ASBOG Field Geology and Mapping 4 — Questions and Answers
Question 1: During field mapping, a geologist finds that the same formation appears twice on a traverse crossing a valley. Which structure MOST likely explains this repetition?
- Unconformity
- Thrust fault or overturned fold (Correct answer)
- Normal fault
- Laccolith intrusion
Correct answer: Thrust fault or overturned fold
Repetition of the same formation in the same order across a traverse typically indicates a thrust fault or tight fold that has duplicated the section.
Question 2: A Brunton compass is used to measure the trend and plunge of a lineation. After sighting along the lineation downhill, the geologist reads 035° and 28°. This is reported as:
- Strike N35E, dip 28°NW
- Trend 035°, plunge 28° (Correct answer)
- Bearing N35W, inclination 28°
- Azimuth 035°, rake 28°
Correct answer: Trend 035°, plunge 28°
Lineations are described by trend (azimuth of horizontal projection) and plunge (inclination below horizontal), written as trend/plunge.
Question 3: Which of the following best describes the purpose of a pace-and-compass traverse in geologic mapping?
- To measure the dip of rock units
- To locate position and record observations where GPS is unavailable or unreliable (Correct answer)
- To calculate the strike of a fault
- To measure thickness of a stratigraphic unit
Correct answer: To locate position and record observations where GPS is unavailable or unreliable
A pace-and-compass traverse allows the geologist to track position and record observations in areas where GPS signals are poor or unavailable.
Question 4: On a standard USGS 7.5-minute topographic map used for geologic mapping, what does a brown dashed line typically represent?
- A perennial stream
- A depression contour
- An unpaved road or trail (Correct answer)
- A township boundary
Correct answer: An unpaved road or trail
On USGS 7.5-minute topographic maps, brown dashed lines represent unpaved roads, jeep trails, or foot trails.
Question 5: A geologist maps a pluton and finds that the surrounding sedimentary rocks are recrystallized adjacent to the contact. This metamorphic aureole suggests:
- The pluton is older than the sedimentary rocks
- The pluton intruded the sedimentary rocks and baked them (Correct answer)
- A regional metamorphic event unrelated to the pluton
- The sedimentary rocks are xenoliths within the pluton
Correct answer: The pluton intruded the sedimentary rocks and baked them
A metamorphic aureole indicates contact metamorphism, confirming that the pluton intruded the host rocks after they were deposited.
Question 6: Which type of unconformity is characterized by the absence of a tilting or angular relationship between the rocks above and below the erosional surface?
- Angular unconformity
- Disconformity (Correct answer)
- Nonconformity
- Paraconformity
Correct answer: Disconformity
A disconformity occurs where parallel sedimentary beds are separated by an irregular erosional surface representing a time gap.
Question 7: When plotting stereonet data for a series of bedding planes, the pole to each plane plots as a point. A concentration of poles in one area of the stereonet indicates:
- Randomly oriented beds
- Consistently oriented beds with similar strike and dip (Correct answer)
- A fold axis perpendicular to the pole cluster
- A vertical fault surface
Correct answer: Consistently oriented beds with similar strike and dip
A tight cluster of poles on a stereonet indicates that the beds share a consistent orientation, suggesting uniform regional dip.
ASBOG Field Geology and Mapping 4
During field mapping, a geologist finds that the same formation appears twice on a traverse crossing a valley.
Which structure MOST likely explains this repetition?