The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin


Chapter 10 - page 5

Early Paleozoic Events


Ordovician Paleogeography

Global paleogeography during the Cambrian Period, at about 520 mya. Global paleogeography during the Middle Ordovician

LEFT = Global paleogeography during the Cambrian Period
RIGHT = Global paleogeography during the Ordovician Period

Compare the Ordovician and Cambrian paleogeography. The continents are distributed along the equator.

Note the Ouachita Terrane or "Ouachita embayment microcontinent" (orange in Ordovician), shown as the "Ouachita trough segment" in green on the Cambrian map. This part of Laurentia/North America has broken off, and is a microcontinent headed for a collision with South America in the Andes region. This is the missing part of the Appalachian Mountain chain between Alabama and Arkansas.

The Taconic Orogenic Belt lies between Laurentia (North America) and Baltica (Europe and western Russia) in the Ordovician.

global distribution of continents during the Ordovician Legend
Global distribution of continents during the Ordovician, showing water depth and presence of evaporite deposits (E), which indicate arid paleoclimates.

Global sea levels were high. Shallow seas cover large areas of some of the continents, particularly North America and Siberia.

By the Middle Ordovician, Gondwanaland is moving toward the South Pole, leading to glaciation in Africa at the end of the Ordovician.

Glacial deposits are present in NW Africa (Sahara desert region), indicating that this region was located in the South Pole region.

Sea levels fluctuated during the Ordovician, and dropped sharply at the end of the Ordovician, coinciding with the glaciation.

Times of glaciation in geologic history.
Note the several times of glaciation (yellow) in geologic history, including the Late Ordovician-Early Silurian glaciation.


Taconic Orogeny

Sea levels were high in North America during the early Ordovician, and the craton was flooded by the Sauk epicontinental sea. In the Appalachian area, shallow water carbonate rocks were deposited during the Cambrian and early Ordovician. Shallow water deposition is indicated by the presence of mudcracks and stromatolites.

The depositional setting changed dramatically during the Middle Ordovician. Carbonate sedimentation ended. The carbonate platform in eastern North America collapsed or was downwarped.

This was caused by the partial closure of the Iapetus Ocean along a subduction zone.

As the Iapetus Ocean narrowed, a volcanic island arc approached and collided with the North American craton, causing folding, faulting, metamorphism, and mountain building.

Note the volcanoes in the Appalachian region. Volcanic ash deposits are found in Ordovician rocks throughout the eastern US. (Now altered to a clay called bentonite).

This mountain-building event in the Appalachian region is called the Taconic orogeny. 480 - 460 my.

Paleogeographic map of North America during the Ordovician
Paleogeographic map of North America during the Ordovician.

 

Plate tectonic cross-section
Plate tectonic cross-section showing forces that caused the Taconic Orogeny.
A - Eastern North America in the Cambrian and early Ordovician, following the breakup of Rodinia.
B - Large volcanic island arc nears eastern North America.
C - Volcanic island arc collides with eastern North America causing deformation and mountain-building in the Appalachian region (orogenic belt).


Ordovician Sedimentary Deposits

As the Taconic mountains belt eroded, Upper Ordovician to Lower Silurian red sandstones and shales were deposited to the west in huge delta systems.

These sediments formed a wedge-shaped deposit known as the Queenston clastic wedge, or the Queenston delta. These red deltaic sediments coarsen and thicken to the east (toward the mountainous source area), and become thinner and finer grained to the west.

The size of the clastic wedge suggests that the mountains may have been more than 4000 m (13,100 ft) high.

Clastic wedges, or fans of clastic sediment
Clastic wedges, or fans of clastic sediment spread westward, derived from the erosion of the mountain range that developed in eastern North America during the Taconic orogeny.

There are two main highlands areas, the higher of the two is in the northern Appalachians.

The area in eastern North America that had been deep water shales during the Cambrian (see Cambrian map of North America) has been deformed and uplifted to form the Taconic mountain belt.

The shales have been altered to metamorphic and igneous rocks by the high temperatures and pressures associated with mountain building (orogeny).

The Caledonian orogenic belt (which extends along the northwestern edge of Europe) is part of the same trend as the Taconic orogenic belt. The Caledonian orogeny reached its climax slightly later, in the Late Silurian to early Devonian. The Caledonian event is recognized in the Canadian Maritime Provinces, northeastern Greenland, northwestern Great Britain, and Norway.


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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

November 11, 2005