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The Early 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 ... – PowerPoint PPT presentation

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Title: The Early Paleozoic Era

The Early Paleozoic Era
  • Geology 103

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

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

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
  • 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 Goulds 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 patchreefs (not
    like todays 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

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 basins 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 waters 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, its
    responsible for much of the northern Appalachian
  • 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 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
  • No flowers or seeds yet

West coast of North America finally stops being
passive Antler orogeny
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