The Late Paleozoic Era

Geology 103

Sea level changes

• Kaskasia transgression starts in Devonian, continues to the end of the Mississippian

• Absaroka transgression begins in Pennsylvanian, continues to Triassic

Late Paleozoic plate configuration• Basically, Pangea is assembling• Gondwana still exists (over South Pole for most of

this) and Laurentia is equatorial

End Paleozoic plate configuration• At end of Paleozoic, Pangea is complete• Tethys ocean begins opening in the east

Acadian orogeny continues through Devonian

• To the north, Laurentia + Baltica = Laurasia

• To the south, an island continent called Avalonia accretes onto southern Laurentia

Catskill clastic wedge and some crystalline rocks are all that remain

• When the 4000+ meter mountains are eroded away, all that remains are:

• their crystalline roots, both metamorphic and intrusive igneous• Their erosion products in the clastic wedge

Roots of mountain ranges

Alleghenian/Ouchitan orogeny – Miss. - Permian• Alleghenian orogeny is a result of a collision between parts

of northern Laurasia and Gondwana, which spread southward to present-day Oklahoma, Texas, Louisiana, Venezuela (Ouchitan orogeny)

Ancestral Rockies orogeny

• Around the same time (Miss. – Permian), compressive forces in the middle of Laurasia created a series of ranges and basins (the ancestral Rockies)

• Many basins become oil-producing regions (Big Horn Basin), not just in North America (Perm Basin, Russia)

Western North America has been quiet since the Antler orogeny, but in Permian,

Sonoma orogeny begins

Late Paleozoic life diversifies but is marked by two extinctions

• End of Mississippian

• End of Permian (greatest of all extinctions) – 90% of marine species and 70% of land species go extinct

Mississippian in Laurentia was characterized by extensive limestones

Crinoids (modern sea lilies), blastoids, bryozoans and fusilinid forams

Reconstruction of Mississippian sea floor

Tetrapod evolution• Some fish had developed lungs• Changes to fin structure led to feet – land animal• Transitional form – Tiktaalik (375 my)

Amniote egg evolution• Amnion =

“membrane around fetus”

• Group within tetrapods that produce eggs that are surrounded by membrane(s)

• First of this type around 340 my

• Synapsids (which lead to mammals) and sauropsids are part of these

Tetrapod cladogram

Pennsylvanian in Laurentia was a time of extensive coal deposition

• To make so much coal, lots of carbon dioxide was “scrubbed” from the air

• Atmospheric CO2 drops to a tenth of previous (about modern day values)

Massive CO2 drop may have removed some greenhouse warming, resulting in

mid-Carboniferous ice age

Deposition of cyclothems

• Cyclic sedimentary “package” of rocks, ranging from coal to limestone

• Each cycle represents about 400,000 yr

• The standard interpretation is that there are small transgressions and regressions at coastal regions where there are swamps

Permian in Laurentia was a time of extensive deserts

• Pangean continental interiors were dry, so extensive deserts and dunefields formed

• Result are arenites with huge (meter-scale) cross-beds

Therapsids, ancestors of mammals

• Division of synapsids• Still are reptiles but

have some mammalian characteristics like hair, lactation and erect posture

• “apsid” = arch, typically over a hole in the skull (“fenstra”)

Almost for naught – Permian extinction which occurs in less than 1 million years

Causes of P/T extinction – climate change• Supercontinent interior generates extremes in

temperature – 50°C average temperature in the interior• Habitable areas reduced

Causes of P/T extinction – climate change

• At Hallett Cove, South Australia, among other sites, extensive glacial striations exist in exposed bedrock of end Permian times

• Worldwide glaciation was already occuring regularly during Permian

• Regressions caused by glaciations remove habit on continental shelf

Causes of P/T extinction – massive vulcanism

• Siberian traps are a region of huge outpourings of basaltic lava around the end of the Permian

• Increase CO2 in atmosphere, as well as dust

How does a massive volcanic eruption cause enough climate change for a mass extinction?

Initial temperature rise due to eruption CO2 increase warms oceans enough to exsolve trapped methane from ocean floor, which increases the warming

Causes of P/T extinction - impacts• Potential impact sites of the right timing in Australia and

Antarctica• Identified through presence of shocked quartz and stishovite, a

high pressure form of quartz