Ordovician

What is the time scale for ordovician?

485 to 443 mya

Ordovician Life

Ordovician Life
• Many hold-overs from Cambrian time
• Middle-Ordovician saw a great radiation
• Many newly evolved animals were successful for rest of Paleozoic

Early Ordovician Life Swimmers and floaters:

Early Ordovician Life Swimmers and floaters:
– Graptolites

–floated (planktonic)
– Offshore, settle to muddy bottom, useful for dating black shales

– Nautiloids (swim)
– predators; arose in Cambrian,
diversified in Ordovician

Mid-Ordovician Life

Mid-Ordovician Life
• Great radiation; 3-fold increase in
number of marine animal families
• Most dramatic increase in marine
animals, ever

• Life in sediment
– Burrowers expanded
• Pump oxygen-bearing water into
sediment
• Diversification of worms and other soft-burrowers

• Sediments indicate burrowers flourished
-Also an increase in burrowers w/
skeletons

– bivalve mollusks, brachiopods, new trilobites dug shallow burrows, too

Life on the seafloor

• Life on the seafloor
– Diversity of benthic organisms increased
– Jawless fishes
– Grazing snails
– Articulate brachiopods
– Crinoids expanded

Coral-strome reefs

• Coral-strome reefs
– Rugose corals
– Tabulate corals
– Stromatoporoids
(sponges)

No major reef-builders since archeocyathids, which disappeared in the early Cambrian; these were bigger, and thrived thru the Paleozoic

Stromatolites

Stromatolites – rare by end of Ordovician
– Less abundant; more restricted, sub-tidal region
– Weak grazing pressure in intertidal zone

Microbialites

Microbialites: cyanos not necessary
– Thin organic layer in sedimentary rock, trapped sediment in intertidal zone, could tolerate changes in temp, salinity

Thrombolites

Thrombolites
– Thin organic layer in sedimentary rock, trapped sediment, lots of holes from predators, little layering because of burrowers

Phanerozoic Eon increases
Ordovician Life significantly

Phanerozoic Eon increases
Ordovician Life significantly

• Number of genera of marine animals with skeletons that have existed at various times during the
Phanerozoic Eon increases significantly

• Fewer major extinctions through time thought to result from organisms poorly adapted to rapidly changing environment already went extinct.

Plants?

Plants may have invaded land
– Inconclusive evidence
– Probably restricted to moist
habitats like rivers, coasts

Paleogeography

Early Ordovician

Baltica began move from South Pole

End of Ordovician Paleogeography

End of Ordovician
Baltica moved to tropics

• Gondwanaland nearing south pole
– Glaciers expanded to 30  ̊N and S Continental Drift

Glaciers

Glaciers
– Orientation of glacial markings on all continents suggests they were linked
– Du Toit hypothesized that Gondwanaland was a distinct supercontinent from Laurasia, which contained northern continents

– Isotopes suggest sea-level fell significantly
– Orientation of glacial markings on all continents suggests they were linked
– Du Toit hypothesized that Gondwanaland was a distinct supercontinent from Laurasia, which contained northern continents

– Isotopes suggest sea-level fell significantly

Glaciation and Mass Extinction

Glaciation and Mass Extinction
-Ordovician glaciation more extensive than Pleistocene (20 kya), but only lasted ~500 kyrs
-Marine genera died out both at initiation and termination of glaciation

Marine genera died out at initiation, including many:
-brachiopods
-bryozoans
-corals
-acritarchs
-graptolites
-conodonts
-nautiloids

Ordovician Mass extinction first pulse

First pulse preferentially killed tropical taxa, like coral-strom reef communities due to dropping temps
-also killed species restricted to a single, epicontinental sea

Ordovician Mass extinction second pulse

Second pulse killed cool-water species that had
spread towards equator
-Killed half of marine genera; allowed microbial mats and stromatolites to briefly flourish again

Why massive glaciation?

Why massive glaciation?
• Gondwanaland nearing south pole
• Some unknown greenhouse gas forcing..?

Ordovician Events:

Ordovician Events:

 Laurentia
• Transgression
– Yields characteristic sedimentary pattern
• Siliciclastic sediments
– Innermost belt
• Carbonate platform
– Seaward of siliciclastic.

Taconic Orogeny
-Ordovician Mountain building: recorded in modern New England

-Records collision of Laurentia with islands
-Early Ordovician carbonate platform east coast of
Laurentia

 

-Mid-Ordovician carbonate deposition stopped; flysch
(deep water) sedimentation dominated tapering towards northwest; sediment from eastern uplift

Laurentia

Laurentia
• Transgression
– Yields characteristic sedimentary pattern
• Siliciclastic sediments
– Innermost belt
• Carbonate platform
– Seaward of siliciclastic

Taconic Orogeny

Taconic Orogeny
-Ordovician Mountain building: recorded in modern New England

-Records collision of Laurentia with islands
-Early Ordovician carbonate platform east coast of
Laurentia

 

Laurentia: Taconic Orogeny

• Flysch overlain by molasse
• Clastic wedge tapering towards northwest; sediment from eastern uplift

-Exotic terrane; igneous island arc accreted

-Carbonate platform wedged into subduction zone

-With continued collision, foreland basin migrated westward
• Sediment records collisions

-Ocean crust, accretionary wedge rode up over carbonate platform
• Depressed continental margin
became foreland basin

-Wedges of flysch (deep
water seds) thrust further west, forming thick pile of deep deposits atop carbonate

-Radiometric ages from igneous rocks indicate orogeny ended near the end of the Ordovician Period

Exotic terrane

-Exotic terrane; igneous island arc accreted

• Fossils of different fauna
but same age; some from Great
Britain
• Required plate tectonics to
explain

Laurentian Margin

Laurentian Margin
• Western margin was stable,
passive margin.
• Transcontinental Arch
• Stable continental shelf
• Steep carbonate platform edge
– Accumulated thick limestone sequences

Burgess Shale

– Unusual fauna
– Collected by Walcott

-Burgess Shale fauna deposited in oxygen-free deep waters; no
scavengers or bacterial degradation; turbidite deposits
• Came from the carbonate platform above

Laurentia

Laurentia
• Transcontinental Arch
• Stable continental shelf
• Steep carbonate platform edge
– Accumulated thick limestone
sequences.