We have Gondwana (South America, Africa, Antarctica, southern Europe and Australia) lying at the south pole. The sea level is high, and there is lots of shelf space and shallow seas on continental interiors. As for life, this period began with a radiation, the replacement of the Cambrian fauna with "skeletonized benthic suspension-feeding communities." And this period is also marked by the appearance of starfish.

Here are some maps which illustrate the placement of the continents:

• http://zircon.geology.union.edu/gildner/globe.html
• http://vishnu.glg.nau.edu/rcb/Ord.jpg

And here is a nice overview of what we can find there:

• http://www.uky.edu/KGS/coal/webfossl/pages/ordov.htm

North America (Laurentia)

Laurentia is composed of North America and northwest Scotland. And it lies on the equator, where it stays, although it is rotating counterclockwise. (What we think of as the East coast begins the Ordovician period facing south.) Most of the rocks we have from the Ordovician are oceanic crust, which got caught up in collisional events and now reside in mountain belts like the Appalachians. (Besides, there's nothing really interesting going on on the land; all the animals are aquatic.)

miogeosynclinal deposition environment - Passive Continental Shelf deposits

• Dolomites and limestones
• Fossils - Brachiopods, bryozoans, nautiloids, gastropods, corals, algae (and some of this is calcified algae)

eugeosynclinal depositional environment - Continental Slope and Rise deposits

• Volcanic and volcanic clastic sedimentary rocks, signifying island arcs
• Fossils -Graptolites, plankton

Here are some links to pictures of rocks formed during the Ordovician:

• http://www.albion.edu/fac/geol/ScotPics.htm (near the bottom)
• http://www.emory.edu/GEOSCIENCE/HTML/ringgold.htm

Europe (Baltica)

Baltica is composed of most of northwest Europe (from the Urals to the sea). And it lies between 30-50 degrees in the southern hemisphere at the beginning of the Ordovician, and then moves north, while rotating counterclockwise.

This continental block also displays a passive margin depositional setting.

Carbonate rocks become more abundant towards the middle of the Ordovician, showing that this continental block is moving north.

And between them lies the Iapetus Ocean, which begins to close up in the middle of the Ordovician.

In the beginning of the period it is 5,000 km across, but at the middle of the period it is only 1000 to 1500 km. This is supported by faunal data; the fact that some species of trilobites and brachiopods begin to appear on both sides of the ocean.

The subduction zone that begins all of this ocean basin collapse starts out by subducting the margin of the North American plate under an island arc.

Some Other Landmarks in the Ordovician Period, tectonically speaking

To begin with, there were a lot of island arcs and microcontinents floating around. Here are the routes of some North America and Europe related microcontinents.

• Eastern Avolonia (southern Europe) is rifted from western Gondwana (Africa) Mid - Late Ordovician and begins to drift north. It won't meet the rest of Europe until the Silurian.

• Armorica is also rifted off of western Gondwana, and follows Avalonia north. Armorica is also slated to become part of southern Europe (France, Spain, Germany, Iberia, and southern Poland).

• In the late Ordovician Avalon (which is to become Newfoundland, Nova Scotia, and New Brunswick) also rifts from Gondwana and moves north.

• The Taconic Orogeny begins in the Late Ordovician Period. This is the collision of what is now the eastern coast of North America with an island arc, and this arc came to rest in the Central Mobile Belt of the Appalachians.

Although the Iapetus is closing during this period, and Laurentia and Baltic are getting closer to each other, they do not actually collide until the Silurian.

Reflections on the Overall Climate of the Ordovician

During the later part of this period, the supercontinent resting at the south pole is subjected to a large amount of glaciation.

This results in a drop in sea level (of about 70 m) and a drop in global temperatures. This is reflected by the migration of animals from the poles towards the more equatorial regions of the oceans; for example, cold water trilobite faunas appear in North American strata. The drop in sea level also drains many shallow seas, reducing the amount of shelf space over all. And this regression was followed by a transgression.

And this brings us to the end of the Ordovician, and to the extinction

The End Ordovician Event 450 - 440 MA

Some Relevant Numbers:

• For everyone - loss of twelve percent of all families
• Cephalopods - loss of about half of the families and a third of the genera
• Bryozoans - loss of only a tenth of the species (not really effected on a world-wide scale)
• Brachiopods - loss of a third of the families, and less than half of the genera
• Trilobites - loss of two thirds of the families and over half of the genera
• Crinoids - lost twelve families
• Cystoids - loss of about half of their families and seventy percent of their genera
• Conodonts - loss of over seventy percent of the genera and nearly eighty percent of the species

Some general facts about this extinction:

• Organic reefs are consequently quite rare in the following period.
• There was a rather unsurprising reduction in limestone production.
• The trilobites never really recovered from this event.

Some possible explanations of this event:

Loss of habitat - due to the regression of the sea, due to the rapid change in climate, all due, ultimately, to the glaciation of Gondwana. And don't forget that there are many island arcs and microcontinents (and even Baltica and Laurentia themselves) that are slowly approaching each other, as ocean basins close between them.

Anoxia - resulting from a "stepped rise in sea level," which would be caused by the intermittent release of icebergs from glaciers. And this ice would melt as it drifted equator-wards and sink, causing oceanic overturn, which would bring the more nutrient rich (and potentially toxic) waters to the surface.

Bibliography

Composed by Amy Barbour for Geology 204.