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
What kinds of Precambrian
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:
2) The Great Lakes region
Precambrian rocks are the “basement” rocks over which younger
sedimentary rocks were deposited.
The Accretion of North
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.
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 –
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
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)
Modern Stromatolite (Wikipedia
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 –
In North America, flood basalts erupted during the Late
Proterozoic through fissures in the stable craton (far away from any
The flood basalts are believed to be the result of
Wikipedia Commons Image
Oxygen in the Hydrosphere and
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
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.
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).
Banded iron formation from Michigan. Public Domain Image by
Mark A. Wilson (Department of Geology, The College of Wooster).
BIFs consist of iron oxides
(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.
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
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
The Big Bang.
Formation of the Solar Nebula, Formation of meteorites, and cold accretion
Melting and differentiation into Fe-Ni core and peridotite-rich mantle.
First transient crust (via fractional crystallization of molten peridotite).
First stable crust (plates).
Start of oxygen atmosphere.