The younger of the two Precambrian eras, and
the erathem of rocks deposited in this era. The Proterozoic is
divided into several intervals, including the Early or Paleopro-
terozoic (2.5 Ga–1.6 Ga), Middle or Mesoproterozoic (1.6
Ga–1.3 Ga), and Late or Neoproterozoic (1.3 Ga–0.54 Ga).
Proterozoic rocks are widespread on many continents, with
large areas preserved especially well in North America, Africa
and Saudi Arabia, South America, China, and Antarctica.
Like the Archean, Proterozoic terrains are of three basic
types, including rocks preserved in cratonic associations, orogens
(often called mobile belts in Proterozoic literature), and
cratonic cover associations. Many Proterozoic terrains are cut
by wide shear zones, extensive mafic dike swarms, and layered
mafic-ultramafic intrusions. Proterozoic orogens have
long linear belts of arc-like associations, metasedimentary
belts, and widespread, well-developed ophiolites. Many geologists
have suggested that clear records of plate tectonics first
appeared in the Proterozoic, although many others have challenged
this view, placing the operation of plate tectonics earlier,
in the Archean. This later view is supported by the recent
recognition of Archean ophiolites (including the Dongwanzi
ophiolite) in northern China.
The Proterozoic saw the development of many continental-
scale orogenic belts, many of which have been recently recognized
to be parts of global-scale systems that reflect the
formation, breakup, and reassembly of several supercontinents.
Paleoproterozoic orogens include the Wopmay in northern
Canada, interpreted to be a continental margin arc that
rifted from North America, then collided soon afterward, closing
the young back arc basin. There are many 1.9–1.6 Ga orogens
in many parts of the world, including the Cheyenne belt
in the western United States, interpreted as a suture that marks
the accretion of the Proterozoic arc terrains of the southwestern
United States with the Archean Wyoming Province.
The supercontinent Rodinia formed in Mesoproterozoic
times by the amalgamation of Laurentia, Siberia, Baltica,
Australia, India, Antarctica, and the Congo, Kalahari, West
Africa, and Amazonia cratons between 1.1 Ga and 1.0 Ga.
The joining of these cratons resulted in the terminal collisional
events at convergent margins on many of these cratons,
including the circa 1.1–1.0 Ga Ottawan and Rigolet orogenies
in the Grenville Province of Laurentia’s southern margin.
Globally, these events have become known as the Grenville
orogenic period, named after the Grenville orogen of eastern
North America. Grenville-age orogens are preserved along
eastern North America, as the Rodonia-Sunsas belt in Amazonia,
the Irumide and Kibaran belts of the Congo craton,
the Namaqua-Natal and Lurian belts of the Kalahari craton,
the Eastern Ghats of India, and the Albany-Fraser belt of
Australia. Many of these belts now preserve deep-crustal
metamorphic rocks (granulites) that were tectonically buried
to 19–25-mile (30–40-km) depth, then the overlying crust
was removed by erosion, forcing the deeply buried rocks to
the surface. Since 30–40 kilometers of crust still underlie
these regions, it has been surmised that during the peak of
metamorphism, they had double crustal thicknesses. Such
thick crust is today produced in regions of continent-continent
collision, and locally in Andean arc settings. Since the
Grenville-aged orogens are so linear and widely distributed,
they are generally interpreted to mark the sites of continentcontinent
collisions where the various cratonic components
of Rodinia collided between 1.1 Ga and 1.0 Ga.
The Neoproterozoic breakup of Rodinia and the formation
of Gondwana at the end of the Precambrian and the dawn
of the Phanerozoic represents one of the most fundamental
problems being studied in earth sciences today. There have
been numerous and rapid changes in our understanding of
events related to the assembly of Gondwana. One of the most
fundamental and most poorly understood aspects of the formation
of Gondwana is the timing and geometry of closure of
the oceanic basins which separated the continental fragments
that amassed to form the Late Neoproterozoic supercontinent.
It appears that the final collision between East and West Gondwana
most likely followed the closure of the Mozambique
Ocean, forming the East African Orogen. The East African
Orogen encompasses the Arabian-Nubian Shield in the north
and the Mozambique Belt in the south. These and several
other orogenic belts are commonly referred to as Pan-African
belts, recognizing that many distinct belts in Africa and other
continents experienced deformation, metamorphism, and magmatic
activity spanning the period of 800–450 Ma. Pan-
African tectonothermal activity in the Mozambique Belt was
broadly contemporaneous with magmatism, metamorphism,
and deformation in the Arabian-Nubian Shield. The difference
in lithology and metamorphic grade between the two belts has
been attributed to the difference in the level of exposure, with
the Mozambican rocks interpreted as lower crustal equivalents
of the juvenile rocks in the Arabian-Nubian Shield. Recent
geochronologic data indicate the presence of two major “Pan-
African” tectonic events in East Africa. The East African
Orogeny (800–650 Ma) represents a distinct series of events
within the Pan-African of central Gondwana, responsible for
the assembly of greater Gondwana. Collectively, paleomagnetic
and age data indicate that another later event at 550 Ma
(Kuunga orogeny) may represent the final suturing of the Australian
and Antarctic segments of the Gondwana continent.
The Arabian-Nubian shield in the northern part of the East
African orogen preserves many complete ophiolite complexes,
making it one of the oldest orogens with abundant Penrosestyle
ophiolites with crustal thicknesses similar to those of
Phanerozoic orogens.
The Proterozoic record preserves several continental scale
rift systems. Rift systems with associated mafic dike swarms
cut across the North China craton at 2.4 billion and 1.8 billion
years, as well as in many other cratons. One of the bestknown
of Proterozoic rifts is the 1.2–1.0 Ga Keweenawan rift,
a 932-mile (1,500-km) long, 93-mile (150-km) wide trough
that stretches from Lake Superior to Kansas in North America.
This trough, like many Proterozoic rifts, is filled with a
mixture of basalts, rhyolites, arkose, conglomerate, and other,
locally red, immature sedimentary rocks, all intruded by granite
and syenite. Some of the basalt flows in the Keweenawan
rift are 1–4 miles (2–7 km) thick.
Massive Proterozoic diabase dike swarms cut straight
across many continents and may be related to some of the
Proterozoic rift systems, or to mantle plume activity. Some of
the dike swarms are more than 1,865 miles (3,000 km) long,
hundreds of kilometers wide, and made of thousands of individual
dikes ranging from less than three feet to more than
1,600 feet (1–500 m). Some dike swarms, such as the 1.267
Ga Mackenzie swarm of North America, show radial patterns
and point to a source near the Coppermine River
basalts in northern Canada. Other dike swarms are more linear
and may parallel failed or successful rift arms. Magma
flow directions in the dikes is generally parallel to the surface,
except in the central 300–650 miles (500–1,000 km) of the
swarms, suggesting that magma may have fed upward from a
plume that initiated a triple-armed rift system, then the
magma flowed away from the plume head. In some cases,
such as the Mackenzie swarm, one of the rift arms may have
become successful, forming an ocean basin.
Cratonic cover sequences are well-preserved from the Proterozoic
in many parts of the world. In China the Mesoproterozoic
Changcheng Series consists of several-kilometer-thick
accumulations of quartzite, conglomerate, carbonate, and
shale. In North America the Paleoproterozoic Huronian Supergroup
of southern Canada consists of up to 7.5 miles (12 km)
of coarse clastic rocks dominated by clean beach and fluvial
sandstones, interbedded with carbonates and shales. Thick
sequences of continentally derived clastic rocks interbedded
with marine carbonates and shales represent deposition on
passive continental margins, rifted margins of back arc basins,
and as thin cratonic cover sequences from epicontinental seas.
Similar cratonic cover sequences are known from many parts
of the world, showing that continents were stable by the Proterozoic,
that they were at a similar height with respect to sea
level (freeboard), and that the volume of continental crust at
the beginning of the Proterozoic was at least 60 percent of the
present volume of continental crust.
One of the more unusual rock associations from the Proterozoic
record is the 1.75–1.00 Ga granite-anorthosite association.
The anorthosites (rocks consisting essentially of
plagioclase) have chemical characteristics indicating that they
were derived as cumulate rocks from fractional crystallization
of a basaltic magma extracted from the mantle, whereas the
granites were produced by partial melting of lower crustal
rocks. The origin of these rocks is not clearly understood—
some geologists suggest they were produced on the continental
side of a convergent margin, others suggest an extensional
origin, still others suggest an anorogenic association.
Proterozoic life began with very simple organisms similar
to those of the Archean, and by 2.0 Ga planktonic algae and
stromatolitic mounds with prokaryotic filaments and spherical
forms are well preserved in many cherts and carbonates.
The stromatolites are formed by cyanobacteria and form a
wide variety of morphological forms, including columns,
branching columns, mounds, cones, and cauliflower type
forms. In the 1960s many geologists, particularly from the
Russian academies, attempted to correlate different Precambrian
strata based on the morphology of the contained stromatolites,
but this line of research proved futile as all forms
are found in rocks of all ages. The diversity and abundance
of stromatolites peaked about 750 million years ago and
declined rapidly after that time period. The decline is probably
related to the sudden appearance of grazing multicellular
metazoans such as worms at this same time. Eukaryotic cells
(with a membrane-lined cell wall) are preserved from at least
as old as 1.8 Ga, reflecting increased oxygen in the atmosphere
and ocean. The Acritarchs are single-celled spherical
fossils that are interpreted as photosynthetic marine plankton
and are found in a wide variety of rock types. Around 750
million years ago the prokaryotes experienced a sudden
decline, and their niches were replaced by eukaryotic forms.
This dramatic change is not understood, but its timing coincident
with the breakup of Rodinia and the formation of
Gondwana is notable. It could be that tectonic changes
induced atmospheric and environmental changes, favoring
one type of organism over the other.
A wide range of metazoans, complex multicellular organisms,
are recognized from the geological record by 1.0 Ga and
probably evolved along several different lines before the
record was well established. A few metazoans up to 1.7 Ga
old have been recognized from North China, but the fossil
record from this interval is poorly preserved since most animals
were soft-bodied. The transition from the Proterozoic
fauna to the Paleozoic is marked by a remarkable group of
fossils known as the Ediacaran fauna, first described from
the Ediacara Hills in the Flinders Ranges of southern Australia.
These 550–540-million-year-old fauna show an
extremely diverse group of multicelled complex metazoa
including jellyfish-like forms, flatworm-like forms, soft-bodied
arthropods, echinoderms, and many other species. The
ages of these fauna overlap slightly with the sudden appearance
and explosion of shelly fauna in Cambrian strata at
540 million years ago, showing the remarkable change in life
coincident with the formation of Gondwana at the end of
the Proterozoic.
See also ARCHEAN; CHINA’S DONGWANZI OPHIOLITE;
SUPERCONTINENT CYCLE.
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