Local Stratigraphic Sections

Historical Geology

Local Stratigraphic Sections

In the heart of Colonial Williamsburg, there is a deep groundwater well that was installed by the Virginia Department of Environmental Quality. The water drawn from this well is used by Colonial Williamsburg largely for geothermal heating and cooling. We are interested in this well because with it being so deep, we have a unique chance to learn about the stratigraphy underneath the Peninsula region. (If you’d like to see specific data about this well, click HERE)

Below are the notes from the drilling operations, which identified 1) the units in the ground, and 2) the depth of the contacts between each formation.

Unit Depth (feet) Elevation (feet above sea-level) Thickness (in feet) Scaled thickness (in cm)
Pleistocene units, undifferentiated 0 80
Pleistocene units, undifferentiated 25 55 25 1.25
Yorktown Formation 80
Cobham Bay Member of the Eastover Formation 105
Claremont Manor Member of the Eastover Formation 180
St Mary’s Formation 230
Calvert Formation 275
Piney Point Formation 290
Nanjemoy Formation 340
Marlboro Clay 365
Aquia Clay 395
Potomac Formation 475

You’ll notice that all the measurements provided to you are in feet below the surface of the earth (i.e. depth). However, sometimes it is helpful to know the elevation of these units (using the elevation, rather than depth, makes it much easier to compare units over different areas). You may want to copy and paste the table above into Excel to us it as a template to calculate the next few answers. Eventually we will use these to draw a stratigraphic section. Here are the steps to follow:

A. The starting elevation of the well is 80-feet above sea-level. Use this number to determine the elevation of each contact and type your answers into the Canvas quiz. (10 pts)

B. Determine the thickness of each layer (in feet) and type your answers in to the Canvas quiz. (10 pts)

C. Now we will determine how thick each layer should be on your stratigraphic section. The vertical scale we’ll use is 1 cm (on your drawing) is equal to 20 feet (in the real world). Enter these values into the Canvas quiz. (10 pts)

D. Using all the numbers you just calculated, create a scaled stratigraphic section. Be sure to label each unit, and include the elevation and depth of each contact. Here is an example of a stratigraphic section. Take a picture of your drawing and upload it as the answer to the quiz in Canvas. (25 pts)

Questions (25 pts)

1. The Eastover Formation shows up twice in the section above, but with different members. What is the relationship between a formation and a member? (2 pts)

2. What type of stratigraphy are these units an example of? (1 pt)

3. Based on your answer to the question above, what sort of characteristics would differentiate one unit from another? (2 pts)

4. If we defined the difference between the Yorktown and Eastover formations based on the index fossils Isognomon maxillita and Chesapecten jeffersonious, then what type of stratigraphy would we be doing? (1 pt)

5. Aside from using fossils or minerals and grain size, what other characteristics could a geologist use to define distinctly different layers of sediments or sedimentary rock? (2 pts)

Digging a little deeper. Use the website below to answer the following questions:

https://macrostrat.org/sift/#/

then, go to the search window and select “Stratigraphic names”. Use the names from the units in your stratigraphic section to complete the following questions

6. Choose any two formations. Which ones did you choose and what are the main differences between them? (3 pts)

7. What is the elevation of your home? How did you determine this? (2 pts)

8. Based on the stratigraphic section from above, which unit would you expect to find at the surface of your home? (1 pt)

9. Do a little research on this unit using the Macrostrat link above. What is the age of this unit? What units are directly above and below and what the ages of these units?(3 pts)

10. In what sort of depositional environment were these sediments deposited (be more specific than marine, terrestrial, or transitional). (2 pts)

11. Are there fossils within this unit? If so, which ones? (2 pts)

12. What else did you find out about that unit? (2 pts)

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