highly deformed linear belts that represent ancient orogenies.

 

There are two main types of Archean belts:

 

1) Greenstone belts– These are mildly metamorphosed and deformed basaltic rocks and associated sediments.  These represent oceanic rocks interlayered with sediments derived from land. The tectonic environment is interpreted to be oceanic and related to a subduction-volcanic arc complex.

 

File:Oceanic-continental convergence Fig21oceancont.svg

Ocean-continental convergence

 

 

2) Gneiss belts– These are highly metamorphosed and deformed rocks, characterized by granitic plutons (early continental crust deformed to gneiss), shales (transformed to schist), graywacke (transformed to quartzites), and minor limestone (transformed to marble).

 

The sedimentary component of the gneiss belts may represent temporary passive margin deposits prior to tectonic collisions.

 

 

The bottom line: It appears that gneiss belts represent granitic protocontinents and the greenstone belts represent basaltic ocean basins.  Collisions of protocontinents caused the sandwiching of greenstone belts between the gneiss belts.

 

 

 

Early to Middle Proterozoic Rocks (2.5 – 1 bya)

 

Proterozoic rocks include abundant mature pure quartz sandstones (not just graywackes) that are metamorphosed to quartzites.  The quartzites show cross-stratification and ripple marks. 

 

They also include limestones have wavy laminated fossil structures called stromatolites.  The stromatolites are trace fossils formed by the actions of cyanobacteria (blue-green algae).  These are the earliest known fossils!

 

Fossil stromatolite (National Park Service Photo)

 

Image:Lake Thetis-Stromatolites-LaRuth.jpg

Modern Stromatolite (Wikipedia Photo)

 

The occurrences of sandstone and limestone suggest that:

 

1) Continental processes such as weathering, sorting, and deposition were commonplace.

2) A shallow sea had developed (probably a passive margin).

 

 

 

Late Proterozic Rocks (1 – 0.6 bya)

 

In North America, flood basalts erupted during the Late Proterozoic through fissures in the stable craton (far away from any orogenic belts).

 

The flood basalts are believed to be the result of continental rifting.

 

File:Ottawabonnecheregrabenmap.png

Wikipedia Commons Image

 

 

 

Oxygen in the Hydrosphere and Atmosphere

 

In Archean rocks, metals tend to occur in low oxidation states (for example, Fe2+ instead of Fe3+) indicating a high metal:oxygen ratio in the oceans and atmosphere.  The sediments are essentially rust-free.

 

 

After the late Proterozoic, sedimentary deposits often have reddish colors and are called red beds due to the presence of iron-oxide coatings between sand grains.  From the later Proterozoic onward, enough free oxygen has been available to oxidize iron in sediments.

 

File:Butte pdphoto roadtrip 24 bg 021604.jpg

A sandstone butte outside of Sedona, Arizona.  Public domain image by Jon Sullivan.

 

Late Archean and early Proterozoic rocks contain rock formations called banded iron formations (BIFs).

 

File:MichiganBIF.jpg

Banded iron formation from Michigan. Public Domain Image by Mark A. Wilson (Department of Geology, The College of Wooster).

 

BIFs consist of iron oxides such as magnetite (Fe3O4) and hematite (Fe2O3) interlayered with chert (SiO2).

 

These formations are believed to have precipitated from early ocean water that contained dissolved oxygen produced by cyanobacteria.  The banding may represent cyclic changes in the availability of oxygen as our atmosphere was getting established.

 

 

 

Precambrian Climate

 

Higher Precambrian CO2 concentrations due to volcanic outgassing may have resulted in warmer average temperatures.  Evaporite deposits and mud cracks provide evidence of arid conditions in some localities.  Windblown deposits are indicative of large dune fields.

 

In the late Proterozoic, there is evidence of glaciation in the form of ancient glacial tills found today on most continents.

 

Why was the Earth so cold?

 

1) Maybe the Earths orbit was extremely elliptical, causing great temperature contrasts between winter and summer.

 

2) Most of the landmasses during the late Proterozoic were located near the equator (away from the poles).  Maybe the Earth’s axis was extremely tilted, causing the poles to be warm and the equator to be relatively cool.

 

3) Instead of heat being absorbed by tropical oceans, heat may have been reflected by equatorial land masses (the albedo effect).  The reflection of sunlight may have led Earth into a cooling period.

 

 

 

The End of the Precambrian:

 

The boundary between the Precambrian and Cambrian is typically defined by an unconformity.  The deformed and metamorphosed Precambrian rocks underlie Paleozoic sedimentary strata.

 

 

 

Timeline of Precambrian Events

 

15 BYA- The Big Bang.

4.6 BYA- Formation of the Solar Nebula, Formation of meteorites, and cold accretion of planets.

4.5 BYA- Melting and differentiation into Fe-Ni core and peridotite-rich mantle.

4.4 BYA- First transient crust (via fractional crystallization of molten peridotite).

3.5 BYA- First stable crust (plates).

3.5 BYA- First cyanobacteria.

3.5 BYA- Start of oxygen atmosphere.