Ordovician Period Notes

ORDOVICIAN

History of Earth Through Deep Time

  • The Ordovician period is a significant part of Earth's history, falling within the early Paleozoic era.
  • Key events during this time include:
    • Glaciation
    • Trilobite mass extinctions
    • Sea level fluctuations
    • The rise and fall of trilobites
    • The appearance of land plants
    • General marine adaptive radiation
    • Radiation and mass extinction of nautiloids

Ordovician Time Scale

  • The Ordovician period spanned from approximately 485 to 444 million years ago (MA).
  • It is divided into several stages or ages:
    • Hirnantian: 443.8 to 445.2 ± 1.4 million years ago.
    • Katian: 453.0 ± 0.7 million years ago.
    • Sandbian: 458.4 ± 0.9 million years ago.
    • Darriwilian: 467.3 ± 1.1 million years ago.
    • Dapingian: 470.0 ± 1.4 million years ago.
    • Floian: 477.7 ± 1.4 million years ago.
    • Tremadocian: 485.4 ± 1.9 million years ago.

Ordovician Paleogeography

  • Laurentia rotated and moved into temperate waters in the southern hemisphere.
  • Gondwana was located over the South Pole.
  • Significant tectonic activity occurred, including the Taconic Orogeny and the beginning of the closure of the Iapetus Ocean.
  • Early Ordovician (c. 480 Ma):
    • Laurentia was positioned near Scotland and NW Ireland.
    • Baltica was located near England, Wales and SE Ireland.
    • Gondwana was situated at the South Pole.
  • Late Ordovician (c. 450 Ma):
    • The Rheic Ocean began to form.

Taconic Orogeny

  • The Taconic Orogeny involved the collision of island arcs and terranes with Laurentia.
  • This orogeny led to the deposition of nearshore sandstones and other clastic sediments.
  • Widespread fossiliferous carbonates, including reefs, were formed.
  • Evaporites were deposited in the Michigan Basin during the Tippecanoe regression in the Silurian period.
  • Shallow water carbonates were formed along the margin of ancestral North America (Laurentia).
  • The process included:
    • Subduction zone activity
    • Formation of a Taconian volcanic island arc
    • Crustal flexure
    • Development of an accretionary wedge
    • Turbidity currents

Sea Level Changes

  • The Sauk-Tippecanoe sequence boundary represents a period of low sea level and erosion.
  • The Tippecanoe transgression began in the Middle Ordovician and lasted into the Early Devonian.
  • High sea levels promoted the formation of shallow marine reefs.

Climate Change

  • The Early Ordovician had high sea surface temperatures.
  • Temperatures began to fall in the Middle Ordovician.
  • Glaciation occurred in the Middle-Late Ordovician, with a south pole ice cap on Gondwana.
  • Landmasses over the poles are necessary for glaciation to begin and grow.

Great Ordovician Biodiversification Event (GOBE)

  • The GOBE, also known as the Ordovician Radiation, saw a rapid increase in biodiversity.
  • Phyla that appeared during the Cambrian Explosion diversified at lower taxonomic levels.
  • There were significant increases in plankton, flat bottom communities (dominated by brachiopods), and reefs.
  • This event led to the rise of filter-feeding lifestyles and swimming predators (nautiloid cephalopods).

Late Ordovician Mass Extinction

  • This was the second-most severe of the Big 5 mass extinction events.
  • Approximately 75% of marine species went extinct.
  • The extinction occurred in two phases: the first linked to cooling (glaciation) and the second linked to warming (deglaciation).
  • Groups strongly affected included trilobites, conodonts, graptolites, bryozoans, and brachiopods.
  • Causes are disputed but typically linked to short-lived and intense glaciation and anoxia.
  • Volcanic eruptions may have played a role in causing both glaciation (related to weathering of Taconic mountains) and anoxia.
  • Extinction 1 shows the melting glaciation
  • Extinction 2 shows the ice sheet collapse
  • Sea Surface Temperatures went from 40°C to 28°C
  • The carbon isotope curve changed from -2δ¹³C(‰ VPDB) to 6δ¹³C(‰ VPDB)