Earth History
GEOL 2110
The Paleozoic Era
Late Ordovician Period
Limestone, Limestone, and more Limestone
and The Emergence of the Appalachians
Major Concepts
• The great Sauk sea retreated to the edge of the NA craton about
480 Ma, when it transgressed the craton again (the Tippecanoe
Transgression) over the next 50 million years, a whole new
diversity of marine live came with it.
• While Cambrian sands were initially spread over the craton as the
Tippecanoe beach trangressed over the land, creating another
ultrapure quartz sandstone formation – the St. Peter, this was
quickly followed extensive deposition of fossil-rich limestone.
• In the late Ordovician, the eastern margin of Laurentia transformed
from a passive continental slope margin to a tectonically active
volcanic arc – the Taconic.
• Immature sediments shed westward from this arc accumulated
between the continent and the volcanic arc and eventually became
deformed and metamorphosed when the arc eventually collided
and was thrust onto the continental edge –Penokean-like.
The Tippecanoe Transgression
Mid- to Late Ordovician
The Ordovician “Explosion”
Changes in Ordovician
from Cambrian Fauna
• more complex food
chain
• extend higher above
the seabed
• hard part made of
calcite rather than
phosphate
Ecological Diversification of Ordovician Fauna
Increasingly Complex Food Web
Multi-tiered Feeding Levels
Arrival of Classic “Paleozoic” Fauna
Articulated
Brachiopods
Sea Shells
Bryzoans
Twigs
Crinoids
“Sea Lillies”
Cheerios
…and the world’s best Index fossils!
Conodonts
Graptolites
Ostracodes
Carniverous Nautiloid
Jawless Bony Fish
New Kids on the Block
Nautiloid fragment found last year on
field trip
Why This Explosive Radiation?
An Early Ordovician
regression followed by
the most extensive
transgression up to that
point in Earth history
Enormous areas of
shallow marine
environments - niches
Atmospheric oxygen
reaches modern-day
levels
The Tippecanoe Trangression
St Peter SS – Platteville LS, St. Paul
Capping the Craton in Limestone
Except
Here
The Taconic Orogeny
The Taconic Orogeny
The Taconic Orogeny
ModernDay
Analog
Prior-Day Analog
1.85 Ga Penokean Orogeny
Next Lecture
The Paleozoic Era
Cambrian and Ordovician History
of Minnesota and Wisconsin
Have a Great Spring Break!!!