The Ordovician Period

Paleogeography & Tectonics

High sea level. Cratons flooded.
North America covered by shallow epicontinental seas.
Several sea level fluctuations (trans & regr)

Baltica nears Laurentia causing Taconic Orogeny & volcanism.

Collapse of Cambro-Ordovician carbonate platform and deposition of clastic wedge in eastern North America.

First "tectonic cycle" in Appalachians:

• SS molasse (red SS)
• SH flysch (black shale & turbidites)
• LS shallow shelf deposits (carb. platform)

Evidence for volcanism found in GA - layer of volcanic ash - bentonite - interbedded with carbonates; has caused silicification of underlying beds (green chert)

Paleoclimatology

Life

1. Early Ord - a time of adaptive radiation of many faunal groups, following the mass extinction of trilobites and nautiloids at end of Cambrian.
   150 families -> 400 families

2. Representative fauna:
   brachiopods, rugose & tabulate corals, bryozoans, crinoids, conodont animals, JAWLESS fishes

3. Most marine organisms were epifaunal; few infaunal

4. Predators
   Evolution of increasing effective predators.
   Major predators were invertebrates
   
   • nautiloids
   • starfish

5. Decline of stromatolites

6. New kinds of reefs appeared
   bryozoans, stromatoporoids, tabulate corals

7. Plants probably colonized the land; land plants were well established by the Silurian.

8. Mass extinctions of tropical marine faunas at end of Ord.
   100 families extinct

   more than half of the bryozoan and brachiopod species died out

   Causes?
   

   • climatic cooling
   • major glaciation
   • sea level drop
   • Iapetus Ocean (proto-Atlantic) closed, eliminating habitats
   • Cambro-Ord platform collapsed
   • Taconic Orogeny

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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
October 1995.
Modified November 12, 1996
Last modified July 17, 1999