Field Geology and Mapping Flashcards
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Read the first 7 Field Geology and Mapping flashcards as text
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?
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.
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:
Answer: Trend 035°, plunge 28°
Lineations are described by trend (azimuth of horizontal projection) and plunge (inclination below horizontal), written as trend/plunge.
Which of the following best describes the purpose of a pace-and-compass traverse in geologic mapping?
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.
On a standard USGS 7.5-minute topographic map used for geologic mapping, what does a brown dashed line typically represent?
Answer: An unpaved road or trail
On USGS 7.5-minute topographic maps, brown dashed lines represent unpaved roads, jeep trails, or foot trails.
A geologist maps a pluton and finds that the surrounding sedimentary rocks are recrystallized adjacent to the contact. This metamorphic aureole suggests:
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.
Which type of unconformity is characterized by the absence of a tilting or angular relationship between the rocks above and below the erosional surface?
Answer: Disconformity
A disconformity occurs where parallel sedimentary beds are separated by an irregular erosional surface representing a time gap.
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:
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.