The Early Paleozoic Era

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The Early Paleozoic Era

• Geology 103

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Precambrian Washingtondoesnt exist

• In fact, youd 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

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Whats 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

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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
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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)

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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

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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

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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 Goulds Wonderful Life accounts for
  events surrounding the Burgess Shale, the most
  famous outcrop of the Cambrian explosion.

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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

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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

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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

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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 patchreefs (not
  like todays scleractinian corals that make
  linear reefs)

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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

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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

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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

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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

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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 basins seawater evaporated

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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

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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

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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

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Silurian land plants

• Transition between non-vascular and vascular
  plants, could be a bit further from waters edge
• Cooksonia is the name of this extinct group of
  plants fossils found globally
• Used spores for reproduction

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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, its
  responsible for much of the northern Appalachian
  uplift
• In Britain, called the Caledonian orogeny
  (Scottish highlands)

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The Devonian period starts the Kaskasia
transgressive sequence
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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
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First forests

• Restricted to the waters 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

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West coast of North America finally stops being
passive Antler orogeny