ASBOG - Association of State Boards of Geology Field Geology and Mapping Questions and Answers — Questions and Answers
Question 1: A geologist is mapping a sedimentary sequence in an area with a known magnetic declination of 12° East. When using a Brunton compass to measure the strike of a sandstone bed, which of the following adjustments is necessary to obtain the true geologic strike?
- The compass must be held perfectly level using the bull's-eye level.
- The clinometer must be set to zero before taking the measurement.
- No adjustment is needed; the raw reading provides the true strike.
- The azimuth ring must be rotated so the index pin is set to 12°. (Correct answer)
Correct answer: The azimuth ring must be rotated so the index pin is set to 12°.
To correct for magnetic declination, the azimuth ring on the Brunton compass must be adjusted. For an East declination, the index pin is turned the corresponding number of degrees counter-clockwise (to the East on the dial) from 0°. This adjustment ensures that the compass needle's reading relative to the adjusted dial will indicate true north, thereby providing an accurate geologic strike measurement.
Question 2: While creating a geologic cross-section from a map, which of the following is the most critical first step to ensure the accurate depiction of subsurface structures?
- Drawing a topographic profile along the line of section without vertical exaggeration. (Correct answer)
- Coloring the different geologic units on the map before starting the section.
- Assuming all contacts are vertical to simplify the initial drawing.
- Extrapolating the surface geology to a depth of at least 5,000 feet.
Correct answer: Drawing a topographic profile along the line of section without vertical exaggeration.
The foundational step for a geologically accurate cross-section is establishing the ground surface. This is done by creating a topographic profile along the chosen line of section. It is crucial that this profile has no vertical exaggeration (a 1:1 scale) initially, because exaggerating the vertical scale will distort the apparent dip of geologic contacts and structures, leading to an incorrect interpretation of the subsurface geology.
Question 3: On a standard geologic map, what does a strike and dip symbol that looks like a 'T' with the number '35' next to the shorter line represent?
- A horizontal bed with an elevation of 35 feet.
- A planar feature striking parallel to the long line and dipping 35 degrees in the direction of the short line. (Correct answer)
- A vertical fault with 35 feet of offset.
- A fold axis trending in the direction of the long line with a plunge of 35 degrees.
Correct answer: A planar feature striking parallel to the long line and dipping 35 degrees in the direction of the short line.
The standard symbol for strike and dip on a geologic map uses a long line to represent the strike direction of a planar feature (like a sedimentary bed or fault). The shorter, perpendicular line points in the direction of the dip. The accompanying number indicates the angle of the dip in degrees from the horizontal.
Question 4: A geologist is mapping a contact between a sandstone and a shale unit on a hillside. The outcrop width of the sandstone unit on the map is 200 meters. The beds dip at 30 degrees into the hill, and the ground surface is perfectly flat. What is the true stratigraphic thickness of the sandstone unit?
- 173 meters
- 400 meters
- 200 meters
- 100 meters (Correct answer)
Correct answer: 100 meters
The true stratigraphic thickness (T) can be calculated using the formula: T = W * sin(d), where W is the outcrop width on a horizontal surface and d is the dip angle. In this case, T = 200 meters * sin(30°). Since sin(30°) = 0.5, the true stratigraphic thickness is 200 * 0.5 = 100 meters.
Question 5: Which of the following represents the most common type of error or 'blunder' made when transferring structural data from field maps to a final published geologic map?
- Incorrectly calculating the true stratigraphic thickness.
- Misidentifying the rock type of a mapped unit.
- Using the wrong symbol for a measurement (e.g., bedding vs. cleavage) or recording an incorrect dip value. (Correct answer)
- Failing to correct for magnetic declination during fieldwork.
Correct answer: Using the wrong symbol for a measurement (e.g., bedding vs. cleavage) or recording an incorrect dip value.
Studies analyzing the transfer of data from field notes to published maps have shown that the most frequent blunders are human errors in transcription. These include using the wrong symbol for a structural feature (e.g., mistaking a bedding symbol for a cleavage or joint symbol) and errors in transcribing the numerical value of the dip angle.
Question 6: During a field mapping project in a structurally complex area, a geologist encounters an outcrop where the bedding planes are contorted, and a consistent strike and dip measurement is impossible. What is the BEST practice for documenting this location in the field notes and on the map?
- Leave the area blank on the map as no reliable data can be collected.
- Average the various orientations to create a single representative strike and dip.
- Use a specific symbol for contorted or convoluted bedding and provide a detailed sketch and description in the field notebook. (Correct answer)
- Record the measurement of the most dominant fold limb and ignore the others.
Correct answer: Use a specific symbol for contorted or convoluted bedding and provide a detailed sketch and description in the field notebook.
In areas of complex deformation where a single planar measurement is not representative, the best practice is to use a specific map symbol designated for such conditions (e.g., convoluted bedding symbol). This should be supplemented with detailed field notes, including sketches, photographs, and descriptions of the structural style. This approach accurately documents the complexity without oversimplifying or misrepresenting the geology.
ASBOG - Association of State Boards of Geology Field Geology and Mapping Questions and Answers
A geologist is mapping a sedimentary sequence in an area with a known magnetic declination of 12° East.
When using a Brunton compass to measure the strike of a sandstone bed, which of the following adjustments is necessary to obtain the true geologic strike?