If rocks have been broken down and moved away over time, they will no longer be part of the stratigraphic record. Describe at least 2 ways to recognize unconformities. How can you tell rocks are missing?

1.      If rocks have been broken down and moved away over time, they will no longer be part of the stratigraphic record. Describe at least 2 ways to recognize unconformities. How can you tell rocks are missing?

 

Figure 1: Stratigraphic section containing 3 strata (rock layers) and 5 fossils, labeled as A, B, C, D, and E.  Some fossils appear in more than one layer.

 

2. 3.  Geologic maps + cross-sections.According to the principle of superposition, which layer is the oldest?

Fossil “A” is found in layers 1 and 3. Make a hypothesis about that fossil and/or the rock layers based on that observation.

To test your hypothesis and determine the absolute age of the layers, you can:

a.      Identify each of fossil.  These fossils are associated with specific periods of geologic time.  This has been done for you, and the period of time associated with each fossil is shown in Table 1.

b.      Analyze the fossil assemblages for each layer to see if we narrow time of formation based on the overlap of associated fossil time periods.  See Table 1.

c.       Determine the time period of formation for each layer, and look for unconformities.

 

In what time period did layer 1 form? (see Table 1) 

 In what time period did layer 2 forms?

In what time period did layer 3 forms? 

An unconformity exists between layers 2 and 3.  What period of time is missing? 

 

3.  Geologic maps + cross-sections.

Often geologists do not get to see cross-sections of the inside of the Earth (though we do love road cuts for this reason!) Instead, geologists look at the rocks exposed at the Earth’s surface to determine the geologic history of an area.  Often, geologists take cores to get a look inside the Earth to test their hypotheses. 

In this section, you will see cross-sections such as the ones you saw in Parts 1&2 of the lab, and corresponding surface features.  Next week’s lab will focus on what we see from the surface view only.


                         a                                                                         b

 

 Put the layers shown in figures a. and b. above in order from oldest to youngest:

 

a.      Figure a: __________________________

 

b.      Figure b:__________________________

 

Compare and contrast what is seen on the map view for each.  How are they the same?  How are they different?

 

 


4. Assume that in the image to the left, “1” represents the oldest layer and “3” represents the youngest/most recent layer.  Would the “cross section view” of this diagram look more like figure a. or figure b. above?

 

 

If rocks have been broken down and moved away over time, they will no longer be part of the stratigraphic record. Describe at least 2 ways to recognize unconformities. How can you tell rocks are missing?

 

Fa21_Lab2_RelativeDating_2

 

 

 

 

APA

 

 

 

 

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