The Early Paleozoic World:

1
The Early Paleozoic World the paleogeography of
the Cambrian Earth
EPSC-233 Earth Life History (Fall 2002)
2
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)
3
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.
4
Possible shape of Rodinia.
5
By the end of the Proterozoic, Rodinia had broken
up and most continents were quite close to the
equator.
6
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.

7
Paleomagnetism
The core of the Earth is largely iron. Its outer
shell (shown in yellow) is liquid. Motions within
this liquid generate the Earths magnetic field.
8
Paleogeographic reconstructions
The Earths magnetic field is almost like that of
giant bar magnet. A compass needle points to
the north pole of that magnetic field.
9

• -Remanent magnetism is the fossil magnetism
  acquired by a rock during its formation, under
  the influence of the Earths magnetic field.
• - igneous rock containing magnetite (Fe3O4)
• sedimentary rock containing magnetite grains
• The crystals settling from the magma or seawater
  will orient themselves relative to the Earths
  magnetic field, like small compass needles.

(such as the banded iron formations)
10
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.
11
The inclination of the magnetic field, relative
to the Earths surface, is high in a rock formed
near the pole, and it decreases for rocks closer
to the equator.
12
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).
13
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 Earths surface,
the rock formed near the tropics. Specimens with
a field at higher inclination must have formed at
higher latitudes.
14
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.
15
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.
16
Laurentia and Baltica (joined in Rodinia) rifted
along a zone which became a mid-ocean ridge.
17
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.
18
Mid-ocean ridges are regions where new crust
forms continuously from magma rising from the
upper mantle.
19
Cambrian concentric belts of sandstones,
limestones and shales accumulated all around the
margins of Laurentia.
20
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.
21
How an unconformity may form...
1) sediments laid flat 1 billion years ago
2) sedimentary rocks tilted (and faulted)
3) erosion
4) Cambrian sediments
22
In Ontario and Quebec, the Precambrian shield is
exposed on much of the territory because later
rocks were scraped away by recent glaciers.
23
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.
24
Most Cambrian sandstones are poor in fossils. -
the nearshore environment is a high-energy 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.
25

• trace fossil Skolithos, a simple vertical
  dwelling burrow dug by a filter-feeding 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.
26
Climactichnites, an enigmatic Cambrian crawler
immortalized on the Potsdam sandstone.
Slab collected by William Logan (the discoverer
of Eozoon canadense).
27
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.