Part D

Fossil Dating

Fossilized animal and plant material is commonly used to determine the relative age of rock units. Single fossils or index fossils can be powerful age-dating tools (see Figure 6-11), but combinations of fossils are more commonly used. A fossil assemblage is a group of fossils collected from the same rock unit (from the same depositional horizon, if possible). The relative age of the sedimentary host rock may be determined once the fossils are identified and their geologic ranges can be plotted. The area of overlap of all the ranges defines the interval of time in which all of the organisms coexisted.

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Index Fossils

Some fossils are especially helpful in dating rock. These fossils must satisfy several different conditions to be considered an index fossil:

> abundant (lots and lots of them around)

> short-lived (lived for a relatively brief span of geologic time so have a small geologic age range)

> widespread geographic distribution (not affected by local changes in sedimentary environments (facies changes))

> distinctive in appearance (easily recognizable)

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Geologic Age Ranges

The life span of an organism throughout geologic time is represented by its geologic age range (Figure 6-12). A geologic age range diagram also shows the relative abundance of the organism with time (the wider the geologic range stripe, the greater the abundance of that critter).

Quiz Me! / D31

 

Quiz Me! / D32

 

Quiz Me! / D33

 

Quiz Me! / D34

 

Quiz Me! / D35

 

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Using Fossil Assemblages to Date a Rock Unit

When using fossils to date a rock unit, a group of fossils (a.k.a. a fossil assemblage) must be collected from the same spot in the rock unit (from the same depositional horizon, if possible). The relative age of the sedimentary host rock may be determined once certain information (phylum, genus, etc.) is known about one or more fossils. Once the fossils are identified, their geologic ranges can be plotted (see Figure 6-13). The area of overlap of all the ranges defines the interval of time in which all of the organisms coexisted when the rock was deposited.

Here is a summary of the fossil assemblage in Figure 6-13:

> Fossil 5 is the best single index fossil because it has the smallest geologic range.

> Fossil 6 is the worst single index fossil because it has the largest geologic range.

> The combination of Fossil 3 and Fossil 4 is the most definitive because their geologic ranges have the smallest amount of overlap.

 

Questions D36 through D39 refer to the geologic age range diagram in Figure 6-14. To answer the following questions, print (or draw) the PDF of Figure 6-14 below (see PDF link) and then draw the geologic age range lines for each fossil using the fossil assemblage geologic range information given.

Quiz Me! / D36   

 

Quiz Me! / D37 

 

Quiz Me! / D38 

 

Quiz Me! / D39

 

Questions D40 through D43 refer to the geologic age range diagram in Figure 6-15. To answer the following questions, print )or draw) the PDF of Figure 6-15 below (see PDF link) and then draw the geologic age range lines for each fossil using the fossil assemblage geologic range information given.

 

Quiz Me! / D40

 

 

Quiz Me! / D41

 

Quiz Me! / D42

 

Quiz Me! / D43

 

Questions D44 through D50 refer to the stratigraphic column in Figure 6-16 and the geologic age range diagram in Figure 6-17.

You suspect that there is a disconformity between the Grant Formation and Benson Sandstone, but there is no way to tell without looking at the fossils in the two units. You collect more fossils from each unit (see Figure 6-17) to determine if there is a significant age difference. To answer the following questions, print (or draw) the PDF of Figure 6-17 below (see PDF link) and then draw the geologic age range lines for each fossil using the fossil assemblage geologic range information given.

 

 

 

Quiz Me! / D44

 

Quiz Me! / D45

 

Quiz Me! / D46

 

Quiz Me! / D47

 

Quiz Me! / D48  

 

Quiz Me! / D49

 

Quiz Me! / D50