Download Proterozoic History

Earth History
3. The Proterozoic
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What to Look For:
• Proterozoic plate tectonics involved true continental drift. By the end
of the Archaean there were large enough cratons to behave like fullsized continents, by the end of the Proterozoic the modern cratons
were recognizable and incorporated all the modern types of plate
margins and stable interiors in their structure.
• Oxygen production and build-up, which had begun in the late
Archaean was quickly finalized in the early Proterozoic. Ever since
there has been free O2 in the atmosphere.
• Sedimentary rocks were consequently characterized by minerals
with completely oxidized metals. Hematite is particularly
conspicuous in redbeds and ironstones.
• This abundance of free oxygen also allowed blossoming of aerobic
life: more stromatolites, aerobic bacteria, and finally eukaryotic
protists. By the latest interval of the Eon (the Ediacaran) there were
multicellular organisms whose relationship to later life is uncertain.
Proterozoic History
1 – Plate Tectonics
By the end of Archaean time there were large areas of aggregated island arcs
that began to behave as continental crust rather than island arcs. You are
familiar with this type of tectonics from earlier in the course, and we will elaborate
on how it actually progressed when we get to the Phanerozoic.
The Proterozoic Eon saw continued aggregation of these cratons into even
bigger landmasses. For example, early in the Eon the various provinces of the
Canadian Shield collided and built mountain chains between them (next slide).
They have remained as one cratonic unit ever since.
The green bands
represent
metamorphic rocks
dated at 1.8 to 2.0
by – in the middle
Early Proterozoic.
The Late Archaean
provinces are also
labeled. The green
rocks were
metamorphosed as
the small cratons
accreted during this
interval, continuing
the growth of the
craton seen in the
Archaean.
Rae
Province
Greenland
Province
Slave
Province
Hearne
Province
Superior
Province
Wyoming
Province
Of course, another way to
think of this is that by a
little less than 2 by ago
North America looked
roughly like the red area
on this map.
This area covers all the
rocks in the continent 1.8
by in age or older and the
youngest of these are
sediments and/or
volcanics deformed as the
older pieces accreted at
just prior to about 1.8 by
or so.
With the passage of more
time these continental
cores grew even larger,
by both arc volcanism on
their edges and by
accretion of offshore arcs
and small continents.
Between about 1.8 and
1.75 by, accretionary
events added the
material shown in green
on this map.
Then between about 1.75
and 1.6 by the region in
blue was similarly added.
This is the rough outline
of our continent at the
end of the Middle
Proterozoic.
Precambrian continental accretion
culminated with a series of
collisions among essentially all the
continents in the neighborhood of
a billion years ago – Late
Proterozoic but pre-Ediacaran.
The region in yellow on this map
was the consequence of that
collision in North America – the
Grenville Mountains. The
Grenvilles were a Himalayan-type
chain of great size. The
sediments that came off them as
they eroded away were later
incorporated into younger
mountains, in part. The rocks of
the Cohutta Mountains in GA, the
Great Smokeys in TN/NC, and the
other high chains of the western
Blue Ridge are mainly built of
these rocks, metamorphosed.
This was our continent about 900
my ago. Georgia was mostly not
there yet.
It would form, in part, during a
series of additional volcanic arcs
and a collision in the Paleozoic,
and from a piece of Africa left
behind when the Atlantic Ocean
rifted.
It is still forming as its rivers bring
sediment out of the stubs of the
Appalachians and deposit it along
the coast and the continental
shelf edge.
X
You are
here.
Within the landmass that resulted from the mass pile-up, the modern cratons were
recognizable, though they were aggregated into that one large landmass. We call
that “supercontinent” Rhodinia.
Look over the map on the next page and pick out where they are. Note that some
continents have more than one craton (or shield) and so are not always
recognizable as their modern shapes.
Ediacaran paleogeography (modified from Scotese)
sChn
nChn
Aust
Arab
Panthalassia
Ind
Ant
Rhodinia
nAf
sAf
N Asia
NAm
wAf
Grn
nSAm
Eur
Grenville Mts
By the very end of the Ediacaran, Rhodinia had rifted to produce the Cambrian continents
whose subsequent history we will follow more closely through the Phanerozoic Eon.
Proterozoic History
2 – Rock Types and their
environmental implications
The BIFs that had begun appearing late in the Archaean became more and more abundant
through the first half-billion years of the Proterozoic. Most are between 2500 and 2000 my
(2.5 and 2 by) in age.
After that, sediments are dominated by fully oxidized minerals such a hematite. In other
words, redbeds and non-BIF ironstones became prevalent. In other words, besides the BIFs,
Proterozoic rocks look almost exactly like Cenozoic rocks. All that is missing is the great
diversity of fossil organisms.
Proterozoic History
3 – Proterozoic Life
These are Proterozoic stromatolites, and they are uncharacteristically small. They are usually
much larger, frequently quite a bit taller than you are.
1 cm
The increasing abundance of BIFs suggests continued accumulation of free oxygen in the
atmosphere. This was accompanied, beginning about 2.3 by ago, by a blossoming of larger
and more complex stromatolites of a sort presently constructed by aerobic Cyanobacteria.
The oldest known eukaryotic cells (those with nuclei) are about 2.1 by old. These cells are
preserved with the nucleus clearly visible, often caught in the act of dividing in preparation
for cell division. Because all eukaryotic cells are obligatory aerobes (they must have free
O2) their existence necessarily implies that this gas was continuously abundant in the
atmosphere by that time.
At about 1.4 by ago there is a noticeable increase in size in these protists, to the size of
modern algae and protozoans. There is still no sign of plants or animals, not even trace
fossils.
Two interesting types of fossils (animals?) appear in the Ediacaran rocks of most continents.
Body fossils take the form of carbon films of rather large organisms. Their shapes are odd,
in comparison to any living organisms we know, and even in comparison to any later fossils
we know. They have been interpreted as belonging to nearly all of the kingdoms (except the
bacterial ones), or even to their own unique one.
The Ediacaran is also where the first trace fossils are found. There are no burrows to any
great depth, but things were clearly moving around on and in the uppermost parts of
sedimentary beds. Thus both body and trace fossils indicate the beginnings of multicellular
life in the latest Proterozoic.
The diagram on the next page summarizes in detail the major events in Phanerozoic history
along with radiometrically derived ages of those events. You do not need to know this level of
detail, but if I ask about the Precambrian time scale you should be able to give a rough outline
of the Eons, the main characteristics, and the boundary ages. This summary will help:
~542 my ----- End of Proterozoic and Ediacaran/Beginning of Phanerozoic and Cambrian
Ediacaran – multicellular organisms evident. Rhodinia.
~900my – 1 by
Proterozoic Eon – Assembly of modern cratons and, finally Rhodinia.
Abundant free oxygen in the atmosphere, based on:
Abundance of aerobic organisms, including Protista,
and Abundance of redbeds and non-BIF ironstones.
~2.5 by ----- End of Archaean and Beginning of Proterozoic.
Archaean Eon – Tectonic system operating with mostly oceanic crust and island arcs.
Limited accretion of these arcs into larger cratonic pieces.
O2-free atmosphere based on abundance of reduced metal minerals and lack of
apparent aerobic fossils.
~3.9 by (but subject to “oldward” revision) – Beginning of rock record and Archaean Eon.
No rock record.
~4.6 by – Existence of differentiated Earth.
Take-Home Message:
• Proterozoic plate tectonics involved true continental drift. By the end
of the Archaean there were large enough cratons to behave like fullsized continents, by the end of the Proterozoic the modern cratons
were recognizable and incorporated all the modern types of plate
margins and stable interiors in their structure.
• Oxygen production and build-up, which had begun in the late
Archaean was quickly finalized in the early Proterozoic. Ever since
there has been free O2 in the atmosphere.
• Sedimentary rocks were consequently characterized by minerals
with completely oxidized metals. Hematite is particularly
conspicuous in redbeds and ironstones.
• This abundance of free oxygen also allowed blossoming of aerobic
life: more stromatolites, aerobic bacteria, and finally eukaryotic
protists. By the latest interval of the Eon (the Ediacaran) there were
multicellular organisms whose relationship to later life is uncertain.