Geology 12 - Historical Geology
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Ordovician

Reading: Chapter 11 (excluding the section on Ordovician life)

We'll be doing the history fo life later, so you can skip the section on Ordovician life (p.230-238).

This chapter has a lot of references to old geosynclinal theory (p. 245-246). If you find it confusing, don't worry too much about it. We are interested in the plate tectonic interpretation of events.

You can follow the plate tectonic history of North America here, by looking for the yellow chunks of contnent in each map. Bookmark this site to look at later in the semester.

Background

Terms you should know:

Key Concepts and Questions:

1. Ordovician regression and transgression

What is the evidence that the sea regressed in Mid-Ordovician, and again transgressed in the Late Ordovician?

 

 

Why is the St. Peter Sandstone such pure quartz?

 

 

2. Ordovician of the Appalachian region

Look carefully at Figure 11.15 (the key to the map symbols is on p. 213. Note that red dots in the sandstone indicate a redbed; marine sanstones are yellow dots, as in Figure 10.8).

Do the blue lines bounding the white areas of the map (e.g., where it says "trans-continental arch") represent shorelines of the Ordovician sea, or do they represent places where there once was Ordovician rock that has since eroded away? How do you know?

 

 

 

What tectonic basin type (rift, passive margin, forearc, etc.) do each of these areas on the map correspond to?

• carbonate rocks of the interior of the continent

 

• redbeds in the Appalchian region

 

Note that the numbers in boxes are isotopic ages for granite intrusions. Given the basin types and granites, what kind of plate boundary existed here during the Late Ordovician?

 

 

What part of the modern world is most like the Ordovician Appalachians?

 

 

 

Assessment

1. Ordovician sea level

You should be able to:

• describe the evidence and effects of changes in sea level during the Ordovician

2. Appalachians

You should be able to:

• describe how the Ordocivician rocks of the Appalachian region tell us the plate setting at that time