Reference: McDowell, R.C. and A.P. Schultz. 1990. Structural and stratigraphic framework of the Giles County area, a part of the Appalachian Basin of Virginia and West Virginia. U.S. Geological Survey Bulletin 1839-E. 24 p.
This report covers all of Giles Co., VA, plus parts of adjacent VA and WV counties. The geology of this area is typical of the central Appalachians.
Exercise:
You should familiarize yourself with the geology of Giles Co. by reading the report and studying the geologic map. The map is a newer version of the one we normally use, which is not conducive to the online format. It can be accessed by using the link: https://ngmdb.usgs.gov/Prodesc/proddesc_39822.htm
The map can be viewed by clicking on it and using the zoom buttons or your mouse wheel. Even though this is only a preview, you will be able to see all of the relevant symbols and map units which are defined in the lower right quarter of the map.
Next, compare Figure 6 with the geologic map and determine for yourself on the geologic map the location of all the faults and folds shown in Figure 6.
Questions: (neatly write your answers on separate pages, or use a word processor)
- According to Figure 1, in which two structural provinces is Giles Co. Situated? What is the age range of the rocks in the two provinces? What feature “marks the boundary?
- Calculate the total thickness of the stratigraphic section in Giles Co. Use the mid-value of the thickness range given for each stratigraphic unit in Figure 5.
- List the names, from oldest to youngest, of the Mississippian formations.
Structural Geology
- Name the four major thrust faults (Figure 6).
- Name the six major folds (Figure 6).
- Why is there no anticline between the Pearisburg and Glen Lyn synclines?
- Why does the New River flow perpendicular to strike at the Narrows, whereas it meanders through the central part of Giles Co.?
- What is the oldest formation outcropping in the map area? Where does it outcrop?
- What kind of unit is C? It is labelled with the light bluish-green with whitish dots.
- According to the explanation (key), what is the age relationship between unit C and the bedrock in Giles Co.? Locate Butt Mountain (Yes, Butt Mountain). It is located in the purple Stk unit east of the New River to southeast of the Narrows Fault label on the eastern side of the map. How does the map pattern support the the age relationship?
- What are some typical dips on the hanging (head) wall of the St. Clair thrust fault? (Look for strike and dip symbols on the geologic map.)
- What are some typical dips on the foot wall of the St. Clair thrust fault? (Look for strike and dip symbols on the geologic map.)
- Sketch a rough geologic cross section of the St. Clair Fault, along the straight stretch of the New River in the area in Giles county that juts out to the northwest like a tooth. Your section should include units OC through Oj in the headwall, and units Du through Mh on the footwall. You can use a strip of paper to mark the contacts just like we did in class. Label the section line N-N’. You can ignore topography.
Seismicity
- What happened on May 31, 1897? Could it happen again?
- Describe/discuss the potential seismic hazards for Giles Co. and surrounding areas.
- What is the cause of the seismic zone underneath Giles Co.?
Mineral Resources
- Which formation has significant manganese deposits? Are these deposits currently of commercial value?
- Which formation has been quarried for crushed stone?
- Which formation has mineable coal beds? What is its rank? Why is very little coal currently being mined?
- Is there any commercial production of natural gas or oil in Giles Co.?
Geologic Hazards - What potential geologic hazards exist in Giles Co.? List them.
- What is the origin of the name Sinking Creek in the lower right of Giles Co.?
- If you were concerned about possible landslides or sinkholes that could affect roads or buildings, what reference would you consult to find a map of possible hazards?
- The town of Staffordsville, along Walker Creek near the southern edge of the map needs a location for a sanitary landfill. Landfills must be removed from towns and out of sight if possible, easily excavated, and they are often situated in shale or mudrock which limits the leaking leachate into the ground or streams. Leachates are liquids produced by seepage of rainwater and groundwater through the landfill, which then leaches chemicals from the buried waste. Based on your knowledge of geology, and the above restrictions, propose a good site for a landfill.