Transcript No Slide Title

The Early Paleozoic World:
the paleogeography of
the Cambrian Earth
EPSC-233 Earth & Life History (Fall 2002)
Recommended reading:
STANLEY “Earth System History”
Chapter 13, pp. 355-357.
See also pp. 285-286 about
paleomagnetism.
Keywords: correlation, paleomagnetism
(remanent magnetism), inclination of
magnetic field, unconformity, sandstone,
trace fossils (burrows and tracks)
Towards the end of the
Precambrian, most supercontinents
were probably clustered in a single
supercontinent.
This gathering of most continents
in a single huge supercontinent may
have happened more than once
during the Proterozoic.
Possible
shape of
Rodinia.
By the end of the Proterozoic, Rodinia had
broken up and most continents were quite
close to the equator.
What lines of evidence can we use to
reconstruct the position of continents in
the past?
-the position of sedimentary belts that
formed along the edges of continents
when they broke up again
- the dates of igneous intrusions that are
part of mountain chains along ancient
continental margins
- the “fossil” (or remanent) magnetism of
some Fe-rich Precambrian rocks.
Paleomagnetism
The core of the Earth
is largely iron. Its
outer shell (shown in
yellow) is liquid.
Motions within this
liquid generate the
Earth’s magnetic field.
Paleogeographic reconstructions
The Earth’s magnetic
field is almost like
that of giant bar
magnet.
A compass needle
points to the north
pole of that magnetic
field.
-Remanent magnetism is the “fossil magnetism”
acquired by a rock during its formation, under the
influence of the Earth’s magnetic field.
- igneous rock containing magnetite (Fe3O4)
- sedimentary rock containing magnetite grains
(such as the banded iron formations)
The crystals settling from the magma or seawater
will orient themselves relative to the Earth’s
magnetic field, like small compass needles.
If it has not been altered, this ancient
magnetism (called remanent magnetism)
provides 2 pieces of information.
1) polarity of the magnetic field: was the
magnetic north pole pointing towards the
geographic North pole, or to the geographic
South pole? (... yes, the magnetic poles
“switch” from time to time!)
2) inclination of the magnetic field, which
varies with latitude.
The inclination
of the
magnetic field,
relative to the
Earth’s
surface, is high
in a rock
formed near
the pole, and it
decreases for
rocks closer to
the equator.
This inclination (which depends on the latitude at
which the rock formed) is not the same as the
declination (slight deviation of magnetic North pole
from geographic North pole).
This magnetism can be preserved for hundreds of
millions of years, as long as the rock is not strongly
reheated again. This “fossilized magnetism” is
called remanent magnetism or paleomagnetism.
The temperature at which the magnetism of a
magnetite-rich rock could be modified again is
called the Curie point (about 550 º C for
magnetite).
Reconstructions of the movements of tectonic
plates take into account this paleomagnetic
information.
Rocks are taken back to the lab, after their
exact orientation in the field is recorded, and
their paleomagnetism is measured in laboratory.
If the paleomagnetism shows a very low
inclination relative to the Earth’s surface, the
rock formed near the tropics. Specimens with
a field at higher inclination must have formed
at higher latitudes.
The break-up of Rodinia led to the
repositioning of large continental
surfaces at low latitudes (tropical and
subtropical zones).
In most areas, this Precambrian-Cambrian
interval left a gap in the sedimentary
(and fossil) record. Such gaps (periods of
erosion and non-deposition) are called
UNCONFORMITIES.
This indicates that the surface of most
cratons stood largely ABOVE sea level.
Drift of continents
during the Cambrian:
600, 540 and 525
million years ago.
A “proto-Atlantic”
called Iapetus is
created along east
coast of N. America.
Laurentia and Baltica (joined in Rodinia) rifted
along a zone which became a mid-ocean ridge.
This is how all oceans are born... (They are
not “dug up” by rivers.... In fact rivers
brings sediment and salt to the oceans.)
New oceans form when crust rifts apart,
and some of the faulted blocks sink down
(because it is cooler and denser than the
hot mafic magma underneath).
Sinking large blocks of crust creates a very
large “hole” that water gradually fills in.
As the opening widens, it connects with
other oceans and becomes full of seawater.
Mid-ocean ridges are regions where new
crust forms continuously from magma rising
from the upper mantle.
Cambrian: concentric belts of sandstones,
limestones and shales accumulated all around
the margins of Laurentia.
Grand Canyon:
Angular unconformity between 1 billion
year old Dox Proterozoic sandstone and
the Cambrian Tapeats sandstone.
Layers are at
different angles in
Tapeats and in Dox.
How an unconformity may form...
1) sediments laid flat
1 billion years ago
3) erosion
2) sedimentary rocks
tilted (and faulted)
4) Cambrian sediments
In Ontario and Quebec, the Precambrian
shield is exposed on much of the territory
because later rocks were scraped away by
“recent” glaciers.
In southern Ontario and Quebec, there
are a few places where we can see the
Precambrian-Cambrian contact.
The first sedimentary rock is a
sandstone, derived by weathering of
continental rocks.
Most Cambrian sandstones are poor in
fossils.
- the nearshore environment is a highenergy zone where the remains of
dead animals are rarely preserved
intact.
- a sandstone can remain quite porous
during sedimentary burial. The decay
of organic matter tends to make pore
waters acidic, and shells tend to
dissolve.
-trace fossil Skolithos, a
simple vertical dwelling
burrow dug by a filterfeeding animal
- a few simple tracks are the
most common Cambrian
fossils preserved in the
Potsdam sandstone (Ontario,
Quebec, NE United States).
The Potsdam sandstone itself is widely quarried
and used as flagstone for paths and facing stones
for buildings.
Climactichnites, an enigmatic Cambrian
crawler immortalized on the Potsdam
sandstone.
Slab
collected by
William Logan
(the
discoverer of
“Eozoon
canadense”).
Near Kingston, the Potsdam
is eolian (sand wind-blown
into dunes that were
preserved).
This year,
footprints were
discovered in
some eolian
Potsdam...
Something crawled
out of the water
about 500 Ma ago.