Transcript The Paleozoic Era

The Early Paleozoic Era
Geology 103
Precambrian Washington…doesn’t exist
• In fact, you’d have to go to
the Idaho-Montana border
to see rocks of Precambrian
age
• Typical rocks are the Belt
Supergroup, which consist
of rocks of a clastic wedge
developing off the edge of
the then-North American
craton
What’s a clastic wedge?
• A clastic wedge comprises the rocks that represent the
deltaic/fluvial deposits made by large stream systems
• Sand, silt and clay
• Tells you which direction the uplands were
• Belt supergroup represents a clastic wedge 1.45 by old
Precambrian life (besides Ediacaran fauna)
is mostly stromatolites
Basically, they are algal mats of various bacteria that produce a
biofilm that binds sedimentary grains that help preserve the mat;
later the structure is mineralized, so there is no organic stuff
At the start of the Paleozoic…
• The supercontinent
Pannotia has broken up,
and the Earth is leaving
extreme ice (“Snowball
Earth”) conditions
• Most reconstructions show
two continents: Laurentia
(modern North America and
Europe) and Gondwana
(everything else)
The continued rifting of continents
increases continental shelf area
• Continental shelf seas are
shallow, so plenty of light
for photosynthetic
organisms and all that feed
on them and each other
• Due to the Sauk
transgression, the ocean
invaded low-lying
continental areas: “epeiric
seas”
The base of the Cambrian
• What defines the start of
the Cambrian period?
• Used to be where trilobites
were found, but “small
shelly fauna” (SSF) was
found under the trilobites
• Many modern phyla are
represented
• Defined now as the first
appearance of Trichophycus
(formerly Phycodes) pedum
• Ediacaran fauna is gone
Cambrian “explosion”
For reasons unclear, 13 million
years after the start of the
Cambrian (so 530 my), there
was a tremendous
diversification of marine life
forms, some of which have
never been reproduced.
Stephen Jay Gould’s Wonderful
Life accounts for events
surrounding the Burgess Shale,
the most famous outcrop of
the Cambrian explosion.
The Burgess shale Lagerstatten
• Burgess shale quarry was discovered by Alexander Wolcott in
the early 1900s
• The diversity of Cambrian fossils there is due to a Lagerstatten
– an exceptionally well-preserved fossil locality
• Depositional environment: bottom of an algal reef
Other Cambrian life
• Trilobites – type of
arthropod (“jointed feet”),
major predator, swimmer
(nekton)
• Archaeocyathids – related
to sponges
• Inarticulate brachipods
(“lamp shells”)
• End of the Cambrian mass
extinction got rid of many
trilobites and all
archeocyaths; cause may be
the end of the Sauk
transgression
Tippecanoe transgressive sequence
• Ordovician period begins
with this sea level rise
• In North America, first
major Paleozoic orogeny on
eastern margin – the
Taconic orogeny, which was
the result of the Iapetus
(proto-Atlantic) Ocean
closing
• Queenston clastic wedge
lays down sandstones in
Ohio, evidence for 4000
meter mountains along
North American east coast
Reef-builders begin, a new ecosystem
• In the Cambrian (and
before), algae and
archeocyathids made large
carbonate structures, not
particularly extensive
• In the Ordovician, tabulate
and rugose corals
developed and made
extensive “patch”reefs (not
like today’s scleractinian
corals that make linear
reefs)
Ordovician life
• Graptolites (Graptolithnia)
are creatures that made
their first appearance in the
Cambrian but are
considered index fossils of
the Ordovician
• Look like “rock writing” ,
hence their name
• Are hemichordates!
• Go extinct during
Carboniferous
First land “plants”
• Some recent research
(2001) has given some
evidence that there were
some land-based lichen or
fungi as early as 1.1 by
• More conventionally, there
is good evidence for
liverwort-like plants during
the Ordovician - no vascular
tissue, so short, near water
• The presence of plants will
alter not only atmosphere
chemistry, but also the rate
of weathering
End of Ordovician extinction
• Second largest mass extinction (except for the end of
Permian)
• 450 to 440 my
• Cause: massive ice age as Gondwana moves over South Pole
• Sea level falls as glaciers grow, and expose much shelf area
Mid-Paleozoic climate
• Except for the 30 my long
ice age at the end of the
Ordovician, both the
Ordovician and Silurian
were greenhouse Earth
times – sea level was 200 m
higher than today
Silurian reefs and basins
• In what is now
Michigan, coral reefs
developed toward the
north and south (and
west, though these are
gone), trapping sea
water in between
• Subsidence dropped
the level of this basin,
allowing deposition of
various salts as the
restricted basin’s
seawater evaporated
What did these reefs look like?
• Similar to modern structures, since reefs exist in clear
continental shelf water, at the edge of much deeper water
• Can restrict the flow of ocean currents, strong enough to
withstand even storm-driven waves
Evaporite basin
• Repeated inundation by
seawater and plenty of time
resulted in 600 m of halite
deposits in the center of the
Michigan basin
• Maximum deposition during
the late Silurian
Eurypterids are the major marine predator
• Mistakenly called “sea
scorpions”, they are not
true scorpions or
entirely marine, but
they are among the
largest arthropods ever
on Earth
Silurian land plants
• Transition between
non-vascular and
vascular plants, could
be a bit further from
water’s edge
• Cooksonia is the name
of this extinct group of
plants; fossils found
globally
• Used spores for
reproduction
Acadian-Caledonian orogeny
• At the end of the Silurian,
the Tippecanoe
transgression comes to an
end due to the accretion of
the Avalonia terrane to
eastern North America
(Laurentia)
• Called the Acadian orogeny
in North America, it’s
responsible for much of the
northern Appalachian uplift
• In Britain, called the
Caledonian orogeny
(Scottish highlands)
The Devonian period starts the Kaskasia
transgressive sequence
Devonian life
• Though jawless fishes
appear in the late Cambrian
and jawed fishes in the
Silurian, fishes (including
sharks and bony fishes)
diversify during the
Devonian
Also, first amphibians and insects
First forests
• Restricted to the water’s
edge, the first true trees
were comparable to
modern trees in height
• Lycopsids and
progymnosperms (both
extinct) dominate, look like
tree ferns, but they are not
ferns (structures are a result
of convergent evolution)
• No flowers or seeds yet
West coast of North America finally stops
being passive – Antler orogeny