Transition to the Proterozoic
the late Archean life developed as single-celled bacteria, plate
tectonics constructed large Precambrian shields with sedimentary rocks
along their margins.
Proterozoic, plate tectonics processes continue at a slower rate with
cratons growing larger, new oceans basins & new mountains form.
the first evidence of radically different climates and
the oceans begins to evolve. First multi-cellular &
soft-bodied organism to the appearance of animals with hard external
landmass during this time by a process called continental accretion.
Microcontinent are sutured to the craton as volcanic arcs
collide during subduction.
mountain building thickens the crust and hardens unconsolidated
materials. Regional and contact metamorphism results forming more
greenstone belts with granitic and gneissic intrusions.
2.0 and 1.85 bya, at least six small continents were accreted to the
proto-North American continent and parts of Scandinavia and Baltica.
This large continent was called Laurentia. It is believed that a
number of island arc terrains were suture together to form this large
the Laurentia by Continental Accretion
Tectonics 2.0 – 1.85 bya
Rift of N. America
Mid-Continent Rift began to form between 1.2 and 1 bya.
rocks formed during this rift event include basalts, redbed
(mudstones) and alluvial fan conglomerates & sandstones.
rifting stopped sometime late in this time period therefore, it is
referred to as a "failed rift zone"
Grenville Orogeny was the last major accretion stage during the
Proterozoic Rocks of the Grenville underlie much of the eastern US
(including Ohio). They include sandstones, shales and carbonate rocks
like those that form along a passive continental margin.
and breakup of Supercontinents
bya and 550 mya there were several events of suturing and
fragmentation of the continents.
Rodinia was formed as a result of Grenville orogeny and later
attachment of Laurentia, southern Africa, Australia and Antarctica.
events between 2.5 and 1.0 bya
Proterozoic glaciation spreads over Canadian Shield about 2 bya and is
represented by fine laminations or varved mudstone (Gowganda
called tillite (lithified glacial sediments or till). Similar evidence
of glaciation is found in Finland, southern Africa, and India.
Proterozoic life is represented by stromatolites (appearing about 2.5
by ago, gaining maximum diversity about 1.2 by ago). These deposits
form when algal materials accumulate on the sea floor trapping
calcite-rich sediments between the layers. Stromatolites are forming
today off the western coast of Australia.
atmospheric oxygen started to rise about 3.5 bya as photosynthetic
prokaryotes began to release free oxygen.
concentration in the ocean increased, allowing more banded iron
formations (BIF's) to form.
ago the oceans began dissolving more atmospheric oxygen, oxygenating
the deep ocean waters which ceased the formation of BIF's.
By the late Proterozoic, highly oxidized soils also form indicating an
Life in the
end of the proterozoic more modern forms of life appear.
species (single-celled algae that later might have evolved into
dinoflagellates). Stromatolites declined in abundance and diversity.
Multicellular organism with soft body parts evolve.
a site in the Ediacara Hills, Australia provide evidence for a massive
radiation of species occur at the end of the Proterozoic.
Fauna and Evolution of animals
three major types of the fossils that bring our understanding of that
era: trace fossils (there were not trace fossils in earlier times, the
oldest trace fossils represent tubes made worm like species), soft
body fossils (Cnidaran floating jellyfish and attached to the seafloor
sea pens, preserved in sandy well-oxidized sea floor), and skeletal
fossils (tube like microscopic Cloudina). The reason for the life
evolution of this time is not quit clear.