ASBOG Field Geology and Mapping 5 — Questions and Answers
Question 1: A geologist observes graded bedding in a turbidite sequence. In which direction does grain size increase within a single graded bed?
- Grain size increases upward
- Grain size increases downward toward the base (Correct answer)
- Grain size is uniform throughout
- Grain size increases laterally
Correct answer: Grain size increases downward toward the base
Graded bedding in turbidites shows coarser grains at the base and progressively finer grains toward the top of each bed.
Question 2: During reconnaissance mapping of a volcanic terrane, a geologist should use which feature FIRST to distinguish lava flows from pyroclastic deposits?
- Absolute age dating
- Vesicularity, welding texture, and presence of lithic fragments (Correct answer)
- Geochemical analysis of major elements
- Seismic reflection data
Correct answer: Vesicularity, welding texture, and presence of lithic fragments
Field textures such as vesicles in flows, welding and eutaxitic texture in ignimbrites, and abundant lithic fragments in fall deposits allow rapid discrimination.
Question 3: On a geologic cross-section, the geologist must project a contact from the surface to depth. Which information is MOST critical for this projection?
- The formation's color and fossil content
- The true dip and dip direction of the contact (Correct answer)
- The formation's resistance to weathering
- The age of the overlying unit
Correct answer: The true dip and dip direction of the contact
True dip and dip direction are the geometric data needed to accurately project a planar contact from surface to subsurface depth.
Question 4: A geologist recognizes a set of parallel joints in an outcrop. To determine if the joints are systematic (tectonic), which observation is MOST diagnostic?
- Joints cut through multiple lithologies and maintain consistent orientation (Correct answer)
- Joints are curved and irregular
- Joints are filled with calcite
- Joints are present only in igneous rocks
Correct answer: Joints cut through multiple lithologies and maintain consistent orientation
Systematic joints are planar, parallel, and persistent across multiple rock types, reflecting a regional stress field rather than local effects.
Question 5: Which field method best determines the thickness of a steeply dipping stratigraphic unit in the absence of a drill hole?
- Measuring the map width and applying a dip correction using t = w × sin(dip) (Correct answer)
- Counting annual varves in the unit
- Using a Jacob's staff along a horizontal traverse
- Extrapolating from seismic velocity data
Correct answer: Measuring the map width and applying a dip correction using t = w × sin(dip)
True thickness is calculated from map width (w) and dip angle using the formula t = w × sin(dip), corrected for topographic slope if needed.
Question 6: A geologist maps a series of en echelon fractures oriented obliquely to a fault zone. These are most consistent with which stress condition?
- Pure tensional stress perpendicular to the fault
- Right-lateral or left-lateral shear along the fault zone (Correct answer)
- Vertical compressional loading
- Isotropic hydrostatic pressure
Correct answer: Right-lateral or left-lateral shear along the fault zone
En echelon fractures form at an angle to a shear zone and are classic kinematic indicators of strike-slip (right-lateral or left-lateral) faulting.
Question 7: When preparing a final geologic map for submission, which element is considered MANDATORY for professional standards?
- A photograph of every outcrop
- A north arrow, scale bar, legend, and correlation of map units (Correct answer)
- A list of all GPS waypoints
- Chemical analyses of all igneous rocks
Correct answer: A north arrow, scale bar, legend, and correlation of map units
Professional geologic maps require a north arrow, graphic scale bar, legend defining all symbols and map units, and a correlation chart showing relative age.
ASBOG Field Geology and Mapping 5
A geologist observes graded bedding in a turbidite sequence.
In which direction does grain size increase within a single graded bed?